*********************************************************************** United States Navy Electricity & Electronics Training Series - NEETS *********************************************************************** The Navy Electricity and Electronics Training Series [NEETS] was developed for use by personnel in many electrical and electronic related Navy ratings. Written by, and with the advice of, senior technicians in these ratings, this series provides beginners with fundamental electrical and electronic concepts through self-study. The presentation of this series is not oriented to any specific rating structure, but is divided into modules containing related information organized into traditional paths of instruction. The series is designed to give small amounts of information that can be easily digested before advancing further into the more complex material. For a student just becoming acquainted with electricity or electronics, it is highly recommended that the modules be studied in their suggested sequence. While there is a listing of NEETS by module title, the following brief descriptions give a quick overview of how the individual modules flow together. *********************************************************************** Module 1 - Introduction to Matter, Energy and Direct Current Introduces the course with a short history of electricity and electronics and proceeds into the characteristics of matter, energy and direct current (DC). It also describes some of the general safety precautions and first-aid procedures that should be common knowledge for a person working in the field of electricity. Related safety hints are located throughout the rest of the series, as well. 2.00 MB *********************************************************************** Module 2 - Introduction to Alternating Current and Transformers An introduction to alternating current (AC) and transformers, including basic AC theory and fundamentals of electromagnetism, inductance, capacitance, impedance and transformers. 3.87 MB *********************************************************************** Module 3 - Introduction to Circuit Protection, Control and Measurement Encompasses circuit breakers, fuses and current limiters used in circuit protection, as well as the theory and use of meters as electrical measuring devices. 2.36 MB ***********************************************************************
*********************************************************************** Module 4 - Introduction to Electrical Conductors, Wiring Techniques and Schematic Reading Presents conductor usage, insulation used as wire covering, splicing, termination of wiring, soldering and reading electrical wiring diagrams. 1.49 MB *********************************************************************** Module 5 - Introduction to Generators and Motors Is an introduction to generators and motors and covers the uses of AC and DC generators and motors in the conversion of electrical and mechanical energies. 1.28 MB *********************************************************************** Module 6 - Introduction to Electronic Emission, Tubes and Power Supplies Ties the first five modules together in an introduction to vacuum tubes and vacuum-tube power supplies. 1.53 MB *********************************************************************** Module 7 - Introduction to Solid-State Devices and Power Supplies Similar to module 6, but it is in reference to solid-state devices. 2.41 MB *********************************************************************** Module 8 - Introduction to Amplifiers Covers amplifiers. 1.19 MB *********************************************************************** Module 9 - Introduction to Wave-Generation and Wave-Shaping Circuits Discusses wave generation and wave-shaping circuits. 1.81 MB *********************************************************************** Module 10 - Introduction to Wave Propagation, Transmission Lines and Antennas Presents the characteristics of wave propagation, transmission lines and antennas. 2.40 MB *********************************************************************** Module 11 - Microwave Principles Explains microwave oscillators, amplifiers and waveguides. 4.28 MB *********************************************************************** Module 12 - Modulation Principles Discusses the principles of modulation. 1.65 MB ***********************************************************************
*********************************************************************** Module 13 - Introduction to Number Systems and Logic Circuits Presents the fundamental concepts of number systems, Boolean algebra and logic circuits, all of which pertain to digital computers. 1.14 MB *********************************************************************** Module 14 - Introduction to Microelectronics Covers microelectronics technology and miniature and micro miniature circuit repair. 5.96 MB *********************************************************************** Module 15 - Principles of Synchros, Servos and Gyros Provides the basic principles, operations, functions, and applications of synchro, servo and gyro mechanisms. 1.62 MB *********************************************************************** Module 16 - Introduction to Test Equipment Is an introduction to some of the more commonly used test equipments and their applications. 1.89 MB *********************************************************************** Module 17 - Radio-Frequency Communications Principles Presents the fundamentals of a radio frequency communications system. 5.61 MB *********************************************************************** Module 18 - Radar Principles Covers the fundamentals of a radar system. 1.65 MB *********************************************************************** Module 19 - The Technician's Handbook A handy reference of commonly used general information, such as electrical and electronic formulas, color coding and naval supply system data. 1.27 MB *********************************************************************** Module 20 - Master Glossary Is the glossary of terms for the series. 505 KB *********************************************************************** Module 21 - Test Methods and Practices Describes basic test methods and practices. 1.95 MB ***********************************************************************
*********************************************************************** Module 22 - Introduction to Digital Computers Is an introduction to digital computers. 3.59 MB *********************************************************************** Module 23 - Magnetic Recording Is an introduction to the use and maintenance of magnetic recorders and the concepts of recording on magnetic tape and disks. 2.95 MB *********************************************************************** Module 24 - Introduction to Fiber Optics Is an introduction to fiber optics. 1.67 MB ***********************************************************************
NONRESIDENT TRAINING COURSE SEPTEMBER 1998
Navy Electricity and Electronics Training Series Module 1—Introduction to Matter, Energy, and Direct Current NAVEDTRA 14173
DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.
Although the words “he,” “him,” and “his” are used sparingly in this course to enhance communication, they are not intended to be gender driven or to affront or discriminate against anyone.
DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.
PREFACE By enrolling in this self-study course, you have demonstrated a desire to improve yourself and the Navy. Remember, however, this self-study course is only one part of the total Navy training program. Practical experience, schools, selected reading, and your desire to succeed are also necessary to successfully round out a fully meaningful training program. COURSE OVERVIEW: To introduce the student to the subject of Matter, Energy, and Direct Current who needs such a background in accomplishing daily work and/or in preparing for further study. THE COURSE: This self-study course is organized into subject matter areas, each containing learning objectives to help you determine what you should learn along with text and illustrations to help you understand the information. The subject matter reflects day-to-day requirements and experiences of personnel in the rating or skill area. It also reflects guidance provided by Enlisted Community Managers (ECMs) and other senior personnel, technical references, instructions, etc., and either the occupational or naval standards, which are listed in the Manual of Navy Enlisted Manpower Personnel Classifications and Occupational Standards, NAVPERS 18068. THE QUESTIONS: The questions that appear in this course are designed to help you understand the material in the text. VALUE: In completing this course, you will improve your military and professional knowledge. Importantly, it can also help you study for the Navy-wide advancement in rate examination. If you are studying and discover a reference in the text to another publication for further information, look it up.
1998 Edition Prepared by ETCS(SW) Donnie Jones
Published by NAVAL EDUCATION AND TRAINING PROFESSIONAL DEVELOPMENT AND TECHNOLOGY CENTER
NAVSUP Logistics Tracking Number 0504-LP-026-8260
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Sailor’s Creed “I am a United States Sailor. I will support and defend the Constitution of the United States of America and I will obey the orders of those appointed over me. I represent the fighting spirit of the Navy and those who have gone before me to defend freedom and democracy around the world. I proudly serve my country’s Navy combat team with honor, courage and commitment. I am committed to excellence and the fair treatment of all.”
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TABLE OF CONTENTS CHAPTER
PAGE
1. Matter, Energy, and Electricity.................................................................................
1-1
2. Batteries....................................................................................................................
2-1
3. Direct Current...........................................................................................................
3-1
APPENDIX I. Glossary..................................................................................................................
AI-1
II. Laws of Exponents ................................................................................................. AII-1 III. Square and Square Roots........................................................................................ AIII-1 IV. Comparison of Units in Electric and Magnetic Circuits; and Carbon Resistor Size Comparison by Wattage Rating...................................................................... AIV-1 V. Useful Formulas for I.C. Circuits ........................................................................... AV-1 INDEX
.........................................................................................................................
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INDEX-1
NAVY ELECTRICITY AND ELECTRONICS TRAINING SERIES The Navy Electricity and Electronics Training Series (NEETS) was developed for use by personnel in many electrical- and electronic-related Navy ratings. Written by, and with the advice of, senior technicians in these ratings, this series provides beginners with fundamental electrical and electronic concepts through self-study. The presentation of this series is not oriented to any specific rating structure, but is divided into modules containing related information organized into traditional paths of instruction. The series is designed to give small amounts of information that can be easily digested before advancing further into the more complex material. For a student just becoming acquainted with electricity or electronics, it is highly recommended that the modules be studied in their suggested sequence. While there is a listing of NEETS by module title, the following brief descriptions give a quick overview of how the individual modules flow together. Module 1, Introduction to Matter, Energy, and Direct Current, introduces the course with a short history of electricity and electronics and proceeds into the characteristics of matter, energy, and direct current (dc). It also describes some of the general safety precautions and first-aid procedures that should be common knowledge for a person working in the field of electricity. Related safety hints are located throughout the rest of the series, as well. Module 2, Introduction to Alternating Current and Transformers, is an introduction to alternating current (ac) and transformers, including basic ac theory and fundamentals of electromagnetism, inductance, capacitance, impedance, and transformers. Module 3, Introduction to Circuit Protection, Control, and Measurement, encompasses circuit breakers, fuses, and current limiters used in circuit protection, as well as the theory and use of meters as electrical measuring devices. Module 4, Introduction to Electrical Conductors, Wiring Techniques, and Schematic Reading, presents conductor usage, insulation used as wire covering, splicing, termination of wiring, soldering, and reading electrical wiring diagrams. Module 5, Introduction to Generators and Motors, is an introduction to generators and motors, and covers the uses of ac and dc generators and motors in the conversion of electrical and mechanical energies. Module 6, Introduction to Electronic Emission, Tubes, and Power Supplies, ties the first five modules together in an introduction to vacuum tubes and vacuum-tube power supplies. Module 7, Introduction to Solid-State Devices and Power Supplies, is similar to module 6, but it is in reference to solid-state devices. Module 8, Introduction to Amplifiers, covers amplifiers. Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits, discusses wave generation and wave-shaping circuits. Module 10, Introduction to Wave Propagation, Transmission Lines, and Antennas, presents the characteristics of wave propagation, transmission lines, and antennas.
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Module 11, Microwave Principles, explains microwave oscillators, amplifiers, and waveguides. Module 12, Modulation Principles, discusses the principles of modulation. Module 13, Introduction to Number Systems and Logic Circuits, presents the fundamental concepts of number systems, Boolean algebra, and logic circuits, all of which pertain to digital computers. Module 14, Introduction to Microelectronics, covers microelectronics technology and miniature and microminiature circuit repair. Module 15, Principles of Synchros, Servos, and Gyros, provides the basic principles, operations, functions, and applications of synchro, servo, and gyro mechanisms. Module 16, Introduction to Test Equipment, is an introduction to some of the more commonly used test equipments and their applications. Module 17, Radio-Frequency Communications Principles, presents the fundamentals of a radiofrequency communications system. Module 18, Radar Principles, covers the fundamentals of a radar system. Module 19, The Technician's Handbook, is a handy reference of commonly used general information, such as electrical and electronic formulas, color coding, and naval supply system data. Module 20, Master Glossary, is the glossary of terms for the series. Module 21, Test Methods and Practices, describes basic test methods and practices. Module 22, Introduction to Digital Computers, is an introduction to digital computers. Module 23, Magnetic Recording, is an introduction to the use and maintenance of magnetic recorders and the concepts of recording on magnetic tape and disks. Module 24, Introduction to Fiber Optics, is an introduction to fiber optics. Embedded questions are inserted throughout each module, except for modules 19 and 20, which are reference books. If you have any difficulty in answering any of the questions, restudy the applicable section. Although an attempt has been made to use simple language, various technical words and phrases have necessarily been included. Specific terms are defined in Module 20, Master Glossary. Considerable emphasis has been placed on illustrations to provide a maximum amount of information. In some instances, a knowledge of basic algebra may be required. Assignments are provided for each module, with the exceptions of Module 19, The Technician's Handbook; and Module 20, Master Glossary. Course descriptions and ordering information are in NAVEDTRA 12061, Catalog of Nonresident Training Courses.
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Throughout the text of this course and while using technical manuals associated with the equipment you will be working on, you will find the below notations at the end of some paragraphs. The notations are used to emphasize that safety hazards exist and care must be taken or observed.
WARNING
AN OPERATING PROCEDURE, PRACTICE, OR CONDITION, ETC., WHICH MAY RESULT IN INJURY OR DEATH IF NOT CAREFULLY OBSERVED OR FOLLOWED.
CAUTION
AN OPERATING PROCEDURE, PRACTICE, OR CONDITION, ETC., WHICH MAY RESULT IN DAMAGE TO EQUIPMENT IF NOT CAREFULLY OBSERVED OR FOLLOWED.
NOTE
An operating procedure, practice, or condition, etc., which is essential to emphasize.
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INSTRUCTIONS FOR TAKING THE COURSE assignments. To submit your answers via the Internet, go to:
ASSIGNMENTS The text pages that you are to study are listed at the beginning of each assignment. Study these pages carefully before attempting to answer the questions. Pay close attention to tables and illustrations and read the learning objectives. The learning objectives state what you should be able to do after studying the material. Answering the questions correctly helps you accomplish the objectives.
assignment
http://courses.cnet.navy.mil Grading by Mail: When you submit answer sheets by mail, send all of your assignments at one time. Do NOT submit individual answer sheets for grading. Mail all of your assignments in an envelope, which you either provide yourself or obtain from your nearest Educational Services Officer (ESO). Submit answer sheets to:
SELECTING YOUR ANSWERS Read each question carefully, then select the BEST answer. You may refer freely to the text. The answers must be the result of your own work and decisions. You are prohibited from referring to or copying the answers of others and from giving answers to anyone else taking the course.
COMMANDING OFFICER NETPDTC N331 6490 SAUFLEY FIELD ROAD PENSACOLA FL 32559-5000 Answer Sheets: All courses include one “scannable” answer sheet for each assignment. These answer sheets are preprinted with your SSN, name, assignment number, and course number. Explanations for completing the answer sheets are on the answer sheet.
SUBMITTING YOUR ASSIGNMENTS To have your assignments graded, you must be enrolled in the course with the Nonresident Training Course Administration Branch at the Naval Education and Training Professional Development and Technology Center (NETPDTC). Following enrollment, there are two ways of having your assignments graded: (1) use the Internet to submit your assignments as you complete them, or (2) send all the assignments at one time by mail to NETPDTC.
Do not use answer sheet reproductions: Use only the original answer sheets that we provide—reproductions will not work with our scanning equipment and cannot be processed.
Grading on the Internet: Advantages to Internet grading are:
Follow the instructions for marking your answers on the answer sheet. Be sure that blocks 1, 2, and 3 are filled in correctly. This information is necessary for your course to be properly processed and for you to receive credit for your work.
•
COMPLETION TIME
•
you may submit your answers as soon as you complete an assignment, and you get your results faster; usually by the next working day (approximately 24 hours).
Courses must be completed within 12 months from the date of enrollment. This includes time required to resubmit failed assignments.
In addition to receiving grade results for each assignment, you will receive course completion confirmation once you have completed all the
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PASS/FAIL ASSIGNMENT PROCEDURES
For subject matter questions:
If your overall course score is 3.2 or higher, you will pass the course and will not be required to resubmit assignments. Once your assignments have been graded you will receive course completion confirmation.
E-mail: Phone:
[email protected] Comm: (850) 452-1001, ext. 1728 DSN: 922-1001, ext. 1728 FAX: (850) 452-1370 (Do not fax answer sheets.) Address: COMMANDING OFFICER NETPDTC N315 6490 SAUFLEY FIELD ROAD PENSACOLA FL 32509-5237
If you receive less than a 3.2 on any assignment and your overall course score is below 3.2, you will be given the opportunity to resubmit failed assignments. You may resubmit failed assignments only once. Internet students will receive notification when they have failed an assignment--they may then resubmit failed assignments on the web site. Internet students may view and print results for failed assignments from the web site. Students who submit by mail will receive a failing result letter and a new answer sheet for resubmission of each failed assignment.
For enrollment, shipping, completion letter questions
grading,
or
E-mail: Phone:
[email protected] Toll Free: 877-264-8583 Comm: (850) 452-1511/1181/1859 DSN: 922-1511/1181/1859 FAX: (850) 452-1370 (Do not fax answer sheets.) Address: COMMANDING OFFICER NETPDTC N331 6490 SAUFLEY FIELD ROAD PENSACOLA FL 32559-5000
COMPLETION CONFIRMATION After successfully completing this course, you will receive a letter of completion.
NAVAL RESERVE RETIREMENT CREDIT
ERRATA If you are a member of the Naval Reserve, you will receive retirement points if you are authorized to receive them under current directives governing retirement of Naval Reserve personnel. For Naval Reserve retirement, this course is evaluated at 6 points. (Refer to Administrative Procedures for Naval Reservists on Inactive Duty, BUPERSINST 1001.39, for more information about retirement points.)
Errata are used to correct minor errors or delete obsolete information in a course. Errata may also be used to provide instructions to the student. If a course has an errata, it will be included as the first page(s) after the front cover. Errata for all courses can be accessed and viewed/downloaded at: http://www.advancement.cnet.navy.mil
STUDENT FEEDBACK QUESTIONS We value your suggestions, questions, and criticisms on our courses. If you would like to communicate with us regarding this course, we encourage you, if possible, to use e-mail. If you write or fax, please use a copy of the Student Comment form that follows this page.
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Student Comments Course Title:
NEETS Module 1 Introduction to Matter, Energy, and Direct Current
NAVEDTRA:
14173
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NETPDTC 1550/41 (Rev 4-00)
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CHAPTER 1
MATTER, ENERGY, AND ELECTRICITY LEARNING OBJECTIVES Learning objectives are stated at the beginning of each chapter. These learning objectives serve as a preview of the information you are expected to learn in the chapter. The comprehensive check questions are based on the objectives. By successfully completing the NRTC, you indicate that you have met the objectives and have learned the information. The learning objectives are listed below. Upon completing this chapter, you will be able to: 1. State the meanings of and the relationship between matter, element, nucleus, compound, molecule, mixture, atom, electron, proton, neutron, energy, valence, valence shell, and ion. 2. State the meanings of and the relationship between kinetic energy, potential energy, photons, electron orbits, energy levels, and shells and subshells. 3. State, in terms of valence, the differences between a conductor, an insulator, and a semiconductor, and list some materials which make the best conductors and insulators. 4. State the definition of static electricity and explain how static electricity is generated. 5. State the meanings of retentivity, reluctance, permeability, ferromagnetism, natural magnet, and artificial magnet as used to describe magnetic materials. 6. State the Weber and domain theories of magnetism and list six characteristics of magnetic lines of force (magnetic flux), including their relation to magnetic induction, shielding, shape, and storage. 7. State, using the water analogy, how a difference of potential (a voltage or an electromotive force) can exist. Convert volts to microvolts, to millivolts, and to kilovolts. 8. List six methods for producing a voltage (emf) and state the operating principles of and the uses for each method. 9. State the meanings of electron current, random drift, directed drift, and ampere, and indicate the direction that an electric current flows. 10. State the relationship of current to voltage and convert amperes to milliamperes and microamperes. 11. State the definitions of and the terms and symbols for resistance and conductance, and how the temperature, contents, length and cross-sectional area of a conductor affect its resistance and conductance values. 12. List the physical and operating characteristics of and the symbols, ratings, and uses for various types of resistors; use the color code to identify resistor values.
1-1
INTRODUCTION The origin of the modern technical and electronic Navy stretches back to the beginning of naval history, when the first navies were no more than small fleets of wooden ships, using wind-filled sails and manned oars. The need for technicians then was restricted to a navigator and semiskilled seamen who could handle the sails. As time passed, larger ships that carried more sail were built. These ships, encouraging exploration and commerce, helped to establish world trade routes. Soon strong navies were needed to guard these sea lanes. Countries established their own navies to protect their citizens, commercial ships, and shipping lanes against pirates and warring nations. With the addition of mounted armament, gunners joined the ship’s company of skilled or semiskilled technicians. The advent of the steam engine signaled the rise of an energy source more practical than either wind and sails or manpower. With this technological advancement, the need for competent operators and technicians increased. However, the big call for operators and technicians in the U.S. Navy came in the early part of the 20th century, when power sources, means of communication, modes of detection, and armaments moved with amazing rapidity toward involved technical development. Electric motors and generators by then had become the most widely used sources of power. Telephone systems were well established on board ship, and radio was being used more and more to relay messages from ship to ship and from ship to shore. Listening devices were employed to detect submarines. Complex optical systems were used to aim large naval rifles. Mines and torpedoes became highly developed, effective weapons, and airplanes joined the Navy team. During the years after World War I, the Navy became more electricity and electronic minded. It was recognized that a better system of communications was needed aboard each ship, and between the ships, planes, submarines, and shore installations; and that weaponry advances were needed to keep pace with worldwide developments in that field. This growing technology carried with it the awareness that an equally skilled force of technicians was needed for maintenance and service duties. World War II proved that all of the expense of providing equipment for the fleet and of training personnel to handle that equipment paid great dividends. The U. S. Navy had the modern equipment and highly trained personnel needed to defeat the powerful fleets of the enemy. Today there is scarcely anyone on board a Navy ship who does not use electrical or electronic equipment. This equipment is needed in systems of electric lighting and power, intercommunications, radio, radar, sonar, loran, remote metering, weapon aiming, and certain types of mines and torpedoes. The Navy needs trained operators and technicians in this challenging field of electronics and electricity. It is to achieve this end that this module, and others like it, are published. MATTER, ENERGY, AND ELECTRICITY If there are roots to western science, they no doubt lie under the rubble that was once ancient Greece. With the exception of the Greeks, ancient people had little interest in the structure of materials. They accepted a solid as being just that a continuous, uninterrupted substance. One Greek school of thought believed that if a piece of matter, such as copper, were subdivided, it could be done indefinitely and still only that material would be found. Others reasoned that there must be a limit to the number of subdivisions that could be made and have the material still retain its original characteristics. They held fast to the idea that there must be a basic particle upon which all substances are built. Recent experiments have revealed that there are, indeed, several basic particles, or building blocks within all substances.
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The following paragraphs explain how substances are classified as elements and compounds, and are made up of molecules and atoms. This, then, will be a learning experience about protons, electrons, valence, energy levels, and the physics of electricity.
MATTER Matter is defined as anything that occupies space and has weight; that is, the weight and dimensions of matter can be measured. Examples of matter are air, water, automobiles, clothing, and even our own bodies. Thus, we can say that matter may be found in any one of three states: SOLID, LIQUID, and GASEOUS. ELEMENTS AND COMPOUNDS An ELEMENT is a substance which cannot be reduced to a simpler substance by chemical means. Examples of elements with which you are in everyday contact are iron, gold, silver, copper, and oxygen. There are now over 100 known elements. All the different substances we know about are composed of one or more of these elements. When two or more elements are chemically combined, the resulting substance is called a COMPOUND. A compound is a chemical combination of elements which can be separated by chemical but not by physical means. Examples of common compounds are water which consists of hydrogen and oxygen, and table salt, which consists of sodium and chlorine. A MIXTURE, on the other hand, is a combination of elements and compounds, not chemically combined, that can be separated by physical means. Examples of mixtures are air, which is made up of nitrogen, oxygen, carbon dioxide, and small amounts of several rare gases, and sea water, which consists chiefly of salt and water. Q1. What is matter, and in what three states is it found? Q2. What is an element? Q3. What is a compound? Q4. What is the difference between a compound and a mixture? MOLECULES A MOLECULE is a chemical combination of two or more atoms, (atoms are described in the next paragraph). In a compound the molecule is the smallest particle that has all the characteristics of the compound. Consider water, for example. Water is matter, since it occupies space and has weight. Depending on the temperature, it may exist as a liquid (water), a solid (ice), or a gas (steam). Regardless of the temperature, it will still have the same composition. If we start with a quantity of water, divide this and pour out one half, and continue this process a sufficient number of times, we will eventually end up with a quantity of water which cannot be further divided without ceasing to be water. This quantity is called a molecule of water. If this molecule of water divided, instead of two parts of water, there will be one part of oxygen and two parts of hydrogen (H 2 O). ATOMS Molecules are made up of smaller particles called ATOMS. An atom is the smallest particle of an element that retains the characteristics of that element. The atoms of one element, however, differ from 1-3
the atoms of all other elements. Since there are over 100 known elements, there must be over 100 different atoms, or a different atom for each element. Just as thousands of words can be made by combining the proper letters of the alphabet, so thousands of different materials can be made by chemically combining the proper atoms. Any particle that is a chemical combination of two or more atoms is called a molecule. The oxygen molecule consists of two atoms of oxygen, and the hydrogen molecule consists of two atoms of hydrogen. Sugar, on the other hand, is a compound composed of atoms of carbon, hydrogen, and oxygen. These atoms are combined into sugar molecules. Since the sugar molecules can be broken down by chemical means into smaller and simpler units, we cannot have sugar atoms. The atoms of each element are made up of electrons, protons, and, in most cases, neutrons, which are collectively called subatomic particles. Furthermore, the electrons, protons, and neutrons of one element are identical to those of any other element. The reason that there are different kinds of elements is that the number and the arrangement of electrons and protons within the atom are different for the different elements The electron is considered to be a small negative charge of electricity. The proton has a positive charge of electricity equal and opposite to the charge of the electron. Scientists have measured the mass and size of the electron and proton, and they know how much charge each possesses. The electron and proton each have the same quantity of charge, although the mass of the proton is approximately 1837 times that of the electron. In some atoms there exists a neutral particle called a neutron. The neutron has a mass approximately equal to that of a proton, but it has no electrical charge. According to a popular theory, the electrons, protons, and neutrons of the atoms are thought to be arranged in a manner similar to a miniature solar system. The protons and neutrons form a heavy nucleus with a positive charge, around which the very light electrons revolve. Figure 1-1 shows one hydrogen and one helium atom. Each has a relatively simple structure. The hydrogen atom has only one proton in the nucleus with one electron rotating about it. The helium atom is a little more complex. It has a nucleus made up of two protons and two neutrons, with two electrons rotating about the nucleus. Elements are classified numerically according to the complexity of their atoms. The atomic number of an atom is determined by the number of protons in its nucleus.
Figure 1-1.—Structures of simple atoms.
In a neutral state, an atom contains an equal number of protons and electrons. Therefore, an atom of hydrogen—which contains one proton and one electron—has an atomic number of 1; and helium, with 1-4
two protons and two electrons, has an atomic number of 2. The complexity of atomic structure increases with the number of protons and electrons. Q5. What is a molecule? Q6. What are the three types of subatomic particles, and what are their charges? Energy Levels Since an electron in an atom has both mass and motion, it contains two types of energy. By virtue of its motion the electron contains KINETIC ENERGY. Due to its position it also contains POTENTIAL ENERGY. The total energy contained by an electron (kinetic plus potential) is the factor which determines the radius of the electron orbit. In order for an electron to remain in this orbit, it must neither GAIN nor LOSE energy. It is well known that light is a form of energy, but the physical form in which this energy exists is not known. One accepted theory proposes the existence of light as tiny packets of energy called PHOTONS. Photons can contain various quantities of energy. The amount depends upon the color of the light involved. Should a photon of sufficient energy collide with an orbital electron, the electron will absorb the photon’s energy, as shown in figure 1-2. The electron, which now has a greater than normal amount of energy, will jump to a new orbit farther from the nucleus. The first new orbit to which the electron can jump has a radius four times as large as the radius of the original orbit. Had the electron received a greater amount of energy, the next possible orbit to which it could jump would have a radius nine times the original. Thus, each orbit may be considered to represent one of a large number of energy levels that the electron may attain. It must be emphasized that the electron cannot jump to just any orbit. The electron will remain in its lowest orbit until a sufficient amount of energy is available, at which time the electron will accept the energy and jump to one of a series of permissible orbits. An electron cannot exist in the space between energy levels. This indicates that the electron will not accept a photon of energy unless it contains enough energy to elevate itself to one of the higher energy levels. Heat energy and collisions with other particles can also cause the electron to jump orbits.
Figure 1-2.—Excitation by a photon.
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Once the electron has been elevated to an energy level higher than the lowest possible energy level, the atom is said to be in an excited state. The electron will not remain in this excited condition for more than a fraction of a second before it will radiate the excess energy and return to a lower energy orbit. To illustrate this principle, assume that a normal electron has just received a photon of energy sufficient to raise it from the first to the third energy level. In a short period of time the electron may jump back to the first level emitting a new photon identical to the one it received. A second alternative would be for the electron to return to the lower level in two jumps; from the third to the second, and then from the second to the first. In this case the electron would emit two photons, one for each jump. Each of these photons would have less energy than the original photon which excited the electron. This principle is used in the fluorescent light where ultraviolet light photons, which are not visible to the human eye, bombard a phosphor coating on the inside of a glass tube. The phosphor electrons, in returning to their normal orbits, emit photons of light that are visible. By using the proper chemicals for the phosphor coating, any color of light may be obtained, including white. This same principle is also used in lighting up the screen of a television picture tube. The basic principles just developed apply equally well to the atoms of more complex elements. In atoms containing two or more electrons, the electrons interact with each other and the exact path of any one electron is very difficult to predict. However, each electron lies in a specific energy band and the orbits will be considered as an average of the electron’s position. Q7. What is energy of motion called? Q8. How is invisible light changed to visible light in a fluorescent light? Shells and Subshells The difference between the atoms, insofar as their chemical activity and stability are concerned, is dependent upon the number and position of the electrons included within the atom. How are these electrons positioned within the atom? In general, the electrons reside in groups of orbits called shells. These shells are elliptically shaped and are assumed to be located at fixed intervals. Thus, the shells are arranged in steps that correspond to fixed energy levels. The shells, and the number of electrons required to fill them, may be predicted by the employment of Pauli’s exclusion principle. Simply stated, this principle specifies that each shell will contain a maximum of 2n2electrons, where n corresponds to the shell number starting with the one closest to the nucleus. By this principle, the second shell, for example, would contain 2(2) 2 or 8 electrons when full. In addition to being numbered, the shells are also given letter designations, as pictured in figure 1-3. Starting with the shell closest to the nucleus and progressing outward, the shells are labeled K, L, M, N, O, P, and Q, respectively. The shells are considered to be full, or complete, when they contain the following quantities of electrons: two in the K shell, eight in the L shell, 18 in the M shell, and so on, in accordance with the exclusion principle. Each of these shells is a major shell and can be divided into subshells, of which there are four, labeled s, p, d, and f. Like the major shells, the subshells are also limited as to the number of electrons which they can contain. Thus, the "s" subshell is complete when it contains two electrons, the "p" subshell when it contains 10, and the "f" subshell when it contains 14 electrons.
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Figure 1-3.—Shell designation.
Inasmuch as the K shell can contain no more than two electrons, it must have only one subshell, the s subshell. The M shell is composed of three subshells: s, p, and d. If the electrons in the s, p, and d subshells are added, their total is found to be 18, the exact number required to fill the M shell. Notice the electron configuration for copper illustrated in figure 1-4. The copper atom contains 29 electrons, which completely fill the first three shells and subshells, leaving one electron in the "s" subshell of the N shell.
Figure 1-4.—Copper atom.
Valence The number of electrons in the outermost shell determines the valence of an atom. For this reason, the outer shell of an atom is called the VALENCE SHELL; and the electrons contained in this shell are called VALENCE ELECTRONS. The valence of an atom determines its ability to gain or lose an electron, which in turn determines the chemical and electrical properties of the atom. An atom that is
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lacking only one or two electrons from its outer shell will easily gain electrons to complete its shell, but a large amount of energy is required to free any of its electrons. An atom having a relatively small number of electrons in its outer shell in comparison to the number of electrons required to fill the shell will easily lose these valence electrons. The valence shell always refers to the outermost shell. Q9. What determines the valence of an atom? Ionization When the atom loses electrons or gains electrons in this process of electron exchange, it is said to be IONIZED. For ionization to take place, there must be a transfer of energy which results in a change in the internal energy of the atom. An atom having more than its normal amount of electrons acquires a negative charge, and is called a NEGATIVE ION. The atom that gives up some of its normal electrons is left with less negative charges than positive charges and is called a POSITIVE ION. Thus, ionization is the process by which an atom loses or gains electrons. Q10. What is an ion? CONDUCTORS, SEMICONDUCTORS, AND INSULATORS In this study of electricity and electronics, the association of matter and electricity is important. Since every electronic device is constructed of parts made from ordinary matter, the effects of electricity on matter must be well understood. As a means of accomplishing this, all elements of which matter is made may be placed into one of three categories: CONDUCTORS, SEMICONDUCTORS, and INSULATORS, depending on their ability to conduct an electric current. CONDUCTORS are elements which conduct electricity very readily, INSULATORS have an extremely high resistance to the flow of electricity. All matter between these two extremes may be called SEMICONDUCTORS. The electron theory states that all matter is composed of atoms and the atoms are composed of smaller particles called protons, electrons, and neutrons. The electrons orbit the nucleus which contains the protons and neutrons. It is the valence electrons that we are most concerned with in electricity. These are the electrons which are easiest to break loose from their parent atom. Normally, conductors have three or less valence electrons; insulators have five or more valence electrons; and semiconductors usually have four valence electrons. The electrical conductivity of matter is dependent upon the atomic structure of the material from which the conductor is made. In any solid material, such as copper, the atoms which make up the molecular structure are bound firmly together. At room temperature, copper will contain a considerable amount of heat energy. Since heat energy is one method of removing electrons from their orbits, copper will contain many free electrons that can move from atom to atom. When not under the influence of an external force, these electrons move in a haphazard manner within the conductor. This movement is equal in all directions so that electrons are not lost or gained by any part of the conductor. When controlled by an external force, the electrons move generally in the same direction. The effect of this movement is felt almost instantly from one end of the conductor to the other. This electron movement is called an ELECTRIC CURRENT. Some metals are better conductors of electricity than others. Silver, copper, gold, and aluminum are materials with many free electrons and make good conductors. Silver is the best conductor, followed by copper, gold, and aluminum. Copper is used more often than silver because of cost. Aluminum is used where weight is a major consideration, such as in high-tension power lines, with long spans between supports. Gold is used where oxidation or corrosion is a consideration and a good conductivity is
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required. The ability of a conductor to handle current also depends upon its physical dimensions. Conductors are usually found in the form of wire, but may be in the form of bars, tubes, or sheets. Nonconductors have few free electrons. These materials are called INSULATORS. Some examples of these materials are rubber, plastic, enamel, glass, dry wood, and mica. Just as there is no perfect conductor, neither is there a perfect insulator. Some materials are neither good conductors nor good insulators, since their electrical characteristics fall between those of conductors and insulators. These in-between materials are classified as SEMICONDUCTORS. Germanium and silicon are two common semiconductors used in solid-state devices. Q11. What determines whether a substance is a conductor or an insulator? ELECTROSTATICS Electrostatics (electricity at rest) is a subject with which most persons entering the field of electricity and electronics are somewhat familiar. For example, the way a person’s hair stands on end after a vigorous rubbing is an effect of electrostatics. While pursuing the study of electrostatics, you will gain a better understanding of this common occurrence. Of even greater significance, the study of electrostatics will provide you with the opportunity to gain important background knowledge and to develop concepts which are essential to the understanding of electricity and electronics. Interest in the subject of static electricity can be traced back to the Greeks. Thales of Miletus, a Greek philosopher and mathematician, discovered that when an amber rod is rubbed with fur, the rod has the amazing characteristic of attracting some very light objects such as bits of paper and shavings of wood. About 1600, William Gilbert, an English scientist, made a study of other substances which had been found to possess qualities of attraction similar to amber. Among these were glass, when rubbed with silk, and ebonite, when rubbed with fur. Gilbert classified all the substances which possessed properties similar to those of amber as electrics, a word of Greek origin meaning amber. Because of Gilbert’s work with electrics, a substance such as amber or glass when given a vigorous rubbing was recognized as being ELECTRIFIED, or CHARGED with electricity. In the year 1733, Charles Dufay, a French scientist, made an important discovery about electrification. He found that when a glass was rubbed with fur, both the glass rod and the fur became electrified. This realization came when he systematically placed the glass rod and the fur near other electrified substances and found that certain substances which were attracted to the glass rod were repelled by the fur, and vice versa. From experiments such as this, he concluded that there must be two exactly opposite kinds of electricity. Benjamin Franklin, American statesman, inventor, and philosopher, is credited with first using the terms POSITIVE and NEGATIVE to describe the two opposite kinds of electricity. The charge produced on a glass rod when it is rubbed with silk, Franklin labeled positive. He attached the term negative to the charge produced on the silk. Those bodies which were not electrified or charged, he called NEUTRAL. STATIC ELECTRICITY In a natural, or neutral state, each atom in a body of matter will have the proper number of electrons in orbit around it. Consequently, the whole body of matter composed of the neutral atoms will also be
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electrically neutral. In this state, it is said to have a "zero charge." Electrons will neither leave nor enter the neutrally charged body should it come in contact with other neutral bodies. If, however, any number of electrons are removed from the atoms of a body of matter, there will remain more protons than electrons and the whole body of matter will become ELECTRICALLY POSITIVE. Should the positively charged body come in contact with another body having a normal charge, or having a NEGATIVE (too many electrons) charge, an electric current will flow between them. Electrons will leave the more negative body and enter the positive body. This electron flow will continue until both bodies have equal charges. When two bodies of matter have unequal charges and are near one another, an electric force is exerted between them because of their unequal charges. However, since they are not in contact, their charges cannot equalize. The existence of such an electric force, where current cannot flow, is referred to as static electricity. ("Static" in this instance means "not moving.") It is also referred to as an electrostatic force. One of the easiest ways to create a static charge is by friction. When two pieces of matter are rubbed together, electrons can be "wiped off" one material onto the other. If the materials used are good conductors, it is quite difficult to obtain a detectable charge on either, since equalizing currents can flow easily between the conducting materials. These currents equalize the charges almost as fast as they are created. A static charge is more easily created between nonconducting materials. When a hard rubber rod is rubbed with fur, the rod will accumulate electrons given up by the fur, as shown in figure 1-5. Since both materials are poor conductors, very little equalizing current can flow, and an electrostatic charge builds up. When the charge becomes great enough, current will flow regardless of the poor conductivity of the materials. These currents will cause visible sparks and produce a crackling sound.
Figure 1-5.—Producing static electricity by friction.
Q12. How is a negative charge created in a neutral body? Q13. How are static charges created? Nature of Charges When in a natural, or neutral state, an atom has an equal number of electrons and protons. Because of this balance, the net negative charge of the electrons in orbit is exactly balanced by the net positive charge of the protons in the nucleus, making the atom electrically neutral.
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An atom becomes a positive ion whenever it loses an electron, and has an overall positive charge. Conversely, whenever an atom acquires an extra electron, it becomes a negative ion and has a negative charge. Due to normal molecular activity, there are always ions present in any material. If the number of positive ions and negative ions is equal, the material is electrically neutral. When the number of positive ions exceeds the number of negative ions, the material is positively charged. The material is negatively charged whenever the negative ions outnumber the positive ions. Since ions are actually atoms without their normal number of electrons, it is the excess or the lack of electrons in a substance that determines its charge. In most solids, the transfer of charges is by movement of electrons rather than ions. The transfer of charges by ions will become more significant when we consider electrical activity in liquids and gases. At this time, we will discuss electrical behavior in terms of electron movement. Q14. What is the electrical charge of an atom which contains 8 protons and 11 electrons? Charged Bodies One of the fundamental laws of electricity is that LIKE CHARGES REPEL EACH OTHER and UNLIKE CHARGES ATTRACT EACH OTHER. A positive charge and negative charge, being unlike, tend to move toward each other. In the atom, the negative electrons are drawn toward the positive protons in the nucleus. This attractive force is balanced by the electron’s centrifugal force caused by its rotation about the nucleus. As a result, the electrons remain in orbit and are not drawn into the nucleus. Electrons repel each other because of their like negative charges, and protons repel each other because of their like positive charges. The law of charged bodies may be demonstrated by a simple experiment. Two pith (paper pulp) balls are suspended near one another by threads, as shown in figure 1-6.
Figure 1-6.—Reaction between charged bodies.
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If a hard rubber rod is rubbed with fur to give it a negative charge and is then held against the righthand ball in part (A), the rod will give off a negative charge to the ball. The right-hand ball will have a negative charge with respect to the left-hand ball. When released, the two balls will be drawn together, as shown in figure 1-6(A). They will touch and remain in contact until the left-hand ball gains a portion of the negative charge of the right-hand ball, at which time they will swing apart as shown in figure 1-6(C). If a positive or a negative charge is placed on both balls (fig. 1-6(B)), the balls will repel each other. Coulomb’s Law of Charges The relationship between attracting or repelling charged bodies was first discovered and written about by a French scientist named Charles A. Coulomb. Coulomb’s Law states that CHARGED BODIES ATTRACT OR REPEL EACH OTHER WITH A FORCE THAT IS DIRECTLY PROPORTIONAL TO THE PRODUCT OF THEIR INDIVIDUAL CHARGES, AND IS INVERSELY PROPORTIONAL TO THE SQUARE OF THE DISTANCE BETWEEN THEM. The amount of attracting or repelling force which acts between two electrically charged bodies in free space depends on two things—(1) their charges and (2) the distance between them. Electric Fields The space between and around charged bodies in which their influence is felt is called an ELECTRIC FIELD OF FORCE. It can exist in air, glass, paper, or a vacuum. ELECTROSTATIC FIELDS and DIELECTRIC FIELDS are other names used to refer to this region of force. Fields of force spread out in the space surrounding their point of origin and, in general, DIMINISH IN PROPORTION TO THE SQUARE OF THE DISTANCE FROM THEIR SOURCE. The field about a charged body is generally represented by lines which are referred to as ELECTROSTATIC LINES OF FORCE. These lines are imaginary and are used merely to represent the direction and strength of the field. To avoid confusion, the lines of force exerted by a positive charge are always shown leaving the charge, and for a negative charge they are shown entering. Figure 1-7 illustrates the use of lines to represent the field about charged bodies.
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Figure 1-7.—Electrostatic lines of force.
Figure 1-7(A) represents the repulsion of like-charged bodies and their associated fields. Part (B) represents the attraction of unlike-charged bodies and their associated fields. Q15. What is the relationship between charged bodies? Q16. What is an electrostatic field? Q17. In what direction are electrostatic lines of force drawn? MAGNETISM In order to properly understand the principles of electricity, it is necessary to study magnetism and the effects of magnetism on electrical equipment. Magnetism and electricity are so closely related that the study of either subject would be incomplete without at least a basic knowledge of the other. Much of today’s modern electrical and electronic equipment could not function without magnetism. Modern computers, tape recorders, and video reproduction equipment use magnetized tape. High-fidelity speakers use magnets to convert amplifier outputs into audible sound. Electrical motors use magnets to convert electrical energy into mechanical motion; generators use magnets to convert mechanical motion into electrical energy. Q18. What are some examples of electrical equipment which use magnetism? MAGNETIC MATERIALS Magnetism is generally defined as that property of a material which enables it to attract pieces of iron. A material possessing this property is known as a MAGNET. The word originated with the ancient Greeks, who found stones possessing this characteristic. Materials that are attracted by a magnet, such as iron, steel, nickel, and cobalt, have the ability to become magnetized. These are called magnetic materials.
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Materials, such as paper, wood, glass, or tin, which are not attracted by magnets, are considered nonmagnetic. Nonmagnetic materials are not able to become magnetized. Q19. What are magnetic materials? Ferromagnetic Materials The most important group of materials connected with electricity and electronics are the ferromagnetic materials. Ferromagnetic materials are those which are relatively easy to magnetize, such as iron, steel, cobalt, and the alloys Alnico and Permalloy. (An alloy is made from combining two or more elements, one of which must be a metal). These new alloys can be very strongly magnetized, and are capable of obtaining a magnetic strength great enough to lift 500 times their own weight. Natural Magnets Magnetic stones such as those found by the ancient Greeks are considered to be NATURAL MAGNETS. These stones had the ability to attract small pieces of iron in a manner similar to the magnets which are common today. However, the magnetic properties attributed to the stones were products of nature and not the result of the efforts of man. The Greeks called these substances magnetite. The Chinese are said to have been aware of some of the effects of magnetism as early as 2600 B.C. They observed that stones similar to magnetite, when freely suspended, had a tendency to assume a nearly north and south direction. Because of the directional quality of these stones, they were later referred to as lodestones or leading stones. Natural magnets, which presently can be found in the United States, Norway, and Sweden, no longer have any practical use, for it is now possible to easily produce more powerful magnets. Q20. What characteristics do all ferromagnetic materials have in common? Artificial Magnets Magnets produced from magnetic materials are called ARTIFICIAL MAGNETS. They can be made in a variety of shapes and sizes and are used extensively in electrical apparatus. Artificial magnets are generally made from special iron or steel alloys which are usually magnetized electrically. The material to be magnetized is inserted into a coil of insulated wire and a heavy flow of electrons is passed through the wire. Magnets can also be produced by stroking a magnetic material with magnetite or with another artificial magnet. The forces causing magnetization are represented by magnetic lines of force, very similar in nature to electrostatic lines of force. Artificial magnets are usually classified as PERMANENT or TEMPORARY, depending on their ability to retain their magnetic properties after the magnetizing force has been removed. Magnets made from substances, such as hardened steel and certain alloys which retain a great deal of their magnetism, are called PERMANENT MAGNETS. These materials are relatively difficult to magnetize because of the opposition offered to the magnetic lines of force as the lines of force try to distribute themselves throughout the material. The opposition that a material offers to the magnetic lines of force is called RELUCTANCE. All permanent magnets are produced from materials having a high reluctance. A material with a low reluctance, such as soft iron or annealed silicon steel, is relatively easy to magnetize but will retain only a small part of its magnetism once the magnetizing force is removed. Materials of this type that easily lose most of their magnetic strength are called TEMPORARY MAGNETS. The amount of magnetism which remains in a temporary magnet is referred to as its
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RESIDUAL MAGNETISM. The ability of a material to retain an amount of residual magnetism is called the RETENTIVITY of the material. The difference between a permanent and a temporary magnet has been indicated in terms of RELUCTANCE, a permanent magnet having a high reluctance and a temporary magnet having a low reluctance. Magnets are also described in terms of the PERMEABILITY of their materials, or the ease with which magnetic lines of force distribute themselves throughout the material. A permanent magnet, which is produced from a material with a high reluctance, has a low permeability. A temporary magnet, produced from a material with a low reluctance, would have a high permeability. Q21. What type of magnetic material should be used to make a temporary magnet? Q22. What is retentivity? MAGNETIC POLES The magnetic force surrounding a magnet is not uniform. There exists a great concentration of force at each end of the magnet and a very weak force at the center. Proof of this fact can be obtained by dipping a magnet into iron filings (fig. 1-8). It is found that many filings will cling to the ends of the magnet while very few adhere to the center. The two ends, which are the regions of concentrated lines of force, are called the POLES of the magnet. Magnets have two magnetic poles and both poles have equal magnetic strength.
Figure 1-8.—Iron filings cling to the poles of a magnet.
Law of Magnetic Poles If a bar magnet is suspended freely on a string, as shown in figure 1-9, it will align itself in a north and south direction. When this experiment is repeated, it is found that the same pole of the magnet will always swing toward the north magnetic pole of the earth. Therefore, it is called the north-seeking pole or simply the NORTH POLE. The other pole of the magnet is the south-seeking pole or the SOUTH POLE. 1-15
Figure 1-9.—A bar magnet acts as a compass.
A practical use of the directional characteristic of the magnet is the compass, a device in which a freely rotating magnetized needle indicator points toward the North Pole. The realization that the poles of a suspended magnet always move to a definite position gives an indication that the opposite poles of a magnet have opposite magnetic polarity. The law previously stated regarding the attraction and repulsion of charged bodies may also be applied to magnetism if the pole is considered as a charge. The north pole of a magnet will always be attracted to the south pole of another magnet and will show a repulsion to a north pole. The law for magnetic poles is: Like poles repel, unlike poles attract. Q23. How does the law of magnetic poles relate to the law of electric charges? The Earth’s Magnetic Poles The fact that a compass needle always aligns itself in a particular direction, regardless of its location on earth, indicates that the earth is a huge natural magnet. The distribution of the magnetic force about the earth is the same as that which might be produced by a giant bar magnet running through the center of the earth (fig. 1-10). The magnetic axis of the earth is located about 15º IURPLWVJHRJUDSKLFDOD[LVWKHUHE\ locating the magnetic poles some distance from the geographical poles. The ability of the north pole of the compass needle to point toward the north geographical pole is due to the presence of the magnetic pole nearby. This magnetic pole is named the magnetic North Pole. However, in actuality, it must have the polarity of a south magnetic pole since it attracts the north pole of a compass needle. The reason for this conflict in terminology can be traced to the early users of the compass. Knowing little about magnetic effects, they called the end of the compass needle that pointed towards the north geographical pole, the north pole of a compass. With our present knowledge of magnetism, we know the north pole of a compass needle (a small bar magnet) can be attracted only by an unlike magnetic pole, that is, a pole of south magnetic polarity.
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Figure 1-10.—The earth is a magnet.
Q24. A compass is located at the geographical North Pole. In which direction would its needle point? THEORIES OF MAGNETISM Weber’s Theory A popular theory of magnetism considers the molecular alignment of the material. This is known as Weber’s theory. This theory assumes that all magnetic substances are composed of tiny molecular magnets. Any unmagnetized material has the magnetic forces of its molecular magnets neutralized by adjacent molecular magnets, thereby eliminating any magnetic effect. A magnetized material will have most of its molecular magnets lined up so that the north pole of each molecule points in one direction, and the south pole faces the opposite direction. A material with its molecules thus aligned will then have one effective north pole, and one effective south pole. An illustration of Weber’s Theory is shown in figure 111, where a steel bar is magnetized by stroking. When a steel bar is stroked several times in the same direction by a magnet, the magnetic force from the north pole of the magnet causes the molecules to align themselves.
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Figure 1-11.—Weber's molecular theory of magnetism.
Q25. Using Weber’s molecular theory of magnetism, describe the polarity of the magnetic poles produced by stroking a magnetic material from right to left with the south pole of a magnet. Domain Theory A more modern theory of magnetism is based on the electron spin principle. From the study of atomic structure it is known that all matter is composed of vast quantities of atoms, each atom containing one or more orbital electrons. The electrons are considered to orbit in various shells and subshells depending upon their distance from the nucleus. The structure of the atom has previously been compared to the solar system, wherein the electrons orbiting the nucleus correspond to the planets orbiting the sun. Along with its orbital motion about the sun, each planet also revolves on its axis. It is believed that the electron also revolves on its axis as it orbits the nucleus of an atom. It has been experimentally proven that an electron has a magnetic field about it along with an electric field. The effectiveness of the magnetic field of an atom is determined by the number of electrons spinning in each direction. If an atom has equal numbers of electrons spinning in opposite directions, the magnetic fields surrounding the electrons cancel one another, and the atom is unmagnetized. However, if more electrons spin in one direction than another, the atom is magnetized. An atom with an atomic number of 26, such as iron, has 26 protons in the nucleus and 26 revolving electrons orbiting its nucleus. If 13 electrons are spinning in a clockwise direction and 13 electrons are spinning in a counterclockwise direction, the opposing magnetic fields will be neutralized. When more than 13 electrons spin in either direction, the atom is magnetized. An example of a magnetized atom of iron is shown in figure 1-12.
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Figure 1-12.—Iron atom.
Q26. What is the difference between the domain theory and Weber’s theory of magnetism? MAGNETIC FIELDS The space surrounding a magnet where magnetic forces act is known as the magnetic field. A pattern of this directional force can be obtained by performing an experiment with iron filings. A piece of glass is placed over a bar magnet and the iron filings are then sprinkled on the surface of the glass. The magnetizing force of the magnet will be felt through the glass and each iron filing becomes a temporary magnet. If the glass is now tapped gently, the iron particles will align themselves with the magnetic field surrounding the magnet just as the compass needle did previously. The filings form a definite pattern, which is a visible representation of the forces comprising the magnetic field. Examination of the arrangements of iron filings in figure 1-13 will indicate that the magnetic field is very strong at the poles and weakens as the distance from the poles increases. It is also apparent that the magnetic field extends from one pole to the other, constituting a loop about the magnet.
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Figure 1-13.—Pattern formed by iron filings.
Q27. Refer to figure 1-13. For what purpose would you sprinkle iron filings on the glass plate? Q28. Refer to figure 1-13. What pattern would be formed if sawdust was sprinkled on the glass instead of iron filings? Lines of Force To further describe and work with magnet phenomena, lines are used to represent the force existing in the area surrounding a magnet (refer to fig. 1-14). These lines, called MAGNETIC LINES OF FORCE, do not actually exist but are imaginary lines used to illustrate and describe the pattern of the magnetic field. The magnetic lines of force are assumed to emanate from the north pole of a magnet, pass through surrounding space, and enter the south pole. The lines of force then travel inside the magnet from the south pole to the north pole, thus completing a closed loop.
Figure 1-14.—Bar magnet showing lines of force.
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When two magnetic poles are brought close together, the mutual attraction or repulsion of the poles produces a more complicated pattern than that of a single magnet. These magnetic lines of force can be plotted by placing a compass at various points throughout the magnetic field, or they can be roughly illustrated by the use of iron filings as before. A diagram of magnetic poles placed close together is shown in figure 1-15.
Figure 1-15.—Magnetic poles in close proximity.
Although magnetic lines of force are imaginary, a simplified version of many magnetic phenomena can be explained by assuming the magnetic lines to have certain real properties. The lines of force can be compared to rubber bands which stretch outward when a force is exerted upon them and contract when the force is removed. The characteristics of magnetic lines of force can be described as follows: 1. Magnetic lines of force are continuous and will always form closed loops. 2. Magnetic lines of force will never cross one another. 3. Parallel magnetic lines of force traveling in the same direction repel one another. Parallel magnetic lines of force traveling in opposite directions tend to unite with each other and form into single lines traveling in a direction determined by the magnetic poles creating the lines of force. 4. Magnetic lines of force tend to shorten themselves. Therefore, the magnetic lines of force existing between two unlike poles cause the poles to be pulled together. 5. Magnetic lines of force pass through all materials, both magnetic and nonmagnetic. 6. Magnetic lines of force always enter or leave a magnetic material at right angles to the surface. Q29. What is a magnetic line of force? Q30. In what way do magnetic lines of force differ from electrostatic lines of force?
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MAGNETIC EFFECTS MAGNETIC FLUX. The total number of magnetic lines of force leaving or entering the pole of a magnet is called MAGNETIC FLUX. The number of flux lines per unit area is known as FLUX DENSITY. FIELD INTENSITY. The intensity of a magnetic field is directly related to the magnetic force exerted by the field. ATTRACTION/REPULSION. The intensity of attraction or repulsion between magnetic poles may be described by a law almost identical to Coulomb’s Law of Charged Bodies. The force between two poles is directly proportional to the product of the pole strengths and inversely proportional to the square of the distance between the poles. Magnetic Induction It has been previously stated that all substances that are attracted by a magnet are capable of becoming magnetized. The fact that a material is attracted by a magnet indicates the material must itself be a magnet at the time of attraction. With the knowledge of magnetic fields and magnetic lines of force developed up to this point, it is simple to understand the manner in which a material becomes magnetized when brought near a magnet. As an iron nail is brought close to a bar magnet (fig. 1-16), some flux lines emanating from the north pole of the magnet pass through the iron nail in completing their magnetic path. Since magnetic lines of force travel inside a magnet from the south pole to the north pole, the nail will be magnetized in such a polarity that its south pole will be adjacent to the north pole of the bar magnet. There is now an attraction between the two magnets.
Figure 1-16.—Magnetized nail.
If another nail is brought in contact with the end of the first nail, it would be magnetized by induction. This process could be repeated until the strength of the magnetic flux weakens as distance from
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the bar magnet increases. However, as soon as the first iron nail is pulled away from the bar magnet, all the nails will fall. The reason being that each nail becomes a temporary magnet, and as soon as the magnetizing force is removed, their domains once again assume a random distribution. Magnetic induction will always produce a pole polarity on the material being magnetized opposite that of the adjacent pole of the magnetizing force. It is sometimes possible to bring a weak north pole of a magnet near a strong magnet north pole and note attraction between the poles. The weak magnet, when placed within the magnetic field of the strong magnet, has its magnetic polarity reversed by the field of the stronger magnet. Therefore, it is attracted to the opposite pole. For this reason, you must keep a very weak magnet, such as a compass needle, away from a strong magnet. Magnetism can be induced in a magnetic material by several means. The magnetic material may be placed in the magnetic field, brought into contact with a magnet, or stroked by a magnet. Stroking and contact both indicate actual contact with the material but are considered in magnetic studies as magnetizing by INDUCTION. Magnetic Shielding There is no known INSULATOR for magnetic flux. If a nonmagnetic material is placed in a magnetic field, there is no appreciable change in flux—that is, the flux penetrates the nonmagnetic material. For example, a glass plate placed between the poles of a horseshoe magnet will have no appreciable effect on the field although glass itself is a good insulator in an electric circuit. If a magnetic material (for example, soft iron) is placed in a magnetic field, the flux may be redirected to take advantage of the greater permeability of the magnetic material, as shown in figure 1-17. Permeability, as discussed earlier, is the quality of a substance which determines the ease with which it can be magnetized.
Figure 1-17.—Effects of a magnetic substance in a magnetic field.
The sensitive mechanisms of electric instruments and meters can be influenced by stray magnetic fields which will cause errors in their readings. Because instrument mechanisms cannot be insulated against magnetic flux, it is necessary to employ some means of directing the flux around the instrument. This is accomplished by placing a soft-iron case, called a MAGNETIC SCREEN or SHIELD, about the instrument. Because the flux is established more readily through the iron (even though the path is longer) than through the air inside the case, the instrument is effectively shielded, as shown by the watch and softiron shield in figure 1-18.
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Figure 1-18.—Magnetic shield.
MAGNETIC SHAPES Because of the many uses of magnets, they are found in various shapes and sizes. However, magnets usually come under one of three general classifications: bar magnets, horseshoe magnets, or ring magnets. The bar magnet is most often used in schools and laboratories for studying the properties and effects of magnetism. In the preceding material, the bar magnet proved very helpful in demonstrating magnetic effects. Another type of magnet is the ring magnet, which is used for computer memory cores. A common application for a temporary ring magnet would be the shielding of electrical instruments. The shape of the magnet most frequently used in electrical and electronic equipment is called the horseshoe magnet. A horseshoe magnet is similar to a bar magnet but is bent in the shape of a horseshoe. The horseshoe magnet provides much more magnetic strength than a bar magnet of the same size and material because of the closeness of the magnetic poles. The magnetic strength from one pole to the other is greatly increased due to the concentration of the magnetic field in a smaller area. Electrical measuring devices quite frequently use horseshoe-type magnets. CARE OF MAGNETS A piece of steel that has been magnetized can lose much of its magnetism by improper handling. If it is jarred or heated, there will be a disalignment of its domains resulting in the loss of some of its effective magnetism. Had this piece of steel formed the horseshoe magnet of a meter, the meter would no longer be
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operable or would give inaccurate readings. Therefore, care must be exercised when handling instruments containing magnets. Severe jarring or subjecting the instrument to high temperatures will damage the device. A magnet may also become weakened from loss of flux. Thus when storing magnets, one should always try to avoid excess leakage of magnetic flux. A horseshoe magnet should always be stored with a keeper, a soft iron bar used to join the magnetic poles. By using the keeper while the magnet is being stored, the magnetic flux will continuously circulate through the magnet and not leak off into space. When bar magnets are stored, the same principle must be remembered. Therefore, bar magnets should always be stored in pairs with a north pole and a south pole placed together. This provides a complete path for the magnetic flux without any flux leakage. Q31. How should a delicate instrument be protected from a magnetic field? Q32. How should bar magnets be stored? ELECTRICAL ENERGY In the field of physical science, work must be defined as the PRODUCT OF FORCE AND DISPLACEMENT. That is, the force applied to move an object and the distance the object is moved are the factors of work performed. It is important to notice that no work is accomplished unless the force applied causes a change in the position of a stationary object, or a change in the velocity of a moving object. A worker may tire by pushing against a heavy wooden crate, but unless the crate moves, no work will be accomplished. ENERGY In our study of energy and work, we must define energy as THE ABILITY TO DO WORK. In order to perform any kind of work, energy must be expended (converted from one form to another). Energy supplies the required force, or power, whenever any work is accomplished. One form of energy is that which is contained by an object in motion. When a hammer is set in motion in the direction of a nail, it possesses energy of motion. As the hammer strikes the nail, the energy of motion is converted into work as the nail is driven into the wood. The distance the nail is driven into the wood depends on the velocity of the hammer at the time it strikes the nail. Energy contained by an object due to its motion is called KINETIC ENERGY. Assume that the hammer is suspended by a string in a position one meter above a nail. As a result of gravitational attraction, the hammer will experience a force pulling it downward. If the string is suddenly cut, the force of gravity will pull the hammer downward against the nail, driving it into the wood. While the hammer is suspended above the nail it has ability to do work because of its elevated position in the earth’s gravitational field. Since energy is the ability to do work, the hammer contains energy. Energy contained by an object due to its position is called POTENTIAL ENERGY. The amount of potential energy available is equal to the product of the force required to elevate the hammer and the height to which it is elevated. Another example of potential energy is that contained in a tightly coiled spring. The amount of energy released when the spring unwinds depends on the amount of force required to wind the spring initially.
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Q33. What is the definition of energy? Q34. What type of energy does a rolling stone have? Q35. What kind of energy does the stone have if it is at rest at the top of a hill? Electrical Charges From the previous study of electrostatics, you learned that a field of force exists in the space surrounding any electrical charge. The strength of the field is directly dependent on the force of the charge. The charge of one electron might be used as a unit of electrical charge, since charges are created by displacement of electrons; but the charge of one electron is so small that it is impractical to use. The practical unit adopted for measuring charges is the COULOMB, named after the scientist Charles Coulomb. One coulomb is equal to the charge of 6,280,000,000,000,000,000 (six quintillion two hundred and eighty quadrillion) or (6.28 x 1018 ) electrons. When a charge of one coulomb exists between two bodies, one unit of electrical potential energy exists, which is called the difference of potential between the two bodies. This is referred to as ELECTROMOTIVE FORCE, or VOLTAGE, and the unit of measure is the VOLT. Electrical charges are created by the displacement of electrons, so that there exists an excess of electrons at one point, and a deficiency at another point. Consequently, a charge must always have either a negative or positive polarity. A body with an excess of electrons is considered to be negative, whereas a body with a deficiency of electrons is positive. A difference of potential can exist between two points, or bodies, only if they have different charges. In other words, there is no difference in potential between two bodies if both have a deficiency of electrons to the same degree. If, however, one body is deficient of 6 coulombs (representing 6 volts), and the other is deficient by 12 coulombs (representing 12 volts), there is a difference of potential of 6 volts. The body with the greater deficiency is positive with respect to the other. In most electrical circuits only the difference of potential between two points is of importance and the absolute potentials of the points are of little concern. Very often it is convenient to use one standard reference for all of the various potentials throughout a piece of equipment. For this reason, the potentials at various points in a circuit are generally measured with respect to the metal chassis on which all parts of the circuit are mounted. The chassis is considered to be at zero potential and all other potentials are either positive or negative with respect to the chassis. When used as the reference point, the chassis is said to be at GROUND POTENTIAL. Occasionally, rather large values of voltage may be encountered, in which case the volt becomes too small a unit for convenience. In a situation of this nature, the kilovolt (kV), meaning 1,000 volts, is frequently used. As an example, 20,000 volts would be written as 20 kV. In other cases, the volt may be too large a unit, as when dealing with very small voltages. For this purpose the millivolt (mV), meaning one-thousandth of a volt, and the microvolt (µV), meaning one-millionth of a volt, are used. For example, 0.001 volt would be written as 1 mV, and 0.000025 volt would be written as 25 µV. See Appendix II for exponential symbology. When a difference in potential exists between two charged bodies that are connected by a conductor, electrons will flow along the conductor. This flow is from the negatively charged body to the positively charged body, until the two charges are equalized and the potential difference no longer exists.
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An analogy of this action is shown in the two water tanks connected by a pipe and valve in figure 1-19. At first the valve is closed and all the water is in tank A. Thus, the water pressure across the valve is at maximum. When the valve is opened, the water flows through the pipe from A to B until the water level becomes the same in both tanks. The water then stops flowing in the pipe, because there is no longer a difference in water pressure between the two tanks.
Figure 1-19.—Water analogy of electric differences of potential.
Electron movement through an electric circuit is directly proportional to the difference in potential or electromotive force (emf), across the circuit, just as the flow of water through the pipe in figure 1-19 is directly proportional to the difference in water level in the two tanks. A fundamental law of electricity is that the ELECTRON FLOW IS DIRECTLY PROPORTIONAL TO THE APPLIED VOLTAGE. If the voltage is increased, the flow is increased. If the voltage is decreased, the flow is decreased. Q36. What term describes voltage or emf? Q37. Convert 2.1 kV to volts. Q38. Express the following in more simple terms. (a) 250,000 volts, (b) 25,000,000 microvolts, (c) 0.001 millivolt. HOW VOLTAGE IS PRODUCED It has been demonstrated that a charge can be produced by rubbing a rubber rod with fur. Because of the friction involved, the rod acquires electrons from the fur, making it negative; the fur becomes positive due to the loss of electrons. These quantities of charge constitute a difference of potential between the rod and the fur. The electrons which make up this difference of potential are capable of doing work if a discharge is allowed to occur. To be a practical source of voltage, the potential difference must not be allowed to dissipate, but must be maintained continuously. As one electron leaves the concentration of negative charge, another must be immediately provided to take its place or the charge will eventually diminish to the point where no further work can be accomplished. A VOLTAGE SOURCE, therefore, is a device which is capable of supplying and maintaining voltage while some type of electrical apparatus is connected to its terminals. The internal action of the source is such that electrons are continuously removed from one terminal, keeping it positive, and simultaneously supplied to the second terminal which maintains a negative charge.
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Presently, there are six known methods for producing a voltage or electromotive force (emf). Some of these methods are more widely used than others, and some are used mostly for specific applications. Following is a list of the six known methods of producing a voltage. 1. FRICTION—Voltage produced by rubbing certain materials together. 2. PRESSURE (piezoelectricity)—Voltage produced by squeezing crystals of certain substances. 3. HEAT (thermoelectricity)—Voltage produced by heating the joint (junction) where two unlike metals are joined. 4. LIGHT (photoelectricity)—Voltage produced by light striking photosensitive (light sensitive) substances. 5. CHEMICAL ACTION—Voltage produced by chemical reaction in a battery cell. 6. MAGNETISM—Voltage produced in a conductor when the conductor moves through a magnetic field, or a magnetic field moves through the conductor in such a manner as to cut the magnetic lines of force of the field. Voltage Produced by Friction The first method discovered for creating a voltage was that of generation by friction. The development of charges by rubbing a rod with fur is a prime example of the way in which a voltage is generated by friction. Because of the nature of the materials with which this voltage is generated, it cannot be conveniently used or maintained. For this reason, very little practical use has been found for voltages generated by this method. In the search for methods to produce a voltage of a larger amplitude and of a more practical nature, machines were developed in which charges were transferred from one terminal to another by means of rotating glass discs or moving belts. The most notable of these machines is the Van de Graaff generator. It is used today to produce potentials in the order of millions of volts for nuclear research. As these machines have little value outside the field of research, their theory of operation will not be described here. Q39. A device which supplies a voltage is commonly referred to by what name? Voltage Produced by Pressure One specialized method of generating an emf utilizes the characteristics of certain ionic crystals such as quartz, Rochelle salts, and tourmaline. These crystals have the remarkable ability to generate a voltage whenever stresses are applied to their surfaces. Thus, if a crystal of quartz is squeezed, charges of opposite polarity will appear on two opposite surfaces of the crystal. If the force is reversed and the crystal is stretched, charges will again appear, but will be of the opposite polarity from those produced by squeezing. If a crystal of this type is given a vibratory motion, it will produce a voltage of reversing polarity between two of its sides. Quartz or similar crystals can thus be used to convert mechanical energy into electrical energy. This phenomenon, called the PIEZOELECTRIC EFFECT, is shown in figure 1-20. Some of the common devices that make use of piezoelectric crystals are microphones, phonograph cartridges, and oscillators used in radio transmitters, radio receivers, and sonar equipment. This method of generating an emf is not suitable for applications having large voltage or power requirements, but is widely used in sound and communications systems where small signal voltages can be effectively used.
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Figure 1-20.—(A) Noncrystallized structure; (B) crystallized structure; (C) compression of a crystal; (D) decompression of a crystal.
Crystals of this type also possess another interesting property, the "converse piezoelectric effect." That is, they have the ability to convert electrical energy into mechanical energy. A voltage impressed across the proper surfaces of the crystal will cause it to expand or contract its surfaces in response to the voltage applied. Voltage Produced by Heat When a length of metal, such as copper, is heated at one end, electrons tend to move away from the hot end toward the cooler end. This is true of most metals. However, in some metals, such as iron, the opposite takes place and electrons tend to move TOWARD the hot end. These characteristics are illustrated in figure 1-21. The negative charges (electrons) are moving through the copper away from the heat and through the iron toward the heat. They cross from the iron to the copper through the current meter to the iron at the cold junction. This device is generally referred to as a THERMOCOUPLE.
Figure 1-21.—Voltage produced by heat.
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Thermocouples have somewhat greater power capacities than crystals, but their capacity is still very small if compared to some other sources. The thermoelectric voltage in a thermocouple depends mainly on the difference in temperature between the hot and cold junctions. Consequently, they are widely used to measure temperature, and as heat-sensing devices in automatic temperature control equipment. Thermocouples generally can be subjected to much greater temperatures than ordinary thermometers, such as the mercury or alcohol types. Voltage Produced by Light When light strikes the surface of a substance, it may dislodge electrons from their orbits around the surface atoms of the substance. This occurs because light has energy, the same as any moving force. Some substances, mostly metallic ones, are far more sensitive to light than others. That is, more electrons will be dislodged and emitted from the surface of a highly sensitive metal, with a given amount of light, than will be emitted from a less sensitive substance. Upon losing electrons, the photosensitive (light-sensitive) metal becomes positively charged, and an electric force is created. Voltage produced in this manner is referred to as a PHOTOELECTRIC VOLTAGE. The photosensitive materials most commonly used to produce a photoelectric voltage are various compounds of silver oxide or copper oxide. A complete device which operates on the photoelectric principle is referred to as a "photoelectric cell." There are many different sizes and types of photoelectric cells in use, and each serves the special purpose for which it is designed. Nearly all, however, have some of the basic features of the photoelectric cells shown in figure 1-22.
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Figure 1-22.—Voltage produced by light.
The cell (fig. 1-22 view A) has a curved light-sensitive surface focused on the central anode. When light from the direction shown strikes the sensitive surface, it emits electrons toward the anode. The more intense the light, the greater the number of electrons emitted. When a wire is connected between the filament and the back, or dark side of the cell, the accumulated electrons will flow to the dark side. These electrons will eventually pass through the metal of the reflector and replace the electrons leaving the lightsensitive surface. Thus, light energy is converted to a flow of electrons, and a usable current is developed. The cell (fig. 1-22 view B) is constructed in layers. A base plate of pure copper is coated with lightsensitive copper oxide. An extremely thin semitransparent layer of metal is placed over the copper oxide. This additional layer serves two purposes: 1. It permits the penetration of light to the copper oxide. 2. It collects the electrons emitted by the copper oxide. An externally connected wire completes the electron path, the same as in the reflector-type cell. The photocell’s voltage is used as needed by connecting the external wires to some other device, which amplifies (enlarges) it to a usable level. The power capacity of a photocell is very small. However, it reacts to light-intensity variations in an extremely short time. This characteristic makes the photocell very useful in detecting or accurately 1-31
controlling a great number of operations. For instance, the photoelectric cell, or some form of the photoelectric principle, is used in television cameras, automatic manufacturing process controls, door openers, burglar alarms, and so forth. Voltage Produced by Chemical Action Voltage may be produced chemically when certain substances are exposed to chemical action. If two dissimilar substances (usually metals or metallic materials) are immersed in a solution that produces a greater chemical action on one substance than on the other, a difference of potential will exist between the two. If a conductor is then connected between them, electrons will flow through the conductor to equalize the charge. This arrangement is called a primary cell. The two metallic pieces are called electrodes and the solution is called the electrolyte. The voltaic cell illustrated in figure 1-23 is a simple example of a primary cell. The difference of potential results from the fact that material from one or both of the electrodes goes into solution in the electrolyte, and in the process, ions form in the vicinity of the electrodes. Due to the electric field associated with the charged ions, the electrodes acquire charges.
Figure 1-23.—Voltaic cell.
The amount of difference in potential between the electrodes depends principally on the metals used. The type of electrolyte and the size of the cell have little or no effect on the potential difference produced. There are two types of primary cells, the wet cell and the dry cell. In a wet cell the electrolyte is a liquid. A cell with a liquid electrolyte must remain in an upright position and is not readily transportable. An automotive battery is an example of this type of cell. The dry cell, much more commonly used than the wet cell, is not actually dry, but contains an electrolyte mixed with other materials to form a paste. Flashlights and portable radios are commonly powered by dry cells. Batteries are formed when several cells are connected together to increase electrical output.
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Voltage Produced by Magnetism Magnets or magnetic devices are used for thousands of different jobs. One of the most useful and widely employed applications of magnets is in the production of vast quantities of electric power from mechanical sources. The mechanical power may be provided by a number of different sources, such as gasoline or diesel engines, and water or steam turbines. However, the final conversion of these source energies to electricity is done by generators employing the principle of electromagnetic induction. These generators, of many types and sizes, are discussed in other modules in this series. The important subject to be discussed here is the fundamental operating principle of ALL such electromagnetic-induction generators. To begin with, there are three fundamental conditions which must exist before a voltage can be produced by magnetism. 1. There must be a CONDUCTOR in which the voltage will be produced. 2. There must be a MAGNETIC FIELD in the conductor’s vicinity. 3. There must be relative motion between the field and conductor. The conductor must be moved so as to cut across the magnetic lines of force, or the field must be moved so that the lines of force are cut by the conductor. In accordance with these conditions, when a conductor or conductors MOVE ACROSS a magnetic field so as to cut the lines of force, electrons WITHIN THE CONDUCTOR are propelled in one direction or another. Thus, an electric force, or voltage, is created. In figure 1-24, note the presence of the three conditions needed for creating an induced voltage.
Figure 1-24.—Voltage produced by magnetism.
1. A magnetic field exists between the poles of the C-shaped magnet. 2. There is a conductor (copper wire). 3. There is a relative motion. The wire is moved back and forth ACROSS the magnetic field.
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In figure 1-24 view A, the conductor is moving TOWARD the front of the page and the electrons move from left to right. The movement of the electrons occurs because of the magnetically induced emf acting on the electrons in the copper. The right-hand end becomes negative, and the left-hand end positive. The conductor is stopped at view B, motion is eliminated (one of the three required conditions), and there is no longer an induced emf. Consequently, there is no longer any difference in potential between the two ends of the wire. The conductor at view C is moving away from the front of the page. An induced emf is again created. However, note carefully that the REVERSAL OF MOTION has caused a REVERSAL OF DIRECTION in the induced emf. If a path for electron flow is provided between the ends of the conductor, electrons will leave the negative end and flow to the positive end. This condition is shown in part view D. Electron flow will continue as long as the emf exists. In studying figure 1-24, it should be noted that the induced emf could also have been created by holding the conductor stationary and moving the magnetic field back and forth. The more complex aspects of power generation by use of mechanical motion and magnetism are discussed later in this series, under the heading "Generators and Motors." Q40. Name the six methods of producing a voltage. Q41. The piezoelectric effect is an example of a voltage being produced by what method? Q42. A thermocouple is a device that produces voltage by what method? Q43. A battery uses what method to produce a voltage? Q44. A generator uses what method to produce a voltage? ELECTRIC CURRENT It has been proven that electrons (negative charges) move through a conductor in response to an electric field. ELECTRON CURRENT FLOW will be used throughout this explanation. Electron current is defined as the directed flow of electrons. The direction of electron movement is from a region of negative potential to a region of positive potential. Therefore electric current can be said to flow from negative to positive. The direction of current flow in a material is determined by the polarity of the applied voltage. NOTE: In some electrical/electronic communities, the direction of current flow is recognized as being from positive to negative. Q45. According to electron theory, an electric current flows from what potential to what potential? Random Drift All materials are composed of atoms, each of which is capable of being ionized. If some form of energy, such as heat, is applied to a material, some electrons acquire sufficient energy to move to a higher energy level. As a result, some electrons are freed from their parent atom’s which then becomes ions. Other forms of energy, particularly light or an electric field, will cause ionization to occur. The number of free electrons resulting from ionization is dependent upon the quantity of energy applied to a material, as well as the atomic structure of the material. At room temperature some materials, classified as conductors, have an abundance of free electrons. Under a similar condition, materials classified as insulators have relatively few free electrons. In a study of electric current, conductors are of major concern. Conductors are made up of atoms that contain loosely bound electrons in their outer orbits. Due to the effects of increased energy, these outermost electrons frequently break away from their atoms and freely drift throughout the material. The 1-34
free electrons, also called mobile electrons, take a path that is not predictable and drift about the material in a haphazard manner. Consequently such a movement is termed RANDOM DRIFT. It is important to emphasize that the random drift of electrons occurs in all materials. The degree of random drift is greater in a conductor than in an insulator. Directed Drift Associated with every charged body there is an electrostatic field. Bodies that are charged alike repel one another and bodies with unlike charges attract each other. An electron will be affected by an electrostatic field in exactly the same manner as any negatively charged body. It is repelled by a negative charge and attracted by a positive charge. If a conductor has a difference in potential impressed across it, as shown in figure 1-25, a direction is imparted to the random drift. This causes the mobile electrons to be repelled away from the negative terminal and attracted toward the positive terminal. This constitutes a general migration of electrons from one end of the conductor to the other. The directed migration of mobile electrons due to the potential difference is called DIRECTED DRIFT.
Figure 1-25.—Directed drift.
The directed movement of the electrons occurs at a relatively low VELOCITY (rate of motion in a particular direction). The effect of this directed movement, however, is felt almost instantaneously, as explained by the use of figure 1-26. As a difference in potential is impressed across the conductor, the positive terminal of the battery attracts electrons from point A. Point A now has a deficiency of electrons. As a result, electrons are attracted from point B to point A. Point B has now developed an electron deficiency, therefore, it will attract electrons. This same effect occurs throughout the conductor and repeats itself from points D to C. At the same instant the positive battery terminal attracted electrons from point A, the negative terminal repelled electrons toward point D. These electrons are attracted to point D as it gives up electrons to point C. This process is continuous for as long as a difference of potential exists across the conductor. Though an individual electron moves quite slowly through the conductor, the effect of a directed drift occurs almost instantaneously. As an electron moves into the conductor at point D, an electron is leaving at point A. This action takes place at approximately the speed a light (186,000 miles Per Second).
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Figure 1-26.—Effect of directed drift.
Q46. The effects of directed drift take place at what rate of speed? Magnitude of Current Flow Electric current has been defined as the directed movement of electrons. Directed drift, therefore, is current and the terms can be used interchangeably. The expression directed drift is particularly helpful in differentiating between the random and directed motion of electrons. However, CURRENT FLOW is the terminology most commonly used in indicating a directed movement of electrons. The magnitude of current flow is directly related to the amount of energy that passes through a conductor as a result of the drift action. An increase in the number of energy carriers (the mobile electrons) or an increase in the energy of the existing mobile electrons would provide an increase in current flow. When an electric potential is impressed across a conductor, there is an increase in the velocity of the mobile electrons causing an increase in the energy of the carriers. There is also the generation of an increased number of electrons providing added carriers of energy. The additional number of free electrons is relatively small, hence the magnitude of current flow is primarily dependent on the velocity of the existing mobile electrons. The magnitude of current flow is affected by the difference of potential in the following manner. Initially, mobile electrons are given additional energy because of the repelling and attracting electrostatic field. If the potential difference is increased, the electric field will be stronger, the amount of energy imparted to a mobile electron will be greater, and the current will be increased. If the potential difference is decreased, the strength of the field is reduced, the energy supplied to the electron is diminished, and the current is decreased. Q47. What is the relationship of current to voltage in a circuit? Measurement of Current The magnitude of current is measured in AMPERES. A current of one ampere is said to flow when one coulomb of charge passes a point in one second. Remember, one coulomb is equal to the charge of 6.28 x 1018 electrons. 1-36
Frequently, the ampere is much too large a unit for measuring current. Therefore, the MILLIAMPERE (mA), one-thousandth of an ampere, or the MICROAMPERE ( $ RQHPLOOLRQWKRIDQ ampere, is used. The device used to measure current is called an AMMETER and will be discussed in detail in a later module. Q48. Convert 350 mA to amperes. ELECTRICAL RESISTANCE It is known that the directed movement of electrons constitutes a current flow. It is also known that the electrons do not move freely through a conductor’s crystalline structure. Some materials offer little opposition to current flow, while others greatly oppose current flow. This opposition to current flow is known as RESISTANCE (R), and the unit of measure is the OHM. The standard of measure for one ohm is the resistance provided at zero degrees Celsius by a column of mercury having a cross-sectional area of one square millimeter and a length of 106.3 centimeters. A conductor has one ohm of resistance when an applied potential of one volt produces a current of one ampere. The symbol used to represent the ohm is WKH*UHHNOHWWHURPHJD Resistance, although an electrical property, is determined by the physical structure of a material. The resistance of a material is governed by many of the same factors that control current flow. Therefore, in a subsequent discussion, the factors that affect current flow will be used to assist in the explanation of the factors affecting resistance. Q49. What is the symbol for ohm? Factors That Affect Resistance The magnitude of resistance is determined in part by the "number of free electrons" available within the material. Since a decrease in the number of free electrons will decrease the current flow, it can be said that the opposition to current flow (resistance) is greater in a material with fewer free electrons. Thus, the resistance of a material is determined by the number of free electrons available in a material. A knowledge of the conditions that limit current flow and, therefore, affect resistance can now be used to consider how the type of material, physical dimensions, and temperature will affect the resistance of a conductor. TYPE OF MATERIAL.—Depending upon their atomic structure, different materials will have different quantities of free electrons. Therefore, the various conductors used in electrical applications have different values of resistance. Consider a simple metallic substance. Most metals are crystalline in structure and consist of atoms that are tightly bound in the lattice network. The atoms of such elements are so close together that the electrons in the outer shell of the atom are associated with one atom as much as with its neighbor. (See fig. 1-27 view A). As a result, the force of attachment of an outer electron with an individual atom is practically zero. Depending on the metal, at least one electron, sometimes two, and in a few cases, three electrons per atom exist in this state. In such a case, a relatively small amount of additional electron energy would free the outer electrons from the attraction of the nucleus. At normal room temperature materials of this type have many free electrons and are good conductors. Good conductors will have a low resistance.
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Figure 1-27.—Atomic spacing in conductors.
If the atoms of a material are farther apart, as illustrated in figure 1-27 view B, the electrons in the outer shells will not be equally attached to several atoms as they orbit the nucleus. They will be attracted by the nucleus of the parent atom only. Therefore, a greater amount of energy is required to free any of these electrons. Materials of this type are poor conductors and therefore have a high resistance. Silver, gold, and aluminum are good conductors. Therefore, materials composed of their atoms would have a low resistance. The element copper is the conductor most widely used throughout electrical applications. Silver has a lower resistance than copper but its cost limits usage to circuits where a high conductivity is demanded. Aluminum, which is considerably lighter than copper, is used as a conductor when weight is a major factor. Q50. When would silver be used as a conductor in preference to copper? EFFECT OF CROSS-SECTIONAL AREA.—Cross-sectional area greatly affects the magnitude of resistance. If the cross-sectional area of a conductor is increased, a greater quantity of electrons are available for movement through the conductor. Therefore, a larger current will flow for a given amount of applied voltage. An increase in current indicates that when the cross-sectional area of a conductor is increased, the resistance must have decreased. If the cross-sectional area of a conductor is decreased, the number of available electrons decreases and, for a given applied voltage, the current through the conductor decreases. A decrease in current flow indicates that when the cross-sectional area of a conductor is decreased, the resistance must have increased. Thus, the RESISTANCE OF A CONDUCTOR IS INVERSELY PROPORTIONAL TO ITS CROSS-SECTIONAL AREA. The diameter of conductors used in electronics is often only a fraction of an inch, therefore, the diameter is expressed in mils (thousandths of an inch). It is also standard practice to assign the unit circular mil to the cross-sectional area of the conductor. The circular mil is found by squaring the diameter when the diameter is expressed in mils. Thus, if the diameter is 35 mils (0.035 inch), the circular
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mil area is equal to (35) 2 or 1225 circular mils. A comparison between a square mil and a circular mil is illustrated in figure 1-28.
Figure 1-28.—Square and circular mils.
EFFECT OF CONDUCTOR LENGTH.—The length of a conductor is also a factor which determines the resistance of a conductor. If the length of a conductor is increased, the amount of energy given up increases. As free electrons move from atom to atom some energy is given off as heat. The longer a conductor is, the more energy is lost to heat. The additional energy loss subtracts from the energy being transferred through the conductor, resulting in a decrease in current flow for a given applied voltage. A decrease in current flow indicates an increase in resistance, since voltage was held constant. Therefore, if the length of a conductor is increased, the resistance increases. THE RESISTANCE OF A CONDUCTOR IS DIRECTLY PROPORTIONAL TO ITS LENGTH. Q51. Which wire has the least resistance? Wire A-copper, 1000 circular mils, 6 inches long. Wire B-copper, 2000 circular mils, 11 inches long. EFFECT OF TEMPERATURE.—Temperature changes affect the resistance of materials in different ways. In some materials an increase in temperature causes an increase in resistance, whereas in others, an increase in temperature causes a decrease in resistance. The amount of change of resistance per unit change in temperature is known as the TEMPERATURE COEFFICIENT. If for an increase in temperature the resistance of a material increases, it is said to have a POSITIVE TEMPERATURE COEFFICIENT. A material whose resistance decreases with an increase in temperature has a NEGATIVE TEMPERATURE COEFFICIENT. Most conductors used in electronic applications have a positive temperature coefficient. However, carbon, a frequently used material, is a substance having a negative temperature coefficient. Several materials, such as the alloys constantan and manganin, are considered to have a ZERO TEMPERATURE COEFFICIENT because their resistance remains relatively constant for changes in temperature. Q52. Which temperature coefficient indicates a material whose resistance increases as temperature increases?
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Q53. What term describes a material whose resistance remains relatively constant with changes in temperature? CONDUCTANCE Electricity is a study that is frequently explained in terms of opposites. The term that is the opposite of resistance is CONDUCTANCE. Conductance is the ability of a material to pass electrons. The factors that affect the magnitude of resistance are exactly the same for conductance, but they affect conductance in the opposite manner. Therefore, conductance is directly proportional to area, and inversely proportional to the length of the material. The temperature of the material is definitely a factor, but assuming a constant temperature, the conductance of a material can be calculated. The unit of conductance is the MHO (G), which is ohm spelled backwards. Recently the term mho has been redesignated SIEMENS (S). Whereas the symbol used to represent resistance (R) is the Greek OHWWHURPHJD WKHV\PEROXVHGWRUHSUHVHQWFRQGXFWDQFH* LV6 7KHUHODWLRQVKLSWKDWH[LVWV between resistance (R) and conductance (G) or (S) is a reciprocal one. A reciprocal of a number is one divided by that number. In terms of resistance and conductance:
Q54. What is the unit of conductance and what other term is sometimes used? Q55. What is the relationship between conductance and resistance? ELECTRICAL RESISTORS Resistance is a property of every electrical component. At times, its effects will be undesirable. However, resistance is used in many varied ways. RESISTORS are components manufactured to possess specific values of resistance. They are manufactured in many types and sizes. When drawn using its schematic representation, a resistor is shown as a series of jagged lines, as illustrated in figure 1-29.
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Figure 1-29.—Types of resistors.
Q56. What is schematic symbol for a resistor? Composition of Resistors One of the most common types of resistors is the molded composition, usually referred to as the carbon resistor. These resistors are manufactured in a variety of sizes and shapes. The chemical composition of the resistor determines its ohmic value and is accurately controlled by the manufacturer in the development process. They are made in ohmic values that range from one ohm to millions of ohms. The physical size of the resistor is related to its wattage rating, which is the ability of resistor to dissipate heat caused by the resistance. Carbon resistors, as you might suspect, have as their principal ingredient the element carbon. In the manufacturer of carbon resistors, fillers or binders are added to the carbon to obtain various resistor values. Examples of these fillers are clay, bakelite, rubber, and talc. These fillers are doping agents and cause the overall conduction characteristics to change. Carbon resistors are the most common resistors found because they are easy to manufacturer, inexpensive, and have a tolerance that is adequate for most electrical and electronic applications. Their prime disadvantage is that they have a tendency to change value as they age. One other disadvantage of carbon resistors is their limited power handling capacity. The disadvantage of carbon resistors can be overcome by the use of WIREWOUND resistors (fig. 1-29 (B) and (C)). Wirewound resistors have very accurate values and possess a higher current handling capability than carbon resistors. The material that is frequently used to manufacture wirewound resistors
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is German silver which is composed of copper, nickel, and zinc. The qualities and quantities of these elements present in the wire determine the resistivity of the wire. (The resistivity of the wire is the measure or ability of the wire to resist current. Usually the percent of nickel in the wire determines the resistivity.) One disadvantage of the wirewound resistor is that it takes a large amount of wire to manufacture a resistor of high ohmic value, thereby increasing the cost. A variation of the wirewound resistor provides an exposed surface to the resistance wire on one side. An adjustable tap is attached to this side. Such resistors, sometimes with two or more adjustable taps, are used as voltage dividers in power supplies and other applications where a specific voltage is desired to be "tapped" off. Q57. What does the wattage rating of a resistor indicate? Q58. What are the two disadvantages of carbon-type resistors? Q59. What type resistor should be used to overcome the disadvantages of the carbon resistor? Fixed and Variable Resistors There are two kinds of resistors, FIXED and VARIABLE. The fixed resistor will have one value and will never change (other than through temperature, age, etc.). The resistors shown in A and B of figure 129 are classed as fixed resistors. The tapped resistor illustrated in B has several fixed taps and makes more than one resistance value available. The sliding contact resistor shown in C has an adjustable collar that can be moved to tap off any resistance within the ohmic value range of the resistor. There are two types of variable resistors, one called a POTENTIOMETER and the other a RHEOSTAT (see views D and E of fig. 1-29.) An example of the potentiometer is the volume control on your radio, and an example of the rheostat is the dimmer control for the dash lights in an automobile. There is a slight difference between them. Rheostats usually have two connections, one fixed and the other moveable. Any variable resistor can properly be called a rheostat. The potentiometer always has three connections, two fixed and one moveable. Generally, the rheostat has a limited range of values and a high current-handling capability. The potentiometer has a wide range of values, but it usually has a limited current-handling capability. Potentiometers are always connected as voltage dividers. (Voltage dividers are discussed in Chapter 3.) Q60. Describe the differences between the rheostat connections and those of the potentiometer. Q61. Which type of variable resistor should you select for controlling a large amount of current? Wattage Rating When a current is passed through a resistor, heat is developed within the resistor. The resistor must be capable of dissipating this heat into the surrounding air; otherwise, the temperature of the resistor rises causing a change in resistance, or possibly causing the resistor to burn out. The ability of the resistor to dissipate heat depends upon the design of the resistor itself. This ability to dissipate heat depends on the amount of surface area which is exposed to the air. A resistor designed to dissipate a large amount of heat must therefore have a large physical size. The heat dissipating capability of a resistor is measured in WATTS (this unit will be explained later in chapter 3). Some of the more common wattage ratings of carbon resistors are: one-eighth watt, one-fourth watt, one-half watt, one watt, and two watts. In some of the newer state-of-the-art circuits of today, much smaller wattage resistors are used. Generally, the type that you will be able to physically work with are of the values given. The higher the wattage rating of the resistor the larger is the physical size. Resistors that dissipate very large amounts of power (watts) are usually wirewound resistors. Wirewound resistors with wattage ratings up to 50
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watts are not uncommon. Figure 1-30 shows some resistors which have different wattage ratings. Notice the relative sizes of the resistors.
Figure 1-30.—Resistors of different wattage ratings.
Standard Color Code System In the standard color code system, four bands are painted on the resistor, as shown in figure 1-31.
Figure 1-31.—Resistor color codes.
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Examples of resistor color codes.
The color of the first band indicates the value of the first significant digit. The color of the second band indicates the value of the second significant digit. The third color band represents a decimal multiplier by which the first two digits must be multiplied to obtain the resistance value of the resistor. The colors for the bands and their corresponding values are shown in Table 1-1.
7DEOH 6WDQGDUG&RORU&RGHIRU5HVLVWRUV
COLOR
SIGNIFICANT FIGURE
DECIMAL MULTIPLIER
TOLERANCE PERCENT
BLACK BROWN RED ORANGE YELLOW GREEN BLUE VIOLET GRAY WHITE GOLD SILVER NO COLOR
0 1 2 3 4 5 6 7 8 9 — — —
1 10 100 1,000 10,000 100,000 1,000,000 10,000,000 100,000,000 1,000,000,000 .1 .01 —
PERCENT 1 2 — — — — — — 5 10 20
RELIABILITY LEVEL PER 1,000 HRS. 1.0% .1% .01% .001% — — — — — — — — —
Use the example colors shown in figure 1-31. Since red is the color of the first band, the first significant digit is 2. The second band is violet, therefore the second significant digit is 7. The third band is orange, which indicates that the number formed as a result of reading the first two bands is multiplied by 1000. In this case 27 x 1000 = 27,000 ohms. The last band on the resistor indicates the tolerance; that is, the manufacturer's allowable ohmic deviation above and below the numerical value indicated by the resistor's color code. In this example, the color silver indicates a tolerance of 10 percent. In other words, 1-44
the actual value of the resistor may fall somewhere within 10 percent above and 10 percent below the value indicated by the color code. This resistor has an indicated value of 27,000 ohms. Its tolerance is 10 percent x 27,000 ohms, or 2,700 ohms. Therefore, the resistor’s actual value is somewhere between 24,300 ohms and 29,700 ohms. When measuring resistors, you will find situations in which the quantities to be measured may be extremely large, and the resulting number using the basic unit, the ohm, may prove too cumbersome. Therefore, a metric system prefix is usually attached to the basic unit of measurement to provide a more manageable unit. Two of the most commonly used prefixes are kilo and mega. Kilo is the prefix used to represent thousand and is abbreviated k. Mega is the prefix used to represent million and is abbreviated M. In the example given above, the 27,000-ohm resistor could have been written as 27 kilohms or 27 N 2WKHUH[DPSOHVare: 1,000 ohms = 1 N ; 10,000 ohms = 10 N ; 100,000 ohms = 100 N . /LNHZLVHRKPVLVZULWWHQDVPHJRKPRU0 DQGRKPV 0 Q62. A carbon resistor has a resistance of 50 ohms, and a tolerance of 5 percent. What are the colors of bands one, two, three, and four, respectively? SIMPLIFYING THE COLOR CODE.—Resistors are the most common components used in electronics. The technician must identify, select, check, remove, and replace resistors. Resistors and resistor circuits are usually the easiest branches of electronics to understand. The resistor color code sometimes presents problems to a technician. It really should not, because once the resistor color code is learned, you should remember it for the rest of your life. Black, brown, red, orange, yellow, green, blue, violet, gray, white—this is the order of colors you should know automatically. There is a memory aid that will help you remember the code in its proper order. Each word starts with the first letter of the colors. If you match it up with the color code, you will not forget the code. Bad Boys Run Over Yellow Gardenias Behind Victory Garden Walls, or: Black Brown Red Orange Yellow Green Blue Violet Gray White
— — — — — — — — — —
Bad Boys Run Over Yellow Gardenias Behind Victory Garden Walls
There are many other memory aid sentences that you might want to ask about from experienced technicians. You might find one of the other sentences easier to remember. There is still a good chance that you will make a mistake on a resistor's color band. Most technicians do at one time or another. If you make a mistake on the first two significant colors, it usually is not too
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serious. If you make a miscue on the third band, you are in trouble, because the value is going to be at least 10 times too high or too low. Some important points to remember about the third band are: When the third band is . . . . Black, the resistor’s value is less than 100 ohms. Brown, the resistor’s value is in hundreds of ohms. Red, the resistor’s value is in thousands of ohms. Orange, the resistor’s value is in tens of thousands of ohms. Yellow, the resistor’s value is in hundreds of thousands of ohms. Green, the resistor’s value is in megohms. Blue, the resistor’s value is in tens of megohms or more. Although you may find any of the above colors in the third band, red, orange, and yellow are the most common. In some cases, the third band will be silver or gold. You multiply the first two bands by 0.01 if it is silver, and 0.1 if it is gold. The fourth band, which is the tolerance band, usually does not present too much of a problem. If there is no fourth band, the resistor has a 20-percent tolerance; a silver fourth band indicates a 10-percent tolerance; and a gold fourth band indicates a 5-percent tolerance. Resistors that conform to military specifications have a fifth band. The fifth band indicates the reliability level per 1,000 hours of operation as follows: Fifth band color Brown Red Orange Yellow
Level 1.0% 0.1% 0.01% 0.001%
For a resistor whose the fifth band is color coded brown, the resistor’s chance of failure will not exceed 1 percent for every 1,000 hours of operation. In equipment such as the Navy’s complex computers, the reliability level is very significant. For example, in a piece of equipment containing 10,000 orange fifth-band resistors, no more than one resistor will fail during 1,000 hours of operation. This is very good reliability. More information on resistors is contained in NEETS Module 19. Q63. A carbon resistor has the following color bands: The first band is yellow, followed by violet, yellow, and silver. What is the ohmic value of the resistor? Q64. The same resistor mentioned in question 63 has a yellow fifth band. What does this signify? Q65. A resistor is handed to you for identification with the following color code: the first band is blue, followed by gray, green, gold, and brown. What is the resistor’s value?
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Some resistors, both wirewound and composition, will not use the resistor color code. These resistors will have the ohmic value and tolerance imprinted on the resistor itself.
SUMMARY With the completion of this chapter, you now have gained the necessary information which is the foundation for the further study of electricity. The following is a summary of the important parts in the chapter. In describing the composition of matter, the following terms are important for you to remember: MATTER is defined as anything that occupies space and has weight. AN ELEMENT is a substance which cannot be reduced to a simpler substance by chemical means. A COMPOUND is a chemical combination of elements which can be separated by chemical means, but not by physical means. It is created by chemically combining two or more elements. A MIXTURE is a combination of elements or compounds that can be separated by physical means. A MOLECULE is the chemical combination of two or more atoms. In a compound, the molecule is the smallest particle that has all the characteristics of the compound. AN ATOM is the smallest particle of an element that retains the characteristics of that element. An atom is made up of electrons, protons, and neutrons. The number and arrangement of these subatomic particles determine the kind of element.
AN ELECTRON is considered to be a negative charge of electricity. A PROTON is considered to be a positive charge of electricity. A NEUTRON is a neutral particle in that it has no electrical charge. ENERGY in an electron is of two types—kinetic (energy of motion) and potential (energy of position).
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ENERGY LEVELS of the electron exist because the electron has mass and motion. The motion gives it kinetic energy and its position gives it potential energy. Energy balance keeps the electron in orbit and should it gain energy it will assume an orbit further from the center of the atom. It will remain at that level for only a fraction of a second before it radiates the excess energy and goes back to a lower orbit.
SHELLS AND SUBSHELLS of electrons are the orbits of the electrons in the atom. Each shell contains a maximum of 2 times its number squared electrons. Shells are lettered K through Q, starting with K, which is the closest to the nucleus. The shell can be split into 4 subshells labeled s, p, d, and f, which can contain 2, 6, 10, and 14 electrons, respectively.
VALENCE OF AN ATOM is determined by the number of electrons in the outermost shell. The shell is referred to as the valence shell, and the electrons within it are valence electrons. An atom with few valence electrons requires little energy to free the valence electrons. IONIZATION refers to the electrons contained in an atom. An atom with a positive charge has lost some of its electrons, and is called a positive ion. A negatively charged atom is a negative ion. 1-48
CONDUCTORS, SEMICONDUCTORS, AND INSULATORS are categorized as such by the number of valence electrons in their atoms. The conductor normally has 3 or less valence electrons and offers little opposition to the flow of electrons (electric current). The insulator contains 5 or more valence electrons and offers high opposition to electron flow. The semiconductor usually has four valence electrons of conductivity and is in the midrange. The best conductors in order of conductance are silver, copper, gold, and aluminum. CHARGED BODIES affect each other as follows: When two bodies having unequal charges are brought close to each other, they will tend to attract each other in an attempt to equalize their respective charges. When two bodies, both having either positive or negative charges, are brought close together, they tend to repel each other as no equalization can occur. When the charge on one body is high enough with respect to the charge on an adjacent body, an equalizing current will flow between the bodies regardless of the conductivity of the material containing the bodies.
A NEUTRAL BODY may be attracted to either a positively or negatively charged body due to the relative difference in their respective charges. CHARGED BODIES will attract or repel each other with a force that is directly proportional to the product of their individual charges and inversely proportional to the square of the distance between the bodies.
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ELECTROSTATIC LINES of force are a graphic representation of the field around a charged body. These lines are imaginary. Lines from a positively charged body are indicated as flowing out from the body, while lines from a negatively charged body are indicated as flowing into the body. MAGNETISM is that property of a material which enables it to attract pieces of iron. A material with this property is called a magnet. Any material that is attracted to a magnet can be made into a magnet itself. FERROMAGNETIC MATERIALS are materials that are easy to magnetize; e.g., iron, steel, and cobalt. NATURAL MAGNETS, called magnetite, lodestones, or leading stones, were the first magnets to be studied. Most magnets in practical use are artificial or man-made magnets, and are made either by electrical means or by stroking a magnetic material with a magnet. RELUCTANCE is defined as the opposition of a material to being magnetized. PERMEABILITY is defined as the ease with which a material accepts magnetism. A material which is easy to magnetize does not hold its magnetism very long, and vice versa. RETENTIVITY is defined as the ability of a material to retain magnetism. A MAGNETIC POLE is located at each end of a magnet. The majority of the magnetic force is concentrated at these poles and is approximately equal at both poles.
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THE NORTH POLE, or north seeking pole, of a magnet freely suspended on a string always points toward the north geographical pole. THE LIKE POLES of magnets repel each other, while the unlike poles attract each other.
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WEBER’S THEORY OF MAGNETISM assumes that all magnetic material is made up of magnetic molecules which, if lined up in north to south pole order, will be a magnet. If not lined up, the magnetic fields about the molecules will neutralize each other and no magnetic effect will be noted.
THE DOMAIN THEORY OF MAGNETISM states that if the electrons of the atoms in a material spin more in one direction than in the other, the material will become magnetized.
A MAGNETIC FIELD is said to exist in the space surrounding a magnet.
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MAGNETIC LINES OF FORCE are imaginary lines used to describe the patterns of the magnetic field about a magnet. These lines are assumed to flow externally from the north pole and into the south pole.
MAGNETIC FLUX is the total number of magnetic lines of force leaving or entering the pole of a magnet.
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FLUX DENSITY is the number of flux lines per unit area. FIELD INTENSITY or the intensity of a magnetic field is directly related to the magnetic force exerted by the field. THE INTENSITY OF ATTRACTION/REPULSION between magnetic poles may be described by a law almost identical to Coulomb’s Law of Charged Bodies, that is, the force between two poles is directly proportional to the product of the pole strengths and inversely proportional to the square of the distance between the poles. MAGNETIC SHIELDING can be accomplished by placing a soft iron shield around the object to be protected, thus directing the lines of force around the object.
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MAGNETS ARE CLASSIFIED BY SHAPE and include the bar magnet, the horseshoe magnet, and the ring magnet. The ring magnet is used in computer memory circuits; the horseshoe magnet in some meter circuits. ENERGY may be defined as the ability to do work. THE COULOMB (C) is the basic unit used to indicate an electrical charge. One coulomb is equal to a charge of 6.28 x 1018 electrons. When one coulomb of charge exists between two bodies, the electromotive force (or voltage) is one volt. VOLTAGE is measured as the difference of potential of two charges of interest. VOLTAGE MEASUREMENTS may be expressed in the following units: volts (V), kilovolts (kV), millivolts (mV), or microvolts ( 9 For example: 1 kV = 1,000 V 1 mV = 0.001 V 1 9 9 METHODS OF PRODUCING A VOLTAGE include: 1. Friction 2. Pressure (piezoelectricity)
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3. Heat (thermoelectricity)
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4. Light (photoelectricity)
5. Chemical action (battery)
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6. Magnetism (electromagnetic induction generator) ELECTROMAGNETIC INDUCTION GENERATOR To produce voltage by use of magnetism, three conditions must be met: There must be a CONDUCTOR in which the voltage will be produced; there must be a MAGNETIC FIELD in the conductor’s vicinity; and there must be relative motion between the field and conductor. When these conditions are met, electrons WITHIN THE CONDUCTOR are propelled in one direction or another, creating an electromotive force, or voltage.
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ELECTRON CURRENT is based on the assumption that electron current flow is from negative to positive through a circuit. AN ELECTRIC CURRENT is a directed movement of electrons in a conductor or circuit. THE AMPERE is the basic unit used to indicate an electric current. A current of one ampere is said to flow when one coulomb of charge (6.28 x 1018 electrons) passes a given point in one second of time. Current measurements may be expressed in the following units: ampere (A), milliampere (mA), and microampere ( $ &XUUHQWLQDFLUFXLWLQFUHDVHVLQGLUHFWSURSRUWLRQWRWKHYROWDJHemf) applied across the circuit. RESISTANCE is the opposition to current. The ohm is the basic unit of resistance and is UHSUHVHQWHGE\WKH*UHHNOHWWHURPHJD $FRQGXFWRULVVDLGWRKDYHRQHRKPRIUHVLVWDQFHZKHQDQ emf of one volt causes one ampere of current to flow in the conductor. Resistance may be expressed in the IROORZLQJXQLWVRKP kilohm (N DQGPHJRKPV0 )RUH[DPSOH N 0 THE RESISTANCE OF A MATERIAL is determined by the type, the physical dimensions, and the temperature of the material that is, 1. 2. 3. 4.
A good conductor contains an abundance of free electrons. As the cross-sectional area of a given conductor is increased, the resistance will decrease. As the length of a conductor is increased, the resistance will increase. In a material having a positive temperature coefficient, the resistance will increase as the temperature is increased.
THE CONDUCTANCE OF A MATERIAL is the reciprocal of resistance. THE UNIT OF CONDUCTANCE is the mho and the symbol is V. G or S. THE RESISTOR is manufactured to provide a specific value of resistance. THE CARBON RESISTOR is made of carbon, with fillers and binders blended in to control the ohmic value.
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THE RESISTANCE OF A WIREWOUND RESISTOR is determined by the metal content of the wire and the wire’s length. Wirewound resistors may be tapped so two or more different voltage values may be taken off the same resistor.
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THE POTENTIOMETER AND THE RHEOSTAT are variable resistors and can be adjusted to any resistance value within their ohmic range. The rheostat is usually used for relatively high current applications and has two connections; the potentiometer has 3 connections and is a relatively highresistance, low-current device.
Two examples of potentiometers.
Example of a rheostat.
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Table 1-1.—Standard Color Code for Resistors
COLOR BLACK BROWN RED ORANGE YELLOW GREEN BLUE VIOLET GRAY WHITE GOLD SILVER NO COLOR
SIGNIFICANT FIGURE 0 1 2 3 4 5 6 7 8 9 — — —
DECIMAL MULTIPLIER 1 10 100 1,000 10,000 100,000 1,000,000 10,000,000 100,000,000 1,000,000,000 .1 .01 —
TOLERANCE PERCENT PERCENT 1 2 . — — — — — — 5 10 20
RELIABILITY LEVEL PER 1,000 HRS. 1.0% .1% .01% 001% — — — — — — — — —
THE WATTAGE RATING OF A RESISTOR is related to the resistor’s physical size, that is, the greater the surface area exposed to the air, the larger the rating. THE STANDARD COLOR CODE for resistors is used to determine the following: 1. Ohmic value 2. Tolerance 3. Reliability level (on some resistors)
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ANSWERS TO QUESTIONS Q1. THROUGH Q65. A1. Anything that occupies space and has weight. Solids, liquids, gases. A2. A substance which cannot be reduced to a simpler substance by chemical means. A3. A substance consisting of two or more elements. A4. A compound is a chemical combination of elements that cannot be separated by physical means. A mixture is a physical combination of elements and compounds that are not chemically combined. A5. A chemical combination of two or more atoms. A6. Electrons-negative, protons-positive, and neutrons-neutral. A7. Kinetic energy. A8. Invisible light photons (ultraviolet) bombard the phosphor atom in the light tube. The phosphor atoms emit visible light photons. A9. The number of electrons in the outer shell. A10. An atom with more or less than its normal number of electrons. A11. The number of valence electrons. A12. Through the accumulation of excess electrons. A13. By friction. A14. Negative. A15. Like charges repel, and unlike charges attract with a force directly proportional to the product of their charges and inversely proportional to the square of the distance between them. A16. The space between and around charged bodies. A17. Leaving positive, entering negative. A18. Motors, generators, speakers, computers, televisions, tape recorders, and many others. A19. Those materials that are attracted by magnets and have the ability to become magnetized. A20. The relative ease with which they are magnetized. A21. A material that exhibits low reluctance and high permeability, such as iron or soft steel. A22. The ability of a material to retain magnetism. A23. They are very similar; like charges repel, unlike charges attract, like poles repel —unlike poles attract. A24. To the magnetic north pole. A25. South pole at the right, north pole at the left. 1-63
A26. The domain theory is based upon the electron spin principle; Weber’s theory uses the concept of tiny molecular magnets. A27. To enable you to "see" the magnetic field. A28. No specific pattern, sawdust is a nonmagnetic material. A29. An imaginary line used to illustrate magnetic effects. A30. Electrostatic lines of force do not form closed loops. A31. By shielding or surrounding the instrument with a soft iron case, called a magnetic shield or screen. A32. In pairs, with opposite poles together to provide a complete path for magnetic flux. A33. The ability to do work. A34. Kinetic energy. A35. Potential energy. A36. Difference of potential. A37. 2100 volts. A38. (a) 250 kV, (b) 25 V, (c) 1 9 A39. A voltage source. A40. Friction, pressure, heat, light, chemical action, and magnetism. A41. Pressure. A42. Heat. A43. Chemical. A44. Magnetic. A45. Electron theory assumes that electron flow is from negative to positive. A46. The speed of light (186,000 miles per second, 300,000,000 meters per second). A47. Current increases as voltage increases. A48. 0.35 amperes. A49. A50. When the need for conductivity is great enough to justify the additional expense. A51. Wire B. A52. Positive.
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A53. Zero temperature coefficient. A54. The mho (v), siemens. A55. They are reciprocals of each other. A56.
A57. Its ability to dissipate heat. A58. 1. Change value with age. 2. Limited power capacity. A59. The wirewound resistor. A60. The rheostat may have two connections, one fixed and one moveable; the potentiometer always has three connections, one moveable and two fixed. A61. The rheostat. A62. The bands are green, black, black, and gold. A63. 470,000 ohms (470 kilohms). A64. The resistor’s chance of failure is 0.001 percent for 1000 hours of operation. A65. 6,800,000 ohms (6.8 megohms), with 5% tolerance, and a 1% reliability level.
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CHAPTER 2
BATTERIES LEARNING OBJECTIVES Upon completing this chapter, you will be able to: 1. State the purpose of a cell. 2. State the purpose of the three parts of a cell. 3. State the difference between the two types of cells. 4. Explain the chemical process that takes place in the primary and secondary cells. 5. Recognize and define the terms electrochemical action, anode, cathode, and electrolyte. 6. State the causes of polarization and local action and describe methods of preventing these effects. 7. Identify the parts of a dry cell. 8. Identify the various dry cells in use today and some of their capabilities and limitations. 9. Identify the four basic secondary cells, their construction, capabilities, and limitations. 10. Define a battery, and identify the three ways of combining cells to form a battery. 11. Describe general maintenance procedures for batteries including the use of the hydrometer, battery capacity, and rating and battery charging. 12. Identify the five types of battery charges. 13. Observe the safety precautions for working with and around batteries.
INTRODUCTION The purpose of this chapter is to introduce and explain the basic theory and characteristics of batteries. The batteries which are discussed and illustrated have been selected as representative of many models and types which are used in the Navy today. No attempt has been made to cover every type of battery in use, however, after completing this chapter you will have a good working knowledge of the batteries which are in general use. First, you will learn about the building block of all batteries, the CELL. The explanation will explore the physical makeup of the cell and the methods used to combine cells to provide useful voltage, current, and power. The chemistry of the cell and how chemical action is used to convert chemical energy to electrical energy are also discussed.
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In addition, the care, maintenance, and operation of batteries, as well as some of the safety precautions that should be followed while working with and around batteries are discussed. Batteries are widely used as sources of direct-current electrical energy in automobiles, boats, aircraft, ships, portable electric/electronic equipment, and lighting equipment. In some instances, they are used as the only source of power; while in others, they are used as a secondary or standby power source. A battery consists of a number of cells assembled in a common container and connected together to function as a source of electrical power. THE CELL A cell is a device that transforms chemical energy into electrical energy. The simplest cell, known as either a galvanic or voltaic cell, is shown in figure 2-1. It consists of a piece of carbon (C) and a piece of zinc (Zn) suspended in a jar that contains a solution of water (H20) and sulfuric acid (H2S0 4) called the electrolyte.
Figure 2-1.—Simple voltaic or galvanic cell.
The cell is the fundamental unit of the battery. A simple cell consists of two electrodes placed in a container that holds the electrolyte. In some cells the container acts as one of the electrodes and, in this case, is acted upon by the electrolyte. This will be covered in more detail later. ELECTRODES The electrodes are the conductors by which the current leaves or returns to the electrolyte. In the simple cell, they are carbon and zinc strips that are placed in the electrolyte; while in the dry cell (fig. 2-2), they are the carbon rod in the center and zinc container in which the cell is assembled.
2-2
Figure 2-2.—Dry cell, cross-sectional view.
ELECTROLYTE The electrolyte is the solution that acts upon the electrodes. The electrolyte, which provides a path for electron flow, may be a salt, an acid, or an alkaline solution. In the simple galvanic cell, the electrolyte is in a liquid form. In the dry cell, the electrolyte is a paste. CONTAINER The container which may be constructed of one of many different materials provides a means of holding (containing) the electrolyte. The container is also used to mount the electrodes. In the voltaic cell the container must be constructed of a material that will not be acted upon by the electrolyte. Q1. What is the purpose of a cell? Q2. What are the three parts of a cell? Q3. What is the purpose of each of the three parts of a cell? PRIMARY CELL A primary cell is one in which the chemical action eats away one of the electrodes, usually the negative electrode. When this happens, the electrode must be replaced or the cell must be discarded. In the galvanic-type cell, the zinc electrode and the liquid electrolyte are usually replaced when this happens. In the case of the dry cell, it is usually cheaper to buy a new cell. SECONDARY CELL A secondary cell is one in which the electrodes and the electrolyte are altered by the chemical action that takes place when the cell delivers current. These cells may be restored to their original condition by forcing an electric current through them in the direction opposite to that of discharge. The automobile storage battery is a common example of the secondary cell.
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Q4. What are the two types of cells? Q5. What is the main difference between the two types of cells?
ELECTROCHEMICAL ACTION If a load (a device that consumes electrical power) is connected externally to the electrodes of a cell, electrons will flow under the influence of a difference in potential across the electrodes from the CATHODE (negative electrode), through the external conductor to the ANODE (positive electrode). A cell is a device in which chemical energy is converted to electrical energy. This process is called ELECTROCHEMICAL action. The voltage across the electrodes depends upon the materials from which the electrodes are made and the composition of the electrolyte. The current that a cell delivers depends upon the resistance of the entire circuit, including that of the cell itself. The internal resistance of the cell depends upon the size of the electrodes, the distance between them in the electrolyte, and the resistance of the electrolyte. The larger the electrodes and the closer together they are in the electrolyte (without touching), the lower the internal resistance of the cell and the more current the cell is capable of supplying to the load. Q6. What is electrochemical action? Q7. What is another name for the (a) positive electrode, and the (b) negative electrode? PRIMARY CELL CHEMISTRY When a current flows through a primary cell having carbon and zinc electrodes and a diluted solution of sulfuric acid and water (combined to form the electrolyte), the following chemical reaction takes place. The current flow through the load is the movement of electrons from the negative electrode of the cell (zinc) and to the positive electrode (carbon). This causes fewer electrons in the zinc and an excess of electrons in the carbon. Figure 2-1 shows the hydrogen ions (H2) from the sulfuric acid being attracted to the carbon electrode. Since the hydrogen ions are positively charged, they are attracted to the negative charge on the carbon electrode. This negative charge is caused by the excess of electrons. The zinc electrode has a positive charge because it has lost electrons to the carbon electrode. This positive charge attracts the negative ions (S04) from the sulfuric acid. The negative ions combine with the zinc to form zinc sulfate. This action causes the zinc electrode to be eaten away. Zinc sulfate is a grayish-white substance that is sometimes seen on the battery post of an automobile battery. The process of the zinc being eaten away and the sulfuric acid changing to hydrogen and zinc sulfate is the cause of the cell discharging. When the zinc is used up, the voltage of the cell is reduced to zero. In figure 2-1 you will notice that the zinc electrode is labeled negative and the carbon electrode is labeled positive. This represents the current flow outside the cell from negative to positive. The zinc combines with the sulfuric acid to form zinc sulfate and hydrogen. The zinc sulfate dissolves in the electrolyte (sulfuric acid and water) and the hydrogen appears as gas bubbles around the carbon electrode. As current continues to flow, the zinc gradually dissolves and the solution changes to zinc sulfate and water. The carbon electrode does not enter into the chemical changes taking place, but simply provides a return path for the current.
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Q8. In the primary cell, why are negative ions attracted to the negative terminal of the cell? Q9. How do electrons get from the negative electrode to the positive electrode? Q10. What causes the negative electrode to be eaten away? SECONDARY CELL CHEMISTRY As stated before, the differences between primary and secondary cells are, the secondary cell can be recharged and the electrodes are made of different materials. The secondary cell shown in figure 2-3 uses sponge lead as the cathode and lead peroxide as the anode. This is the lead-acid type cell and will be used to explain the general chemistry of the secondary cell. Later in the chapter when other types of secondary cells are discussed, you will see that the materials which make up the parts of a cell are different, but that the chemical action is essentially the same.
Figure 2-3.—Secondary cell.
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Figure 2-3 view A shows a lead-acid secondary cell that is fully charged. The cathode is pure sponge lead, the anode is pure lead peroxide, and the electrolyte is a mixture of sulfuric acid and water. Figure 2-3 view B shows the secondary cell discharging. A load is connected between the cathode and anode; current flows negative to positive as shown. This current flow creates the same process as was explained for the primary cell with the following exceptions. In the primary cell the zinc cathode was eaten away by the sulfuric acid. In the secondary cell the sponge-like construction of the cathode retains the lead sulfate formed by the chemical action of the sulfuric acid and the lead. In the primary cell the carbon anode was not chemically acted upon by the sulfuric acid. In the secondary cell the lead peroxide anode is chemically changed to lead sulfate by the sulfuric acid. When the cell is fully discharged it will be as shown in figure 2-3 view C. The anode and cathode retain some lead peroxide and sponge lead but the amounts of lead sulfate in each is maximum. The electrolyte has a minimum amount of sulfuric acid. With this condition no further chemical action can take place within the cell. As you know, the secondary cell can be recharged. Recharging is the process of reversing the chemical action that occurs as the cell discharges. To recharge the cell, a voltage source, such as a generator, is connected as shown in figure 2-3 view D. The negative terminal of the voltage source is connected to the cathode of the cell and the positive terminal of the voltage source is connected to the anode of the cell. With this arrangement the lead sulfate is chemically changed back to sponge lead in the cathode, lead peroxide in the anode, and sulfuric acid in the electrolyte. After all the lead sulfate is chemically changed, the cell is fully charged as shown in figure 2-3 view A. Once the cell has been charged, the discharge-charge cycle may be repeated. Q11. Refer to figure 2-3(B). Why is the sulfuric acid decreasing? Q12. Refer to figure 2-3(D). How is it possible for the sulfuric acid to be increasing? Q13. Refer to figure 2-3(D). When all the lead sulfate has been converted, what is the condition of the cell? POLARIZATION OF THE CELL The chemical action that occurs in the cell while the current is flowing causes hydrogen bubbles to form on the surface of the anode. This action is called POLARIZATION. Some hydrogen bubbles rise to the surface of the electrolyte and escape into the air, some remain on the surface of the anode. If enough bubbles remain around the anode, the bubbles form a barrier that increases internal resistance. When the internal resistance of the cell increases, the output current is decreased and the voltage of the cell also decreases. A cell that is heavily polarized has no useful output. There are several methods to prevent polarization or to depolarize the cell. One method uses a vent on the cell to permit the hydrogen to escape into the air. A disadvantage of this method is that hydrogen is not available to reform into the electrolyte during recharging. This problem is solved by adding water to the electrolyte, such as in an automobile battery. A second method is to use material that is rich in oxygen, such as manganese dioxide, which supplies free oxygen to combine with the hydrogen and form water.
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A third method is to use a material that will absorb the hydrogen, such as calcium. The calcium releases hydrogen during the charging process. All three methods remove enough hydrogen so that the cell is practically free from polarization. LOCAL ACTION When the external circuit is removed, the current ceases to flow, and, theoretically, all chemical action within the cell stops. However, commercial zinc contains many impurities, such as iron, carbon, lead, and arsenic. These impurities form many small electrical cells within the zinc electrode in which current flows between the zinc and its impurities. Thus, the chemical action continues even though the cell itself is not connected to a load. Local action may be prevented by using pure zinc (which is not practical), by coating the zinc with mercury, or by adding a small percentage of mercury to the zinc during the manufacturing process. The treatment of the zinc with mercury is called amalgamating (mixing) the zinc. Since mercury is many times heavier than an equal volume of water, small particles of impurities weighing less than mercury will float to the surface of the mercury. The removal of these impurities from the zinc prevents local action. The mercury is not readily acted upon by the acid. When the cell is delivering current to a load, the mercury continues to act on the impurities in the zinc. This causes the impurities to leave the surface of the zinc electrode and float to the surface of the mercury. This process greatly increases the storage life of the cell. Q14. Describe three ways to prevent polarization. Q15. Describe local action . TYPES OF CELLS The development of new and different types of cells in the past decade has been so rapid that it is virtually impossible to have a complete knowledge of all the various types. A few recent developments are the silver-zinc, nickel-zinc, nickel-cadmium, silver-cadmium, organic and inorganic lithium, and mercury cells. PRIMARY DRY CELL The dry cell is the most popular type of primary cell. It is ideal for simple applications where an inexpensive and noncritical source of electricity is all that is needed. The dry cell is not actually dry. The electrolyte is not in a liquid state, but is a moist paste. If it should become totally dry, it would no longer be able to transform chemical energy to electrical energy. Construction of a Dry Cell The construction of a common type of dry cell is shown in figure 2-4. These dry cells are also referred to as Leclanche' cells. The internal parts of the cell are located in a cylindrical zinc container. This zinc container serves as the negative electrode (cathode) of the cell. The container is lined with a nonconducting material, such as blotting paper, to separate the zinc from the paste. A carbon electrode is located in the center, and it serves as the positive terminal (anode) of the cell. The paste is a mixture of several substances such as ammonium chloride, powdered coke, ground carbon, manganese dioxide, zinc chloride, graphite, and water.
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Figure 2-4.—Cutaway view of the general-purpose dry cell.
This paste, which is packed in the space between the anode and the blotting paper, also serves to hold the anode rigid in the center of the cell. When the paste is packed in the cell, a small space is left at the top for expansion of the electrolytic paste caused by the depolarization action. The cell is then sealed with a cardboard or plastic seal. Since the zinc container is the cathode, it must be protected with some insulating material to be electrically isolated. Therefore, it is common practice for the manufacturer to enclose the cells in cardboard and metal containers. The dry cell (fig. 2-4) is basically the same as the simple voltaic cell (wet cell) described earlier, as far as its internal chemical action is concerned. The action of the water and the ammonium chloride in the paste, together with the zinc and carbon electrodes, produces the voltage of the cell. Manganese dioxide is added to reduce polarization when current flows and zinc chloride reduces local action when the cell is not being used. A cell that is not being used (sitting on the shelf) will gradually deteriorate because of slow internal chemical changes (local action). This deterioration is usually very slow if cells are properly stored. If unused cells are stored in a cool place, their shelf life will be greatly preserved. Therefore, to minimize deterioration, they should be stored in refrigerated spaces. The blotting paper (paste-coated pulpboard separator) serves two purposes—(1) it keeps the paste from making actual contact with the zinc container and (2) it permits the electrolyte from the paste to filter through to the zinc slowly. The cell is sealed at the top to keep air from entering and drying the electrolyte. Care should be taken to prevent breaking this seal. Q16. What serves as the cathode of a dry cell? Q17. Why is a dry cell called a DRY cell? Q18. What does the term "shelf life" mean?
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Mercuric-Oxide Zinc Cell The mercuric-oxide zinc cell (mercury cell) is a primary cell that was developed during World War II. Two important assets of the mercury cell are its ability to produce current for a long period of time and a long shelf life when compared to the dry cell shown in figure 2-4. The mercury cell also has a very stable output voltage and is a power source that can be made in a small physical size. With the birth of the space program and the development of small transceivers and miniaturized equipment, a power source of small size was needed. Such equipment requires a small cell which is capable of delivering maximum electrical energy at a constant discharge voltage. The mercury cell, which is one of the smallest cells, meets these requirements. Present mercury cells are manufactured in three basic types as shown in figure 2-5. The woundanode type, shown in figure 2-5 view A, has an anode composed of a corrugated zinc strip with a paper absorbent. The zinc is mixed with mercury, and the paper is soaked in the electrolyte which causes it to swell and press against the zinc and make positive contact. This process ensures that the electrolyte makes contact with the anode.
Figure 2-5.—Mercury cells.
In the pressed-powder cells, shown in figure 2-5 views B and C, the zinc powder for the anode is mixed prior to being pressed into shape. The absorbent shown in the figure is paper soaked in the electrolyte. The space between the inner and outer containers provides passage for any gas generated by an improper chemical balance or impurities present within the cell. If the anode and cathode of a cell are connected together without a load, a SHORT CIRCUIT condition exists. Short circuits (shorts) can be very dangerous. They cause excessive heat, pressure, and current flow which may cause serious damage to the cell or be a safety hazard to personnel.
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WARNING Do not short the mercury cell. Shorted mercury cells have exploded with considerable force. Other Types of Cells There are many different types of primary cells. Because of such factors as cost, size, ease of replacement, and voltage or current needs, many types of primary cells have been developed. The following is a brief description of some of the primary cells in use today. The Manganese Dioxide-Alkaline-Zinc Cell is similar to the zinc-carbon cell except for the electrolyte used. This type of cell offers better voltage stability and longer life than the zinc-carbon type. It also has a longer shelf life and can operate over a wide temperature range. The manganese dioxidealkaline-zinc cell has a voltage of 1.5 volts and is available in a wide range of sizes. This cell is commonly referred to as the alkaline cell. The Magnesium-Manganese Dioxide Cell uses magnesium as the anode material. This allows a higher output capacity over an extended period of time compared to the zinc-carbon cell. This cell produces a voltage of approximately 2 volts. The disadvantage of this type of cell is the production of hydrogen during its operation. The Lithium-Organic Cell and the Lithium-Inorganic Cell are recent developments of a new line of high-energy cells. The main advantages of these types of cells are very high power, operation over a wide temperature range, they are lighter than most cells, and have a remarkably long shelf life of up to 20 years. CAUTION Lithium cells contain toxic materials under pressure. Do not puncture, recharge, short-circuit, expose to excessively high temperatures, or incinerate. Use these batteries/cells only in approved equipment. Do not throw in trash. Q19. Why should a mercury cell NOT be shorted? Q20. What factors should be considered when selecting a primary cell for a power source? SECONDARY WET CELLS Secondary cells are sometimes known as wet cells. There are four basic type of wet cells, the leadacid, nickel-cadmium, silver-zinc, and silver-cadmium. Lead-Acid Cell The lead-acid cell is the most widely used secondary cell. The previous explanation of the secondary cell describes exactly the manner in which the lead-acid cell provides electrical power. The discharging and charging action presented in electrochemical action describes the lead-acid cell. You should recall that the lead-acid cell has an anode of lead peroxide, a cathode of sponge lead, and the electrolyte is sulfuric acid and water.
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Nickel-Cadmium Cell The nickel-cadmium cell (NICAD) is far superior to the lead-acid cell. In comparison to lead-acid cells, these cells generally require less maintenance throughout their service life in regard to the adding of electrolyte or water. The major difference between the nickel-cadmium cell and the lead-acid cell is the material used in the cathode, anode, and electrolyte. In the nickel-cadmium cell the cathode is cadmium hydroxide, the anode is nickel hydroxide, and the electrolyte is potassium hydroxide and water. The nickel-cadmium and lead-acid cells have capacities that are comparable at normal discharge rates, but at high discharge rates the nickel-cadmium cell can deliver a larger amount of power. In addition the nickel-cadmium cell can: 1. Be charged in a shorter time, 2. Stay idle longer in any state of charge and keep a full charge when stored for a longer period of time, and 3. Be charged and discharged any number of times without any appreciable damage. Due to their superior capabilities, nickel-cadmium cells are being used extensively in many military applications that require a cell with a high discharge rate. A good example is in the aircraft storage battery. Silver-Zinc Cells The silver-zinc cell is used extensively to power emergency equipment. This type of cell is relatively expensive and can be charged and discharged fewer times than other types of cells. When compared to the lead-acid or nickel-cadmium cells, these disadvantages are overweighed by the light weight, small size, and good electrical capacity of the silver-zinc cell. The silver-zinc cell uses the same electrolyte as the nickel-cadmium cell (potassium hydroxide and water), but the anode and cathode differ from the nickel-cadmium cell. The anode is composed of silver oxide and the cathode is made of zinc. Silver-Cadmium Cell The silver-cadmium cell is a fairly recent development for use in storage batteries. The silvercadmium cell combines some of the better features of the nickel-cadmium and silver-zinc cells. It has more than twice the shelf life of the silver-zinc cell and can be recharged many more times. The disadvantages of the silver-cadmium cell are high cost and low voltage production. The electrolyte of the silver-cadmium cell is potassium hydroxide and water as in the nickelcadmium and silver-zinc cells. The anode is silver oxide as in the silver-zinc cell and the cathode is cadmium hydroxide as in the nicad cell. You may notice that different combinations of materials are used to form the electrolyte, cathode, and anode of different cells. These combinations provide the cells with different qualities for many varied applications. Q21. What are the four basic types of secondary (wet) cells? Q22. What are the advantages of a nicad cell over a lead-acid cell? Q23. What type of cell is most commonly used for emergency systems? Q24. What three cells use the same electrolyte? 2-11
BATTERIES A battery is a voltage source that uses chemical action to produce a voltage. In many cases the term battery is applied to a single cell, such as the flashlight battery. In the case of a flashlight that uses a battery of 1.5 volts, the battery is a single cell. The flashlight that is operated by 6 volts uses four cells in a single case and this is a battery composed of more than one cell. There are three ways to combine cells to form a battery. COMBINING CELLS In many cases, a battery-powered device may require more electrical energy than one cell can provide. The device may require either a higher voltage or more current, and in some cases both. Under such conditions it is necessary to combine, or interconnect, a sufficient number of cells to meet the higher requirements. Cells connected in SERIES provide a higher voltage, while cells connected in PARALLEL provide a higher current capacity. To provide adequate power when both voltage and current requirements are greater than the capacity of one cell, a combination SERIES-PARALLEL network of cells must be used. Series-Connected Cells Assume that a load requires a power supply of 6 volts and a current capacity of 1/8 ampere. Since a single cell normally supplies a voltage of only 1.5 volts, more than one cell is needed. To obtain the higher voltage, the cells are connected in series as shown in figure 2-6.
Figure 2-6.—(A) Pictorial view of series-connected cells; (B) Schematic of series connection. Figure 2-6 view B is a schematic representation of the circuit shown in figure 2-6 view A. The load is shown by the resistance symbol and the battery is indicated by one long and one short line per cell. In a series hookup, the negative electrode (cathode) of the first cell is connected to the positive electrode (anode) of the second cell, the negative electrode of the second to the positive of the third, etc. 2-12
The positive electrode of the first cell and negative electrode of the last cell then serve as the terminals of the battery. In this way, the voltage is 1.5 volts for each cell in the series line. There are four cells, so the output terminal voltage is 1.5 × 4, or 6 volts. When connected to the load, 1/8 ampere flows through the load and each cell of the battery. This is within the capacity of each cell. Therefore, only four series-connected cells are needed to supply this particular load. CAUTION When connecting cells in series, connect alternate terminals together (− − to +, − to +, etc.) Always have two remaining terminals that are used for connection to the load only. Do not connect the two remaining terminals together as this is a short across the battery and would not only quickly discharge the cells but could cause some types of cells to explode. Parallel-Connected Cells In this case, assume an electrical load requires only 1.5 volts, but will require 1/2 ampere of current. (Assume that a single cell will supply only 1/8 ampere.) To meet this requirement, the cells are connected in parallel, as shown in figure 2-7 view A and schematically represented in 2-7 view B. In a parallel connection, all positive cell electrodes are connected to one line, and all negative electrodes are connected to the other. No more than one cell is connected between the lines at any one point; so the voltage between the lines is the same as that of one cell, or 1.5 volts. However, each cell may contribute its maximum allowable current of 1/8 ampere to the line. There are four cells, so the total line current is 1/8 × 4, or 1/2 ampere. In this case four cells in parallel have enough capacity to supply a load requiring 1/2 ampere at 1.5 volts.
Figure 2-7.—(A) Pictorial view of parallel-connected cells; (B) Schematic of parallel connection.
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Series-Parallel-Connected Cells Figure 2-8 depicts a battery network supplying power to a load requiring both a voltage and a current greater than one cell can provide. To provide the required 4.5 volts, groups of three 1.5-volt cells are connected in series. To provide the required 1/2 ampere of current, four series groups are connected in parallel, each supplying 1/8 ampere of current.
Figure 2-8.—Schematic of series-parallel connected cells.
The connections shown have been used to illustrate the various methods of combining cells to form a battery. Series, parallel, and series-parallel circuits will be covered in detail in the next chapter, "Direct Current." Some batteries are made from primary cells. When a primary-cell battery is completely discharged, the entire battery must be replaced. Because there is nothing else that can be done to primary cell batteries, the rest of the discussion on batteries will be concerned with batteries made of secondary cells. Q25. What does the term battery normally refer to? Q26. What are the three ways of combining cells, and what is each used for? BATTERY CONSTRUCTION Secondary cell batteries are constructed using the various secondary cells already described. The lead-acid battery is one of the most common batteries in use today and will be used to explain battery construction. The nickel-cadmium battery is being used with increasing frequency and will also be discussed. Figure 2-9 shows the makeup of a lead-acid battery. The container houses the separate cells. Most containers are hard rubber, plastic, or some other material that is resistant to the electrolyte and mechanical shock and will withstand extreme temperatures. The container (battery case) is vented through vent plugs to allow the gases that form within the cells to escape. The plates in the battery are the cathodes and anodes that were discussed earlier. In figure 2-10 the negative plate group is the cathode of the individual cells and the positive plate group is the anode. As shown in the figure, the plates are interlaced with a terminal attached to each plate group. The terminals of the individual cells are connected together by link connectors as shown in figure 2-9. The cells are connected in series in the battery and the
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positive terminal of one end cell becomes the positive terminal of the battery. The negative terminal of the opposite end cell becomes the negative terminal of the battery.
Figure 2-9.—Lead-acid battery construction.
Figure 2-10.—Lead-acid battery plate arrangement.
The terminals of a lead-acid battery are usually identified from one another by their size and markings. The positive terminal, marked (+) is sometimes colored red and is physically larger than the negative terminal, marked (−).
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The individual cells of the lead-acid battery are not replaceable, so in the event one cell fails the battery must be replaced. The nickel-cadmium battery is similar in construction to the lead-acid battery with the exception that it has individual cells which can be replaced. The cell of the nicad battery is shown in figure 2-11.
Figure 2-11.—Nickel-cadmium cell.
The construction of secondary cell batteries is so similar, that it is difficult to distinguish the type of battery by simply looking at it. The type of battery must be known to properly check or recharge the battery. Each battery should have a nameplate that gives a description of its type and electrical characteristics. Q27. Other than the type of cell used, what is the major difference between the construction of the leadacid and nicad battery? Q28. How is the type of battery most easily determined? BATTERY MAINTENANCE The following information concerns the maintenance of secondary-cell batteries and is of a general nature. You must check the appropriate technical manuals for the specific type of battery prior to performing maintenance on any battery. Specific Gravity For a battery to work properly, its electrolyte (water plus active ingredient) must contain a certain amount of active ingredient. Since the active ingredient is dissolved in the water, the amount of active ingredient cannot be measured directly. An indirect way to determine whether or not the electrolyte contains the proper amount of active ingredient is to measure the electrolyte's specific gravity. Specific gravity is the ratio of the weight of a certain amount of a given substance compared to the weight of the same amount of pure water. The specific gravity of pure water is 1.0. Any substance that floats has a specific gravity less than 1.0. Any substance that sinks has a specific gravity greater than 1.0.
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The active ingredient in electrolyte (sulfuric acid, potassium hydroxide, etc.) is heavier than water. Therefore, the electrolyte has a specific gravity greater than 1.0. The acceptable range of specific gravity for a given battery is provided by the battery's manufacturer. To measure a battery's specific gravity, use an instrument called a HYDROMETER. The Hydrometer A hydrometer, shown in figure 2-12, is a glass syringe with a float inside it. The float is a hollow glass tube sealed at both ends and weighted at the bottom end, with a scale calibrated in specific gravity marked on its side. To test an electrolyte, draw it into the hydrometer using the suction bulb. Draw enough electrolyte into the hydrometer to make the float rise. Do not draw in so much electrolyte that the float rises into the suction bulb. The float will rise to a point determined by the specific gravity of the electrolyte. If the electrolyte contains a large amount of active ingredient, its specific gravity will be relatively high. The float will rise higher than it would if the electrolyte contained only a small amount of active ingredient.
Figure 2-12.—Hydrometer.
To read the hydrometer, hold it in a vertical position and read the scale at the point that surface of the electrolyte touches the float. Refer to the manufacturer's technical manual to determine whether or not the battery's specific gravity is within specifications. Note: Hydrometers should be flushed with fresh water after each use to prevent inaccurate readings. Storage battery hydrometers must not be used for any other purpose. Q29. What is the purpose of the hydrometer?
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Q30. Which electrolyte has more active ingredient? Electrolyte A, specific gravity 1.015? Electrolyte B, specific gravity 1.125? Other Maintenance The routine maintenance of a battery is very simple. Terminals should be checked periodically for cleanliness and good electrical connection. The battery case should be inspected for cleanliness and evidence of damage. The level of electrolyte should be checked and if the electrolyte is low, distilled water should be added to bring the electrolyte to the proper level. Maintenance procedures for batteries are normally determined by higher authority and each command will have detailed procedures for battery care and maintenance. Safety Precautions With Batteries All types of batteries should be handled with care: 1. NEVER SHORT THE TERMINALS OF A BATTERY. 2. CARRYING STRAPS SHOULD BE USED WHEN TRANSPORTING BATTERIES. 3. PROTECTIVE CLOTHING, SUCH AS RUBBER APRON, RUBBER GLOVES, AND A FACE SHIELD SHOULD BE WORN WHEN WORKING WITH BATTERIES. 4. NO SMOKING, ELECTRIC SPARKS, OR OPEN FLAMES SHOULD BE PERMITTED NEAR CHARGING BATTERIES. 5. CARE SHOULD BE TAKEN TO PREVENT SPILLING OF THE ELECTROLYTE. In the event electrolyte is splashed or spilled on a surface, such as the floor or table, it should be diluted with large quantities of water and cleaned up immediately. If the electrolyte is spilled or splashed on the skin or eyes, IMMEDIATELY flush the skin or eyes with large quantities of fresh water for a minimum of 15 minutes. If the electrolyte is in the eyes, be sure the upper and lower eyelids are pulled out sufficiently to allow the fresh water to flush under the eyelids. The medical department should be notified as soon as possible and informed of the type of electrolyte and the location of the accident. CAPACITY AND RATING OF BATTERIES The CAPACITY of a battery is measured in ampere-hours. The ampere-hour capacity is equal to the product of the current in amperes and the time in hours during which the battery will supply this current. The ampere-hour capacity varies inversely with the discharge current. For example, a 400 ampere-hour battery will deliver 400 amperes for 1 hour or 100 amperes for 4 hours. Storage batteries are RATED according to their rate of discharge and ampere-hour capacity. Most batteries are rated according to a 20-hour rate of discharge. That is, if a fully charged battery is completely discharged during a 20-hour period, it is discharged at the 20-hour rate. Thus, if a battery can deliver 20 amperes continuously for 20 hours, the battery has a rating of 20 amperes × 20 hours, or 400 ampere-hours. Therefore, the 20-hour rating is equal to the average current that a battery is capable of supplying without interruption for an interval of 20 hours. (Note: Aircraft batteries are rated according to a 1-hour rate of discharge.)
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All standard batteries deliver 100 percent of their available capacity if discharged in 20 hours or more, but they will deliver less than their available capacity if discharged at a faster rate. The faster they discharge, the less ampere-hour capacity they have. The low-voltage limit, as specified by the manufacturer, is the limit beyond which very little useful energy can be obtained from a battery. This low-voltage limit is normally a test used in battery shops to determine the condition of a battery. Q31. When should safety precautions pertaining to batteries be observed? Q32. How long should a 200 ampere-hour battery be able to deliver 5 amperes? BATTERY CHARGING It should be remembered that adding the active ingredient to the electrolyte of a discharged battery does not recharge the battery. Adding the active ingredient only increases the specific gravity of the electrolyte and does not convert the plates back to active material, and so does not bring the battery back to a charged condition. A charging current must be passed through the battery to recharge it. Batteries are usually charged in battery shops. Each shop will have specific charging procedures for the types of batteries to be charged. The following discussion will introduce you to the types of battery charges. The following types of charges may be given to a storage battery, depending upon the condition of the battery: 1. Initial charge 2. Normal charge 3. Equalizing charge 4. Floating charge 5. Fast charge Initial Charge When a new battery is shipped dry, the plates are in an uncharged condition. After the electrolyte has been added, it is necessary to charge the battery. This is accomplished by giving the battery a long lowrate initial charge. The charge is given in accordance with the manufacturer's instructions, which are shipped with each battery. If the manufacturer's instructions are not available, reference should be made to the detailed instructions for charging batteries found in current Navy directives. Normal Charge A normal charge is a routine charge that is given in accordance with the nameplate data during the ordinary cycle of operation to restore the battery to its charged condition. Equalizing Charge An equalizing charge is a special extended normal charge that is given periodically to batteries as part of a maintenance routine. It ensures that all the sulfate is driven from the plates and that all the cells
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are restored to a maximum specific gravity. The equalizing charge is continued until the specific gravity of all cells, corrected for temperature, shows no change for a 4-hour period. Floating Charge In a floating charge, the charging rate is determined by the battery voltage rather than by a definite current value. The floating charge is used to keep a battery at full charge while the battery is idle or in light duty. It is sometimes referred to as a trickle charge and is accomplished with low current. Fast Charge A fast charge is used when a battery must be recharged in the shortest possible time. The charge starts at a much higher rate than is normally used for charging. It should be used only in an emergency, as this type charge may be harmful to the battery. Charging Rate Normally, the charging rate of Navy storage batteries is given on the battery nameplate. If the available charging equipment does not have the desired charging rates, the nearest available rates should be used. However, the rate should never be so high that violent gassing (explained later in this text) occurs. Charging Time The charge must be continued until the battery is fully charged. Frequent readings of specific gravity should be taken during the charge and compared with the reading taken before the battery was placed on charge. Gassing When a battery is being charged, a portion of the energy breaks down the water in the electrolyte. Hydrogen is released at the negative plates and oxygen at the positive plates. These gases bubble up through the electrolyte and collect in the air space at the top of the cell. If violent gassing occurs when the battery is first placed on charge, the charging rate is too high. If the rate is not too high, steady gassing develops as the charging proceeds, indicating that the battery is nearing a fully charged condition. WARNING A mixture of hydrogen and air can be dangerously explosive. No smoking, electric sparks, or open flames should be permitted near charging batteries. Q33. Can a battery be recharged by adding more electrolyte? Q34. If violent gassing occurs during a battery charge, what action should be taken?
SUMMARY In this chapter you learned that batteries are widely used as sources of direct-current. You were introduced to electrochemical action and the way it works in a cell, the cell itself, the type and parts of a cell, and how cells are connected together to form batteries. You learned the construction and maintenance of batteries and some of the safety precautions in handling and working with batteries. 2-20
Several new terms were introduced in this chapter. The following is a summary of the chapter on batteries. A CELL is a device that transforms chemical energy into electrical energy. The cell has three parts; the electrodes, the electrolyte, and the container. There are two basic cells: primary and secondary.
THE ELECTRODES are the current conductors of the cell. THE ELECTROLYTE is the solution that acts upon the electrodes. THE CONTAINER holds the electrolyte and provides a means of mounting the electrodes. THE PRIMARY CELL is a cell in which the chemical action finally destroys one of the electrodes, usually the negative. The primary cell cannot be recharged. THE SECONDARY CELL is a cell in which the chemical action alters the electrodes and electrolyte. The electrodes and electrolyte can be restored to their original condition by recharging the cell.
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ELECTROCHEMICAL ACTION is the process of converting chemical energy into electrical energy. THE ANODE is the positive electrode of a cell. THE CATHODE is the negative electrode of a cell. PRIMARY CELL CHEMISTRY is the process in which electrons leaving the cathode to the load cause a positive charge which attracts negative ions from the electrolyte. The negative ions combine with the material of the cathode and form a substance such as lead-sulfate. Electrons from the load to the anode create a negative charge which attracts positive ions (hydrogen) from the electrolyte.
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SECONDARY CELL CHEMISTRY is the process in which the electrolyte acts upon and chemically changes both electrodes. This process also depletes the amount of active material in the electrolyte. A charging current applied to the cell reverses the process and restores the cell to its original condition. POLARIZATION is the effect of hydrogen surrounding the anode of a cell which increases the internal resistance of the cell. Polarization can be prevented by venting the cell, adding a material rich in oxygen, or adding a material that will absorb hydrogen. LOCAL ACTION is the continuation of current flow within the cell when there is no external load. It is caused by impurities in the electrode and can be prevented by the use of mercury amalgamated with the material of the electrode. DRY CELL is the type commonly referred to as the "flashlight battery." Since the electrolyte is not in liquid form, but is a paste, the term dry cell is used. In most dry cells the case is the cathode.
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SHELF LIFE is the period the cell may be stored and still be usable. MERCURY CELLS should never be shorted because of the danger of explosion.
DRY CELLS are of many types, each having advantages and disadvantages. The type selected for use depends on such factors as cost, size, ease of replacement, and voltage or current needs. THE LEAD-ACID CELL is the most widely used secondary cell. The lead-acid cell produces electricity by electrochemical action. The anode is lead peroxide, the cathode is sponge lead, and the electrolyte is sulfuric acid and water.
THE NICKEL-CADMIUM CELL, commonly called the NICAD, has the following advantages over the lead-acid cell; charges in a shorter period of time, delivers a larger amount of power, stays idle longer, and can be charged and discharged many times. The anode is nickel hydroxide, the cathode is cadmium hydroxide, and the electrolyte is potassium hydroxide and water.
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THE SILVER-ZINC CELL is used mostly for emergency equipment. It is light, small, and has a large power capacity for its size. The anode is silver oxide, the cathode is zinc, and the electrolyte is potassium hydroxide and water. THE SILVER-CADMIUM CELL combines the better features of the nickel-cadmium and silverzinc cells. The anode is silver-oxide, the cathode is cadmium hydroxide, and the electrolyte is potassium hydroxide. A BATTERY is a voltage source in a single container made from one or more cells. The cells can be combined in series, parallel, or series-parallel.
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SERIES CONNECTED CELLS provide a higher voltage than a single cell, with no increase in current.
PARALLEL CONNECTED CELLS provide a higher current than a single cell, with no increase in voltage.
SERIES-PARALLEL CONNECTED CELLS provide a higher voltage and a higher current than a single cell.
TYPES OF BATTERIES can be determined from nameplate data. HYDROMETER provides the means to check the specific gravity of the electrolyte.
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SAFETY PRECAUTIONS should always be followed when working with or around batteries. CAPACITY is an indication of the current-supplying capability of the battery for a specific period of time; e.g., 400 ampere-hour. RATING is the capacity of the battery for a specific rate of discharge. In most batteries the rating is given for a 20 hour discharge cycle; e.g., 20 amperes for 20 hours. BATTERY CHARGE is the process of reversing the current flow through the battery to restore the battery to its original condition. The addition of active ingredient to the electrolyte will not recharge the battery. There are five types of charges: 1. Initial charge 2. Normal charge 3. Equalizing charge 4. Floating charge 5. Fast charge GASSING is the production of hydrogen gas caused by a portion of the charge current breaking down the water in the electrolyte. Steady gassing is normal during the charging process. Violent gassing indicates that the charge rate is too high.
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ANSWERS TO QUESTIONS Q1. THROUGH Q34. A1.A cell is a device that converts chemical energy to electrical energy. A2.The electrodes, the electrolyte, and the container. A3.The electrodes are the current conductors of the cell. The electrolyte is the solution that acts upon the electrodes. The container holds the electrolyte and provides a means of mounting the electrodes. A4.Primary and secondary. A5.The secondary cell can be restored to its original condition by an electric current. The primary cell cannot. A6.The process of converting chemical energy into electrical energy. A7.(a) The anode, (b) the cathode. A8.The positive charge caused by electrons leaving the negative electrode attracts the negative ions. A9.By current flow through the load. A10.The chemical action between the negative electrode and the electrolyte. A11.The sulfuric acid is chemically acting upon the anode and cathode which creates a current flow through the load. A12.The charging currents causes the lead sulfate in the anode and cathode to be changed back to lead peroxide, sponge lead, and sulfuric acid. A13.Fully charged. A14.Vent the cell, add a material rich in oxygen, and use a material that will absorb hydrogen. A15.Current flow in a cell with no external load. A16.The zinc container. A17.The electrolyte is not a liquid but is in the form of a paste. A18.The period that a cell can be stored and still be useable. A19.The danger of explosion. A20.Cost, size, ease of replacement, and voltage or current needs. A21.Lead-acid, nickel-cadmium (NICAD), silver-zinc, and silver-cadmium. A22.Can be charged in a shorter time, can deliver a larger amount of power, and stays idle longer. A23.Silver-zinc cell. A24.Silver-cadmium, silver-zinc, and nickel-cadmium. A25.A voltage source in a single container made from one or more cells. 2-28
A26.Series, to increase voltage but not current. Parallel, to increase current but not voltage. SeriesParallel, to increase both current and voltage. A27.The cells in the nicad battery can be replaced. A28.By looking at the nameplate data. A29.To measure the amount of active ingredient in the electrolyte. A30.Electrolyte B. It is heavier per unit volume. A31.At all times. A32.Forty hours. A33.No, a current must be passed through the battery. A34.Reduce the charging rate.
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CHAPTER 3
DIRECT CURRENT LEARNING OBJECTIVES Upon completing this chapter, you will be able to: 1. Identify the term schematic diagram and identify the components in a circuit from a simple schematic diagram. 2. State the equation for Ohm’s law and describe the effects on current caused by changes in a circuit. 3. Given simple graphs of current versus power and voltage versus power, determine the value of circuit power for a given current and voltage. 4. Identify the term power, and state three formulas for computing power. 5. Compute circuit and component power in series, parallel, and combination circuits. 6. Compute the efficiency of an electrical device. 7. Solve for unknown quantities of resistance, current, and voltage in a series circuit. 8. Describe how voltage polarities are assigned to the voltage drops across resistors when Kirchhoff’s voltage law is used. 9. State the voltage at the reference point in a circuit. 10. Define open and short circuits and describe their effects on a circuit. 11. State the meaning of the term source resistance and describe its effect on a circuit. 12. Describe in terms of circuit values the circuit condition needed for maximum power transfer. 13. Compute efficiency of power transfer in a circuit. 14. Solve for unknown quantities of resistance, current, and voltage in a parallel circuit. 15. State the significance of the polarity assigned to a current when using Kirchhoff’s current law. 16. State the meaning of the term equivalent resistance. 17. Compute resistance, current, voltage, and power in voltage dividers. 18. Describe the method by which a single voltage divider can provide both positive and negative voltages. 19. Recognize the safety precautions associated with the hazard of electrical shock. 20. Identify the first aid procedures for a victim of electrical shock.
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INTRODUCTION The material covered in this chapter contains many new terms that are explained as you progress through the material. The basic dc circuit is the easiest to understand, so the chapter begins with the basic circuit and from there works into the basic schematic diagram of that circuit. The schematic diagram is used in all your future work in electricity and electronics. It is very important that you become familiar with the symbols that are used. This chapter also explains how to determine the total resistance, current, voltage, and power in a series, parallel, or combination circuit through the use of Ohm’s and Kirchhoff’s laws. The voltage divider network, series, parallel, and series-parallel practice problem circuits will be used for practical examples of what you have learned.
THE BASIC ELECTRIC CIRCUIT The flashlight is an example of a basic electric circuit. It contains a source of electrical energy (the dry cells in the flashlight), a load (the bulb) which changes the electrical energy into a more useful form of energy (light), and a switch to control the energy delivered to the load. Before you study a schematic representation of the flashlight, it is necessary to define certain terms. The LOAD is any device through which an electrical current flows and which changes this electrical energy into a more useful form. Some common examples of loads are a lightbulb, which changes electrical energy to light energy; an electric motor, which changes electrical energy into mechanical energy; and the speaker in a radio, which changes electrical energy into sound. The SOURCE is the device which furnishes the electrical energy used by the load. It may consist of a simple dry cell (as in a flashlight), a storage battery (as in an automobile), or a power supply (such as a battery charger). The SWITCH, which permits control of the electrical device, interrupts the current delivered to the load. SCHEMATIC REPRESENTATION The technician’s main aid in troubleshooting a circuit in a piece of equipment is the SCHEMATIC DIAGRAM. The schematic diagram is a "picture" of the circuit that uses symbols to represent the various circuit components; physically large or complex circuits can be shown on a relatively small diagram. Before studying the basic schematic, look at figure 3-1. This figure shows the symbols that are used in this chapter. These, and others like them, are referred to and used throughout the study of electricity and electronics.
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Figure 3-1.—Symbols commonly used in electricity.
The schematic in figure 3-2 represents a flashlight. View A of the figure shows the flashlight in the off or deenergized state. The switch (S1) is open. There is no complete path for current (I) through the circuit, and the bulb (DS1) does not light. In figure 3-2 view B, switch S1 is closed. Current flows in the direction of the arrows from the negative terminal of the battery (BAT), through the switch (S1), through the lamp (DS1), and back to the positive terminal of the battery. With the switch closed the path for current is complete. Current will continue to flow until the switch (S1) is moved to the open position or the battery is completely discharged.
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Figure 3-2.—Basic flashlight schematic.
Q1. In figure 3-2, what part of the circuit is the (a) load and (b) source? Q2. What happens to the path for current when S1 is open as shown in figure 3-2(A)? Q3. What is the name given to the "picture" of a circuit such as the one shown in figure 3-2?
OHM’S LAW In the early part of the 19th century, George Simon Ohm proved by experiment that a precise relationship exists between current, voltage, and resistance. This relationship is called Ohm’s law and is stated as follows: The current in a circuit is DIRECTLY proportional to the applied voltage and INVERSELY proportional to the circuit resistance. Ohm’s law may be expressed as an equation:
As stated in Ohm’s law, current is inversely proportional to resistance. This means, as the resistance in a circuit increases, the current decreases proportionately.
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In the equation
if any two quantities are known, the third one can be determined. Refer to figure 3-2(B), the schematic of the flashlight. If the battery (BAT) supplies a voltage of 1.5 volts and the lamp (DS1) has a resistance of 5 ohms, then the current in the circuit can be determined. Using this equation and substituting values:
If the flashlight were a two-cell flashlight, we would have twice the voltage, or 3.0 volts, applied to the circuit. Using this voltage in the equation:
You can see that the current has doubled as the voltage has doubled. This demonstrates that the current is directly proportional to the applied voltage. If the value of resistance of the lamp is doubled, the equation will be:
The current has been reduced to one half of the value of the previous equation, or .3 ampere. This demonstrates that the current is inversely proportional to the resistance. Doubling the value of the resistance of the load reduces circuit current value to one half of its former value. APPLICATION OF OHM’S LAW By using Ohm’s law, you are able to find the resistance of a circuit, knowing only the voltage and the current in the circuit. In any equation, if all the variables (parameters) are known except one, that unknown can be found. For example, using Ohm’s law, if current (I) and voltage (E) are known, resistance (R) the only parameter not known, can be determined: 1. Basic formula:
2. Remove the divisor by multiplying both sides by R:
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3. Result of step 2: R x I = E 4. To get R alone (on one side of the equation) divide both sides by I:
5. The basic formula, transposed for R, is:
Refer to figure 3-3 where E equals 10 volts and I equals 1 ampere. Solve for R, using the equation just explained. Given:
E = 10 volts I = 1 ampere
Solution:
Figure 3-3.—Determining resistance in a basic circuit.
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This equation can be used to find the voltage for the circuit shown in figure 3-4.
Figure 3-4.—Determining voltage in a basic circuit.
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The Ohm’s law equation and its various forms may be obtained readily with the aid of figure 3-5. The circle containing E, I, and R is divided into two parts, with E above the line and with I and R below the line. To determine the unknown quantity, first cover that quantity with a finger. The position of the uncovered letters in the circle will indicate the mathematical operation to be performed. For example, to find I, cover I with a finger. The uncovered letters indicate that E is to be divided by R, or
To find the formula for E, cover E with your finger. The result indicates that I is to be multiplied by R, or E = IR. To find the formula for R, cover R. The result indicates that E is to be divided by I, or
Figure 3-5.—Ohm's law in diagram form.
You are cautioned not to rely wholly on the use of this diagram when you transpose the Ohm’s law formulas. The diagram should be used to supplement your knowledge of the algebraic method. Algebra is a basic tool in the solution of electrical problems. Q4. According to Ohm’s law, what happens to circuit current if the applied voltage (a) increases, (b) decreases? Q5. According to Ohm’s law, what happens to circuit current if circuit resistance (a) increases, (b) decreases? Q6. What is the equation used to find circuit resistance if voltage and current values are known? GRAPHICAL ANALYSIS OF THE BASIC CIRCUIT One of the most valuable methods of analyzing a circuit is by constructing a graph. No other method provides a more convenient or more rapid way to observe the characteristics of an electrical device.
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The first step in constructing a graph is to obtain a table of data. The information in the table can be obtained by taking measurements on the circuit under examination, or can be obtained theoretically through a series of Ohm’s law computations. The latter method is used here. Since there are three variables (E, I, and R) to be analyzed, there are three distinct graphs that may be constructed. To construct any graph of electrical quantities, it is standard practice to vary one quantity in a specified way and note the changes which occur in a second quantity. The quantity which is intentionally varied is called the independent variable and is plotted on the horizontal axis. The horizontal axis is known as the X-AXIS. The second quantity, which varies as a result of changes in the first quantity, is called the dependent variable and is plotted on the vertical, or Y-AXIS. Any other quantities involved are held constant. For example, in the circuit shown in figure 3-6, if the resistance was held at 10 ohms and the voltage was varied, the resulting changes in current could then be graphed. The resistance is the constant, the voltage is the independent variable, and the current is the dependent variable.
Figure 3-6.—Three variables in a basic circuit.
Figure 3-7 shows the graph and a table of values. This table shows R held constant at 10 ohms as E is varied from 0 to 20 volts in 5-volt steps. Through the use of Ohm’s law, you can calculate the value of current for each value of voltage shown in the table. When the table is complete, the information it contains can be used to construct the graph shown in figure 3-7. For example, when the voltage applied to the 10-ohm resistor is 10 volts, the current is 1 ampere. These values of current and voltage determine a point on the graph. When all five points have been plotted, a smooth curve is drawn through the points.
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Figure 3-7.—Volt-ampere characteristic.
Through the use of this curve, the value of current through the resistor can be quickly determined for any value of voltage between 0 and 20 volts. Since the curve is a straight line, it shows that equal changes of voltage across the resistor produce equal changes in current through the resistor. This fact illustrates an important characteristic of the basic law—the current varies directly with the applied voltage when the resistance is held constant. When the voltage across a load is held constant, the current depends solely upon the resistance of the load. For example, figure 3-8 shows a graph with the voltage held constant at 12 volts. The independent variable is the resistance which is varied from 2 ohms to 12 ohms. The current is the dependent variable. Values for current can be calculated as:
Figure 3-8.—Relationship between current and resistance.
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This process can be continued for any value of resistance. You can see that as the resistance is halved, the current is doubled; when the resistance is doubled, the current is halved. This illustrates another important characteristic of Ohm's law—current varies inversely with resistance when the applied voltage is held constant. Q7. Using the graph in figure 3-7, what is the approximate value of current when the voltage is 12.5 volts? Q8. Using the graph in figure 3-8, what is the approximate value of current when the resistance is 3 ohms?
POWER Power, whether electrical or mechanical, pertains to the rate at which work is being done. Work is done whenever a force causes motion. When a mechanical force is used to lift or move a weight, work is done. However, force exerted WITHOUT causing motion, such as the force of a compressed spring acting between two fixed objects, does not constitute work. Previously, it was shown that voltage is an electrical force, and that voltage forces current to flow in a closed circuit. However, when voltage exists but current does not flow because the circuit is open, no work is done. This is similar to the spring under tension that produced no motion. When voltage causes electrons to move, work is done. The instantaneous RATE at which this work is done is called the electric power rate, and is measured in WATTS. A total amount of work may be done in different lengths of time. For example, a given number of electrons may be moved from one point to another in 1 second or in 1 hour, depending on the RATE at which they are moved. In both cases, total work done is the same. However, when the work is done in a
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short time, the wattage, or INSTANTANEOUS POWER RATE, is greater than when the same amount of work is done over a longer period of time. As stated, the basic unit of power is the watt. Power in watts is equal to the voltage across a circuit multiplied by current through the circuit. This represents the rate at any given instant at which work is being done. The symbol P indicates electrical power. Thus, the basic power formula is P = E x I, where E is voltage and I is current in the circuit. The amount of power changes when either voltage or current, or both voltage and current, are caused to change. In practice, the ONLY factors that can be changed are voltage and resistance. In explaining the different forms that formulas may take, current is sometimes presented as a quantity that is changed. Remember, if current is changed, it is because either voltage or resistance has been changed. Figure 3-9 shows a basic circuit using a source of power that can be varied from 0 to 8 volts and a graph that indicates the relationship between voltage and power. The resistance of this circuit is 2 ohms; this value does not change. Voltage (E) is increased (by increasing the voltage source), in steps of 1 volt, from 0 volts to 8 volts. By applying Ohm’s law, the current (I) is determined for each step of voltage. For instance, when E is 1 volt, the current is:
Figure 3-9.—Graph of power related to changing voltage.
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Power (P), in watts, is determined by applying the basic power formula:
and P=ExI P = 2 volts x 1 ampere P = 2 watts
You should notice that when the voltage was increased to 2 volts, the power increased from .5 watts to 2 watts or 4 times. When the voltage increased to 3 volts, the power increased to 4.5 watts or 9 times. This shows that if the resistance in a circuit is held constant, the power varies directly with the SQUARE OF THE VOLTAGE. Another way of proving that power varies as the square of the voltage when resistance is held constant is:
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Another important relationship may be seen by studying figure 3-10. Thus far, power has been calculated with voltage and current (P = E x I), and with voltage and resistance
Referring to figure 3-10, note that power also varies as the square of current just as it does with voltage. Thus, another formula for power, with current and resistance as its factors, is P = I 2R. This can be proved by:
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Figure 3-10.—Graph of power related to changing current.
Up to this point, four of the most important electrical quantities have been discussed. These are voltage (E), current (I), resistance (R), and power (P). You must understand the relationships which exist among these quantities because they are used throughout your study of electricity. In the preceding paragraphs, P was expressed in terms of alternate pairs of the other three basic quantities E, I, and R. In practice, you should be able to express any one of these quantities in terms of any two of the others. Figure 3-11 is a summary of 12 basic formulas you should know. The four quantities E, I, R, and P are at the center of the figure. Adjacent to each quantity are three segments. Note that in each segment, the basic quantity is expressed in terms of two other basic quantities, and no two segments are alike.
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Figure 3-11.—Summary of basic formulas.
For example, the formula wheel in figure 3-11 could be used to find the formula to solve the following problem: A circuit has a voltage source that delivers 6 volts and the circuit uses 3 watts of power. What is the resistance of the load? Since R is the quantity you have been asked to find, look in the section of the wheel that has R in the center. The segment
contains the quantities you have been given. The formula you would use is
The problem can now be solved.
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Q9. What is the term applied to the rate at which a mechanical or electrical force causes motion? Q10. How can the amount of current be changed in a circuit? Q11. What are the three formulas for electrical power? POWER RATING Electrical components are often given a power rating. The power rating, in watts, indicates the rate at which the device converts electrical energy into another form of energy, such as light, heat, or motion. An example of such a rating is noted when comparing a 150-watt lamp to a 100-watt lamp. The higher wattage rating of the 150-watt lamp indicates it is capable of converting more electrical energy into light energy than the lamp of the lower rating. Other common examples of devices with power ratings are soldering irons and small electric motors. In some electrical devices the wattage rating indicates the maximum power the device is designed to use rather than the normal operating power. A 150-watt lamp, for example, uses 150 watts when operated at the specified voltage printed on the bulb. In contrast, a device such as a resistor is not normally given a voltage or a current rating. A resistor is given a power rating in watts and can be operated at any combination of voltage and current as long as the power rating is not exceeded. In most circuits, the actual power used by a resistor is considerably less than the power rating of the resistor because a 50% safety factor is used. For example, if a resistor normally used 2 watts of power, a resistor with a power rating of 3 watts would be used. Resistors of the same resistance value are available in different wattage values. Carbon resistors, for example, are commonly made in wattage ratings of 1/8, 1/4, 1/2, 1, and 2 watts. The larger the physical size of a carbon resistor the higher the wattage rating. This is true because a larger surface area of material radiates a greater amount of heat more easily. When resistors with wattage ratings greater than 5 watts are needed, wirewound resistors are used. Wirewound resistors are made in values between 5 and 200 watts. Special types of wirewound resistors are used for power in excess of 200 watts. As with other electrical quantities, prefixes may be attached to the word watt when expressing very large or very small amounts of power. Some of the more common of these are the kilowatt (1,000 watts), the megawatt (1,000,000 watts), and the milliwatt (1/1,000 of a watt). Q12. What is the current in a circuit with 5 ohms of resistance that uses 180 watts of power? (refer to figure 3-12) Q13. What type of resistor should be used in the circuit described in question 12? Q14. What is the power used in a circuit that has 10 amperes of current through a 10-ohm resistor?
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Figure 3-12.—Circuit for computing electrical quantities.
POWER CONVERSION AND EFFICIENCY The term power consumption is common in the electrical field. It is applied to the use of power in the same sense that gasoline consumption is applied to the use of fuel in an automobile. Another common term is power conversion. Power is used by electrical devices and is converted from one form of energy to another. An electrical motor converts electrical energy to mechanical energy. An electric light bulb converts electrical energy into light energy and an electric range converts electrical energy into heat energy. Power used by electrical devices is measured in energy. This practical unit of electrical energy is equal to 1 watt of power used continuously for 1 hour. The term kilowatt hour (kWh) is used more extensively on a daily basis and is equal to 1,000 watt-hours. The EFFICIENCY of an electrical device is the ratio of power converted to useful energy divided by the power consumed by the device. This number will always be less than one (1.00) because of the losses in any electrical device. If a device has an efficiency rating of .95, it effectively transforms 95 watts into useful energy for every 100 watts of input power. The other 5 watts are lost to heat, or other losses which cannot be used. Calculating the amount of power converted by an electrical device is a simple matter. You need to know the length of time the device is operated and the input power or horsepower rating. Horsepower, a unit of work, is often found as a rating on electrical motors. One horsepower is equal to 746 watts. Example: A 3/4-hp motor operates 8 hours a day. How much power is converted by the motor per month? How many kWh does this represent? Given:
t = 8 hrs x 30 days P = 3/4 hp
Solution:
Convert horsepower to watts P = hp x 746 watts P = 3/4 x 746 watts
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P = 559 watts Convert watts to watt-hours P = work x time P = 559 watts x 8 x 30 P = 134,000 watt-hours per month (NOTE: These figures are rounded to the nearest 1000.) To convert to kWh
If the motor actually uses 137 kWh per month, what is the efficiency of the motor? Given:
Power converted = 134 kWh per month Power used = 137 kWh per month
Solution:
Q15. How much power is converted by a 1-horsepower motor in 12 hours? Q16. What is the efficiency of the motor if it actually uses 9.5 kWh in 12 hours?
SERIES DC CIRCUITS When two unequal charges are connected by a conductor, a complete pathway for current exists. An electric circuit is a complete conducting pathway. It consists not only of the conductor, but also includes the path through the voltage source. Inside the voltage source current flows from the positive terminal, through the source, emerging at the negative terminal.
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SERIES CIRCUIT CHARACTERISTICS A SERIES CIRCUIT is defined as a circuit that contains only ONE PATH for current flow. To compare the basic circuit that has been discussed and a more complex series circuit, figure 3-13 shows two circuits. The basic circuit has only one lamp and the series circuit has three lamps connected in series.
Figure 3-13.—Comparison of basic and series circuits.
Resistance in a Series Circuit Referring to figure 3-13, the current in a series circuit must flow through each lamp to complete the electrical path in the circuit. Each additional lamp offers added resistance. In a series circuit, THE TOTAL CIRCUIT RESISTANCE (RT) IS EQUAL TO THE SUM OF THE INDIVIDUAL RESISTANCES. As an equation: RT = R1 + R2 + R3 + . . . R n
NOTE: The subscript n denotes any number of additional resistances that might be in the equation. Example: In figure 3-14 a series circuit consisting of three resistors: one of 10 ohms, one of 15 ohms, and one of 30 ohms, is shown. A voltage source provides 110 volts. What is the total resistance?
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Figure 3-14.—Solving for total resistance in a series circuit.
In some circuit applications, the total resistance is known and the value of one of the circuit resistors has to be determined. The equation RT = R1 + R2 + R 3 can be transposed to solve for the value of the unknown resistance. Example: In figure 3-15 the total resistance of a circuit containing three resistors is 40 ohms. Two of the circuit resistors are 10 ohms each. Calculate the value of the third resistor (R3).
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Figure 3-15.—Calculating the value of one resistance in a series circuit.
Current in a Series Circuit Since there is only one path for current in a series circuit, the same current must flow through each component of the circuit. To determine the current in a series circuit, only the current through one of the components need be known. The fact that the same current flows through each component of a series circuit can be verified by inserting meters into the circuit at various points, as shown in figure 3-16. If this were done, each meter would be found to indicate the same value of current.
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Figure 3-16.—Current in a series circuit.
Voltage in a Series Circuit The voltage dropped across the resistor in a circuit consisting of a single resistor and a voltage source is the total voltage across the circuit and is equal to the applied voltage. The total voltage across a series circuit that consists of more than one resistor is also equal to the applied voltage, but consists of the sum of the individual resistor voltage drops. In any series circuit, the SUM of the resistor voltage drops must equal the source voltage. This statement can be proven by an examination of the circuit shown in figure 3-17. In this circuit a source potential (ET) of 20 volts is dropped across a series circuit consisting of two 5-ohm resistors. The total resistance of the circuit (R T) is equal to the sum of the two individual resistances, or 10 ohms. Using Ohm’s law the circuit current may be calculated as follows:
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Figure 3-17.—Calculating individual voltage drops in a series circuit.
Since the value of the resistors is known to be 5 ohms each, and the current through the resistors is known to be 2 amperes, the voltage drops across the resistors can be calculated. The voltage (E1) across R1 is therefore:
By inspecting the circuit, you can see that R2 is the same ohmic value as R1 and carries the same current. The voltage drop across R2 is therefore also equal to 10 volts. Adding these two 10-volts drops together gives a total drop of 20 volts, exactly equal to the applied voltage. For a series circuit then: ET = E1 = E 2 + E3 = . . . En Example: A series circuit consists of three resistors having values of 20 ohms, 30 ohms, and 50 ohms, respectively. Find the applied voltage if the current through the 30 ohm resistor is 2 amps. (The abbreviation amp is commonly used for ampere.) To solve the problem, a circuit diagram is first drawn and labeled (fig 3-18).
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Figure 3-18.—Solving for applied voltage in a series circuit.
Substituting:
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NOTE: When you use Ohm’s law, the quantities for the equation MUST be taken from the SAME part of the circuit. In the above example the voltage across R2 was computed using the current through R2 and the resistance of R2. The value of the voltage dropped by a resistor is determined by the applied voltage and is in proportion to the circuit resistances. The voltage drops that occur in a series circuit are in direct proportion to the resistances. This is the result of having the same current flow through each resistor—the larger the ohmic value of the resistor, the larger the voltage drop across it. Q17. A series circuit consisting of three resistors has a current of 3 amps. If R 1 = 20 ohms, R2= 60 ohms, and R3 = 80 ohms, what is the (a) total resistance and (b) source voltage of the circuit? Q18. What is the voltage dropped by each resistor of the circuit described in question 17? Q19. If the current was increased to 4 amps, what would be the voltage drop across each resistor in the circuit described in question 17? Q20. What would have to be done to the circuit described in question 17 to increase the current to 4 amps? Power in a Series Circuit Each of the resistors in a series circuit consumes power which is dissipated in the form of heat. Since this power must come from the source, the total power must be equal to the power consumed by the circuit resistances. In a series circuit the total power is equal to the SUM of the power dissipated by the individual resistors. Total power (PT) is equal to: PT = P1 + P2 + P3 . . .Pn Example: A series circuit consists of three resistors having values of 5 ohms, 10 ohms, and 15 ohms. Find the total power when 120 volts is applied to the circuit. (See fig. 3-19.)
Figure 3-19.—Solving for total power in a series circuit.
3-26
Given:
Solution: The total resistance is found first.
By using the total resistance and the applied voltage, the circuit current is calculated.
By means of the power formulas, the power can be calculated for each resistor:
To check the answer, the total power delivered by the source can be calculated:
3-27
The total power is equal to the sum of the power used by the individual resistors. SUMMARY OF CHARACTERISTICS The important factors governing the operation of a series circuit are listed below. These factors have been set up as a group of rules so that they may be easily studied. These rules must be completely understood before the study of more advanced circuit theory is undertaken. Rules for Series DC Circuits 1. The same current flows through each part of a series circuit. 2. The total resistance of a series circuit is equal to the sum of the individual resistances. 3. The total voltage across a series circuit is equal to the sum of the individual voltage drops. 4. The voltage drop across a resistor in a series circuit is proportional to the ohmic value of the resistor. 5. The total power in a series circuit is equal to the sum of the individual powers used by each circuit component. SERIES CIRCUIT ANALYSIS To establish a procedure for solving series circuits, the following sample problems will be solved. Example: Three resistors of 5 ohms, 10 ohms, and 15 ohms are connected in series with a power source of 90 volts as shown in figure 3-20. Find the total resistance, circuit current, voltage drop of each resistor, power of each resistor, and total power of the circuit.
Figure 3-20.—Solving for various values in a series circuit.
3-28
In solving the circuit the total resistance will be found first. Next, the circuit current will be calculated. Once the current is known, the voltage drops and power dissipations can be calculated.
3-29
Example: Four resistors, R1 = 10 ohms, R2 = 10 ohms, R3 = 50 ohms, and R4 = 30 ohms, are connected in series with a power source as shown in figure 3-21. The current through the circuit is 1/2 ampere. a. What is the battery voltage? b. What is the voltage across each resistor? c. What is the power expended in each resistor? d. What is the total power?
Figure 3-21.—Computing series circuit values.
3-30
Given:
Solution (a):
Solution (b):
Solution (c):
3-31
Solution (d):
An important fact to keep in mind when applying Ohm’s law to a series circuit is to consider whether the values used are component values or total values. When the information available enables the use of Ohm’s law to find total resistance, total voltage, and total current, total values must be inserted into the formula. To find total resistance:
3-32
To find total voltage:
To find total current:
NOTE: IT is equal to I in a series circuit. However, the distinction between IT and I in the formula should be noted. The reason for this is that future circuits may have several currents, and it will be necessary to differentiate between IT and other currents. To compute any quantity (E, I, R, or P) associated with a single given resistor, the values used in the formula must be obtained from that particular resistor. For example, to find the value of an unknown resistance, the voltage across and the current through that particular resistor must be used. To find the value of a resistor:
To find the voltage drop across a resistor:
To find current through a resistor:
Q21. A series circuit consists of two resistors in series. R1 = 25 ohms and R2 = 30 ohms. The circuit current is 6 amps. What is the (a) source voltage, (b) voltage dropped by each resistor, (c) total power, and (d) power used by each resistor?
KIRCHHOFF’S VOLTAGE LAW In 1847, G. R. Kirchhoff extended the use of Ohm’s law by developing a simple concept concerning the voltages contained in a series circuit loop. Kirchhoff’s voltage law states: "The algebraic sum of the voltage drops in any closed path in a circuit and the electromotive forces in that path is equal to zero."
3-33
To state Kirchhoff’s law another way, the voltage drops and voltage sources in a circuit are equal at any given moment in time. If the voltage sources are assumed to have one sign (positive or negative) at that instant and the voltage drops are assumed to have the opposite sign, the result of adding the voltage sources and voltage drops will be zero. NOTE: The terms electromotive force and emf are used when explaining Kirchhoff’s law because Kirchhoff’s law is used in alternating current circuits (covered in Module 2). In applying Kirchhoff’s law to direct current circuits, the terms electromotive force and emf apply to voltage sources such as batteries or power supplies. Through the use of Kirchhoff’s law, circuit problems can be solved which would be difficult, and often impossible, with knowledge of Ohm’s law alone. When Kirchhoff’s law is properly applied, an equation can be set up for a closed loop and the unknown circuit values can be calculated. POLARITY OF VOLTAGE To apply Kirchhoff’s voltage law, the meaning of voltage polarity must be understood. In the circuit shown in figure 3-22, the current is shown flowing in a counterclockwise direction. Notice that the end of resistor R1, into which the current flows, is marked NEGATIVE (í 7KHHQGRI51 at which the current leaves is marked POSITIVE (+). These polarity markings are used to show that the end of R1 into which the current flows is at a higher negative potential than the end of the resistor at which the current leaves. Point A is more negative than point B.
Figure 3-22.—Voltage polarities.
Point C, which is at the same potential as point B, is labeled negative. This is to indicate that point C is more negative than point D. To say a point is positive (or negative) without stating what the polarity is based upon has no meaning. In working with Kirchhoff’s law, positive and negative polarities are assigned in the direction of current flow.
3-34
APPLICATION OF KIRCHHOFF’S VOLTAGE LAW Kirchhoff’s voltage law can be written as an equation, as shown below: Ea + Eb + Ec + . . . En = 0 where Ea, Eb, etc., are the voltage drops or emf’s around any closed circuit loop. To set up the equation for an actual circuit, the following procedure is used. 1. Assume a direction of current through the circuit. (The correct direction is desirable but not necessary.) 2. Using the assumed direction of current, assign polarities to all resistors through which the current flows. 3. Place the correct polarities on any sources included in the circuit. 4. Starting at any point in the circuit, trace around the circuit, writing down the amount and polarity of the voltage across each component in succession. The polarity used is the sign AFTER the assumed current has passed through the component. Stop when the point at which the trace was started is reached. 5. Place these voltages, with their polarities, into the equation and solve for the desired quantity. Example: Three resistors are connected across a 50-volt source. What is the voltage across the third resistor if the voltage drops across the first two resistors are 25 volts and 15 volts? Solution: First, a diagram, such as the one shown in figure 3-23, is drawn. Next, a direction of current is assumed (as shown). Using this current, the polarity markings are placed at each end of each resistor and also on the terminals of the source. Starting at point A, trace around the circuit in the direction of current flow, recording the voltage and polarity of each component. Starting at point A and using the components from the circuit:
Substituting values from the circuit:
3-35
Figure 3-23.—Determining unknown voltage in a series circuit.
Using the same idea as above, you can solve a problem in which the current is the unknown quantity. Example: A circuit having a source voltage of 60 volts contains three resistors of 5 ohms, 10 ohms, and 15 ohms. Find the circuit current. Solution: Draw and label the circuit (fig. 3-24). Establish a direction of current flow and assign polarities. Next, starting at any point—point A will be used in this example—write out the loop equation.
Figure 3-24.—Correct direction of assumed current.
3-36
Since the current obtained in the above calculations is a positive 2 amps, the assumed direction of current was correct. To show what happens if the incorrect direction of current is assumed, the problem will be solved as before, but with the opposite direction of current. The circuit is redrawn showing the new direction of current and new polarities in figure 3-25. Starting at point A the loop equation is:
3-37
Figure 3-25.—Incorrect direction of assumed current.
Notice that the AMOUNT of current is the same as before. The polarity, however, is NEGATIVE. The negative polarity simply indicates the wrong direction of current was assumed. Should it be necessary to use this current in further calculations on the circuit using Kirchhoff’s law, the negative polarity should be retained in the calculations. Series Aiding and Opposing Sources In many practical applications a circuit may contain more than one source of emf. Sources of emf that cause current to flow in the same direction are considered to be SERIES AIDING and the voltages are added. Sources of emf that would tend to force current in opposite directions are said to be SERIES OPPOSING, and the effective source voltage is the difference between the opposing voltages. When two opposing sources are inserted into a circuit current flow would be in a direction determined by the larger source. Examples of series aiding and opposing sources are shown in figure 3-26.
3-38
Figure 3-26.—Aiding and opposing sources.
A simple solution may be obtained for a multiple-source circuit through the use of Kirchhoff’s voltage law. In applying this method, the same procedure is used for the multiple-source circuit as was used above for the single-source circuit. This is demonstrated by the following example. Example: Using Kirchhoff’s voltage equation, find the amount of current in the circuit shown in fig 3-27.
Figure 3-27.-Solving for circuit current using Kirchhoff's voltage equation.
3-39
Solution: As before, a direction of current flow is assumed and polarity signs are placed on the drawing. The loop equation will be started at point A. E2 + ER1 + E 1 + E3 + ER2 = 0
Q22. When using Kirchhoff’s voltage law, how are voltage polarities assigned to the voltage drops across resistors? Q23. Refer to figure 3-27, if R1 was changed to a 40-ohm resistor, what would be the value of circuit current (IT)? Q24. Refer to figure 3-27. What is the effective source voltage of the circuit using the 40-ohm resistor?
CIRCUIT TERMS AND CHARACTERISTICS Before you learn about the types of circuits other than the series circuit, you should become familiar with some of the terms and characteristics used in electrical circuits. These terms and characteristics will be used throughout your study of electricity and electronics. REFERENCE POINT A reference point is an arbitrarily chosen point to which all other points in the circuit are compared. In series circuits, any point can be chosen as a reference and the electrical potential at all other points can be determined in reference to that point. In figure 3-28 point A shall be considered the reference point. Each series resistor in the illustrated circuit is of equal value. The applied voltage is equally distributed across each resistor. The potential at point B is 25 volts more positive than at point A. Points C and D are 50 volts and 75 volts more positive than point A respectively.
3-40
Figure 3-28.—Reference points in a series circuit.
When point B is used as the reference, as in figure 3-29, point D would be positive 50 volts in respect to the new reference point. The former reference point, A, is 25 volts negative in respect to point B.
Figure 3-29.—Determining potentials with respect to a reference point.
3-41
As in the previous circuit illustration, the reference point of a circuit is always considered to be at zero potential. Since the earth (ground) is said to be at a zero potential, the term GROUND is used to denote a common electrical point of zero potential. In figure 3-30, point A is the zero reference, or ground, and the symbol for ground is shown connected to point A. Point C is 75 volts positive in respect to ground.
Figure 3-30.—Use of ground symbols.
In most electrical equipment, the metal chassis is the common ground for the many electrical circuits. When each electrical circuit is completed, common points of a circuit at zero potential are connected directly to the metal chassis, thereby eliminating a large amount of connecting wire. The electrons pass through the metal chassis (a conductor) to reach other points of the circuit. An example of a chassis grounded circuit is illustrated in figure 3-31.
3-42
Figure 3-31.—Ground used as a conductor.
Most voltage measurements used to check proper circuit operation in electrical equipment are taken in respect to ground. One meter lead is attached to a grounded point and the other meter lead is moved to various test points. Circuit measurement is explained in more detail in NEETS Module 3. OPEN CIRCUIT A circuit is said to be OPEN when a break exists in a complete conducting pathway. Although an open occurs when a switch is used to deenergize a circuit, an open may also develop accidentally. To restore a circuit to proper operation, the open must be located, its cause determined, and repairs made. Sometimes an open can be located visually by a close inspection of the circuit components. Defective components, such as burned out resistors, can usually be discovered by this method. Others, such as a break in wire covered by insulation or the melted element of an enclosed fuse, are not visible to the eye. Under such conditions, the understanding of the effect an open has on circuit conditions enables a technician to make use of test equipment to locate the open component. In figure 3-32, the series circuit consists of two resistors and a fuse. Notice the effects on circuit conditions when the fuse opens.
3-43
Figure 3-32.—Normal and open circuit conditions. (A) Normal current; (B) Excessive current.
Current ceases to flow; therefore, there is no longer a voltage drop across the resistors. Each end of the open conducting path becomes an extension of the battery terminals and the voltage felt across the open is equal to the applied voltage (EA). An open circuit has INFINITE resistance. INFINITY represents a quantity so large it cannot be measured. The symbol for infinity is ,QDQRSHQFLUFXLW5T = SHORT CIRCUIT A short circuit is an accidental path of low resistance which passes an abnormally high amount of current. A short circuit exists whenever the resistance of a circuit or the resistance of a part of a circuit drops in value to almost zero ohms. A short often occurs as a result of improper wiring or broken insulation. In figure 3-33, a short is caused by improper wiring. Note the effect on current flow. Since the resistor has in effect been replaced with a piece of wire, practically all the current flows through the short and very little current flows through the resistor. Electrons flow through the short (a path of almost zero resistance) and the remainder of the circuit by passing through the 10-ohm resistor and the battery. The amount of current flow increases greatly because its resistive path has decreased from 10,010 ohms to 10 ohms. Due to the excessive current flow the 10-ohm resistor becomes heated. As it attempts to dissipate this heat, the resistor will probably be destroyed. Figure 3-34 shows a pictorial wiring diagram, rather than a schematic diagram, to indicate how broken insulation might cause a short circuit.
3-44
Figure 3-33.—Normal and short circuit conditions.
Figure 3-34.—Short due to broken insulation.
3-45
SOURCE RESISTANCE A meter connected across the terminals of a good 1.5-volt battery reads about 1.5 volts. When the same battery is inserted into a complete circuit, the meter reading decreases to something less than 1.5 volts. This difference in terminal voltage is caused by the INTERNAL RESISTANCE of the battery (the opposition to current offered by the electrolyte in the battery). All sources of electromotive force have some form of internal resistance which causes a drop in terminal voltage as current flows through the source. This principle is illustrated in figure 3-35, where the internal resistance of a battery is shown as Ri. In the schematic, the internal resistance is indicated by an additional resistor in series with the battery. The battery, with its internal resistance, is enclosed within the dotted lines of the schematic diagram. With the switch open, the voltage across the battery terminals reads 15 volts. When the switch is closed, current flow causes voltage drops around the circuit. The circuit current of 2 amperes causes a voltage drop of 2 volts across Ri. The 1-ohm internal battery resistance thereby drops the battery terminal voltage to 13 volts. Internal resistance cannot be measured directly with a meter. An attempt to do this would damage the meter.
Figure 3-35.—Effect of internal resistance.
The effect of the source resistance on the power output of a dc source may be shown by an analysis of the circuit in figure 3-36. When the variable load resistor (RL) is set at the zero-ohm position (equivalent to a short circuit), current (I) is calculated using the following formula:
3-46
This is the maximum current that may be drawn from the source. The terminal voltage across the short circuit is zero volts and all the voltage is across the resistance within the source.
Figure 3-36.—Effect of source resistance on power output.
3-47
If the load resistance (RL) were increased (the internal resistance remaining the same), the current drawn from the source would decrease. Consequently, the voltage drop across the internal resistance would decrease. At the same time, the terminal voltage applied across the load would increase and approach a maximum as the current approaches zero amps. POWER TRANSFER AND EFFICIENCY Maximum power is transferred from the source to the load when the resistance of the load is equal to the internal resistance of the source. This theory is illustrated in the table and the graph of figure 3-36. When the load resistance is 5 ohms, matching the source resistance, the maximum power of 500 watts is developed in the load. The efficiency of power transfer (ratio of output power to input power) from the source to the load increases as the load resistance is increased. The efficiency approaches 100 percent as the load resistance approaches a relatively large value compared with that of the source, since less power is lost in the source. The efficiency of power transfer is only 50 percent at the maximum power transfer point (when the load resistance equals the internal resistance of the source). The efficiency of power transfer approaches zero efficiency when the load resistance is relatively small compared with the internal resistance of the source. This is also shown on the chart of figure 3-36. The problem of a desire for both high efficiency and maximum power transfer is resolved by a compromise between maximum power transfer and high efficiency. Where the amounts of power involved are large and the efficiency is important, the load resistance is made large relative to the source resistance so that the losses are kept small. In this case, the efficiency is high. Where the problem of matching a source to a load is important, as in communications circuits, a strong signal may be more important than a high percentage of efficiency. In such cases, the efficiency of power transfer should be only about 50 percent; however, the power transfer would be the maximum which the source is capable of supplying. You should now understand the basic concepts of series circuits. The principles which have been presented are of lasting importance. Once equipped with a firm understanding of series circuits, you hold the key to an understanding of the parallel circuits to be presented next. Q25. A circuit has a source voltage of 100 volts and two 50-ohm resistors connected in series. If the reference point for this circuit is placed between the two resistors, what would be the voltage at the reference point? Q26. If the reference point in question 25 were connected to ground, what would be the voltage level of the reference point? Q27. What is an open circuit? Q28. What is a short circuit? Q29. Why will a meter indicate more voltage at the battery terminal when the battery is out of a circuit than when the battery is in a circuit? Q30. What condition gives maximum power transfer from the source to the load? Q31. What is the efficiency of power transfer in question 30? Q32. A circuit has a source voltage of 25 volts. The source resistance is 1 ohm and the load resistance is 49 ohms. What is the efficiency of power transfer?
3-48
PARALLEL DC CIRCUITS The discussion of electrical circuits presented up to this point has been concerned with series circuits in which there is only one path for current. There is another basic type of circuit known as the PARALLEL CIRCUIT with which you must become familiar. Where the series circuit has only one path for current, the parallel circuit has more than one path for current. Ohm’s law and Kirchhoff’s law apply to all electrical circuits, but the characteristics of a parallel dc circuit are different than those of a series dc circuit. PARALLEL CIRCUIT CHARACTERISTICS A PARALLEL CIRCUIT is defined as one having more than one current path connected to a common voltage source. Parallel circuits, therefore, must contain two or more resistances which are not connected in series. An example of a basic parallel circuit is shown in figure 3-37.
Figure 3-37.—Example of a basic parallel circuit.
Start at the voltage source (Es) and trace counterclockwise around the circuit. Two complete and separate paths can be identified in which current can flow. One path is traced from the source, through resistance R1, and back to the source. The other path is from the source, through resistance R2, and back to the source. Voltage in a Parallel Circuit You have seen that the source voltage in a series circuit divides proportionately across each resistor in the circuit. IN A PARALLEL CIRCUIT, THE SAME VOLTAGE IS PRESENT IN EACH BRANCH. (A branch is a section of a circuit that has a complete path for current.) In figure 3-37 this voltage is equal to the applied voltage (Es). This can be expressed in equation form as: ES = ER1 = ER2 Voltage measurements taken across the resistors of a parallel circuit, as illustrated by figure 3-38 verify this equation. Each meter indicates the same amount of voltage. Notice that the voltage across each resistor is the same as the applied voltage.
3-49
Figure 3-38.—Voltage comparison in a parallel circuit.
Example: Assume that the current through a resistor of a parallel circuit is known to be 4.5 milliamperes (4.5 mA) and the value of the resistor is 30,000 ohms (30 N 'HWHUPLQHWKHVRXUFH voltage. The circuit is shown in figure 3-39. Given:
Solution:
Figure 3-39.—Example problem parallel circuit.
3-50
Since the source voltage is equal to the voltage of a branch:
To simplify the math operation, the values can be expressed in powers of ten as follows:
If you are not familiar with the use of the powers of 10 or would like to brush up on it, Mathematics, Vol. 1, NAVEDTRA 10069-C, will be of great help to you. Q33. What would the source voltage (ES) in figure 3-39 be if the current through R2 were 2 milliamps? Current in a Parallel Circuit Ohm’s law states that the current in a circuit is inversely proportional to the circuit resistance. This fact is true in both series and parallel circuits. There is a single path for current in a series circuit. The amount of current is determined by the total resistance of the circuit and the applied voltage. In a parallel circuit the source current divides among the available paths. The behavior of current in parallel circuits will be shown by a series of illustrations using example circuits with different values of resistance for a given value of applied voltage. Part (A) of figure 3-40 shows a basic series circuit. Here, the total current must pass through the single resistor. The amount of current can be determined.
3-51
Figure 3-40.—Analysis of current in parallel circuit.
Given:
Solution:
Part (B) of figure 3-40 shows the same resistor (R1) with a second resistor (R2) of equal value connected in parallel across the voltage source. When Ohm’s law is applied, the current flow through each resistor is found to be the same as the current through the single resistor in part (A).
3-52
Given:
Solution:
It is apparent that if there is 5 amperes of current through each of the two resistors, there must be a TOTAL CURRENT of 10 amperes drawn from the source. The total current of 10 amperes, as illustrated in figure 3-40(B), leaves the negative terminal of the battery and flows to point a. Since point a is a connecting point for the two resistors, it is called a JUNCTION. At junction a, the total current divides into two currents of 5 amperes each. These two currents flow through their respective resistors and rejoin at junction b. The total current then flows from junction b back to the positive terminal of the source. The source supplies a total current of 10 amperes and each of the two equal resistors carries one-half the total current. Each individual current path in the circuit of figure 3-40(B) is referred to as a BRANCH. Each branch carries a current that is a portion of the total current. Two or more branches form a NETWORK. From the previous explanation, the characteristics of current in a parallel circuit can be expressed in terms of the following general equation: IT = I1 + I 2 + . . . In
3-53
Compare part (A) of figure 3-41 with part (B) of the circuit in figure 3-40. Notice that doubling the value of the second branch resistor (R2) has no effect on the current in the first branch (IR1), but does reduce the second branch current (IR2) to one-half its original value. The total circuit current drops to a value equal to the sum of the branch currents. These facts are verified by the following equations. Given:
Solution:
3-54
Figure 3-41.—Current behavior in parallel circuits.
The amount of current flow in the branch circuits and the total current in the circuit shown in figure 3-41(B) are determined by the following computations. Given:
3-55
Solution:
Notice that the sum of the ohmic values in each circuit shown in figure 3-41 is equal (30 ohms), and that the applied voltage is the same (50 volts). However, the total current in 3-41(B) (15 amps) is twice the amount in 3-41(A) (7.5 amps). It is apparent, therefore, that the manner in which resistors are connected in a circuit, as well as their actual ohmic values, affect the total current. The division of current in a parallel network follows a definite pattern. This pattern is described by KIRCHHOFF’S CURRENT LAW which states: 3-56
"The algebraic sum of the currents entering and leaving any junction of conductors is equal to zero." This law can be stated mathematically as: Ia + lb + . . . I n + 0 where: Ia, Ib, etc., are the currents entering and leaving the junction. Currents ENTERING the junction are considered to be POSITIVE and currents LEAVING the junction are considered to be NEGATIVE. When solving a problem using Kirchhoff’s current law, the currents must be placed into the equation WITH THE PROPER POLARITY SIGNS ATTACHED. Example: Solve for the value of I3 in figure 3-42. Given:
Solution: Ia + lb + . . . I a + 0
Figure 3-42.—Circuit for example problem.
The currents are placed into the equation with the proper signs.
3-57
I3 has a value of 2 amperes, and the negative sign shows it to be a current LEAVING the junction. Example. Using figure 3-43, solve for the magnitude and direction of I3.
Figure 3-43.—Circuit for example problem.
Given:
Solution:
3-58
I3 is 2 amperes and its positive sign shows it to be a current entering the junction. Q34. There is a relationship between total current and current through the individual components in a circuit. What is this relationship in a series circuit and a parallel circuit? Q35. In applying Kirchhoff’s current law, what does the polarity of the current indicate? Resistance in a Parallel Circuit In the example diagram, figure 3-44, there are two resistors connected in parallel across a 5-volt battery. Each has a resistance value of 10 ohms. A complete circuit consisting of two parallel paths is formed and current flows as shown.
Figure 3-44.—Two equal resistors connected in parallel.
Computing the individual currents shows that there is one-half of an ampere of current through each resistance. The total current flowing from the battery to the junction of the resistors, and returning from the resistors to the battery, is equal to 1 ampere. The total resistance of the circuit can be calculated by using the values of total voltage (ET) and total current (IT ). NOTE: From this point on the abbreviations and symbology for electrical quantities will be used in example problems. Given:
Solution:
3-59
This computation shows the total resistance to be 5 ohms; one-half the value of either of the two resistors. Since the total resistance of a parallel circuit is smaller than any of the individual resistors, total resistance of a parallel circuit is not the sum of the individual resistor values as was the case in a series circuit. The total resistance of resistors in parallel is also referred to as EQUIVALENT RESISTANCE (Req). The terms total resistance and equivalent resistance are used interchangeably. There are several methods used to determine the equivalent resistance of parallel circuits. The best method for a given circuit depends on the number and value of the resistors. For the circuit described above, where all resistors have the same value, the following simple equation is used:
This equation is valid for any number of parallel resistors of EQUAL VALUE. Example: Four 40-ohm resistors are connected in parallel. What is their equivalent resistance? Given:
Solution:
Figure 3-45 shows two resistors of unequal value in parallel. Since the total current is shown, the equivalent resistance can be calculated.
3-60
Figure 3-45.—Example circuit with unequal parallel resistors.
Given:
Solution:
The equivalent resistance of the circuit shown in figure 3-45 is smaller than either of the two resistors (R1, R2). An important point to remember is that the equivalent resistance of a parallel circuit is always less than the resistance of any branch. Equivalent resistance can be found if you know the individual resistance values and the source voltage. By calculating each branch current, adding the branch currents to calculate total current, and dividing the source voltage by the total current, the total can be found. This method, while effective, is somewhat lengthy. A quicker method of finding equivalent resistance is to use the general formula for resistors in parallel:
If you apply the general formula to the circuit shown in figure 3-45 you will get the same value for HTXLYDOHQWUHVLVWDQFH DVZDVREWDLQHGLQWKHSUHYLRXVFDOFXODWLRQWKDWXVHGVRXUFHYROWDJHDQGWRWDO current.
3-61
Given:
Solution:
Convert the fractions to a common denominator.
Since both sides are reciprocals (divided into one), disregard the reciprocal function.
The formula you were given for equal resistors in parallel
is a simplification of the general formula for resistors in parallel
There are other simplifications of the general formula for resistors in parallel which can be used to calculate the total or equivalent resistance in a parallel circuit. RECIPROCAL METHOD.—This method is based upon taking the reciprocal of each side of the equation. This presents the general formula for resistors in parallel as:
3-62
This formula is used to solve for the equivalent resistance of a number of unequal parallel resistors. You must find the lowest common denominator in solving these problems. If you are a little hazy on finding the lowest common denominator, brush up on it in Mathematics Volume 1, NAVEDTRA 10069 (Series). Example: Three resistors are connected in parallel as shown in figure 3-46. The resistor values are: R1 = 20 ohms, R2 = 30 ohms, R3 = 40 ohms. What is the equivalent resistance? (Use the reciprocal method.)
Figure 3-46.—Example parallel circuit with unequal branch resistors.
Given:
Solution:
3-63
PRODUCT OVER THE SUM METHOD.—A convenient method for finding the equivalent, or total, resistance of two parallel resistors is by using the following formula.
This equation, called the product over the sum formula, is used so frequently it should be committed to memory. Example: What is the equivalent resistance of a 20-ohm and a 30-ohm resistor connected in parallel, as in figure 3-47?
Figure 3-47.—Parallel circuit with two unequal resistors.
3-64
Given:
Solution:
Q36. Four equal resistors are connected in parallel, each resistor has an ohmic value of 100 ohms, what is the equivalent resistance? Q37. Three resistors connected in parallel have values of 12 N N DQGN :KDWLVWKH equivalent resistance? Q38. Two resistors connected in parallel have values of 10 N DQGN :KDWLVWKHHTXLYDOHQW resistance? Power in a Parallel Circuit Power computations in a parallel circuit are essentially the same as those used for the series circuit. Since power dissipation in resistors consists of a heat loss, power dissipations are additive regardless of how the resistors are connected in the circuit. The total power is equal to the sum of the power dissipated by the individual resistors. Like the series circuit, the total power consumed by the parallel circuit is:
Example: Find the total power consumed by the circuit in figure 3-48.
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Figure 3-48.—Example parallel circuit.
Given:
Solution:
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Since the total current and source voltage are known, the total power can also be computed by: Given:
Solution:
Equivalent Circuits In the study of electricity, it is often necessary to reduce a complex circuit into a simpler form. Any complex circuit consisting of resistances can be redrawn (reduced) to a basic equivalent circuit containing the voltage source and a single resistor representing total resistance. This process is called reduction to an EQUIVALENT CIRCUIT. Figure 3-49 shows a parallel circuit with three resistors of equal value and the redrawn equivalent circuit. The parallel circuit shown in part A shows the original circuit. To create the equivalent circuit, you must first calculate the equivalent resistance.
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Figure 3-49.—Parallel circuit with equivalent circuit.
Given:
Solution:
Once the equivalent resistance is known, a new circuit is drawn consisting of a single resistor (to represent the equivalent resistance) and the voltage source, as shown in part B. Rules for Parallel DC Circuits 1. The same voltage exists across each branch of a parallel circuit and is equal to the source voltage.
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2. The current through a branch of a parallel network is inversely proportional to the amount of resistance of the branch. 3. The total current of a parallel circuit is equal to the sum of the individual branch currents of the circuit. 4. The total resistance of a parallel circuit is found by the general formula:
or one of the formulas derived from this general formula. 5. The total power consumed in a parallel circuit is equal to the sum of the power consumptions of the individual resistances. SOLVING PARALLEL CIRCUIT PROBLEMS Problems involving the determination of resistance, voltage, current, and power in a parallel circuit are solved as simply as in a series circuit. The procedure is the same — (1) draw the circuit diagram, (2) state the values given and the values to be found, (3) select the equations to be used in solving for the unknown quantities based upon the known quantities, and (4) substitute the known values in the equation you have selected and solve for the unknown value. Example: A parallel circuit consists of five resistors. The value of each resistor is known and the current through R1 is known. You are asked to calculate the value for total resistance, total power, total current, source voltage, the power used by each resistor, and the current through resistors R2, R3, R4, and R5. Given:
Find:
This may appear to be a large amount of mathematical manipulation. However, if you use the stepby-step approach, the circuit will fall apart quite easily. The first step in solving this problem is for you to draw the circuit and indicate the known values as shown in figure 3-50.
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Figure 3-50.—Parallel circuit problem.
There are several ways to approach this problem. With the values you have been given, you could first solve for RT, the power used by R1, or the voltage across R1, which you know is equal to the source voltage and the voltage across each of the other resistors. Solving for RT or the power used by R1 will not help in solving for the other unknown values. Once the voltage across Rl is known, this value will help you calculate other unknowns. Therefore the logical unknown to solve for is the source voltage (the voltage across R1). Given:
Solution:
Now that source voltage is known, you can solve for current in each branch. Given:
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Solution:
Since R3 = R4 = R5 and the voltage across each branch is the same:
Solving for total resistance. Given:
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Solution:
An alternate method for solving for RT can be used. By observation, you can see that R 3, R 4, and R5 are of equal ohmic value. Therefore an equivalent resistor can be substituted for these three resistors in solving for total resistance. Given:
Solution:
The circuit can now be redrawn using a resistor labeled Req1 in place of R3, R4, and R5 as shown in figure 3-51.
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Figure 3-51.—First equivalent parallel circuit.
An equivalent resistor can be calculated and substituted for Rl and R2 by use of the product over the sum formula. Given:
Solution:
The circuit is now redrawn again using a resistor labeled Req2 in place of R1 and R2 as shown in figure 3-52.
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Figure 3-52.—Second equivalent parallel circuit.
You are now left with two resistors in parallel. The product over the sum method can now be used to solve for total resistance. Given:
Solution:
This agrees with the solution found by using the general formula for solving for resistors in parallel. The circuit can now be redrawn as shown in figure 3-53 and total current can be calculated.
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Figure 3-53.—Parallel circuit redrawn to final equivalent circuit.
Given:
Solution:
This solution can be checked by using the values already calculated for the branch currents. Given:
Solution:
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Now that total current is known, the next logical step is to find total power. Given:
Solution:
Solving for the power in each branch. Given:
Solution:
Since IR3 = IR4 = IR5 then, PR3 = PR4 = PR5 = 1800 W. The previous calculation for total power can now be checked.
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Given:
Solution:
Q39. What term identifies a single resistor that represents total resistance of a complex circuit? Q40. The total power in both series and parallel circuits is computed with the formula: PT = P1 + P2 + P3 +...Pn. Why can this formula be used for both series and parallel circuits? Q41. A circuit consists of three resistors connected in parallel across a voltage source. Rl 52 = 53 DQG3R3 = 360 watts. Solve for RT, ES and IR2. (Hint: Draw and label the circuit first.)
SERIES-PARALLEL DC CIRCUITS In the preceding discussions, series and parallel dc circuits have been considered separately. The technician will encounter circuits consisting of both series and parallel elements. A circuit of this type is referred to as a COMBINATION CIRCUIT. Solving for the quantities and elements in a combination circuit is simply a matter of applying the laws and rules discussed up to this point. SOLVING COMBINATION-CIRCUIT PROBLEMS The basic technique used for solving dc combination-circuit problems is the use of equivalent circuits. To simplify a complex circuit to a simple circuit containing only one load, equivalent circuits are substituted (on paper) for the complex circuit they represent. To demonstrate the method used to solve combination circuit problems, the network shown in figure 3-54(A) will be used to calculate various circuit quantities, such as resistance, current, voltage, and power.
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Figure 3-54.—Example combination circuit.
Examination of the circuit shows that the only quantity that can be computed with the given information is the equivalent resistance of R2 and R3. Given:
Solution:
Now that the equivalent resistance for R2 and R3 has been calculated, the circuit can be redrawn as a series circuit as shown in figure 3-54(B).
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The equivalent resistance of this circuit (total resistance) can now be calculated. Given:
Solution:
The original circuit can be redrawn with a single resistor that represents the equivalent resistance of the entire circuit as shown in figure 3-54(C). To find total current in the circuit: Given:
Solution:
To find total power in the circuit: Given:
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Solution:
To find the voltage dropped across Rl, R2 , and R3 , refer to figure 3-54(B). Req1 represents the parallel network of R2 and R3. Since the voltage across each branch of a parallel circuit is equal, the voltage across Req1 (Eeq1) will be equal to the voltage across R2 (ER2) and also equal to the voltage across R 3 (ER3). Given:
Solution:
To find power used by Rl: Given:
Solution:
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To find the current through R2 and R3, refer to the original circuit, figure 3-54(A). You know ER2 and ER3 from previous calculation. Given:
Solution:
To find power used by R2 and R3, using values from previous calculations: Given:
Solution:
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Now that you have solved for the unknown quantities in this circuit, you can apply what you have learned to any series, parallel, or combination circuit. It is important to remember to first look at the circuit and from observation make your determination of the type of circuit, what is known, and what you are looking for. A minute spent in this manner may save you many unnecessary calculations. Having computed all the currents and voltages of figure 3-54, a complete description of the operation of the circuit can be made. The total current of 3 amps leaves the negative terminal of the battery and flows through the 8-ohm resistor (R1). In so doing, a voltage drop of 24 volts occurs across resistor R1. At point A, this 3-ampere current divides into two currents. Of the total current, 1.8 amps flows through the 20-ohm resistor. The remaining current of 1.2 amps flows from point A, down through the 30-ohm resistor to point B. This current produces a voltage drop of 36 volts across the 30-ohm resistor. (Notice that the voltage drops across the 20- and 30-ohm resistors are the same.) The two branch currents of 1.8 and 1.2 amps combine at junction B and the total current of 3 amps flows back to the source. The action of the circuit has been completely described with the exception of power consumed, which could be described using the values previously computed. It should be pointed out that the combination circuit is not difficult to solve. The key to its solution lies in knowing the order in which the steps of the solution must be accomplished. Practice Circuit Problem Figure 3-55 is a typical combination circuit. To make sure you understand the techniques of solving for the unknown quantities, solve for ER1.
Figure 3-55.—Combination practice circuit.
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It is not necessary to solve for all the values in the circuit to compute the voltage drop across resistor R1 (E R1). First look at the circuit and determine that the values given do not provide enough information to solve for ER1 directly. If the current through R1 (IR1) is known, then ER1 can be computed by applying the formula:
The following steps will be used to solve the problem. 1. The total resistance (RT) is calculated by the use of equivalent resistance. Given:
Solution:
Redraw the circuit as shown in figure 3-55(B). Given:
Solution:
Solution:
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Redraw the circuit as shown in figure 3-55(C). Given:
Solution:
2. The total current (IT) is now computed. Given:
Solution:
3. Solve for the voltage dropped across Req2. This represents the voltage dropped across the network R1, R2, and R3 in the original circuit. Given:
Solution:
4. Solve for the current through Req1. (Req1 represents the network R1 and R2 in the original circuit.) Since the voltage across each branch of a parallel circuit is equal to the voltage across the equivalent resistor representing the circuit: 3-84
Given:
Solution:
5. Solve for the voltage dropped across R1 (the quantity you were asked to find). Since Req1 represents the series network of R1 and R2 and total current flows through each resistor in a series circuit, IR1 must equal IReq1. Given:
Solution:
Q42. Refer to figure 3-55(A). If the following resistors were replaced with the values indicated: R 1 = 53 = lk ZKDWLVWKHWRWDOSRZHULQWKHFLUFXLW":KDWLV(R2? REDRAWING CIRCUITS FOR CLARITY You will notice that the schematic diagrams you have been working with have shown parallel circuits drawn as neat square figures, with each branch easily identified. In actual practice the wired circuits and more complex schematics are rarely laid out in this simple form. For this reason, it is important for you to recognize that circuits can be drawn in a variety of ways, and to learn some of the techniques for redrawing them into their simplified form. When a circuit is redrawn for clarity or to its simplest form, the following steps are used. 1. Trace the current paths in the circuit. 2. Label the junctions in the circuit. 3. Recognize points which are at the same potential. 3-85
4. Visualize a rearrangement, "stretching" or "shrinking," of connecting wires. 5. Redraw the circuit into simpler form (through stages if necessary). To redraw any circuit, start at the source, and trace the path of current flow through the circuit. At points where the current divides, called JUNCTIONS, parallel branches begin. These junctions are key points of reference in any circuit and should be labeled as you find them. The wires in circuit schematics are assumed to have NO RESISTANCE and there is NO VOLTAGE drop along any wire. This means that any unbroken wire is at the same voltage all along its length, until it is interrupted by a resistor, battery, or some other circuit component. In redrawing a circuit, a wire can be "stretched" or "shrunk" as much as you like without changing any electrical characteristic of the circuit. Figure 3-56(A) is a schematic of a circuit that is not drawn in the box-like fashion used in previous illustrations. To redraw this circuit, start at the voltage source and trace the path for current to the junction marked (a). At this junction the current divides into three paths. If you were to stretch the wire to show the three current paths, the circuit would appear as shown in figure 3-56(B).
Figure 3-56.—Redrawing a simple parallel circuit.
While these circuits may appear to be different, the two drawings actually represent the same circuit. The drawing in figure 3-56(B) is the familiar box-like structure and may be easier to work with. Figure 3-57(A) is a schematic of a circuit shown in a box-like structure, but may be misleading. This circuit in reality is a series-parallel circuit that may be redrawn as shown in figure 3-57(B). The drawing in part (B) of the figure is a simpler representation of the original circuit and could be reduced to just two resistors in parallel.
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Figure 3-57.—Redrawing a simple series-parallel circuit.
Redrawing a Complex Circuit Figure 3-58(A) shows a complex circuit that may be redrawn for clarification in the following steps.
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Figure 3-58.—Redrawing a complex circuit.
NOTE: As you redraw the circuit, draw it in simple box-like form. Each time you reach a junction, a new branch is created by stretching or shrinking the wires. Start at the negative terminal of the voltage source. Current flows through R1 to a junction and divides into three paths; label this junction (a). Follow one of the paths of current through R2 and R3 to a junction where the current divides into two more paths. This junction is labeled (b). The current through one branch of this junction goes through R5 and back to the source. (The most direct path.) Now that you have completed a path for current to the source, return to the last junction, (b). Follow current through the other branch from this junction. Current flows from junction (b) through R4 to the source. All the paths from junction (b) have been traced. Only one path from junction (a) has been completed. You must now return to junction (a) to complete the other two paths. From junction (a) the current flows through R7 back to the source. (There are no additional branches on this path.) Return to junction (a) to trace the third path from this junction. Current flows through R6 and R8 and comes to a junction. Label this junction (c). From junction (c) one path for current is through R9 to the source. The other path for current from junction (c) is through R10 to the source. All the junctions in this circuit have
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now been labeled. The circuit and the junction can be redrawn as shown in figure 3-58(C). It is much easier to recognize the series and parallel paths in the redrawn circuit. Q43. What is the total resistance of the circuit shown in figure 3-59? (Hint: Redraw the circuit to simplify and then use equivalent resistances to compute for RT.)
Figure 3-59.—Simplification circuit problem.
Q44. What is the total resistance of the circuit shown in figure 3-60?
Figure 3-60.—Source resistance in a parallel circuit.
Q45. What effect does the internal resistance have on the rest of the circuit shown in figure 3-60? EFFECTS OF OPEN AND SHORT CIRCUITS Earlier in this chapter the terms open and short circuits were discussed. The following discussion deals with the effects on a circuit when an open or a short occurs. 3-89
The major difference between an open in a parallel circuit and an open in a series circuit is that in the parallel circuit the open would not necessarily disable the circuit. If the open condition occurs in a series portion of the circuit, there will be no current because there is no complete path for current flow. If, on the other hand, the open occurs in a parallel path, some current will still flow in the circuit. The parallel branch where the open occurs will be effectively disabled, total resistance of the circuit will INCREASE, and total current will DECREASE. To clarify these points, figure 3-61 illustrates a series parallel circuit. First the effect of an open in the series portion of this circuit will be examined. Figure 3-61(A) shows the normal circuit, RT = 40 ohms and IT = 3 amps. In figure 3-61(B) an open is shown in the series portion of the circuit, there is no complete path for current and the resistance of the circuit is considered to be infinite.
Figure 3-61.—Series-parallel circuit with opens.
In figure 3-61(C) an open is shown in the parallel branch of R3. There is no path for current through R3. In the circuit, current flows through R1 and R2 only. Since there is only one path for current flow, R1 and R2 are effectively in series. Under these conditions RT DQG,T = 1 amp. As you can see, when an open occurs in a parallel branch, total circuit resistance increases and total circuit current decreases. A short circuit in a parallel network has an effect similar to a short in a series circuit. In general, the short will cause an increase in current and the possibility of component damage regardless of the type of
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circuit involved. To illustrate this point, figure 3-62 shows a series-parallel network in which shorts are developed. In figure 3-62 (A) the normal circuit is shown. RT = 40 ohms and IT = 3 amps.
Figure 3-62.—Series-parallel circuit with shorts.
In figure 3-62 (B), R1 has shorted. R1 now has zero ohms of resistance. The total of the resistance of the circuit is now equal to the resistance of the parallel network of R2 and R3, or 20 ohms. Circuit current has increased to 6 amps. All of this current goes through the parallel network (R2, R3) and this increase in current would most likely damage the components. In figure 3-62 (C), R3 has shorted. With R3 shorted there is a short circuit in parallel with R2 . The short circuit routes the current around R2, effectively removing R2 from the circuit. Total circuit resistance is now equal to the resistance of R1, or 20 ohms. As you know, R2 and R3 form a parallel network. Resistance of the network can be calculated as follows: Given:
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Solution:
The total circuit current with R3 shorted is 6 amps. All of this current flows through R1 and would most likely damage R1. Notice that even though only one portion of the parallel network was shorted, the entire paralleled network was disabled. Opens and shorts alike, if occurring in a circuit, result in an overall change in the equivalent resistance. This can cause undesirable effects in other parts of the circuit due to the corresponding change in the total current flow. A short usually causes components to fail in a circuit which is not properly fused or otherwise protected. The failure may take the form of a burned-out resistor, damaged source, or a fire in the circuit components and wiring. Fuses and other circuit protection devices are installed in equipment circuits to prevent damage caused by increases in current. These circuit protection devices are designed to open if current increases to a predetermined value. Circuit protection devices are connected in series with the circuit or portion of the circuit that the device is protecting. When the circuit protection device opens, current flow ceases in the circuit. A more thorough explanation of fuses and other circuit protection devices is presented in Module 3, Introduction to Circuit Protection, Control, and Measurement. Q46. What is the effect on total resistance and total current in a circuit if an open occurs in (a) a parallel branch, and (b) in a series portion? Q47. What is the effect on total resistance and total current in a circuit if a short occurs in (a) a parallel branch, and (b) in a series portion? Q48. If one branch of a parallel network is shorted, what portion of circuit current flows through the remaining branches?
VOLTAGE DIVIDERS Most electrical and electronics equipment use voltages of various levels throughout their circuitry. One circuit may require a 90-volt supply, another a 150-volt supply, and still another a 180-volt supply. These voltage requirements could be supplied by three individual power sources. This method is expensive and requires a considerable amount of room. The most common method of supplying these voltages is to use a single voltage source and a VOLTAGE DIVIDER. Before voltage dividers are explained, a review of what was discussed earlier concerning voltage references may be of help.
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As you know, some circuits are designed to supply both positive and negative voltages. Perhaps now you wonder if a negative voltage has any less potential than a positive voltage. The answer is that 100 volts is 100 volts. Whether it is negative or positive does not affect the feeling you get when you are shocked. Voltage polarities are considered as being positive or negative in respect to a reference point, usually ground. Figure 3-63 will help to illustrate this point.
Figure 3-63.—Voltage polarities.
Figure 3-63(A) shows a series circuit with a voltage source of 100 volts and four 50-ohm resistors connected in series. The ground, or reference point, is connected to one end of resistor R1. The current in this circuit determined by Ohm’s law is .5 amp. Each resistor develops (drops) 25 volts. The five tap-off points indicated in the schematic are points at which the voltage can be measured. As indicated on the schematic, the voltage measured at each of the points from point A to point E starts at zero volts and becomes more positive in 25 volt steps to a value of positive 100 volts. In figure 3-63(B), the ground, or reference point has been moved to point B. The current in the circuit is still .5 amp and each resistor still develops 25 volts. The total voltage developed in the circuit remains at 100 volts, but because the reference point has been changed, the voltage at point A is negative 25 volts. Point E, which was at positive 100 volts in figure 3-63(A), now has a voltage of positive 75 volts. As you can see the voltage at any point in the circuit is dependent on three factors; the current through the resistor, the ohmic value of the resistor, and the reference point in the circuit. A typical voltage divider consists of two or more resistors connected in series across a source voltage (Es). The source voltage must be as high or higher than any voltage developed by the voltage divider. As the source voltage is dropped in successive steps through the series resistors, any desired 3-93
portion of the source voltage may be "tapped off" to supply individual voltage requirements. The values of the series resistors used in the voltage divider are determined by the voltage and current requirements of the loads. Figure 3-64 is used to illustrate the development of a simple voltage divider. The requirement for this voltage divider is to provide a voltage of 25 volts and a current of 910 milliamps to the load from a source voltage of 100 volts. Figure 3-64(A) provides a circuit in which 25 volts is available at point B. If the load was connected between point B and ground, you might think that the load would be supplied with 25 volts. This is not true since the load connected between point B and ground forms a parallel network of the load and resistor R1. (Remember that the value of resistance of a parallel network is always less than the value of the smallest resistor in the network.)
Figure 3-64.—Simple voltage divider.
Since the resistance of the network would now be less than 25 ohms, the voltage at point B would be less than 25 volts. This would not satisfy the requirement of the load. To determine the size of resistor used in the voltage divider, a rule-of-thumb is used. The current in the divider resistor should equal approximately 10 percent of the load current. This current, which does not flow through any of the load devices, is called bleeder current. Given this information, the voltage divider can be designed using the following steps. 1. Determine the load requirement and the available voltage source.
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2. Select bleeder current by applying the 10% rule-of-thumb.
3. Calculate bleeder resistance.
The value of R1 may be rounded off to 275 ohms:
4. Calculate the total current (load plus bleeder).
5. Calculate the resistance of the other divider resistor(s).
The voltage divider circuit can now be drawn as shown in figure 3-64(B). Q49. What information must be known to determine the component values for a voltage divider?
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Q50. If a voltage divider is required for a load that will use 450 mA of current, what should be the value of bleeder current? Q51. If the load in question 50 requires a voltage of +90 V, what should be the value of the bleeder resistor? Q52. If the source voltage for the voltage divider in question 50 supplies 150 volts, what is the total current through the voltage divider? MULTIPLE-LOAD VOLTAGE DIVIDERS A multiple-load voltage divider is shown in figure 3-65. An important point that was not emphasized before is that when using the 10% rule-of-thumb to calculate the bleeder current, you must take 10% of the total load current.
Figure 3-65.—Multiple-load voltage divider.
Given the information shown in figure 3-65, you can calculate the values for the resistors needed in the voltage-divider circuits. The same steps will be followed as in the previous voltage divider problem.
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Given:
The bleeder current should be 10% of the total load current. Solution:
Since the voltage across R1 (ER1) is equal to the voltage requirement for load 1, Ohm’s law can be used to calculate the value for R1. Solution:
The current through R2 (IR2) is equal to the current through R1 plus the current through load 1. Solution:
The voltage across R2 (ER2) is equal to the difference between the voltage requirements of load 1 and load 2.
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Ohm’s law can now be used to solve for the value of R 2 . Solution:
The current through R3 (IR3) is equal to the current through R2 plus the current through load 2.
The voltage across R3 (ER3) equals the difference between the voltage requirement of load 3 and load 2.
Ohm’s law can now be used to solve for the value of R3 . Solution:
The current through R4 (IR4) is equal to the current through R3 plus the current through load 3. IR4 is equal to total circuit current (I T).
The voltage across R4 (ER4) equals the difference between the source voltage and the voltage requirement of load 3. 3-98
Ohm’s law can now be used to solve for the value of R4 . Solution:
With the calculations just explained, the values of the resistors used in the voltage divider are as follows:
POWER IN THE VOLTAGE DIVIDER Power in the voltage divider is an extremely important quantity. The power dissipated by the resistors in the voltage divider should be calculated to determine the power handling requirements of the resistors. Total power of the circuit is needed to determine the power requirement of the source. The power for the circuit shown in figure 3-65 is calculated as follows: Given:
Solution:
The power in each resistor is calculated just as for R1. When the calculations are performed, the following results are obtained:
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To calculate the power for load 1: Given:
Solution:
The power in each load is calculated just as for load 1. When the calculations are performed, the following results are obtained.
Total power is calculated by summing the power consumed by the loads and the power dissipated by the divider resistors. The total power in the circuit is 15.675 watts. The power used by the loads and divider resistors is supplied by the source. This applies to all electrical circuits; power for all components is supplied by the source. Since power is the product of voltage and current, the power supplied by the source is equal to the source voltage multiplied by the total circuit current (Es x IT). In the circuit of figure 3-65, the total power can be calculated by: Given:
Solution:
VOLTAGE DIVIDER WITH POSITIVE AND NEGATIVE VOLTAGE REQUIREMENTS In many cases the load for a voltage divider requires both positive and negative voltages. Positive and negative voltages can be supplied from a single source voltage by connecting the ground (reference point) between two of the divider resistors. The exact point in the circuit at which the reference point is placed depends upon the voltages required by the loads.
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For example, a voltage divider can be designed to provide the voltage and current to three loads from a given source voltage. Given:
The circuit is drawn as shown in figure 3-66. Notice the placement of the ground reference point. The values for resistors R1, R3, and R4 are computed exactly as was done in the last example. IR1 is the bleeder current and can be calculated as follows:
Figure 3-66.—Voltage divider providing both positive and negative voltages.
Solution:
Calculate the value of R1.
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Solution:
Calculate the current through R2 using Kirchhoff’s current law. At point A:
(or 195mA leaving point A) Since ER2 = E load 2, you can calculate the value of R2. Solution:
Calculate the current through R3.
The voltage across R3 (ER3) equals the difference between the voltage requirements of loads 3 and 2. Solution:
Calculate the value of R3.
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Solution:
Calculate the current through R4.
The voltage across ER4 equals the source voltage (Es) minus the voltage requirement of load 3 and the voltage requirement of load 1. Remember Kirchhoff’s voltage law which states that the sum of the voltage drops and emfs around any closed loop is equal to zero. Solution:
Calculate the value of R4. Solution:
With the calculations just explained, the values of the resistors used in the voltage /divider are as follows:
From the information just calculated, any other circuit quantity, such as power, total current, or resistance of the load, could be calculated. 3-103
PRACTICAL APPLICATION OF VOLTAGE DIVIDERS In actual practice the computed value of the bleeder resistor does not always come out to an even value. Since the rule-of-thumb for bleeder current is only an estimated value, the bleeder resistor can be of a value close to the computed value. (If the computed value of the resistance were 510 ohms, a 500ohm resistor could be used.) Once the actual value of the bleeder resistor is selected, the bleeder current must be recomputed. The voltage developed by the bleeder resistor must be equal to the voltage requirement of the load in parallel with the bleeder resistor. The value of the remaining resistors in the voltage divider is computed from the current through the remaining resistors and the voltage across them. These values must be used to provide the required voltage and current to the loads. If the computed values for the divider resistors are not even values; series, parallel, or series-parallel networks can be used to provide the required resistance. Example: A voltage divider is required to supply two loads from a 190.5 volts source. Load 1 requires +45 volts and 210 milliamps; load 2 requires +165 volts and 100 milliamps. Calculate the bleeder current using the rule-of-thumb. Given:
Solution:
Calculate the ohmic value of the bleeder resistor. Given:
Solution:
Since it would be difficult to find a resistor of 1451.6 ohms, a practical choice for R1 is 1500 ohms. Calculate the actual bleeder current using the selected value for R1. 3-104
Given:
Solution:
Using this value for IR1, calculate the resistance needed for the next divider resistor. The current (IR2) is equal to the bleeder current plus the current used by load 1. Given:
Solution:
The voltage across R2 (ER2) is equal to the difference between the voltage requirements of loads 2 and 1, or 120 volts. Calculate the value of R2. Given:
Solution:
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The value of the final divider resistor is calculated with I R3 (IR2 + I load 2) equal to 340 mA and E R3 (Es - E load 2) equal to 25.5V. Given:
Solution:
A 75-ohm resistor may not be easily obtainable, so a network of resistors equal to 75 ohms can be used in place of R3. Any combination of resistor values adding up to 75 ohms could be placed in series to develop the required network. For example, if you had two 37.5-ohm resistors, you could connect them in series to get a network of 75 ohms. One 50-ohm and one 25-ohm resistor or seven 10-ohm and one 5-ohm resistor could also be used. A parallel network could be constructed from two 150-ohm resistors or three 225-ohm resistors. Either of these parallel networks would also be a network of 75 ohms. The network used in this example will be a series-parallel network using three 50-ohm resistors. With the information given, you should be able to draw this voltage divider network. Once the values for the various divider resistors have been selected, you can compute the power used by each resistor using the methods previously explained. When the power used by each resistor is known, the wattage rating required of each resistor determines the physical size and type needed for the circuit. This circuit is shown in figure 3-67.
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Figure 3-67.—Practical example of a voltage divider.
Q53. In figure 3-67, why is the value of R1 calculated first? Q54. In figure 3-67, how is (a) the current through R2 and (b) the voltage drop across R2 computed? Q55. In figure 3-67, what is the power dissipated in R1? Q56. In figure 3-67, what is the purpose of the series-parallel network R3, R 4, and R5? Q57. In figure 3-67, what should be the minimum wattage ratings of R3 and R5? Q58. If the load requirement consists of both positive and negative voltages, what technique is used in the voltage divider to supply the loads from a single voltage source?
EQUIVALENT CIRCUIT TECHNIQUES The circuit solutions that you have studied up to this point have been obtained mainly through the use of formulas derived from Ohm’s law. As in many other fields of science, electricity has its share of special shortcut methods. Some of the special circuit analysis techniques are: THEVENIN’S THEOREM, which uses a process of circuit reduction to Thevenin’s equivalent circuit; and NORTON’S THEOREM, which is reduction of a circuit to Norton’s equivalent. Another method is called LOOP ANALYSIS. This uses Kirchhoff’s voltage law to simultaneously solve problems in parallel branches of a circuit. The use of 3-107
these methods should be reserved until you have become thoroughly familiar with the methods covered thus far in this chapter. You may want to explore some of the special techniques later in your career.
ELECTRICAL SAFETY Safety precautions must always be observed by persons working around electric circuits and equipment to avoid injury from electric shock. Detailed safety precautions are contained in NAVMAT P-5l00, Safety Precautions for Shore Activities and OPNAVINST 5l00-19, Navy Safety Precautions for Forces Afloat. The danger of shock from a 450-volt ac electrical service system is well recognized by operating personnel. This is shown by the relatively low number of reports of serious shock received from this voltage, despite its widespread use. On the other hand, a number of fatalities have been reported due to contact with low-voltage circuits. Despite a fairly widespread, but totally unfounded, popular belief to the contrary, low-voltage circuits (115 volts and below) are very dangerous and can cause death when the resistance of the body is lowered. Fundamentally, current, rather than voltage, is the measure of shock intensity. The passage of even a very small current through a vital part of the human body can cause DEATH. The voltage necessary to produce the fatal current is dependent upon the resistance of the body, contact conditions, the path through the body, etc. For example, when a 60-hertz alternating current, is passed through a human body from hand to hand or from hand to foot, and the current is gradually increased, it will cause the following effects: At about 1 milliampere (0.001 ampere), the shock can be felt; at about 10 milliamperes (0.01 ampere), the shock is of sufficient intensity to prevent voluntary control of the muscles; and at about 100 milliamperes (0.1 ampere) the shock is fatal if it lasts for 1 second or more. The above figures are the results of numerous investigations and are approximate because individuals differ in their resistance to electrical shock. It is most important to recognize that the resistance of the human body cannot be relied upon to prevent a fatal shock from 115 volts or less— FATALITIES FROM VOLTAGES AS LOW AS 30 VOLTS HAVE BEEN RECORDED. Tests have shown that body resistance under unfavorable conditions may be as low as 300 ohms, and possibly as low as 100 ohms from temple to temple if the skin is broken. Conditions aboard ship add to the chance of receiving an electrical shock. Aboard ship the body is likely to be in contact with the metal structure of the ship and the body resistance may be low because of perspiration or damp clothing. Extra care and awareness of electrical hazards aboard ship are needed. Short circuits can be caused by accidentally placing or dropping a metal tool, rule, flashlight case, or other conducting article across an energized line. The arc and fire which result, even on relatively lowvoltage circuits, may cause extensive damage to equipment and serious injury to personnel. Since ship service power distribution systems are designed to be ungrounded, many persons believe it is safe to touch one conductor, since no electrical current would flow. This is not true, since the distribution system is not totally isolated from the hull of the ship. If one conductor of an ungrounded electrical system is touched while the body is in contact with the hull of the ship or other metal equipment enclosure, a fatal electric current may pass through the body. ALL LIVE ELECTRIC CIRCUITS SHALL BE TREATED AS POTENTIAL HAZARDS AT ALL TIMES. DANGER SIGNALS Personnel should constantly be on the alert for any signs which might indicate a malfunction of electric equipment. Besides the more obvious visual signs, the reaction of other senses, such as hearing, smell, and touch, should also make you aware of possible electrical malfunctions. Examples of signs which you must be alert for are: fire, smoke, sparks, arcing, or an unusual sound from an electric motor. 3-108
Frayed and damaged cords or plugs; receptacles, plugs, and cords which feel warm to the touch; slight shocks felt when handling electrical equipment; unusually hot running electric motors and other electrical equipment; an odor of burning or overheated insulation; electrical equipment which either fails to operate or operates irregularly; and electrical equipment which produces excessive vibrations are also indications of malfunctions. When any of the above signs are noted, they are to be reported immediately to a qualified technician. DO NOT DELAY. Do not operate faulty equipment. Above all, do not attempt to make any repairs yourself if you are not qualified to do so. Stand clear of any suspected hazard and instruct others to do likewise.
• • •
•
Warning Signs—They have been placed for your protection. To disregard them is to invite personal injury as well as possible damage to equipment. Switches and receptacles with a temporary warning tag, indicating work is being performed, are not to be touched. Working Near Electrical Equipment—When work must be performed in the immediate vicinity of electrical equipment, check with the technician responsible for the maintenance of the equipment so you can avoid any potential hazards of which you may not be immediately aware. Authorized Personnel Only—Because of the danger of fire, damage to equipment, and injury to personnel, all repair and maintenance work on electrical equipment shall be done only by authorized persons. Keep your hands off of all equipment which you have not been specifically authorized to handle. Particularly stay clear of electrical equipment opened for inspection, testing, or servicing. Circuit Breakers and Fuses—Covers for all fuse boxes, junction boxes, switch boxes, and wiring accessories should be kept closed. Any cover which is not closed or is missing should be reported to the technician responsible for its maintenance. Failure to do so may result in injury to personnel or damage to equipment in the event accidental contact is made with exposed live circuits.
ELECTRICAL FIRES Carbon dioxide (CO2) is used in fighting electrical fires. It is nonconductive and, therefore, the safest to use in terms of electrical safety. It also offers the least likelihood of damaging equipment. However, if the discharge horn of a CO2 extinguisher is allowed to accidentally touch an energized circuit, the horn may transmit a shock to the person handling the extinguisher. The very qualities which cause CO2 to be a valuable extinguishing agent also make it dangerous to life. When it replaces oxygen in the air to the extent that combustion cannot be sustained, respiration also cannot be sustained. Exposure of a person to an atmosphere of high concentration of CO2 will cause suffocation.
FIRST AID FOR ELECTRIC SHOCK A person who has stopped breathing is not necessarily dead, but is in immediate danger. Life is dependent upon oxygen, which is breathed into the lungs and then carried by the blood to every body cell. Since body cells cannot store oxygen, and since the blood can hold only a limited amount (and that only for a short time), death will surely result from continued lack of breathing. However, the heart may continue to beat for some time after breathing has stopped, and the blood may still be circulated to the body cells. Since the blood will, for a short time, contain a small supply of 3-109
oxygen, the body cells will not die immediately. For a very few minutes, there is some chance that the person’s life may be saved. The process by which a person who has stopped breathing can be saved is called ARTIFICIAL VENTILATION (RESPIRATION). The purpose of artificial ventilation is to force air out of the lungs and into the lungs, in rhythmic alternation, until natural breathing is reestablished. Artificial ventilation should be given only when natural breathing has stopped; it should NOT be given to any person who is breathing naturally. You should not assume that an individual who is unconscious due to electrical shock has stopped breathing. To tell if someone suffering from an electrical shock is breathing, place your hands on the person’s sides, at the level of the lowest ribs. If the victim is breathing, you will usually be able to feel the movement. Remember: DO NOT GIVE ARTIFICIAL VENTILATION TO A PERSON WHO IS BREATHING NATURALLY. Records show that seven out of ten victims of electric shock were revived when artificial respiration was started in less than 3 minutes. After 3 minutes, the chances of revival decrease rapidly. Once it has been determined that breathing has stopped, the person nearest the victim should start the artificial ventilation without delay and send others for assistance and medical aid. The only logical, permissible delay is that required to free the victim from contact with the electricity in the quickest, safest way. This step, while it must be taken quickly, must be done with great care; otherwise, there may be two victims instead of one. In the case of portable electric tools, lights, appliances, equipment, or portable outlet extensions, this should be done by turning off the supply switch or by removing the plug from its receptacle. If the switch or receptacle cannot be quickly located, the suspected electrical device may be pulled free of the victim. Other persons arriving on the scene must be clearly warned not to touch the suspected equipment until it is deenergized. Aid should be enlisted to unplug the device as soon as possible. The injured person should be pulled free of contact with stationary equipment (such as a bus bar) if the equipment cannot be quickly deenergized, or if considerations of military operation or unit survival prevent immediate shutdown of the circuits. This can be done quickly and safely by carefully applying the following procedures: 1. Protect yourself with dry insulating material. 2. Use a dry board, belt, clothing, or other available nonconductive material to free the victim from electrical contact. DO NOT TOUCH THE VICTIM UNTIL THE SOURCE OF ELECTRICITY HAS BEEN REMOVED. Once the victim has been removed from the electrical source, it should be determined, if the person is breathing. If the person is not breathing, a method of artificial ventilation is used. Sometimes victims of electrical shock suffer cardiac arrest (heart stoppage) as well as loss of breathing. Artificial ventilation alone is not enough in cases where the heart has stopped. A technique known as Cardiopulmonary Resuscitation (CPR) has been developed to provide aid to a person who has stopped breathing and suffered a cardiac arrest. Because you most likely will be working in the field of electricity, the risk of electrical shock is higher than most other Navy occupations. You should, at your earliest opportunity, learn the technique of CPR. CPR is relatively easy to learn and is taught in courses available from the American Red Cross, some Navy Medical Departments, and in the Standard First Aid Training Course (NAVEDTRA 12081).
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Q59. Is it considered safe for a person to touch any energized low-voltage conductor with the bare hand? Q60. What should you do if you become aware of a possible malfunction in a piece of electrical equipment? Q61. Who should perform CPR?
SUMMARY
With the completion of this chapter you have gained a basic understanding of dc circuits. The information you have learned will provide you with a firm foundation for continuing your study of electricity. The following is a summary of the important points in the chapter. A BASIC ELECTRIC CIRCUIT consists of a source of electrical energy connected to a load. The load uses the energy and changes it to a useful form.
A SCHEMATIC DIAGRAM is a "picture" of a circuit, which uses symbols to represent components. The space required to depict an electrical or electronic circuit is greatly reduced by the use of a schematic.
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VOLTAGE (E) is the electrical force or pressure operating in a circuit. AN AMPERE (A) represents the current flow produced by one volt working across one ohm of resistance. RESISTANCE (R)LVWKHRSSRVLWLRQWRFXUUHQW,WLVPHDVXUHGLQRKPV 2QHRKPRIUHVLVWDQFH will limit the current produced by one volt to a level of one ampere. THE OHM’S FORMULA can be transposed to find one of the values in a circuit if the other two values are known. You can transpose the Ohm’s law formula
mathematically, or you can use the Ohm’s law figure to determine the mathematical relationship between R, E, and I.
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GRAPHICAL ANALYSIS of the relationship between R, E, and I can be studied by plotting these quantities on a graph. Such a graph is useful for observing the characteristics of an electrical device.
POWER is the rate of doing work per unit of time. The time required to perform a given amount of work will determine the power expended. As a formula, P = E x I, where P = power in watts, E = voltage in volts, and I = current in amperes. THE FOUR BASIC ELECTRICAL QUANTITIES are P, I, E, R. Any single unknown quantity can be expressed in terms of any two of the other known quantities. The formula wheel is a simple representation of these relationships.
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POWER RATING in watts indicates the rate at which a device converts electrical energy into another form of energy. The power rating of a resistor indicates the maximum power the resistor can withstand without being destroyed. POWER USED by an electrical device is measured in watt-hours. One watt-hour is equal to one watt used continuously for one hour. THE EFFICIENCY of an electrical device is equal to the electrical power converted into useful energy divided by the electrical power supplied to the device.
HORSEPOWER is a unit of measurement often used to rate electrical motors. It is a unit of work. One horsepower is equal to 746 watts. A SERIES CIRCUIT is defined as a circuit that has only one path for current flow.
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RULES FOR SERIES DC CIRCUITS:
•
The same current flows through each part of a series circuit.
•
The total resistance of a series circuit is equal to the sum of the individual resistances.
•
The total voltage across a series circuit is equal to the sum of the individual voltage drops.
• •
The voltage drop across a resistor in a series circuit is proportional to the ohmic value of the resistor. The total power in a series circuit is equal to the sum of the individual power used by each circuit component.
KIRCHHOFF’S VOLTAGE LAW states: The algebraic sum of the voltage drops in any closed path in a circuit and the electromotive forces in that path is equal to zero, or Ea + Eb + Ec +...En = 0.
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VOLTAGE POLARITIES must be used when applying Kirchhoff’s voltage law. The point at which current enters a load (resistor) is considered negative with respect to the point at which current leaves the load. SERIES AIDING VOLTAGES cause current to flow in the same direction; thus the voltages are added.
SERIES OPPOSING VOLTAGES tend to force current to flow in opposite directions; thus the equivalent voltage is the difference between the opposing voltages. A REFERENCE POINT is a chosen point in a circuit to which all other points are compared.
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AN OPEN CIRCUIT is one in which a break exists in the complete conducting pathway. A SHORT CIRCUIT is an accidental path of low resistance which passes an abnormally high amount of current. INTERNAL RESISTANCE causes a drop in the terminal voltage of a source as current flows through the source. The decrease in terminal voltage is caused by the voltage drop across the internal resistance. All sources of electromotive force have some form of internal resistance.
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HIGH EFFICIENCY in a circuit is achieved when the resistance of the load is high with respect to the resistance of the source.
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POWER TRANSFER in a circuit is highest when the resistance of the load equals the resistance of the source. A PARALLEL CIRCUIT is a circuit having more than one current path connected to a common voltage source.
RULES FOR PARALLEL DC CIRCUITS:
• • • • •
The same voltage exists across each branch of a parallel circuit and is equal to the source voltage. The current through a branch of a parallel network is inversely proportional to the amount of resistance of the branch. The total current of a parallel circuit is equal to the sum of the currents of the individual branches of the circuit. The total resistance of a parallel circuit is equal to the reciprocal of the sum of the reciprocals of the individual resistances of the circuit. The total power consumed in a parallel circuit is equal to the sum of the power consumptions of the individual resistances.
THE SOLUTION OF A COMBINATION CIRCUIT is a matter of applying the laws and rules for series and parallel circuits as applicable.
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ALL PARALLEL CIRCUITS ARE COMBINATION CIRCUITS when the internal resistance of the source is taken into account. REDRAWING CIRCUITS FOR CLARITY is accomplished in the following steps: 1. Trace the current paths in the circuit. 2. Label the junctions in the circuit. 3. Recognize points which are at the same potential. 4. Visualize rearrangements, "stretching" or "shrinking," of connecting wires. 5. Redraw the circuit into simpler form (through stages if necessary).
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EQUIPMENT PROTECTION from short-circuit current is accomplished by use of fuses and other circuit protection devices. A VOLTAGE DIVIDER is a series circuit in which desired portions of the source voltage may be tapped off for use in equipment. Both negative and positive voltage can be provided to the loads by the proper selection of the reference point (ground).
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ELECTRICAL SAFETY PRECAUTIONS must be observed. A fatal shock can occur from 0.1 ampere of current. Voltages as low as 30 volts have been recorded as causing sufficient current to be fatal. ALL LIVE ELECTRICAL CIRCUITS shall be treated as potential hazards at all times. ELECTRONIC OR ELECTRICAL EQUIPMENT discovered to be faulty or unsafe should be reported immediately to proper authority. ELECTRICAL OR ELECTRONIC EQUIPMENT should be used and repaired by authorized personnel only. A CO2 EXTINGUISHER should be used to extinguish electrical fires. FIRST AID FOR ELECTRICAL SHOCK includes the following actions:
•
Remove the victim from the source of the shock.
•
Check the victim to see if the person is breathing.
• •
If the victim is not breathing, give artificial ventilation. The preferred method is mouth-to-mouth. CPR may be necessary if the heartbeat has stopped, but do not attempt this unless you have been trained in its use. OBTAIN MEDICAL ASSISTANCE AS SOON AS POSSIBLE.
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ANSWERS TO QUESTIONS Q1. THROUGH Q61. A1. (a) DS1, the flashlight bulb (b) BAT, the dry cell A2. The path for current is incomplete; or, there is no path for current with S1 open. A3. A schematic diagram. A4. (a) Current increases (b) Current decreases A5. (a) Current decreases (b) Current increases A6.
A7. 1.25 amperes. A8. 4 amperes. A9. Power. A10. By changing the circuit resistance or the voltage of the power source. A11.
A12. 6 amperes. A13. A wirewound resistor. A14. 1 kilowatt. A15. 8,952 watt hours or 8.952 kWh. A16. 942 (rounded to 3 places). A17.
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A18.
A19.
A20. The source voltage would have to be increased to 640 volts. A21.
A22. The point at which current enters the resistor is assigned a negative polarity and the point at which current leaves the resistor is assigned a positive polarity. A23. 2 amperes. A24. 120 volts. A25. 50 volts. A26. Zero volts. A27. A circuit where there is no longer a complete path for current flow. A28. An accidental path of low resistance which passes an abnormally high amount of current. A29. The internal (source) resistance of the battery will drop some of the voltage. A30. When the load resistance equals the source resistance. A31. 50 percent.
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A32.
A33. 60 volts. A34. Total current in a series circuit flows through every circuit component but in a parallel circuit total current divides among the available paths. A35. Whether the current is entering the junction (+) or leaving the junction (-). A36.
A37.
A38.
A39. Equivalent resistor or Req. A40. In both cases all the power used in the circuit must come from the source. A41.
A42. PT = 60 W, ER2 = 10 V. A43. A44.
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A45. Because of the 2-volt drop across the internal resistance, only 48 volts is available for the rest of the circuit. A46. (a) Total resistance increases, total current decreases (b) Total resistance becomes infinite, total current is equal to zero A47. (a) Total resistance decreases, total current increases (b) Total resistance decreases, total current increases A48. None. A49. The source voltage and load requirements (voltage and current). A50. 45 mA rule-of-thumb. A51. 2 N A52. 495 mA. A53. R1 is the bleeder resistor. Bleeder current must be known before any of the remaining divider resistor ohmic values can be computed. A54. (a) By adding the bleeder current (IR1) and the current through load 1(b) By subtracting the voltage of load 1 from the voltage of load 2. A55. 1.35 watts. A56. The series-parallel network drops the remaining source voltage and is used to take the place of a single resistor (75 ohms) when the required ohmic value is not available in a single resistor. A57. R 3 = 2 watts; R5 = 6 watts. A58. The ground (reference point) is placed in the proper point in the voltage divider so that positive and negative voltages are supplied. A59. NEVER! All energized electric circuits should be considered potentially dangerous. A60. You should immediately report this condition to a qualified technician. A61. Only trained, qualified personnel.
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APPENDIX I
GLOSSARY AMMETER—An instrument for measuring the amount of electron flow in amperes. AMPERE—The basic unit of electrical current. ANODE—A positive electrode of an electrochemical device (such as a primary or secondary electric cell) toward which the negative ions are drawn. ATTRACTION—The force that tends to make two objects approach each other. Attraction exists between two unlike magnetic poles (North and South) or between two unlike static charges (plus and minus). BATTERY—A device for converting chemical energy into electrical energy. BATTERY CAPACITY—The amount of energy available from a battery. Battery capacity is expressed in ampere-hours. BLEEDER CURRENT—The current through a bleeder resistor. In a voltage divider, bleeder current is usually determined by the 10 percent rule of thumb. BLEEDER RESISTOR—A resistor which is used to draw a fixed current. BRANCH—An individual current path in a parallel circuit. CATHODE—The general name for any negative electrode. CELL—A single unit that transforms chemical energy into electrical energy. Batteries are made up of cells. CHARGE—Represents electrical energy. A material having an excess of electrons is said to have a negative charge. A material having a deficiency of electrons is said to have a positive charge. CIRCUIT—The complete path of an electric current. CIRCULAR MIL—An area equal to that of a circle with a diameter of 0.001 inch. It is used for measuring the cross-sectional area of wires. COMBINATION CIRCUIT—A series-parallel circuit. CONDUCTANCE—The ability of a material to conduct or carry an electric current. It is the reciprocal of the resistance of the material, and is expressed in mhos or siemans. CONDUCTIVITY—Ease with which a substance transmits electricity. CONDUCTOR—(1) A material with a large number of free electrons. (2) A material which easily permits electric current to flow. COULOMB—A measure of the quantity of electricity. One coulomb is equal to 6.28 × 1018 electrons.
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COULOMB’S LAW—Also called the law of electric charges or the law of electrostatic attraction. Coulomb's Law states that charged bodies attract or repel each other with a force that is directly proportional to the product of their individual charges and inversely proportional to the square of the distance between them. CPR—Cardio-Pulminary Resuscitation. CROSS-SECTIONAL AREA—The area of a "slice" of an object. When applied to electrical conductors, it is usually expressed in circular mils. CURRENT—The flow of electrons past a reference point. The passage of electrons through a conductor. Measured in amperes. DEAD SHORT—A short circuit having minimum resistance. DIELECTRIC FIELD—The space between and around charged bodies in which their influence is felt. Also called Electric Field of Force or an Electrostatic Field. DIRECT CURRENT—An electric current that flows in one direction only. DOMAIN THEORY—A theory of magnetism based upon the electron-spin principle. Spinning electrons have a magnetic field. If more electrons spin in one direction than another, the atom is magnetized. DRY CELL—An electrical cell in which the electrolyte is not a liquid. In most dry cells the electrolyte is in the form of a paste. EFFICIENCY—The ratio of output power to input power, generally expressed as a percentage. ELECTRIC CURRENT—The flow of electrons. ELECTRICAL CHARGE—Symbol Q, q. Electric energy stored on or in an object. The negative charge is caused by an excess of electrons; the positive charge is caused by a deficiency of electrons. ELECTROCHEMICAL—The action of converting chemical energy into electrical energy. ELECTRODE—The terminal at which electricity passes from one medium into another, such as in an electrical cell where the current leaves or returns to the electrolyte. ELECTROLYTE—A solution of a substance which is capable of conducting electricity. An electrolyte may be in the form of either a liquid or a paste. ELECTROMAGNET—An electrically excited magnet capable of exerting mechanical force, or of performing mechanical work. ELECTROMAGNETIC—The term describing the relationship between electricity and magnetism. Having both magnetic and electric properties. ELECTROMAGNETIC INDUCTION—The production of a voltage in a coil due to a change in the number of magnetic lines of force (flux linkages) passing through the coil. ELECTRON—The elementary negative charge that revolves around the nucleus of an atom. ELECTRON SHELL—A group of electrons which have a common energy level that forms part of the outer structure (shell) of an atom. AI-2
ELECTROSTATIC—Pertaining to electricity at rest, such as charges on an object (static electricity). ELEMENT—A substance, in chemistry, that cannot be divided into simpler substances by any means ordinarily available. EMF—(Electromotive Force) The force which causes electricity to flow between two points with different electrical charges or when there is a difference of potential between the two points. The unit of measurement in volts. ENERGY—The ability or capacity to do work. EQUIVALENT RESISTANCE—(Req) A resistance that represents the total ohmic values of a circuit component or group of circuit components. Usually drawn as a single resistor when simplifying complex circuits. FERROMAGNETIC MATERIAL—A highly magnetic material, such as iron, cobalt, nickel, or alloys, make up these materials. FIELD OF FORCE—A term used to describe the total force exerted by an action-at-a-distance phenomenon such as gravity upon matter, electric charges acting upon electric charges, magnetic forces acting upon other magnets or magnetic materials. FIXED RESISTOR—A resistor having a definite resistance value that cannot be adjusted. FLUX—In electrical or electromagnetic devices, a general term used to designate collectively all the electric or magnetic lines of force in a region. FLUX DENSITY—The number of magnetic lines of force passing through a given area. GAS—One of the three states of matter having no fixed form or volume. (Steam is a gas.) GRAPH—A pictorial presentation of the relation between two or more variable quantities, such as between an applied voltage and the current it produces in a circuit. GROUND POTENTIAL—Zero potential with respect to the ground or earth. HORSEPOWER—The English unit of power, equal to work done at the rate of 550 foot-pounds per second. Equal to 746 watts of electrical power. HORSESHOE MAGNET—A permanent magnet or electromagnet bent into the shape of a horseshoe or having a U-shape to bring the two poles near each other. HYDROMETER—An instrument used to measure specific gravity. In batteries hydrometers are used to indicate the state of charge by the specific gravity of the electrolyte. INDUCED CHARGE—An electrostatic charge produced on an object by the electric field that surrounds a nearby object. INDUCED CURRENT—Current due to the relative motion between a conductor and a magnetic field. INDUCED ELECTROMOTIVE FORCE—The electromotive force induced in a conductor due to the relative motion between a conductor and a magnetic field. INDUCED VOLTAGE—See Induced Electromotive Force.
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INDUCTION—The act or process of producing voltage by the relative motion of a magnetic field across a conductor. INFINITE—(1) Extending indefinitely, endless. (2) Boundless having no limits. (3) An incalculable number. INSULATION—(1) A material used to prevent the leakage of electricity from a conductor and to provide mechanical spacing or support to protect against accidental contact. (2) Use of material in which current flow is negligible to surround or separate a conductor to prevent loss of current. INSULATOR—(1) Material of such low conductivity that the flow of current through it can usually be neglected. (2) Device having high-electric resistance, used for supporting or separating conductors so as to prevent undesired flow of current from the conductors to other objects. INVERSELY—Inverted or reversed in position or relationship. ION—An electrically charged atom or group of atoms. Negative ions have an excess of electrons; positive ions have a deficiency of electrons. IONIZE—To make an atom or molecule of an element lose an electron, as by X-ray bombardment, and thus be converted into a positive ion. The freed electron may attach itself to a neutral atom or molecule to form a negative ion. JUNCTION—(1) The connection between two or more conductors. (2) The contact between two dissimilar metals or materials, as is in a thermocouple. KILO—A prefix meaning one thousand. KINETIC ENERGY—Energy which a body possesses by virtue of its motion. KIRCHHOFF’S LAWS—(1) The algebraic sum of the currents flowing toward any point and the current flowing from that point in an electric network is zero. (2) The algebraic sum of the products of the current and resistance in each of the conductors in any closed path in a network is equal to the algebraic sum of the electromotive forces in the path. LAW OF MAGNETISM—Like poles repel; unlike poles attract. LEAD-ACID CELL—A cell in an ordinary storage battery, in which electrodes are grids of lead containing an active material consisting of certain lead oxides that change in composition during charging and discharging. The electrodes or plates are immersed in an electrolyte of diluted sulfuric acid. LINE OF FORCE—A line in an electric or magnetic field that shows the direction of the force. LIQUID—One of the three states of matter which has a definite volume but no definite form. (Water is a liquid.) LOAD—(1) A device through which an electric current flows and which changes electrical energy into another form. (2) Power consumed by a device or circuit in performing its function. LOCAL ACTION—A continuation of current flow within an electrical cell when there is no external load. Caused by impurities in the electrode. MAGNETIC FIELD—The space in which a magnetic force exists.
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MAGNETIC POLES—The section of a magnet where the flux lines are concentrated; also where they enter and leave the magnet. MAGNETISM—The property possessed by certain materials by which these materials can exert mechanical force on neighboring masses of magnetic materials; and can cause currents to be induced in conducting bodies moving relative to the magnetized bodies. MATTER—Any physical entity which possesses mass. MEGA—A prefix meaning one million, also Meg. MHO—Unit of conductance: the reciprocal of the ohm. Replaced by siemens. MICRO—A prefix meaning one-millionth. MILLI—A prefix meaning one-thousandth. NEGATIVE ELECTRODE—A terminal or electrode having more electrons than normal. Electrons flow out of the negative terminal of a voltage source. NEGATIVE TEMPERATURE COEFFICIENT—The temperature coefficient expressing the amount of reduction in the value of a quantity, such as resistance for each degree of increase in temperature. NETWORK—A combination of electrical components. In a parallel circuit it is composed of two or more branches. NEUTRAL—In a normal condition, hence neither positive nor negative. A neutral object has a normal number of electrons. OHM—The unit of electrical resistance. It is that value of electrical resistance through which a constant potential difference of 1 volt across the resistance will maintain a current flow of 1 ampere through the resistance. OHMIC VALUE—Resistance in ohms. OHM’S LAW—The current in an electric circuit is directly proportional to the electromotive force in the circuit. The most common form of the law is E = IR, where E is the electromotive force or voltage across the circuit, I is the current flowing in the circuit, and R is the resistance of the circuit. OPEN CIRCUIT—(1) The condition of an electrical circuit caused by the breaking of continuity of one or more conductors of the circuit; usually an undesired condition. (2) A circuit which does not provide a complete path for the flow of current. PARALLEL CIRCUIT—Two or more electrical devices connected to the same pair of terminals so separate currents flow through each; electrons have more than one path to travel from the negative to the positive terminal. PERMEABILITY—The measure of the ability of a material to act as a path for magnetic lines of force. PHOTOELECTRIC VOLTAGE—A voltage produced by light. PICO—A prefix adopted by the National Bureau of Standards meaning 10 –12.
AI-5
PIEZOELECTRIC EFFECT—The effect of producing a voltage by placing a stress, either by compression, expansion, or twisting, on a crystal and, conversely, producing a stress in a crystal by applying a voltage to it. PLATE—One of the electrodes in a storage battery. POLARITY—(1) The condition in an electrical circuit by which the direction of the flow of current can be determined. Usually applied to batteries and other direct voltage sources. (2) Two opposite charges, one positive and one negative. (3) A quality of having two opposite magnetic poles, one north and the other south. POLARIZATION—The effect of hydrogen surrounding the anode of a cell which increases the internal resistance of the cell. POTENTIAL ENERGY—Energy due to the position of one body with respect to another body or to the relative parts of the same body. POTENTIOMETER—A 3-terminal resistor with one or more sliding contacts, which functions as an adjustable voltage divider. POWER—The rate of doing work or the rate of expending energy. The unit of electrical power is the watt. PRIMARY CELL—An electrochemical cell in which the chemical action eats away one of the electrodes, usually the negative electrode. RECIPROCAL—The value obtained by dividing the number 1 by any quantity. REFERENCE POINT—A point in a circuit to which all other points in the circuit are compared. RELUCTANCE—A measure of the opposition that a material offers to magnetic lines of force. REPULSION—The mechanical force tending to separate bodies having like electrical charges or like magnetic polarity. RESIDUAL MAGNETISM—Magnetism remaining in a substance after removal of the magnetizing force. RESISTANCE—(1) The property of a conductor which determines the amount of current that will flow as the result of the application of a given electromotive force. All conductors possess some resistance, but when a device is made especially for the purpose of limiting current flow, it is called a resistor. A resistance of 1 ohm will allow a current of 1 ampere to flow through it when a potential of 1 volt is applied. (2) The opposition which a device or material offers to the flow of current. The effect of resistance is to raise the temperature of the material or device carrying the current. (3) A circuit element designed to offer a predetermined resistance to current flow. RESISTOR—The electrical component which offers resistance to the flow of current. It may be a coil of fine wire or a composition rod. RETENTIVITY—The ability of a material to retain its magnetism. RHEOSTAT—(1) A resistor whose value can be varied. (2) A variable resistor which is used for the purpose of adjusting the current in a circuit.
AI-6
SCHEMATIC CIRCUIT DIAGRAM—A circuit diagram in which component parts are represented by simple, easily drawn symbols. May be called schematic. SCHEMATIC SYMBOLS—A letter, abbreviation, or design used to represent specific characteristics or components on a schematic diagram. SECONDARY CELL—A cell that can be recharged by passing a current through the cell in a direction opposite to the discharge current. SERIES CIRCUIT—An arrangement where electrical devices are connected so that the total current must flow through all the devices; electrons have one path to travel from the negative terminal to the positive terminal. SERIES-PARALLEL CIRCUIT—A circuit that consists of both series and parallel networks. SHELF LIFE—The period of time that a cell or battery may be stored and still be useful. SHIELDING—A metallic covering used to prevent magnetic or electromagnetic fields from effecting an object. SHORT CIRCUIT—A low resistance connection between two points of different potential in a circuit, usually accidental and usually resulting in excessive current flow that may cause damage. SIEMANS—The new and preferred term for mho. SOLID—One of the three states of matter which has definite volume and shape. (Ice is a solid.) SOURCE VOLTAGE—The device which furnishes the electrical energy used by a load. SPECIFIC GRAVITY—The ratio between the density of a substance and that of pure water at a given temperature. STATIC ELECTRICITY—Stationary electricity that is in the form of a charge. The accumulated electric charge on an object. SWITCH—A device to connect, disconnect, or change the connections in an electrical circuit. TAPPED RESISTOR—A wire-wound, fixed resistor having one or more additional terminals along its length, generally for voltage-divider applications. TEMPERATURE COEFFICIENT—The amount of change of resistance in a material per unit change in temperature. TERMINAL—An electrical connection. THERMOCOUPLE—A junction of two dissimilar metals that produces a voltage when heated. TOLERANCE—(1) The maximum error or variation from the standard permissible in a measuring instrument. (2) A maximum electrical or mechanical variation from specifications which can be tolerated without impairing the operation of a device.
AI-7
TOTAL RESISTANCE—(RT) The equivalent resistance of an entire circuit. For a series circuit: RT = R1 + R2 + R3 . . . Rn. For parallel circuits:
UNIDIRECTIONAL—In one direction only. VALENCE—The measure of the extent to which an atom is able to combine directly with other atoms. It is believed to depend on the number and arrangement of the electrons in the outermost shell of the atom. VALENCE SHELL—The electrons that form the outermost shell of an atom. VARIABLE RESISTOR—A wire-wound or composition resistor, the value of which may be changed. VOLT—The unit of electromotive force or electrical pressure. One volt is the pressure required to send 1 ampere of current through a resistance of 1 ohm. VOLTAGE—(1) The term used to signify electrical pressure. Voltage is a force which causes current to flow through an electrical conductor. (2) The voltage of a circuit is the greatest effective difference of potential between any two conductors of the circuit VOLTAGE DIVIDER—A series circuit in which desired portions of the source voltage may be tapped off for use in equipment. VOLTAGE DROP—The difference in voltage between two points. It is the result of the loss of electrical pressure as a current flows through a resistance. WATT—The practical unit of electrical power. It is the amount of power used when one ampere of dc flows through a resistance of one ohm. WATTAGE RATING—A rating expressing the maximum power that a device can safely handle. WATT-HOUR—A practical unit of electrical energy equal to one watt of power for one hour. WEBER’S THEORY—A theory of magnetism which assumes that all magnetic material is composed of many tiny magnets. A piece of magnetic material that is magnetized has all of the tiny magnets aligned so that the north pole of each magnet points in one direction. WIRE—A solid or stranded group of solid, cylindrical conductors having low resistance to current flow, with any associated insulation. WORK—The product of force and motion.
AI-8
APPENDIX II
LAWS OF EXPONENTS The International Symbols Committee has adopted prefixes for denoting decimal multiples of units. The National Bureau of Standards has followed the recommendations of this committee, and has adopted the following list of prefixes:
Numbers
Powers of ten
Prefixes
Symbols
1,000,000,000,000
1012
tera
T
1,000,000,000
109
giga
G
1,000,000
106
mega
M
1,000
103
kilo
k
100
102
hecto
h
10
10
deka
da
.1
10-1
deci
d
.01
10-2
centi
c
.001
10-3
milli
m
.000001
10-6
micro
u
.000000001
10-9
nano
n
.000000000001
10-12
Pico
p
.000000000000001
10-15
femto
F
.000000000000000001
10-18
atto
a
To multiply like (with same base) exponential quantities, add the exponents. In the language of algebra the rule is am × a n = am+n
AII-1
To divide exponential quantities, subtract the exponents. In the language of algebra the rule is
*Generally used with electrical quantities.
To raise an exponential quantity to a power, multiply the exponents. In the language of algebra (xm )n = xmn.
Any number (except zero) raised to the zero power is one. In the language of algebra x0 = 1
Any base with a negative exponent is equal to 1 divided by the base with an equal positive exponent. In the language of algebra x –a = 1/xa
To raise a product to a power, raise each factor of the product to that power.
AII-2
To find the nth root of an exponential quantity, divide the exponent by the index of the root. Thus, the nth root of am = am/n.
AII-3
APPENDIX III
SQUARE AND SQUARE ROOTS
AIII-1
AIII-2
APPENDIX IV
COMPARISON OF UNITS IN ELECTRIC AND MAGNETIC CIRCUITS; AND CARBON RESISTOR SIZE COMPARISON BY WATTAGE RATING
AIV-1
APPENDIX V
USEFUL FORMULAS FOR DC CIRCUITS
AV-1
MODULE 1 INDEX A
C
Artificial magnets, 1-14, 1-15 Atoms, matter, 1-4 to 1-8 energy levels, 1-5, 1-6 ionization, 1-8 shells and subshells, 1-6 valence, 1-7
Cell, batteries, 2-2 to 2-4 Cells, types of, 2-8 to 2-13 batteries, 2-13 battery charging, 2-20, 2-21 battery construction, 2-16 to 2-18 battery maintenance, 2-18 to 2-20 hydrometer, 2-19 other maintenance, 2-19 safety precautions with batteries, 2-19 capacity and rating of batteries, 2-20, 2-21 combining cells, 2-13 to 2-15 primary dry, 2-8, 2-9 secondary wet, 2-11, 2-12 Charged bodies, electrostatics, 1-11 Charges, electrical, 1-26 to 1-31 Circuit terms and characteristics, 3-41 to 3-48 open circuit, 3-43, 3-44 power transfer and efficiency, 3-48 reference point, 3-41, 3-42 short circuit, 3-44, 3-45 source resistance, 3-46 to 3-48 Color code, simplifying, 1-45, 1-46 Combination-circuit problems, solving, 3-76 to 3-84 Combining cells, 2-13 to 2-15 series-connected cells, 2-13, 2-14 series-parallel-connected cells, 2-15 to 2-16 Composition of resistors, 1-41, 1-42 Conductors, semiconductors, and insulators, 1-8 Container, cell, batteries, 2-3 Coulomb’s Law of Charges, 1-12 Current, electric, 1-34, 1-35
B
Batteries, 2-2 to 2-29 cell, 2-2 container, 2-3 electrodes, 2-2 electrolyte, 2-3 primary cell, 2-3 secondary cell, 2-3 electrochemical action, 2-4 local action, 2-7 polarization of the cell, 2-7 primary cell chemistry, 2-4 secondary cell chemistry, 2-5, 2-6 summary, 2-22 to 2-29 types of cells, 2-8 to 2-16 batteries, 2-13 battery charging, 2-20, 2-21 battery construction, 2-16 to 2-18 battery maintenance, 2-18, 2-19 capacity and rating of batteries, 2-20, 2-22 combining cells, 2-13 to 2-15 primary dry, 2-8 to 2-11 secondary wet, 2-11 to 2-12 Battery charging, 2-20, 2-22 charging rate, 2-22 charging time, 2-22 equalizing, 2-21 fast, 2-22 floating, 2-21 gassing, 2-22 initial, 2-21 normal, 2-21
D
Direct current, 3-1 to 3-126 basic electric circuit, 3-2 schematic representation, 3-2, 3-3 circuit terms and characteristics, 3-40 open circuit, 3-43, 3-44
INDEX-1
Direct current—Continued power transfer and efficiency, 3-48, 3-49 reference point, 3-48, 3-49 short circuit, 3-48 to 3-49 source resistance, 3-48 to 3-50 electrical safety, 3-108 to 3-111 danger signals, 3-108, 3-109 electrical fires, 3-109 equivalent circuit techniques, 3-107, 3-108 first aid for electric shock, 3-108, 3-109 Kirchhoff's voltage law, 3-35 to 3-43 application, 3-35, 3-37 polarity of voltage, 3-34 Ohm’s law, 3-8 to 3-9 application, 3-5 to 3-9 graphical analysis of the basic circuit, 3-9 to 3-12 parallel d.c. circuits, 3-49 to 3-78 parallel circuit characteristics, 3-49 to 3-70 solving parallel circuit problems, 3-68 to 3-76 power, 3-11 to 3-17 power conversion and efficiency, 3-17 to 3-19 power rating, 3-16, 3-17 series d.c. circuits, 3-19 to 3-33 analysis, 3-28 to 3-33 characteristics, 3-19 summary, 3-28 series-parallel d.c. circuits, 3-76 to 3-84 effects of open and short circuits, 3-89 to 3-91 redrawing circuits for clarity, 3-86 to 3-90 solving combination-circuit problems, 3-76 to 3-84 summary, 3-110 to 3-121 voltage dividers, 3-91 to 3-107 multiple-load voltage dividers, 3-95 to 3-98 positive and negative voltage requirements, 3-100 to 3-103 power, 3-98, 3-99 practical application, 3-103 to 3-107
Directed drift electric current, 1-35 to 1-37 Domain Theory, magnetism, 1-18, 1-19 E
Electric circuit, basic, 3-2 schematic representation, 3-2 Electrical energy, 1-25, 1-26 conductance, 1-40 electric current, 1-34 to 1-37 directed drift, 1-35 to 1-37 magnitude of current flow, 1-36 measurement of current, 1-37 random drift, 1-34 electrical charges, 1-26, 1-27 electrical resistance, 1-37 factors that affect resistance, 1-37, 1-38 electrical resistors, 1-40 to 1-46 composition of resistors, 1-41, 1-42 fixed and variable resistors, 1-42 simplifying the color code, 1-45, 1-47 standard color code system, 1-43 to 1-47 wattage rating, 1-42 how voltage is produced, 1-27 to 1-34 by chemical action, 1-32, 1-33 by friction, 1-28 by heat, 1-29 by light, 1-30 to 1-32 by magnetism, 1-33, 1-34 by pressure, 1-28 Electrical safety, 3-108 to 3-111 danger signals, 3-107, 3-108 electrical fires, 3-108 Electrochemical action, 2-4 local action, 2-7 polarization of the cell, 2-7 primary cell chemistry, 2-8 secondary cell chemistry, 2-3, 2-4 Electrodes, cell, batteries, 2-2 Electrolyte, cell, batteries, 2-3 Electrostatics, 1-9 static electricity 1-10, 1-11 charged bodies, 1-11 Coulomb’s Law of Charges, 1-12 electric fields, 1-12, 1-13
INDEX-2
Electrostatics—Continued nature of charges, 1-11 F
Ferromagnetic materials, 1-14 First aid for electric shock, 3-109, 3-110 Fixed and variable resistors, 1-42 Formulas, d.c. circuits, AV-1 G
Glossary, Al-I to AI-9 H
Hydrometer, battery, 2-19 K
Kirchhoff’s voltage law, 3-34 to 3-43 application, 3-35, 3-36 series aiding and opposing sources, 3-39 to 3-40 polarity of voltage, 3-34 L
Law of exponents, AII-l to AII-3 Lead acid cell, 2-11 Lines of force, magnetic fields, 1-20 to 1-22 M
Magnetism, 1-13 care of magnets, 1-24 magnetic effects, 1-22 magnetic induction, 1-22, 1-23 magnetic shielding, 1-23 to 1-25 magnetic fields, 1-19, 1-20 lines of force, 1-20 to 1-22 magnetic materials, 1-14 artificial magnets, 1-14, 1-15 ferromagnetic materials, 1-14 natural magnets, 1-14 magnetic poles, 1-15 to 1-17 earth’s magnetic poles, 1-16, 1-17 law of, 1-16, 1-17
Magnetism—Continued magnetic shapes, 1-24 theories of magnetism, 1-17 to 1-19 Domain Theory, 1-18, 1-19 Weber’s Theory, 1-17, 1-18 Magnitude of current flow, 1-36 Matter, energy, and electricity, 1-2 to 1-71 conductors, semiconductors, and insulators, 1-8 electrical energy, 1-25 to 1-38 conductance, 1-40 electric current, 1-34 to 1-37 electrical charges, 1-26 to 1-28 electrical resistance, 1-37 to 1-40 electrical resistors, 1-40 to 1-46 how voltage is produced, 1-27 to 1-34 electrostatics, 1-9 static electricity, 1-10, 1-11 magnetism, 1-13 care of magnets, 1-24 magnetic effects, 1-22 magnetic fields, 1-19, 1-20 magnetic materials, 1-14 magnetic poles, 1-15 to 1-17 magnetic shapes, 1-24 theories of magnetism, 1-17 to 1-19 matter, 1-3 to 1-8 atoms, 1-4 to 1-8 molecules, 1-3, 1-4 summary, 1-47 to 1-62 Measurement of current, 1-37 Mercuric-oxide zinc cell, 2-10, 2-11 Molecules, matter, 1-3, 1-4 Multiple-load voltage dividers, 3-95 to 3-98 N Natural magnets, 1-14 Nickel-cadmium cell, 2-12, 2-13 O
Ohm’s law, 3-4 to 3-9 application, 3-5 to 3-9 graphical analysis of the basic circuit, 3-9, 3-12 Open circuit, 3-43, 3-44 INDEX-3
P
Parallel d.c. circuits, 3-49 to 3-78 characteristics, 3-49 to 3-70 current, 3-51 to 3-60 equivalent circuits, 3-66, 3-70 power, 3-65 to 3-67 resistance, 3-58 to 3-64 rules, 3-68 voltage, 3-49 to 3-51 solving parallel circuit problems, 3-68 to 3-76 Polarization of the cell, 2-7, 2-8 Positive and negative voltage requirements, 3-100 to 3-103 Power, direct current, 3-11 to 3-19 power conversion and efficiency, 3-17 to 3-19 power rating, 3-16, 3-17 Power transfer and efficiency, circuit, 3-48, 3-49 Power, voltage dividers, 3-98, 3-99 Primary dry cell, 2-8 construction of a dry cell, 2-8, 2-9 mercuric-oxide zinc cell, 2-10, 2-11 other types, 2-11 Primary cell, batteries, 2-3 Primary cell, chemistry, 2-4, 2-5 R
Random drift electric current, 1-34 Reference Point, Circuit, 3-41, 3-42 Redrawing circuits for clarity, 3-84 to 3-88 Resistance, electrical, 1-37 S
Secondary wet cells, 2-11, 2-12 lead-acid cell, 2-11 nickel-cadmium cell, 2-12, 2-13 silver-cadmium cell, 2-12 silver-zinc cells, 2-12 Secondary cell, batteries, 2-3 Secondary cell chemistry, 2-5, 2-6 Series-connected cells, 2-13, 2-14 Series d.c. circuits, 3-19 to 3-33
Series d.c. circuits—Continued analysis, 3-28 to 3-33 characteristics, 3-19 current, 3-22, 3-23 power, 3-26 to 3-28 resistance, 3-20 to 3-22 voltage, 3-23 to 3-26 summary, 3-28 rules for series, 3-28 Series-parallel-connected cells, 2-14 Series-parallel d.c. circuits, 3-76 to 3-91 effects of open and short circuits, 3-89 to 3-91 redrawing circuits for clarity, 3-84 to 3-88 redrawing a complex circuit, 3-86 to 3-88 solving combination-circuit problems, 3-76 to 3-84 practice circuit problem, 3-81 to 3-84 Short circuit, 3-44, 3-45 Silver-cadmium cell, 2-12 Silver-zinc cells, 2-12 Source resistance, 3-46 to 3-48 Square and square roots, AIII-l, AIII-2 Standard color code system, 1-43 to 1-45 V
Voltage dividers, 3-91 to 3-107 multiple-load voltage dividers, 3-95 to 3-98 positive and negative voltage requirements, 3-100 to 3-103 power, 3-98, 3-99 practical application, 3-103 to 3-107 Voltage is produced, 1-27 to 1-34 by chemical action, 1-32, 1-33 by friction, 1-28, 1-29 by heat, 1-29 by light, 1-30 to 1-32 by magnetism, 1-33, 1-34 by pressure, 1-28 W
Wattage rating, 1-42 Weber’s Theory, magnetism, 1-17, 1-18
INDEX-4
Assignment Questions
Information: The text pages that you are to study are provided at the beginning of the assignment questions.
ASSIGNMENT 1 Textbook assignment: Chapter 1, Turning to Electricity, pages 1-1 through 1-65. _______________________________________________________________________________________ 1-6. What subatomic particle has a positive charge and a large mass?
1-1. Matter can be found in which of the following forms? 1. 2. 3. 4.
1. 2. 3. 4.
Solid Liquid Gaseous Each of the above
1-7. What subatomic particle has no charge?
1-2. A substance that CANNOT be reduced to a simpler substance by chemical means is called a/an 1. 2. 3. 4.
1. 2. 3. 4.
element mixture compound solution
1. 2. 3. 4.
An element A mixture A compound A solution
1. The electron will move around the same orbit faster 2. The electron will jump to an orbit further from the nucleus 3. The electron will jump to an orbit closer to the nucleus 4. The electron will merge with the nucleus
An element A mixture A compound A solution
1-5. What subatomic particle has a negative charge and a small mass? 1. 2. 3. 4.
Angstroms Photons Wavelengths Frequencies
1-9. If light energy collides with an orbiting electron, what happens to the electron?
1-4. An atom is the smallest possible particle that retains the characteristic of which of the following substances? 1. 2. 3. 4.
Proton Electron Positron Neutron
1-8. When light is represented as a tiny packet of energy, what are these packets of energy called?
1-3. A molecule is the smallest possible particle that retains the characteristic of which of the following substances? 1. 2. 3. 4.
Proton Electron Positron Neutron
Proton Electron Positron Neutron
1
1-15. Which of following actions describes the easiest way to accumulate a static electric charge?
1-10. After the action described in question 1-9 occurs, the electron will return to the condition it had before being acted upon by the light. When the electron returns to this condition, which of the following actions occurs? 1. 2. 3. 4.
1. 2. 3. 4.
The nucleus becomes lighter The atom becomes an ion Light energy is emitted The valence of the atom changes
1-16. An atom that contains 6 protons and 5 electrons has what electrical charge? 1. 2. 3. 4.
1-11. The number of electrons in the outermost shell of an atom determines which of the following characteristics of the atom? 1. 2. 3. 4.
Valence Atomic weight Atomic number Number of shells
1. Unlike charges repel each other, like charges repel each other 2. Unlike charges attract each other, like charges attract each other 3. Unlike charges repel each other, like charges attract each other 4. Unlike charges attract each other, like charges repel each other
Unbalanced Lightened Neutral Ionized
1-18. What is/are the term(s) applied to the space between and around charged bodies in which their influence is felt?
1-13. What is the main difference between conductors, semiconductors, and insulators? 1. 2. 3. 4.
1. 2. 3. 4.
The temperature differences The physical state of their mass The number of free electrons The designations of the outer shells
Electric field of force Electrostatic field Dielectric field Each of the above
1-19. Electrostatic lines of force are drawn in which of the following manners?
1-14. A substance with an excess of electrons is considered to be in what electrical state? 1. 2. 3. 4.
Positive Negative Neutral Intermediate
1-17. How do "like" and "unlike" charges react to one another?
1-12. When an atom gains or loses an electron, which of the following terms applies? 1. 2. 3. 4.
Friction between two conductors Friction between two insulators Pressure between two conductors Pressure between two insulators
1. Entering negative charge, entering positive charge 2. Entering negative charge, leaving positive charge 3. Leaving negative charge, leaving positive charge 4. Leaving negative charge, entering positive charge
Neutral Positive Negative Discharged
2
1-26. The north indicating pole of a compass needle is attracted to which of the following poles of the earth?
1-20. Which of the following devices use magnetism? 1. 2. 3. 4.
Batteries Light bulbs High-fidelity speakers Each of the above
1. 2. 3. 4.
1-21. Magnetic materials have which of the following qualities? 1. 2. 3. 4.
1-27. Weber's theory of magnetism assumes that magnetic material is composed of
They are attracted by magnets They can be magnetized Both 1 and 2 above They are electrical insulators
1. tiny molecular magnets 2. domains of magnetic influence 3. large blocks of material acting as magnets 4. atoms with electrons spinning different directions
1-22. Ferromagnetic materials have which of the following qualities? 1. 2. 3. 4.
1-28. According to the domain theory, if an atom with 26 electrons has 20 electrons spinning counterclock-wise, the atom is considered to be
They are all alloys They all contain nickel They make very weak magnets They are relatively easy to magnetize
1. 2. 3. 4.
1-23. A material with low reluctance and high permeability such as iron or soft steel is used to make what type of magnet? 1. 2. 3. 4.
Temporary Permanent Residual Natural
1. 2. 3. 4.
permeability retentivity reluctance ionization
The magnetic field The electrostatic field The piezoelectric effect The chemical reaction of the magnet and the filings
1-30. An imaginary line used to illustrate a magnetic effect is known as a/an
1-25. The law of magnetic poles states which of the following relationships? 1. 2. 3. 4.
charged insulated neutralized magnetized
1-29. If a glass plate is placed over a magnet and iron filings are sprinkled over the glass, a pattern will be visible. What does this pattern indicate?
1-24. The ability of a material to retain magnetism is called 1. 2. 3. 4.
The geographic north pole The magnetic north pole The geographic south pole The magnetic south pole
1. 2. 3. 4.
Like poles attract, unlike poles attract Like poles attract, unlike poles repel Like poles repel, unlike poles repel Like poles repel, unlike poles attract
3
magnetic pole force field pole magnetic line of force electrostatic line of force
1-36. A book sitting on a shelf has what kind of energy?
1-31. Which of the following is NOT a property of magnetic lines of force?
1. 2. 3. 4.
1. They form closed loops around the magnet 2. They leave the magnetic material at right angles to the surface 3. They cross each other at right angles 4. They leave the north pole and enter the south pole of the magnet
1-37. Which of the following term(s) apply(ies) to the difference of potential between two bodies?
1-32. A magnetic shield or screen used to protect a delicate instrument should be made of which of the following materials? 1. 2. 3. 4.
1. 2. 3. 4.
Plastic Copper Soft iron Aluminum
1. 210 V 2. 2100 V 3. 21,000 V 4. 2.1 x 106 V
Separately In pairs at 90 degree angles In pairs with north poles together In pairs with a north pole and a south pole together
1-39. 250µV is equal to which of the following terms? 1. .25 mV 2. .00025 V 3. 250 x 10-6 V 4. All of the above
1-34. What is the term applied to the ability to do work? 1. 2. 3. 4.
Power Energy Voltage Current
1-40. What is the general term that describes a device which supplies a voltage? 1. 2. 3. 4.
1-35. An object that is in motion has what type of energy? 1. 2. 3. 4.
Voltage Electromotive force Both 1 and 2 above Current
1-38. Which of the following terms is equal to "2.1 kV?"
1-33. Bar magnets should be stored in which of the following manners? 1. 2. 3. 4.
Kinetic Potential Newtonian Magnetic
Kinetic Magnetic Newtonian Potential
A voltage source A voltage supply A voltage generator A voltage producer
1-41. In addition to friction, magnetism, and chemical action, which of the following methods can be used to produce a voltage? 1. 2. 3. 4.
4
Pressure Heat Light Each of the above
1-50. Which of the following values is equal to 100mA?
________________________________________ IN ANSWERING QUESTIONS 1-42 THROUGH 1-46, MATCH THE VOLTAGE PRODUCING METHOD LISTED IN COLUMN B TO THE DEVICE LISTED IN COLUMN A. COLUMN A
1. 1.0 ampere 2. 10.0 amperes 3. 0.10 ampere 4. 0.01 ampere
COLUMN B
1-42. Radio receiver's oscillator 1-43. Thermocouple
1. Heat
1-44. Automobile battery
3. Magnetism
1-45. Automobile generator
4. Chemical action
1-51. What symbol is used to represent the ohm? 1. 2. 3. 4.
2. Pressure
1-52. If low weight is the major factor, which of the following materials should be used as a conductor?
1-46. Flashlight cell ________________________________________
1. 2. 3. 4.
1-47. Current in an electric circuit is caused by which of the following actions? 1. Electrons moving from negative to positive 2. Electrons moving from positive to negative 3. Protons moving from negative to positive 4. Protons moving from positive to negative
1. 2. 3. 4.
Aluminum Copper Silver Gold
1-54. Resistance of a conductor will increase with which of the following changes to the cross-sectional area and length of the conductor?
100,000 miles per hour 186,000 miles per second 300,000 meters per hour 500,000 meters per second
1. Cross-sectional area is increased, length is increased 2. Cross-sectional area is increased, length is decreased 3. Cross-sectional area is decreased, length is increased 4. Cross-sectional area is decreased, length is decreased
1-49. If the voltage in a circuit increases, what happens to the current? 1. 2. 3. 4.
Aluminum Copper Silver Gold
1-53. What material is MOST widely used as a conductor in electrical equipment?
1-48. When directed drift takes place, at what speed does the effect take place? 1. 2. 3. 4.
A O µ !
Current increases Current decreases Current remains the same Current fluctuates rapidly
5
1-59. Which of the following schematic symbols is used to represent a resistor?
1-55. A material whose resistance decreases as the temperature increases has what temperature coefficient? 1. 2. 3. 4.
Positive Negative Zero Neutral
1-56. A material whose resistance remains constant as the temperature increases has what temperature coefficient? 1. 2. 3. 4.
Positive Negative Zero Neutral
1-57. Which of the following units is NOT a unit of conductance? 1. 2. 3. 4.
1-60. How is the ability of a resistor to dissipate heat indicated?
Siemens S G Ohm
1. 2. 3. 4.
1-58. Resistance bears which, if any, of the following relationships to conductance? 1. 2. 3. 4.
By the wattage rating By the voltage rating By the resistance rating By the tolerance
1-61. Carbon resistors have which of the following disadvantages?
A direct relationship A reciprocal relationship An inverse square relationship None
1. 2. 3. 4.
A high cost factor An extremely large physical size The resistance value changes with age A limited range of resistance values
1-62. Which of the following types of resistors will overcome the disadvantages of a carbon resistor?
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1. 2. 3. 4.
Rheostat Potentiometer Molded composition Wirewound resistor
1-63. What is the total number of connections on (a) a rheostat and (b) a potentiometer? 1. 2. 3. 4.
6
(a) Two (a) Two (a) Three (a) Three
(b) two (b) three (b) two (b) three
1-64. Which, if any, of the following types of variable resistors is used to control a large amount of current? 1. 2. 3. 4.
Rheostat Potentiometer Wirewound potentiometer None of the above
1-65. A carbon resistor is color-coded orange, orange, orange. What is the resistance value of this resistor? 1. 2.2 k! 2. 3.3 k! 3. 33.0 k! 4. 440.0 k!
Figure 1A.—Resistor with color coding.
IN ANSWERING QUESTIONS 1-68 THROUGH 1-70, REFER TO FIGURE 1A.
1-66. What are the allowable limits of ohmic value in a resistor color coded blue, green, yellow, gold?
1-68. What is the ohmic value of the resistor? 1. 8! 2. 79! 3. 790! 4. 800!
1. 682.5 k! to 617.5 k! 2. 715.0 k! to 585.0 k! 3. 7.98 M! to 7.22 M! 4. 8.36 M! to 6.84 M!
1-69. What is the specified tolerance of the resistor?
1-67. Of the following, which color of the fifth band on a resistor indicates the LEAST chance of failure? 1. 2. 3. 4.
1. 1% 2. 5% 3. 10% 4. 20%
Red Brown Yellow Orange
1-70. What is the specified reliability of the resistor?
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1. 2. 3. 4.
7
1.0% 0.1% 0.01% 0.001%
ASSIGNMENT 2 Textbook assignment: Chapter 2, “Batteries,” pages 2-1 through 2-29. _____________________________________________________________________________________________
2-7. What term is given to the process that takes place inside a cell?
2-1. Which of the following is the purpose of an electrical cell?
1. 2. 3. 4.
1. To change mechanical energy to electrical energy 2. To change chemical energy to electrical energy 3. To change electrical energy to mechanical energy 4. To change electrical energy to chemical energy
2-8. With respect to recharging a primary or secondary cell, of the following statements, which one is correct? 1. The secondary cell can be recharged by passing current through it in the proper direction 2. The primary cell can be recharged by passing current through it in the proper direction 3. The secondary cell can only be recharged by changing the electrodes 4. The primary cell can only be recharged by changing the electrolyte
2-2. What are the three basic parts of a cell? 1. Electrodes, electrolyte, container 2. Electrodes, acid, water 3. Anode, cathode, ions 4. Anode, load, depolarizer _______________________________________ IN ANSWERING QUESTIONS 2-3 THROUGH 2-6, SELECT THE PHRASE FROM THE FOLLOWING LIST THAT DESCRIBES THE PART OF A CELL IN THE QUESTION. A. PARTS OF A CELL
2-9. What determines the amount of current that a cell can deliver to the external circuit?
B. DESCRIPTIVE PHRASE
2-3. Electrolyte
1. negative electrode
2-4. Container
2. positive electrode
2-5. Anode
3. solution acting upon the electrode
Electromagnetic action Piezoelectric action Electromechanical action Electrochemical action
1. The internal resistance of the cell only 2. The resistance of the external load only 3. The circuit resistance and the internal resistance of the cell 4. The circuit capacitance and number of free electrons in the load
2-6. Cathode
4. mounting for the electrode _______________________________________
8
2-14. The primary cell is completely discharged when which of the following conditions exists?
2-10. Which of the following actions will lower the internal resistance of a cell? 1. Decreasing the size of the electrodes 2. Increasing the size of the electrodes 3. Increasing the spacing between the electrodes 4. Increasing the resistance of the electrolyte
1. The cathode is completely eaten away 2. The active ingredient in the electrolyte is used up 3. The voltage of the cell is reduced to zero 4. Each of the above
2-11. What causes negative ions to be attracted to the cathode of a primary cell while the cell is discharging?
2-15. In a zinc-carbon primary cell, what is the function of the carbon electrode? 1. 2. 3. 4.
1. A negative charge caused by a loss of electrons 2. A negative charge caused by an excess of electrons 3. A positive charge caused by a loss of electrons 4. A positive charge caused by an excess of electrons
To generate electrons To supply a return path for current To speed electrolysis To collect hydrogen
2-16. The lead-acid cell is an example of which of the following types of cells? 1. 2. 3. 4.
2-12. What causes hydrogen to be attracted to the anode of a primary cell when the cell is discharging?
The dry cell The voltaic cell The primary cell The secondary cell
2-17. In a fully charged lead-acid cell, what is the composition of the anode, cathode, and electrolyte respectively?
1. A negative charge caused by a loss of electrons 2. A negative charge caused by an excess of electrons 3. A positive charge caused by a loss of electrons 4. A positive charge caused by an excess of electrons
1. Zinc, carbon, and water 2. Carbon, lead, sulfuric acid and water 3. Lead peroxide, sponge lead, sulfuric acid, and water 4. Nickel, cadmium, potassium hydroxide, and water
2-13. What causes the cathode to be "eaten away" in the primary cell while the cell is discharging? 1. The material of the cathode combines with the negative ions to form a new substance. 2. The material of the cathode dissolves in the electrolyte. 3. The material of the cathode leaves the negative terminal of the cell and goes through the load to the anode. 4. Bacteria in the electrolyte erodes the material in the cathode.
9
2-20. The cell is charging.
2-18. Which of the following actions will recharge a secondary cell?
1. 2. 3. 4.
1. Adding more water to the electrolyte 2. Adding more active ingredient to the electrolyte 3. Connecting the negative terminal of a voltage source to the cathode of the cell and the positive terminal of the voltage source to the anode of the cell 4. Connecting the negative terminal of a voltage source to the anode of the cell and the positive terminal of the voltage source to the cathode of the cell
A, C, F, H B, C, F, H A, D, F, G B, D, F, G
2-21. When all the lead sulfate in a lead-acid cell is converted to sulfuric acid, lead peroxide, and sponge lead, what is the condition of the cell? 1. 2. 3. 4.
Fully charged Discharged Sulfated Unusable
A. Sulfuric acid decreasing 2-22. Polarization has what effects on an electrical cell?
B. Sulfuric acid increasing
1. Decreases internal resistance, thereby increasing the output voltage 2. Decreases internal resistance, thereby decreasing the output voltage 3. Increases internal resistance, thereby increasing the output voltage 4. Increases internal resistance, thereby decreasing the output voltage
C. Sponge lead decreasing D. Sponge lead increasing E. Lead peroxide decreasing F. Lead peroxide increasing G. Lead sulfate decreasing
2-23. Which of the following methods is used to control polarization in a cell?
H. Lead sulfate increasing
1. Venting the cell 2. Heating the electrolyte 3. Adding mercury to the electrode material 4. Using an electrolyte that absorbs oxygen
Figure 2A.—Lead acid chemical actions.
IN ANSWERING QUESTIONS 2-19 AND 2-20, REFER TO FIGURE 2A. SELECT THE CORRECT CHEMICAL ACTIONS WITHIN A LEAD-ACID CELL FOR THE CONDITION STATED IN EACH QUESTION.
2-24. Which of the following is caused by local action in a cell?
2-19. The cell is discharging. 1. 2. 3. 4.
1. Shelf life is reduced 2. Hydrogen is generated in large quantities 3. Impurities rise to the surface of the electrolyte 4. Mercury coating of the zinc electrode is worn away
A, C, E, H A, D, E, G B, C, F, G B, D, F, H
10
2-31. What is/are the advantages(s) of using a manganese-dioxide-alkaline- zinc cell over the zinc-carbon cell?
2-25. In a dry cell, what is the consistency of the electrolyte? 1. 2. 3. 4.
Solid Liquid Paste Powder
1. Better voltage stability 2. Longer storage life 3. Operates over a wide temperature range 4. All the above
2-26. What serves as the cathode in a common type of dry cell? 1. 2. 3. 4.
2-32. What is the common name for manganesedioxide-alkaline-zinc cell?
Carbon electrode Zinc container Steel cover Nickel terminal
1. 2. 3. 4.
2-27. How should the dry cell be stored to obtain maximum shelf life? 1. 2. 3. 4.
2-33. Which of the following factors should be considered when selecting a primary cell as a power source?
In a dark container In a heated cabinet In a ventilated area In a refrigerated space
1. 2. 3. 4.
2-28. The blotting paper in a dry cell serves which of the following purposes?
Power requirement Type of electrolyte used Container material All of the above
2-34. Of the following types of cells, which one is a primary cell?
1. Separates the paste from the zinc 2. Permits the electrolyte from the paste to filter through to the zinc slowly 3. Both 1 and 2 above 4. Keeps the electrolyte dry
1. 2. 3. 4.
2-29. Of the following characteristics, which one describes the mercury cell? 1. 2. 3. 4.
Alkaline cell Long-life cell Moz cell Manganese-dioxide cell
Nickel cadmium Silver zinc Lithium organic Silver cadmium
2-35. Which of the following is/are the difference(s) in the construction of a NICAD cell as compared to a lead-acid cell?
It is physically one of the largest cells It has a very stable output voltage It is designed to be rechargeable It produces a large amount of current but has a short shelf life
1. 2. 3. 4.
2-30. Which of the following describes the shorting of a cell? 1. Decreasing the length of a cell 2. Connecting the anode and cathode together without a load 3. Using the cell below its full potential 4. Providing a recharge voltage that is not sufficient to recharge the cell
11
The electrolyte used The material of the anode The material of the cathode All of the above
2-40. What is the (a) voltage output and (b) current capacity of the circuit?
2-36. What is the most common use of a silverzinc cell? 1. 2. 3. 4.
1. 2. 3. 4.
Flashlight batteries Automobile batteries Aircraft storage batteries Emergency equipment batteries
(a) 1.5 volts (a) 1.5 volts (a) 7.5 volts (a) 7.5 volts
(b) 1/8 ampere (b) 5/8 ampere (b) 1/8 ampere (b) 5/8 ampere
2-37. In addition to the nickel-cadmium and silver-zinc cells, which of the following cells uses potassium hydroxide as the active ingredient in the electrolyte? 1. 2. 3. 4.
Lead-acid cell Silver-cadmium Lithium-inorganic cell Magnesium-manganese dioxide cell Figure 2C.—Five cells connected to form a battery.
2-38. What is the minimum number of cells necessary to form a battery? 1. 2. 3. 4.
IN ANSWERING QUESTIONS 2-41 AND 2-42, REFER TO FIGURE 2C. EACH CELL IS 1.5 VOLTS AND HAS A CAPACITY OF 1/8 AMPERE.
One Two Three Four
2-41. What type of connection is used to combine the cells? 1. Series 2. Parallel 3. Series-parallel 2-42. What is the (a) voltage output and (b) current capacity of the circuit?
Figure 2B.—Battery consisting of five cells.
1. 2. 3. 4.
IN ANSWERING QUESTIONS 2-39 AND 2-40, REFER TO FIGURE 2B. EACH CELL IS 1.5 VOLTS AND HAS A CAPACITY OF 1/8 AMPERE. 2-39. What type of connection is used to combine the cells? 1. Series 2. Parallel 3. Series-parallel
12
(a) 1.5 volts (a) 1.5 volts (a) 7.5 volts (a) 7.5 volts
(b) 1/8 ampere (b) 5/8 ampere (b) 1/8 ampere (b) 5/8 ampere
2-45. What is the (a) voltage output and (b) current capacity of the circuit?
2-43. Which of the following diagrams shows the proper connections for obtaining 6 volts at 1/4 ampere? (Each cell is 1.5 volts and has a capacity of 1/8 amp.)
1. 2. 3. 4.
(a) 1.5 volts (a) 4.5 volts (a) 9 volts (a) 18 volts
(b) 1.5 amperes (b) 1/2 ampere (b) 1/4 ampere (b) 1/8 ampere
2-46. What is the first step in performing maintenance on a secondary-cell battery? 1. Check the level of the electrolyte 2. Check the technical manual for information on the specific type of battery 3. Check the terminals for cleanliness and good electrical connection 4. Check the battery case for cleanliness and evidence of damage 2-47. When a hydrometer is used to check the specific gravity of the electrolyte in a battery, to what level should the electrolyte be drawn? 1. Enough to just wet the float 2. Enough so the float will rise without entering the suction bulb 3. Enough so the top one-third of the float will rise into the suction bulb 4. Enough so the float is completely covered by the electrolyte 2-48. To flush a hydrometer, which of the following liquids should be used? Figure 2D.—Battery consisting of 12 cells.
1. 2. 3. 4.
IN ANSWERING QUESTIONS 2-44 AND 2-45, REFER TO FIGURE 2D. EACH CELL EQUALS 1.5 VOLTS AND HAS A CAPACITY OF 1/8 AMPERE.
Sulfuric acid Salt water Fresh water A solution of baking soda and water
2-49. If the electrolyte level in a battery is low, what should be added to the electrolyte to bring it to the proper level?
2-44. What type of connection is used to combine the cells?
1. 2. 3. 4.
1. Series 2. Parallel 3. Series-parallel
13
Tap water Sulfuric acid Potassium hydroxide Distilled water
2-54. Which of the following types of routine charges follows the nameplate data in restoring a battery to its charged condition during the ordinary cycle of operation?
2-50. Which one of the following safety precautions for batteries is NOT correct? 1. Terminals should be electrically connected together before transporting a battery 2. Care should be taken to prevent the spilling of electrolyte 3. Smoking, open flames, and electrical sparks are prohibited around charging batteries 4. Protective clothing, such as rubber apron, rubber gloves, and face shield, should be worn when working on batteries
1. 2. 3. 4.
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2-51. If electrolyte comes in contact with the skin, what first aid treatment should be given immediately to the affected area? 1. 2. 3. 4.
Cover with petroleum jelly Wrap with a sterile bandage Apply an antiseptic lotion Flush with fresh water
2-52. A battery with a capacity of 600 ampere-hours should provide 3 amperes for a maximum of how many hours? 1. 2. 3. 4.
100 hr 200 hr 300 hr 600 hr
2-53. A battery is rated according to a 20-hour rate of discharge at 300 ampere-hours. Which of the following currents is the maximum current that will allow the battery to deliver its rated capacity? 1. 2. 3. 4.
Initial Floating Normal Fast
15 amperes 20 amperes 25 amperes 30 amperes
14
_______________________________________ IN ANSWERING QUESTIONS 2-55 THROUGH 2-58, MATCH THE DESCRIPTION GIVEN IN THE FOLLOWING LIST WITH THE TYPE OF BATTERY CHARGE IN THE QUESTION. A. TYPE OF CHARGE
B. DESCRIPTION
2-55. Initial charge
1. Used in emergency only
2-56. Equalizing charge
2. Used periodically as part of a maintenance routine
2-57. Floating charge
3. Used to keep a battery at full charge while the battery is idle
2-58. Fast charge
4. Used after electrolyte is added to a dry-shipped battery _______________________________________ 2-59. If violent gassing occurs during the charging of a battery, which of the following actions should be taken? 1. 2. 3. 4.
Increase the room ventilation Decrease the room temperature Increase the charging rate Decrease the charging rate
2-60. If a battery is being charged at the proper rate, which, if any of the following types of gassing should occur? 1. 2. 3. 4.
Steady gassing Intermittent gassing Violent gassing None
15
ASSIGNMENT 3 Textbook assignment: Chapter 3, Direct Current, pages 3-1 through 3-126. _____________________________________________________________________________________ 3-4. If circuit voltage is held constant, circuit current will react in what manner as the resistance (a) increases, and (b) decreases? 1. 2. 3. 4.
1. 2. 3. 4.
IN ANSWERING QUESTIONS 3-1 THROUGH 3-3, REFER TO FIGURE 3A. 3-1. What parts of the circuit represent the (a) source and (b) load? (a) Es (a) Es (a) S1 (a) S1
(b) S1 (b) R1 (b) R1 (b) Es
Partially shorted Partially open Shorted Open
3-3. Which of the following terms describes the figure 3A? 1. 2. 3. 4.
(a) Increase (a) Increase (a) Decrease (a) Decrease
(b) decrease (b) increase (b) decrease (b) increase
3-6. According to Ohm's law, what formula should be used to calculate circuit voltage if resistance and current value are known?
3-2. Which of the following terms describes the circuit condition? 1. 2. 3. 4.
(b) decrease (b) increase (b) decrease (b) increase
3-5. If circuit resistance is held constant, circuit current will react in what manner as the voltage (a) increases, and (b) decreases?
Figure 3A.—Basic circuit.
1. 2. 3. 4.
(a) Increase (a) Increase (a) Decrease (a) Decrease
Parts layout Exploded view Wiring diagram Schematic diagram
16
3-10. Which of the following circuit quantities can be varied ONLY by varying one of the other circuit quantities? 1. 2. 3. 4.
Voltage Current Resistance Each of the above
3-11. Which of the following is a correct formula for determining power in an electrical circuit? Figure 3B.—Graph of current and voltage.
IN ANSWERING QUESTIONS 3-7 AND 3-8, REFER TO FIGURE 3B. 3-7. If the current is 15 amperes, what is the value of the voltage? 1. 50 V 2. 75 V 3. 100 V 4. 150 V
3-12. What is the current in a circuit with 15 ohms of resistance that uses 135 watts of power?
3-8. If the voltage is 200 volts, what is the value of the current? 1. 2. 3. 4.
1. 10 A 2. 15 A 3. 3 A 4. 9 A
10 A 20 A 30 A 40 A
3-13. What is the total power used by a 15-ohm resistor with 4 amps of current?
3-9. Which of the following terms applies to the rate at which an electrical force causes motion? 1. 2. 3. 4.
1. 60 W 2. 240 W 3. 360 W 4. 900 W
Power Energy Inertia Each of the above
3-14. What type of resistor should be used in question 3-13? 1. 2. 3. 4.
17
Carbon Wirewound Precision Composition
3-19. What is the total voltage dropped by each resistor in question 3-18?
3-15. How much total energy is converted by a l-horsepower motor in 10 hours? 1. 2. 3. 4.
1. 20 V 2. 60 V 3. 180 V 4. 540 V
7.46 kWh 8.32 kWh 8.59 kWh 9.32 kWh
3-20. If the current decreases to 2 amps, what is the total voltage drop across each resistor?
3-16. If the energy used by the motor in question 3-15 is 9.5 kWh, what is the efficiency of the motor? 1. 2. 3. 4.
1. 2. 3. 4.
.981 .904 .876 .785
120 V 230 V 310 V 400 V
3-21. What would have to be done to the circuit to cause the current to decrease to 2 amps? 1. The source voltage would have to be increased 2. The source voltage would have to be decreased 3. The resistance of R1 would have to be decreased 4. One of the resistors would have to be removed from the circuit 3-22. If the circuit current is 2 amps, what is the total power used by each resistor?
Figure 3C.—Series circuit.
1. 2. 3. 4.
IN ANSWERING QUESTIONS 3-17 THROUGH 3-23, REFER TO FIGURE 3C. 3-17. What is the total circuit resistance (R)?
240 W 460 W 620 W 800 W
3-23. What is the total power used in the circuit if Es = 360 V?
1. 20! 2. 60! 3. 180! 4. 240!
1. 720 W 2. 1380 W 3. 1860 W 4. 2400 W
3-18. If the circuit current is 3 amps, what is the source voltage (Es)? 1. 60 V 2. 180 V 3. 540 V 4. 720 V
18
3-27. Which of the following terms applies to a circuit in which there is NO complete path for current?
3-24. When Kirchoff's voltage law is used to assign polarities to the voltage drop across a resistor, which of the following references is used to indicate the end of the resistor that the current enters? 1. 2. 3. 4.
1. 2. 3. 4.
Ground Neutral Negative Positive
Open Short Closed Grounded
3-28. A circuit in which the resistance is almost zero ohms is referred to by which of the following terms? 1. 2. 3. 4.
Open Short Closed Broken
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Figure 3D.—Multiple source circuit.
IN ANSWERING QUESTIONS 3-25 AND 3-26, REFER TO FIGURE 3D. 3-25. What is the effective source voltage? 1. 2. 3. 4.
15 V 25 V 50 V 75 V
3-26. What is the total amount and direction of current through R 3? 1. 1.0 A from Y to X 2. 1.0 A from X to Y 3. .33 A from Y to X 4. .33 A from X to Y
19
3-32. To achieve maximum power transfer in the circuit, which of the following conditions must be met? 1. 2. 3. 4.
Ri = RL Is = IL Es = EL Ks = KL
3-33. Maximum power is transferred from a source to a load when the value of the load resistance is of what value when compared to the source resistance? Figure 3E.—Series circuit and source resistance.
1. 2. 3. 4.
IN ANSWERING QUESTIONS 3-29 THROUGH 3-32, REFER TO FIGURE 3E. 3-29. If R2 has a short circuit, what will most likely happen to the circuit? 1. 2. 3. 4.
3-34. When maximum power is transferred from a source to a load, what is the efficiency of power transfer?
R1 will be destroyed Es will increase V will indicate O volts S1 will automatically open
1. 5% 2. 25% 3. 50% 4. 95%
3-30. What is the total voltage drop across Ri when the switch is closed?
3-35. A circuit consists of three resistors connected in parallel. R1 = 30 ohms, R2 = 15 ohms, and R 3 = 10 ohms. If the current through R2 = 4 amperes, what is the total source voltage?
1. 2.5 V 2. 6.5 V 3. 97.5 V 4. 100.0 V
1. 20 V 2. 60 V 3. 120 V 4. 220 V
3-31. What will the meter indicate with (a) S1 open, and (b) S1 closed? 1. 2. 3. 4.
(a) 100 V (a) 97.5 V (a) 100 V (a) 97.5 V
Equal Twice One-half Several times
(b) 100 V (b) 100 V (b) 97.5 V (b) 97.5 V
3-36. What is the relationship of total current to the current through a component in (a) a series circuit, and (b) a parallel circuit? 1. 2. 3. 4.
20
(a) Divides (a) Divides (a) Equals (a) Equals
(b) divides (b) equals (b) equals (b) divides
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3-37. If a current has a negative polarity when Kirchoff's current law is applied, which of the following, statements is true of the current? 1. 2. 3. 4.
It is from a battery It is from a generator It is entering a junction It is leaving a junction
3-38. Three equal resistors are connected in parallel and each resistor has an ohmic value of 300 ohms. What is the equivalent resistance of the circuit? 1. 2. 3. 4.
100! 150! 600! 900!
3-39. Three resistors with ohmic values of 120 ohms, 60 ohms, and 40 ohms are connected in parallel. What is the equivalent resistance of the circuit? 1. 2. 3. 4.
10! 20! 30! 40!
3-40. Two resistors with ohmic values of 90 ohms and 45 ohms are connected in parallel. What is the equivalent resistance of the circuit? 1. 2. 3. 4.
10! 20! 30! 40!
3-41. Which of the following terms describes a single resistor that represents a complex circuit? 1. 2. 3. 4.
Equal resistor Phantom resistor Schematic resistor Equivalent resistor
21
3-46. What is the total power consumed by R 3? 1. 108 W 2. 240 W 3. 360 W 4. 1200 W
Figure 3F.—Parallel circuit.
IN ANSWERING QUESTIONS 3-42 THROUGH 3-46, REFER TO FIGURE 3F. 3-42. What is the value of Es? 1. 2. 3. 4.
Figure 3G.—Series-parallel circuit.
336 V 300 V 240 V 120 V
IN ANSWERING QUESTIONS 3-47 THROUGH 3-49, REFER TO FIGURE 3G. 3-47. What is the value of the total resistance?
3-43. What is the value of current through R2? 1. 2. 3. 4.
1. 3.6! 2. 15! 3. 34! 4. 40!
1A 2A 3A 4A
3-48. What is the total power used in the circuit?
3-44. What is the approximate value of total resistance?
1. 22.5 W 2. 26.5 W 3. 60.0 W 4. 250.0 W
1. 8! 2. 37! 3. 112! 4. 257!
3-49. What is the total voltage drop across R 3?
3-45. What is the value of total power? 1. 2. 3. 4.
1. 8 V 2. 12 V 3. 18 V 4. 30 V
1.2 kW 1.5 kW 1.8 kW 2.0 kW
22
3-53. If an open occurs in a parallel branch of a circuit, what is the effect on (a) total resistance, and (b) total current? 1. 2. 3. 4.
(a) Increases (a) Increases (a) Decreases (a) Decreases
(b) decreases (b) increases (b) decreases (b) increases
3-54. If a short circuit occurs in a series portion of a circuit, what is the effect on (a) total resistance, and (b) total current? Figure 3H.—Complex circuit.
1. 2. 3. 4.
IN ANSWERING QUESTIONS 3-50 AND 3-51, REFER TO FIGURE 3H. 3-50. What is the value of total resistance?
(a) Increases (a) Increases (a) Decreases (a) Decreases
(b) decreases (b) increases (b) decreases (b) increases
3-55. If a short circuit occurs in a parallel branch of a circuit, what is the effect in (a) total resistance, and (b) total current?
1. 5! 2. 8! 3. 13! 4. 15!
1. 2. 3. 4.
3-51. If an equivalent resistor is used to represent the network of R1, R2, R 3, R 4, R5, and R6, what is the total voltage drop across this resistor?
(a) Increases (a) Increases (a) Decreases (a) Decreases
(b) decreases (b) increases (b) decreases (b) increases
3-56. If one branch of a parallel network shorts, what portion of the circuit current, if any, will flow through the remaining branches?
1. 8V 2. 26V 3. 52V 4. 60V
1. An amount determined by the combined resistance of the remaining branches 2. All 3. One-half 4. None
3-52. If an open occurs in a series portion of a circuit, what is the effect on (a) total resistance, and (b) total current? 1. (a) Decreases to zero (b) Becomes infinite 2. (a) Decreases to zero (b) Decreases to zero 3. (a) Becomes infinite (b) Becomes infinite 4. (a) Becomes infinite (b) Decreases to zero
3-57. Which of the following circuit quantities need NOT be known before designing a voltage divider? 1. 2. 3. 4.
23
The current of the source The voltage of the source The current requirement of the load The voltage requirement of the load
______________________________________ THE FOLLOWING INFORMATION IS TO BE USED IN ANSWERING QUESTIONS 3-58 THROUGH 3-60: A VOLTAGE DIVIDER IS REQUIRED TO SUPPLY A SINGLE LOAD WITH +150 VOLTS AND 300 MILLIAMPS OF CURRENT. THE SOURCE VOLTAGE IS 250 VOLTS. (HINT: DRAW THE CIRCUIT.) ______________________________________ 3-58. What should be the value of the bleeder current? 1. 3A 2. 300 mA 3. 30 mA 4. 3 mA
Figure 3I.—Voltage divider.
3-59. What should be the ohmic value of the bleeder resistor?
IN ANSWERING QUESTIONS 3-61 THROUGH 3-66, REFER TO FIGURE 3I.
1. 50 2. 500 3. 5k 4. 50 k
3-61. Why must the value of R1 be calculated first? 1. For convenience 2. The current through R2 depends on the value of R1 3. The voltage drop across R1depends on the value of load 1 4. In any circuit, values for resistors labeled R1 are calculated first
3-60. What is the value of total current? 1. 303 mA 2. 330 mA 3. 600 mA 4. 3300 mA
3-62. How is the current through R2 calculated? 1. By adding IR1 and the current requirement of load 1 2. By adding the current requirements of load 1 and load 2 3. By subtracting the current requirement of load 1 from the current requirement of load 2 4. By subtracting the current requirement of load 2 from the current requirement of load 1
24
3-67. A single voltage divider provides both negative and positive voltages from a single source voltage through the use of a
3-63. How is the voltage drop across R2 calculated? 1. By adding the voltage requirements of load 1 and load 2 2. By subtracting the voltage drops across R5 and R 3 from the source voltage 3. By subtracting the voltage requirement of load 1 from the voltage requirement of load 2 4. By subtracting the voltage requirements of load 1 and load 2 from the source voltage
1. ground between two of the dividing resistors 2. ground to the positive terminal of the source 3. ground to the negative terminal of the source 4. ground to the input of all loads requiring a negative voltage 3-68. Which of the following voltages are considered dangerous?
3-64. What is the minimum wattage rating required for R5? 1. 2. 3. 4.
1. 2. 3. 4.
1W 2W 1/2 W 1/4 W
3-69. If you discover a possible malfunction in an electric circuit, which of the following actions should be taken?
3-65. What is the total power supplied by the source? 1. 2. 3. 4.
Voltages above 115 volts only Voltages above 230 volts only Voltages above 450 volts only All voltages
1. Attempt repairs yourself 2. Report the malfunction to a qualified technician 3. Ignore the malfunction unless you were assigned to repair it 4. Secure the circuit immediately by removing power at the nearest switch
3.765 W 7.965 W 8.209 W 8.965 W
3-66. What is the purpose of using the seriesparallel network consisting of R 3, R4, and R5 in place of a single resistor?
3-70. If a person has stopped breathing and there is NO detectable heartbeat, who should perform CPR?
1. It provides the desired resistance with resistor values that are easily obtainable 2. It provides the close tolerance required for the circuit 3. It is more reliable than the use of a single resistor 4. It costs less by using three resistors of lower wattage rating than a single, large power resistor
1. 2. 3. 4.
25
Medical personnel only The first person on the scene Emergency Medical Technicians only Trained, qualified personnel only
NONRESIDENT TRAINING COURSE SEPTEMBER 1998
Navy Electricity and Electronics Training Series Module 2—Introduction to Alternating Current and Transformers NAVEDTRA 14174
DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.
Although the words “he,” “him,” and “his” are used sparingly in this course to enhance communication, they are not intended to be gender driven or to affront or discriminate against anyone.
DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.
PREFACE By enrolling in this self-study course, you have demonstrated a desire to improve yourself and the Navy. Remember, however, this self-study course is only one part of the total Navy training program. Practical experience, schools, selected reading, and your desire to succeed are also necessary to successfully round out a fully meaningful training program. COURSE OVERVIEW: To introduce the student to the subject of Alternating Current and Transformers who needs such a background in accomplishing daily work and/or in preparing for further study. THE COURSE: This self-study course is organized into subject matter areas, each containing learning objectives to help you determine what you should learn along with text and illustrations to help you understand the information. The subject matter reflects day-to-day requirements and experiences of personnel in the rating or skill area. It also reflects guidance provided by Enlisted Community Managers (ECMs) and other senior personnel, technical references, instructions, etc., and either the occupational or naval standards, which are listed in the Manual of Navy Enlisted Manpower Personnel Classifications and Occupational Standards, NAVPERS 18068. THE QUESTIONS: The questions that appear in this course are designed to help you understand the material in the text. VALUE: In completing this course, you will improve your military and professional knowledge. Importantly, it can also help you study for the Navy-wide advancement in rate examination. If you are studying and discover a reference in the text to another publication for further information, look it up.
1998 Edition Prepared by DSC Ray A. Jackson
Published by NAVAL EDUCATION AND TRAINING PROFESSIONAL DEVELOPMENT AND TECHNOLOGY CENTER
NAVSUP Logistics Tracking Number 0504-LP-026-8270
i
Sailor’s Creed “I am a United States Sailor. I will support and defend the Constitution of the United States of America and I will obey the orders of those appointed over me. I represent the fighting spirit of the Navy and those who have gone before me to defend freedom and democracy around the world. I proudly serve my country’s Navy combat team with honor, courage and commitment. I am committed to excellence and the fair treatment of all.”
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TABLE OF CONTENTS CHAPTER
PAGE
1. Concepts of Alternating Current ..............................................................................
1-1
2. Inductance ................................................................................................................
2-1
3. Capacitance ..............................................................................................................
3-1
4. Inductive and Capacitive Reactance.........................................................................
4-1
5. Transformers ............................................................................................................
5-1
APPENDIX I. Glossary..................................................................................................................
AI-1
II. Greek Alphabet....................................................................................................... AII-1 III. Square and Square Roots........................................................................................ AIII-1 IV. Useful AC Formulas............................................................................................... AIV-1 V. Trigonometric Functions ........................................................................................ AV-1 VI. Trigonometric Tables ............................................................................................. AVI-1 INDEX
.........................................................................................................................
iii
INDEX-1
NAVY ELECTRICITY AND ELECTRONICS TRAINING SERIES The Navy Electricity and Electronics Training Series (NEETS) was developed for use by personnel in many electrical- and electronic-related Navy ratings. Written by, and with the advice of, senior technicians in these ratings, this series provides beginners with fundamental electrical and electronic concepts through self-study. The presentation of this series is not oriented to any specific rating structure, but is divided into modules containing related information organized into traditional paths of instruction. The series is designed to give small amounts of information that can be easily digested before advancing further into the more complex material. For a student just becoming acquainted with electricity or electronics, it is highly recommended that the modules be studied in their suggested sequence. While there is a listing of NEETS by module title, the following brief descriptions give a quick overview of how the individual modules flow together. Module 1, Introduction to Matter, Energy, and Direct Current, introduces the course with a short history of electricity and electronics and proceeds into the characteristics of matter, energy, and direct current (dc). It also describes some of the general safety precautions and first-aid procedures that should be common knowledge for a person working in the field of electricity. Related safety hints are located throughout the rest of the series, as well. Module 2, Introduction to Alternating Current and Transformers, is an introduction to alternating current (ac) and transformers, including basic ac theory and fundamentals of electromagnetism, inductance, capacitance, impedance, and transformers. Module 3, Introduction to Circuit Protection, Control, and Measurement, encompasses circuit breakers, fuses, and current limiters used in circuit protection, as well as the theory and use of meters as electrical measuring devices. Module 4, Introduction to Electrical Conductors, Wiring Techniques, and Schematic Reading, presents conductor usage, insulation used as wire covering, splicing, termination of wiring, soldering, and reading electrical wiring diagrams. Module 5, Introduction to Generators and Motors, is an introduction to generators and motors, and covers the uses of ac and dc generators and motors in the conversion of electrical and mechanical energies. Module 6, Introduction to Electronic Emission, Tubes, and Power Supplies, ties the first five modules together in an introduction to vacuum tubes and vacuum-tube power supplies. Module 7, Introduction to Solid-State Devices and Power Supplies, is similar to module 6, but it is in reference to solid-state devices. Module 8, Introduction to Amplifiers, covers amplifiers. Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits, discusses wave generation and wave-shaping circuits. Module 10, Introduction to Wave Propagation, Transmission Lines, and Antennas, presents the characteristics of wave propagation, transmission lines, and antennas.
iv
Module 11, Microwave Principles, explains microwave oscillators, amplifiers, and waveguides. Module 12, Modulation Principles, discusses the principles of modulation. Module 13, Introduction to Number Systems and Logic Circuits, presents the fundamental concepts of number systems, Boolean algebra, and logic circuits, all of which pertain to digital computers. Module 14, Introduction to Microelectronics, covers microelectronics technology and miniature and microminiature circuit repair. Module 15, Principles of Synchros, Servos, and Gyros, provides the basic principles, operations, functions, and applications of synchro, servo, and gyro mechanisms. Module 16, Introduction to Test Equipment, is an introduction to some of the more commonly used test equipments and their applications. Module 17, Radio-Frequency Communications Principles, presents the fundamentals of a radiofrequency communications system. Module 18, Radar Principles, covers the fundamentals of a radar system. Module 19, The Technician's Handbook, is a handy reference of commonly used general information, such as electrical and electronic formulas, color coding, and naval supply system data. Module 20, Master Glossary, is the glossary of terms for the series. Module 21, Test Methods and Practices, describes basic test methods and practices. Module 22, Introduction to Digital Computers, is an introduction to digital computers. Module 23, Magnetic Recording, is an introduction to the use and maintenance of magnetic recorders and the concepts of recording on magnetic tape and disks. Module 24, Introduction to Fiber Optics, is an introduction to fiber optics. Embedded questions are inserted throughout each module, except for modules 19 and 20, which are reference books. If you have any difficulty in answering any of the questions, restudy the applicable section. Although an attempt has been made to use simple language, various technical words and phrases have necessarily been included. Specific terms are defined in Module 20, Master Glossary. Considerable emphasis has been placed on illustrations to provide a maximum amount of information. In some instances, a knowledge of basic algebra may be required. Assignments are provided for each module, with the exceptions of Module 19, The Technician's Handbook; and Module 20, Master Glossary. Course descriptions and ordering information are in NAVEDTRA 12061, Catalog of Nonresident Training Courses.
v
Throughout the text of this course and while using technical manuals associated with the equipment you will be working on, you will find the below notations at the end of some paragraphs. The notations are used to emphasize that safety hazards exist and care must be taken or observed.
WARNING
AN OPERATING PROCEDURE, PRACTICE, OR CONDITION, ETC., WHICH MAY RESULT IN INJURY OR DEATH IF NOT CAREFULLY OBSERVED OR FOLLOWED.
CAUTION
AN OPERATING PROCEDURE, PRACTICE, OR CONDITION, ETC., WHICH MAY RESULT IN DAMAGE TO EQUIPMENT IF NOT CAREFULLY OBSERVED OR FOLLOWED.
NOTE
An operating procedure, practice, or condition, etc., which is essential to emphasize.
vi
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assignment
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SUBMITTING YOUR ASSIGNMENTS To have your assignments graded, you must be enrolled in the course with the Nonresident Training Course Administration Branch at the Naval Education and Training Professional Development and Technology Center (NETPDTC). Following enrollment, there are two ways of having your assignments graded: (1) use the Internet to submit your assignments as you complete them, or (2) send all the assignments at one time by mail to NETPDTC.
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•
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•
you may submit your answers as soon as you complete an assignment, and you get your results faster; usually by the next working day (approximately 24 hours).
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In addition to receiving grade results for each assignment, you will receive course completion confirmation once you have completed all the
vii
PASS/FAIL ASSIGNMENT PROCEDURES
For subject matter questions:
If your overall course score is 3.2 or higher, you will pass the course and will not be required to resubmit assignments. Once your assignments have been graded you will receive course completion confirmation.
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If you receive less than a 3.2 on any assignment and your overall course score is below 3.2, you will be given the opportunity to resubmit failed assignments. You may resubmit failed assignments only once. Internet students will receive notification when they have failed an assignment--they may then resubmit failed assignments on the web site. Internet students may view and print results for failed assignments from the web site. Students who submit by mail will receive a failing result letter and a new answer sheet for resubmission of each failed assignment.
For enrollment, shipping, completion letter questions
grading,
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COMPLETION CONFIRMATION After successfully completing this course, you will receive a letter of completion.
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ERRATA If you are a member of the Naval Reserve, you will receive retirement points if you are authorized to receive them under current directives governing retirement of Naval Reserve personnel. For Naval Reserve retirement, this course is evaluated at 10 points. (Refer to Administrative Procedures for Naval Reservists on Inactive Duty, BUPERSINST 1001.39, for more information about retirement points.)
Errata are used to correct minor errors or delete obsolete information in a course. Errata may also be used to provide instructions to the student. If a course has an errata, it will be included as the first page(s) after the front cover. Errata for all courses can be accessed and viewed/downloaded at: http://www.advancement.cnet.navy.mil
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viii
Student Comments Course Title:
NEETS Module 2 Introduction to Alternating Current and Transformers
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14174
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NETPDTC 1550/41 (Rev 4-00)
ix
CHAPTER 1
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—REBALANCE ON L(Q) . In other words, if the dissipation of an inductor, as read on the D dial when using the Hay bridge (FUNCTION switch set to L(D) position), exceeds 0.05, then you should change to the Maxwell bridge (FUNCTION switch set to L(Q) position), which is discussed in the following paragraph. The loss factor of the inductor under test is then balanced in terms of the Q of the inductor. Q-15.
A Hay bridge measures inductance by comparing an inductor to what component?
1-20
MAXWELL BRIDGE The Maxwell bridge, shown in view C of figure 1-14, measures inductance by comparing it with a capacitance and (effectively) two resistances.] This bridge circuit is employed for measuring inductances having losses greater than 0.05 (expressed by the D dial reading). For such inductors it is necessary to introduce, in place of the series control (D dial), a new loss control (Q dial), which shunts the standard capacitor. This control, which becomes effective when the FUNCTION switch is turned to the L(Q) position, is conveniently calibrated in values of Q, the storage factor of the inductor under measurement. The balance for inductance is the same for either bridge circuit. This permits the use of the same markings on the RANGE switch for both the L(D) and L(Q) positions of the FUNCTION switch. REACTANCE MEASURING EQUIPMENT The reactance type of inductance measuring equipment makes use of the following principle: If an ac voltage of fixed frequency is applied across an inductor (and a resistor in series), the voltage drop produced across the reactance of the inductor by the resulting current flow is directly proportional to the value of the inductance. An inductance measurement using the reactance method is identical to capacitance measurements using the same method, except that current flow is directly proportional to the value of inductance, rather than inversely proportional as in the case of capacitance. It follows then that if a reactance-type capacitance measuring equipment is provided with a chart that converts the capacitance readings to equivalent inductance values and a proper range multiplying factor, the same test setup can be used to measure both capacitance and inductance. In practice, test equipment using the reactance method for capacitance measurements usually provides an inductance conversion chart. Because the current flowing through the inductance under test is directly proportional to the value of inductance, the reciprocals of the capacitance range multipliers must be used; for example, a multiplier of 0.1 becomes
and a multiplier of 100 becomes
The reactance-type equipment gives approximate values only. Like the analog multimeter, it is used only when portability and speed are more important than precision. If the ohmic resistance of the inductor is low, the inductance value obtained from the conversion chart can be used directly. If the ohmic value (as measured with an ohmmeter) is appreciable, a more accurate value of inductance can be obtained by use of the following formula:
1-21
Q-16.
Is the current flow through an inductor directly proportional or inversely proportional to its inductance value?
MEASUREMENT OF INDUCTANCE USING THE VTVM If you do not have a 250DE+1325 at your disposal, the inductance of a coil can be determined by using a vtvm and a decade resistance box, as shown in figure 1-15. In the following example the inductance of an unknown coil in the secondary winding of a 6.3-volt filament transformer will be determined with a vtvm and decade resistance box. The unknown coil must be connected in series with the decade resistance box. The voltage across the decade box and across the coil must be monitored as the decade box is adjusted. When equal voltages are reached, read the resistance of the decade box. Since the voltage across the inductor equals the voltage across the decade box, the XL of the coil must be equal to the resistance read on the decade box. For example, assume that the resistance reading on the decade box is 4 kilohms and the frequency is 60 hertz. This must mean that the XL of the coil is also equal to 4,000 ohms. The inductance formula L = XL IFDQEHXVHGWRILQGWKHLQGXFWDQFHRIWKHFRLOLQKHQULHV
Figure 1-15.—Determining inductance with a vtvm and decade resistance box.
1-22
SUMMARY This chapter has presented information on basic measurements. The information that follows summarizes the important points of this chapter. The five basic measurements are VOLTAGE, CURRENT, RESISTANCE, CAPACITANCE, and INDUCTANCE. The accuracy of all measurements depends upon YOUR SKILL as a technician and the accuracy of your TEST EQUIPMENT. Accuracy of different types of test equipment varies greatly and depends on design characteristics, tolerances of individual components, and YOUR KNOWLEDGE of test equipment applications. The METCAL program ensures that your calibrated test equipment meets established specifications. Most equipment technical manuals contain VOLTAGE CHARTS which list correct voltages that should be obtained at various test points. It is important to remember that the INPUT IMPEDANCE of your test equipment must be high enough to prevent circuit loading. When you are performing ac voltage measurements, an additional consideration that greatly affects the accuracy of your measurements is the FREQUENCY LIMITATIONS of your test equipment. Ac and dc CURRENT MEASUREMENTS can be performed using a wide variety of test equipment. Most current measurements require you to break the current path by unsoldering components and wires and inserting an ammeter in series with the current path. One alternative method is to compute (using OHM’S LAW) the current through a circuit by measuring the voltage drop across a known resistance. Another alternative is to use a CURRENT PROBE that requires no unsoldering.
1-23
When performing resistance measurements, your primary concerns are the RANGE AND DEGREE OF ACCURACY of your test equipment. In most instances, an analog multimeter is accurate enough to perform basic troubleshooting. When measuring extremely large resistances, you are sometimes required to use a MEGGER or a DIFFERENTIAL VOLTMETER.
1-24
When testing current-sensitive devices, you must be certain that the current produced by your test equipment does not exceed the current limitations of the device being tested. Capacitance and inductance measurements are seldom required in the course of troubleshooting. These measurements are usually performed with various types of BRIDGES or with a reactance type of measuring device. The bridge -measuring techniques are more commonly used and are more accurate than reactance types of measurements.
REFERENCES 8000A Digital Multimeter, NAVSEA 0969-LP-279-9010, Naval Sea Systems Command, Washington, D.C., undated. EIMB, Test Methods and Practices Handbook, NAVSEA 0967-LP-000-0130, Naval Sea Systems Command, Washington, D.C., 1980. EIBM, General, NAVSEA 0967-000-0100, Naval Sea Systems Command, Washington, D.C., 1983. Instruction Manual for Universal Impedance Bridge, Model 250DE, 13202, Electro Scientific Industries, 13900 N. W. Science Park Drive, Portland, Oregon 97229, March 1971.
1-25
Instruction Manual, Model 893A/AR AC-DC Differential Voltmeter, NAVSEA 0969-LP-279-7010, Naval Sea Systems Command, Washington, D.C., 1969. Operation and Maintenance Instruction, Current Tracer 547A, NAVAIR 16-45-3103, Naval Air Systems Command, Washington, D.C., 1979. Operation and Maintenance Instructions, Volt-Ohm-Milliammeter, 260 Series 6P, NAVSEA 0969-LP286-1010, Naval Sea Systems Command, Washington, D.C., 1974.
ANSWERS TO QUESTIONS Q1. THROUGH Q16. A-1.
Its calibration.
A-2.
10 to 1
A-3.
Increased input impedance, greater accuracy, and increased voltage range.
A-4.
Midscale.
A-5.
Accuracy and high input impedance.
A-6.
The range of frequencies that can accurately be measured.
A-7.
At least 60% of the vertical trace.
A-8.
Decreased internal meter resistance, greater accuracy, and greater current range.
A-9.
Current probes enable you to perform current measurements without disconnecting wires. Current probes are clamped around the insulated wire.
A-10.
By zeroing the meter with the test leads shorted.
A-11.
The current flow through the component is limited to 1 milliamp.
A-12.
True.
A-13.
Bridge type.
A-14.
Magnetic-metal core.
A-15.
A capacitor.
A-16.
Directly proportional.
1-26
CHAPTER 2
COMPONENT TESTING LEARNING OBJECTIVES Upon completion of this chapter, you will be able to do the following: 1. Explain the importance of testing individual electronic components. 2. Identify the various methods of testing electron tubes. 3. Identify the various methods of testing semiconductors. 4. Identify the various methods of testing integrated circuits. 5. Identify the various types of testing batteries and their characteristics. 6. Identify the various methods of testing rf attenuators and resistive loads. 7. Identify the various methods of testing fiber-optic devices.
INTRODUCTION TO COMPONENT TESTING It is imperative that you be able to troubleshoot an equipment failure to the component level. In the majority of cases, Navy technicians are expected to troubleshoot and identify faulty components. This chapter, "Component Testing," will acquaint you with alternative methods of testing various components and their parameters. A quick glance at the Navy’s mission and concept of operation explains why we, in most cases, must be able to troubleshoot to the faulty component level. A ship must be a self-sustaining unit when deployed. Storage space is a primary consideration on most ships and a limiting factor for storage of bulky items or electronic modules as ready spares. Therefore, it is practical to store only individual components common to a great number of equipment types. This of course, limits the larger replacement modules available to you during troubleshooting. Q-1.
Why are most ships limited in their ability to stock replacement modules for repair of electronic equipment?
TESTING ELECTRON TUBES In equipment that uses vacuum tubes, faulty tubes are responsible for more than 50% of all electronic equipment failures. As a result, testing of electronic tubes is important to you. You can determine the condition of a tube by substituting an identical tube known to be good for the questionable one. However, indiscriminate substitution of tubes is to be avoided for at least the following two reasons: (1) detuning of circuits may result and (2) a tube may not operate properly in a high-frequency circuit even though it performs well in a low-frequency circuit. Therefore, your knowledge of tube-testing devices and their limitations, as well as correct interpretation of the test results obtained, is indispensable for accurate and rapid maintenance.
2-1
Because the operating capabilities and design features of a tube are demonstrated by its electrical characteristics, a tube is tested by measuring those characteristics and comparing them with representative values established for that type of tube. Tubes that read abnormally high or low with respect to the standard are suspect. Practical considerations, which take into account the limitations of the tube test in predicting actual tube performance in a particular circuit, make it unnecessary to use complex and costly test equipment with laboratory accuracy. For most applications, testing of a single tube characteristic is good enough to determine tube performance. Some of the more important factors affecting the life expectancy of an electron tube are listed below: • The circuit function of the tube • Deterioration of the cathode coating • A decrease in emission of impregnated emitters in aging filament-type tubes • Defective seals that permit air to leak into the envelope and oxidize the emitting surface • Internal short circuits and open circuits caused by vibration or excessive voltage If the average receiving tube is not overdriven or operated continuously at maximum rating, it can have a life of at least 2,000 hours before the filament opens. Because of the expansion and contraction of tube elements during the process of heating and cooling, electrodes may lean or sag, which causes excessive noise or microphonics to develop. Other electron-tube defects are cathode-to-heater leakage and nonuniform electron emission of the cathode. These common tube defects contribute to about 50% of all electronic equipment failures. For this reason you should immediately eliminate any tube known to be faulty. However, avoid blind or random replacement of good tubes with fresh spares. The most common cause of tube failure is open filaments. Evidence of a tube defect is often obvious when the filament is open in glass-envelope tubes. You will also notice the brighter-than-normal cherry-red glow of the plate when the plate current is excessive. Also, when the tube becomes gassy or when arcing occurs between electrodes, you will probably have visual indication. Metal-encased tubes can be felt for warmth to determine if the heater is operating. You can tap a tube while it is operating in a circuit to reveal an aural indication of loose elements within the tube or microphonics, which are produced by loose elements. Most tubes are extremely fragile and subject to damage during shipment. When you replace a tube, never make the assumption that the new tube is good because it’s new. You should always test tubes before installing them. Q-2.
What is the most common cause of electron tube failure?
SUBSTITUTION METHODS Substituting with a tube known to be in good condition is a simple method of testing a questionable tube. However, in high-frequency circuits tube substitution should be carried out in a logical sequence. Replace tubes one at a time so that you can observe the effect of differences in interelectrode capacitance in the substituted tubes on tuned circuits. The tube substitution test method cannot be used to advantage in locating more than one faulty tube in a single circuit for two reasons: (1) If both an rf amplifier tube and IF amplifier tube are defective in a receiver, replacing either one will not correct the trouble; and (2) if all the tubes are replaced, there is no way for you to know what tubes were defective. Under these conditions, using test equipment designed for testing the quality of a tube saves you valuable time. Q-3.
What is the most accurate method of determining the condition of an electron tube?
2-2
NOTE ON SYMBOLS USED IN THE FOLLOWING SECTIONS: IEEE and ANSI standards (see inside front cover) are used to define various terms, such as anode (plate) current, anode voltage, and anode resistance. This book uses Ea for anode voltage, Ia for anode current, and ra for anode resistance. These are the same as E, Ip, and rp that you will see elsewhere. This module uses the terms anode and plate interchangeably. ELECTRON TUBE TESTERS A representative field type of electron tube tester designed to test all common low-power tubes is shown in figure 2-1. The tube test conditions are as close as possible to actual tube operating conditions and are programmed on a prepunched card. The card switch (S101, fig. 2-1) automatically programs the tube test conditions when it is actuated by a card. A card compartment on the front panel of the tester provides storage for the most frequently used cards. The cover of the tester (not shown) contains the operating instructions, the brackets for storing the technical manual, the power cord, the calibration cell for checking the meter and short tests, the calibration cards, the blank cards, and a steel hand punch.
Figure 2-1.—Electron tube tester.
Front Panel When a prepunched card is fully inserted into the card switch (S101), a microswitch is actuated that energizes a solenoid, causing the card switch contacts to complete the circuit. The card switch has 187 single-pole, single-throw switches arranged in 17 rows with 11 switches in each row. The card is used to push the switches closed; thus, the absence of a hole in the card is required to actuate a switch.
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The meter (M301) contains four scales. The upper scale is graduated from 0 to 100 for direct numerical readings. The three lower scales, numbered 1, 2, and 3, are read for LEAKAGE, QUALITY, and GAS, respectively. Each numbered scale includes green and red areas marked GOOD and REPLACE. Inside a shield directly in front of the meter are five neon lamps (DS301 through DS305), which indicate shorts between tube elements. The number 2 pushbutton (MP6) is used for transconductance, emission, and other quality tests (described later). The number 3 pushbutton (MP7) is used to test for the presence of gas in the tube envelope. The number 4 pushbutton (MP8) is used for tests on dual tubes. A neon lamp (DS203) lights when pushbutton number 4 is to be used. Eleven tube test sockets are located on the panel, plus tube pin straighteners for the 7- and 9-pin miniature tubes. The power ON-OFF spring-return toggle switch (S105) turns the tester on by energizing a line relay. The pilot light (DS107) lights when this relay closes. Above the power ON-OFF switch are five fuses. Fuses F101, F201, and F202 protect circuits in the tester not protected by other means and have neon lamps to indicate when they have blown. Fuses F102 and F103 protect both sides of the power line. Auxiliary Compartment A group of auxiliary controls covered by a hinged panel is used for special tests and for calibration of the tester. Two of these controls, labeled SIGNAL CAL (R152 and R155, fig. 2-2), are used with special test cards for adjusting the regulation and amplitude of the signal voltage. A pushbutton labeled CATH ACT (S302D) is used for making cathode activity tests. When this button is pressed, DS106 on the front panel (fig. 2-1) lights, and the filament voltage of the tube under test is reduced by 10%. Results of the test are read as a change in reading on the numerical meter scale.
Figure 2-2.—Auxiliary compartment.
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Pushbutton S302E and potentiometers R401 and R405 (fig. 2-2) are used for balancing the transconductance (Gm) bridge circuit under actual tube operating current. Pressing S302E removes the grid signal and allows a zero balance to be made with one potentiometer or the other, depending upon whether the tube under test is passing high or low plate current. Lamp DS108 on the front panel lights when S302E is pressed. Pushbutton S302C is used for checking grid-to-cathode shorts at a sensitivity much higher than the normal tests. Results of this test are indicated by the short test lamps on the front panel. Certain special tests require the use of a continuously adjustable auxiliary power supply. By pressing pushbutton S302B, you may use meter M301 to read the voltage of the auxiliary power supply on meter M301. This voltage may be adjusted by the use of the potentiometer R142. The rest of the potentiometer controls are calibration controls and are adjusted by the use of special calibration cards and a calibration test cell. All circuits in the tester, except the filament supply, are electronically regulated to compensate for line voltage fluctuations. The filament supply voltage is adjusted by pressing pushbutton S302A and rotating the filament standardization adjustment switch S106 until meter M301 reads midscale. Program Cards The circuits to be used in testing are selected by a prepunched card. These cards are made of tough vinyl plastic material. The tube numbers are printed in color on the tabs of the cards and also at the edge of the card for convenience in filing. A special card is provided to use as a marker when a card is removed for use. Blank cards are provided so that additional test cards may be punched for new tubes that are developed or to replace cards that have become unserviceable. Operation Before operating the tester for the first time, and periodically thereafter, you should calibrate it using the calibration test cards as described in the equipment technical manual. NORMAL TESTS.—The tester is equipped with a three-conductor power cord, one wire of which is chassis ground. It should be plugged into a grounded 105- to 125-volt, 50- to 400-hertz outlet. Before operating the tester, open the auxiliary compartment (fig. 2-2) and ensure that the FILAMENT STD ADJ and the Gm BAL knobs are in the NOM position. The GRID SIG and CATH ACT buttons (S302E and S302D) should be up and lamps DS108 and DS106 on the front panel should be out. Turn on the tester and allow it to warm up for 5 to 10 minutes, then press the CARD REJECT KNOB (fig. 2-1) down until it locks. If a nontest card is installed in the card switch, remove it. This card is used to keep the switch pins in place during shipment and should be inserted before transporting the tester. Plug the tube to be tested into its proper socket. (Use the pin straighteners before plugging in 7- and 9-pin miniature tubes.) Select the proper card or cards for the tube to be tested. Insert the card selected into the slot in the card switch until the CARD REJECT KNOB pops up. The card will operate the tester only if it is fully inserted and the printing is up and toward the operator. Do not put paper or objects other than program cards into the card switch, because they will jam the switch contacts. If the overload shuts off the tester when the card is inserted in the switch, check to see that the proper card is being used for the tube under test and that the tube under test has a direct interelement short.
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As soon as the card switch is actuated, the tube under test is automatically subjected to an interelement short test and a heater-to-cathode leakage test. A blinking or steady glow of any of the short test lamps is an indication of an interelement short. If the short test lamps remain dark, no interelement shorts exist within the tube. If a short exists between two or more elements, the short test lamp or lamps connected between these elements remain dark, and the remaining lamps light. The abbreviations for the tube elements are located on the front panel just below the short test shield so that the neon lamps are between them. This enables the operator to tell which elements are shorted. Heater-to-cathode shorts are indicated as leakage currents on the #1 meter scale. If the meter reads above the green area, the tube should be replaced. A direct heater-to-cathode short causes the meter to read full scale. To make the QUALITY test, push the number 2 button (fig. 2-1) and read the number 2 scale on meter M301 to determine if the tube is good. (This test may be one of various types, such as transconductance, emission, plate current, or voltage drop, depending upon the type of tube under test.) To test the tube for GAS, press the number 3 button and read the number 3 meter scale. The number 2 button also goes down when number 3 is pressed. If a dual tube having two identical sections is being tested, the neon lamp (DS203) will light, indicating that both sections of the tube may be tested with one card. To do this, check the tube for shorts, leakage, quality, and gas as described previously; then hold down button number 4 and repeat these tests to test the second section of the tube. Dual tubes with sections that are not identical require two cards for testing. A second card is also provided to make special tests on certain tubes. AUXILIARY TEST.—As mentioned previously, two special tests (cathode activity and sensitive grid shorts) may be made by use of controls located in the auxiliary compartment (fig. 2-2). The cathode activity test (CATH ACT) is used to indicate the amount of useful life remaining in the tube. By reducing the filament voltage by 10 percent and allowing the cathode to cool off slightly, the ability of the cathode as an emitter of electrons can be estimated. This test is made in conjunction with the normal quality test. To make the CATH ACT test, allow the tube under test to warm up, press button number 2 (fig. 2-1), and note the reading of scale number 2 on meter M301. Note also the numerical scale reading on M301. Next, lock down the CATH ACT button (fig. 2-2), wait for about 1.5 minutes, then press button number 2 (fig. 2-1) again and note the numerical and number 2 scale readings on meter M301. The tube should be replaced if the numerical reading on M301 differs from the first reading by more than 10 percent or if the reading is in the red area on the number 2 scale. It is sometimes desirable to check certain tubes for shorts at a sensitivity greater than normal. To make the SENSITIVE GRID SHORTS test, push S302C (fig. 2-2) and note if any short test lamps (fig. 2-1) light. HIGH-POWER HF AMPLIFIER TUBE TESTS You normally test high-power amplifier tubes, which operate in the low-to-high frequency range, in the transmitter in which they are to be used. When you operate the tube in a transmitter, its condition can be determined by using built-in meters to measure the grid current, plate current, and power output and comparing those values with those obtained when using tubes known to be good. Q-4.
Normally, how are high-power rf tubes tested?
Klystron Tube Tests You can check low-power klystron tubes for gas, frequency of the output signal, and output power by placing them in the equipment where they are to be used. You measure the beam current, output
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frequency, and output power with the transmitter’s built-in test equipment. You can check the output of klystrons used as receiver local oscillators by measuring the current in the crystal mixer unit. Klystron tubes that remain inoperative for more than 6 months may become gassy. This condition occurs in klystrons installed in stored or spare equipment as well as in klystrons stored as stock supplies. Operation of a gassy klystron at its rated voltages will ionize the gas molecules and may cause excessive beam current to flow. This excessive beam current may shorten the life of the klystron or produce immediate failure. You can detect gas in a klystron tube by setting the applied reflector voltage to zero and slowly increasing the beam voltage while observing a meter that indicates the beam current excessive beam current for a specific value of voltage indicates that the tube is gassy. A gassy klystron tube can usually be restored to serviceable condition if you temporarily operate it at reduced beam voltage. Eight hours or more of reduced voltage operation may be required for klystrons that have been inoperative for periods in excess of 6 months. The beam current is also an indication of the power output of the klystron. As klystrons age they normally draw less beam current; when this current decreases to a minimum value for a specific beam voltage, the tube must be replaced. You can usually determine the power output of transmitter klystrons by measuring the transmitter power output during equipment performance checks. Q-5.
What should you do if a klystron becomes gassy?
Traveling-Wave Tube You can usually test a traveling-wave tube (twt) in the equipment in which it is used. When the twt is installed, you can usually measure the collector current and voltage and check the power output for various inputs. Any deviation greater than 10% from normal specifications may be considered to be an indication of a defective tube. Most amplifiers are supplied with built-in panel meters and selector switches so that the cathode, anode, helix, focus, and collector currents may be measured. Thus, continuous monitoring of amplifier operation and tube evaluation is possible. Adjustments usually are provided for you to set the helix, grid bias, and collector voltages for optimum operation. If variation of these controls will not produce normal currents and if all voltages are normal, you should consider the tube to be defective and replace it with a new tube or one known to be in good operating condition. To avoid needless replacement of tubes, however, you should make an additional check by measuring the input power and output power and determining the tube gain. If, with normal operating conditions, the gain level drops below the minimum indicated value in the equipment technical manual, the tube is defective. Q-6.
When used as an amplifier, what is the best indication that a twt is operating properly?
In the absence of special field-test sets, you may construct a laboratory test mock-up similar to that shown in figure 2-3. Because of the variations in power and gain between tubes and the large frequency ranges offered, we can illustrate only a general type of equipment. The equipment you select must have the proper range, impedance, and attenuation to make the test for a specific type of twt. To make gain measurements, you turn the switch shown in figure 2-3 to position 1 and set the precision attenuator to provide a convenient level of detector output. Then turn the switch to position 2 and insert attenuation until the detector output level is identical to that obtained without the twt in the circuit. The gain of the traveling-wave tube is equal to the amount of added attenuation.
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Figure 2-3.—Traveling-wave tube test arrangement.
When you use the twt as an oscillator, failure of the tube to break into oscillations when all other conditions are normal usually indicates a defective tube. In the case of a tube used as a receiving amplifier, an increase of noise with a normal or reduced output can indicate that the tube is failing but is still usable. All the general rules applying to klystron tubes mentioned previously are also applicable to the twt. Magnetron Tube Tests You test a magnetron tube while it is in the transmitter equipment in which it is to be used. When you install the magnetron in the transmitter, the condition of the tube can be determined by the normal plate-current measurement and the power, frequency spectrum, and standing-wave-ratio tests of the output signal. An unusual value for any of these measurements may indicate a defective tube. Crossed-Field Amplifier You usually test a crossed-field amplifier (cfa) tube while it is in the equipment in which it is used. Like the klystron, if you do not operate the cfa for more than a few months, the tube may become gassy. If a cfa tube is suspected of being gassy, we recommend that you consult the technical manual for the particular piece of equipment in which the crossed-field amplifier is used.
TESTING SEMICONDUCTORS Unlike vacuum tubes, transistors are very rugged in that they can tolerate vibration and a rather large degree of shock. Under normal operating conditions, they will provide dependable operation for a long period of time. However, transistors are subject to failure when they are subjected to relatively minor overloads. Crystal detectors are also subject to failure or deterioration when subjected to electrical overloads and will deteriorate from a long period of normal use. To determine the condition of semiconductors, you can use various test methods. In many cases you may substitute a transistor of known good quality for a questionable one to determine the condition of a suspected transistor. This method is highly accurate and sometimes efficient. However, you should avoid indiscriminate substitution of semiconductors in critical circuits. When transistors are soldered into equipment, substitution becomes impractical - generally, you should test these transistors while they are in their circuits. Q-7.
What is the major advantage of a transistor over a tube?
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Since certain fundamental characteristics indicate the condition of semiconductors, test equipment is available that allows you to test these characteristics with the semiconductors in or out of their circuits. Crystal-rectifier testers normally allow you to test only the forward-to-reverse current ratio of the crystal. Transistor testers, however, allow you to measure several characteristics, such as the collector leakage current (Ic), collector to base current gain (β), and the four-terminal network parameters. The most useful test characteristic is determined by the type of circuit in which the transistor will be used. Thus, the alternating-current beta measurement is preferred for ac amplifier or oscillator applications; and for switching-circuit applications, a direct-current beta measurement may prove more useful. Many common transistors are extremely heat sensitive. Excess heat will cause the semiconductor to either fail or give intermittent operation. You have probably experienced intermittent equipment problems and know them to be both time consuming and frustrating. You know, for example, that if a problem is in fact caused by heat, simply opening the equipment during the course of troubleshooting may cause the problem to disappear. You can generally isolate the problem to the faulty printed-circuit board (pcb) by observing the fault indications. However, to further isolate the problem to a faulty component, sometimes you must apply a minimal amount of heat to the suspect pcb by carefully using a low wattage, heat shrink gun; an incandescent drop light; or a similar heating device. Be careful not to overheat the pcb. Once the fault indication reappears, you can isolate the faulty component by spraying those components suspected as being bad with a nonconductive circuit coolant, such as Freon. If the alternate heating and cooling of a component causes it to operate intermittently, you should replace it. Q-8.
Name two major disadvantages of transistors.
TRANSISTOR TESTING When trouble occurs in solid-state equipment, you should first check power supplies and perform voltage measurements, waveform checks, signal substitution, or signal tracing. If you isolate a faulty stage by one of these test methods, then voltage, resistance, and current measurements can be made to locate defective parts. When you make these measurements, the voltmeter impedance must be high enough that it exerts no appreciable effect upon the voltage being measured. Also, current from the ohmmeter you use must not damage the transistors. If the transistors are not soldered into the equipment, you should remove the transistors from the sockets during a resistance test. Transistors should be removed from or reinserted into the sockets only after power has been removed from the stage; otherwise damage by surge currents may result. Transistor circuits, other than pulse and power amplifier stages, are usually biased so that the emitter current is from 0.5 milliampere to 3 milliamperes and the collector voltage is from 3 to 15 volts. You can measure the emitter current by opening the emitter connector and inserting a milliammeter in series. When you make this measurement, you should expect some change in bias because of the meter resistance. You can often determine the collector current by measuring the voltage drop across a resistor in the collector circuit and calculating the current. If the transistor itself is suspected, it can be tested by one or more of the methods described below. Resistance Test You can use an ohmmeter to test transistors by measuring the emitter-collector, base-emitter, and base-collector forward and reverse resistances. A back-to-forward resistance ratio on the order of 100 to 1 or greater should be obtained for the collector-to-base and emitter-to-base measurements. The forward and reverse resistances between the emitter and collector should be nearly equal. You should make all three measurements for each transistor you test, because experience has shown that transistors can develop shorts between the collector and emitter and still have good forward and reverse resistances for the other two measurements. Because of shunting resistances in transistor circuits, you will normally have
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to disconnect at least two transistor leads from the associated circuit for this test. Exercise caution during this test to make certain that current during the forward resistance tests does not exceed the rating of the transistor — ohmmeter ranges requiring a current of more than 1 milliampere should not be used for testing transistors. Many ohmmeters are designed such that on the R × 1 range, 100 milliamperes or more can flow through the electronic part under test. For this reason, you should use a digital multimeter. Be sure you select a digital multimeter that produces enough voltage to properly bias the transistor junctions. Q-9.
When you are using an ohmmeter to test a transistor, what range settings should be avoided?
Transistor Testers Laboratory transistor test sets are used in experimental work to test all characteristics of transistors. For maintenance and repair, however, it is not necessary to check all of the transistor parameters. A check of two or three performance characteristics is usually sufficient to determine whether a transistor needs to be replaced. Two of the most important parameters used for transistor testing are the transistor current gain (beta) and the collector leakage or reverse current (Ic). The semiconductor test set (fig. 2-4) is a rugged, field type of tester designed to test transistors and semiconductor diodes. The set measures the beta of a transistor, resistance appearing at the electrodes, reverse current of a transistor or semiconductor diode, shorted or open conditions of a diode, forward transconductance of a field-effect transistor, and condition of its own batteries.
Figure 2-4.—Semiconductor test set.
In order to assure that accurate and useful information is gained from the transistor tester, the following preliminary checks of the tester should be made prior to testing any transistors. With the POLARITY switch (fig. 2-4) in the OFF position, the meter pointer should indicate exactly zero. (When required, rotate the meter adjust screw on the front of the meter to fulfill this requirement.) When measurements are not actually being made, the POLARITY switch must always be left in the OFF position to prevent battery drain.
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Always check the condition of the test set batteries by disconnecting the test set power cord, placing the POLARITY switch in the PNP position and placing the FUNCTION switch first to BAT.1, then to BAT.2. In both BAT positions the meter pointer should move so as to indicate within the red BAT range. BETA MEASUREMENTS.—If the transistor is to be tested out of the circuit, plug it into the test jack located on the right-hand side below the meter shown in figure 2-4. If the transistor is to be tested in the circuit, it is imperative that at least 300 ohms exist between E-B, C-B, and C-E for accurate measurement. Initial settings of the test set controls are as follows: 1. FUNCTION switch to BETA 2. POLARITY switch to PNP or NPN (dependent on type of transistor under test) 3. RANGE switch to X10 4. Adjust METER ZERO for zero meter indication (transistor disconnected) NOTE: The POLARITY switch should remain OFF while the transistor is connected to or disconnected from the test set. If you determine that the beta reading is less than 10, reset the RANGE switch to X1 and reset the meter to zero. After connecting the yellow test lead to the emitter, the green test lead to the base, and the blue test lead to the collector, plug the test probe (not shown) into the jack located at the lower right-hand corner of the test set. When testing grounded equipment, unplug the 115 vac line cord and use battery operation. The beta reading is attained by multiplying the meter reading times the RANGE switch setting. Refer to the transistor characteristics book provided with the tester to determine if the reading is normal for the type of transistor under test. ELECTRODE RESISTANCE MEASUREMENTS.—Connect the in-circuit probe test leads to the transistor with the yellow lead to the emitter, the green lead to the base, and the blue lead to the collector. Set the FUNCTION switch to the OHMS E-B position, and read the resistance between the emitter and base electrode on the center scale of the meter. To read the resistance between the collector and base and the collector and emitter, set the FUNCTION switch to OHMS C-B and OHMS C-E. These in-circuit electrode resistance measurements are used to correctly interpret the in-circuit beta measurements. The accuracy of the BETA X1, X10 range is ±15 percent only when the emitter-to-base load is equal to or greater than 300 ohms. Ic MEASUREMENTS.—Adjust the METER ZERO control for zero meter indication. Plug the transistor to be tested into the jack or connect test leads to the device under test. Set the PNP/NPN switch to correspond with the transistor under test. Set the FUNCTION switch to Ic and the RANGE switch to X0.1, X1, or X10 as specified by the transistor data book for allowable leakage. Read the amount of leakage on the bottom scale, and multiply this by the range setting figure as required. DIODE MEASUREMENTS.—Diode qualitative in-circuit measurements are attained by connecting the green test lead to the cathode and the yellow test lead to the anode. Set the FUNCTION switch to DIODE IN/CKT and the RANGE switch to X1. (Ensure that the meter has been properly zeroed on this scale.) If the meter reads down scale, reverse the POLARITY switch. If the meter reads less than midscale, the diode under test is either open or shorted. The related circuit impedance of this test is less than 25 ohms.
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PRECAUTIONS.—Transistors, although generally more rugged mechanically than electron tubes, are susceptible to damage by excessive heat and electrical overload. The following precautions should be taken in servicing transistorized equipment: 1. Test equipment and soldering irons must be checked to make certain that there is no leakage current from the power source. If leakage current is detected, isolation transformers must be used. 2. Ohmmeter ranges that require a current of more than 1 milliampere in the test circuit are not to be used for testing transistors. 3. Battery eliminators should not be used to furnish power for transistor equipment because they have poor voltage regulation and, possibly, high ripple voltage. 4. The heat applied to a transistor, when soldered connections are required, should be kept to a minimum by using a low-wattage soldering iron and heat shunts (such as long-nose pliers) on the transistor leads. 5. All circuits should be checked for defects before a transistor is replaced. 6. The power should be removed from the equipment before replacing a transistor or other circuit part. 7. When working on equipment with closely spaced parts, you will find that conventional test probes are often the cause of accidental short circuits between adjacent terminals. Momentary short circuits, which rarely cause damage to an electron tube, may ruin a transistor. To avoid accidental shorts, a test probe can be covered with insulation for all but a very short length of the tip. Electrostatic Discharge Sensitive (ESDS) Care Devices that are sensitive to electrostatic discharge (ESD) require special handling. You can readily identify ESD-sensitive (ESDS) devices by the symbols shown in figure 2-5. Static electricity is created whenever two substances (solid or fluid) are rubbed together or separated. The rubbing or separating of substances causes the transfer of electrons from one substance to the other; one substance then becomes positively charged, and the other becomes negatively charged. When either of these charged substances comes in contact with a grounded conductor, an electrical current flows until that substance is at the same electrical potential as ground.
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Figure 2-5.—Warning symbols for ESDS devices.
You commonly experience static build-up during the winter months when you walk across a vinyl or carpeted floor. (Synthetics, especially plastics, are excellent generators of static electricity.) If you then touch a doorknob or any other conductor, an electrical arc to ground may result, and you may receive a slight shock. For you to experience such a shock, the electrostatic potential created must be 3,500 to 4,000 volts. Lesser voltages, although present and similarly discharged, normally are not apparent to your nervous system. Some typical measured static charges caused by various actions are shown in table 2-1.
Table 2-1.—Typical Measured Static Charges (in volts)
ITEM
RELATIVE HUMIDITY LOW (10 - 20%) HIGH (65 - 90%) 35,000V 1,500V 12,000V 250V 6,000V 100V 7,000V 600V 20,000V 1,200V 18,000 V 1,500 V
WALKING ACROSS CARPET WALKING OVER VINYL FLOOR WORKER AT BENCH VINYL ENVELOPES FOR WORK INSTRUCT. POLY BAG PICKED UP FROM BENCH WORK CHAIR PADDED WITH URETHANE FORM
Q-10.
At approximately what minimum voltage potential should you be able to feel an electrostatic discharge?
Metal oxide semiconductor (MOS) devices are the most susceptible to damage from ESD. For example, an MOS field-effect transistor (MOSFET) can be damaged by a static voltage potential of as little as 35 volts. Commonly used discrete bipolar transistors and diodes (often used in ESD-protective circuits), although less susceptible to ESD, can be damaged by voltage potentials of less than 3,000 electrostatic volts. Damage does not always result in sudden device failure but sometimes results in device degradation and early failure. Table 2-1 clearly shows that electrostatic voltages well in excess of 2-13
3,000 volts can be easily generated, especially under low-humidity conditions. ESD damage of ESDS parts or circuit assemblies is possible whenever two or more pins of any of these devices are electrically exposed or have low impedance paths. Similarly, an ESDS device in a printed-circuit board or even in another pcb that is electrically connected in a series can be damaged if it provides a path to ground. ESD damage can occur during the manufacture of equipment or during the servicing of the equipment. Damage can occur anytime devices or assemblies are handled, replaced, tested, or inserted into a connector. Q-11.
A MOSFET can be damaged by an electrostatic discharge at approximately what minimum potential?
ESD-sensitive devices can be grouped by their sensitivity to ESD. Semiconductors fall within the following categories: • VERY SENSITIVE DEVICES. These include MOS and CMOS devices without input diode protection circuitry on all input circuits; dielectrically isolated semiconductors with internal capacitor contacts connected to external pins; and microcircuits using N + guard-ring construction (with metalization crossing over the guard ring). • SENSITIVE DEVICES. These include all low-power Schottky-barrier and Schottky-TTL devices; all ECL devices; high input-impedance linear microcircuits; all small-signal transistors that operate at 500 MHz or higher; all discrete semiconductors that use silicon dioxide to insulate metal paths over other active areas; MOS or CMOS devices with input diode protection on all input terminals; junction field-effect transistors; and precision resistive networks. • MODERATELY SENSITIVE DEVICES. These include all microcircuits and small-signal discrete semiconductors with less than 10 watts dissipation at 25º C, and thick-film resistors. The following procedure is an example of some of the protective measures used to prevent ESD damage: 1.
Before servicing equipment, you should be grounded to discharge any static electricity from your body. This can be accomplished with the use of a test lead (a single-wire conductor with a series resistance of 1 megohm) equipped with alligator clips on each end. After the equipment has been completely de-energized, one clip end is connected to the grounded equipment frame; the other clip end is touched with your bare hand. Figure 2-6 shows a more refined ground strap, which frees both hands for work.
NOTE: When wearing a wrist strap, you should never use ac-powered test equipment because of your increased chance of receiving an electrical shock.
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Figure 2-6.—ESD wrist strap.
2.
Equipment technical manuals and packaging material should be checked for ESD warnings and instructions.
3.
Prior to opening an electrostatic unit package of an ESDS device or assembly, clip the free end of the grounded test lead to the package. This will cause any static electricity that may have built up on the package to discharge. The other end remains connected to the equipment frame or other ESD ground. Keep the unit package grounded until the replacement device or assembly is placed in the unit package.
4.
Minimize handling of ESDS devices and assemblies. Keep replacement devices or assemblies, with their connector-shorting bars, clips, and so forth, intact in their electrostatic-free packages until needed. Place removed repairable ESDS devices or assemblies, with their connector shorting bars or clips installed, in electrostatic-free packages as soon as they are removed from the equipment. ESDS devices or assemblies should be transported and stored only in protective packaging.
5.
Always avoid unnecessary physical movement, such as scuffing the feet, when handling ESDS devices or assemblies. Such movement will generate additional charges of static electricity.
6.
When removing or replacing an ESDS device or assembly in the equipment, hold the device or assembly through the electrostatic-free wrap if possible. Otherwise, pick up the device or assembly by its body only. DO NOT TOUCH component leads, connector pins, or any other electrical connections or paths on boards, even though they are covered by conformal coating.
7.
Do not permit ESDS devices or assemblies to come in contact with clothing or other ungrounded materials that could have an electrostatic charge. The charges on a nonconducting material are not equal. A plastic storage bag may have a −10,000 volt potential one-half inch from a +15,000 volt potential, with many other such charges all over the bag. Placing a circuit card inside the bag allows the charges to equalize through the pcb conductive paths and components, thereby causing failures. Do not hand an ESDS device or assembly to another person until the device or assembly is protectively packaged.
8.
When moving an ESDS device or assembly, always touch (with your bare skin) the surface on which it rests for at least 1 second before picking it up. Before placing it on any surface, touch the
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surface with your free hand for at least 1 second. The bare skin contact provides a safe discharge path for electrostatic charges accumulated while you are moving around. 9.
While servicing equipment containing ESDS devices, do not handle or touch materials such as plastic, vinyl, synthetic textiles, polished wood, fiber glass, or similar items that could create static charges; or, be sure to repeat the grounding action with the bare hands after contacting these materials. These materials are prime electrostatic generators.
10.
If possible, avoid repairs that require soldering at the equipment level. Soldering irons must have heater and tip assemblies grounded to ac electrical ground. Do not use ordinary plastic solder suckers (special antistatic solder suckers are commercially available).
11.
Ground the leads of test equipment momentarily before you energize the test equipment and before you probe ESDS items.
Q-12.
Why should you avoid using ac-powered test equipment when wearing a wrist strap?
DIODE TESTING Because of the reliability of semiconductor devices, servicing techniques developed for transistorized equipment differ from those used for electron-tube circuits. Electron tubes are usually considered to be the circuit component most susceptible to failure and are normally the first to be tested. Transistors, however, are capable of operating in excess of 30,000 hours at maximum ratings without appreciable degradation. They are often soldered into equipment in the same manner as resistors and capacitors. Substitution of a diode or transistor known to be in good condition is a simple method of determining the quality of a questionable semiconductor device. You should use this technique only after voltage and resistance measurements indicate that no circuit defect exists that might damage the substituted semiconductor device. If more than one defective semiconductor is present in the equipment section where trouble has been localized, substitution becomes cumbersome since several semiconductors may have to be replaced before the trouble is corrected. To determine which stages failed and which semiconductors are not defective, you must test all of the removed semiconductors. This can be accomplished by observing whether the equipment operates correctly as each of the removed semiconductor devices is reinserted into the equipment. Q-13.
Prior to substituting a diode, what measurements should you take to determine its condition?
DIODE TESTERS Diodes, such as general-purpose germanium and silicon diodes, power silicon diodes, and microwave silicon diodes, are most effectively tested under actual operating conditions. However, rectifier testers are available for you to determine direct-current characteristics that provide an indication of diode quality. Rf Diode Test A common type of diode test set is a combination ohmmeter-ammeter. You can make measurements of forward resistance, back resistance, and reverse current with this equipment. You can determine the condition of the rectifier under test by comparing its actual values with typical values obtained from test information furnished with the test set or from the manufacturer’s data sheets. Comparing the diode’s back and forward resistance at a specified voltage provides you with a rough indication of the rectifying property of a diode. A typical back-to-forward resistance ratio is on the order of 10 to 1, and a forwardresistance value of 50 to 80 ohms is common.
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Switching Diode Test To effectively test diodes used for computer applications, you must obtain back-resistance measurements at a large number of different voltage levels. This can be done efficiently by using a dynamic diode tester in conjunction with an oscilloscope, which is used to display the diode’s backcurrent-versus-voltage curve. You can easily interpret diode characteristics, such as flutter, hysteresis, and negative resistance, through use of the dynamic current and voltage display. DIODE CHARACTERISTIC GRAPHICAL DISPLAY You can use an oscilloscope to graphically display the forward- and back-resistance characteristics of a diode. A test circuit used in conjunction with an oscilloscope is shown in figure 2-7. This circuit uses an audio-signal generator as the test signal. It should be adjusted for an approximate 2-volt, 60-hertz signal, as measured across R1.
Figure 2-7.—Display circuit used with an oscilloscope.
The test signal you apply to the diode is also connected to the horizontal input of the oscilloscope. The horizontal sweep will then display the voltage applied to the diode under test. The voltage developed across current-measuring resistor R2 is applied to the vertical input of the oscilloscope. Since this voltage is proportional to the current through the diode under test, the vertical deflection will indicate diode current. The resulting oscilloscope trace will be similar to the curve shown in figure 2-8.
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Figure 2-8.—Typical characteristic curve of a silicone diode.
Reverse Voltage-Current Analysis You can make an analysis of the reverse voltage-current portion of the characteristic curve for a diode with the method described above or with a diode test set. This test is very important for diodes used in computer applications, where stability of operation is essential. Various diode conditions that may be detected by this test are shown in figure 2-9, view A, view B, view C, and view D.
Figure 2-9A.—Diode reverse current-voltage characteristics. GOOD DIODE TRACE.
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Figure 2-9B.—Diode reverse current-voltage characteristics. HYSTERESIS CHARACTERISTIC.
Figure 2-9C.—Diode reverse current-voltage characteristics. FLUTTER (OR DRIFT) CHARACTERISTIC.
Figure 2-9D.—Diode reverse current-voltage characteristics. NEGATIVE RESISTANCE TRACE.
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Zener Diode Test An audio signal generator may not be able to produce a high enough voltage for you to test Zener diodes. You can, however, make this test with a diode test set or with the circuit shown in figure 2-10. In this circuit, R1 is used to adjust the input voltage to a suitable value for the Zener diode being tested. Resistor R2 limits the current through the diode. The signal voltage applied to the diode is also connected to the horizontal input of the oscilloscope. The voltage developed across current-measuring resistor R3 is applied to the vertical input of the oscilloscope. The horizontal sweep will therefore represent the applied voltage, and the vertical deflection will indicate the current through the diode under test. Figure 2-11 shows the characteristic pattern of a Zener diode (note the sharp increase in current at the avalanche breakdown point). For the Zener diode to be acceptable, this voltage must be within the limits specified by the manufacturer.
Figure 2-10.—Zener diode test circuit.
Figure 2-11.—Zener diode characteristic pattern.
STATIC RESISTANCE MEASUREMENTS One convenient method of testing a diode requires only your ohmmeter. The forward and back resistances can be measured at a voltage determined by the battery potential of the ohmmeter and the resistance range at which the meter is set. When the test leads of the ohmmeter are connected to the diode, a resistance will be measured that is different from the resistance indicated if the leads are reversed. The smaller value is called the forward resistance, and the larger value is called the back resistance. If the ratio of back-to-forward resistance is greater than 10 to 1, the diode should be capable of functioning as a rectifier. This is a very limited test, which does not take into account the action of the diode at voltages of
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different magnitudes and frequencies. Some diodes may be damaged by the excessive current produced by some range settings of a standard multimeter. Therefore, you should use a digital multimeter when performing this measurement. Q-14.
As a rule of thumb, what is an acceptable ratio of back-to-forward resistance for a diode?
SILICON-CONTROLLED RECTIFIERS (SCR) Many naval electronic equipments use silicon-controlled rectifiers (SCRs) for the control of power. Like other solid-state components, SCRs are subject to failure. You can test most SCRs with a standard ohmmeter, but you must understand just how the SCR functions. As shown in figure 2-12, the SCR is a three-element, solid-state device in which the forward resistance can be controlled. The three active elements shown in the figure are the anode, cathode, and gate. Although they may differ in outward appearance, all SCRs operate in the same way. The SCR acts like a very high-resistance rectifier in both forward and reverse directions without requiring a gate signal. However, when the correct gate signal is applied, the SCR conducts only in the forward direction, the same as any conventional rectifier. To test an SCR, you connect an ohmmeter between the anode and cathode, as shown in figure 2-12. Start the test at R × 10,000 and reduce the value gradually. The SCR under test should show a very high resistance, regardless of the ohmmeter polarity. The anode, which is connected to the positive lead of the ohmmeter, must now be shorted to the gate. This will cause the SCR to conduct; as a result, a low-resistance reading will be indicated on the ohmmeter. Removing the anodeto-gate short will not stop the SCR from conducting; but removing either of the ohmmeter leads will cause the SCR to stop conducting — the resistance reading will then return to its previous high value. Some SCRs will not operate when you connect an ohmmeter. This is because the ohmmeter does not supply enough current. However, most of the SCRs in Navy equipment can be tested by the ohmmeter method. If an SCR is sensitive, the R × 1 scale may supply too much current to the device and damage it. Therefore, try testing it on the higher resistance scales.
Figure 2-12.—Testing an SCR with an ohmmeter.
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Q-15.
When testing an SCR with an ohmmeter, the SCR will conduct if what two elements are shorted together?
TRIAC Triac is General Electric’s trade name for a silicon, gate-controlled, full-wave, ac switch, as shown in figure 2-13. The device is designed to switch from a blocking state to a conducting state for either polarity of applied voltages and with either positive or negative gate triggering. Like a conventional SCR, the Triac is an excellent solid-state device for controlling current flow. You can make the Triac conduct by using the same method used for an SCR, but the Triac has the advantage of being able to conduct equally well in either the forward or reverse direction.
Figure 2-13.—Testing a Triac with an ohmmeter.
To test the Triac with an ohmmeter (R × 1 scale), you connect the ohmmeter’s negative lead to anode 1 and the positive lead to anode 2, as shown in figure 2-13. The ohmmeter should indicate a very high resistance. Short the gate to anode 2; then remove it. The resistance reading should drop to a low value and remain low until either of the ohmmeter leads is disconnected from the Triac. This completes the first test. The second test involves reversing the ohmmeter leads between anodes 1 and 2 so that the positive lead is connected to anode 1 and the negative lead is connected to anode 2. Again, short the gate to anode 2; then remove it. The resistance reading should again drop to a low value and remain low until either of the ohmmeter leads is disconnected. Q-16.
When a Triac is properly gated, what is/are the direction(s) of current flow between anodes 1 and 2?
UNIJUNCTION TRANSISTORS (UJTs) The unijunction transistor (UJT), shown in figure 2-14, is a solid-state, three-terminal semiconductor that exhibits stable open-circuit, negative-resistance characteristics. These characteristics enable the UJT
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to serve as an excellent oscillator. Testing a UJT is a relatively easy task if you view the UJT as being a diode connected to the junction of two resistors, as shown in figure 2-15. With an ohmmeter, measure the resistance between base 1 and base 2; then reverse the ohmmeter leads and take another reading. Readings should show the same high resistance regardless of meter lead polarity. Connect the negative lead of the ohmmeter to the emitter of the UJT. Using the positive lead, measure the resistance from the emitter to base 1 and then from the emitter to base 2. Both readings should indicate high resistances that are approximately equal to each other. Disconnect the negative lead from the emitter and connect the positive lead to it. Using the negative lead, measure the resistance from the emitter to base 1 and then from the emitter to base 2. Both readings should indicate low resistances approximately equal to each other.
Figure 2-14.—Unijunction transistor.
Figure 2-15.—Unijunction transistor equivalent circuit.
JUNCTION FIELD-EFFECT TRANSISTOR (JFET) TESTS The junction field-effect transistor (JFET) has circuit applications similar to those of a vacuum tube. The JFET has a voltage-responsive characteristic with a high input impedance. Two types of JFETs that you should become familiar with are the junction p-channel and the junction n-channel types, as shown in figure 2-16. Their equivalent circuits are shown in figures 2-17 and 2-18, respectively. The only difference in your testing of these two types of JFETs involves the polarity of the meter leads. 2-23
Figure 2-16.—Junction FETs.
Figure 2-17.—N-channel JFET equivalent circuit.
Figure 2-18.—P-channel JFET equivalent circuit.
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N-Channel Test Using an ohmmeter set to the R × 100 scale, measure the resistance between the drain and the source; then reverse the ohmmeter leads and take another reading. Both readings should be equal (in the 100- to 10,000-ohm range), regardless of the meter lead polarity. Connect the positive meter lead to the gate. Using the negative lead, measure the resistance between the gate and the drain; then measure the resistance between the gate and the source. Both readings should indicate a low resistance and be approximately the same. Disconnect the positive lead from the gate and connect the negative lead to the gate. Using the positive lead, measure the resistance between the gate to the drain; then measure the resistance between the gate and the source. Both readings should show infinity. P-Channel Test Using an ohmmeter set to the R × 100 scale, measure the resistance between the drain and the source; then reverse the ohmmeter leads and take another reading. Both readings should be the same (100 to 10,000 ohms), regardless of meter lead polarity. Next, connect the positive meter lead to the gate. Using the negative lead, measure the resistance between the gate and the drain; then measure it between the gate and the source. Both readings should show infinity. Disconnect the positive lead from the gate and connect the negative lead to the gate. Using the positive lead, measure the resistance between the gate and the drain; then measure it between the gate and the source. Both readings should indicate a low resistance and be approximately equal. MOSFET TESTING Another type of semiconductor you should become familiar with is the metal oxide semiconductor field-effect transistor (MOSFET), as shown in figures 2-19 and 2-20. You must be extremely careful when working with MOSFETs because of their high degree of sensitivity to static voltages. As previously mentioned in this chapter, the soldering iron should be grounded. A metal plate should be placed on the workbench and grounded to the ship’s hull through a 250-kilohm to 1-megohm resistor. You should also wear a bracelet with an attached ground strap and ground yourself to the ship’s hull through a 250-kilohm to 1-megohm resistor. You should not allow a MOSFET to come into contact with your clothing, plastics, or cellophane-type materials. A vacuum plunger (solder sucker) must not be used because of the high electrostatic charges it can generate. Solder removal by wicking is recommended. It is also good practice to wrap MOSFETs in metal foil when they are out of a circuit. To ensure MOSFET safety under test, use a portable volt-ohm-milliammeter (vom) to make MOSFET resistance measurements. A vtvm must never be used in testing MOSFETs. You must be aware that while you are testing a MOSFET, you are grounded to the ship’s hull or station’s ground. Use of a vtvm would cause a definite safety hazard because of the 115-volt, 60-hertz power input. When the resistance measurements are complete and the MOSFET is properly stored, unground both the plate on the workbench and yourself. You will understand MOSFET testing better if you visualize it as equivalent to a circuit using diodes and resistors, as shown in figures 2-21 and 2-22.
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Figure 2-19.—MOSFET (depletion/enhancement type).
Figure 2-20.—MOSFET (enhancement type).
Figure 2-21.—MOSFET (depletion/enhancement type) equivalent circuit.
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Figure 2-22.—MOSFET (enhancement type) equivalent circuit.
Q-17.
Why is it not advisable to use a solder sucker when working on MOSFETs?
MOSFET (Depletion/Enhancement Type) Test Using an ohmmeter set to the R × 100 scale, measure the resistance between the MOSFET drain and the source; then reverse the ohmmeter leads and take another reading. The readings should be equal, regardless of meter lead polarity. Connect the positive lead of the ohmmeter to the gate. Using the negative lead, measure the resistance between the gate and the drain and between the gate and the source. Both readings should show infinity. Disconnect the positive lead from the gate and connect the negative lead to the gate. Using the positive lead, measure the resistance between the gate and the drain; then measure it between the gate and the source. Both readings should show infinity. Disconnect the negative lead from the gate and connect it to the substrate. Using the positive lead, measure the resistance between the substrate and the drain and between the substrate and the source. Both of these readings should indicate infinity. Disconnect the negative lead from the substrate and connect the positive lead to the substrate. Using the negative lead, measure the resistance between the substrate and the drain and between the substrate and the source. Both readings should indicate a low resistance (about 1,000 ohms). MOSFET (Enhancement Type) Test Using an ohmmeter set to the R × 100 scale, measure the resistance between the drain and the source; then reverse the leads and take another reading between the drain and the source. Both readings should show infinity, regardless of meter lead polarity. Connect the positive lead of the ohmmeter to the gate. Using the negative lead, measure the resistance between the gate and the drain and then between the gate and the source. Both readings should indicate infinity. Disconnect the positive lead from the gate and connect the negative lead to the gate. Using the positive lead, measure the resistance between the gate and the drain and then between the gate and the source. Both readings should indicate infinity. Disconnect the negative lead from the gate and connect it to the substrate. Using the positive lead, measure the resistance between the substrate and the drain and between the substrate and the source. Both readings should indicate infinity. Disconnect the negative lead from the substrate and connect the positive lead to the substrate. Using the negative lead, measure the resistance between the substrate and the drain and between the substrate and the source. Both readings should indicate a low resistance (about 1,000 ohms).
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INTEGRATED CIRCUIT (IC) TESTING Integrated circuits (ICs) constitute an area of microelectronics in which many conventional electronic components are combined into high-density modules. Integrated circuits are made up of active and passive components, such as transistors, diodes, resistors, and capacitors. Because of their reduced size, use of integrated circuits can simplify otherwise complex systems by reducing the number of separate components and interconnections. Their use can also reduce power consumption, reduce the overall size of the equipment, and significantly lower the overall cost of the equipment concerned. Many types of integrated circuits are ESDS devices and should be handled accordingly. Q-18.
Name two advantages in using ICs.
Your IC testing approach needs to be somewhat different from that used in testing vacuum tubes and transistors. The physical construction of ICs is the prime reason for this different approach. The most frequently used ICs are manufactured with either 14 or 16 pins, all of which may be soldered directly into the circuit. It can be quite a job for you to unsolder all of these pins, even with the special tools designed for this purpose. After unsoldering all of the pins, you then have the tedious job of cleaning and straightening all of them. Although there are a few IC testers on the market, their applications are limited. Just as transistors must be removed from the circuit to be tested, some ICs must also be removed to permit testing. When ICs are used in conjunction with external components, the external components should first be checked for proper operation. This is particularly important in linear applications where a change in the feedback of a circuit can adversely affect operating characteristics of the component. Any linear (analog) IC is sensitive to its supply voltage. This is especially the case among ICs that use bias and control voltages in addition to a supply voltage. If you suspect a linear IC of being defective, all voltages coming to the IC must be checked against the manufacturer’s circuit diagram of the equipment for any special notes on voltages. The manufacturer’s handbook will also give you recommended voltages for any particular IC. When troubleshooting ICs (either digital or linear), you cannot be concerned with what is going on inside the IC. You cannot take measurements or conduct repairs inside the IC. You should, therefore, consider the IC as a black box that performs a certain function. You can check the IC, however, to see that it can perform its design functions. After you check static voltages and external components associated with the IC, you can check it for dynamic operation. If it is intended to function as an amplifier, then you can measure and evaluate its input and output. If it is to function as a logic gate or combination of gates, it is relatively easy for you to determine what inputs are required to achieve a desired high or low output. Examples of different types of ICs are provided in figure 2-23.
Figure 2-23.—Types of ICs.
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Q-19.
Why should you consider an IC as a black box?
Digital ICs are relatively easy for you to troubleshoot and test because of the limited numbers of input/output combinations involved. When using positive logic, the logic state of the inputs and outputs of a digital IC can only be represented as either a high (also referred to as a 1 state) or as a low (also referred to as a 0 state). In most digital circuitry, a high is a steady 5-vdc level, and a low is a 0-vdc level. You can readily determine the logic state of an IC by using high-input-impedence measuring devices, such as an oscilloscope. Because of the increased use of ICs in recent years, numerous pieces of test equipment have been designed specifically for testing ICs. They are described in the following paragraphs. Q-20.
What are the two logic states of an IC?
LOGIC CLIPS Logic clips, as shown in figure 2-24, are spring-loaded devices that are designed to clip onto a dualin-line package IC while the IC is mounted in its circuit. It is a simple device that usually has 16 light emitting diodes (LEDs) mounted at the top of the clips. The LEDs correspond to the individual pins of the IC, and any lit LED represents a high logic state. An unlit LED represents a low logic state. Logic clips require no external power connections, and they are small and lightweight. Their ability to simultaneously monitor the input and output of an IC is very helpful when you are troubleshooting a logic circuit.
Figure 2-24.—Logic clip.
Q-21.
A lighted LED on a logic clip represents what logic level?
LOGIC COMPARATORS The logic comparator, as shown in figure 2-25, is designed to detect faulty, in-circuit-DIP ICs by comparing them with ICs that are known to be good (reference ICs). The reference IC is mounted on a small printed-circuit board and inserted into the logic comparator. You then attach the logic comparator to the IC under test by a test lead, which is connected to a spring-loaded device similar in appearance to a logic clip. The logic comparator is designed to detect differences in logic states of the reference IC and the IC being tested. If any difference in logic states does exist on any pin, an LED corresponding to the pin in question will be lit on the logic comparator. The logic comparator is powered by the IC under test.
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Figure 2-25.—Logic comparator.
Q-22.
What does a lighted LED indicate on a logic comparator?
LOGIC PROBES Logic probes, as shown in figure 2-26, are extremely simple and useful devices that are designed to help you detect the logic state of an IC. Logic probes can show you immediately whether a specific point in the circuit is low, high, open, or pulsing. A high is indicated when the light at the end of the probe is lit and a low is indicated when the light is extinguished. Some probes have a feature that detects and displays high-speed transient pulses as small as 5 nanoseconds wide. These probes are usually connected directly to the power supply of the device being tested, although a few also have internal batteries. Since most IC failures show up as a point in the circuit stuck either at a high or low level, these probes provide a quick, inexpensive way for you to locate the fault. They can also display that single, short-duration pulse that is so hard to catch on an oscilloscope. The ideal logic probe will have the following characteristics:
Figure 2-26.—Logic probe.
1. Be able to detect a steady logic level 2. Be able to detect a train of logic levels 3. Be able to detect an open circuit 4. Be able to detect a high-speed transient pulse 2-30
5. Have overvoltage protection 6. Be small, light, and easy to handle 7. Have a high input impedance to protect against circuit loading Q-23.
What is the purpose of a logic probe?
LOGIC PULSERS Another extremely useful device for troubleshooting logic circuits is the logic pulser. It is similar in shape to the logic probe and is designed to inject a logic pulse into the circuit under test. Logic pursers are generally used in conjunction with a logic clip or a logic probe to help you trace the pulse through the circuit under test or verify the proper operation of an IC. Some logic pursers have a feature that allows a single pulse injection or a train of pulses. Logic pursers are usually powered by an external dc power supply but may, in some cases, be connected directly to the power supply of the device under test. View A of figure 2-27 shows a typical logic pulser. View B shows a logic pulser (right) used with a logic probe (left).
Figure 2-27A.—Logic pulser.
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Figure 2-27B.—Logic pulser.
LOGIC ANALYZER A relatively new device on the test equipment scene is the logic analyzer. A logic analyzer provides various functions that can assist you in maintenance, testing, and troubleshooting of equipment using digital circuitry. From your standpoint, they are extremely useful in performing timing analysis. Most logic analyzers have crt displays that can monitor up to 32 timing signals at the same time. A large percentage of today’s digital equipment is designed with the logic analyzer in mind and have built-in status or bus lines for your convenience in monitoring multiple signals at the same time. When monitoring a bus line, you can readily determine, through visual displays, such things as the presence of master clock signals or sequential timing events.
BATTERY MEASUREMENTS As a technician, you are primarily concerned with the uses of batteries; however, checking or testing of storage and dry cell batteries is an important part of your maintenance program. Proper preventive maintenance of batteries can significantly extend the useful life of a battery. STORAGE BATTERIES When you check a lead-acid type of storage battery for its condition of charge or discharge, you take a specific gravity reading of the electrolyte by using a hydrometer. A specific gravity reading between 1.275 and 1.300 indicates a full-charge condition and assures you that the battery is in good condition. A hydrometer reading of approximately 1.175 indicates a normal discharge condition, and a reading of approximately 1.250 indicates that the battery is half-discharged. Since the acids used in various batteries do not always have the same specific gravity and since electrode composition may differ, the hydrometer reading you obtain at the charged and discharged conditions will vary with the type of electrolyte and battery composition. A general rule for you to follow is not to discharge a battery more than 100 points (.100 specific gravity) before recharging. Although readings of specific gravity are a reliable measure of the condition of a storage battery, cells that indicate normal may prove useless under load. This is usually caused by a high internal resistance. A load-voltage check of the cells with the use of a cell tester indicates the actual voltage 2-32
charge held by each battery cell. Cell voltages should not differ by more than 0.15 volt for 6-volt or 12-volt batteries. Use extreme caution whenever testing or working around lead-acid storage batteries. OPNAVINST 5100.23B emphatically states that you must wear eye protection devices at all times and that emergency eyewash facilities must be immediately adjacent to, or within 10 feet of, any eye-hazard area. Smoking and spark-producing tools or devices are also prohibited in enclosed spaces that contain lead-acid storage batteries. When charging, these batteries produce sufficient quantities of hydrogen to produce large explosions. Lead-acid storage batteries should only be charged in well-ventilated spaces. Q-24.
Emergency eyewash facilities must be located within what minimum number of feet of an eyehazard area?
DRY BATTERIES You must periodically check dry cell batteries that are used for test instruments and portable or field equipments for loss of power. For actual voltages of dry batteries, you should measure with a battery tester for a minimum acceptable voltage before installation. The TS-183/U series of battery testers incorporate a multiple-range voltmeter, battery-loading resistors, multiplier resistors, and a jack-switching arrangement that connects the load resistors across the voltmeter for a total of 32 different voltmeter-load resistor combinations. This type of tester permits you to complete a rapid and accurate measurement of battery potentials under load conditions, ranging in voltages from 1.5 to 180 volts. A data chart supplied with the battery tester provides information regarding the jack to be used and minimum acceptable voltages of various batteries used in Navy equipments. Q-25.
What is the advantage of using a battery test set versus a voltmeter to test batteries?
Table 2-2 shows general standards of tolerance for dry batteries. Whenever practical, dry cell batteries that are not in use should be stored in a refrigerated area to extend their shelf life.
Table 2-2.—Typical voltage Tolerances for Dry Cell Batteries
RATED VOLTAGE
MAX. VOLTAGE TOLERANCE
1 to 2 3 to 10 11 to 15 16 to 25 26 to 50 50 to 70 70 to 99
0.1 0.3 0.5 1.0 2.0 3.0 5.0
CARBON-ZINC AND ALKALINE BATTERIES Carbon-zinc and alkaline cells are used primarily in portable test equipment, vom’s, flashlights, some portable radios, and beacon equipment. The carbon-zinc cell provides 1.5 volts and holds its charge for approximately 1 year in normal service. The alkaline cell provides 1.2 volts and has about twice the stored energy of the carbon-zinc cell of the same size. It also has a longer life at a higher discharge rate than the carbon-zinc cell. You should discard both types of batteries at the first indication of weakness.
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MERCURY CELLS The storage life of a mercury cell varies but is generally classified as long. The working life of the cell is extremely long relative to other types of batteries; and it maintains its full rated voltage (1.34 volts) until just before it is ready to expire, at which point its voltage will drop off sharply. Recharging of mercury cells is possible, but is not recommended because the recharging cycle can vary from one cell to another; and, after being recharged, their operating lifetime is uncertain. NICKEL-CADMIUM BATTERIES (NICAD) Nickel-cadmium batteries have very high efficiency. They can be recharged hundreds of times; given the proper conditions, they may even be recharged thousands of times. They can be stored for a number of years with no significant loss of performance. After just a few charge and discharge cycles, NICAD cells can be recharged to the point that they are just as good as new batteries. Since they are sealed, they are maintenance free and can be installed in any position. There are two types of nickel-cadmium batteries — vented and nonvented. This description deals with the nonvented exclusively because a vented NICAD would have extremely limited application in a shipboard environment. The voltage at the terminals of a NICAD will normally be between 1.25 and 1.30 volts in an opencircuit condition. This value will vary, of course, depending on the state of charge. If the charge has dropped to a low of 1.1 volts, the NICAD should be regarded as being completely discharged and should not be permitted to be discharged further. The majority of small NICADs are rated in milliampere hours; the large ones are rated in ampere hours. The small NICAD is the one the technician will almost always be concerned with. Q-26.
At what voltage is a NICAD battery considered to be fully discharged?
As a general rule, if the charging current is held to 10% of the milliampere-hour rating for the NICAD and the time of charge is held at 150% of the time required to establish its full milliampere-hour rating, you will encounter no difficulty in maintaining NICADs at their maximum charge. For example, you should charge a battery rated at 300 milliampere hours for 15 hours at 30 milliamperes. You can leave the battery on extended charge for years, provided the charge rate is lowered to less than 10% of the NICAD's milliampere rating. You should never place a NICAD in your pocket, because metal objects (such as keys) can short the cell and cause extreme heat. Never dispose of a NICAD by fire, because it can explode. Never solder a connection directly to the cell, because the heat of an iron can damage it. Never overcharge a NICAD cell, because an accumulation of gases within its case can destroy it. NICADs are also subject to a phenomenon commonly referred to as cell memory. If a NICAD is consistently discharged to a minor extent (for example, 30 minutes per day) and then recharged after each use, the useful capacity of the cell will eventually be reduced to that level. To keep this from happening, you should fully discharge (1.1 volts) NICADs on a regular basis. In fact, some maintenance requirement cards and calibration laboratory procedures require this periodic full discharge of equipment containing NICADs.
RF ATTENUATORS AND RESISTIVE LOAD TESTS All rf attenuators, decade or step attenuators, decade resistors, and 50/75-ohm loads are clearly marked to show their attenuation factor or resistance. In the case of precision rf attenuators, they are usually marked to show their useful frequency ranges. They are all basically resistive devices and are
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designed for a multitude of applications. None of these devices are user-repairable; however, you should be aware of the different methods of determining whether or not they are functioning properly. FIXED RF ATTENUATORS Fixed rf attenuators (shown in fig. 2-28), such as the ones commonly found in power-measuring sets, are designed to provide a fixed-signal attenuation over a specific frequency range. Frequency ranges can be in excess of 30 gigahertz, and attenuation factors are typically in 1-, 3-, 6-, and 10-dB steps. Fixed attenuators can be connected in series to provide you with the desired attenuation. Most fixed rf attenuators are designed to handle only small amounts of rf power and are extremely susceptible to damage because of overloading. To test a fixed rf attenuator, you can either substitute it with a known good attenuator or perform basic measurements on the attenuator itself. With the rf substitution method, you connect an rf signal generator to a power meter and establish a suitable reference point on the meter by adjusting the power output of the signal generator. Once you establish the reference point, insert the rf attenuator between the signal generator and the power meter. You then determine the attenuation by noting the difference between the power meter reading and the initial reference point.
Figure 2-28.—Fixed attenuator set.
Q-27.
What is the most common method of testing a fixed rf attenuator?
DECADE RESISTORS Decade resistors (also referred to as decade boxes) typically are precision devices. Depending on the make and model of the decade resistor, it may be capable of providing you with a selection of resistors ranging in value from a small fraction of an ohm to hundreds of megohms. Decade resistors are commonly used in calibration laboratories and in engineering design applications. Like the fixed rf attenuator, most decade resistors are capable of handling only small amounts of current. They are very limited in respect to frequency capabilities and are commonly used in dc-circuit applications. You may encounter specific equipment that requires the use of a decade resistor in performing your maintenance tests or alignments. To test a decade resistor, you can connect a standard multimeter or digital multimeter directly across its resistance terminals and read its resistance on the meter. This test will only indicate gross errors in the decade resistor such as an open or a badly damaged resistor. If you are performing a precision measurement or an alignment using a decade resistor and have any doubt as to its accuracy, you should submit it to your servicing calibration laboratory. Figure 2-29 shows a typical decade resistor.
2-35
Figure 2-29.—Decade resistor.
DECADE (STEP) ATTENUATORS Decade attenuators (also referred to as step attenuators) are common devices that may be designed as either a stand-alone piece of test equipment or as an integral part of an operational piece of electronic equipment. As the name implies, they are used to attenuate rf signals in incremental steps. Like the fixed rf attenuator, you can easily test them by using the rf substitution method, as previously described. Views A and B of figure 2-30 show two types of decade attenuators.
Figure 2-30.—Step attenuators.
50/75-OHM TERMINATIONS Terminations of 50 and 75 ohms are designed as either feedthrough, impedance-matching devices, or as rf loading devices. They are precision resistors sealed in small plastic or metal enclosures and are designed to be mounted on various rf connectors. In the case of feedthrough terminations, they are designed with rf connectors at both ends, which allows the rf signal to pass through them. They are impedance-matching devices designed primarily to reduce the voltage standing-wave ratio (vswr) that is produced when two pieces of equipment with dissimilar impedances are connected together. You can test a feedthrough termination by measuring the resistance between the center conductor and the shield of either rf connector with an ohmmeter. As mentioned above, some terminations are manufactured as loading devices that are designed to shunt an rf signal to ground. A perfectly matched termination can be compared to a transmitting antenna in that it absorbs all of the rf signal with only a
2-36
small amount of power being reflected back to the transmitting device. When using a termination as a load, you should ensure that its wattage rating exceeds the power output of the equipment to which it is connected. You can also measure this type of termination by using a standard ohmmeter to read the resistance between the center conductor and the shield of the rf connector. Q-28.
What is the most common method of testing resistive terminations?
FIBER-OPTIC TESTING Fiber optics are a relatively new type of transmission media. Figure 2-31 depicts a typical fiber-optic cable design. The core of the fiber-optic cable is the optical transmission path, which carries data from the optical transmitter to the optical receiver. The core is usually made of plastic, glass, or plastic-clad silica (PCS). Glass-core fibers are usually smaller in diameter than plastic or PCS cores. The major disadvantages of glass cores are that they have high attenuation (25 dB/km), require precision tools and connectors, and are extremely susceptible to mechanical damage. Plastic cores are typically more rugged than other types of cores, but their attenuation is high (35 dB/km). PCS cores are fairly rugged and have a relatively low attenuation (10 dB/km). A fiber-optic cable may consist of one fiber, multiples of singleoptical fibers, or bundles of optical fibers. Fiber-optic cables are well suited for the transmission of highspeed data over relatively short distances. They are virtually immune to crosstalk or interference through inductance. (Interference is a characteristic of metallic cables.)
Figure 2-31.—Typical fiber-optic cable.
Testing techniques and the principles of measurement for fiber-optic and conventional cable are similar. For example, if both ends of the cable are exposed and can be used for testing, relatively unsophisticated equipment can be used to measure cable parameters, such as continuity and attenuation. This includes equipment such as optical multimeters and optical power meters (OPM). If only one cable end is available, then more sophisticated equipment such as an optical time-domain reflectometer (OTDR), is used. The following section lists and defines some common optical test equipment. Q-29.
What is the main disadvantage of using fiber-optic cables? 2-37
OPTICAL TIME-DOMAIN REFLECTOMETER (OTDR) The portable optical time-domain reflectometer (OTDR) is used to check loss at each splice, at each connector, and of the entire system. Loss measurements are figured by using the same methods you would use for wire loss measurements. The OTDR injects a short, intense laser pulse into the fiber and monitors reflections caused by breaks, inclusions, microcracks, and discontinuities. Discontinuities appear as a spike on the OTDR display. The loss at the discontinuity point is directly related to the distance between the major pulse triggered by the laser and the spike. The manufacturer’s manual provides you with conversion factors to figure actual losses and locations of the discontinuities. OSCILLOSCOPE An oscilloscope is used with an OTDR to provide visual evidence of fiber faults, connector and splice locations, and attenuation locations. OPTICAL MULTIMETER The optical multimeter measures light sources and light in cable and at the detector, fiber cable transmission loss, and connector splice loss. For cable transmission measurements, transmission through a short length of cable is compared with transmission through a known longer length. OPTICAL OHMMETER The optical ohmmeter measures the input versus the output of light in an optical fiber. It displays attenuation losses based on a comparison of known and unknown cable signals. It can be used in manufacturing, connecting, and installing cable. It is as simple to use as a digital voltmeter. OPTICAL POWER METER The optical power meter measures current by converting light power from plug-in units, such as light emitting diodes, into electrical current. In some models, the readout is in power units, watts. In other models, the readout is in absolute power levels and attenuation. Some units operate with a variety of power sensors for conventional coaxial and waveguide systems and fiber-optic systems. RADIOMETER/PHOTOMETER The radiometer/photometer measures light power in watts from dc to unlimited ac response. It uses plug-in sensor heads and, for low-light displays, it uses spectrometers and fiber-optic measurements. AUTOMATIC TEST EQUIPMENT Automatic Test Equipment (ATE) is test equipment designed to evaluate the operational performance of a piece of equipment or printed circuit board (pcb). ATE assists you in troubleshooting a fault to the defective component. Basically, ATEs are state-of-the-art, computer devices in which software programs are specifically tailored to meet the requirements of the device being tested. The AN/USM-465 Portable Service Processor (psp), shown in figure 2-32, is the Navy’s standard ATE for testing digital pcb’s.
2-38
Figure 2-32.—AN/USM-465 Portable Service Processor.
The AN/USM-465 is part of the Support and Test Equipment Engineering Program (STEEP). It provides on-site screen testing and fault isolation of digital pcb’s and modules. The psp is presently available on most ships and shore intermediate maintenance activities (SIMA) with Mini/Micro maintenance stations (2M). Psp’s come with maintenance-assist modules (spare parts kit) and diagnostic kits. The psp is easy to use. You have a choice of three pcb connectors (located on the top panel of the test set) into which you insert the pcb being tested. The software program, which is provided on magnetic tape cartridges, is then loaded into the test set. The test set automatically tests the pcb by applying input signals to the appropriate pins while monitoring the output signal for a correct indication. An LED display will give you a pass or fail indication. If a pcb fails the operational test, the psp tells you (via LED display) what troubleshooting steps must be taken. The psp uses a guided probe fault isolation technique that tells you what test points to check on the faulty pcb. The software program guides you from the faulty output backwards toward the input until the fault is located. The probe is a standard 10 megohm, 10 to 1 oscilloscope probe. The guided probe circuitry and software is also unique because it is capable of locating faults within feedback loops and can sense when you have placed the probe at an incorrect test point. An interesting advantage is that if the psp itself fails, the faulty board inside the psp can be identified by the test set’s own capability. After you replace the faulty pcb with a good one from the spare parts kit, you can use the psp to identify the faulty component on its own pcb. HUNTRON TRACKER 2000 The Huntron Tracker 2000, shown in figure 2-33, is a versatile troubleshooting tool used to statically test resistors, capacitors, inductors, diodes, transistors, multiple-component circuits, and integrated circuits. Its built-in features eliminate the use of multiple pieces of test equipment. These features and its lightweight portability make the 2000 a widely used tool for troubleshooting.
2-39
Figure 2-33.—Huntron Tracker 2000.
We recommend you review setup and operating procedures discussed in NEETS Module 16, Introduction to Test Equipment, NAVEDTRA B72-16-00-95, before continuing with this chapter. Since the 2000 was covered in depth in module 16, we will cover only the most common troubleshooting procedures and provide a few troubleshooting tips. Q-30.
What two features make the Huntron Tracker 2000 a widely used troubleshooting tool?
The Huntron Tracker 2000 has the following features: • Multiple-test signal frequencies (2000 Hz, 400 Hz, and 50/60 Hz). • Four impedance ranges (low, medium 1, medium 2, high). • Automatic range scanning. • Range control: High Lockout. • Rate-of-channel alteration and/or range scanning is/are adjustable. • Dual-polarity pulse generator for dynamic testing of three terminal devices. • LED indicators for all functions. • Dual-channel capability for easy comparison. • Large CRT display with easy-to-operate controls. CAUTION The device to be tested must have all power turned off, and have all high voltage capacitors discharged before connecting the Tracker 2000 to the device.
2-40
Testing Components by Comparison Testing components by comparison is the most preferred method for troubleshooting. The ALT (alternate) mode setup is the most commonly used mode for this method. This mode allows the technician to compare a known good component to a suspect component. This is accomplished by connecting channel A to a known good device, channel B to the device under test, and a common test lead to COM as illustrated in figure 2-34. Select the ALT button, and the 2000 will alternately display the signature of the known good device and the device under test. By examining the signature differences, you can detect a defective component. Figure 2-35 is a typical example of the CRT display on the 2000 while testing the base to emitter on a good transistor. Figure 2-36 illustrates a defective transistor under the same test setup. Note that in the low range, the transistor appears to be good. Sometimes component defects are more obvious in one range than another, so is a suspect device appears normal for one range, try the other ranges.
Figure 2-34.—Alternate mode setup.
2-41
Figure 2-35.—Signatures between base-emitter of a good transistor.
Figure 2-36.—Signatures between base-emitter of a defective transistor.
Q-31.
What is the most preferred method of troubleshooting?
Q-32.
Why is it recommended to use more than one range while troubleshooting a device?
Troubleshooting Tips When you are testing individual components in a circuit, a parallel resistor or diode of similar value may cause a defective component to appear good. Therefore, you should, in most cases, electrically isolate the suspected component from the circuit while testing individual components. The best way to do this is to desolder all but one lead on the suspected component.
2-42
Q-33.
When you are testing individual components in a circuit, what may cause a defective component to appear good?
You should be aware that devices made by different manufacturers may appear to have slightly different signatures. This is normal, especially with digital integrated circuits, and does not necessarily indicate a failed device. When this occurs, the best way to verify this is to compare the outputs of the device under test with the equipment specifications to ensure the signals are adequate for proper equipment operation.
SUMMARY The information that follows summarizes the important points of this chapter. ELECTRON TUBES are usually tested for SHORTS, TRANSCONDUCTANCE, and the presence of GAS. Several different types of tubes (i.e., twt’s, magnetrons, and klystrons) are normally tested in-circuit.
Most TRANSISTORS can be tested by measuring the forward-to-back resistance of their junctions using a standard ohmmeter. The resistance scale of the ohmmeter must be carefully selected to ensure that the current rating of the transistor is not exceeded. ESD-SENSITIVE DEVICES are components that require special handling. Some of the more sensitive devices can be damaged by static charges as small as 35 volts.
2-43
Most DIODES and MOSFETs can be tested by measuring the forward-to-back resistance of their junctions using a standard ohmmeter. MOSFETS, however, are classed as ESD-sensitive devices; and care should be exercised when handling or testing them. INTEGRATED CIRCUITS (ICs) have revolutionized the electronics industry. They are rugged, compact, and inexpensive. There is a wide assortment of equipment on the market designed for testing ICs.
BATTERIES are common to a large number of both electronic test equipment and operational equipment. You should be familiar with the different types of batteries, their test requirements, and the safety precautions to be followed. RF ATTENUATORS and RESISTIVE LOADS are common devices that are widely used for attenuating rf signals and impedance matching. Resistive loads can be tested with a standard ohmmeter, and rf attenuators are normally tested through the rf substitution method.
2-44
FIBER-OPTIC CABLES are used primarily for the transmission of high-speed data over short distances. Their construction and theory of operation require that they be tested with a light source, usually a laser beam. There is a wide assortment of test equipment designed specifically for testing fiberoptic cables.
AUTOMATIC TEST EQUIPMENT (ATE) is test equipment designed to evaluate the operational performance of a piece of equipment or printed circuit board (pcb).
2-45
The HUNTRON TRACKER 2000 is a versatile troubleshooting tool commonly used for statically testing resistors, capacitors, inductors, diodes, transistors, multiple-component circuits, and integrated circuits.
REFERENCES EIMB, Test Methods and Practices, NAVSEA 0967-LP-000-0130, Naval Sea Systems Command, Washington, D.C., 1980. Fiber Optic Communication Cables and Connectors, (Navy) EE169-CA-GYD-010/E110 TSER E & I, (Published under the authority of the Secretaries of the Air Force, Army, and Navy), 1983. Fire Control Technician G 3 & 2, NAVEDTRA 10207-B, Naval Education and Training Professional Development and Technology Center, Pensacola, Fla., 1981. Huntron Tracker 2000 Operation and Maintenance Manual, P/N 21-1052, Huntron Instruments, Inc., 15720 Mill Creek Blvd., Mill Creek, WA 98012. Introduction to Microelectronics, NAVEDTRA 172-14-00-84, Naval Education and Training Professional Development and Technology Center, Pensacola, Fla., 1984. Logic Clip 548A, NAVAIR 16-45-3102, Naval Air Systems Command, Washington, D.C., 1979. Logic Comparator 10529A, NAVAIR 16-45-3100, Naval Air Systems Command, Washington, D.C., 1979. Logic Probe 545A, NAVAIR 16-45-3105, Naval Air Systems Command, Washington, D.C., 1979. Logic Pulser 546A, NAVAIR 16-45-3104, Naval Air Systems Command, Washington, D.C., 1979. 2-46
ANSWERS TO QUESTIONS Q1. THROUGH Q33. A-1.
Lack of adequate storage space.
A-2.
Open filaments.
A-3.
Testing the tube in its circuit.
A-4.
In their circuit.
A-5.
Restore it to serviceable condition by operating it temporarily at reduced beam voltage.
A-6.
Correct gain figure.
A-7.
Rugged design.
A-8.
Sensitive to heat and minor overloads.
A-9.
Any range setting that produces a current flow through the transistor that exceeds 1 milliamp (usually R x 1 range).
A-10.
3,500 to 4,000 volts.
A-11.
35 volts.
A-12.
For your own safety.
A-13.
Voltages and resistances.
A-14.
Greater than 10 to 1.
A-15.
Gate and anode.
A-16.
Current is allowed to flow in either direction.
A-17.
Solder suckers create an electrostatic charge capable of damaging a MOSFET.
A-18.
Low power consumption, compact size, and lower cost.
A-19.
ICs cannot be repaired. All you need to test is output versus input.
A-20.
A "1" or "0."
A-21.
A "1" state.
A-22.
A difference in logic states between the reference IC and the IC under test.
A-23.
They provide you with a visual indication of the logic state at any point you choose in the circuit.
A-24.
10 feet.
A-25.
A battery test set will test batteries under load conditions.
A-26.
At 1.1 volts.
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A-27.
Rf substitution method.
A-28.
Reading their resistances with a standard ohmmeter.
A-29.
High attenuation.
A-30.
It eliminates the need for multiple pieces of test equipment and it is lightweight and portable.
A-31.
Testing components by comparison.
A-32.
Some defective devices may appear to be good in certain ranges.
A-33.
A parallel resistor or diode of similar value.
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CHAPTER 3
QUANTITATIVE MEASUREMENTS LEARNING OBJECTIVES Upon completion of this chapter, you will be able to do the following: 1. Explain the purposes and benefits of performing quantitative measurements. 2. Identify the various methods of performing impedance measurements. 3. Identify the various methods of performing power measurements. 4. Identify the various methods of performing frequency measurements.
INTRODUCTION TO QUANTITATIVE MEASUREMENTS You have already studied the basics of performing electronics measurements and how to determine if a component is or is not functioning properly. This chapter will cover techniques used in measurements of specific impedance, frequency, and power. These measurements are extremely important to you in evaluating the performance of a piece of electronic equipment. IMPEDANCE MEASUREMENTS Impedance measurements are often used during routine test procedures. Impedance-measuring equipment, such as impedance bridges, are mainly used in determining the capacitance and inductance of component parts. However, the values of combined circuit constants also may be obtained and used in direct calculations of impedance. An impedance measurement effectively totals the inductive and capacitive reactance together with the resistance in a circuit. In addition, impedance measurements are useful in testing and analyzing antenna and transmission line performance and for determining the figure of merit (Q) of electrical parts and resonant circuits. Q meters are impedance-measuring instruments that determine the ratio of reactance to resistance of capacitors or inductors and resistors. Details of Q meters and impedance bridges as well as a number of other methods of measuring circuit impedance are described in the following paragraphs. Also discussed are methods of measuring the impedance of antennas and transmission lines. BRIDGE METHODS Bridges are among the most accurate types of measuring devices used in the measurement of impedance. In addition, bridges are also used to measure dc resistance, capacitance, and inductance. Certain types of bridges are more suitable for measuring a specific characteristic, such as capacitance or inductance. Basic schematics for the various bridge circuits are shown in figure 3-1. The bridge circuits shown are similar in that they usually contain two branches in the measuring circuit, two branches in the comparing circuit, a detector circuit, and a power circuit, as shown in figure 3-2. The bridge shown in figure 3-2 is actually the dc Wheatstone bridge; however, the general principles of circuit operation for ac remain the same.
3-1
Figure 3-1.—Basic bridge circuits.
Figure 3-2.—Typical bridge circuit configuration.
3-2
The comparing circuit contains branches A and B and has provisions for changing the ratios of the branches with respect to each other, which enables various measuring ranges to be obtained. Comparison of figures 3-1 and 3-2 shows that either or both branches of the comparing circuit do not necessarily contain resistors alone. Branch B of the Hay bridge, containing CB and RB in series connection, provides a striking contrast with the parallel connection of CB and RB of the Maxwell bridge. The measuring circuit in figure 3-2 also contains two branches. The resistance, capacitance, or inductance to be measured is connected to branch X of the bridge-measuring circuit. The subscript X is also used in figure 3-1 to designate the circuit parameters involved in computing the values of various electronic parts. Branch S contains the variable control used to bring the bridge into a balanced condition. A potentiometer is used for this purpose in most bridge equipment, because it offers a wide range of smoothly variable current changes within the measuring circuit. The third arm of the bridge is the detector circuit. The detector circuit may use a galvanometer for sensitive measurements that require high accuracy. In the case of bridges using ac as the power source, the galvanometer must be adapted for use in an ac circuit. In many practical bridge circuits using ac to operate the bridge, an electron-ray indicating tube is used to indicate the balanced condition by opening and closing the shadow area of the tube. Headsets are also used for audible balance detection, but this method reduces the accuracy obtainable with the bridge. Switches are used in bridge circuits to control the application of operating power to the bridge and to complete the detector circuit. Frequently, the two switching functions are combined into a single key, called a bridge key, so that the operating power is applied to the bridge prior to the detector circuit. This sequence reduces the effects of inductance and capacitance during the process of measurement. The most unfavorable condition for making a measurement occurs when the resistance, capacitance, or inductance to be measured is completely unknown. In these cases, the galvanometer cannot be protected by setting the bridge arms for approximate balance. To reduce the possibility of damage to the galvanometer, you should use an adjustable shunt circuit across the meter terminals. As the bridge is brought closer to the balanced condition, the resistance of the shunt can be increased; when the bridge is in balance, the meter shunt can be removed to obtain maximum detector sensitivity. Bridges designed specifically for capacitance measurements provide a dc source of potential for electrolytic capacitors. The electrolytic capacitors often require the application of dc polarizing voltages in order for them to exhibit the same capacitance values and dissipation factors that would be obtained in actual circuit operation. The dc power supply and meter circuits used for this purpose are connected so that there is no interference with the normal operation of the capacitance-measuring bridge circuit. The dissipation factor of the capacitor may be obtained while the capacitor is polarized. In figure 3-2, the signal voltage in the A and B branches of the bridge will be divided in proportion to the resistance ratios of its component members, RA and RB, for the range of values selected. The same signal voltage is impressed across the branches S and X of the bridge. The variable control, RS, is rotated to change the current flowing through the S and X branches of the bridge. When the voltage drop across branch S is equal to the voltage drop across branch A, the voltage drop across branch X is equal to the voltage drop across branch B. At this time the potentials across the detector circuit are the same, resulting in no current flow through the detector circuit and an indication of zero-current flow. The bridge is balanced at these settings of its operating controls, and they cannot be placed at any other setting and still maintain this balanced condition. The ability of the bridge circuit to detect a balanced condition is not impaired by the length or the leads connecting the bridge to the electronic part to be measured. However, the accuracy of the measurement is not always acceptable, because the connecting leads exhibit capacitive and inductive
3-3
characteristics, which must be subtracted from the total measurement. Hence, the most serious errors affecting accuracy of a measurement are because of the connecting leads. Stray wiring capacitance and inductance, called residuals, that exist between the branches of the bridge also cause errors. The resistance-ratio bridge, for example, is redrawn in figure 3-3 to show the interfering residuals that must be eliminated or taken into consideration. Fortunately, these residuals can be reduced to negligible proportions by shielding and grounding. A method of shielding and grounding a bridge circuit to reduce the effects of interfering residuals is through the use of a Wagner ground, as shown in figure 3-4. Observe that with switch S in position Y, the balanced condition can be obtained by adjusting Z1 and Z2. With switch S in position X, the normal method of balancing the bridge applies. You should be able to reach a point where there is no deflection of the meter movement for either switch position (X or Y) by alternately adjusting Z1 and Z2 when the switch is at position Y and by adjusting RS when the switch is at position X. Under these conditions, point 1 is at ground potential; and the residuals at points 2, 3, and 4 are effectively eliminated from the bridge. The main disadvantage of the Wagner ground is that two balances must be made for each measurement. One is to balance the bridge, and the other is to balance the Wagner ground. Both adjustments are interacting because RA and RB are common to both switch positions X and Y.
Figure 3-3.—Resistance-ratio bridge residual elements.
3-4
Figure 3-4.—Wagner ground.
Many bridge instruments provide terminals for external excitation potentials; however, do not use a voltage in excess of that needed to obtain reliable indicator deflection because the resistivity of electronic parts varies with heat, which is a function of the power applied. Q-1.
What conditions must be met in order to balance a bridge circuit?
Q-2.
When you are measuring a component using a bridge, what is the most common cause of inaccurate measurements?
Wheatstone Bridge The Wheatstone bridge, shown in figure 3-1, is often used to measure resistance. These instruments are usually portable because they require only a small, dc source to power the bridge, which is easily obtained from flashlight batteries. In those cases where an external supply voltage is desirable for the operation of the bridge, use the minimum voltage that will give a reliable indication by the galvanometer. Increasing the supply voltage any further results in uncompensated thermal variations and decreased bridge accuracy. If greater bridge sensitivity is needed, use a galvanometer with greater sensitivity. A number of other considerations are involved in the choice of a galvanometer. For example, the galvanometer should not be subjected to false or erratic indications because of external magnetic fields. This requirement dictates the choice of a shielded meter mechanism. It is also desirable to use a critically dampened meter movement to ensure decisive movement of the meter pointer during conditions of bridge unbalance. Thermal agitation sometimes produces voltages that interfere with the balancing of the bridge. For this reason, the Wheatstone bridge usually includes a polarity-reversing switch in the detector circuit. When a measurement is required, note the reading for both positive and negative indications, and figure the average of both readings. With the exception of inaccuracies introduced by thermal variations (caused by excessive supply voltages), the accuracy of the Wheatstone bridge is, otherwise, independent of the value of supply voltages. The units used in calibrating the galvanometer are unimportant to the accuracy of the bridge, since a 0 indication is desired at the balanced condition.
3-5
Resistance values ranging from 1 ohm to 1 megohm can be measured with an accuracy of approximately 0.1%. However, difficulties are encountered when very high and very low resistances are measured. Resistances less than 1 ohm are difficult to measure accurately because of uncertainty arising from the contact resistance present between the resistor to be measured and the binding posts of the bridge. Measurement of resistances greater than 1 megohm becomes difficult because of two factors: (1) The ratio of standard resistances RA and RB involve a ratio on the order of 1,000 to 1, and (2) the voltage applied to the bridge must be substantially increased to obtain definite galvanometer action. The result is that an increase in the supply voltage increases the power dissipation (heat) of the bridge resistors. The change in resistance RB, because of the heat, is sufficient to produce an appreciable error. A Kelvin bridge is recommended for measuring resistances lower than 1 ohm. An electronic multimeter is recommended for the indicating device in bridges used for the measurement of very high resistances. One of the most elementary precautions concerning the use of a bridge, when measuring low resistance, is to tighten the binding posts securely so that the contact resistance between the binding posts and the resistance to be measured is minimum. Leakage paths between the resistor leads along the outside surface of the resistor body must be avoided when resistances greater than 0.1 megohm are measured. Search for defective solder joints or broken strands in stranded wire leads; these defects can cause erratic galvanometer indications. In those cases where wire leads must be used to reach from the resistance under test to the bridge terminals, measure the ohmic value of those leads prior to further measurements. Q-3.
How does the supply voltage affect the accuracy of Wheatstone bridge measurements?
Kelvin Bridge It is often necessary to make rapid measurements of low resistances, such as samples of wire or low values of meter shunt resistors. A frequently used instrument that is capable of good precision is the Kelvin bridge, shown in figure 3-1. Note the similarity between this and the Wheatstone bridge. Two additional resistances, R1 and R2, are connected in series and shunted across resistance R, which is the circuit resistance existing between the standard and unknown resistances, RS and R X, respectively. In performing the adjustment for balance, you must make the ratio of R1 to R2 equal to the ratio of RA to R B. When this is done, the unknown resistance can be computed in the same manner as that for the Wheatstone bridge, because resistance R is effectively eliminated. In using a Kelvin bridge, you must follow precautions similar to those given for the Wheatstone bridge. A rheostat is usually placed in series with the battery so that bridge current can be conveniently limited to the maximum current allowable. This value of current, which affects the sensitivity of the bridge, is determined by the largest amount of heat that can be sustained by the bridge resistances without causing a change in their values. All connections must be firm and electrically perfect so that contact resistances are held to a minimum. The use of point and knife-edge clamps is recommended. Commercially manufactured Kelvin bridges have accuracies of approximately 2% for resistance ranges from 0.001 ohm to 25 ohms. Q-4.
Kelvin bridges are well suited for what type of measurements?
Resistance-Ratio Bridge The resistance-ratio bridge, shown in figure 3-1, may be used to measure capacitance, inductance, or resistance so long as the electronic part to be measured is compared with a similar standard. The measurement of the value of a capacitor must be made in terms of another capacitor of known characteristics, termed the STANDARD CAPACITOR. The same requirement is necessary for an inductance measurement. The standard of comparison is designated as XX, and the losses of the standard are represented as R X. If you experience difficulty in obtaining a balanced bridge condition, insert
3-6
additional resistance in series with branch S of the bridge. This adjustment becomes necessary because the Q of the unknown capacitor or inductor in branch X is higher than the comparable Q of the standard in branch S. Schering Bridge The Schering bridge, shown in figure 3-1, is a commonly used type of bridge for the measurement of capacitors and dielectric losses. The Q of a capacitor is defined as the reciprocal of the dissipation factor, which is the ratio of the capacitor's dielectric constant to its conductivity at a given frequency. Accordingly, capacitor Q is determined by the frequency used to conduct the measurement and the value of the capacitor, CB, required to obtain bridge balance. The accuracy of this type of bridge is excellent, about 2% for dissipation factors ranging from 0.00002 to 0.6. Typical accuracies for capacitive reactances in the range of 100 picofarads to 1 microfarad are 0.2%. Hay Bridge The Hay bridge, shown in figure 3-1, is used for the measurement of inductance and the Q of the inductor. It is interesting to note that this type of bridge measures inductance by comparing it with a standard capacitor of known characteristics. This arrangement provides the advantage of a wide measurement range with the minimum use of electronic parts as comparison standards. A typical range of values that can be measured with the Hay bridge is from 1 microhenry to 100 henries. The accuracy of the measurements made with this bridge is about 2%. The frequency used in conducting the inductance measurement must be taken into account because of the series reactance of capacitor CB. The loss factor of the inductor under test is balanced in terms of the Q of the inductor. The Hay bridge, then, is used for measurement of inductances having a Q greater than 10. For instance, a Q of 10 gives a calibration error of 1%, whereas a Q of 30 gives a calibration error of 0.1%. Q-5.
When you are testing an inductor with a Hay bridge, the characteristics of the inductor are compared with what type of device?
Maxwell Bridge The Maxwell bridge, shown in figure 3-1, is used for the measurement of inductance and inductive Q. This bridge is similar to the Hay bridge because it also measures inductance by comparison with a standard capacitor of known characteristics. Notice, in particular, that capacitor C B is connected in parallel with resistor R B. In connection with this difference, the requirement of an accurately known frequency is removed. This bridge circuit is employed for measuring the inductance of inductors having large losses; i.e., low Q. The range of this type of instrument is much greater than that of the Hay bridge; values ranging from 1 microhenry to 1,000 henries are measurable, with an error of only 2%. VECTOR BRIDGES The basic bridges described up to now determined the resistive and reactive components of the unknown impedance; however, the vector bridge indicates the magnitude and phase angle. Typically, vector bridges require two null readings. Consider the basic bridge circuit of figure 3-5. The magnitude of the unknown impedance (Z X) is determined by the voltages applied across R and ZX and to the bases of emitter followers Q1 and Q2, which bias the balanced rectifiers, CR1 and CR2. Resistors A and B are equal in value. When R is adjusted to equal ZX, the voltages between points 1 and 2 and between points 1 and 4 are equal in magnitude, and the vtvm will indicate 0 volts.
3-7
Figure 3-5.—Typical vector-bridge configuration (amplitude).
The absolute value of Z X is determined from the dial calibration of R. Without altering the amplitude balance, you reconnect the external circuits as shown in figure 3-6. Note that the voltage between points 1 and 3 is being compared to the voltage between points 1 and 2. Potentiometer R, calibrated in degrees, is adjusted for a null indication on the vtvm; and the phase angle is read directly. If Z X is purely resistive, the voltage between points 1 and 3 will be zero and the setting of R will be 0 volts. If ZX is purely reactive (capacitive or inductive), the setting of R will be at maximum voltage. For phase angles between 0º and 90º, the scale of R may be calibrated directly in degrees. The sign of the phase angle can be determined by changing the signal frequency slightly and observing the change in impedance. The presence of harmonics in the signal input will severely hamper the measurements. If a pure frequency source is not available, suitable low-pass filters will have to be employed in the output leads from the bridge.
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Figure 3-6.—Typical vector-bridge configuration (phase).
CONSTANT-CURRENT, IMPEDANCE-MEASURING TECHNIQUE This technique employs an oscillator circuit and a vtvm, as shown in figure 3-7.
Figure 3-7.—Constant-current, impedance-measuring method.
A large value of resistance, R, is selected so that IC is virtually independent of the range of Z X to be measured. Thus, ICZ X represents the value of voltage measured by the vtvm. If R is chosen so that the voltage drop across ZX corresponds to a full-scale reading on the vtvm, a direct reading impedance meter is realized. For example, assume that the audio oscillator open-circuit voltage is 10 volts (rms) and that the full-scale reading of the vtvm is 0.05 volt. If you want to measure ZX values ranging up to a maximum of 5,000 ohms, you should use a 1-megohm resistor for R. This will result in a full-scale, 0.05-volt deflection. An oscillator that does not produce harmonics should be used. IMPEDANCE-ANGLE METER Like vector bridges, impedance-angle meters determine an unknown impedance in terms of magnitude and phase angle. However, a non-bridge technique is used. The simplified circuit of a commercial instrument is shown in figure 3-8. With switches S1 and S2 at the BAL position, the variable
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standard resistor, R, is adjusted until the balanced rectifier outputs of Q1 and Q2 are equal (indicated by a null in the deflection of the voltmeter connected between the emitters of Q3 and Q4). The dial setting of R gives the value of ZX. For phase angle determination, the circuit is switched to CAL and the input voltage is adjusted for full-scale voltmeter deflection. The circuit is then switched to PHASE; thus, the paralleled outputs of Q1 and Q2 are applied to rectifier CR1 only. With S2 in the phase position, there is no input to the base of Q4. If Z is purely resistive, the outputs of Q1 and Q2 cancel, and the voltmeter indicates zero deflection. For a complex impedance, the base of Q3 will be unbalanced with respect to the base of Q4; and the voltmeter deflection, calibrated in degrees, determines the phase angle of the unknown impedance. Typical commercial impedance angle meters, operating at 2 MHz, are accurate to within 4% for impedances of from 10 to 500 ohms.
Figure 3-8.—Impedance-angle meter.
Q-6.
What do impedance-angle meters and vector bridges have in common?
IMPEDANCE TESTING OF ANTENNAS AND TRANSMISSION LINES The amount of current that flows in an antenna is one of the most important factors affecting the performance of transmitter equipment. As much of the rf energy generated as possible must be efficiently transferred to the antennas to secure the maximum radiated power from a transmitter. Also, for best reception, maximum transfer of energy from the antenna to the receiver must occur. Efficient transmission and reception conditions prevail whenever the transmitter (or receiver) is properly matched to the transmission line and the transmission line is properly matched to the antenna. Normally, performance tests concerning impedance match consist primarily of taking standing-wave measurements. In certain instances, it may be found that a change in antenna impedance has resulted in an undesirably high standing-wave ratio. This could be the result of a new antenna installation or an interfering structure near the antenna that influences antenna characteristics. In practice, the antenna-matching network is varied to match the new antenna characteristics, since the transmission line is designed to match equipment impedance. This can best be done by making a series of standing-wave-ratio checks and antenna-matching adjustments until an acceptable standing-wave 3-10
ratio is reached. It must be understood, however, that the antenna does have a specific impedance at a given frequency and that, when necessary, this impedance may be determined by use of an rf impedance bridge. A typical rf impedance bridge circuit is shown in figure 3-9. Rf impedance bridge measurements require an rf signal generator, a detector, and a calibrated rf bridge to determine transmission-line impedance. The bridge compares the parallel resistive-reactive combination with the series combination and can typically measure impedance over a frequency range of 500 kHz to 60 MHz.
Figure 3-9.—Typical rf bridge.
Basically, the bridge is balanced with a known capacitance under short-circuit conditions. The unknown impedance is then inserted in lieu of the short bus, and the bridge is rebalanced. The difference between the known impedance under short-circuit conditions and the balance measurements obtained with the unknown impedance inserted in lieu of the short is the value of the unknown impedance. Q-7.
What is the result of an impedance mismatch between a receiver or transmitter and its transmission line or antenna?
POWER MEASUREMENTS It is often necessary to check the input and output signal power levels of electronic equipment. The determination of dc power is computed by using a derivative of Ohm's law (P = IE = I2R = E 2/R). However, the presence of a reactive component in ac circuits means that apparent power is being measured or calculated unless the rms voltage-current value is multiplied by a power factor to obtain true lower. The measurement of ac power is further complicated by the frequency limitations of various power meters. If there is no phase difference, ac power may be computed in the same manner as dc power by determining the average value of the product of the voltage and current. In practical ac circuits, the apparent power must be multiplied by the cosine of the phase angle between the voltage and current in order to compute true power. In the repeated measurement of audio-frequency (af) power, you may use a normal power meter calibrated directly in watts. However, when reactive components of dissipative impedance introduce a
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phase angle, a device that is proportional to both the power factor and the apparent power must be used. Because power-level measurements are concerned with decibel units, a working knowledge of decibels is required for proper interpretation of power tests. The decibel is used to determine the ratio of power changes or to indicate the power level in a circuit with respect to either 0 or a standard reference level. AF POWER In the electrical transmission of speech or music, rapidly fluctuating amplitudes and frequencies are involved. The average power-level measurement and its variation rate depend on the signal characteristics and time interval over which this average is taken. Power measurements for af circuits are usually indicated in terms of decibels (dB), decibels referenced to 1 milliwatt (dBm), or volume units (vu). For example, the power gain of an amplifier can be expressed in dB; the power level of a sinusoidal signal compared to a 1-milliwatt reference is indicated in dBm; and the power level of a complex signal, such as voice, music, or multiplexed information, compared to a reference level of 1 milliwatt, is indicated in vu. Q-8.
What are the three units of measure most commonly used when referring to af power measurements?
DECIBEL METERS A dB meter is a form of ac electronic voltmeter calibrated in dB's. These meters are useful for making measurements where direct indication in decibels is desired. However, remember that these are voltmeters, and power measurements are not meaningful unless the circuit impedance is known. When the dB meter is calibrated, a reference point, based on a specific power or value of voltage across a specified resistance, is selected to represent 0 dB. Many electronic voltmeters use a single dB scale based on 1 milliwatt into a 600-ohm load to represent 0 dBm. Based on this reference point, various voltage readings could be made on the low ac-voltage scale. The +dB numbers corresponding to voltage ratios that exist between successive ranges and the low ac range have been computed for each range. These numbers, shown on the front panel of the instrument, are added algebraically to each successive range reading to produce the correct value for the range. The term decibel does not, in itself, indicate power. It indicates a ratio or comparison between two power levels that permits you to calculate the power. Often, it is more desirable to express performance measurements in terms of decibels using a fixed power level as a reference. The original standard reference level was 6 milliwatts, but to simplify calculations a standard reference level of 1 milliwatt has been adopted. Q-9.
In reference to dB meters, 0 dBm represents 1 milliwatt into what value of load?
VOLUME UNIT METERS The volume unit (vu) meter is used in audio equipment to indicate input power to a transmitter or to a transmission line. This type of meter has special characteristics, such as a standardized speed of pointer movement, speed of return, and calibration. The measurement of the average power level and its rate of variation with respect to time depends not only on the signal characteristics, but also on the time interval over which the average is being taken. Accordingly, the speed of response of the instrument used to measure average power is of particular concern. The unit of measurement is the volume unit (vu), which is numerically equal to the number of dB above or below the reference level of 1 milliwatt into a 600-ohm load (provided the standard instrument was calibrated under constant-amplitude, sine-wave conditions). A change of one vu is the same as a change of one decibel. Therefore, the vu value obtained represents averages of instantaneous power of speech or music obtained by an instrument having particular dynamic characteristics. The vu readings are equivalent to the power level in decibels only if the sinusoidal waveform is of constant amplitude. Q-10.
What is the main difference between a vu and a dB meter? 3-12
ELECTRODYNAMIC WATTMETER The electrodynamic wattmeter is used to measure power taken from ac or dc power sources. The electrodynamic wattmeter, shown in figure 3-10, uses the reaction between the magnetic fields of two current-carrying coils (or sets of coils), one fixed and the other movable. When the current through the fixed-position field winding(s) is the same as current through the load and the current through the moving coil is proportional to the load voltage, then the instantaneous pointer deflection is proportional to the instantaneous power. Since the moving pointer cannot follow the rapid variations in torque because of its momentum, it assumes a deflection proportional to the average power. The dynamometer-type wattmeter automatically compensates for the power factor error of the circuit under test. It indicates only the instantaneous power resulting from in-phase values of current and voltage. With out-of-phase relationships, a current peak through the moving coil never occurs at the same instant as the voltage peak across the load, resulting in less pointer deflection than when the current and voltage are in phase. The simple meter shown in figure 3-10 is not compensated. When the load is disconnected, this meter will still indicate that power is being consumed in the circuit. This difficulty can be eliminated by incorporating two compensating windings, mounted with the primary fixed-coil current windings, as shown in figure 311. These stationary windings are used to produce a magnetic flux proportional to the current through the movable coil. As shown by the arrows, the currents through the primary movable coil and the compensating coil flow in opposite directions, producing a torque caused by the opposing magnetic fields. These opposing fields cancel. Hence, with the load removed from the circuit, the meter will indicate zero power through the load.
Figure 3-10.—Typical electrodynamic wattmeter.
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Figure 3-11.—Electrical equivalent of the compensated electrodynamic wattmeter.
Electrodynamic wattmeters are subject to errors arising from various factors, such as temperature and frequency characteristics and vibration. Heat through the control mechanism can cause the springs to lengthen and lose tension; as a result, deflection errors are produced. Figure 3-12 illustrates the mechanical equivalent of the electrodynamic wattmeter. Large currents within the circuit will also produce errors. Therefore, the maximum current range of electrodynamic wattmeters is normally restricted to about 20 amperes. When larger load currents are involved, a current transformer of suitable range is used in conjunction with the wattmeter. However, a current transformer cannot be used if the ac circuit under test contains a dc component.
Figure 3-12.—Mechanical equivalent of the electrodynamic wattmeter.
The voltage range of wattmeters is generally limited to several hundred volts because of heat dissipation within the voltage circuit. However, the voltage range can be extended by using external voltage dividers. Wattmeters used as laboratory standards have an accuracy of 0.1%, high-grade portable wattmeters an accuracy of 0.2% to 0.25%, and high-grade switchboard wattmeters an accuracy of 1% of
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full-scale value. Because electrodynamic wattmeter errors increase with frequency, they are used primarily for measuring 60-hertz line power. Unshielded electrodynamic wattmeters should not be placed in the vicinity of stray magnetic fields. A wattmeter has current, voltage, and power ratings; therefore, damage may result when any of these ratings is exceeded. The electrodynamic wattmeter may be converted into an instrument for measuring reactive power by replacing the resistance normally in series with the voltage coil with a large inductance. A 90-degree current lag within the voltage coil provides a direct reading proportional to the reactive power in the circuit. Compensating networks must be used to cause the phase shift to be exactly 90º. Q-11.
What type of device is used to extend the current-measuring capability of electrodynamic wattmeters?
IRON-CORE, COMPOSITE-COIL, AND TORSION-HEAD WATTMETERS Iron-core wattmeters are primarily used as switchboard instruments and employ the induction principle. Voltage and current coils are wound around a laminated iron core shaped to produce a mutually perpendicular magnetic field across an air gap. Eddy currents induced in a thin metal cylinder rotating in this air gap interact with the magnetic field to produce a torque proportional to the instantaneous power. This type of construction provides the advantages of increased operating torque, larger angles of rotation, ruggedness, compactness, and freedom from errors caused by stray fields. It has the disadvantage of a very narrow frequency range. The composite-coil wattmeter uses the upscale torque, produced by the ac power being measured, in opposition to the torque produced by an adjustable dc current in a set of windings intermingled or wound within the ac windings. Greater reading precision is obtained with this method than is possible with straightforward wattmeters, and errors caused by elasticity of the spring suspension carrying the movingcoil system are avoided. The torsion-head wattmeter is used to restore the movable coil to its original position after deflection and to remove the mutual inductance error. ELECTRONIC WATTMETER Electronic wattmeters are used for direct, small power measurements or for power measurements at frequencies beyond the range of electrodynamometer-type instruments. A simplified electronic wattmeter circuit is shown in figure 3-13. The matched triodes are operated in the nonlinear portion of their characteristic grid-voltage, plate-current curves. The symmetrical resistive T network between the generator and load will provide V1 and V2 voltages proportional to, and in phase with, the load current and voltage, respectively. A source of ac power is connected to the load through the series resistors R1 and R2. These two resistors are of equal value and are made small to prevent the voltage drop across them from reducing the load voltage appreciably. R3 is made large enough to have negligible power consumption. Therefore, the R3 voltage is equal to the load voltage, and the voltage across either series resistor is proportional to the difference in the output currents of the tubes. The average value of the difference could be measured by a dc meter connected to read the voltage potential between the grids of V1 and V2. This method is adequate only at low frequencies. As the frequency increases, the stray capacitances and inductances also increase. The frequency range of the electronic wattmeter can be extended up to 20 megahertz by using pentodes instead of triode tubes. The operating conditions in a pentode are adjusted so that plate current is proportional to the product of a linear function of plate voltage and an exponential function of grid voltage.
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Figure 3-13.—Simple electronic wattmeter circuit.
Q-12.
For power measurements, what advantage does an electronic wattmeter have over an electrodynamic wattmeter?
ABSORPTION POWER METERS Absorption power meters absorb either all or part of the source power. They require means of dissipating the absorbed power, sensing the power thus dissipated, and indicating the amount of power absorbed by the sensing network. Output power meters, in-line wattmeters, and meters employing bolometers are examples of absorption power meters used by the Navy. Output Power Meters Figure 3-14 shows a common output power meter used in vhf-uhf applications. It has a 0- to 150watt range covered in two steps: 0-50 watts and 0-150 watts. Attenuator AT1 provides a 50-ohm nominal resistive (dummy) load and uses metal film on glass construction. This dummy load is tapped to provide the proper operating voltage to the meter. Resistors R3 and R5 form a calibration network at 50 watts; R7 and R8 form a calibration network at 150 watts. Accuracy, at approximately 20º C, is ±5% for frequencies between 30 MHz and 600 MHz, ±10% for frequencies between 0.6 GHz and 0.8 GHz, and ±20% for frequencies between 0.8 and 1.0 GHz. When radio-frequency (rf) power is applied to AT1, this attenuator minimizes the effects of power factors generated by any reactive components. The rf energy is then detected and filtered by CR1 and C1, respectively. The resultant dc voltage, which is proportional to the input power, is applied to a sensitive microammeter via one of the calibration networks. This meter has a scale provided with two ranges: 0-50 watts and 0-150 watts. To protect the meter, you should always try the higher range first. If the value proves to be under 50 watts, a shift to the lower scale would provide improved accuracy.
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Figure 3-14.—Vhf-uhf wattmeter.
In-Line Wattmeters The AN/URM-120 in-line wattmeter, shown in figure 3-15, measures power applied to a 50-ohm impedance load and the power reflected from that load. The internal directional coupler is oriented such that it responds only to a wave traveling in one direction on the transmission line. The coupler can be rotated to accommodate either incidental or reflected power. The rf is then rectified, filtered, and applied to the meter, which is scaled in watts. The rf power of 50 to 1,000 watts can be measured between the frequencies of 2 MHz to 30 MHz; and 10 to 500 watts, between the frequencies of 30 MHz to 1,000 MHz.
Figure 3-15.—Typical in-line wattmeter.
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Q-13.
What is the advantage of using in-line wattmeters over output power meters?
Bolometer A bolometer features a specially constructed element of temperature-sensitive material. The active material is a semiconductor bead supported between two pigtail leads. When rf power is applied to a bolometer element, the power absorption by the element heats the element and causes a change in its electrical resistance. Thus, a bolometer can be used in a bridge circuit so that small resistance changes can be easily detected and power measurement can be accomplished by the substitution method (that is, substitution of dc or low-frequency power to produce an equivalent heating effect). A D'Arsonval meter movement is usually employed as the null indicator. According to one principle of measurement (the principle used in the balanced bridge), the bridge is initially balanced with low-frequency bias power. Rf power is then applied to the bolometer and the bias power is gradually removed until the bridge is again balanced. The actual rf power is then equal to the bias power removed. According to another principle of measurement (the principle used in the unbalanced bridge), the bridge is not rebalanced after the rf power is applied. Rather, the indicator reading is converted directly into power by calibration previously performed. Figure 3-16 illustrates the basic bolometer bridge circuit. The bolometer element must be physically small to be highly sensitive; it must be equally responsive to low-frequency and rf power; and it must be matched to the rf-input power line. The cross-sectional dimension of the bolometer element is approximately equal to the skin depth of rf current penetration at the highest frequency of operation. This condition permits the dc and rf resistivities to be essentially equal with the reactive component of the bolometer impedance at a minimum. Thermistors, which are a type of bolometer, use semiconductor material shaped like a bead, with a thicker skin depth and shorter length to minimize standing-wave effects. These physical properties assure correspondence between lengthwise low-frequency and rf power distribution to provide the necessary inherent accuracy of the bolometer.
Figure 3-16.—Basic bolometer bridge circuit.
An air-mounted bolometer provides a power sensitivity 100 or more times greater than that provided by static calorimetric devices. Additional sensitivity may be obtained by mounting the element within an evacuated envelope to eliminate convective heat loss. The small size of bolometer elements is associated with small thermal mass and short thermal time constants. The thermal time constant varies directly with the volume-to-area ratio of the element for a particular shape and composition. Typical time is up to 0.1 3-18
second for thermistor beads. The thermistor type of bolometer element is usually composed of a ceramiclike mixture of metallic oxides having a large negative temperature coefficient of resistance. Two fine platinum-alloy wires are embedded in the bead, after which the bead is heated and coated with a glass film. Typical dimensions of a thermistor bead used for microwave measurements are 0.015 inch along its major axis and 0.010 inch along its minor axis. The thermistor bead may be operated at high temperatures; it is rugged, both electrically and mechanically; it has high resistance-power sensitivity; and it has a good temperature-power sensitivity. In addition, it can endure large pulse energies; it has a sluggish thermal response; and it has negligible pulsed-power measurement errors. The more sensitive thermistor requires thermal shielding or heat compensation for best operation. Q-14.
What type of material is used in the construction of bolometers and thermistors?
Bolometer Power Meter The standard power meter used in the Navy (Hewlett-Packard 431 C) is an automatic self-balancing instrument employing dual-bridge circuits. It is designed to operate with temperature-compensated thermistor mounts that enable you to measure power in a 50-ohm coaxial system from 10 MHz to 18 GHz and in a waveguide system from 2.6 GHz to 40 GHz. This power meter can be operated from either an ac or a dc primary power source. The ac source can be either 115 or 230 volts at 50 to 400 hertz. The dc source is a 24-volt rechargeable battery. A seven-position range switch allows full-scale power measurements of 10 microwatts to 10 milliwatts or of −20 dBm to +10 dBm. These ranges can be further extended with the aid of attenuators. The thermistor mount (as shown in fig. 3-17) contains two thermistors: one in the detection bridge, which absorbs the microwave power to be measured, and the other in the compensation and metering bridge, which supplies temperature compensation and converts the measured rf power to a meter indication. Each bridge includes its respective thermistor element as a bridge arm.
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Figure 3-17.—Power meter.
Basically, the power meter circuit consists of two bridges; each bridge includes one of the thermistor elements as a bridge arm. The bridges are made self-balancing through the use of feedback loops. Positive or regenerative feedback is used in feedback loop 1; degenerative (negative) feedback is used in feedback 3-20
loop 2. Both bridges are excited by a common 10-kHz source. The 10-kHz amplifier-oscillator supplies 10-kHz power to bias the thermistor in feedback loop 1 to produce the resistance required to balance the rf bridge. An equal amount of 10-kHz power is supplied by the same oscillator to the second thermistor in feedback loop 2 through two series-connected transformers. Feedback loop 2 balances the meter bridge. When rf is applied to the thermistor in the detection bridge (but not to the compensation and metering bridge), an amount of 10-kHz power is present, equal to the rf power being removed from the detection bridge by the self-balancing action of the bridge. Since the rf power replaced the 10-kHz power, the detection bridge is in balance; however, the metering bridge must be balanced by its separate feedback loop. Sufficient dc power to equal the 10-kHz power lost by the metering bridge is automatically replaced, balancing this loop. Hence the dc power applied to the metering bridge thermistor is equal to the microwave power applied to the detection bridge. The meter circuit senses the magnitude of the feedback current. The resultant meter current passes through a differential amplifier to the indicating meter. The two thermistors are matched with respect to their temperature characteristics; therefore, there is only a very small amount of drift of the zero point with ambient temperature changes. When there is a change in temperature, there is a change in the electrical power needed by the thermistors to maintain constant operating resistances. This change is automatically performed by feedback loop 1, which changes the amount of 10-kHz power for both thermistors by the proper amount. The dc power in feedback loop 2 is not changed; and since it is this dc power that is metered, the temperature change has not affected the meter indication. CALORIMETERS The calorimeters are the most accurate of all instruments for measuring high power. Calorimeters depend on the complete conversion of the input electromagnetic energy into heat. Direct heating requires the measurement of the heating effect on the medium, or load, terminating the line. Indirect heating requires the measurement of the heating effect on a medium or body other than the original powerabsorbing material. Power measurement with true calorimeter methods is based solely on temperature, mass, and time. Substitution methods use a known, low-frequency power to produce the same physical effect as an unknown rf power being measured. Calorimeters are classified as STATIC (nonflow) types and CIRCULATING (flow) types. Q-15.
Power measurements performed with calorimeters are based on what three variables?
Static Calorimeters The static calorimeter uses a thermally shielded body. Since an isolated body loses little heat to a surrounding medium, the temperature increase of the body is in direct proportion to the time of applied power. The product of the rate of temperature rise in the calorimetric body and its heat capacity equals applied power. Figure 3-18 illustrates a static-type calorimeter.
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Figure 3-18.—Static calorimeter using low-frequency power substitution.
The most common type of static calorimeter is the ADIABATIC calorimeter. In the adiabatic meter, power is applied directly to a thermally isolated body; and the rate of temperature rise is determined from a temperature change measurement during a sufficiently long, known time interval. Figure 3-19 illustrates an adiabatic calorimeter using water as the body contained in a covered Dewar flask. A tapered-wall, open-ended waveguide contains a sealed, inclined glass partition to create a wedge-shaped water load of low-reflection coefficient. Thorough mixing of the water is accomplished with a stirrer, and a sensitive thermometer measures the temperature rise. A heating coil is wound around the waveguide inside the calorimeter and is used for calibrating purposes when low-frequency power is applied. This type of meter can be used for accurate measurement of several hundred watts of average power and can withstand 50 kilowatts of peak power.
Figure 3-19.—Adiabatic calorimeter.
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The NONADIABATIC calorimeter uses an rf termination with a resistive film strip or LOSSY dielectric materials (solids or liquids that are designed to efficiently dissipate the applied power) as a load. Temperature indication can be accomplished with thermocouples, thermopiles, thermistors, thermometers, bimetallic strips, and manometers. Calibration is against a power standard or known lowfrequency power. Based on the above principle, a coaxial calorimeter of good sensitivity with a short, 50-ohm resistive film on a lava (dielectric) center conductor, enclosed within a tapered, thin-walled outer conductor, is used for frequencies between 0 and 1.2 GHz. The rf termination is electrically connected to, but thermally isolated from, a massive mounting plate by a short section of silvered-lava coaxial line with a high thermal resistance. The steady-state temperature rise of the outer casing of the load with respect to the mounting plate is measured by a differential platinum-resistance thermometer in a Wheatstone bridge. Low-frequency power applied to the termination provides a method of calibration. Power in the range of 0 to 2.5 watts may be measured. A 70-second time constant and steady-state temperatures are attained in about 6 minutes. The small physical size of termination (to keep convective and radiative heat losses low) provides high sensitivity. Calibration with lower frequency power is extremely accurate, because the termination is broadband and should exhibit the same power distribution from dc to 10 gigahertz. A twin calorimeter provides a method of using two calorimetric bodies thermally shielded against ambient temperature variations and improves sensitivity. Figure 3-20 illustrates this type of calorimetric device. The power to be measured is applied to one calorimetric body; the other calorimetric body acts as a temperature reference. The steady-state temperature difference between the two calorimeters is used as a measure of rf power. Calibration is performed by applying low-frequency power. A differential-air, thermometer-type temperature difference indicator, shown in figure 3-21, is used with a twin calorimeter to measure microwave power in the 0.1-mW range. This instrument consists of two similar glass cells connected by a capillary tube containing a liquid pellet. Each glass cell contains a tapered, carbon-coated strip; and the entire assembly is mounted in a rectangular waveguide. Balancing dc power heats one strip; the other strip is heated by rf power. The liquid pellet, which indicates the differential expansion of the air within the two cells, is viewed through an aperture in the waveguide wall, preferably with a microscopy for highest sensitivity. This procedure permits a 2% accuracy at 10 mW.
Figure 3-20.—Twin calorimetric system.
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Figure 3-21.—Differential-air, thermometer-type calorimeter.
Flow Calorimeters Flow calorimeters are classified by the type of circulating method used (open or closed), the type of heating used (direct or indirect), and the type of measurement performed (true calorimetric or substitution). Water or other calorimetric fluid is used only once in an open system. An overflow system is used to maintain a constant rate of flow. Closed systems recirculate the fluid continuously by means of a pump, and a cooling system restores the fluid to ambient temperatures prior to its return to the calorimeter. Closed systems are more elaborate and permit the use of fluids other than water. Flow calorimeters provide the primary standards for the measurement of high power levels; and, in conjunction with calibrated directional couplers, attenuators, power dividers, or other similar devices serve to standardize medium- and low-power measuring instruments. The measurement time depends on the required time for the entering fluid to reach the outlet, where the rise in temperature is measured. The circulating fluid may serve in a dual capacity as the dissipative medium and coolant, using the direct heating method, or solely as a coolant, using the indirect heating method. Because of its excellent thermal properties and high dielectric losses at 1 GHz or higher, water is normally used in both heating methods. Water is rarely used as the fluid at frequencies lower than 100 MHz, because of insufficient dielectric losses. The indirect heating method offers a wider frequency and power-range coverage and can be used in substitution-type measurements. True calorimetric measurements contain appreciable error, because of nonuniformity of flow rate, air bubbles, flow-rate measurement inaccuracies, and temperature rise. Flow regulators, bubble traps, and good thermal insulation are required to eliminate the majority of these errors. Substitution methods do not involve direct heat dissipation measurement of moving fluid. Greater accuracy is obtained because known low-frequency power is substituted for the unknown rf power, with all other measurement parameters remaining constant. The accuracy depends on the exactness of the low-frequency power determination and the degree to which factors remain fixed during the substitution of one type of power with another. Figure 3-22 illustrates a flow calorimeter using low-frequency power substitution. Two different measurement techniques are possible with this type of meter: the calibration technique and the balance technique. The CALIBRATION TECHNIQUE uses an adjustable known power to exactly reproduce the same temperature indication originally obtained by the unknown rf power measurement. The BALANCE TECHNIQUE uses an initial low-frequency power (P1) to provide a steady-state temperature rise in the calorimetric fluid. When unknown rf power is applied, the original power (P 1) is reduced to a new power (P2) to maintain the same temperature indication. Therefore, the actual power equals P1 minus P 2. Figure 3-23 illustrates a widely used method of power measurement using a balanced-flow calorimeter. Temperature-sensitive resistors are bridge-connected as the thermometric elements and are balanced at ambient temperature prior to the application of power. Low-frequency balancing power and the unknown 3-24
rf power are applied to maintain the bridge at null. This occurs when the temperature rise caused by the unknown rf power equals the temperature rise caused by the known low-frequency power.
Figure 3-22.—Flow calorimetric system using substitution at low-frequency power.
Figure 3-23.—Balanced-flow calorimeter.
Q-16.
What is the result of applying power to a calorimeter?
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FREQUENCY MEASUREMENTS Frequency measurements are an essential part of preventive and corrective maintenance for communication and electronic equipment. Rotation frequencies of some mechanical devices must be determined; the output frequency of electric power generators is checked when the engine is started and during preventive maintenance routines; carrier equipment that operates in the audio-frequency range must be adjusted to operate at the correct frequencies; and radio transmitters must be accurately tuned to the assigned frequencies to provide reliable communications and to avoid interfering with radio circuits operating on other frequencies. These are only a few of the applications for making frequency measurements. FREQUENCY-MEASUREMENT METHODS Frequency-measuring equipment and devices, particularly those used to determine radio frequencies, constitute a distinct class of test equipment, because of the important and critical nature of such measurements. The requirement of precise calibration is extremely important in all frequency-measuring work. To provide accurate measurements, every type of frequency-measuring device must be calibrated against some frequency standard. FREQUENCY STANDARDS Of considerable importance in the measurements of frequency or wavelength are the standards against which frequency-measuring devices are compared and calibrated. Frequency standards belong to two general categories: primary and secondary standards. The PRIMARY FREQUENCY STANDARD maintained by the U.S. National Bureau of Standards has long-term stability and an accuracy of 1 part in 1012, using an atomic clock. A SECONDARY FREQUENCY STANDARD is a highly stable and accurate standard that has been calibrated against the primary standard. Secondary standards are maintained by calibration laboratories that service your test equipment. The National Bureau of Standards provides time and frequency standards from station WWV at Fort Collins, Colorado, and from station WWVH at Kekaha, Kauai, Hawaii. The following technical radio services are given continuously by these stations: • Standard radio frequencies • Standard audio frequencies • Standard time intervals • Standard musical pitch • Time signals • Radio propagation notices (WWV only) • Geophysical alerts • Universal Time Coordinated (UTC) • + UT1 Corrections The UTC scale uses the ATOMIC SECOND as a time interval. UT 1 is based on the earth's uniform rate of rotation. Since the earth's rotation is not precisely uniform, UT1 is an adjustable interval.
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To ensure reliable coverage of the United States and extensive coverage of other parts of the world, radio stations WWV and WWVH provide the primary standard radio frequencies listed in table 3-1. The transmission of WWV and WWVH are interrupted for 5 minutes of each hour. The silent period begins at 15 minutes past the hour for station WWVH and 45 minutes past the hour for station WWV. These silent periods are provided to eliminate errors caused by interference.
Table 3-1.—NBS Frequency Standards and Time Transmission
TRANSMISSION RF Signal Frequency MHz Frequency Stability Frequency Deviation Seconds Frequency and Duration Audio Tones Frequency Accuracy Propagation Forecast
WWV 5, 10, and 15 1 part in 1011 1 part in 1012 per day 5 cycles of 1000Hz for .005 seconds 600Hz and 500Hz with 440Hz to mark the hour 1 part in 1012 14 min. past the hour (in voice)
WWVH 5, 10, and 15 1 part in 1011 1 part in 1012 per day 6 cycles of 1200Hz for .005 seconds 600Hz and 500Hz with 440Hz to mark the hour 1 part in 1012 None
Two primary standard audio-frequency tones (440 Hz and 600 Hz) are broadcast on all WWV and WWVH carrier frequencies. In the absence of a message, a 500-Hz tone is broadcast during the message interval. The 440-Hz signal that denotes the 1-hour mark is the standard musical pitch, A above middle C. The 600-Hz tone provides a frequency standard for checking the 60-Hz power-line frequency. The standard time pulse marking interval of 1 second consists of five cycles of a 1,000-Hz tone at WWV and six cycles of a 1,200-Hz tone at WWVH. These marker pulses are heard as clock ticks. Intervals of 1 minute are marked by a 0.8-second, 100-Hz tone for WWV and a 0.8-second, 1,200-Hz tone for WWVH. Each hour is marked by a 0.8-second, 1,500-Hz tone on both stations. Universal Time Coordinated (UTC) is announced on WWVH between the 45 and 52.5 seconds of each minute and on WWV between the 52.5 and 60 seconds of each minute. An announcement of radio propagation conditions (geophysical alert) for the North Atlantic area is broadcast by station WWV in voice at 18 minutes after each hour. For example, these short-term announcements might state, "The radio propagation quality forecast at ... (normal, unsettled, disturbed)." The propagation format is repeated phonetically and in numerical code to ensure clarity. The letter designations N, U, and W, signifying "normal," unsettled," and "disturbed," respectively, classify the radio propagation conditions at the time of the broadcast. The digits from 1 to 9 indicate the expected radio propagation conditions during the next 6 hours; refer to table 3-2 for code interpretations. The National Bureau of Standards forecasts are based on information obtained from a worldwide network of geophysical and solar observations.
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Table 3-2.—NBS Radio Propagation Coding
PHONETIC Whiskey Uniform Normal NUMERAL 1 2 3 4 5 6 7 8 9
Q-17.
PROPAGATION CONDITION Disturbed Unsettled Normal Useless Very poor Poor Poor to fair Fair Fair to good Good Very good Excellent
What government agency is responsible for monitoring our primary frequency standards?
MECHANICAL ROTATION AND VIBRATION METHODS There are many instances when you are very much concerned with the question of rotational or vibratory speeds. Knowledge of rotational speeds is necessary where the output of a direct current generator has fallen below a minimum desired output or where the speed of a motor (such as the motor in a teletypewriter or radar antenna) must be maintained at a constant value. There are many instruments that you can use for this purpose, such as tuning forks, stroboscopes, vibrating-reed meters, and electromechanical counters. The oscilloscope and the frequency counter are two of the other devices which may be used, but their use may require the employment of accessory equipment. Tuning Fork Methods A tuning fork is generally used in conjunction with the measurement of the rotational speed of a teletypewriter or facsimile motor but is not limited to this application. However, you must remember that the tuning fork can be used at only one frequency, the frequency of vibration for which it was manufactured, and therefore cannot be used on variable-speed motors. To use the tuning fork, you direct a source of light upon the point to be observed. In the case of a teletypewriter, a black-and-white segmented target is painted on the outer circumference of the motor governor. Radial spokes in a flywheel could be used equally well. Permit the motor to reach operational speed under normal load conditions; otherwise, the motor will slow down considerably when the normal load is applied. Strike the tuning fork against the side of your hand to set it into vibration. Then observe the target through the slots in the plates attached to the tines of the fork. The correct speed is obtained when the segments of the target appear to be stationary. If the segments seem to move backward, apparently against the known motor rotational direction, the speed is too low. If the segments seem to move forward, the speed is too high. There is also the possibility that the target segments will appear to jump back and forth or to disappear suddenly. Such erratic action is often because of governor malfunctioning. The correct speed adjustment is reached when the targets appear to be stationary. Q-18.
What is the primary measurement application for tuning forks?
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Stroboscope Methods When using a stroboscope to measure the speed of rotating or reciprocating mechanisms, hold the instrument so that the light from the stroboscope lamp falls directly on the part to be observed. If the part is uniform, or symmetrical, place an identification mark with chalk or a grease pencil on the portion to be observed. This method provides a positive means of identification, because if only one reference mark is observed during measurement, you can be sure that either the fundamental synchronization or a submultiple thereof has been obtained. If the approximate speed of rotation is known, the stroboscope controls may be set to the appropriate positions prior to actual measurement. The main frequency control that determines the rate of the flashing light is then varied until the reference mark on the moving part appears to be standing still. The calibrated scale of the stroboscope will then show the speed directly in revolutions per minute (rpm). If you have no idea of the speed of the moving part, it is best to start the measurement procedure at the highest frequency that the stroboscope can deliver. The flashing rate of the stroboscope can then be gradually reduced until a single stationary image of the reference mark is obtained. This is the point of fundamental synchronism that corresponds to the speed of the moving part. Do not continue to reduce the flashing rate of the instrument beyond this point without a valid reason for doing so. If you do continue the reduction, a stationary image will still be observed, but the stroboscope will indicate a submultiple of the true rotational speed; thus, a measurement error will be introduced. Stroboscopes generally have a high- and low-range switch. The typical low range is from 600 to 3,600 rpm, and the upper range is from 3,600 to 15,000 rpm; there is a slight overlap in ranges to ensure reliable frequency coverage. In view of the limitation imposed by flasher tube life, the stroboscope should always be operated at a flashing rate that is as low as possible, consistent with the rotational speed of the observed part. If you should be required to operate this instrument over a long period of time, use a submultiple of the fundamental synchronous speed. The pattern will remain just as stationary, and the tube life will be greatly extended. In addition, the quality of the light is better at the lower ranges than at the upper end of the scale. Sometimes you will encounter a rotating or vibrating device that is moving faster (or slower) than the measuring range of the stroboscope will accommodate. Although such speeds can still be measured, you must use the multiple or submultiple synchronism points. There are two methods of measuring high speeds. The first method is to obtain a single stationary image of the rotating object at a subharmonic speed relationship and to record that value as A. Then obtain a second single stationary image at the next lower subharmonic speed relationship, and record this value as B. The unknown speed may then be computed from the following formula:
For example, assume reading A was 4,000 rpm and reading B was 3,500 rpm. The computation would be as follows:
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The second method is used where the value of A × B becomes progressively smaller. The A reading is obtained as in the previous example (for the sake of easier computation, suppose that the A reading is still 4,000 rpm). Then obtain another submultiple reading for B, keeping in mind the number of times a stationary single image was observed. If a stationary single image was observed seven different times and the final B reading was 2,000 rpm, the calculation would become as follows:
At speeds lower than the lowest range of the stroboscope, multiple images will be observed. For example, assume a dial reading of 900 rpm was obtained when two stationary images were observed. Then dividing the rpm by the number of images will give the unknown shaft speed, as shown below:
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WARNING Exercise caution in using a stroboscope. The illusion of stopped motion is very convincing. Do not attempt to touch the moving equipment. Q-19.
If you are required to monitor the speed of a device with a stroboscope over an extended period of time, what step should be taken to prolong the life of the flasher tube?
Frequency Counter Methods Various frequency counters have found application as an ELECTRONIC TACHOMETER to obtain accurate measurements of high-speed rotating machinery. A tachometer pickup may be used to produce signals that are fed directly to the frequency counter. If the tachometer pickup is designed to generate 1 signal per revolution, the counter will indicate directly in revolutions per second; if the pickup is designed to produce 60 signals per revolution, the counter will indicate directly in revolutions per minute. AUDIO-FREQUENCY MEASUREMENTS Audio-frequencies can be measured with a variety of nonelectronic and electronic devices. Examples of nonelectronic measuring devices are the vibrating-reed meter and the moving-disk frequency meter. (Both of these devices were discussed in NEETS, module 3.) They are used primarily to measure the frequency of ac power, 60 Hz. However, such instruments do not have a wide frequency range. The most common instruments available for the measurement of audio frequencies are oscilloscopes and frequency counters. OSCILLOSCOPE METHOD The frequency of a waveform can readily be determined by using an oscilloscope. The most common oscilloscope method of measuring a frequency is accomplished by first measuring the time duration of the waveform. Frequency is the reciprocal of time
and may be easily computed, as shown in figure 3-24.
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Figure 3-24.—Oscilloscope method of determining frequency.
Another common method of determining the frequency of a waveform is by using Lissajous patterns. This method was discussed in NEETS, module 19. FREQUENCY COUNTER METHOD While oscilloscopes can be used to compare rectangular waveforms for the purpose of measuring the frequency of a signal, frequency counters, as shown in figure 3-25, are much more useful for this purpose. The fundamental measurement of frequency is accomplished by totaling the number of cycles into the counter for a precise period of time. The result is then displayed as an exact digital readout. The audiofrequency signal must be of sufficient amplitude to trigger the counter. The AUTO-MANUAL switch provides two methods of frequency counter operation. One method is to initiate the count simultaneously with the initiation of the signal to be measured. With this method, the AUTO-MANUAL switch should be set to the MANUAL position. The second method assumes that the signal to be measured has been operating over some indefinite period of time and that it will continue to do so after a measurement has been taken (hence, only that segment of the signal required to make the frequency measurement is important). With this method, the AUTO-MANUAL switch is to be set to the AUTO position.
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Figure 3-25.—Frequency counter.
RADIO-FREQUENCY (RF) MEASUREMENTS Radio-frequency measurements are primarily made with frequency counters. Most oscilloscopes are limited in use to approximately 100 MHz. Frequency meters, such as the Hewlett-Packard 530 series, are widely used but lack the accuracy of frequency counters. Frequency Meters Prior to the invention of the frequency counter, most frequency measurements above the af range were made primarily with frequency meters. This process involved heterodyning the frequency to be measured against the calibrated output of the frequency meter to obtain a zero beat from which the measured frequency was then read. This method proved inaccurate because of reading errors. Frequency meters as we know them today are entirely different from their predecessors. Today's frequency meters (fig. 3-26) contain waveguide or coaxial lines coupled to quarter-wavelength resonant cavities. The meter is adjusted until the cavity is tuned to the resonant frequency of the signal being measured. At resonance, power is absorbed by the cavity and produces a dip in the output-power level, as measured at the frequency meter's output connector. The resonant frequency is read directly from the frequency meter dial and is accurate, in most cases, to approximately ±0.2%. Frequency meters are capable of measuring frequencies in the range of 1 to 40 gigahertz, far exceeding the frequency limitations of the average frequency counter. Q-20.
What happens when a frequency meter is adjusted to the frequency of the signal being measured?
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Figure 3-26.—Frequency counter.
Frequency Counters In the early 1950s, the frequency counter was developed. The device could measure and accurately indicate frequencies up to 10 MHz. Present-day frequency counters can accurately read frequencies as high as 40 GHz. In addition to direct frequency measurement indication, some types of frequency counters can measure the WAVE PERIOD, which is the inverse of frequency; RATIO, which compares one frequency against another; and TIME INTERVAL, the time between two events or the time between two functions of an event. In addition, frequency counters can totalize event indications. This is similar to measuring the frequency except that a manual or an electronic start-stop gate controls the time over which the measurement is taken. Frequency counters can also provide scaling in the form of a digital output signal from the frequency counter that represents a frequency-related division of the input frequency. All of the above functions have useful applications. For pulse timing, the period function is used; totalizing is used in digital applications; and ratio is used in comparing harmonic-related signals. Scaling is used for triggering other test equipment used in conjunction with the frequency counter; and timeinterval capability is used in measuring the interval between two pulses or between two sets of pulses. Because of the wide variety of frequency counters in use, the technical manual for a specific frequency counter should be consulted to determine the instrument's full capabilities. Frequency Counter Accuracy All frequency counter measurements are measured with 1 part in 108 of accuracy. However, frequency counters have provisions for input from external frequency standards. This extends the accuracy of the frequency to that of the standard. A frequency self-check capability is provided to determine if the counting and lighting circuits are operating properly.
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Wavemeters Wavemeters are calibrated resonant circuits used to measure frequency. Although the accuracy of wavemeters is not as high as that of heterodyne frequency meters, they have the advantage of being comparatively simple and can be easily carried about. Any type of resonant circuit may be used in wavemeter applications. The exact kind of circuit employed depends on the frequency range for which the meter is intended. Resonant circuits consisting of coils and capacitors are used for low-frequency wavemeters. Butterfly circuits, adjustable transmission line sections, and resonant cavities are used in vhf and microwave instruments. There are three basic kinds of wavemeters: the absorption, the reaction, and the transmission types. Absorption wavemeters are composed of the basic resonant circuit, a rectifier, and a meter for indicating the amount of current induced into the wavemeter. In use, this type of wavemeter is loosely coupled to the circuit to be measured. The resonant circuit of the wavemeter is then adjusted until the current meter shows a maximum deflection. The frequency of the circuit under test is then determined from the calibrated dial of the wavemeter. The reaction type derives its name from the fact that it is adjusted until a marked reaction occurs in the circuit being measured. For example, the wavemeter is loosely coupled to an oscillator, and the resonant circuit of the meter is adjusted until it is in resonance with the oscillator frequency. The setting of the wavemeter dial is made by observing the output current of the oscillator. At resonance, the wavemeter circuit takes energy from the oscillator, causing the current to dip sharply. The frequency of the oscillator is then determined from the calibrated dial of the wavemeter. The transmission wavemeter is an adjustable coupling link. When it is inserted between a source of rf energy and an indicator, energy is transmitted to the indicator only when the wavemeter is tuned to the frequency of the source. Transmission wavemeters are widely used in measuring microwave frequencies. In figure 3-27, a typical cavity wavemeter is illustrated.
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Figure 3-27.—Typical cavity wavemeter.
The wavemeter illustrated is of the type commonly used for the measurement of microwave frequencies. The device employs a resonant cavity, which effectively acts as a high-Q LC tank circuit. The resonant frequency of the cavity is varied by means of a plunger that is mechanically connected to a micrometer mechanism. Movement of the plunger into the cavity reduces the cavity size and increases the resonant frequency. Conversely, an increase in the size of the cavity (made by withdrawing the plunger) lowers the resonant frequency. The microwave energy from the equipment under test is fed into the wavemeter through one of two inputs, A or D. A crystal rectifier then detects or rectifies the signal, and the rectified current is indicated on the current meter, M. The instrument can be used as either a transmission type or an absorption type of wavemeter. When used as a transmission wavemeter, the unknown signal is coupled into the circuit by means of input A. When the cavity is tuned to the resonant frequency of the signal, energy is coupled through coupling loop B into the cavity and out through loop C to the crystal rectifier where it is rectified and indicated on the meter. At frequencies off resonance little or no current flows in the detector and the meter reading is small. Therefore, the micrometer and attached plunger are varied until a maximum meter reading is obtained. The micrometer setting is then compared with a calibration chart supplied with the wavemeter to determine the unknown frequency. When the unknown signal is relatively weak, such as the signal from a klystron oscillator, the wavemeter is usually used as an absorption type of device. Connection is made to the instrument at input D. Rf loop C then acts as an injection loop to the cavity. When the cavity is tuned to the resonant frequency of the klystron, maximum energy is absorbed by the cavity, and the current indicated on the meter dips. When the cavity is not tuned to the frequency of the klystron, high current is indicated on the current meter. Therefore, the cavity is tuned for a minimum reading, or dip, in the meter; and the resonant frequency is determined from the micrometer setting and the calibration chart. 3-36
The potentiometer, R1, is used to adjust the sensitivity of the meter from the front panel of the instrument. J1 is a video jack and is provided for observing video waveforms with a test oscilloscope.
SUMMARY The following is a brief summary of the important points of this chapter. All IMPEDANCE BRIDGES have several things in common. Each type of bridge has a comparing circuit and a measuring circuit. They measure an unknown impedance by comparing the characteristics of the device under test with the characteristics of components within the test set. POWER METERS that are designed to measure af power can be separated into two distinct groups. Power meters that are designed for measuring sine waves are basically electronic voltmeters calibrated in dB or dBm. VU METERS are designed to measure the average value of complex waveforms, such as a voice.
The most common type of test equipment used to measure rf power is the ABSORPTION POWER METER. Absorption power meters are designed to absorb all or part of the signal being measured. Examples of absorption power meters are output power meters, in-line wattmeters, and meters employing bolometers.
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CALORIMETERS are the most accurate type of test equipment used for measuring high power. As power is applied to a calorimeter, its medium (either liquid or solid) is heated. The heat that is produced is directly proportional to the amount of applied power. The amount of applied power is determined by measuring the change in temperature of the medium. Today's ELECTRONIC FREQUENCY COUNTERS are capable of measuring frequencies from dc to 40 GHz. Most have added features that enable period averaging, time-interval measurements, and scaling. Frequency counter accuracy can be extended by using an external frequency standard in lieu of its internal frequency standard.
REFERENCES Coaxial Frequency Meter, NAVSHIPS 0969-092-3010, Naval Ships Systems Command, Washington, D.C., 1978. EIMB, Test Methods and Practices, NAVSEA 08967-LP-000-0130, Naval Sea Systems Command, Washington, D.C., 1980. Fire Control Technician (M) 3 & 2, NAVEDTRA 10209-A, Naval Education Training and Program Development Center, Pensacola, Fla., 1974. 3-38
NEETS, Module 16, Introduction to Test Equipment, NAVEDTRA 172-16-00-84, Naval Education and Training Professional Development and Technology Center, Pensacola, Fla., 1984.
ANSWERS TO QUESTIONS Q1 THROUGH Q20. A-1.
A bridge circuit is balanced when the opposite legs of the comparing and measuring circuits exhibit the same voltage drop.
A-2.
The capacitive and inductive characteristics of the test leads.
A-3.
As the supply voltage increases, bridge components may heat up and become less accurate.
A-4.
Small values of resistances.
A-5.
A standard capacitor.
A-6.
Both measure phase angle and magnitude in determining impedance.
A-7.
High vswr, which equates to poor reception or a loss of power output.
A-8.
DB, dBm, and vu.
A-9.
600-ohm load.
A-10.
DB meters are used for measuring sine waves. Vu meters are used to measure the average value of complex waveforms.
A-11.
Current transformers.
A-12.
Electronic wattmeters are capable of measuring high-fequency signals.
A-13.
Most in-line wattmeters are capable of measuring both forward and reflected power.
A-14.
Temperature-sensitive material that exhibits a large negative temperature coefficient.
A-15.
Temperature, mass, and time.
A-16.
As power is applied, the medium heats up in proportion to the applied power.
A-17.
The National Bureau of Standards.
A-18.
They are used to monitor fixed motor speeds.
A-19.
Monitor a submultiple frequency to prolong the flasher-tube life.
A-20.
Power is absorbed by the frequency meter cavity; and a pronounced dip in power, at the output, will be observed.
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CHAPTER 4
QUALITATIVE MEASUREMENTS LEARNING OBJECTIVES Upon completion of this chapter, you will be able to do the following: 1. Identify the various methods of measuring standing-wave ratios. 2. Identify the various methods of determining electrical losses caused by deterioration of transmission lines. 3. Identify the methods of measuring intermodulation distortion.
INTRODUCTION TO QUALITATIVE MEASUREMENTS As a technician, you are responsible for repairing and maintaining complex electronic systems. The basic ability to repair a specific piece of equipment is only the first step in becoming a qualified technician. Your ultimate goal should be to become proficient at systems fault isolation — in other words, to know the entire system like the back of your hand. To reach this goal you will need to be familiar with all parts of the system and know how they are interconnected and interact with each other. There are numerous shortcuts or tricks of the trade that can only be learned through experience on any system, but the most practical thing for you to remember is to approach all problems in a logical manner. Various combinations of electronic equipment are interconnected to form a system capable of performing specific functions. You must be able to apply general test methods and practices to installation, tuning, maintenance, and repair of the system. This requires you to have a thorough knowledge of many types of electronic equipment. When radar, communication, and digital computers are interconnected, they require different maintenance procedures than when they are operated separately. Revised test procedures may be necessary. Detrimental interactions between equipment or facilities must be corrected and effective preventive maintenance procedures must be planned for all equipment within the system. System quality figures, such as sensitivity and coverage, must be determined and measured during equipment preventive maintenance checks to assure efficient operation. System monitoring at specific test points is often used to help localize a problem. System testing and monitoring are frequently accomplished by using an external piece of electronic equipment, which is designed specifically for testing a particular system. Some computers and computer systems build in their own monitoring and testing devices and will inform the operator when and where failure has occurred. You must realize that any equipment designed to test, monitor, or repair another system is itself subject to malfunction and will require periodic checks and preventive maintenance. This chapter will cover some of the basic test methods and practices associated with system-level troubleshooting.
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STANDING-WAVE RATIO (SWR) MEASUREMENTS Standing-wave ratio (swr) is the ratio of the maximum voltage or current to the minimum voltage or current at any point along a transmission line. Swr measurements are used to determine the matching quality of the termination of the line. A variety of methods and test equipments may be used to measure the voltage or current distribution along a transmission line. An open transmission line is accessible for coupling to many types of voltagemeasuring devices, such as a wavemeter or a grid-dip meter. However, at higher frequencies where coaxial cables or waveguides are used to minimize skin effect losses, (discussed in NEETS, module 10) access is more complicated. Access to the interior of the waveguide or center conductor of the coaxial cable must be gained by using a unidirectional or bidirectional coupler, which is inserted into the transmission line. The coupler contains a slot into which an rf probe is inserted and positioned with respect to directivity. The conditions that produce standing waves and their adverse effects are discussed in detail in NEETS, module 10. The different methods of detecting and measuring standing waves are discussed in the following paragraphs. PROBES A magnetic or electric probe can be used to observe the standing wave on a short-circuited, terminated line. The wavelength is obtained by measuring the distance between alternate maximum or minimum current points along the line. A typical setup operating at 300 megahertz might use two 10-foot lengths of number 14 phosphor-bronze wires, which are spaced 1 inch apart and supported parallel to a set of probe guide rails. The line should be partially matched to the source generator by means of a parallelwire shorting stub connected in parallel with the transmission line and the oscillator output line. Figure 41, view A and view B, illustrates the types of probes required for this method of measurement.
Figure 4-1A.—Typical electromagnetic probe.
4-2
Figure 4-1B.—Typical electromagnetic probe.
NEON-LAMP AND MILLIAMMETER METHODS In this method of measurement, a neon bulb or milliammeter is moved along the two-wire parallel transmission line. Points of maximum voltage (standing-wave voltage peaks) with the lamp or points of maximum current (standing-wave current peaks) with the indicator will have maximum brilliance or indication, respectively. Q-1.
At what points along a transmission line will a neon lamp glow the brightest?
BRIDGE METHODS The bridge method permits measurement of the standing-wave ratio without actually measuring the standing waves. The bridge method is applicable because the input impedance of a line terminated in its characteristic impedance is a pure resistance equal to the characteristic impedance. A line terminated in this way can be used as the unknown resistance in a bridge circuit and a null can be obtained in the indicating device when the other resistance arms of the bridge are properly adjusted. Many types of bridges can be used. For example, an ac bridge that is independent of the applied frequency can be used. The bridge will become unbalanced when the line is no longer properly terminated. Improper termination will produce a reactive component as well as a resistive component in the input impedance of the line and result in a standing wave. The reading of the indicating device depends on the degree of imbalance, which becomes more severe as the mismatch caused by the termination becomes worse. The indicating device can be calibrated directly to indicate the standing-wave ratio. The most common indicator consists of a crystal rectifier, a filtering circuit, and a sensitive dc meter movement in series with a high resistance. RESISTANCE-CAPACITANCE BRIDGE A resistance-capacitance bridge circuit is shown in view A of figure 4-2. The bridge is theoretically independent of the applied frequency.
4-3
Figure 4-2A.—Resistance-capacitance bridge circuit for measuring standing-wave ratio.
However, the applied frequency must be low enough to avoid skin effect, stray inductance, capacitance, and coupling between circuit elements and wiring. The leads must be kept short to eliminate stray reactance, which causes bridge imbalance. The rectifier circuit wiring must be isolated from other bridge component fields so that induced voltages do not cause an erroneous indication. You should only use resistors having negligible capacitance and inductance effects. Before you calibrate a newly constructed bridge, the following procedure must be followed if residual readings caused by stray effects are to be held to a minimum: 1. Connect a noninductive resistor (RL in view B) that is equal to the characteristic impedance of the line to the output terminals of the bridge.
Figure 4-2B.—Resistance-capacitance bridge circuit for measuring standing-wave ratio.
2. Apply an rf voltage to the input terminals and adjust the variable capacitor for a minimum reading on the meter. 3. Reconnect the resistor (R L) to the input terminals and connect the rf power source to the output terminals. 4-4
4. Adjust the rf voltage amplitude applied to the bridge until a full-scale meter reading is obtained. 5. Reconnect the bridge in the normal manner (resistor RL to the output terminals, etc.). If the meter reading is now more than 1% or 2% of the full-scale reading, different arrangements (lead dress) of the internal wiring must be tried until the null is reduced to 0 or as close as possible to the 0 point. The bridge can be calibrated after completion of the preceding check. Connect the transmission line under investigation to the output terminals of the bridge and connect a succession of noninductive resistors (RO in view C) to the load end of the transmission line until the bridge is balanced. Assuming that the bridge was originally balanced for the characteristic impedance of the line, the standing-wave ratio can be computed from the following equation:
Figure 4-2C.—Resistance-capacitance bridge circuit for measuring standing-wave ratio.
Select the formula that yields a ratio greater than unity. The swr calibration can be recorded on the meter scale directly, recorded on a chart in terms of the meter deflection, or plotted on a graph against the meter deflection. The variable capacitor, in turn, can be calibrated for various characteristic impedances. This is accomplished by applying suitable resistors (RO) across the output terminals and noting the capacitor settings at the respective balance points. A range of 50 to 300 ohms should prove attainable.
4-5
ACCURACY OF BRIDGE MEASUREMENTS To assure accurate measurements, the rf signal applied to the bridge must be properly adjusted each time a calibrated instrument is used. Essentially, this adjustment is a repetition of the previously described reversed-bridge procedure. The following steps are to be performed: 1. Connect the line to the input terminals of the bridge and connect the transmitter to the output terminals. 2. Adjust the transmitter coupling until full-scale deflection is obtained. From this point on, the coupling must be left untouched. 3. Reconnect the bridge in the usual way and proceed with the measurement. POWER OUTPUT VERSUS IMPEDANCE MATCHING For maximum transfer of the power out of an rf source, with minimum heating from reflected power, the total output impedance sensed by the rf source must be equal to the internal impedance of that source. A perfect impedance match between transmitter and load would exist if the swr were 1 to 1. As discussed in NEETS, module 10, test equipment designed to measure the instantaneous voltage of a standing wave will give you a voltage standing-wave ratio (vswr). Test equipment designed to measure the instantaneous current component of a standing wave will give you the current standing-wave ratio (iswr). Regardless of the type of test equipment selected, both ratios will be the same. Q-2.
What vswr is a perfect match between a transmitter and its load?
SWR METERS The Hewlett-Packard Model 415E swr meter, shown in figure 4-3, is a commonly used swr meter. It is extremely accurate, sensitive, lightweight, easy to use, and portable. It is essentially a high-gain, tuned audio amplifier with a square-law meter that is calibrated to read swr directly. The meter is designed to be operated at a mean center frequency of 1,000 hertz.
4-6
Figure 4-3.—Typical swr meter.
Figure 4-4 shows a typical swr measurement setup using the swr meter. The signal source is usually a sinusoidal wave that is square-wave modulated at 1,000 hertz.
Figure 4-4.—Typical setup for measuring swr.
The swr meter usually gets its input from a detector, either a barretter or a crystal diode. This detector must be a square-law device (its output voltage is proportional to the applied rf power) to ensure
4-7
the accuracy of the meter. The input is amplified and applied directly to the meter. To perform the measurement as shown in figure 4-4, you move the detector along the slotted line so that its probe is at a voltage maximum and adjust the gain of the meter with the RANGE-DB, GAIN, and VERNIER controls (EXPAND switch to NORM) for full-scale deflection (1.0 on the 1.0 to 4 SWR scale). Then move the probe toward a minimum. If the meter drops below 3.2, rotate the RANGE-DB switch one position clockwise and read on the 3.2 to 10 SWR scale. If the pointer drops below this scale, rotate the RANGEDB switch one more position clockwise and read on the 1.0 to 4 scale and multiply by 10. This pattern continues for still higher swr readings. The dB scales can be used for a standing-wave-ratio measurement by setting the meter to full scale at a voltage maximum, then turning the RANGE-DB switch clockwise for an on-scale reading at a voltage minimum and noting the difference in dB reading at the maximum and minimum. A dB reading is obtained by adding the RANGE-DB switch setting and meter indication. The swr meter may also be used for high resolution insertion loss measurements. The setup for performing insertion loss or attenuation measurements is shown in figure 4-5. It requires that you initially establish a convenient reference on the DB scale of the meter. This is accomplished by connecting the signal source directly to the detector and using the GAIN and VERNIER controls to adjust the meter pointer to a convenient reference. Then you can insert the device to be measured between the signal source and the detector and note the change in dB, as shown on the meter.
Figure 4-5.—Typical setup for measuring attenuation or insertion loss.
ATTENUATION AND INSERTION LOSS MEASUREMENTS OF TRANSMISSION LINES Transmission lines are sometimes subjected to extremes of weather and the corrosive effects of salt water. You should be aware of the adverse effects of this environment on transmission lines and how to determine electrical losses caused by transmission-line deterioration. Q-3.
What are the two common causes of transmission-line deterioration?
LOSS MEASUREMENT Insertion loss measurement of transmission lines requires the use of a good signal generator and an accurate power meter. The method is identical to the insertion loss measurements used on most couplers. When a known frequency, at a predetermined level of power, is inserted into one end of a transmission
4-8
line, then the same frequency and the same level of power should be transmitted to the other end of a transmission line. Because all transmission lines contain some degree of resistance, some loss of power will occur during the test. Exposure to the elements over a period of time causes transmission-line deterioration. To determine the accuracy of this test, you should use the power meter to measure the output of the signal generator at the end of the test cable to be attached to the transmission line. Any power loss associated with the test cables should be recorded and subtracted from the measurement taken with the transmission line connected. You should note that transmission lines, like all other electronic components, are designed to operate over a specific range of frequencies. It is not uncommon for a transmission line to operate improperly at one frequency, yet operate properly over the remainder of its frequency spectrum. You should check transmission-line losses over their entire frequency range. Insertion loss measurements are normally taken when a system is first installed or the transmission line is replaced. Periodic measurements should be performed to enable you to determine if system performance is being degraded by transmission-line deterioration. Q-4.
Is it possible for a transmission line to operate improperly at certain frequencies and properly at others?
TRANSMISSION-LINE FORMULAS Transmission lines are engineered and manufactured to meet certain specifications. The most important of these specifications relates to frequency, power-handling capabilities, and characteristic impedance. The dielectric constant (K) of the insulating material is probably the manufacturer’s most important consideration and is the primary factor that affects the size of the coaxial cable. The formulas in the following sections discuss some aspects of coaxial transmission-line engineering. A cross section of a coaxial line is shown in figure 4-6. The characteristic impedance of a coaxial line can be determined by the following formula:
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Figure 4-6.—Coaxial line.
Table 4-1.—Dielectric Constants of Materials
Material
Air Amber Asbestos Fiber Bakelite (asbestos base) Bakelite (mica filled) Barium Titanate Beeswax Cambric (varnished) Carbon Tetrachloride Celluloid Cellulose Acetate Durite Ebonite Epoxy Resin Ethyl Alcohol (absolute) Fiber Formica Glass (electrical) Glass (photographic) Glass (Pyrex) Glass (window) Gutta Percha Isolantite Selenium (amorphous) Shellac (natural) Silicone (glass) (molding) Silicone (glass) (laminate) Slate Soil (dry) Steatite (ceramic) Stearite (low loss)
Dielectric constant (Approx.) 1.0 2.6-2.7 3.1-4.8 5.0-22 4.5-4.8 100-1250 2.4-2.8 4.0 2.17 4.0 2.9-4.5 4.7-5.1 2.7 3.4-3.7 6.5-25 5.0 3.6-6.0 3.8-14.5 7.5 4.6-5.0 7.6 2.4-2.6 6.1 6.0 2.9-3.9 3.2-4.7 3.7-4.3 7.0 2.4-2.9 5.2-6.3 4.4
Material
Lucite Mica (electrical) Mica (clear India) Mica (filled phenolic) Micaglass (titanium dioxide) Micarta Mycalex Neoprene Nylon Paper (dry) Paper (coated) Paraffin (solid) Plexiglas Polycarbonate Polyethylene Polyimide Polystyrene Porcelain (dry process) Porcelain (wet process) Quartz Quartz (fused) Rubber (hard) Ruby Mica Styrofoam Teflon Titanium Dioxide Vaseline Vinylite Water (distilled) Waxes, mineral Wood (dry)
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Dielectric constant (Approx.) 2.5 4.0-9.0 7.5 4.2-5.2 9.0-9.3 3.2-5.5 7.3-9.3 4.0-6.7 3.4-22.4 1.5-3.0 2.5-4.0 2.0-3.0 2.6-3.5 2.9-3.2 2.5 3.4-3.5 2.4-3.0 5.0-6.5 5.8-6.5 5.0 3.78 2.0-4.0 5.4 1.03 2.1 100 2.16 2.7-7.5 34-78 2.2-2.3 1.4-2.9
Attenuation in a coaxial line in terms of decibels per foot can be determined by the following formula:
As a technician, you need not be concerned with designing coaxial transmission lines. It is, however, our feeling that you should be familiar with the parameters that go into making a transmission line. It can readily be seen by the above formulas that transmission lines cannot be randomly selected without consideration of system requirements. NAVSHIPS 0967-000-0140, EIMB, Reference Data, section 3, lists the characteristics of most common transmission lines. Q-5.
What factor has the greatest effect on the physical size of a coaxial cable?
Q-6.
Is the attenuation of a coaxial cable independent of frequency?
INTERMODULATION DISTORTION MEASUREMENTS Intermodulation distortion occurs when two or more frequencies become mixed across a nonlinear device. The resultants are the difference frequency and the sum frequency, both components of the originals. Undesirable frequencies can be generated by a mixing of two discrete frequencies. Spurious radiation, arising from close spacing of transmitter and receiver, is a prime source of an undesirable frequency that can cause intermodulation distortion in an electronic circuit. This is particularly the case when antenna couplers are employed. Cross modulation and parasitic generation (described in the next section) are two other sources of undesirable frequencies that may cause intermodulation distortion. Q-7.
What is the main cause of intermodulation distortion?
CROSS MODULATION AND PARASITIC GENERATION CROSS MODULATION occurs when a signal from an adjacent channel crosses over into a second channel and modulates the frequency of the second channel. PARASITIC GENERATION occurs when regenerative feedback is sufficient to cause a circuit to oscillate, even though it is not designed to oscillate. Both types of distortion are common to systems that are misaligned. INTERMODULATION DISTORTION DETECTION The presence of intermodulation distortion is determined by a two-tone test method. Two sinusoidal frequencies of equal amplitude are introduced into the system under test. The two frequencies are spaced
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close together with reference to the unit under test. The output of the system under test (an amplifier, receiver, or transmitter) is monitored on a spectrum analyzer that is comparable in characteristics to the suspect system. The resultant display should be an exact reproduction of the input frequencies. If not, some form of intermodulation distortion is present. To determine if external sources are causing the intermodulation distortion, you can use a single-frequency signal. If the display on the spectrum analyzer does not show the single frequency, then intermodulation distortion is present. Intermodulation distortion cannot be entirely suppressed, but it can be minimized by shielding components and circuitry, parasitic suppression circuitry, and antenna spacing. These factors are incorporated in the design of the system and are tested during production. Any shields or parasitic suppressors that are removed by the technician must be replaced before troubleshooting and/or repair can be effective. Antenna locations also pose a consideration when installing a new system. Ship alteration specifications must be observed when new antenna systems are being installed. Q-8.
When you are testing a piece of equipment for intermodulation distortion, what should the output of the equipment look like?
SUMMARY The important points of this chapter are summarized in the following paragraphs: STANDING WAVES are the result of an impedance mismatch between a transmission line and its load. If a transmission line is not properly terminated, it will cause a percentage of the transmitter power to be reflected back to the source. The reflected wave or standing wave will increase in magnitude as the mismatch becomes greater. VSWR refers to the voltage ratio of the incident wave (that which is transmitted to the load) and the reflected wave (that which is reflected by the load back to the transmitter). An ideal vswr is considered to be 1 to 1.
4-12
Standing waves that are present on a transmission line can be used to determine the TRANSMITTER FREQUENCY. Voltage or current peaks are present at half-wavelength intervals. By measuring the distance between peaks, you can compute frequency mathematically. TWO-WIRE, PARALLEL TRANSMISSION LINES are usually tested for standing waves with test devices that are inductively coupled to the line. These test devices vary greatly in their complexity, ranging from bridge circuits to simple neon lamps. INSERTION LOSS MEASUREMENTS are performed by injecting a signal of a known amplitude into a transmission line and then monitoring the signal at the far end of the cable with a power meter. Loss measurements must be taken at various frequencies to determine if the transmission line is good across its frequency range.
The most common cause of INTERMODULATION DISTORTION is improper spacing of transmitters and receivers. CROSS MODULATION is common to equipment that is misaligned. Intermodulation distortion can be tested by injecting two signals (different frequencies) into a piece of equipment and then monitoring its output for distortion using a spectrum analyzer. Intermodulation distortion is usually caused by improper antenna spacing or by poorly shielded components or circuits.
REFERENCES EIMB, Test Methods and Practices Handbook, NAVSEA 0967-LP-000-0130, Naval Sea Systems Command, Washington, D.C., 1980. NEETS, Module 10, Wave Propagation, Transmission Lines, and Antennas, NAVEDTRA 172-10-00-83, Naval Education Training and Program Development Center, Pensacola, Fla., 1983. SWR Meter 415E, NAVSHIPS 0969-139-2010, Hewlett-Packard Co., Palo Alto, Calif. 1968.
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ANSWERS TO QUESTIONS Q1. THROUGH Q8. A-1.
At standing-wave voltage peaks.
A-2.
1 to 1.
A-3.
Corrosive effects of salt water and weather extremes.
A-4.
Yes, it is quite common.
A-5.
The dielectric constant of the insulating material.
A-6.
No.
A-7.
Close spacing of transmitters and receivers.
A-8.
An exact reproduction of the input.
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CHAPTER 5
INTRODUCTION TO WAVEFORM INTERPRETATION LEARNING OBJECTIVES Upon completion of this chapter, you will be able to do the following: 1. Explain the use of waveform interpretation in testing applications. 2. Identify the different types of modulation and methods of measuring modulation. 3. Explain the various uses of spectrum analyzers. 4. Explain the various uses of time-domain reflectometers. 5. Identify the various tests that can be performed with the swept-frequency technique.
INTRODUCTION TO WAVEFORM INTERPRETATION Measurements performed with oscilloscopes, time-domain reflectometers, and spectrum analyzers enable you to view the signal produced by the equipment or circuit under test. However, a visual display is of no value unless you are able to interpret the signal characteristics. A displayed waveform is a representation of a varying signal related to time. You can graphically plot an unknown waveform by using a system of coordinates in which the amplitude of the unknown signal is plotted linearly against time. An analysis of the resultant waveform provides you with valuable information in determining the characteristics of many electronic (and some mechanical) devices. For example, the waveform of a signal may indicate the presence of harmonics or parasitic oscillations, or it may indicate how closely a device is following a desired cycle of operation. As the parts in an amplifier begin to shift in value or deteriorate, waveform distortion often occurs and indicates abnormal operation of a circuit and often precedes circuit breakdown. Malfunctioning of electrical or electronic circuits within equipment can usually be traced, by waveform inspection, to a specific part or parts of the circuit responsible for the distorted signal. On the basis of these facts, it is apparent that there is an important need for test equipment that can provide a visual presentation of a waveform at the instant of its occurrence in a circuit. DISTORTION is a term used by technicians and engineers alike that generally signifies dissatisfaction with the shape of the wave processed by an amplifier. Distortion of a waveform is the undesired change or deviation in the shape of the observed signal with respect to a reference waveform. Classifying any waveform as a distorted wave without reference to the electronic circuitry involved is meaningless. A waveform that can be validly termed distorted with respect to a specific amplifier circuit may be the normal waveform to be expected from another amplifier circuit. One of the most important steps in waveform analysis, the one that usually proves the most difficult for the maintenance personnel, is the interpretation of patterns viewed on the test equipment. This chapter will cover some of the basic test methods and practices associated with waveform interpretation.
5-1
MODULATION MEASUREMENTS Modulation measurements are sometimes required during tuning procedures to adjust transmitting equipment for the proper amount of modulation. During maintenance tests of modulated transmitter equipment, you should determine the amount of distortion in the output signal and the modulation level or index. The modulation level in multiplexing equipment is usually set at the factory or during corrective maintenance procedures. Proper adjustment of the input signal level and automatic signal-level regulation circuits provides the correct amount of modulation. Defects in modulation circuits of a transmitter can be detected by measurements of the quality of the received signals at the receiver. Corrective maintenance analysis of multiplex equipment modulation circuits can usually be made by signal-level measurements. Some radio transmitters, when operating in the AM mode, must be adjusted for correct modulation during normal tuning procedures. If the modulation level is low, the transmitter is not operating at its maximum efficiency. On the other hand, modulation in excess of 100% produces serious distortion. Since neither of these conditions is desirable, amplitude modulation should be maintained between 60% and 95% when possible. The modulation level or index of AM and fm radio transmitters that operate in the vhf range is initially adjusted by the manufacturer or during corrective maintenance. The amplifier gain of the modulator can be initially adjusted by reference to the modulation meter provided on the front panel of the equipment. Pulse modulation of radar and radio beacon signals can be measured by waveform displays presented on a standard oscilloscope. The amount of usable energy in a pulsed waveform, as measured by a spectrum analyzer, is also an indication of the pulse modulation quality. Attaining 100% amplitude modulation of an rf carrier with a sine wave requires a modulating power equal to one-half of the rf carrier power. Under this condition, the average power of the modulated carrier is equal to 1.5 times the average unmodulated carrier power. The added power is divided equally between the upper and lower sidebands. During the peaks of 100% modulation, the amplitude of the carrier is doubled. This will cause the instantaneous peak power to be four times the instantaneous unmodulated peak power P = E2/R. When voice modulation is employed, only the highest amplitude peaks can be allowed to modulate the carrier 100%. Since many speech components do not modulate the carrier 100%, the average power required for voice modulation is less than that required for modulation with a sine wave. Voice peaks usually modulate a carrier 100% when the modulation increases the average carrier output power 25% over its normal value. Q-1.
What is the result of overmodulating an AM signal?
Q-2.
For AM transmissions, the carrier is normally modulated within what range?
AMPLITUDE-MODULATION MEASUREMENTS An increase in the power output of an AM transmitter is indicated by an increase in antenna current. The increase can be taken as a measure of the degree of modulation and can be expressed as a percentage, as shown in figure 5-1. The graph for this figure was developed from the relationship existing between the carrier power and the increased power resulting from the added modulation power. The formula for calculating the PERCENTAGE of MODULATION is as follows:
5-2
Figure 5-1.—Antenna current increase with amplitude modulation.
The use of this formula is based on the assumption that the modulating voltage is a pure sine wave. Normal broadcasting, however, is characterized by complex envelope patterns, as illustrated in figure 5-2. In this light, the previous formula is not so clear. Consequently, the preceding formula should be viewed more correctly as the PERCENTAGE OF POSITIVE PEAK MODULATION. When the minimum voltage (E min) rather than the peak voltage (Emax) is used to compute percentage of modulation, the computed percentage (shown below) is the PERCENTAGE OF NEGATIVE PEAK MODULATION:
5-3
Figure 5-2.—Rf carrier amplitude-modulated by a complex wave envelope.
Since the preceding two modulation percentages often differ, you should define the AVERAGE PERCENTAGE OF MODULATION, as shown below (refer to fig. 5-3):
Figure 5-3.—Rf amplitude percentage modulation wave envelope.
From the preceding definitions of percentage of modulation, you should note that methods of measuring all three types of modulation percentages must be devised. When differing values are obtained, however, the cause may not necessarily be directly related to unequal positive and negative peaks of a complex modulation wave. Another possibility is distortion caused by carrier shift. Distortion may also be produced by effects other than the modulation process — for example, parasitic oscillation, nonlinear radio-frequency amplification of modulated signals, and distortion present in the audio amplifiers. Unfortunately, continuous variations in the percentage of modulation create a number of additional problems. For example, damping is necessary so that a meter can provide an average reading despite fluctuations. An average reading, on the other hand, will not disclose the presence of transient overmodulation. This shortcoming is serious because of the large number of sideband frequencies produced in addition to the normal ones whenever overmodulation occurs. Not only do these extra frequencies interfere drastically with other transmissions, but they also may significantly distort the modulation signal. These considerations account for the importance of using a meter that responds to
5-4
modulation peak; specifically, both positive-peak and negative-peak overmodulation must be indicated. Positive-peak overmodulation occurs when the positive modulation exceeds 100%; negative-peak overmodulation occurs when the negative modulation exceeds 100%. Oscilloscope Measurement Methods The oscilloscope is widely used as an amplitude-modulation monitor and measuring instrument. Since it is capable of presenting visual indications of the modulated output of AM transmitters, the oscilloscope is reliable for detecting overmodulation and determining the percentage of modulation. For example, the relative error of most measurements taken with a 5-inch crt is about 10%. Although such accuracy is adequate for many maintenance checks, the oscilloscope is usually considered more valuable as a monitor of general modulation conditions. It is also used to monitor the amplitude-modulated output of a radio transmitter when photographic records are desired. Types of Modulation Display Two types of modulation patterns are provided by the oscilloscope, depending upon the hookup used. These patterns are the WAVE-ENVELOPE PATTERNS, as shown in figures 5-2 and 5-3, and the TRAPEZOIDAL PATTERN, as shown in figure 5-4.
Figure 5-4.—Trapezoidal modulation patterns.
Figure 5-2 shows an oscilloscope presentation of an rf carrier that is amplitude-modulated by a complex wave, such as that of speech. Figures 5-4 and 5-5 show the effects of over 100% modulation on the carrier wave. The carrier wave envelope pattern (as shown in fig. 5-3) is obtained by applying the rf5-5
modulated wave to the vertical input of the oscilloscope. The trapezoidal pattern is obtained in a similar manner except that the modulation signal from the transmitter is used to horizontally sweep the oscilloscope (instead of having the sweep signal generated internally by the oscilloscope). Both methods are limited by the frequency response of the oscilloscope; therefore, these methods find greater applicability in the lf to hf ranges.
Figure 5-5.—Overmodulated rf carrier.
VHF AND UHF MEASUREMENTS In the vhf and uhf ranges, modulation is normally measured by applying a specific-level, 1-kilohertz tone to the input of the modulator. This, in turn, produces a significant drop in the plate voltage of the final output stage of the modulator. The correct setting of output plate voltage ensures that overmodulation will not occur. SINGLE-SIDEBAND MEASUREMENTS Single-sideband modulation is a form of amplitude modulation in which only one sideband is transmitted with a suppressed carrier. Since balanced modulators are used to provide carrier cancellation, the exact balancing of the carriers to provide cancellation requires a null adjustment. The null can be observed and adjusted by using either a detector and an indicator, such as a voltmeter, or an oscilloscope for observation of the output while tuning the transmitter. Measurements peculiar to sideband technology also include special modulation-amplitude and modulation-distortion checks. If the sideband modulator is overdriven or mistuned or the associated linear amplifiers are improperly loaded or overdriven, spurious output frequencies are produced. These are harmonically related to the driving signals and can cause splatter over a large range of frequencies, thus causing interference to other transmitting stations. To determine the proper amplitude so that the modulation will not cause distortion or splatter, you use the audio two-tone modulation test. The resulting signals are shown in views A, B, and C of figure 56. The two-tone test is used for initial adjustment and for precise checking because it will indicate distortion. The two-tone test corresponds to the wave envelope method of AM modulation checking. Two signals of equal amplitude but of slightly different frequencies beating together are applied to the sideband modulator input to produce a single tone of approximately 1,000 hertz. On an oscilloscope, the
5-6
output appears as a series of fully modulated sine waves and is similar to a 100-percent-amplitudemodulated waveform, as shown in view A. A spectrum analyzer presentation is shown in view B.
Figure 5-6A.—Examples of ideal two-tone test waveforms.
Figure 5-6B.—Examples of ideal two-tone test waveforms.
5-7
Figure 5-6C.—Examples of ideal two-tone test waveforms.
When the trapezoidal method is used, two opposed triangles appear on the oscilloscope, as shown in figure 5-6, view C. When equally balanced modulators are used, the triangles are mirror images. Elliptical or straight-line patterns appear when the phase-distortion check is used. It is also possible to make a rough operating adjustment by varying the audio drive from the microphone so that on peak swings a definite value of final plate current is not exceeded. This check depends upon the initial accuracy of calibration and response characteristics of the ammeter in the final stage, as well as other factors. FREQUENCY MODULATION In frequency modulation, the carrier amplitude remains constant, and the output frequency of the transmitter is varied about the carrier (or mean) frequency at a rate corresponding to the audio frequencies. The extent to which the frequency changes in one direction from the unmodulated (carrier) frequency is called the FREQUENCY DEVIATION. Deviation in frequency is usually expressed in kilohertz. It is equal to the difference between the carrier frequency and either the highest or lowest frequency reached by the carrier in its excursions with modulation. There is no modulation percentage in the usual sense. With suitable circuit design, the frequency deviation may be made as large as desired without encountering any adverse effects that are equivalent to the overmodulation in amplitude-modulation transmissions. However, the maximum permissible frequency deviation is determined by the width of the band assigned for station operation. In frequency modulation, the equivalent of 100% modulation occurs when the frequency deviation is equal to a predetermined maximum value. There are several methods of measuring the modulation in frequency-modulated transmissions. The frequency-deviation measurement of a frequency-modulated signal is normally performed with either a spectrum analyzer or with a modulation analyzer. The modulation analyzer method is more commonly used because of its accuracy. Typical accuracies for a modulation analyzer are within ±1%. Figure 5-7 shows a typical modulation analyzer.
5-8
Figure 5-7.—Typical modulation analyzer.
Q-3.
What is meant by frequency deviation?
SPECTRUM WAVEFORM ANALYSIS AND MEASUREMENTS An analysis of a complex waveform, prepared in terms of a graphic plot of the amplitude versus frequency, is known as SPECTRUM ANALYSIS. Spectrum analysis recognizes the fact that waveforms are composed of the summation of a group of sinusoidal waves, each of an exact frequency and all existing together simultaneously. Three axes of degree (amplitude, time, and frequency) are important when considering varying frequency. The time-domain (amplitude versus time) plot is used to consider phase relationships and basic timing of the signal and is normally observed with an oscilloscope. The frequency-domain (amplitude versus frequency) plot is used to observe frequency response - the spectrum analyzer is used for this purpose. Figure 5-8 illustrates the differences between frequency- and time-domain plots. View A illustrates a three-dimensional coordinate of a fundamental frequency (f1) and its second harmonic (2f1) with respect to time, frequency, and amplitude. View B shows the time-domain display as it would be seen on an oscilloscope. The solid line, f1 + 2f1 is the actual display. The dashed lines, f and 2f1 are drawn to illustrate the fundamental and second harmonic frequency relationship used to formulate the composite signal f1 + 2f1. View C is the frequency-domain display as it would be seen on a spectrum analyzer. Note in view C that the components of the composite signal are clearly seen. Q-4.
A spectrum analyzer is designed to display what signal characteristic?
5-9
Figure 5-8A.—Time versus frequencies.
Figure 5-8B.—Time versus frequencies.
5-10
Figure 5-8C.—Time versus frequencies.
FREQUENCY-DOMAIN DISPLAY CAPABILITIES The frequency domain contains information not found in the time domain. The spectrum analyzer can display signals composed of more than one frequency (complex signals). It can also discriminate between the components of the signal and measure the power level at each one. It is more sensitive to low-level distortion than an oscilloscope. Its sensitivity and wide, dynamic range are also useful for measuring low-level modulation, as illustrated in views A and B of figure 5-9. The spectrum analyzer is useful in the measurement of long- and short-term stability such as noise sidebands of an oscillator, residual fm of a signal generator, or frequency drift of a device during warm-up, as shown in views A, B, and C of figure 5-10.
Figure 5-9A.—Examples of time-domain (left) and frequency-domain (right) low-level signals.
5-11
Figure 5-9B.—Examples of time-domain (left) and frequency-domain (right) low-level signals.
Figure 5-10A.—Spectrum analyzer stability measurements.
5-12
Figure 5-10B.—Spectrum analyzer stability measurements.
Figure 5-10C.—Spectrum analyzer stability measurements.
The swept-frequency response of a filter or amplifier and the swept-distortion measurement of a tuned oscillator are also measurable with the aid of a spectrum analyzer. However, in the course of these measurements, a variable persistence display or an X-Y recorder should be used to simplify readability. Examples of tuned-oscillator harmonics and filter response are illustrated in figure 5-11. Frequencyconversion devices such as mixers and harmonic generators are easily characterized by such parameters as conversion loss, isolation, and distortion. These parameters can be displayed, as shown in figure 5-12, with the aid of a spectrum analyzer.
5-13
Figure 5-11.—Swept-distortion and response characteristics.
Figure 5-12.—Frequency-conversion characteristics.
5-14
Present-day spectrum analyzers can measure segments of the frequency spectra from 0 hertz to as high as 300 gigahertz when used with waveguide mixers. SPECTRUM ANALYZER APPLICATIONS Figure 5-13 shows a typical spectrum analyzer. The previously mentioned measurement capabilities can be seen with a spectrum analyzer. However, you will find that the spectrum analyzer generally is used to measure spectral purity of multiplex signals, percentage of modulation of AM signals, and modulation characteristics of fm and pulse-modulated signals. The spectrum analyzer is also used to interpret the displayed spectra of pulsed rf emitted from a radar transmitter.
Figure 5-13.—Typical spectrum analyzer.
COMPLEX WAVEFORMS Complex waveforms are divided into two groups, PERIODIC WAVES and NONPERIODIC WAVES. Periodic waves contain the fundamental frequency and its related harmonics. Nonperiodic waves contain a continuous band of frequencies resulting from the repetition period of the fundamental frequency approaching infinity and thereby creating a continuous frequency spectrum. MODULATION MEASUREMENTS In all types of modulation, the carrier is varied in proportion to the instantaneous variations of the modulating waveform. The two basic properties of the carrier available for modulation are the AMPLITUDE CHARACTERISTIC and ANGULAR (frequency or phase) CHARACTERISTIC. Amplitude Modulation The modulation energy in an amplitude-modulated wave is contained entirely within the sidebands. Amplitude modulation of a sinusoidal carrier by another sine wave would be displayed as shown in figure 5-14. For 100% modulation, the total sideband power would be one-half of the carrier power; therefore, 5-15
each sideband would be 6 dB less than the carrier, or one-fourth of the power of the carrier. Since the carrier component is not changed with AM transmission, the total power in the 100-percent-modulated wave is 50% higher than in the unmodulated carrier. The primary advantage of the log display that is provided by the spectrum analyzer over the linear display provided by the oscilloscopes for percentage of modulation measurements is that the high dynamic range of the spectrum analyzer (up to 70 dB) allows accurate measurements of values as low as 0.06%. It also allows the measurements of low-level distortion of AM signals. Both capabilities are illustrated in figure 5-15, view A, view B, and view C. The chart in figure 5-16 provides an easy conversion of dB down from carrier into percentage of modulation.
Figure 5-14.—Spectrum analyzer display of an AM signal.
Figure 5-15A.—Spectrum analyzer displays of AM signals.
5-16
Figure 5-15B.—Spectrum analyzer displays of AM signals.
Figure 5-15C.—Spectrum analyzer displays of AM signals.
5-17
Figure 5-16.—Modulation percentage versus sideband levels.
NOTE: Anything greater than -6 dB exceeds 100% modulation and produces distortion, as shown in figure 5-16. In modern, long-range hf communications, the most important form of amplitude modulation is ssb (single-sideband). In ssb either the upper or lower sideband is transmitted, and the carrier is suppressed. Ssb requires only one-sixth of the output power required by AM to transmit an equal amount of intelligence power and less than half the bandwidth. Figure 5-17 shows the effects of balancing the carrier of an AM signal. The most common distortion experienced in ssb is intermodulation distortion, which is caused by nonlinear mixing of intelligence signals. The two-tone test is used to determine if any intermodulation distortion exists. Figure 5-18 illustrates the spectrum analyzer display of the two-tone test with the modulation applied to the upper sideband input.
Figure 5-17.—Double sideband carrier suppressed.
5-18
Figure 5-18.—Two-tone test.
Q-5.
What is the advantage of single-sideband (ssb) transmission over AM transmission?
Frequency Modulation In frequency modulation, the instantaneous frequency of the radio-frequency wave varies with the modulation signal. As mentioned in NEETS, module 12, the amplitude is kept constant. The number of times per second that the instantaneous frequency varies from the average (carrier frequency) is controlled by the frequency of the modulating signal. The amount by which the frequency departs from the average is controlled by the amplitude of the modulating signal. This variation is referred to as the FREQUENCY DEVIATION of the frequency-modulated wave. We can now establish two clear-cut rules for frequency deviation rate and amplitude in frequency modulation: • Amount of frequency shift is proportional to the amplitude of the modulating signal. (This rule simply means that if a 10-volt signal causes a frequency shift of 20 kilohertz, then a 20volt signal will cause a frequency shift of 40 kilohertz.) • Rate of frequency shift is proportional to the frequency of the modulating signal. (This second rule means that if the carrier is modulated with a 1-kilohertz tone, then the carrier is changing frequency 1,000 times each second.) The amplitude and frequency of the signal used to modulate the carrier will determine both the number of significant sidebands (shown in fig. 5-19) and the amplitude of the sidebands (shown in fig. 520). Both the number of significant sidebands and the bandwidth increase as the frequency of the modulating signal increases.
5-19
Figure 5-19.—Distribution of sidebands.
Figure 5-20.—Spectrum distribution for a modulation index of 2.
NEETS, module 12, should be consulted for an in-depth discussion of frequency-modulation principles. Q-6.
What happens to an fm signal as you increase the frequency of the modulating signal?
PULSED WAVES An ideal pulsed radar signal is made up of a train of rf pulses with a constant repetition rate, constant pulse width and shape, and constant amplitude. To receive the energy reflected from a target, the radar receiver requires almost ideal pulse radar emission characteristics. By observing the spectra of a pulsed radar signal, you can easily and accurately measure such characteristics as pulse width, duty cycle, and 5-20
peak and average power. The principles of radar are covered in NEETS, Module 18, Radar Principles, which can be consulted for an explanation of pulsed waves. Rectangular Pulse A rectangular wave is used to pulse-modulate the constant frequency rf carrier to produce the pulse radar output. The rectangular wave is made up of a fundamental frequency and its combined odd and even harmonics. Figure 5-21 shows the development of a rectangular wave.
Figure 5-21.—Rectangular pulse.
Pulsed Wave Analysis In amplitude modulation, sidebands are produced above and below the carrier frequency. A pulse is also produced above and below the carrier frequency, but the pulse is made up of many tones. These tones produce multiple sidebands that are commonly referred to as SPECTRAL LINES, or RAILS, on the spectrum analyzer display. Twice as many rails will be in the pulse-modulated output of the radar as there are harmonics contained in the modulating pulse (upper and lower sidebands), as shown in figure 5-22. In the figure, the pulse repetition frequency (prf) is equal to the pulse interval of 1/T. The actual spectrum analyzer display would show the lower lobes (shown below the reference line in the figure) on top because the spectrum analyzer does not retain any polarity information. Changing the pulse interval, or pulse width, of the modulation signal will change the amount of rails (prf), or number of lobe minima, as illustrated in figure 5-23.
5-21
Figure 5-22.—Pulsed radar output.
Figure 5-23.—Pulsed radar changes caused by modulating signal changes.
ANALYZING THE SPECTRUM PATTERN The leading and trailing edges of the radiated pulse-modulated signal must have a sharp rise time and decay time and a constant amplitude between them. Incorrect pulse shape will cause frequency spread and pulling, which results in less available energy at the frequency to which the receiver is tuned. The primary reason for analyzing the spectrum is to determine the exact amount of amplitude and frequency modulation present. The amount of amplitude modulation determines the increase in the number of sidebands within the applied pulse spectrum; an increase in frequency modulation increases the amplitude of the side-lobe frequencies. In either case, the energy available to the main spectrum lobe is decreased.
5-22
SPECTRUM ANALYZER OPERATION The information desired from the spectra to be analyzed determines the SPECTRUM ANALYZER requirements. Real-time analysis is used if a particular point in the frequency spectrum is to be analyzed, such as a line spectra display. Continuous- or swept-frequency analysis, which is the most common mode of observation, is used to display a wider portion of the frequency spectrum or (in some cases) the entire range of the spectrum analyzer in use. Changing the spectrum analyzer setting from one mode to another is accomplished by varying the scan time and the bandwidth of the spectrum analyzer or a combination of the two. Most real-time spectrum analyzers, however, are preceded by mechanical filters, which limit the input bandwidth of the spectrum analyzer to the desired spectra to be analyzed. Tunable- or sweptspectrum analyzers function basically the same as heterodyne receivers, the difference being that the local oscillator is not used but is replaced by a voltage-controlled oscillator (vco). The vco is swept electronically by a ramp input from a sawtooth generator. The output of the receiver is applied to a crt, which has its horizontal sweep in synchronization with the vco. The lower frequency appears at the left of the crt display. As the trace sweeps to the right, the oscillator increases in frequency. Figure 5-24 is a block diagram of a heterodyne spectrum analyzer.
Figure 5-24.—Block diagram of a heterodyne spectrum analyzer.
Before the frequency of a signal can be measured on a spectrum analyzer, it must be RESOLVED. Resolving a signal means distinguishing it from other signals near it. Resolution is limited by the narrowest bandwidth of the spectrum analyzer because the analyzer traces out its own IF bandwidth shape as it sweeps through a signal. If the narrowest bandwidth is 1 kilohertz, the nearest any two signals can be, and still be resolved, is 1 kilohertz. Reducing the IF bandwidth indefinitely would obtain infinite resolution except that the usable IF bandwidth is limited by the stability of the spectrum analyzer. The smaller the IF bandwidth, the greater the capability of the analyzer to resolve closely spaced signals of unequal amplitudes. Modern spectrum analyzers have been refined to the degree that IF bandwidths are less than 1 hertz. It is important that the spectrum analyzer be more stable in frequency than the signals being measured. The stability of the analyzer depends on the frequency stability of its vco. Scan time of the spectrum analyzer must be long enough, with respect to the amplitude of the signal to be measured, to allow the IF circuitry of the spectrum analyzer to charge and recover. This will prevent amplitude and frequency distortion. 5-23
Q-7.
When referring to spectrum analyzers, what is meant by the term resolving signals?
TIME-DOMAIN REFLECTOMETRY TIME-DOMAIN REFLECTOMETRY is a testing and measurement technique that has found increasing usefulness in testing transmission lines (both metallic and fiber-optic), cables, strip lines, connectors, and other wideband systems or components. Basically, time-domain reflectometry is an extension of an earlier technique in which reflections from an electrical pulse were monitored to locate faults and to determine the characteristics of power transmission lines. You can compare time-domain reflectometry to a closed-loop radar system in which the transmitted signal, a very fast step pulse, is fed into the system and the reflections resulting from discontinuities or impedance deviations in the system are monitored on a crt. The technique used in time-domain reflectometry consists of feeding an impulse of energy into the system and then observing that energy as it is reflected by the system at the point of insertion. When the fast-rise input pulse meets with a discontinuity or impedance mismatch, the resultant reflections appearing at the feed point are compared in phase, time, and amplitude with the original pulse. By analyzing the magnitude, deviation, and shape of the reflected waveform, you can determine the nature of the impedance variation in the transmission system. Also, since distance is related to time and the amplitude of the reflected step is directly related to impedance, the comparison indicates the distance to the fault as well as the nature of the fault. Figure 5-25, view A, view B, view C, and view D, illustrates typical transmission line problems that can easily be identified by using a time-domain reflectometer (tdr). In addition to showing both the distance to and the nature (resistive, inductive, or capacitive) of each line discontinuity, time-domain reflectometry also reveals the characteristic impedance of the line and indicates whether losses are shunt or series. They are also used to locate and analyze connectors and splices.
Figure 5-25A.—Time-domain reflectometer display of transmission line problems.
5-24
Figure 5-25B.—Time-domain reflectometer display of transmission line problems.
Figure 5-25C.—Time-domain reflectometer display of transmission line problems.
5-25
Figure 5-25D.—Time-domain reflectometer display of transmission line problems.
A conventional method of evaluating high-frequency transmission systems and components has been through the use of standing wave ratio (swr) measurements to obtain an overall indication of transmission line performance. This method involves feeding a sine-wave signal into the system and measuring the maximum and minimum amplitudes of the standing waves that result from system discontinuities or load mismatches. The ratio between the minimum and maximum swr values is then taken as the system FIGURE OF MERIT. The swr measurement, however, does not isolate individual discontinuities or mismatches when multiple reflections are present; it only indicates their total effect. Time-domain reflectometry measurements, on the other hand, isolate the line characteristics in time (location). As a result, multiple reflections resulting from more than one discontinuity or impedance variation that are separated in distance on the line are also separated in time at the monitoring point and can be individually analyzed. Prior to the advent of time-domain reflectometers, time-domain reflectometry was performed with the aid of sampling oscilloscopes and pulse generators with very fast rise times. Figure 5-26 shows the earlier type of test setup, which is still an option. However, today's timedomain reflectometers have several advantages over the old pulse-generator and oscilloscope methods. Modern time-domain reflectometers are compact, lightweight, are often supplied with battery pack options for field use, and provide a direct readout of distances instead of time. Some equipments provide a paper-tape recording for a permanent record. Figure 5-27 shows a typical time-domain reflectometer.
5-26
Figure 5-26.—Time-domain reflectometry, basic equipment setup.
Figure 5-27.—Typical time-domain reflectometer.
Q-8.
Why are time-domain reflectometers often compared to a radar system?
Q-9.
What is the main advantage of using a time-domain reflectometer (tdr) to test a transmission line?
SWEPT-FREQUENCY TESTING EQUIPMENT SWEPT-FREQUENCY testing is used to determine the bandwidth, alignment, frequency response, impedance matching, and attenuation in various circuits, systems, and components. Swept-frequency testing can be used to quickly determine the broadband response of a device that otherwise would require a number of separate measurements and manual plotting of the response curve. Swept-frequency
5-27
techniques are applicable over the entire electronic spectrum from vlf to ehf and are generally limited only by your resourcefulness and the basic limitation of the equipment employed. The basic sweptfrequency arrangement is shown in figure 5-28.
Figure 5-28.—Frequency-response test.
The swept-frequency technique can effectively determine the frequency response of an amplifier or filter and is useful in the alignment or bandwidth determination of an IF or rf stage. The test equipment permits direct visual readout on the crt of the spectrum analyzer. The spectrum analyzer can also be connected to an X-Y chart recorder if a permanent record or print is desired. Figure 5-29 shows a spectrum analyzer crt display of the frequency response of a multicoupler. The tracking generator used must be capable of sweeping the desired frequency range of the device under test.
Figure 5-29.—Typical spectrum analyzer frequency-response display.
5-28
Q-10.
What is the purpose of swept-frequency testing?
TRACKING GENERATOR Figure 5-30 shows a typical tracking generator used with the Hewlett-Packard 141 T spectrum analyzer. A TRACKING GENERATOR is basically a sweep generator in which the sweep rate is matched to that of the spectrum analyzer. The output circuitry of the tracking generator contains a network that ensures a constant output amplitude over the entire range being swept. When the fm signal produced by the tracking generator is applied to a device or circuit under test, the instantaneous output amplitude is always proportional to the response of the circuit to the frequency at that instant. Thus, the original fm input signal is changed in passing through the circuit under test. The output signal, therefore, would consist of an fm signal that is also amplitude-modulated. For equal deviations, the positive and negative portions of this envelope are symmetrical, making it necessary to observe only one side of the envelope. After the detection stage in the spectrum analyzer, only the modulation remains to appear on the face of the crt. This presentation will appear as a continuous curve because of the persistence of vision and the phosphor characteristic of the crt. The polarity of the detector determines whether a positive or a negative output is displayed. The frequency at any point on the crt display can be analyzed by arresting the scan of the spectrum analyzer either electronically or manually at the point of interest. For greater accuracy in frequency determination, a frequency counter may be attached to the output of the tracking generator at the point of the arrested scan.
Figure 5-30.—Tracking generator used with a spectrum analyzer.
IMPEDANCE MATCHING Conventional tuners cannot be used successfully to cancel source or load reflections in sweptfrequency measurements. This is because the tuning is effective only at single frequencies; therefore, pads 5-29
or isolators are required. However, by the use of automatic-level control, the power output of the sweep generator can be maintained relatively constant at the point of measurement. The source impedance may thus be maintained very close to the nominal value. With this arrangement, any impedance variation in the connecting cables, connectors, and adapters is effectively cancelled since these components are within the leveling loop. The attenuation of a device under test will be displayed on the associated crt as a continuous response curve as it is scanned. This will result in an attenuation versus frequency plot of the device under test only. IMPEDANCE Circuit impedance is measured conveniently by using the reflectometer principle. The individual values of the incident and reflected signals (swr) in a transmission line feeding an unknown impedance are measured. The ratio between these signals indicates how closely the load impedance matches that of the transmission line. Another method is the use of an auto-mechanical load control to hold the forward power at a constant level while the return load of a specific load is measured. A short is then placed in the circuit, and 100% reflected power is measured. The loss detected is then calculated to obtain swr figures. NOISE FIGURE By using a frequency-sweeping receiver and an automatic noise-figure meter, you can make noisefigure measurements on broadband microwave devices, such as a traveling-wave-tube amplifier. To conduct such a test properly, you must first check the receiver noise figure. Q-11.
In swept-frequency testing the impedance of a transmission line, what electrical characteristic is actually being measured?
SWEEPING ANTENNAS Antenna system testing is one of the more common and useful applications for using the sweptfrequency technique. The main parameters that an antenna system is tested for are vswr, frequency response, and impedance. Figure 5-31 shows a typical test setup for testing a transmitting antenna for vswr. Remember that any transmitting antenna can also act as a receiving antenna and send induced power from adjacent antennas back to the test equipment. You should make an initial power check on the antenna to prevent damage to your test equipment. Figure 5-32 shows a typical hf transmitting antenna vswr display as measured using the swept-frequency technique. The setup for testing a receiving antenna vswr, shown in figure 5-33, is similar, with the exception of the attenuators. The measured vswr (within the operating frequency range) of any broadband antenna should not exceed a vswr of 2.5 to 1. The vswr for any single-tuned antenna should not exceed 1.5 to 1 at the tuned frequency.
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Figure 5-31.—Vswr test for transmitting antennas.
Figure 5-32.—Typical spectrum analyzer vswr display.
5-31
Figure 5-33.—Vswr test for receiving antennas.
Q-12.
What precautions must be taken when sweeping a transmitting antenna?
SUMMARY This chapter has presented information on waveform interpretation. The information that follows summarizes the important points of this chapter. Interpretation of a waveform is best accomplished with test equipment that gives you a visual indication of the waveform. The most common devices used in systems applications are OSCILLOSCOPES and SPECTRUM ANALYZERS. An amplitude-modulated signal can be tested with either an oscilloscope or a spectrum analyzer to determine its percentage of modulation, sideband characteristics, and carrier frequency. Frequencymodulated signals are normally tested with a spectrum analyzer or a modulation analyzer.
5-32
Oscilloscopes are designed to view a time-domain waveform (amplitude versus time). Spectrum analyzers are designed to view a frequency-domain waveform (amplitude versus frequency). One advantage of using a spectrum analyzer is its ability to graphically display the composition of COMPLEX WAVEFORMS.
TIME-DOMAIN REFLECTOMETERS work on a principle similar to that used in radar. A precise signal is produced by the time-domain reflectometer and injected into the device under test (usually a transmission line); and the resulting reflections are displayed to discover such things as
5-33
impedance mismatches, opens, and shorts. The display sections of time-domain reflectometers are calibrated to give you a graphical display of amplitude versus distance.
SWEPT-FREQUENCY TESTING is performed by using a TRACKING GENERATOR to inject a signal into a device and then monitoring the output of the device with a spectrum analyzer. The tracking generator is designed to sweep or scan through the entire frequency range of the device being tested. Its sweep rate must be matched with the sweep rate of the spectrum analyzer.
5-34
REFERENCES Communications Systems, NAVTELCOMINST 2313.1, Naval Telecommunications Command, Washington, D.C., 1984. EIMB, Test Methods and Practices, NAVSEA 0967-LP-000-0130, Naval Sea Systems Command, Washington, D.C., 1980. Modulation Principles, NAVEDTRA 172-12-00-83. Naval Education and Training Professional Development and Technology Center, Pensacola, Fla., 1983.
ANSWERS TO QUESTIONS Q1. THROUGH Q12. A-1.
Distortion.
A-2.
60% to 95%.
A-3.
The difference between the carrier frequency of an fm signal and its maximum frequency excursion when modulated.
A-4.
Amplitude versus frequency (the frequency domain of the signals).
A-5.
The same amount of intelligence can be transmitted with one-sixth of the output power with less than one-half the bandwidth.
A-6.
Both the bandwidth and the number of significant sidebands increase.
A-7.
The ability of the analyzer to discriminate between display signals of slightly different frequencies.
A-8.
Both transmit a pulse and analyze the signal reflection.
A-9.
A Tdr will indicate the nature of and the distance to or location of any faults.
A-10.
To determine various characteristics of a component, piece of equipment, or system over its operational frequency range.
A-11.
Swr on the transmission line.
A-12.
You must ensure that power induced from any adjacent transmitting antennas does not damage your test equipment.
5-35
APPENDIX I
GLOSSARY ABSORPTION WAVEMETER—A device used for measuring frequency, consisting of a tuned circuit or cavity that is loosely coupled to the frequency being measured. Maximum energy is absorbed at the resonant frequency. BOLOMETER—An rf detector that converts rf power to heat, which causes a change in the resistance of the material used in the detector. This change in resistance varies in proportion with the amount of applied power and is used to measure the amount of applied power. CALORIMETER—A device that measures rf power by measuring the heat the rf power generates. CAVITY WAVEMETER—An instrument used to measure microwave frequencies. The resonant frequency of the cavity is determined by its inside dimensions. COAXIAL-LINE WAVEMETERS—A shorted section of a coaxial line used to measure rf frequencies. It is calibrated in either wavelength or frequency. CROSS MODULATION—An intermodulation condition that occurs when a carrier is modulated by an undesired signal. CURRENT PROBE—An inductive device used for measuring the current in a conductor. Probes are designed to be clamped around the insulated conductor. CURRENT TRACER—An inductively coupled device used for tracing current paths to determine the cause of low-impedance faults on a printed-circuit board. DECADE RESISTOR (DECADE RESISTANCE BOX)—It typically has two or more sections, each containing 10 precision resistors wired to selector switches. A piece of test equipment that provides a ready source of various resistances for engineering and measurement applications. DECIBEL (dB)—A standard unit for expressing relative power levels as the ratio of power out to power in. dBm—A unit used to express power levels above or below a l-milliwatt reference level at a designated load impedance (usually 600 ohms). DIFFERENTIAL VOLTMETER—A precision voltmeter that measures an unknown voltage by comparing it to a precision internal-reference voltage supply. ELECTROSTATIC-DISCHARGE SENSITIVE (ESDS) DEVICE—Electronic components that are susceptible to damage from static charges. FIBER OPTICS—Conductors that are usually constructed of plastic or glass fibers that readily pass light. Used primarily for transmission of high-speed data over relatively short distances.
AI-1
FREQUENCY DEVIATION—Refers to the difference between the carrier frequency of an fm signal and the instantaneous frequency of its modulated wave. FREQUENCY DOMAIN—A plot of frequency versus amplitude as shown by a spectrum analyzer display. FREQUENCY RESPONSE—(I) The ability of a component or device to operate over a portion of the frequency spectrum. (2) In reference to test equipment, that portion of the frequency spectrum that the test equipment is capable of sensing and measuring accurately. GALVANOMETER—A meter used to measure small values of current by electromagnetic or electrodynamic means. IMPEDANCE ANGLE METER—A device that measures circuit impedance by comparing the phase angle between voltage and current. INSERTION LOSS—The difference between the amount of power applied to a load before and after the insertion of a device in the line. INTEGRATED CIRCUIT (IC)—(1) A circuit in which many elements are fabricated and interconnected by a single process (into a single chip), as opposed to a nonintegrated circuit in which the transistors, diodes, resistors, and other components are fabricated separately and then assembled. (2) Elements inseparably associated and formed on or within a single substrate. INTERMODULATION DISTORTION—Nonlinear distortion characterized by the appearance (at the system output) of frequencies equal to the sums and differences of two or more frequencies present at the input. LOAD—(I) A device through which an electric current flows that changes electrical energy into another form. (2) Power consumed by a device or circuit in performing its function. LOGIC CLIPS—A device that can be clipped onto an in-circuit, dual-in-line package (DIP) logic IC to determine the logic state of each pin of the IC. LOGIC PROBE—A hand-held probe used to determine the logic state (high or low) of test points in a logic circuit. A logic high is represented by a lit indicator light on the probe. LOGIC PULSER—A hand-held probe used to pulse, or change the logic state, of in-circuit logic ICs. METROLOGY CALIBRATION (METCAL) PROGRAM—A Navy calibration program designed to ensure the accuracy of test equipment through comparisons with calibration laboratory standards of known accuracy. MICROPHONICS—Electrical noise caused by the mechanical motion of the internal parts of a device. The term is usually associated with vacuum tubes. MODULATION INDEX—When a sine wave is used to modulate an fm signal, the ratio of the frequency deviation to the frequency of the modulating wave.
AI-2
NATIONAL BUREAU OF STANDARDS (NBS)—A bureau of the United States government that is responsible for maintaining the nation's electrical and physical standards. The accuracy of all calibrated test equipment is traceable to NBS through the Navy's METCAL program. OPTICAL TIME-DOMAIN REFLECTOMETER (OTDR)—A piece of test equipment used to test a fiber-optic cable for such things as attenuation, localized losses, and defects. It transmits an optical pulse (usually a laser) into the fiber-optic cable and analyzes the resulting reflections in terms of amplitude versus distance. PARALLAX ERROR—The error in meter readings that results when you look at a meter from some position other than directly in line with the pointer and meter face. A mirror mounted on the meter face aids in eliminating parallax error. PARASITIC—In electronics, an undesirable frequency in an electronic circuit. Usually associated with vacuum-tube amplifiers and oscillators. PERCENT OF MODULATION—In AM signals, the ratio of half the difference between the maximum and minimum amplitudes of a modulated wave to its average amplitude. RF IMPEDANCE BRIDGE—A piece of test equipment used for measuring the combined resistance and reactance of a component, piece of equipment, or system at rf frequencies. SENSITIVITY—In reference to test equipment, the ratio of the response of the test equipment to the magnitude of the measured quantity. Sometimes expressed indirectly by stating the property by which sensitivity is computed (e.g., ohms per volt). STANDARD—An exact value of an electrical quantity (established by international agreement), which serves as a model for measurement of that quantity. STANDING WAVE—The distribution of voltage and current along a transmission line formed by the incident and reflected waves, which has minimum and maximum points on a resultant wave that appears to stand still. STROBOSCOPE—An instrument that allows viewing of rotating or reciprocating objects by producing the optical effect of a slowing or stopping motion. SWEPT-FREQUENCY TESTING—Testing the frequency response of a component or system by applying an rf signal, in which the frequency is varied back and forth through a set frequency range at a steady rate, to the input of a device. The output is then monitored to determine the amplitude of the output with respect to frequency. THERMISTOR—(1) A semiconductor device in which the resistance varies with temperature. (2) A type of bolometer characterized by a decrease in resistance as the dissipated power increases. TIME-DOMAIN REFLECTOMETER—A piece of test equipment used to test a transmission line for defects, such as shorts and opens. It transmits an electrical pulse into the transmission line and analyzes the resulting reflections in terms of emplidute versus distance. TRAIC—A three-terminal device that is similar to two SCRs back-to-back with a common gate and common terminals. Although similar in construction and operation to the SCR, the Triac controls and conducts current flow during both alternations of an ac cycle.
AI-3
TUNING FORK—A two-pronged mechanical device that is designed to vibrate only at its natural frequency. In electronics, it is used primarily to determine the correct speed of a motor. UNIJUNCTION TRANSISTOR (UJT)—A three-terminal, semiconductor device with a negative-resistance characteristic that is used in switching circuits, oscillators, and waveshaping circuits. VOLUME UNIT (VU)—Unit of measurement of a complex audio signal such as voice or music. A 0 level is referenced to 1 milliwatt of power into a 600-ohm load. WAVEMETERS—(1) Calibrated resonant circuits that are used to measure frequency. (2) An instrument for measuring the wavelength of an rf wave. ZENER DIODE—A pn-junction diode designed to operate in the reverse-bias breakdown region.
AI-4
MODULE 21 INDEX A Absorption power meters, 3-16 to 3-21 Ac voltage measurements, 1-7 to 1-8 Accuracy of bridge measurements, 4-6 af power, 3-12 Alkaline and carbon-zinc batteries, 2-33 Amplitude modulation, 5-15 to 5-19 Amplitude-modulation measurements, 5-2 to 5-5 Antennas and transmission lines, impedance testing of, 3-10 Attenuation and insertion loss measurements of transmission lines, 4-8 Audio-frequency measurements, 3-26 to 3-32 B Basic measurements, 1-1 to 1-26 capacitor measurements, 1-15 to 1-18 bridge-type measurements, 1-16 to 1-17 reactance-type measurements, 1-17 current measurements, 1-9 to 1-12 current probes, 1-10 current tracers, 1-9 digital multimeter method, 1-9 multimeter method, 1-9 oscilloscope method, 1-11 to 1-12 inductance measurement, 1-18 to 1-22 hay bridge, 1-19 maxwell bridge, 1-21 measurement of inductance using the vtvm, 1-22 reactance measuring equipment, 1-21 introduction to measurements, 1-1 to 1-3 rcl bridges, 1-14 differential voltmeters, 1-14 meggers, 1-14 references, 1-25 resistance measurements, 1-12 to 1-15 multimeter method, 1-13 vtvm method, 1-22 voltage measurements, 1-3 to 1-8
Basic measurements—Continued ac voltage measurements, 1-7 to 1-8 differential voltmeter method, 1-8 digital multimeter method, 1-8 multimeter method, 1-8 oscilloscope method, 1-8 vacuum tube voltmeter method, 1-22 dc voltage measurements, 1-3 to 1-6 differential voltmeter method, 1-6 digital multimeter method, 1-5 to 1-6 electronic voltmeter method, 1-6 multimeter method, 1-3 to 1-4 oscilloscope method, 1-4 to 1-5 Battery measurements, 2-32 to 2-34 carbon-zinc and alkaline batteries, 2-33 dry batteries, 2-33 mercury cells, 2-34 nickel-cadmium batteries, 2-34 storage batteries, 2-32 Bolometer, 3-18 to 3-19 Bridge methods, 3-1 to 3-9, 4-3 to 4-5 C Calorimeters, 3-21 to 3-25 Capacitor measurements, 1-15 to 1-18 bridge-type, 1-16 to 1-17 reactance-type, 1-17 Carbon-zinc and alkaline batteries, 2-33 Component testing, 2-1 to 2-48 battery measurements, 2-32 to 2-34 carbon-zinc and alkaline batteries, 2-33 dry batteries, 2-33 mercury cells, 2-34 nickel-cadmium batteries (NICAD), 2-34 storage batteries, 2-32 fiber-optic testing, 2-37 to 2-43
INDEX-1
Component testing—Continued automatic test equipment, 2-38 to 2-39 optical multimeter, 2-38 optical ohmmeter, 2-38 optical power meter, 2-38 optical time-domain reflectometer (OTDR), 2-38 oscilloscope, 2-38 radiometer/photometer, 2-38 integrated circuit (IC) testing, 2-28 to 2-34 logic analyzer, 2-32 logic clips, 2-29 logic comparators, 2-29 logic probes, 2-30 to 2-31 logic pulsers, 2-31 introduction to component testing, 2-1 rf attenuators and resistive load tests, 2-34 to 2-37 50/75-ohm terminations, 2-36 decade resistors, 2-35 decade (step) attenuators, 2-36 fixed rf attenuators, 2-35 summary, 2-43 to 2-48 testing electron tubes, 2-1 to 2-8 electron tube testers, 2-3 to 2-6 auxiliary compartment, 2-4 to 2-5 front panel, 2-3 to 2-4 operation, 2-5 to 2-6 program cards, 2-5 high-power hf amplifier tube tests, 2-6 to 2-8 crossed-field amplifier, 2-8 klystron tube tests, 2-6 to 2-7 magnetron tube tests, 2-8 traveling-wave tube, 2-7 to 2-8 substitution methods, 2-2 to 2-3 testing semiconductors, 2-8 to 2-27 diode characteristic graphical display, 2-17 to 2-19 reverse voltage-current analysis, 2-18 to 2-19 zener diode test, 2-20 diode testers, 2-16 to 2-17
Component testing—Continued rf diode test, 2-16 switching diode test, 2-17 diode testing, 2-16 field-effect transistor (FET) tests, 2-23 to 2-27 N-channel test, 2-25 P-channel test, 2-25 MOSFET testing, 2-25 to 2-27 MOSFET (depletion/enhancement type) test, 2-27 MOSFET (enhancement type) test, 2-27 silicon-controlled rectifiers (SCR), 2-21 static resistance measurements, 2-20 transistor testing, 2-9 to 2-16 electrostatic discharge sensitive (ESDS) care, 2-12 to 2-15 resistance test, 2-9 transistor testers, 2-10 to 2-12 triac, 2-22 unijunction transistors (UJTs), 2-22 Composite-coil, iron-core, and torsion-head wattmeters, 3-15 Counter method, frequency, 3-32 Cross-modulation and parasitic generation, 4-11 Cross-field amplifier, 2-8 Current measurement, 1-8 to 1-12 D Dc voltage measurements, 1-3 to 1-6 Decibel meters, 3-12 Differential voltmeters, 1-14 Diode characteristic graphical display, 2-17 to 2-19 reverse voltage-current analysis, 2-18 to 2-19 zener diode test, 2-20 Diode testers, 2-16 to 2-17 rf diode test, 2-16 switching diode test, 2-17 Dry batteries, 2-33
INDEX-2
E Electrodynamic wattmeter, 3-13 to 3-15 Electron tubes, testing, 2-1 to 2-8 electron tube testers, 2-3 to 2-6 high-power hf amplifier tube tests, 2-6 to 2-8 substitution methods, 2-2 to 2-3 Electrostatic discharge sensitive (ESDS) care, 2-12 to 2-16 F Fiber-optic testing, 2-37 to 2-43 automatic test equipment, 2-38 to 2-39 optical multimeter, 2-38 optical ohmmeter, 2-38 optical power meter, 2-38 optical time-domain reflectometer (OTDR), 2-38 oscilloscope, 2-38 radiometer/photometer, 2-38 Field-effect transistor (FET) tests, 2-23 to 2-27 N-channel test, 2-25 P-channel test, 2-25 Flow calorimeters, 3-23 to 3-25 Frequency measurements, 3-26 to 3-32 Frequency modulation, 5-8 Frequency-domain display capabilities, 5-11 to 5-15 G Glossary, AI-1 to AI-4 H Hay bridge, 1-19, 3-7 High-power hf amplifier tube tests, 2-6 to 2-8 crossed-field amplifier, 2-8 klystron tube tests, 2-6 to 2-7 magnetron tube tests, 2-8 traveling-wave tube, 2-7 to 2-8 I Impedance matching, 5-29 to 5-30
Impedance measurements, 3-1, 3-9 to 3-10 Inductance measurement, 1-18 to 1-22 Integrated circuit (IC) testing, 2-28 to 2-34 logic analyzer, 2-32 logic clips, 2-29 logic comparators, 2-29 logic probes, 2-30 to 2-31 logic pulsers, 2-31 Intermodulation, 4-11 distortion detection, 4-11 distortion measurements, 4-11 In-line wattmeters, 3-17 Iron-core, composite-coil, and torsion-head wattmeters, 3-15 K Kelvin bridge, to 3-6 Klystron tube tests, 2-6 to 2-7 L Learning objectives, 1-1, 2-1, 3-1, 4-1, 5-1 Loss measurement, 4-8 M Magnetron tube tests, 2-8 Maxwell bridge, 1-21, 3-7 Measurements, basic, 1-1 to 1-26 Measurements, modulation, 5-2 to 5-8 Mechanical rotation and vibration methods, 3-28 to 3-31 Meggers, 1-14 Mercury cells, 2-34 Milliammeter and neon-lamp methods, 4-3 Modulation display, types of, 5-5 to 5-6 Modulation measurements, 5-2 to 5-8, 5-15 to 5-20 amplitude modulation, 5-15 to 5-19 amplitude-modulation measurements, 5-2 to 5-5 frequency modulation, 5-8 single-sideband measurements, 5-6 to 5-8 VHF and UHF measurements, 5-5 MOSFET testing, 2-25 to 2-27
INDEX-3
N Neon-lamp and milliammeter methods, 4-3 Nickel-cadmium batteries (NICAD), 2-34 O Oscilloscope measurement methods, 5-5 Oscilloscope method, 3-31 Output power meters, 3-16 P Power measurements, 3-11 to 3-25 Power output versus impedance matching, 4-6 Probes, 4-2 Pulsed waves, 5-21 Q Qualitative measurements, 4-1 to 4-14 introduction to qualitative measurements, 4-1 standing-wave ratio, (swr) measurements, 4-2 to 4-8 accuracy of bridge measurements, 4-6 attenuation and insertion loss measurements of transmission lines, 4-8 bridge methods, 4-3 cross-modulation and parasitic generation, 4-11 intermodulation distortion detection, 4-11 intermodulation distortion measurements, 4-11 loss measurement, 4-8 neon-lamp and milliammeter methods, 4-3 power output versus impedance matching, 4-6 probes, 4-2 resistance-capacitance bridge, 4-3 to 4-5 swr meters, 4-6 to 4-8
Qualitative measurements—Continued transmission-line formulas, 4-9 to 4-12 Quantitative measurements, 3-1 to 3-39 frequency measurements, 3-26 to 3-32 audio-frequency measurements, 3-26 to 3-32 frequency counter method, 3-32 frequency standards, 3-26 to 3-28 frequency-measurement methods, 3-26 mechanical rotation and vibration methods, 3-28 to 3-31 frequency counter methods, 3-31 stroboscope methods, 3-29 to 3-31 tuning fork methods, 3-28 oscilloscope method, 3-31 radio-frequency (rf) measurements, 3-33 to 3-37 frequency counter accuracy, 3-34 frequency counters, 3-30 to 3-34 frequency meters, 3-33 wavemeters, 3-35 to 3-37 introduction to quantitative measurements, 3-1 to 3-11 bridge methods, 3-1 to 3-9 Hay bridge, 3-7 Kelvin bridge, 3-6 Maxwell bridge, 3-7 resistance-ratio bridge, 3-6 Schering bridge, 3-7 Wheatstone bridge, 3-5 constant-current impedancemeasuring technique, 3-9 impedance measurements, 3-1 impedance testing of antennas and transmission lines, 3-10 impedance-angle meter, 3-9 vector bridges, 3-7 to 3-8 power measurements, 3-11 to 3-25 absorption power meters, 3-16 to 3-21 bolometer, 3-18 to 3-19
INDEX-4
rcl bridges, 1-14 References, 1-25, 2-46, 3-38, 4-13, 5-35 Resistance measurements, 1-12 to 1-15 Resistance-capacitance bridge, 4-3 to 4-5 Resistance-ratio bridge, 3-6 rf attenuators and resistive load tests, 2-34 to 2-37 50/75-ohm terminations, 2-36 decade resistors, 2-35 decade (step) attenuators, 2-36 fixed rf attenuators, 2-35 rf, radio-frequency measurements, 3-33 to 3-37
Triac, 2-22 unijunction transistors (UJTs), 2-22 Silicon-controlled rectifiers (SCR), 2-21 Single-sideband measurements, 5-6 to 5-8 Spectrum analyzer applications, 5-15 Spectrum analyzer operation, 5-23 Spectrum pattern, analyzing the, 5-22 Spectrum waveform analysis and measurements, 5-9 to 5-22 analyzing the spectrum pattern, 5-22 complex waveforms, 5-15 frequency-domain display capabilities, 5-11 to 5-15 modulation measurements, 5-15 to 5-20 pulsed waves, 5-21 spectrum analyzer applications, 5-15 spectrum analyzer operation, 5-23 Standards, frequency, 3-26 to 3-28 Standing-wave ratio, (swr) measurements, 4-2 to 4-8 Static calorimeters, 3-21 to 3-23 Static resistance measurements, 2-20 Storage batteries, 2-32 Stroboscope methods, 3-29 to 3-31 Sweeping antennas, 5-30 to 5-32 Swept-frequency testing equipment, 5-27 to 5-32 impedance matching, 5-29 to 5-30 noise figure, 5-30 tracking generator, 5-29
S
T
Schering bridge, 3-7 Semiconductors, testing, 2-8 to 2-27 diode characteristic graphical display, 2-17 to 2-19 diode testers, 2-16 to 2-17 diode testing, 2-16 field-effect transistor (FET) tests, 2-23 to 2-27 MOSFET testing, 2-25 to 2-27 silicon-controlled rectifiers (SCR), 2-21 static resistance measurements, 2-20 transistor testing, 2-9 to 2-16
Time-domain reflectometry, 5-24 to 5-27 Torsion-head, iron-core, and composite-coil wattmeters, 3-15 Tracking generator, 5-29 Transistor testing, 2-9 to 2-16 electrostatic discharge sensitive (ESDS) care, 2-12 to 2-15 resistance test, 2-9 transistor testers, 2-10 to 2-12 Transmission-line formulas, 4-9 to 4-12 Traveling-wave tube, 2-7 to 2-8 Triac, 2-22 Tuning fork methods, 3-28
Quantitative measurements—Continued bolometer power meter, 3-19 in-line wattmeters, 3-17 output power meters, 3-16 af power, 3-12 calorimeters, 3-21 to 3-25 flow calorimeters, 3-23 to 3-25 static calorimeters, 3-21 to 3-23 decibel meters, 3-12 electrodynamic wattmeter, 3-13 to 3-15 iron-core, composite-coil, and torsion-head wattmeters, 3-15 vacuum tube voltmeter, 3-15 volume unit meters, 3-12 R
INDEX-5
U UHF and VHF measurements, 5-5 Unijunction transistors (UJTs), 2-22 V Vacuum tube voltmeter, 3-15 Vector bridges, 3-7 to 3-8 VHF and UHF measurements, 5-5 Voltage measurements, 1-3 to 1-8 ac, 1-7 to 1-8 dc, 1-3 to 1-6 Volume unit meters, 3-12 W Waveform interpretation, introduction to, 5-1 to 5-35 modulation measurements, 5-2 to 5-8 amplitude-modulation measurements, 5-2 to 5-5 oscilloscope measurement methods, 5-5 types of modulation display, 5-5 to 5-6 frequency modulation, 5-8 single-sideband measurements, 5-6 to 5-8 VHF and UHF measurements, 5-5 references, 5-35 spectrum waveform analysis and measurements, 5-9 to 5-22
Waveform interpretation, introduction to— Continued analyzing the spectrum pattern, 5-22 complex waveforms, 5-15 frequency-domain display capabilities, 5-11 to 5-15 modulation measurements, 5-15 to 5-20 amplitude modulation, 5-15 to 5-19 frequency modulation, 5-19 to 5-20 pulsed waves, 5-20 to 5-22 pulsed wave analysis, 5-21 rectangular pulse, 5-21 spectrum analyzer applications, 5-15 spectrum analyzer operation, 5-23 sweeping antennas, 5-30 to 5-32 swept-frequency testing equipment, 5-27 to 5-32 impedance, 5-30 impedance matching, 5-29 to 5-30 noise figure, 5-30 tracking generator, 5-29 time-domain reflectometry, 5-24 to 5-27 Waveforms, complex, 5-15 Wavemeters, 3-35 to 3-37 Wheatstone bridge, 3-5 Z Zener diode test, 2-20
INDEX-6
Assignment Questions
Information: The text pages that you are to study are provided at the beginning of the assignment questions.
ASSIGNMENT 1 Textbook assignment: Chapter 1, “Basic Measurements,” pages 1-1 through 1-26. Chapter 2, “Component Testing,” pages 2-1 through 2-8. ___________________________________________________________________________________ 1-4. Which, if any, of the following statements correctly describes the effect input impedance of test equipment can have on readings taken?
1-1. What is the purpose of the Navy's Metrology Calibration Program? 1. To provide the fleet with new types of test equipment 2. To provide quality control for your test equipment 3. To improve the efficiency of sophisticated electronic systems 4. To establish test equipment pools from which technicians can borrow
1. The greater the input impedance of your test equipment, the less accurate the readings 2. The lower the input impedance of your test equipment, the more accurate the readings 3. A piece of test equipment with an infinite input impedance will absorb no energy and readings will be more accurate 4. None of the above
1-2. At each higher echelon METCAL calibration laboratory, the accuracy of the test equipment increases by a factor of
1-5. A piece of test equipment with a low input impedance can cause readings taken to be inaccurate. To eliminate this problem, the input impedance of your test equipment should exceed the impedance of the circuit under test by what minimum ratio?
1. 10 2. 2 3. 100 4. 4 1-3. Most equipment technical manuals contain voltage charts. For which of the following purposes are they used?
1. 1 to 1 2. 2 to 1 3. 10 to 1 4. 100 to 1
1. To list the equipment's power supplies 2. To list the input power requirements of the equipment 3. To provide handy reference guides for calculating voltage drops across fixed impedances 4. To list correct voltages at major test points
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1-8. What can you do to reduce the problem of meter-reading errors caused by parallax?
_______________________________________ USE THE FOLLOWING INFORMATION TO ANSWER QUESTION 1-6: YOU NEED TO TAKE A CRITICAL VOLTAGE READING, BUT YOU DO NOT HAVE A HIGH IMPEDANCE METER AVAILABLE. INSTEAD, YOU CONNECT TWO LOWER IMPEDANCE METERS IN SERIES AND PLACE THEM ACROSS THE COMPONENT IN QUESTION. YOU ADD THE READINGS SHOWN ON THE TWO METERS TO GET YOUR MEASUREMENT. _______________________________________
1. Close one eye when reading the meter 2. Use short meter leads 3. Use a meter that has a mirror built into the scale 4. View the meter face from either the left or right side, but not directly in front 1-9. For what primary reason are oscilloscopes used in circuit testing?
1-6. Compared to using just one of the lower impedance meters, what is the advantage of using two meters connected in series?
1. They provide a visual presentation of the signal under test 2. They present a low input impedance to the circuit under test 3. They provide numerical readouts of signals under test 4. They measure voltages more accurately than other pieces of test equipment
1. Input impedance increases and voltage-measuring accuracy increases 2. Frequency response of the test setup doubles 3. Accuracy of current measurements decreases 4. Input impedance decreases and voltage-measuring accuracy increases
1-10. THIS QUESTION HAS BEEN DELETED. 1-11. Digital multimeters effectively eliminate which of the following disadvantages of analog meters?
1-7. On an analog multimeter, where on the scale are the most accurate readings taken? 1. 2. 3. 4.
1. 2. 3. 4.
At the highest end of the scale At the lowest end of the scale Midscale It makes no difference if the meter is properly calibrated
Parallax Low impedance Poor accuracy All of the above
1-12. Which of the following pieces of test equipment is most accurate for measuring dc voltages? 1. 2. 3. 4.
2
Vtvm Oscilloscope Digital voltmeter Differential voltmeter
1-17. When using an oscilloscope to observe a sine wave, what, if anything, must you do to determine the rms voltage?
1-13. If you exceed the frequency limitations of your voltmeter, which of the following results is likely?
1. Divide the observed peak-to-peak voltage by 3.65 2. Multiply the observed peak-to-peak voltage by 2; then divide by 1.414 3. Divide the observed peak-to-peak voltage by 2; then multiply by 0.707 4. Nothing
1. The meter will be destroyed 2. The circuit under test will be damaged 3. The measurement will be inaccurate 4. The meter will indicate average voltage 1-14. When performing measurements with an oscilloscope, you should ensure that the trace extends across what minimum portion of the vertical viewing area? 1. 2. 3. 4.
1-18. The frequency-measuring capabilities of a digital multimeter can be extended by using which of the following devices? 1. 2. 3. 4.
15% 25% 45% 60%
1-19. When performing ac voltage measurements, you should use which of the following pieces of equipment to obtain the most accurate reading?
1-15. When using an oscilloscope to measure a high voltage, you should use which of the following procedures? 1. Use the logic probe instead of the normal probe 2. Use the high voltage probe instead of the normal probe 3. Use two oscilloscopes connected in series 4. Place a 10-ohm shunt across the vertical input of the oscilloscope
1. 2. 3. 4.
A differential voltmeter An oscilloscope A Simpson 260 A wattmeter
1-20. For which of the following purposes would you connect two ammeters in parallel?
1-16. Oscilloscopes are normally calibrated to display which of the following types of voltages? 1. 2. 3. 4.
An rf probe A frequency doubler A high-voltage probe A frequency divider network
1. To perform voltage measurements 2. To increase frequency-measuring capabilities 3. To decrease input impedance 4. To increase input impedance
Peak Average Peak-to-peak Both 2 and 3 above
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1-25. An ohmmeter that is used for field work should meet which of the following criteria?
1-21. When taking measurements with two ammeters connected in parallel, how do you determine the resulting readings?
1. 2. 3. 4.
1. The current equals the sum of both meter readings 2. The current equals the difference of the two meter readings 3. The current equals the product of the two readings divided by their sum 4. Read either meter directly; the same current flows through both meters
1-26. When you use an analog multimeter to measure resistance, which of the following actions should you take first? 1. 2. 3. 4.
1-22. Current tracers indicate the presence of a current in which of the following ways? 1. 2. 3. 4.
By the lighting of an indicator lamp By a clicking noise Both 1 and 2 above By the movement of a meter
Make sure the meter is zeroed Set the meter for dc voltage Set the meter for ac voltage Make sure the meter leads do not exceed 36 inches
1-27. Digital multimeters are used to test semiconductors for which of the following reasons?
1-23. Which of the following is an advantage of using a current probe?
1. They produce voltage sufficient to gate all Zener diodes 2. Their LED displays are easier to read than analog displays 3. They typically limit the current flow through the semiconductor to less than 1 milliamp 4. They produce in excess of the 500 milliamps normally required to gate a PN junction
1. It is the most accurate method of measuring current 2. It senses current by induction without being connected directly into the circuit 3. It is battery operated 4. It is capable of measuring current at frequencies above 40 GHz 1-24. When troubleshooting a specific piece of equipment, you can find an accurate listing of resistance readings for specific test points in which of the following documents? 1. 2. 3. 4.
It should be extremely accurate It should be portable It should be simple to operate Both 2 and 3 above
1-28. Compensation for the resistance in test leads of digital multimeters used to perform resistance measurements is accomplished by which, if any, of the following methods?
In equipment PMS cards In test equipment manuals In equipment technical manuals In Naval Ships Technical Manuals
1. Short the leads, note the lead resistance displayed, and add this value to subsequent resistance measurements 2. Short the leads, note the lead resistance displayed, and subtract the value from subsequent resistance measurements 3. Add 10% to the reading 4. None of the above
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1-33. Which of the following types of inductor core materials produces the greatest inductance?
1-29. Which of the following is a typical use for a megger? 1. 2. 3. 4.
Testing MOSFETs Testing filter capacitors Testing thermistor mounts Testing an ac power cord for insulation breakdown
1. 2. 3. 4.
Mica Magnetic metal Polyparoloxylene Nonmagnetic metal
1-34. As frequency increases, the inherent resistance of the inductor causes which of the following types of losses to become more critical?
1-30. When large capacitors are stored as spare parts, why should their terminals be shorted with a piece of wire? 1. It prevents dielectric leakage 2. It prevents deterioration of the plates 3. It prevents the capacitors from becoming charged when in close proximity to an rf field 4. It prevents electrolytic capacitors from changing value during periods of storage
1. 2. 3. 4.
Hysteresis Skin effect Eddy currents Standing waves
1-35. Most capacitance test sets are capable of testing capacitors and what other type of component?
1-31. Capacitance meters can be grouped into which of the following basic categories?
1. 2. 3. 4.
1. Wheatstone type and Kelvin Varley type 2. Bridge-type and reactance-type 3. Depletion-type and enhancementtype 4. Resistive-type and reactive-type
TRIACS Inductors Resistors Barretters
1-36. When using reactance-type test equipment to measure inductance, what relationship exists between the inductor and the voltage drop across the reactance of the inductor?
1-32. Which of the following statements correctly describes the accuracy and use of a reactance-type capacitance meter?
1. The voltage drop is directly proportional to the value of inductance 2. The voltage drop is inversely proportional to the value of inductance 3. The voltage drop is proportional to the dielectric constant (K) of the inductor 4. The voltage drop is inversely proportional to the frequency of the applied voltage
1. It gives approximate values and is usually portable 2. It gives approximate values and is used in calibration laboratories only 3. It is very accurate and is usually portable 4. It is very accurate and is used to measure capacitors that have a high power factor
.
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1-42. Pushbuttons on the electron tube tester are used to test for which of the following conditions?
1-37. Aboard ship you should be able to troubleshoot equipment failures to the component level for which of the following reasons?
1. 2. 3. 4.
1. Ships must be self-sustaining units when deployed 2. Storage space on board ships limits the number of bulky items or electronic modules that can be stored 3. Individual components may be easier to obtain than modules or larger equipment pieces 4. All of the above
1-43. Which of the following tests is automatically performed when the electron tube tester card switch is first actuated? 1. 2. 3. 4.
1-38. What is the most common cause of electron tube failures? 1. 2. 3. 4.
Vibration damage Open filaments Shorted elements Power supply voltage surges
1. Using tube testers 2. Performing interelectrode resistance checks 3. Making gain measurements with an oscilloscope 4. Observing built-in meters that measure grid and plate current and power output
Using a tube tester Measuring tube element voltages Feeling for signs of overheating Substituting tubes
1-40. Test conditions for the electron tube tester described in the text are set by which of the following methods? 1. 2. 3. 4.
1-45. Which, if any, of the following problems occur when klystrons are left in storage or not used for more than 6 months?
By a technician setting switches By using a magnetic tape program By using a prepunched card program By inserting the tube into the appropriate socket
1. They become gassy 2. The elements become tarnished and ruin the tube 3. All external metallic parts become tarnished and must be cleaned prior to use 4. None
1-41. The electron tube tester can be used to test common low-power tubes for which of the following conditions? 1. 2. 3. 4.
Gas Shorts Opens Quality
1-44. Which of the following methods is normally used to test high-power amplifier tubes?
1-39. The simplest way to test a tube is by which of the following methods? 1. 2. 3. 4.
Emission Transconductance Other quality tests Each of the above
Gas Quality Leakage Each of the above
6
1-50. When using an ohmmeter to test a transistor’s base-to-emitter or base-tocollector junction, what minimum backto-forward resistance ratio should you expect to read?
1-46. Which of the following actions should you take to restore operation if the klystron is gassy? 1. Replace the klystron with a new one 2. Return it to the nearest depot for intermediate maintenance 3. Evacuate the gas by igniting the tube’s getter 4. Operate it at reduced beam voltage for approximately 8 hours
1. 5 to 1 2. 10 to 1 3. 50 to 1 4. 100 to 1 1-51. When taking forward and reverse resistance readings between a transistor’s emitter and collector, what type of reading should you get?
1-47. Traveling-wave tubes (twt) should be replaced if they deviate from design specifications by what minimum percentage?
1. Both the forward and reverse readings should be nearly the same 2. A short in both the forward and reverse directions 3. Less than 15 ohms when measuring in the forward direction and infinite in the reverse direction 4. Less than 15 ohms in the reverse direction and infinite in the forward direction
1. 1% 2. 10% 3. 25% 4. 33% 1-48. If a twt used as an oscillator fails, which of the following indications should you observe? 1. The twt will become noisy 2. Equipment line fuses will blow 3. The twt will have reduced output power 4. The twt will fail to break into oscillation when all other conditions are normal
1-52. When using an ohmmeter to test transistors, you should avoid using R×1 range for which of the following reasons? 1. The R×1 range is not as accurate as the other ranges 2. Most ohmmeters do not produce sufficient voltage on the R×1 range to properly bias a transistor junction 3. Some ohmmeters produce in excess of 100 milliamps of current on the R×1 range and could possibly damage the transistor 4. The R×1 scale is not capable of measuring the high resistances that are typical of a PN junction when forward biased
1-49. Which of the following is an appropriate reason to use transistors instead of electron tubes? 1. Transistors are more rugged than electron tubes 2. Transistors are not as heat sensitive as electron tubes 3. Transistors are not as sensitive as electron tubes to voltage overloads 4. Transistors are capable of handling greater amounts of power than electron tubes
7
1-56. Wearing a grounded wrist strap when repairing electronic circuit boards serves which of the following purposes?
1-53. When using a soldering iron to replace transistors, you must be sure there is no current leakage between the power source and the tip of the iron. Which of the following actions should you take if current leakage is detected?
1. It identifies you as being 2M qualified 2. It protects ESDS devices from damage 3. It protects the technician from electrical shock 4. It protects you from rf burns when working near radar antennas
1. Reduce the wattage of the heating element 2. Use an isolation transformer to power the soldering iron 3. Use a soldering gun instead of a soldering iron 4. Isolate the soldering iron from ground by disconnecting the soldering iron safety ground wire
1-57. What, if any, precaution should you take before you open a package that contains an ESDS device? 1. Rub the package against a dissimilar material 2. Discharge any static electricity by connecting a grounded lead to the package 3. Measure the static charge on the package with an oscilloscope to ensure that it is within tolerance 4. None
1-54. Which of the following is a description of ESDS devices? 1. Components that are sensitive to electrostatic discharge 2. Components that are sensitive to the electromagnetic pulse produced by a nuclear detonation 3. State-of-the-art devices used to detect electronic emissions 4. Devices designed to withstand any type of electromagnetic or electrostatic interference 1-55. MOS and CMOS devices without input diode protection circuitry belong in which, if any, of the following ESDS device categories? 1. 2. 3. 4.
Sensitive devices Very sensitive devices Moderately sensitive devices None of the above
8
ASSIGNMENT 2 Textbook assignment: Chapter 2, “Component Testing,” pages 2-8 through 2-48. Chapter 3, “Quantitative Measurements,” pages 3-1 through 3-15. ___________________________________________________________________________________
2-1. Which of the following servicing techniques applies to semiconductors? 1. Substituting a semiconductor with a known good semiconductor is a simple way to test them 2. Voltage and resistance measurements are taken prior to substituting semiconductors 3. Substituting semiconductors is cumbersome if more than one is bad or if they are soldered into the circuit 4. All of the above 2-2. What minimum ratio of back-to-forward resistance should you expect when testing a diode?
Figure 2A. —Testing an SCR with an ohmmeter.
IN ANSWERING QUESTIONS 2-5 AND 2-6, REFER TO FIGURE 2A. NOTE THAT THE CONNECTIONS OF THE OHMMETER ARE ALREADY MADE.
1. 1 to 1 2. 10 to 1 3. 50 to 1 4. 100 to 1
2-5. To forward bias an SCR, which elements should you short together?
2-3. Which of the following characteristics of a diode cannot be determined by using a multimeter?
1. 2. 3. 4.
1. How the diode reacts to various voltages 2. How the diode reacts to various frequencies 3. Both 1 and 2 above 4. How the diode reacts to forward and reverse dc biasing
2-6. What, if anything, will be the result of removing the short after it has been made? 1. Current flow from the cathode to the anode will stop 2. Current flow from the anode to the cathode will stop 3. Current can flow in either direction between the anode and cathode 4. Nothing
2-4. How are SCRs normally used in the Navy? 1. 2. 3. 4.
The gate and anode The cathode and anode The cathode and gate All three elements
As rectifiers As power control devices As voltage regulators As switching diodes in digital applications
9
Figure 2C. —Unijunction transistor.
IN ANSWERING QUESTIONS 2-8 AND 2-9, REFER TO FIGURE 2C. 2-8. What readings should you expect to find when you measure the resistance between bases 1 and 2 of a UJT?
Figure 2B. —Testing a TRIAC with an ohmmeter.
1. A short regardless of the polarity of the meter leads 2. A high resistance value regardless of the polarity of the meter leads 3. Approximately 15 ohms between base 1 and base 2 with the negative meter lead connected to base 1 4. Approximately 15 ohms between the two bases with the negative meter lead connected to base 2
IN ANSWERING QUESTION 2-7, REFER TO FIGURE 2B. 2-7. With a momentary short connected between the gate and anode 2, the TRIAC will be forward biased and allow current to flow between what elements? 1. 2. 3. 4.
From the gate to anode 1 From the gate to anode 2 From anode 1 to anode 2 only In either direction between the two anodes
2-9. For which, if any, of the following reasons do JFETs have circuit applications similar to those of vacuum tubes?
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1. JFETs have a high input impedance and are voltage-responsive 2. JFETs have a low input impedance and a frequency response comparable to that of vacuum tubes 3. JFETs have a high input impedance and are current-responsive 4. None of the above
10
Figure 2D. —Junction FET.
IN ANSWERING QUESTION 2-10, REFER TO FIGURE 2E.
Figure 2E. —MOSFET (depletion/enhancement type) and equivalent circuit.
2-10. With the negative lead of an ohmmeter attached to the gate and the positive lead attached to the source, which of the JFETs in figure 2D would be good if the meter shows infinity? 1. 2. 3. 4.
IN ANSWERING QUESTIONS 2-11 AND 2-12, REFER TO FIGURE 2E. 2-11. When measuring resistance between the drain and source of a depletion/enhancement type of MOSFET, what readings should you expect?
P-channel N-channel Both 1 and 2 above Neither 1 or 2
1. 15 ohms in one direction and infinity in the other direction 2. The same value of resistance in both directions 3. A short in one direction and infinity in the other 4. Infinite reading regardless of meter lead polarities
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2-12. When measuring resistance between the gate, source, and drain of a depletion/enhancement type of MOSFET with the negative lead attached to the gate, what readings should you expect? 1. Both readings should be infinity 2. Both readings should be between 15 ohms and 100 ohms 3. Both readings should be approximately 1,000 ohms 4. Both readings should be less than 10 ohms
11
2-14. When comparing resistance readings of an enhancement type of MOSFET to those of a depletion/enhancement type of MOSFET, which, if any, of the following differences should you notice?
2-13. When unsoldering a MOSFET from a printed circuit board, you should avoid using a vacuum plunger solder sucker for which of the following reasons? 1. Solder suckers can generate high electrostatic charges that can damage MOSFETs 2. Solder suckers create a vacuum that can physically damage MOSFETs 3. Solder suckers are not authorized for any type of equipment repair 4. Solder suckers require the use of a high wattage soldering iron that may damage MOSFETs
1. The resistance between the substrate and the gate of the enhancement type should be less than 15 ohms 2. The measurement between the drain and source of the enhancement type should read infinite regardless of meter lead polarity 3. The resistances between the gate and the drain and between the gate and the source of the enhancement type should be noticeably higher 4. None of the above 2-15. Which of the following is/are (an) advantage(s) of integrated circuits when compared to circuits made up of separate components and interconnections? 1. 2. 3. 4.
Lower power consumption Smaller size of the equipment Lower equipment cost All of the above
2-16. Which of the following is a characteristic of linear ICs? 1. They do not require regulated power supplies 2. They are typically sensitive to their supply voltages 3. They are never classed as electrostatic discharge sensitive devices 4. They are comparable in size to their equivalent transistor circuits
Figure 2F. —MOSFET (enhancement type) and equivalent circuit.
IN ANSWERING QUESTION 2-14, REFER TO FIGURE 2F.
12
2-20. For which of the following purposes are logic comparators used?
2-17. For which of the following reasons would you classify an IC as a "black box" device?
1. 2. 3. 4.
1. Because ICs are always black in color 2. Because all you can check are the inputs and outputs, not the internal operation of ICs 3. Because printed circuit boards that contain ICs cannot be repaired 4. Because ICs are designed to be repairable components
To test linear ICs To compare different types of ICs To inject pulse trains into digital ICs For in-circuit testing of digital ICs by comparing them with reference ICs
2-21. Which of the following is an advantage of using a logic probe instead of an oscilloscope to test a digital IC? 1. Logic probes are usually larger than an oscilloscope but much lighter 2. Logic probes have a low input impedance 3. Logic probes are battery powered and do not react to line voltage variations 4. Logic probes are capable of detecting short-duration pulses that most oscilloscopes cannot display
2-18. Test equipment used to detect the logic state of a digital IC should have which of the following characteristics? 1. A capability of measuring rms voltages 2. A frequency response in excess of 40 GHz 3. A high input impedance 4. A low input impedance
2-22. For which of the following purposes are logic pulsers used?
2-19. Which of the following statements describes) the use of logic clips?
1. To detect the logic state of digital ICs 2. To detect the logic state of linear ICs 3. To inject a pulse or pulse train into a logic circuit 4. To inject a 1-kHz sine wave into a circuit for signal tracing
1. Logic clips are designed to monitor the input and output of an IC simultaneously 2. Logic clips can only be used to test an IC that is out of the circuit 3. Logic clips can only be used with flat pack ICs 4. All of the above
2-23. Which of the following is a typical application for a logic analyzer? 1. To program EPROMs 2. To test individual logic ICs 3. To analyze the spectral purity at the output of a logic IC 4. To perform timing analysis by monitoring and comparing more than one timing signal simultaneously
13
2-29. Which of the following is a characteristic of fixed rf attenuators?
2-24. Which of the following instruments is used to test the specific gravity of a leadacid battery's electrolyte? 1. 2. 3. 4.
1. They are used to match impedances 2. They are designed to handle small amounts of rf power 3. They are usually built into the equipment in which they are used 4. They are capable of handling several kilowatts of power
Hydrometer Hygrometer Electrometer Gravitometer
2-25. Smoking is prohibited in the vicinity of lead-acid storage batteries for which of the following reasons?
2-30. Which of the following is an easy method of performing an operational test on a decade resistor?
1. Cigarette smoke neutralizes the electrolyte 2. Lead-acid batteries produce explosive hydrogen when they are being charged 3. Fumes produced by a lead-acid battery mixed with cigarette smoke produce a toxic by-product 4. All of the above
1. Use an swr meter 2. Use the resistance substitution method 3. Read the resistance with an ohmmeter 4. Apply an rf voltage across the decade, measuring the voltage drop and computing the resistance 2-31. Which of the following is/are the disadvantages of glass-core, fiber-optic cables?
2-26. When testing dry cell batteries, which of the following procedures should you follow?
1. They are smaller in diameter than plastic-core fibers 2. They are extremely susceptible to mechanical damage 3. They exhibit signal losses as high as 25 dB/km 4. Both 2 and 3 above
1. The battery should be tested under load conditions 2. The battery should not be tested under load conditions 3. The battery should be tested at various temperatures 4. Both 2 and 3 above
2-32. Which of the following types of test equipment should you use to measure the losses in a fiber optic cable if only one end of the cable is accessible?
2-27. Which of the following dry cell batteries is rechargeable? 1. 2. 3. 4.
NICAD Alkaline Carbon-zinc Mercury cells
1. 2. 3. 4.
2-28. Which of the following is the correct maximum charge rate for a NICAD battery rated at 300 milliampere hours? 1. 300 milliamperes for 15 hours 2. 60 milliamperes for 15 hours 3. 30 milliamperes for 15 hours 4. 600 milliamperes for 15 hours
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An optical ohmmeter A Wheatstone bridge An optical power meter An optical time-domain reflectometer
2-37. When approximate values for resistance, capacitance, or inductance to be measured by a bridge are unknown, which, if any, of the following actions should you take?
2-33. When using the AN/USM-465 portable service processor, which of the following procedures is possible? 1. Identifying faulty components on digital printed circuit boards 2. Troubleshooting its own printed circuit boards 3. When using the guided probe, it will tell you if you have placed the probe on the wrong test point 4. All of the above
1. Connect two bridges in parallel to make the measurement 2. Assign a temporary value to the component and set up the bridge accordingly 3. Place an adjustable shunt across the meter terminals 4. None of the above
2-34. Which of the following measurements add resistance and inductive and capacitive reactance? 1. 2. 3. 4.
2-38. The most serious errors affecting the accuracy of bridge measurements can be attributed to which of the following problems?
Q Resonance Impedance Figure of merit
1. The capacitive and inductive characteristics of the connecting leads 2. The resistance of the test leads 3. D'Arsonval meter movements used as detectors 4. Improper selection of meter shunts
2-35. Bridge circuits are used in the measurement of impedance for which of the following reasons? 1. Bridges are one of the most accurate devices for measuring impedance 2. Bridges are only slightly less accurate than vtvm's when measuring impedance 3. Bridges are useful in measuring frequency 4. Both 2 and 3 above
2-39. Which of the following considerations should be given when applying external excitation to a bridge circuit? 1. The voltage applied should equal the maximum voltage rating of the component under test 2. The higher the voltage, the more accurate the measurement 3. Apply only enough voltage to obtain a reliable indicator deflection 4. External excitation should be limited to 115 v 60 Hz
2-36. Bridge circuits typically contain which of the following sections? 1. A measuring circuit and comparing circuit 2. A detector circuit 3. A power circuit 4. All of the above
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2-44. What is the advantage of using a Maxwell bridge over a Hay bridge?
2-40. It is difficult to measure resistances less than 1 ohm with a bridge because of which of the following factors?
1. The Maxwell bridge can measure greater range of inductances 2. The Maxwell bridge can measure much smaller resistances 3. The Maxwell bridge can provide a greater accuracy over a smaller range 4. The Maxwell bridge can measure inductances having a high Q
1. Contact resistance is present between the resistor being measured and the binding posts of the bridge 2. Excessive supply voltage is required to excite the galvanometer 3. The frequency of the excitation source creates excessive skin currents in the resistor under test 4. The excitation voltage causes lowvalue resistors to heat excessively
2-45. Which of the following pieces of test equipment measure(s) the magnitude and phase angle of an unknown impedance?
2-41. What type of bridge is recommended for measuring resistances less than 1 ohm? 1. 2. 3. 4.
1. 2. 3. 4.
Wheatstone bridge Schering bridge Maxwell bridge Kelvin bridge
2-46. Maximum transfer of rf energy between transmitter/receiver and antenna will occur under which of the following circumstances?
2-42. When using resistance-ratio bridges to measure capacitance, inductance, or resistance, you should compare the unknown component with which of the following components? 1. 2. 3. 4.
1. When the transmitter or receiver is properly matched to the antenna 2. When the receiver is tuned one sideband above the transmitter 3. When the transmitter is tuned one sideband above the receiver 4. Both 2 and 3 above
A capacitor An inductor A resistor A similar standard
2-47. Rf impedance bridge measurements require the use of which of the following pieces of equipment?
2-43. A Hay bridge measures unknown inductances by comparing them with which, if any, of the following components? 1. 2. 3. 4.
The vector bridge The impedance-angle meter Both 1 and 2 above The Hay bridge
1. An ac power source, a detector, and a Wheatstone bridge 2. An rf signal generator, an oscilloscope, and a power supply 3. An rf signal generator, an rf bridge, and a detector 4. A Schering bridge, a detector, and an rf power supply
A standard inductor A standard resistor A standard capacitor None of the above
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2-52. What precaution(s), if any, must be taken when checking components with the Huntron Tracker 2000?
2-48. What unit of measure is used to express the power level of a complex voice signal? 1. 2. 3. 4.
1. Voltages must not exceed 5 V dc 2. Voltages must not exceed 5 V ac 3. Device to be tested must have all power turned off and capacitors discharged 4. None of the above
Vu dB dBm Watt
2-49. The function of a dB meter is described in which of the following descriptions?
2-53. When you are testing components by comparison, what is the most common mode used on the Huntron Tracker?
1. A current-measuring device 2. A user-calibrated constant current device 3. An electronic voltmeter calibrated in terms of dB 4. A frequency selective voltmeter calibrated in terms of true power
1. 2. 3. 4.
2-54. Why is it necessary to electrically isolate a component while testing individual components with the Huntron Tracker 2000?
2-50. Electrodynamic wattmeters are used to measure which of the following types of power? 1. 2. 3. 4.
Automatic Pulse generator Single sweep Alternate
Ac power Dc power Both 1 and 2 above Shf power in the 2-32 GHz frequency range
1. A resistor in series may give you an inaccurate signature 2. A diode in series may give you an inaccurate signature 3. A resistor in parallel may give you an inaccurate signature 4. All of the above
2-51. An electrodynamic wattmeter can be converted into an instrument for measuring reactive power by which of the following methods? 1. Installing a capacitor in series with the input 2. Shunting the meter movement with a 0. 1 ufd capacitor 3. Replacing the resistor which is normally in series with the voltage coil with a large inductance 4. Shunting the input terminals with an LC network adjusted to the resonant frequency of the signal being measured
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ASSIGNMENT 3 Textbook assignment: Chapter 3, “Quantitative Measurement,” pages 3-26 through 3-39. Chapter 4, “Qualitative Measurements,” pages 4-1 through 4-14. Chapter 5, “Introduction to Waveform Interpretation,” pages 5-1 through 5-35. ___________________________________________________________________________________ 3-5. Which of the following relationships exist(s) between the temperature increase of the calorimetric body of a static calorimeter and the applied power?
3-1. The AN/URM-120 in-line wattmeter is capable of measuring which of the following values? 1. Rf levels up to 500 watts between 30 MHz and 1,000 MHz 2. Rf levels up to 1 kW between 2 MHz and 30 MHz 3. Both 1 and 2 above 4. Af levels up to 500 watts between 1 kHz and 15 kHz
1. The temperature increase is proportional to the frequency of the applied power 2. The temperature increase is inversely proportional to the amount of applied power 3. The temperature increase is directly proportional to the time of the applied power 4. Both 2 and 3 above
3-2. When rf power is applied to a bolometer, the heat generated by the semiconductor bead results in which of the following characteristic changes? 1. 2. 3. 4.
3-6. Which of the following statements describe(s) the method of using a twin calorimeter?
A capacitive change An inductive change A resistive change A frequency change
1. Rf power is applied to one calorimetric body and the other body acts as a temperature reference 2. The steady-state temperature difference between the two calorimetric bodies is used as a measure of rf power 3. Both 1 and 2 above 4. Power is applied to both calorimetric bodies through a directional coupler
3-3. The Hewlett-Packard 431 C power meter is capable of measuring power within which of the following frequency ranges in a coaxial system? 1. 1 MHz to 9 MHz 2. 10 MHz to 18 GHz 3. 41 GHz to 100 GHz 4. 101 GHz to 1,000 GHz
3-7. Flow calorimeters are classified by the type of measurement performed, the type of heating used, and what other characteristic?
3-4. Calorimeters measure power by converting the input electromagnetic energy into which of the following forms? 1. 2. 3. 4.
1. 2. 3. 4.
Heat Dc power Pulsed rf energy Electrodynamic energy
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Number of calorimetric bodies Type of circulation method used Type of rf loads that they employ Number of calorimetric fluids used
3-11. If you anticipate using a stroboscope over an extended period of time, which of the following actions can you take to extend flasher-tube life?
3-8. When performing measurements above 1 GHz in a flow calorimeter, which of the following dielectrics do you normally use? 1. 2. 3. 4.
1. Operate the stroboscope at a submultiple of the fundamental synchronous speed 2. Lower the plate voltage of the flasher tube 3. Lower the filament voltage of the flasher tube 4. Operate the stroboscope at a multiple of the fundamental synchronous speed
Water Oil MEK H2SO4
3-9. Which of the following government agencies is/are responsible for maintaining our primary? 1. 2. 3. 4.
U. S. National Bureau of Standards U. S. Naval Observatory Department of Weights and Measures All of the above
3-12. Vibrating reed meters and moving disk meters are primarily used to measure which of the following values?
3-10. When using a stroboscope to measure an unknown frequency, which, if any, of the following steps should you take?
1. The frequency of 60-Hz ac power 2. The rotational speed of synchronous motors 3. Frequencies between 1 kHz and 10 MHz 4. The frequencies of multiplexed signals
1. Start the measurement at the lowest frequency that the stroboscope can deliver and increase the flashing rate until a single image is obtained 2. Start the measurement at the highest frequency that the stroboscope can deliver and reduce the flashing rate until a single stationary image is obtained 3. Start the measurement at the midscale range of the stroboscope and adjust the flashing rate, in either direction, until a harmonic of the primary frequency is obtained 4. None of the above
3-13. When using an oscilloscope to measure frequencies, which of the following formulas should you use? 1.
2. 3.
THIS SPACE LEFT BLANK INTENTIONALLY.
4.
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3-18. In becoming a qualified technician, which of the following goals should you seek to achieve?
3-14. Most oscilloscopes are limited in their frequency-measuring capability to which of the following upper frequency limits?
1. To be able to repair a specific piece of equipment 2. To be able to isolate faults in an entire system 3. To demonstrate a basic knowledge of system interconnections 4. To demonstrate minimum maintenance ability on a piece of equipment
1. 50 kHz 2. 100 kHz 3. 500 kHz 4. 100 MHz 3-15. Which of the following indications should you observe when a frequency meter is adjusted to the resonant frequency of the signal under test?
3-19. When attempting to correct a technical problem, which of the following procedures should you follow?
1. An audible beat-frequency signal 2. A pronounced dip in output at resonance 3. A pronounced increase in output power 4. A bright glow of the frequency meter glow lamp
1. 2. 3. 4.
3-16. What is the purpose of the time interval measurement of a frequency counter?
3-20. Efficient operation of equipment is assured by which of the following actions?
1. It indicates the wave period 2. It indicates the time between two events 3. It indicates the time between two functions of an event 4. Both 2 and 3 above
1. Using tricks of the trade 2. Making quick repairs when problems occur 3. Observing system quality figures during preventive maintenance 4. Monitoring all system test points continuously
3-17. What are the three basic categories of wavemeters? 1. 2. 3. 4.
Use short cuts Do random testing Use a logical approach Do a self-test of the equipment
3-21. The standing-wave ratio (swr) in a transmission line is figured by using which of the following ratios?
Resonant, active, and passive Absorption, active, and passive Reaction, resonant, and absorption Absorption, reaction, and transmission
1. Maximum voltage to maximum current 2. Maximum voltage to minimum voltage 3. Maximum current to maximum voltage 4. Minimum voltage to minimum current
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3-26. A milliammeter moved parallel to a twowire transmission line will show its highest indication at which of the following points?
3-22. Swr measurements are taken for which of the following purposes? 1. To determine the output frequency of the system under test 2. To determine the matching quality of the transmission line termination 3. To determine the coupling quality of the transmission line 4. To determine system output power
1. 2. 3. 4.
3-27. Which of the following devices may be used to measure swr without measuring the standing wave?
3-23. Couplers containing slots are used with rf probes to provide access to which of the following components? 1. 2. 3. 4.
1. 2. 3. 4.
Wavemeters Unidirectional couplers Open transmission lines Waveguides
Bridge Rf probe Neon lamps Milliammeter
3-28. When using an RC bridge to measure swr, which of the following factor(s) must you consider as the applied frequency increases?
3-24. The wavelength of a standing wave is measured on a short-circuited, terminated line using a magnetic or electric probe in which of the following ways?
1. 2. 3. 4.
1. By multiplying the average current by the peak current 2. By dividing the average voltage by the peak voltage 3. By measuring the distance between a maximum voltage point and a maximum current point 4. By measuring the distance between alternate maximum or minimum current points along the line
Skin effect Stray inductance Stray capacitance All of the above
3-29. Before a newly constructed bridge can be calibrated, adjustments must be made for which of the following reasons? 1. To determine the frequency range of the bridge 2. To keep stray effects at a minimum 3. To adjust the rf voltage amplitude 4. To determine the characteristic impedance of the circuit
3-25. A neon lamp moved parallel to a twowire parallel transmission line will glow at its brightest at which of the following points? 1. 2. 3. 4.
Maximum voltage points Maximum current points Maximum current and voltage points Maximum and minimum voltage points
Maximum current points Maximum voltage points Maximum voltage and current points Maximum and minimum current points
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3-34. If a 9.5 GHz, 20 watt signal is inserted into a transmission line, approximately what signal should be measured at the other end of the transmission line?
3-30. Which of the following formulas apply(ies) when measuring swr with a bridge? 1.
1. 2. 3. 4.
2.
3.
3-35. To accurately determine transmission line losses, you should perform insertion losses at which of the following frequencies?
Both 1 and 2 above--use the one that yields an swr ratio greater than 1 to 1
4.
1. Midrange of the transmission line’s frequency spectrum 2. At the upper and lower entrances of the transmission line’s frequency spectrum 3. Across the entire frequency spectrum of the transmission line 4. Midrange of the transmission line’s frequency spectrum, plus and minus 10 kHz
3-31. The ideal impedance match between transmitter and load is 1. 2. 3. 4.
5 GHz, 10 watts 5 GHz, 20 watts 0 GHz, 10 watts 0 GHz, 20 watts
1 to 1 2 to 1 3 to 1 4 to 1
3-32. When comparing vswr and iswr, which, if any, of the following is the correct ratio?
3-36. Which of the following transmission line specifications is/are considered important?
1. Vswr will exceed iswr by a minimum of 100% 2. Vswr will exceed iswr by a minimum of 50% 3. Vswr and iswr ratios will be equal 4. None of the above
1. 2. 3. 4.
Frequency Characteristic impedance Power-handling capabilities All of the above
3-37. Mixing two or more frequencies across a nonlinear device produces which of the following signals?
3-33. Electrical losses caused by transmission line deterioration are best measured using which of the following pieces of equipment?
1. 2. 3. 4.
1. A signal generator and a power meter 2. A signal generator and a frequency counter 3. An swr meter and an oscilloscope 4. A frequency counter and a power meter
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Crosstalk Intermodulation distortion Single sideband (ssb) transmission Undesirable carrier frequency deviation
3-42. At what point does an amplitudemodulated signal begin to produce distortion?
3-38. Which of the following statements describes cross modulation? 1. Degenerative feedback that causes a circuit to oscillate 2. Overmodulation that produces an echo 3. The signal from one channel that modulates the signal on an adjacent channel 4. Oscillation that is caused by system misalignment
1. 2. 3. 4.
3-43. To obtain 100% amplitude modulation of an rf carrier with a sine wave, the modulating power must equal what minimum percent of the rf carrier power?
3-39. Distortion caused by excessive regenerative feedback is called 1. 2. 3. 4.
1. 2. 3. 4.
echo crosstalk detected distortion parasitic generation
10% 15% 25% 50%
3-44. The damping of a meter movement that is being used to measure modulation has which of the following disadvantages?
3-40. When using a two-tone test to detect intermodulation distortion, what is the ideal indication you should see on a spectrum analyzer?
1. The frequency response of the meter is reduced 2. The accuracy of the meter movement is reduced 3. An average reading does not disclose transient overmodulation 4. The amount of current required to drive the meter is reduced
1. An exact reproduction of the input frequencies 2. The sum and difference of the input frequencies 3. A single frequency with the amplitude equal to the sum of the input frequencies 4. The beat frequency of the two inputs
3-45. Which of the following modulation patterns can be observed on an oscilloscope?
3-41. Which of the following actions minimizes the effects of intermodulation distortion? 1. 2. 3. 4.
Below 50% modulation At 65% modulation At 95% modulation Above 100% modulation
1. 2. 3. 4.
Using proper antenna spacing Shielding components and circuitry Using parasitic suppression circuits All of the above
Wave-envelope and trapezoidal Lissajous and wave-envelope Time division and frequency division Lissajous and trapezoidal
3-46. The frequency response of most oscilloscopes limits the capability of measuring percentage of modulation to which of the following frequency bands? 1. 2. 3. 4.
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Lf and hf Slf and shf Uhf Vhf
3-52. Frequency-domain plots are used by technicians to graphically view which of the following waveform parameters?
3-47. When using the two-tone test (trapezoidal method) to check a transmitter, you should see what pattern on the oscilloscope?
1. 2. 3. 4.
1. A series of fully modulated sine waves 2. A 100% amplitude-modulated signal 3. Two pulses of equal amplitude and duration 4. Two opposing triangles that are mirror images of each other
3-53. Which of the following pieces of test equipment should you use to determine what signals make up a complex signal? 1. 2. 3. 4.
3-48. Frequency deviation of an fm signal is usually expressed in which of the following units of measurements? 1. 2. 3. 4.
Kilohertz dB dBm Volts
1. 6% 2. 50% 3. 66% 4. 100%
1. The width of the band assigned for station operation 2. The maximum power output rating of the transmitter 3. The distortion that occurs at 100% modulation 4. The transmitting antenna height
3-55. When viewing a 100% amplitudemodulated signal with a spectrum analyzer, what type of display should you observe? 1. A center frequency and both the upper and lower sidebands 6 dB down from the center frequency 2. A center frequency and both the upper and lower sidebands of equal amplitude 3. A center frequency that is -6 dB down from both the upper and lower sidebands 4. A suppressed carrier with both the upper and lower sidebands of equal amplitude
3-50. Spectrum analysis is a graphic plot of amplitude versus time time versus frequency amplitude versus frequency amplitude versus power
3-51. Time-domain plots are used by technicians to graphically view which of the following waveform parameters? 1. 2. 3. 4.
Oscilloscope Sweep oscillator Spectrum analyzer Time-domain reflectometer
3-54. At 100% amplitude modulation, the total power in the sidebands equals what percentage of the carrier power?
3-49. What limits an fm transmitter’s maximum frequency deviation?
1. 2. 3. 4.
Amplitude versus time Amplitude versus frequency Frequency versus distance Amplitude versus power
Amplitude versus time Frequency versus time Frequency versus distance Amplitude versus power
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3-60. The ability of a spectrum analyzer to resolve signals refers to its ability to
3-56. Which of the following is/are an advantage of ssb transmission?
1. distinguish one signal from other signals 2. shape signals through the use of filters 3. determine a receiver’s minimum discernible signal 4. measure the frequency of a signal
1. The voice quality of ssb transmissions is superior to both AM and fm transmissions 2. Ssb transmissions are not susceptible to interference caused by sun spots and atmospherics 3. Ssb requires one-sixth of the output power and less than half the bandwidth required by AM to transmit the same amount of intelligence power 4. All of the above
3-61. The ability of a spectrum analyzer to resolve signals is limited by which of the following factors? 1. The amplitude of the signal under test 2. The narrowest bandwith of the spectrum analyzer 3. The upper frequency limits of the spectrum analyzer 4. The lower frequency limits of the spectrum analyzer
3-57. In fm, the AMOUNT of frequency deviation (shift) is proportional to 1. the frequency of the carrier 2. the amplitude of the modulating signal 3. the frequency of the modulating signal 4. the plate current of the transmitter’s linear amplifier
3-62. Which of the following characteristics of a transmission line fault can be observed using time-domain reflectometry? 1. 2. 3. 4.
3-58. In fm, the RATE of frequency deviation (shift) is proportional to 1. the impedance of the antenna 2. the power output of the transmitter 3. the amplitude of the modulating signal 4. the frequency of the modulating signal
Nature of the fault Distance to the fault Both 1 and 2 above Figure of merit of the fault
3-63. What is the primary application of sweptfrequency testing? 1. To determine the broadband frequency response of a device 2. To determine the characteristics of a device at a specific frequency 3. To determine the impedance of a transmission line 4. To determine the swr of a transmission line
3-59. When analyzing the composition of a rectangular wave with a spectrum analyzer, which of the following types of displays will you see? 1. A fundamental frequency and its odd harmonics only 2. A fundamental frequency and its even harmonics only 3. A fundamental frequency and its combined even and odd harmonics 4. An infinite number of fundamental frequencies
25
3-64. You should perform an initial power check on a transmitting antenna before sweeping the antenna for which of the following reasons? 1. To prevent damage to the test equipment 2. To ensure the transmitter is deenergized 3. To ensure the transmitter is energized 4. To ensure the transmitter is keyed
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NONRESIDENT TRAINING COURSE SEPTEMBER 1998
Navy Electricity and Electronics Training Series Module 22—Introduction to Digital Computers NAVEDTRA 14194
DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.
Although the words “he,” “him,” and “his” are used sparingly in this course to enhance communication, they are not intended to be gender driven or to affront or discriminate against anyone.
DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.
PREFACE By enrolling in this self-study course, you have demonstrated a desire to improve yourself and the Navy. Remember, however, this self-study course is only one part of the total Navy training program. Practical experience, schools, selected reading, and your desire to succeed are also necessary to successfully round out a fully meaningful training program. COURSE OVERVIEW: To introduce the student to the subject of Digital Computers who needs such a background in accomplishing daily work and/or in preparing for further study. THE COURSE: This self-study course is organized into subject matter areas, each containing learning objectives to help you determine what you should learn along with text and illustrations to help you understand the information. The subject matter reflects day-to-day requirements and experiences of personnel in the rating or skill area. It also reflects guidance provided by Enlisted Community Managers (ECMs) and other senior personnel, technical references, instructions, etc., and either the occupational or naval standards, which are listed in the Manual of Navy Enlisted Manpower Personnel Classifications and Occupational Standards, NAVPERS 18068. THE QUESTIONS: The questions that appear in this course are designed to help you understand the material in the text. VALUE: In completing this course, you will improve your military and professional knowledge. Importantly, it can also help you study for the Navy-wide advancement in rate examination. If you are studying and discover a reference in the text to another publication for further information, look it up.
1998 Edition Prepared by FCCM(SW) Robert A. Gray
Published by NAVAL EDUCATION AND TRAINING PROFESSIONAL DEVELOPMENT AND TECHNOLOGY CENTER
NAVSUP Logistics Tracking Number 0504-LP-026-8450
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Sailor’s Creed “I am a United States Sailor. I will support and defend the Constitution of the United States of America and I will obey the orders of those appointed over me. I represent the fighting spirit of the Navy and those who have gone before me to defend freedom and democracy around the world. I proudly serve my country’s Navy combat team with honor, courage and commitment. I am committed to excellence and the fair treatment of all.”
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TABLE OF CONTENTS CHAPTER
PAGE
1. Operational Concepts ...............................................................................................
1-1
2. Hardware ..................................................................................................................
2-1
3. Software....................................................................................................................
3-1
4. Data Representation and Communications...............................................................
4-1
APPENDIX I. Glossary..................................................................................................................
AI-1
II. Reference List......................................................................................................... AII-1 INDEX
.........................................................................................................................
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INDEX-1
NAVY ELECTRICITY AND ELECTRONICS TRAINING SERIES The Navy Electricity and Electronics Training Series (NEETS) was developed for use by personnel in many electrical- and electronic-related Navy ratings. Written by, and with the advice of, senior technicians in these ratings, this series provides beginners with fundamental electrical and electronic concepts through self-study. The presentation of this series is not oriented to any specific rating structure, but is divided into modules containing related information organized into traditional paths of instruction. The series is designed to give small amounts of information that can be easily digested before advancing further into the more complex material. For a student just becoming acquainted with electricity or electronics, it is highly recommended that the modules be studied in their suggested sequence. While there is a listing of NEETS by module title, the following brief descriptions give a quick overview of how the individual modules flow together. Module 1, Introduction to Matter, Energy, and Direct Current, introduces the course with a short history of electricity and electronics and proceeds into the characteristics of matter, energy, and direct current (dc). It also describes some of the general safety precautions and first-aid procedures that should be common knowledge for a person working in the field of electricity. Related safety hints are located throughout the rest of the series, as well. Module 2, Introduction to Alternating Current and Transformers, is an introduction to alternating current (ac) and transformers, including basic ac theory and fundamentals of electromagnetism, inductance, capacitance, impedance, and transformers. Module 3, Introduction to Circuit Protection, Control, and Measurement, encompasses circuit breakers, fuses, and current limiters used in circuit protection, as well as the theory and use of meters as electrical measuring devices. Module 4, Introduction to Electrical Conductors, Wiring Techniques, and Schematic Reading, presents conductor usage, insulation used as wire covering, splicing, termination of wiring, soldering, and reading electrical wiring diagrams. Module 5, Introduction to Generators and Motors, is an introduction to generators and motors, and covers the uses of ac and dc generators and motors in the conversion of electrical and mechanical energies. Module 6, Introduction to Electronic Emission, Tubes, and Power Supplies, ties the first five modules together in an introduction to vacuum tubes and vacuum-tube power supplies. Module 7, Introduction to Solid-State Devices and Power Supplies, is similar to module 6, but it is in reference to solid-state devices. Module 8, Introduction to Amplifiers, covers amplifiers. Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits, discusses wave generation and wave-shaping circuits. Module 10, Introduction to Wave Propagation, Transmission Lines, and Antennas, presents the characteristics of wave propagation, transmission lines, and antennas.
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Module 11, Microwave Principles, explains microwave oscillators, amplifiers, and waveguides. Module 12, Modulation Principles, discusses the principles of modulation. Module 13, Introduction to Number Systems and Logic Circuits, presents the fundamental concepts of number systems, Boolean algebra, and logic circuits, all of which pertain to digital computers. Module 14, Introduction to Microelectronics, covers microelectronics technology and miniature and microminiature circuit repair. Module 15, Principles of Synchros, Servos, and Gyros, provides the basic principles, operations, functions, and applications of synchro, servo, and gyro mechanisms. Module 16, Introduction to Test Equipment, is an introduction to some of the more commonly used test equipments and their applications. Module 17, Radio-Frequency Communications Principles, presents the fundamentals of a radiofrequency communications system. Module 18, Radar Principles, covers the fundamentals of a radar system. Module 19, The Technician's Handbook, is a handy reference of commonly used general information, such as electrical and electronic formulas, color coding, and naval supply system data. Module 20, Master Glossary, is the glossary of terms for the series. Module 21, Test Methods and Practices, describes basic test methods and practices. Module 22, Introduction to Digital Computers, is an introduction to digital computers. Module 23, Magnetic Recording, is an introduction to the use and maintenance of magnetic recorders and the concepts of recording on magnetic tape and disks. Module 24, Introduction to Fiber Optics, is an introduction to fiber optics. Embedded questions are inserted throughout each module, except for modules 19 and 20, which are reference books. If you have any difficulty in answering any of the questions, restudy the applicable section. Although an attempt has been made to use simple language, various technical words and phrases have necessarily been included. Specific terms are defined in Module 20, Master Glossary. Considerable emphasis has been placed on illustrations to provide a maximum amount of information. In some instances, a knowledge of basic algebra may be required. Assignments are provided for each module, with the exceptions of Module 19, The Technician's Handbook; and Module 20, Master Glossary. Course descriptions and ordering information are in NAVEDTRA 12061, Catalog of Nonresident Training Courses.
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Throughout the text of this course and while using technical manuals associated with the equipment you will be working on, you will find the below notations at the end of some paragraphs. The notations are used to emphasize that safety hazards exist and care must be taken or observed.
WARNING
AN OPERATING PROCEDURE, PRACTICE, OR CONDITION, ETC., WHICH MAY RESULT IN INJURY OR DEATH IF NOT CAREFULLY OBSERVED OR FOLLOWED.
CAUTION
AN OPERATING PROCEDURE, PRACTICE, OR CONDITION, ETC., WHICH MAY RESULT IN DAMAGE TO EQUIPMENT IF NOT CAREFULLY OBSERVED OR FOLLOWED.
NOTE
An operating procedure, practice, or condition, etc., which is essential to emphasize.
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INSTRUCTIONS FOR TAKING THE COURSE assignments. To submit your answers via the Internet, go to:
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NEETS Module 22 Introduction to Digital Computers
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CHAPTER 1
OPERATIONAL CONCEPTS LEARNING OBJECTIVES Learning objectives are stated at the beginning of each chapter. These learning objectives serve as a preview of the information you are expected to learn in the chapter. The comprehensive check questions placed throughout the chapters are based on the objectives. By successfully completing the Nonresident Training Course (NRTC), you indicate that you have met the objectives and have learned the information. The learning objectives for this chapter are listed below. Upon completion of this chapter, you will be able to do the following: 1. Describe the history of computers. 2. Describe how computers are classified. 3. Explain how digital computers have changed during each generation. 4. Describe the practical applications of digital computers in the Navy. 5. Describe the initial steps needed to use a microcomputer. 6. Explain storage media handling, backups, and the threats to storage media.
INTRODUCTION Digital computers are used in many facets of today's Navy. It would be impossible for one NEETS module to cover all the ways they are used in any depth. A few of these ways are covered later in this chapter. The purpose of this module is to acquaint you, the trainee, with the basic principles, techniques, and procedures associated with digital computers. We will use a desktop (personal) computer for most of the examples. Personal computers should be more familiar to you than the large mainframes, and the operating principles of personal computers relate directly to the operating principles of mainframe computers. You will learn the basic terminology used in the digital-computer world. When you have completed these chapters satisfactorily, you will have a better understanding of how computers are able to perform the demanding tasks assigned to them. If we were to define the word computer, we would say a computer is an instrument for performing mathematical operations, such as addition, multiplication, division, subtraction, integration, vector resolution, coordinate conversion, and special function generation at very high speeds. But the usage of computers goes well beyond the mathematical-operations level. Computers have made possible military, scientific, and commercial advances that before were considered impossible. The mathematics involved in orbiting a satellite around the earth, for example, would require several teams of mathematicians for a lifetime. Now, with the aid of electronic digital computers, the conquest of space has become reality. 1-1
Computers are employed when repetitious calculations or the processing of large amounts of data are necessary. The most frequent applications are found in the military, scientific, and commercial fields. They are used in many varied projects, ranging from mail sorting, through engineering design, to the identification and destruction of enemy targets. The advantages of digital computers include speed, accuracy, reliability, and man-power savings. Frequently computers are able to take over routine jobs, releasing people for more important work; work that cannot be handled by a computer.
HISTORY OF COMPUTERS The ever increasing need for faster and more efficient computers has created technological advances that can be considered amazing. Ever since humans discovered that it was necessary to count objects, we have been looking for easier ways to do it. Contrary to popular belief, digital computers are not a new idea. The abacus is a manually operated digital computer used in ancient civilizations and used to this day in the Orient (see fig. 1-1). For those who consider the abacus outdated, in a contest between a person using a modern calculator and a person using an abacus, the person using the abacus won.
Figure 1-1.—Abacus.
The first mechanical adding machine (calculator) was invented by Blaise Pascal (French) in 1642. Twenty years later, an Englishman, Sir Samuel Morland, developed a more compact device that could multiply, add, and subtract. In 1682, Wilhelm Liebnitz (German) perfected a machine that could perform all the basic operations (addition, subtraction, division, and multiplication), as well as extract the square root. Liebnitz's principles are still in use today in our modern electronic digital computers. As early as 1919, electronics entered the scene. An article by W. H. Eccles and F. W. Jordan described an electronic "trigger circuit" that could be used for automatic counting. It was the ECCLES-JORDAN multivibrator which was a little ahead of its time because a trigger circuit is one of many components required to make an electronic digital computer. Modern digital computers use these circuits, known as flip-flops, to store information, perform arithmetic operations, and control the timing sequences within the computer. Under the pressure of military needs in World War II, the science of electronic data processing made giant strides forward. In 1944, Harvard University developed a computing system known as the Automatic Sequence Controlled Calculator. After the initial design and construction, several improved models were built.
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Meanwhile, at the University of Pennsylvania, a second system was being developed. This system, completed in 1946, was named ENIAC (Electronic Numerical Integrator and Computer). ENIAC employed 18,000 vacuum tubes in its circuitry; and in spite of these bulky, hot tubes, it worked quite successfully. The first problem assigned to ENIAC was a calculation in nuclear physics that would have taken 100 human-years to solve by conventional methods. The ENIAC solved the problem in 2 weeks, only 2 hours of which were actually spent on the calculation. The remainder of the time was spent checking the results and operational details. All modern computers have their basics in these two early developments conducted at Harvard University and University of Pennsylvania. In 1950, the UNIVAC I was developed. This machine was usually regarded as the most successful electronic data processor of its day. An outstanding feature of the UNIVAC I was that it checked its own results in each step of a problem; thus eliminating the need to run the problems more than once to ensure accuracy. During the first outbreak of publicity about computers (especially when the UNIVAC predicted the outcome of the 1952 presidential election), the term "giant brain" caused much confusion and uneasiness. Many people assumed that science had created a thinking device superior to the human mind. Currently most people know better. By human standards the giant brain is nothing more than a talented idiot that is wholly dependent upon human instructions to perform even the simplest job. A computer is only a machine and definitely cannot think for itself. The field of artificial intelligence, however, is developing computer systems that can "think"; that is, mimic human thought in a specific area and improve performance with experience and operation. The field of digital computers is still in the growing stages. New types of circuitry and new ways of accomplishing things are continuing to be developed at a rapid rate. In the military field, the accomplishments of digital computers are many and varied. One outstanding example is in weapons systems. Most of the controlling is done by digital computers.
CLASSIFICATIONS OF COMPUTERS Computers can be classified in many different ways. They can be classified by the type of technology they use (mechanical, electromechanical, or electronic), the purpose for which they were designed (general purpose or special purpose), by the type of data they can handle (digital or analog), by the amount they cost (from $50 to $10 million and up), and even by their physical size (handheld to room size). We will briefly explain mechanical, electromechanical, and electronic computers; special-purpose and general-purpose computers; and analog and digital computers. MECHANICAL COMPUTERS Mechanical or analog computers are devices used for the computation of mathematical problems. They are made up of components, such as integrators, sliding racks, cams, gears, springs, and driveshafts. Figure 1-2 shows a typical mechanical computer used by the Navy. These computers are analog in nature, and their physical size depends on the number of functions the computer has to perform. In an analog computer, a continuing input will give a constantly updated output. This being perfect for target information, the Navy uses these analog computers primarily for gun fire control. As systems for naval weapons became more and more complex, the need for a different computer was apparent. The functions that had to be performed had increased the size of the computer to an unreasonable scale.
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Figure 1-2.—Bulkhead-type mechanical computer
ELECTROMECHANICAL COMPUTERS Electromechanical computers came next and differ from mechanical computers in that they use electrical components to perform some of the calculations and to increase the accuracy. Because the electrical components are smaller than their mechanical counterparts, the size of the computer was reduced, even though it performs more functions. The components used to perform the calculations are devices such as synchros, servos, resolvers, amplifiers, servo amplifiers, summing networks, potentiometers, and linear potentiometers. Figure 1-3 shows one of the Navy's electromechanical computers. These computers are used in gun fire control and missile fire control. Even though they are better than the mechanical computer, they still have their drawbacks. Of prime importance is that they are special-purpose computers. This means they can only be used for one job, dependent on their design characteristics. By today's Navy standards they are still too large, and the maintenance time on them is excessive. The need for a more accurate, reliable, versatile, and smaller computer was recognized.
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Figure 1-3.—Electromechanical computer.
ELECTRONIC COMPUTERS Next came electronic computers. The early electronic computers' mathematical processes were solved by using electrical voltages only, applied to elements such as amplifiers, summing networks, differentiating, and integrating circuits. The weak link in this type of electrical computation was the vacuum tube. To correct this, transistors which consume less power and last longer than vacuum tubes were used in the amplifiers. Through technological research and development, we have progressed from tubes, to transistors, to miniaturized circuits, to integrated circuitry. These advances have made it possible to reduce the size and weight of our computers. Figure 1-4 is an example of one of our modern electronic digital computers.
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Figure 1-4.—Electronic digital computer.
SPECIAL-PURPOSE COMPUTERS A special-purpose computer, as the name implies, is designed to perform a specific operation and usually satisfies the needs of a particular type of problem. Such a computer system would be useful in weather predictions, satellite tracking, or oil exploration. While a special-purpose computer may have many of the same features found in a general-purpose computer, its applicability to a particular problem is a function of its design rather than to a stored program. The instructions that control it are built directly into the computer, which makes for a more efficient and effective operation. A drawback of this specialization, however, is the computer's lack of versatility. It cannot be used to perform other operations. GENERAL-PURPOSE COMPUTERS General-purpose computers are designed to perform a wide variety of functions and operations. You will probably use this type of computer. A general-purpose computer is able to perform a wide variety of operations because it can store and execute different programs in its internal storage. Unfortunately, having this ability is often achieved at the expense of speed and efficiency. In most situations, however, you will find that having this flexibility makes this compromise a most acceptable one. ANALOG COMPUTERS All analog computers are special-purpose computers. They are designed to measure continuous electrical or physical conditions, such as current, voltage, flow, temperature, length, or pressure. They then convert these measurements into related mechanical or electrical quantities. The early analog computers were strictly mechanical or electromechanical devices. They did not operate on digits (in binary notation, either of the characters, 0 and 1). If digits were involved at all, they were obtained 1-6
indirectly. Your wrist watch (if nondigital); your car's speedometer; and oil pressure, temperature, and fuel gauges are also considered analog computers. The output of an analog computer is often an adjustment to the control of a machine; such as, an adjustment to a valve that controls the flow of steam to a turbine generator or a temperature setting to control the ovens in the ship's galley for baking. Analog computers are also used for controlling processes. To do so, they must convert analog data to digital form, process it, and then convert the digital results back to analog form. You should know that a digital computer can process data with greater accuracy than an analog computer, but an analog computer can process data faster than a digital computer, in some systems. Some computers combine the functions of both analog and digital computers. They are called hybrid computers. DIGITAL COMPUTERS Digital computers perform arithmetic and logic functions on separate discrete data, like numbers, or combinations of discrete data, such as name, rate, and division. This makes them different from analog computers that operate on continuous data, like measuring temperature changes. We generally use digital computers for business and scientific data processing. The following are examples: Accounting—Computers are ideal for keeping payroll records, printing paychecks, billing customers, preparing tax returns, and taking care of many of the other accounting tasks in an organization. Recordkeeping—Computers can record information like inventories and personnel files. They can also keep track of books checked out of a library. Airline ticket counters are much more efficient than they used to be, thanks to centralized reservation computers that can be reached over the telephone lines. Industrial Uses—Industrial computers save considerable time and reduce waste by efficiently performing hundreds of industrial tasks, ranging from filling sales orders and routing parts to various locations on an assembly line, to designing earthquake-resistant structures, and controlling an entire oil refinery. Science—The research and development applications are the most numerous. Digital computers are being used to do lengthy and complicated mathematical calculations millions of times faster than human beings. They are also used to collect, store, and evaluate data from experiments, analyze weather patterns, forecast crop statistics, and, believe it or not, design other computers. Word Processing—Remember, these words were typed into a desktop computer! Word processing is among the most common applications for personal computers. If you have not discovered the advantages of computer writing, it's time to visit a computer dealer for a personalized demonstration. None of this work could be performed by a computer without first instructing the computer how to do it by means of a list of instructions called a program. The instructions in the program must be written in one of the languages the computer understands. The most popular generic term for computer programs is software (this is covered in chapter 3). Hardware (covered in chapter 2), of course, refers to the computer and related equipment. It is easy to say that both computer hardware and software are interdependent because neither can perform useful work without the other. Digital computers may be either special or general purpose. ACCURACY OF COMPUTERS The fundamental difference between analog and digital computers is that digital computers deal with discrete quantities such as beads on an abacus, notches on a toothed wheel, or electrical pulses, while analog computers deal with continuous physical variables such as electrical voltages or mechanical shaft rotations. Computation with analog computers depends on the relation of information to a measurement
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of some physical quantity. For example, you can determine the number of boards in a picket fence by either a digital or an analog system as follows. In the digital method (fig. 1-5, view A), you use an adding machine and count the boards one by one. In the analog method (fig. 1-5, view B), you draw a string (marked off in inches for the width of each board including the gap) over the length of the fence, then measure the length of the string. The number of boards may then be determined by dividing the length of string by the number of inches per board.
Figure 1-5A.—Digital computation.
Figure 1-5B.—Analog computation.
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The accuracy of an analog computer is restricted to the accuracy with which physical quantities can be sensed and displayed. This, in turn, is related to the quality of the components used in constructing the computer; for example, the tolerance of electrical resistors or mechanical shafts and the quality of the output equipment. In an analog computer, for example, if the constant is represented by a voltage, it probably could be read only to the third decimal place. On the other hand, the accuracy of a digital computer is governed by the number of significant figures carried in the computations. This, in turn, is determined by the computer's design. In a digital computer, the number of decimal places in the constant could be many, depending on the design of the computer processing unit. The digital computer is, therefore, capable of higher precision and accuracy. However, a computer, regardless of its accuracy, would do you no good if the wrong one were chosen for a given task. Most of the computer systems you will work with will be general-purpose digital computers. The remainder of this module will be about general-purpose digital computers. Q-1. How are computers classified? Q-2. Mechanical computers are considered to be of what type? Q-3. The Navy uses analog computers primarily for what purpose? Q-4. How do electromechanical computers differ from the mechanical computers? Q-5. In electronic computers, vacuum tubes were replaced by transistors and transistors have been replaced by what device? Q-6. A computer that is designed to perform a specific operation and usually satisfies the needs of a particular type of problem, is said to be what type of computer? Q-7. Rather than using a stored program, a special-purpose computer's applicability to a particular problem is a function of what? Q-8. What is a drawback to the special-purpose computer? Q-9. A general-purpose computer is designed for what purpose? Q-10. How is a general-purpose computer able to perform different operations? Q-11. In a general-purpose computer, the ability to perform a wide variety of operations is achieved at the expense of what capabilities? Q-12. All analog computers are what type of computers? Q-13. What are analog computers designed to measure? Q-14. Early analog computers were what type of devices? Q-15. What are computers called that combine the functions of both analog and digital computers? Q-16. Digital computers are generally used for what purposes? Q-17. What is the fundamental difference between analog and digital computers? Q-18. How is the accuracy of an analog computer restricted? 1-9
Q-19. A constant represented by a voltage can be read to what decimal place? Q-20. The accuracy of a digital computer is governed by what factor? Q-21. In a digital computer, what does the number of decimal places in the constant depend on? Q-22. You will most likely be working with what type of computer?
DIGITAL COMPUTER GENERATIONS In the electronic computer world, we measure technological advancement by generations. A specific system is said to belong to a specific "generation." Each generation indicates a significant change in computer design. The UNIVAC I represents the first generation. Currently we are moving toward the fourth generation. FIRST GENERATION The computers of the first generation (1951-1958) were physically very large machines characterized by the vacuum tube (fig. 1-6). Because they used vacuum tubes, they were very unreliable, required a lot of power to run, and produced so much heat that adequate air conditioning was critical to protect the computer parts. Compared to today's computers, they had slow input and output devices, were slow in processing, and had small storage capacities. Many of the internal processing functions were measured in thousandths of a second (millisecond). The software (computer program) used on first generation computers was unsophisticated and machine oriented. This meant that the programmers had to code all computer instructions and data in actual machine language. They also had to keep track of where instructions and data were stored in memory. Using such a machine language (see chapter 3) was efficient for the computer but difficult for the programmer.
Figure 1-6.—First generation computers used vacuum tubes.
SECOND GENERATION The computers of the second generation (1959-1963), were characterized by transistors (fig. 1-7) instead of vacuum tubes. Transistors were smaller, less expensive, generated almost no heat, and required very little power. Thus second generation computers were smaller, required less power, and produced a lot less heat. The use of small, long lasting transistors also increased processing speeds and reliability. Cost performance also improved. The storage capacity was greatly increased with the introduction of magnetic disk storage and the use of magnetic cores for main storage. High speed card readers, printers, 1-10
and magnetic tape units were also introduced. Internal processing speeds increased. Functions were measured in millionths of a second (microseconds). Like the first generation, a particular computer of the second generation was designed to process either scientific or business oriented problems but not both. The software was also improved. Symbolic machine languages or assembly languages were used instead of actual machine languages. This allowed the programmer to use mnemonic operation codes for instruction operations and symbolic names for storage locations or stored variables. Compiler languages were also developed for the second generation computers (see chapter 3).
Figure 1-7.—Second generation computers used transistors.
THIRD GENERATION The computers of this generation (1964-1970), many of which are still in use, are characterized by miniaturized circuits. This reduces the physical size of computers even more and increases their durability and internal processing speeds. One design employs solid-state logic microcircuits (fig. 1-8) for which conductors, resistors, diodes, and transistors have been miniaturized and combined on half-inch ceramic squares. Another smaller design uses silicon wafers on which the circuit and its components are etched. The smaller circuits allow for faster internal processing speeds resulting in faster execution of instructions. Internal processing speeds are measured in billionths of a second (nanoseconds). The faster computers make it possible to run jobs that were considered impractical or impossible on first or second generation equipment. Because the miniature components are more reliable, maintenance is reduced. New mass storage, such as the data cell, was introduced during this generation, giving a storage capacity of over 100 million characters. Drum and disk capacities and speed have been increased, the portable disk pack has been developed, and faster, higher density magnetic tapes have come into use. Considerable improvements were made to card readers and printers, while the overall cost has been greatly reduced. Applications using online processing, real-time processing, time sharing, multiprogramming, multiprocessing, and teleprocessing have become widely accepted. More on this in later chapters.
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Figure 1-8.—Third generation computers used microcircuits.
Manufacturers of third generation computers are producing a series of similar and compatible computers. This allows programs written for one computer model to run on most larger models of the same series. Most third generation systems are designed to handle both scientific and business data processing applications. Improved program and operating software has been designed to provide better control, resulting in faster processing. These enhancements are of significant importance to the computer operator. They simplify system initialization (booting) and minimize the need for inputs to the program from a keyboard (console intervention) by the operator. FOURTH GENERATION AND BEYOND The computers of the fourth generation are not easily distinguished from earlier generations, yet there are some striking and important differences. The manufacturing of integrated circuits has advanced to the point where thousands of circuits (active components) can be placed on a silicon wafer only a fraction of an inch in size (the computer on a chip). This has led to what is called large scale integration (LSI) and very large scale integration (VLSI). As a result of this technology, computers are significantly smaller in physical size and lower in cost. Yet they have retained large memory capacities and are ultra fast. Large mainframe computers are increasingly complex. Medium sized computers can perform the same tasks as large third generation computers. An entirely new breed of computers called microcomputers (fig. 1-9) and minicomputers are small and inexpensive, and yet they provide a large amount of computing power.
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Figure 1-9.—Fourth generation desktop (personal) computer.
What is in store for the future? The computer industry still has a long way to go in the field of miniaturization. You can expect to see the power of large mainframe computers on a single super chip. Massive data bases, such as the Navy's supply system, may be written into read-only memory (ROM) on a piece of equipment no bigger than a desktop calculator (more about ROM in chapter 2). The future challenge will not be in increasing the storage or increasing the computer's power, but rather in properly and effectively using the computing power available. This is where software (programs such as assemblers, report generators, subroutine libraries, compilers, operating systems, and applications programs) will come into play (see chapter 3). Some believe developments in software and in learning how to use these extraordinary, powerful machines we already possess will be far more important than further developments in hardware over the next 10 to 20 years. As a result, the next 20 years (during your career) may be even more interesting and surprising than the last 20 years. Q-23. Technological advancement is measured by what, in the electronic computer world? Q-24. What does each generation of computer systems indicate? Q-25. What were computers of the first generation characterized by? Q-26. How did vacuum tubes cause a problem for first generation computers? Q-27. In first generation computers, internal processing functions were measured by what division of time? Q-28. The software (computer program) used on first generation computers was what type? Q-29. How were processing speed and reliability increased in second generation computers? Q-30. In second generation computers, how was the storage capacity greatly increased? Q-31. With improvements in software, what kind of computer languages could be used on second generation computers?
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Q-32. What do the smaller circuits in third generation computers allow for? Q-33. On third generation computers, what results are gained by faster internal processing speeds? Q-34. The data cell had a storage capacity of how many characters? Q-35. What type of applications were most third generation computer systems designed to accomplish? Q-36. What type of computers are small and inexpensive yet provide a lot of computing power? Q-37. What does the acronym ROM stand for? Q-38. What will be one of the future challenges involving computer power? Q-39. What term is used for programs such as assemblers, compilers, and operating systems?
USES OF A DIGITAL COMPUTER In the modern computer world of today, the uses of the digital computer are almost as limitless as a person's imagination. New and better programs are being written everyday for easier and greater uses. Consider how many mathematicians it would take to put an astronaut in orbit around the moon, but it only takes one computer. Think back to the days without word processing when a document had to be retyped entirely when any changes were needed. Think back to the days of using an adding machine to prepare and revise budgets and accounting reports. Let's look at three of the primary uses of general-purpose digital computers in the Navy: word processing, accounting/recordkeeping, and work center uses. WORD PROCESSING One of the more widespread uses of the computer is word processing. The word processor can be considered a typewriter with a display screen. To the hundreds of thousands of word processor users, the computer is nothing more than a typewriter. Both have keyboards, and both have a mechanism for making the image of the character you strike on the keyboard appear on some type of visual medium. When using an electric typewriter, the process is strictly mechanical. When you press the key, it causes the type face to strike the paper, and in so doing, it leaves an impression. In the computer, the process is more indirect. A program stored in the computer's memory causes a visual representation to appear on a crt (cathode-ray tube) or at a printer. However, from the view point of the user, the result is the same, a printed document. The great advantage of computers over typewriters is in correcting errors. In the past, correcting a document with a typewriter has meant typing it all over again. Since computers allow the movement of information from one part of memory to another, it is possible to make many changes on a document, and print the result. If the document is still not correct, only the changes need to be entered. The use of computers in this particular way came to be known as word processing. A further breakthrough came with the development of word-processing application programs for microcomputers. These programs cost a fraction of their office machine counterparts, and could be run on general-purpose microcomputers. This was unique because general-purpose microcomputers could be used for functions such as spreadsheets, data base management systems, and programming in common computer languages. The Navy saw the obvious uses to which microcomputers using the word processing programs could be put. Some of these are manuscript writing, memorandum writing, identification-card application filing, and recordkeeping. 1-14
ACCOUNTING AND RECORDKEEPING There are virtually unlimited applications for the computer in today's modern business world, from basic accounting functions to controlling the manufacture of products, and of course, keeping records of these actions. Six standard systems dealing with accounting applications are widely accepted. These systems are (1) order entry; (2) inventory control; (3) accounts receivable; (4) accounts payable; (5) general ledger; and (6) payroll. (Figure 1-10 shows a simplified flowchart of payroll.) The area of recordkeeping has two requirements, legal and audit. The Navy has included similar functions in its Shipboard Non-Tactical ADP Program for work center use.
Figure 1-10.—Programming flowchart used to build a payroll program.
WORK CENTER USES (SNAP II) Every Navy rating has the responsibility for some element of ship's maintenance. And for every rate, recordkeeping has been a "tough nut to turn," an administrative chore that goes along with the work to be done, but takes a "back burner" position to the physical maintenance of the ship and equipment. Today, aboard some ships and soon aboard most, much of that hassle will be done with a SNAP. 1-15
The Navy has looked at the paperwork blizzard of recordkeeping responsibility of the essential records and reports that must be generated, and has offered relief to the fleet. This is in the form of S-N-A-P, which stands for Shipboard Non-Tactical ADP Program. SNAP II is a modern shipboard computer system designed to support shipboard and intermediatelevel maintenance, supply, financial, and administrative functions. If this sounds confusing, it really isn't, for the systems are designed to be user-friendly; that is, operating instructions are written in everyday English. Figure 1-11 shows the AN/UYK-62 (V) Data Processing Set. This is the SNAP II computer and its associated hardware.
Figure 1-11.—AN/UYK-62 (V) Data Processing Set (SNAP II).
Over the next 3 years, new functions will be added to SNAP to support more of the ship's administrative workload. Pay, personnel, food service, ship's store, PMS, training, medical and dental data are all to be added to SNAP systems. The SNAP concept is to take the power of the modern computer, the ability to process information, and put that power in the hands of the work center sailors. The sailors can use the system to reduce the labor associated with the paperwork function. User terminals are placed in the different work centers for use by the work center supervisor. Each work center has a different access code. This access code or password prevents unauthorized entry into the main computer's program. Different levels of entry are also defined. The levels depend on a work center's need. Information stored in the computer for a typical work center normally has the following items that can be updated by the work center supervisor. COSAL (coordinated onboard ship/shore allowance list) is a listing of the repair parts that are allowed to be kept onboard ship, at all times. APL (allowance parts 1-16
list) is the reference for stock numbers, part numbers, and quantity allowed onboard for a specific system. EIC (equipment identification code) identifies a system, sub-system, or equipment. SHIP'S FORCE WORK LIST is a listing of all work to be performed by a certain work center during a given time period. CSMP (current ship's maintenance projects) provides shipboard maintenance managers with a consolidated listing of deferred maintenance to manage and control its accomplishment. These are but a few of the uses of SNAP II that can be updated by the work center supervisor. Although the information is usually viewed on a display screen (cathode-ray tube), printed (hard) copies can be obtained. Today, hard copy output from SNAP can be sent to higher authorities in lieu of written reports. In the future, these hard copy transmittals may be replaced by disks or tapes containing the same data. In some cases, the shipboard computers will have an extra telephone wire to the pier or tender, and information can be exchanged electronically. And there are other important benefits. In practice, the system expedites the storage and retrieval of information the Navy has about its ships. In turn, information that is more accessible means a more timely supply of parts, an improved aid to planners on when and how long to schedule ships' overhauls, and updated information for making decisions whether to place additional or remove unnecessary shipboard equipment. These decisions are now made by laboriously using stacks of printed files. SNAP can sort through these files electronically so Navy planners can make more effective and timely decisions. SNAP II is a system for unclassified use only at present. This cuts the costs of the installation and many of the physical and electronic security requirements. Q-40. What is one of the more widespread uses of the computer? Q-41. What is the great advantage of computers over typewriters? Q-42. How are word processing programs used by the Navy? Q-43. How many systems dealing with accounting applications have been widely accepted? Q-44. What does the acronym S-N-A-P stand for? Q-45. For what purposes is the SNAP II system designed? Q-46. What does user friendly mean in computer terms? Q-47. What does a password prevent? Q-48. In the SNAP II system, how are the different levels of entry defined? Q-49. The work center supervisor can update what items from a user terminal? Q-50. At present what type of classified use is allowed for SNAP II?
USING A DESKTOP COMPUTER To use a desktop (personal) computer effectively, you'll need to learn about the hardware (the equipment) and the software (the programs). You will also need to know how to handle disks and how to back up programs and data files. So let's assume you have a desktop computer system to use. Its hardware consists of a display screen, a keyboard, a computer, two floppy disk drives (A & B), and a printer. Look at the example in figure 1-12. You need software (computer programs) to make the computer operate. The
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first program you need is the operating system. The operating system manages the computer and allows you to run application programs like word processing or recordkeeping programs. So let's begin with the operating system.
Figure 1-12.—Typical microcomputer system with display, keyboard, floppy disk drives, and printer.
OPERATING SYSTEM An operating system is simply a set of programs and routines that lets you and other programs use the computer. A digital computer uses one central set of programs called the operating system to manage execution of other programs and to perform common functions like read, write, or print. Other programs, or you the user, can order the operating system to perform these common functions. These orders are called system calls when other programs use them, or simply commands when you put them through the keyboard. First, you must load the operating system into the computer so we, and our programs, can use the computer. Remember, in our example, we have a desktop computer with two floppy disk drives, named A and B. Booting the System Each desktop computer has a built-in program called "bootstrap loader." When you turn the computer on, this program tries to load, or "boot," an external operating system from disk, usually from drive A, into the computer's internal memory. Disk drive B is usually used for data file disks. The term boot comes from the idea of pulling yourself up by your bootstraps. The computer loads a little program from the disk that tells it how to load a second, bigger program (the operating system). The operating system then tells it how to load another program (an applications program or utility program) to perform a specific job or function. The first thing you need to learn about using a computer is that computers and their programs are very particular. They require complete accuracy and attention to detail on your part. They are not good at guessing what you meant. You'll quickly learn there are a few things that can go wrong at this point, in which case the computer will give you an error message on the display screen similar to this: Device Error
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This means the computer is not reading anything in A drive. Check for: 1. No floppy disk in drive A 2. Floppy disk inserted incorrectly in drive 3. Lock handle on drive A not lowered Another error message you might receive at this time is: No System This means the computer is reading a properly inserted floppy disk, but the disk does not have an operating system on it. Replace the disk with one that does contain the operating system. Once the operating system is properly booted (loaded), you will see a display similar to this: A> You now have what is called a prompt. At this point you can tell the computer what to do next, such as run an application program for example: word processing, accounting, or recordkeeping. Running an Application Program To load an application program into the computer from drive A, you put the disk with the application program in disk drive A. Next you type the name of the program following the operating system prompt (A>). A>WORDPROC This tells the system what program to load and run; in this case Word Processing. The computer then does what the application program tells it. If the application is word processing, the system is ready for you to type a new document, correct an existing document, print a document, and so on. You'll learn more about both the operating system and application programs in chapter 3. Each application program will have its own set of instructions to follow. In addition to printed documentation, many will include online HELP screens you can display while you are working. These will tell you how to perform a given function or operation. Another area that needs your constant attention relates to handling floppy disks and making backup copies to be sure your work is not lost. STORAGE MEDIA HANDLING AND BACKUP Floppy disks (fig. 1-13) are one means by which you will store data (files that you create) either directly or in backing up the data you store on hard (or fixed) disk. For this reason, and because floppy disks are extremely fragile, you should follow certain guidelines to ensure their proper care and handling. This includes properly labeling and backing up disks.
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Figure 1-13.—Floppy disk.
Handling Never touch the exposed surface of a disk. As you know (or will learn), most of the surface of the actual disk is protected most of the time; however, there are areas that are exposed. These areas are the timing hole and the read/write slots. Touching an exposed area can ruin that particular area. If you are familiar with Murphy's Law, you will realize the area you ruin will invariably contain the most important data on that disk. Storage Never bend, fold, or otherwise distort the shape of a disk. Never place heavy objects such as books on top of disks. Store disks in the box they came in, or in filing containers that are specifically designed for storing disks. Try to store disks vertically, but if you do store disks horizontally, do not stack more than 10 disks. Exposure Disks are subject to exposure from magnetic fields, smoke, heat, and sunlight. X-rays may also have a negative effect. MAGNETIC FIELDS.—Disks should never be exposed to anything that could be the source of a magnetic field. Exposure of a disk to a magnetic field could cause the destruction of some or all of the data contained on that disk. Some common sources of magnetic energy are crt's, disk drives, and perhaps the most common, the telephone. SMOKE.—Smoke can cause buildup on disks and on disk drives. DO NOT SMOKE while you work at a terminal or computer.
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HEAT AND SUNLIGHT.—Never expose disks to excessive heat or direct sunlight. Either can cause the disks to become warped or distorted so they cannot be used. Disks are made of a plastic material, and if you have ever seen a phonograph record that has been exposed to heat or sunlight, you have some idea of the damage that can result from exposure. Typically, disks will operate only between 10 and 50 degrees Celsius (50 to 120 degrees Fahrenheit). They will accept a relative humidity of 10% to 80%. X-RAYS.—There is some question about the effect that airport x-ray machines have on disks. It has been the normal experience that the walk-through x-ray machines at airports have no effect on floppy disks; however, this is not to say there will be no effect. It is up to you because these disks contain the data you work with and need. You may not want to take the chance the disks will be affected. Labeling When labeling the outside of a floppy disk, write the label before attaching it to the disk. Never use a pencil or ballpoint pen to write on a label once that label has been attached to a disk. When you use an instrument with a sharp point to write on the label, you can actually etch into the surface of the disk underneath the protective sheath, thereby destroying that disk. If you must write on a label once it has been attached to a disk, use a felt-tip marker. Data Backup In virtually all computer systems, the possibility exists for errors to occur that accidentally alter or destroy the data stored in the data bases or files. This may occur because of natural disasters, such as fire, flood, or power outages. It may occur through operator error. It may occur through equipment malfunction. It is essential, therefore, to provide a means to ensure that any data lost can be recovered. The most common method is backup files. A backup file is merely a copy of a file. If for some reason the file or data base is destroyed or becomes unusable, the backup file can be used to recreate the file or data base. Two media are commonly used for backup: disk or tape. Disk—The most common method of creating a backup for a microcomputer is to use a floppy disk and the diskcopy procedure. This is accomplished by using the original data base or file and copying the information onto a blank floppy disk. The instructions for this procedure will be provided with the particular computer and program you are using. Tape—Another method of creating a backup is to use magnetic tape. The information contained on your disk, whether it is a data base or file, can be copied onto a tape. The instructions for this procedure will also be provided with the particular computer and program you are using. Q-51. What is a central set of programs called that manages the execution of other programs and performs common functions like read, write, and print? Q-52. What is the function of a built-in program called a bootstrap loader? Q-53. When you see the error message NO SYSTEM, what does it mean? Q-54. When an operating system prompt (A>) is displayed on the screen, what do you enter from the keyboard to load an application program? Q-55. If disks are stored horizontally, how many can be stacked? Q-56. What can exposure to a magnetic field do to the data on a disk? 1-21
Q-57. What is the temperature range within which a disk will operate? Q-58. What is the most common method to ensure that any stored data lost can be recovered? Q-59. The most common method of creating a backup for a microcomputer is what? Q-60. Other than disk, what is another media used for backup files?
SUMMARY This chapter has presented information on the history and classification of computers. It introduced you to electronic digital computers, their uses and operation. The information that follows summarizes the important points of this chapter. Early computers were MECHANICAL or ELECTROMECHANICAL. ELECTRONIC COMPUTERS came into use in the 1940s. ANALOG COMPUTERS are special-purpose computers designed to measure continuous electrical or physical conditions. DIGITAL COMPUTERS are special- or general-purpose computers designed to perform arithmetic and logic functions on separate discrete data. They are generally used for business and scientific data processing. Digital computers have evolved through four generations: vacuum tubes, transistors, miniaturized circuits, and integrated circuits. WORD PROCESSING is one of the most widespread uses of desktop computers. ACCOUNTING AND RECORDKEEPING are also major uses of computers. Included are order entry, inventory control, accounts receivable, accounts payable, general ledger, and payroll. The Navy's SHIPBOARD NON-TACTICAL ADP PROGRAM (SNAP) consists of computers used by work center supervisors for logistic and administrative support. This system expedites the storage and retrieval of information the Navy has about its ships. A DESKTOP (PERSONAL) COMPUTER is a microcomputer with at least a display screen, keyboard, floppy disk drive, and printer. It may also have additional devices such as a second floppy or a hard disk drive. An OPERATING SYSTEM is loaded into the computer to let you and other programs use the computer. It also provides common functions like read, write, and print. You can direct the computer to run an APPLICATION PROGRAM by telling the operating system the name of program to run. Common application programs are word processing, accounting, and recordkeeping. You will probably be using FLOPPY DISKS for data storage and backup. To ensure you don't damage a disk, use care in handling, labeling and storing the disks.
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ANSWERS TO QUESTIONS Q1. THROUGH Q60. A-1. Technology (mechanical, electromechanical, electronic), purpose (special or general), type of data they handle (analog or digital), cost, physical size (handheld to room size). A-2. Analog. A-3. Gun fire control. A-4. Electromechanical computers use electrical components to perform some of the calculations. A-5. Integrated circuits. A-6. Special-purpose. A-7. Its design. A-8. Lack of versatility. A-9. To perform a wide variety of functions and operations. A-10. By storing different programs in its internal storage. A-11. Speed and efficiency. A-12. Special-purpose. A-13. Continuous electrical or physical conditions. A-14. Mechanical or electromechanical. A-15. Hybrid computers. A-16. Business and scientific data processing. A-17. Digital computers deal with discrete quantities, while analog computers deal with continuous physical variables. A-18. By the accuracy with which physical quantities can be sensed and displayed. A-19. Third. A-20. The number of significant figures carried in the computations. A-21. Design of the computer processing unit. A-22. General-purpose digital computer. A-23. Generations. A-24. Significant change in computer design. A-25. The vacuum tube. A-26. They were unreliable, required a lot of power to run, and produced so much heat that air conditioning was needed to protect computer parts. 1-23
A-27. Thousandths of a second (millisecond). A-28. Unsophisticated and machine oriented. A-29. By the use of small, long lasting transistors. A-30. With the introduction of magnetic disk storage and the use of core for main storage. A-31. Symbolic machine languages or assembly languages. A-32. Faster internal processing speeds. A-33. Faster execution of instructions. A-34. Over 100 million. A-35. Both scientific and business data processing applications. A-36. Microcomputers and minicomputers. A-37. Read-only memory. A-38. How to properly and effectively use the computing power available. A-39. Software. A-40. Word processing. A-41. Correcting errors. A-42. For manuscript writing, memorandum writing, identification-card application filing, and recordkeeping. A-43. Six. A-44. Shipboard Non-tactical ADP Program. A-45. To support shipboard and intermediate level maintenance, supply, financial, and administrative functions. A-46. Operating instructions are written in everyday English. A-47. Unauthorized entry into the main computer's program. A-48. Dependent on a work center's need. A-49. COSAL, APL, EIC, SHIP'S FORCE WORK LIST, and CSMP. A-50. Unclassified. A-51. Operating system. A-52. To load an external operating system into the computer's internal memory. A-53. The computer is reading a properly inserted floppy disk, but it does not have an operating system on it.
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A-54. The program name. A-55. No more than ten. A-56. Destroy some or all of it. A-57. 10 to 50 degrees Celsius or 50 to 120 degrees Fahrenheit. A-58. Backup files. A-59. Use a floppy disk and the diskcopy procedure. A-60. Magnetic tape.
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CHAPTER 2
HARDWARE LEARNING OBJECTIVES Upon completion of this chapter, you will be able to do the following: 1. Explain the cpu and describe the functions of the different sections. 2. Categorize the types of storage and their functions. 3. Describe how storage is classified. 4. Analyze and compare the input/output devices and explain their functions.
INTRODUCTION Components or tools of a computer system are grouped into one of two categories, hardware or software. We refer to the machines that compose a computer system as hardware. This hardware includes all the mechanical, electrical, electronic, and magnetic devices within the computer itself (the central processing unit) and all related peripheral devices (printers, magnetic tape units, magnetic disk drive units, and so on). These devices will be covered in this chapter to show you how they function and how they relate to one another. Take a few minutes to study figure 2-1. It shows the functional units of a computer system: the inputs, the central processing unit (cpu), and the outputs. The inputs can be on any storage medium from punched cards, paper tape, or magnetic ink to magnetic tape, disk, or drum; or they can be entries from a console keyboard or a cathode-ray tube (crt) terminal. The data from one or more of these inputs will be processed by the central processing unit to produce output. The output may be in punched cards or paper tape, on magnetic tape, disk, or drum, or it may be printed reports or information displayed on a console typewriter or crt terminal. The figure also shows the data flow, instruction flow, and flow of control. We'll start our hardware discussion with the cpu and then move into storage media (disk, tape, and drum). We'll end the chapter with a discussion of input/output devices and how they work.
CENTRAL PROCESSING UNIT (CPU) The brain of a computer system is the central processing unit, which we generally refer to as the cpu or mainframe. The central processing unit IS THE COMPUTER. It is the cpu that processes the data transferred to it from one of the various input devices, and then transfers either the intermediate or final results of the processing to one of many output devices. A central control section and work areas are required to perform calculations or manipulate data. The cpu is the computing center of the system. It consists of a control section, internal storage section (main or primary memory), and arithmetic-logic section (fig. 2-1). Each of the sections within the cpu serves a specific function and has a particular relationship to the other sections within the cpu.
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Figure 2-1.—Functional units of a computer system.
CONTROL SECTION The control section may be compared to a telephone exchange because it uses the instructions contained in the program in much the same manner as the telephone exchange uses telephone numbers. When a telephone number is dialed, it causes the telephone exchange to energize certain switches and control lines to connect the dialing phone with the phone having the number dialed. In a similar manner,
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each programmed instruction, when executed, causes the control section to energize certain control lines, enabling the computer to perform the function or operation indicated by the instruction. The program may be stored in the internal circuits of the computer (computer memory), or it may be read instruction-by-instruction from external media. The internally stored program type of computer, generally referred to only as a stored-program computer, is the most practical type to use when speed and fully automatic operation are desired. Computer programs may be so complex that the number of instructions plus the parameters necessary for program execution will exceed the memory capacity of a stored-program computer. When this occurs, the program may be sectionalized; that is, broken down into modules. One or more modules are then stored in computer memory and the rest in an easily accessible auxiliary memory. Then as each module is executed producing the desired results, it is swapped out of internal memory and the next succeeding module read in. In addition to the commands that tell the computer what to do, the control unit also dictates how and when each specific operation is to be performed. It is also active in initiating circuits that locate any information stored within the computer or in an auxiliary storage device and in moving this information to the point where the actual manipulation or modification is to be accomplished. The four major types of instructions are (1) transfer, (2) arithmetic, (3) logic, and (4) control. Transfer instructions are those whose basic function is to transfer (move) data from one location to another. Arithmetic instructions are those that combine two pieces of data to form a single piece of data using one of the arithmetic operations. Logic instructions transform the digital computer into a system that is more than a high-speed adding machine. Using logic instructions, the programmer may construct a program with any number of alternate sequences. For example, through the use of logic instructions, a computer being used for maintenance inventory will have one sequence to follow if the number of a given item on hand is greater than the order amount and another sequence if it is smaller. The choice of which sequence to use will be made by the control section under the influence of the logic instruction. Logic instructions, thereby, provide the computer with the ability to make decisions based on the results of previously generated data. That is, the logic instructions permit the computer to select the proper program sequence to be executed from among the alternatives provided by the programmer. Control instructions are used to send commands to devices not under direct command of the control section, such as input/output units or devices. ARITHMETIC-LOGIC SECTION The arithmetic-logic section performs all arithmetic operations-adding, subtracting, multiplying, and dividing. Through its logic capability, it tests various conditions encountered during processing and takes action based on the result. As indicated by the solid arrows in figure 2-1, data flows between the arithmetic-logic section and the internal storage section during processing. Specifically, data is transferred as needed from the internal storage section to the arithmetic-logic section, processed, and returned to the internal storage section. At no time does processing take place in the storage section. Data may be transferred back and forth between these two sections several times before processing is completed. The results are then transferred from internal storage to an output unit, as indicated by the solid arrow (fig. 2-1).
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MEMORY (INTERNAL STORAGE) SECTION All memory (internal storage) sections must contain facilities to store computer data or instructions (that are intelligible to the computer) until these instructions or data are needed in the performance of the computer calculations. Before the stored-program computer can begin to process input data, it is first necessary to store in its memory a sequence of instructions, and tables of constants and other data it will use in its computations. The process by which these instructions and data are read into the computer is called loading. Actually, the first step in loading instructions and data into a computer is to manually place enough instructions into memory using the keyboard or electronically using an operating system (discussed in chapter 1), so that these instructions can be used to bring in more instructions as desired. In this manner a few instructions are used to bootstrap more instructions. Some computers make use of an auxiliary (wired) memory that permanently stores the bootstrap program, thereby making manual loading unnecessary. The memory (internal storage) section of a computer is essentially an electronically operated file cabinet. It has a large number (usually several hundred thousand) of storage locations; each referred to as a storage address or register. Every item of data and program instruction read into the computer during the loading process is stored or filed in a specific storage address and is almost instantly accessible. Q-1. What is the brain of a computer system? Q-2. How many sections make up the central processing unit? Q-3. What are the names of the sections that make up the cpu? Q-4. The control section can be compared to what? Q-5. What are the four major types of instructions in the control section? Q-6. What capability allows the arithmetic/logic section to test various conditions encountered during processing and take action based on the result? Q-7. In the arithmetic/logic section, data is returned to what section after processing? Q-8. What is the process by which instructions and data are read into a computer?
TYPES OF INTERNAL STORAGE You already know that the internal storage section is the holding area in which instructions and data are kept. For the control section to control and coordinate all processing activity, it must be able to locate each instruction and data item in storage. About now, you are probably wondering how the control section is able to find these instructions and data items. To understand this, let's look at storage as nothing more than a collection of mailboxes. Each mailbox has a unique address and represents a location in memory as shown in figure 2-2. Like the mail in your mailbox, the contents of a storage location can change, but the number on your mailbox or memory address always remains the same. In this manner, a particular program instruction or data item that is held in storage can be located by knowing its address. Some computers can address each character of data in memory directly. Others address computer words which contain a group of characters at a single address. Each computer word contains a group of characters at a
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single address. Some of the more common types of internal storage media used in today's computers are as follows: magnetic core, semiconductor, and bubble.
Figure 2-2.—Memory locations.
MAGNETIC CORE STORAGE Although magnetic core storage is no longer as popular as it once was, we will cover it in some detail because its concepts are easily understood and apply generally to the more integrated semiconductor and bubble-type memories. Magnetic core storage is made up of tiny doughnut-shaped rings made of ferrite (iron), that are strung on a grid of very thin wires (fig. 2-3). Since data in computers is stored in binary form (refer to NEETS, module 13), a two-state device is needed to represent the two binary digits (bits), 0 for off and 1 for on. In core storage, each ferrite ring can represent a 0 or 1 bit, depending on its magnetic state. If magnetized in one direction, it represents a 1 bit, and if magnetized in the opposite direction, it represents a 0 bit. These cores are magnetized by sending an electric current through the wires on which the core is strung. It is this direction of current that determines the state of each core.
Figure 2-3.—Two-state principle of magnetic storage.
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SEMICONDUCTOR STORAGE (THE SILICON CHIP) Semiconductor memory consists of hundreds of thousands of tiny electronic circuits etched on a silicon chip (fig. 2-4). Each of these electronic circuits is called a bit cell and can be in either an off or on state to represent a 0 or 1 bit, depending on whether or not current is flowing in that cell. Another name you will hear used for semiconductor memory chips is integrated circuits (ICs). Developments in technology have led to large scale integration (LSI), which means that more and more circuits can be squeezed onto the same silicon chip. Companies are even manufacturing very large scale integrated circuits (VLSI), which means even further miniaturization.
Figure 2-4.—A semiconductor memory chip (integrated circuit).
Some of the advantages of semiconductor storage are fast internal processing speeds, high reliability, low power consumption, high density (many circuits), and low cost. However, there is a drawback to this type of storage. It is volatile, which means all data in memory is lost when the power supply is removed. Should the power on your computer fail and you have no backup power supply, all the stored data is lost. This is not the case with magnetic core storage. Core storage is nonvolatile. This means the data is retained even if there is a power failure or breakdown, since the cores store data in the form of magnetic charges rather than electric current. BUBBLE STORAGE One of the latest technological developments in storage media is the introduction of bubble memory. Bubble memory consists of a very thin crystal made of semiconductor material. The molecules of this special crystal act as tiny magnets (fig. 2-5). The polarity of these molecules or "magnetic domains" can be switched in an opposite direction by passing a current through a control circuit imprinted on top of the crystal. In this manner, data can be stored by changing the polarity of the magnetic domains. Since the principle is the same as for magnetic core storage, bubble memory is considered nonvolatile. The data is retained even if there is a power failure. Furthermore, the process of reading from bubble memory is nondestructive, meaning that the data is still present after being read. This is not the case with core storage, which must be regenerated after being read. If we were to view these magnetic domains under a microscope, they would look like tiny bubbles; hence the name, bubble memory. 2-6
Figure 2-5.—Bubble memory.
Q-9. Magnetic core storage is made up of what? Q-10. A semiconductor memory consists of what? Q-11. What is another name for semiconductor memory chips? Q-12. In computer storage, what does volatile mean? Q-13. What type of storage can retain its data even if there is a power failure or breakdown? Q-14. Bubble memory consists of what? Q-15. How are the magnetic domains of a bubble memory switched? Q-16. What do we mean when we say that reading from bubble memory is nondestructive?
CLASSIFICATIONS OF INTERNAL STORAGE Up to this point, you have learned some of the general functions of the cpu, the physical characteristics of memory, and how data is stored in the internal storage section. Now, we will explain yet another way to classify internal (primary or main) storage. This is by the different kinds of memories used within the cpu: read-only memory, random-access memory, programmable read-only memory, and erasable programmable read-only memory. READ-ONLY MEMORY (ROM) In most computers, it is useful to have often used instructions, such as those used to bootstrap (initial system load) the computer or other specialized programs, permanently stored inside the computer. Memory that enables us to do this without the programs and data being lost (even when the computer is powered down) is called read-only memory. Only the computer manufacturer can provide these programs in ROM and once done, they cannot be changed. Consequently, you cannot put any of your own data or programs in ROM. Many complex functions such as routines to extract square roots, translators for programming languages, and operating systems can be placed in ROM memory. Since these instructions are hard wired (permanent), they can be performed quickly and accurately. Another advantage of ROM is
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that your computer facility can order programs tailored for its needs and have them permanently installed in ROM by the manufacturer. Such programs are called microprograms or firmware. RANDOM-ACCESS MEMORY (RAM) Another kind of memory used inside computers is called random-access memory (RAM) or read/write memory. RAM memory is rather like a blackboard on which you can scribble down notes, read them, and rub them out when you are finished with them. In the computer, RAM is the working memory. Data can be read (retrieved) from or written (stored) into RAM just by giving the computer the address of the location where the data is stored or is to be stored. When the data is no longer needed, you can simply write over it. This allows you to use the storage again for something else. Core, semiconductor, and bubble storage all have random access capabilities. PROGRAMMABLE READ-ONLY MEMORY (PROM) An alternative to ROM is programmable read only memory (PROM) that can be purchased already programmed by the manufacturer or in a blank state. By using a blank PROM, you can enter any program into the memory. However, once the PROM has been written into, it can never be altered or changed. Thus you have the advantage of ROM with the additional flexibility to program the memory to meet a unique need. The main disadvantage of PROM is that if a mistake is made and entered into PROM, it cannot be corrected or erased. Also, a special device is needed to "burn" the program into PROM. ERASABLE PROGRAMMABLE READ-ONLY MEMORY (EPROM) The erasable programmable read-only memory (EPROM) was developed to overcome the drawback of PROM. EPROMs can also be purchased blank from the manufacturer and programmed locally at your command/activity. Again, this requires special equipment. The big difference with EPROM is that it can be erased if and when the need arises. Data and programs can be retrieved over and over again without destroying the contents of the EPROM. They will stay there quite safely until you want to reprogram it by first erasing the EPROM with a burst of ultra-violet light. This is to your advantage, because if a mistake is made while programming the EPROM, it is not considered fatal. The EPROM can be erased and corrected. Also, it allows you the flexibility to change programs to include improvements or modifications in the future. Q-17. In what type of memory are often used instructions and programs permanently stored inside the computer? Q-18. Who provides the programs stored in ROM? Q-19. Can programs in ROM be changed? Q-20. What is another name for random-access memory (RAM)? Q-21. How is data read from or written into RAM? Q-22. In what two states can programmable read-only memory (PROM) be purchased? Q-23. What is the main disadvantage of PROM? Q-24. What does EPROM stand for? Q-25. How is EPROM erased?
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SECONDARY STORAGE The last kind of memory we will briefly introduce here is called secondary storage or auxiliary storage. This is memory outside the main body of the computer (cpu) where we store programs and data for future use. When the computer is ready to use these programs and data, they are read into internal storage. Secondary (auxiliary) storage media extends the storage capabilities of the computer system. We need it for two reasons. First, because the computer's internal storage is limited in size, it cannot always hold all the data we need. Second, in secondary storage, data and programs do not disappear when power is turned off. Secondary storage is nonvolatile. This means information is lost only if you, the user, intentionally erase it. The three types of secondary storage we most commonly use are magnetic disk, tape, and drum. MAGNETIC DISK The popularity of disk storage devices is largely because of their direct-access capabilities. Most every system (micro, mini, and mainframe) will have disk capability. Magnetic disks resemble phonograph records (round platters), coated with a magnetizable recording material (iron oxide), but their similarities end there. Magnetic disks come in many different sizes and storage capacities. They range from 3 inches to 4 feet in diameter and can store from 2.5 million to 600 million characters (bytes) of data. They can be portable in that they are removable, or they can be permanently mounted in the storage devices called disk drive units or disk drives. They can be made of rigid metal (hard disks) or flexible plastic (floppy disks or diskettes) as shown in figure 2-6.
Figure 2-6.—Various types and sizes of magnetic disk storage.
Music is stored on a phonograph record in a continuous groove that spirals into the center of the record. But there are no grooves on a magnetic disk. Instead, data is stored on all disks in a number of invisible concentric circles called tracks. Each track has a designated number beginning with track 000 at the outer edge of the disk. The numbering continues sequentially toward the center to track 199, 800, or whatever the highest track number is. No track ever touches another (fig. 2-7). The number of tracks can vary from 35 to 77 on a floppy disk surface and from 200 to over 800 on hard disk surfaces.
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Figure 2-7.—Location of tracks on the disk's recording surface.
Data is written as tiny magnetic bits (or spots) on the disk surface. Eight-bit codes are generally used to represent data. Each code represents a different number, letter, or special character. In chapter 4, you'll learn how the codes are formed. When data is read from the disk, the data on the disk remains unchanged. When data is written on the disk, it replaces any data previously stored on the same area of the disk. Characters are stored on a single track as strings of magnetized bits (0's and 1's) as shown in figure 2-8. The 1 bits indicate magnetized spots or ON bits. The 0 bits represent unmagnetized portions of the track or OFF bits. Although the tracks get smaller as they get closer to the center of the disk platter, each track can hold the same amount of data because the data density is greater on tracks near the center.
Figure 2-8.—A string of bits written to disk on a single track.
A track can hold one or more records. A record is a set of related data treated as a unit. The records on a track are separated by gaps in which no data is recorded, and each of the records is preceded by a disk address. This address indicates the unique position of the record on the track and is used to directly access the record. Figure 2-9 shows a track on which five records have been recorded. Because of the gaps and addresses, the amount of data we can store on a track is reduced as the number of records per track is increased. Records on disk can be blocked (grouped together). Only one disk address is needed
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per block, and as a result, fewer gaps occur. We can use the blocking technique to increase the amount of data we can store on one track.
Figure 2-9.—Data records as they are written to disk on a single track.
The storage capacity of a disk depends on the bits per inch of track and the tracks per inch of surface. Using Winchester technology, the designers of disk drive units were able to increase the data density of a disk by increasing the number of tracks. Winchester was the code name used by IBM during the development of this technology. The designers originally planned to use dual disk drives to introduce the new concept. Each drive was to have a storage capacity of 30 million characters, and thus was expected to be a "30-30." Since that was the caliber of a famous rifle, the new product was nicknamed "Winchester." The designers found that data density could be improved and storage capacity increased by reducing the flying height, the distance of the read/write heads over the disk surfaces when reading and writing. By doing this, smaller magnetized spots could be precisely written and then read. The read/write heads were moved so close to the disk that a human hair looked like a mountain in the path of the flying head. Winchester technology reduces this potential problem by sealing the disks in a contamination-free container. This eliminates foreign objects from coming in contact with the read/write heads. Data can be physically organized in one of two ways on a disk pack, depending on the manufacturer and the model of disk drive you are using. One way uses the cylinder method, and the other uses the sector method. On diskettes, data is organized using the sector method. The cylinder method uses a cylinder as the basic reference point. When you look at figure 2-10, view A, you will see a disk pack containing six disk platters with 10 recording surfaces. Imagine you are looking down through the disk pack from the top. All the tracks with the same number line up vertically. Together they are called a cylinder. These 10 tracks, one on each recording surface, can be referenced by the 10 read/write heads on the five access arms at each discrete location where the access arms can be positioned.
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Figure 2-10A.—Physical organization of data on a disk. CYLINDER METHOD.
Therefore, to physically reference a record stored using the cylinder method, a computer program must specify the cylinder number, the recording surface number, and the record number as shown in figure 2-10, view A. Here, the record is stored in cylinder 25 of recording surface 6 and is the first record on that track. Special data stored on each track specifies the beginning of the track so that the first record, second record, third record, and so on, can be identified. Another way to physically organize data on the disk pack (and on diskettes) is to use the sector method. This requires that each of the tracks be divided into individual storage areas called sectors (shown in figure 2-10, view B). The number of sectors varies with the disk system used; however, there are usually eight or more. Each sector holds a specific number of characters. Before a record can be accessed, a computer program must again give the disk drive the record's address specifying the track number, the surface number, and the sector number of the record. One or more read/write heads are then moved to the proper track, the head over the specified surface is activated, and the data is read from or written to the designated sector as it spins under the head.
Figure 2-10B.—Physical organization of data on a disk. SECTOR METHOD.
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MAGNETIC TAPE Another type of storage device is magnetic tape which is similar to the tape used with commercial tape recorders. It is used mainly for secondary storage. It differs from commercial tape in that it is usually wider (ranging from one-half inch to an inch), and it is manufactured to more rigid quality specifications. It is made of a MYLAR® base coated with a magnetic oxide that can be magnetized to store data. Magnetic tape comes in a variety of lengths (from 600 to 3,000 feet), and is packaged in one of three ways: open reel, cartridge, or cassette, as shown in figure 2-11. Large computers use standard open reels, 1/2-inch wide tape, 2,400 feet in length. Magnetic tape units are categorized by the type of packaging used for the tape. The tape unit (or drive) shown in figure 2-12 uses open reels, while cartridge tape units use tape cartridges and cassette units use tape cassettes. Cartridge tape units are often used on personal computers to provide backup for hard disk.
Figure 2-11.—Various types of magnetic tape storage.
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Figure 2-12.—Mounting a magnetic tape
A standard 1/2-inch tape may have either seven (fig. 2-13, view A) or nine tracks (fig. 2-13, view B) of data stored on it, depending upon the particular read/write heads installed in the tape unit. Read/write heads are usually designed to read (or write) data (in the form of bits) concurrently across the width of the tape.
Figure 2-13.—Multi-track magnetic tape.
The amount of data or the number of binary digits (0 and 1 bits) that can be written (stored) on a linear inch of tape is known as the tape's recording density. Common recording densities for multitrack tapes range from 200 to 6,250 bits/bytes per inch (BPI). Also note that sometimes the density of a tape is
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referred to as the number of frames per inch (FPI) or characters per inch (CPI) rather than BPI. Regardless of which term is used, a frame or byte is a group of related bits that make up a single character written across the width of the tape. Most magnetic tape units are capable of reading and writing in several different densities. Magnetic tapes have many common features and data recording formats. Each tape is physically marked in some manner to indicate where reading and writing on tape is to begin (known as the beginning-of-tape [BOT]), and where it ends (known as the end-of-tape [EOT]). The length of tape between the BOT and EOT is referred to as the usable recording (reading/writing) surface or usable storage area. BOT/EOT markers are usually made of short silver strips of reflective tape (1/4-inch wide by 1/2-inch long) as shown in figure 2-14. The BOT marker is normally placed toward the front edge of the tape (the side nearest you when the tape is mounted on the tape unit). The EOT marker is placed toward the back edge (the side farthest from you when the tape is mounted on the tape unit). They are placed approximately 15 to 20 feet in from each end on the shiny side of the tape. Sometimes, holes or clear plastic inserts are used as markers in place of reflective strips. Regardless of the method used, the BOT/EOT markers are sensed by an arrangement of lamps and/or photodiode sensors to indicate where reading and writing is to begin and end.
Figure 2-14.—Beginning-of-tape (BOT) and end-of-tape (EOT) markers.
We can make records on magnetic tape any size we need to hold the data. We are restricted only by the length of the tape or the capacity of internal storage. For example, a record can be one character, several characters, or thousands of characters in length. The collection of records is called a file. A file containing payroll records is called a payroll file; a file containing supply inventory records is called a supply inventory file. Records can be placed on tape either separately as single records (unblocked) as shown in figure 2-15, view A, or multiple records can be grouped together (blocked) as shown in figure 2-15, view B, to form a record block. The number of records stored in a record block is the blocking factor. In this example, the blocking factor is five.
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Figure 2-15.—Record formats on magnetic tape.
All magnetic tape must be moving at a predetermined speed for data to be read from or written on the tape. Data cannot be read or written while the tape is coming up to speed, slowing down, or stopped. During this time delay, the tape moves a short distance creating a blank spot on the tape. This interrecord gap or interblock gap separates each single record or block of records on the tape. The length of the gap varies, depending upon the particular system and method of recording, but is approximately 2/5 to 3/4 inch in length. If single records are stored on the tape, the interrecord gap may be longer than the portion of tape used to store the record. Therefore, much of the tape's recording surface is wasted. To overcome the inefficiency of storing single data records, we normally block records. In figure 2-15, view B, you will notice the tape is used more efficiently than the tape in figure 2-15, view A. Blocking allows more data to be stored on a reel of tape. During reading, the record begins with the first character sensed following an interrecord or interblock gap and continues until the next gap is reached. All input records read are internally stored in accordance with the amount of storage area set aside by the applications program. Magnetic tape, as a storage media, offers several useful features. We can store large amounts of data in a variety of convenient package sizes (open reels, cartridges, or cassettes). Magnetic tapes are easily interchangeable between similar tape units of different computer systems, and tapes are less prone to damage than other types of storage media. MAGNETIC DRUM Like the magnetic disk, the magnetic drum is another example of a direct-access storage device. Although the magnetic drum was once used as main (or primary) storage, it is now used as secondary (or auxiliary) storage. Unlike some disk packs, the magnetic drum cannot be physically removed. The drum is permanently mounted in the device. Magnetic drum storage devices consist of either a hollow cylinder (thus, the name drum) or a solid cylinder that rotates at a constant velocity (from 600 to 6,000 rpm). The outer surface is coated with an iron-oxide material capable of being magnetized.
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A magnetic drum differs from a magnetic disk in that the tracks in which the data is stored are assigned to channels located around the circumference of the drum as shown in figure 2-16. That is, the channels form circular bands around the drum. The coded representation of data in figure 2-16 is similar to that used on 9-track magnetic tape, 8-bit code. The basic functions of the read/write heads are to place magnetized spots (those little binary 0's and 1's) on the drum during a writing operation and to sense these spots during a reading operation. The read/write heads of a drum perform in a manner similar to the read/write heads of a magnetic tape unit or disk drive unit.
Figure 2-16.—Magnetic drum.
The tracks on each channel are grouped into sectors as illustrated in figure 2-16. Does this sound familiar to you? It sounds almost like the format used on disk packs when referring to tracks (or cylinders) and sectors. As the drum rotates, the reading or writing occurs when the specified sector of a given channel passes under the read/write head for that channel. Some drums are mounted in a horizontal position, such as the one shown in figure 2-16, while others are mounted in a vertical position. Another major difference in the design is the number of read/write heads. Some drums use only one read/write head, which services all channels on the drum. In this case, the head moves back and forth (or up and down) over the surface of the drum as required. Other drums, using multiple read/write heads, have one principal advantage over drums with the single-head type. Since one read/write head is assigned to each channel, no read/write head movement is required. That is, the time required for head positioning is zero. The only significant time required when reading or writing is the rotational delay that occurs in reaching a desired record location. To give you some idea of speed and storage capacities, some high-speed drums are capable of transferring over one million characters of data per second, which is roughly equivalent to reading a stack
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of punched cards 8 feet high in one second. The storage capacities of magnetic drums range from 20 million to more than 150,000 million characters (or bytes) of data. Q-26. Why are disk storage devices popular? Q-27. How is data stored on all disks? Q-28. What precedes each record on a disk? Q-29. How is the storage capacity of a disk determined? Q-30. What two ways can data be physically organized on a disk pack? Q-31. The amount of data that can be stored on a linear inch of tape is known by what term? Q-32. The length of tape between BOT and EOT is referred to by what term? Q-33. How does a magnetic drum differ from a magnetic disk? Q-34. Tracks on each channel of a magnetic drum are grouped into what?
INPUT/OUTPUT DEVICES (EXTERNAL) Input and output devices are similar in operation but perform opposite functions. It is through the use of these devices that the computer is able to communicate with the outside world. Input data may be in any one of three forms: 1. Manual inputs from a keyboard or console 2. Analog inputs from instruments or sensors 3. Inputs from a source on or in which data has previously been stored in a form intelligible to the computer Computers can process hundreds of thousands of computer words or characters per second. Thus, a study of the first method (manual input) reflects the inability of human-operated keyboards or keypunches to supply data at a speed that matches the speed of digital computers. A high average speed for keyboard operation is two or three characters per second, that, when coded to form computer words, would reduce the data input rate to the computer to less than a computer word per second. Since mainframe computers are capable of reading several thousand times this amount of information per second, it is clear that manual inputs should be minimized to make more efficient use of computer time. However, as a rule, the keyboard is the normal input media for microcomputers. Input data that has previously been recorded on paper tapes, magnetic tapes, magnetic disks, or floppy disks in a form understood by the program may also be entered into the computer. These are much faster methods than entering data manually from a keyboard. The most commonly used input devices in this category are magnetic tape units, magnetic disk drive units, and floppy disk drive units. Output information is also made available in three forms: 1. Displayed information: codes, numbers, words, or symbols presented on a display device like a cathode-ray screen
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2. Control signals: information that operates a control device, such as a lever, aileron, or actuator 3. Recordings: information that is stored in a machine language or human language on tapes, disks, or printed media Devices that display, store, or read information include magnetic tape units, magnetic disk drive units, floppy disk drive units, printers, and display devices. MAGNETIC TAPE UNITS (INPUT/OUTPUT) The purpose of any magnetic tape unit (drive or device) is to write data on or read data from a magnetic tape (fig. 2-17). Tape stores data in a sequential manner. In sequential processing, the computer must begin searching at the beginning and check each record until the desired data is found. Like a tape cassette with recorded music, to play the fifth song recorded, you must play or fast forward the tape past the first four songs before you can play the fifth.
Figure 2-17.—Magnetic tape unit.
Two reels are used, tape moves from a supply reel to a take-up reel (both are mounted on hubs). Figure 2-18 shows the basic tape drive mechanism. The magnetic oxide coated side of the tape passes directly over the read/write head assembly, making contact with the heads. The magnetic tape unit reads and writes data in parallel channels or tracks along the length of the tape as shown in figure 2-19, view A. Each channel or track is used by a read/write head (one for each channel), as the tape moves across the magnetic gap of the head. Read/write heads may be either one gap or two gap as shown in figure 2-19, views B and C. The one-gap head has only one magnetic gap at which both reading and writing occur. The two-gap head has one gap for reading and another for writing. Although the one gap is satisfactory, the two-gap head gives increased speed by checking while writing. For example, a tape being written on passes over the write gap where the data is recorded, and then the data is read as it passes over the read gap to make a comparison. With this method, errors are detected almost instantly. When you look closely at figure 2-19, view B (top view), you will notice that there is one read/write coil in the write head for each channel (or track). In this particular case, there are seven. It is the electrical current flowing through these coils that magnetizes the iron-oxide coating on the surface of the tape.
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Figure 2-18.—A basic tape drive mechanism.
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Figure 2-19.—Read/write head assemblies.
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The major differences between magnetic tape units are the speed at which the tape is moved past the read/write head and the density of the recorded information. You know that density describes the number of binary digits, bytes, or frames we can record on an inch of tape. The most common tape densities are 800 and 1,600 BPI (or FPI). Tape speed (or tape movement) varies to a great extent, from less than 50 inches per second to more than 100 inches per second. How fast a tape unit reads and writes is specified as the character transfer rate which is calculated by multiplying the speed of the magnetic tape unit by the character density. MAGNETIC DISK DRIVE UNITS (INPUT/OUTPUT) Magnetic disk drive units are storage devices that read and write information on the magnetized surfaces of rotating disks (fig. 2-20). The disks are made of thin metal, coated on each side so that data can be recorded in the form of magnetized spots. As the disks spin around like music records, characters can be stored on them or retrieved in a direct manner. This direct accessing of data has a big advantage over the sequential accessing of data. It gives us fast, immediate access to specific data without having to examine each and every record from the beginning. You can direct the disk drive to begin reading at any point. This is like the phonograph record, you can place the needle at any point and begin playing at any point.
Figure 2-20.—Magnetic disk drive unit.
Located within each disk drive unit is a drive motor that rotates the disk at a constant speed, normally 3,600 revolutions per minute (rpm); or, if you prefer, 60 revolutions per second. The rotational speed for floppy disks is usually between 300 and 400 rpm because of their plastic base. Data is written on the tracks of a spinning disk surface and read from the surface by one or more (multiple) read/write heads. When reading from and writing to hard disks (rigid disks), the read/write heads float on a cushion of air and do not actually touch the surface of the disk. The distance between the head and the surface varies from a millionth of an inch to one-half millionth of an inch. This distance is called the flying height. When multiple disks (platters) are packaged together as a unit in a disk pack, a number of access arms and read/write heads are used to access both surfaces of each platter (fig. 2-21). The disk pack shown consists of six metal disks mounted on a central spindle. Data can be recorded on all surfaces except the top surface of the top disk, and the bottom surface of the bottom disk. These two surfaces are intentionally left blank for protection.
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Figure 2-21.—Multiple access arms, read/write heads used with disk packs.
FLOPPY DISK DRIVE UNITS (INPUT/OUTPUT) Floppy disk drive units are physically smaller than magnetic disk drive units and are typically used with personal (desktop) computers (fig. 2-22). The unit consists of a disk drive in which the disk rotates and a controller containing the electronic circuitry that feeds signals onto and from the disk. The disk (diskette) is a thin, flexible platter (floppy disk) coated with magnetic material so characters can be recorded on the surface in the form of magnetized spots. Floppy disks come in several sizes from 3 to 8 inches in diameter. The most common are the 8-inch disk, the 5 1/4-inch disk, and the 3 1/2-inch disk.
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Figure 2-22.—Floppy disk drive unit.
PRINTERS (OUTPUT) Printers are widely used output devices that express coded characters on hard (paper document) copy (fig. 2-23). They print out computer program results as numbers, letters, words, symbols, graphics, or drawings. Printers range from electronic typewriters to high-speed printers. High-speed printers are usually used on mainframes and minis to prepare supply requisitions, pay checks, inventory, or financial reports at 10 lines per second and faster. The types of printers we'll discuss are daisy-wheel, dot matrix, ink jet, and laser. These are the ones commonly used with personal computers.
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Figure 2-23.—Printer.
Daisy-Wheel Printers Daisy-wheel printers have the most professional-looking, pleasing-to-the-eye print of all the printers in the character-at-a-time impact printer class. Daisy-wheel printers are often used in an office or word processing environment, where crisp, sharp, high-quality (letter quality) characters are a must. The daisywheel printer uses a round disk, with embossed characters located at the end of each petal-like projection (one character per petal), similar to the petals of a daisy, as shown in figure 2-24. A drive motor spins the wheel at a high rate of speed. When the desired character spins to the correct position, the print hammer strikes that character causing it to be printed on the paper. Once printed, the daisy wheel continues to move, searching out the next character to be printed, until the line is completed. The speeds of daisywheel printers range from 30 to 60 characters per second (cps).
Figure 2-24.—A daisy-wheel print wheel.
Dot-Matrix Printers Dot-matrix printers, (also known as the wire matrix printers) create characters in much the same way you see numbers on the scoreboard at a baseball or football game. In contrast to the daisy-wheel printers, dot-matrix printers use an arrangement of tiny pins or hammers, called a dot matrix, to generate characters a dot-at-a-time. A dot-matrix print head builds characters out of the dots created by the pins in the matrix. Figure 2-25, view A, shows what dot matrix characters look like when printed.
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Figure 2-25.—Dot-matrix printing.
The dot matrix is defined in terms of rows and columns of dots. A 5 by 7 matrix uses up to five vertical columns of seven dots to create a character. An example of a 5 by 7 matrix printing the letter H is shown in figure 2-25, view D. The size of dot matrixes varies from a 5 by 7 matrix to as large as a 58 by 18 matrix. A number of dot-matrix printers use a single vertical column of pins to print characters, as shown in figure 2-25, view B. The characters are printed by moving (stepping) the print head a small amount and printing the vertical columns one at a time until the character is printed as shown in figure 2-25, views C and D. The size of the matrix determines the quality of the printed character. In other words, the more dots used to print a character, the better the character is filled in and the higher its print quality. Dot-matrix printers are faster than the daisy-wheel printers with speeds ranging from 60 to 350 cps, but their print quality is not as good. 2-26
Ink Jet Printers Ink jet printers employ a technique very similar to the way we use a can of spray paint and a stencil. A spray of electrically charged ink is shot (under pressure) toward the paper. Before reaching the paper, the ink is passed through an electrical field which forms the letters in a matrix form. The print resulting from this process consists of easy to read, high-quality characters. Some manufacturers use large droplets of ink for faster printing, while others use small droplets for better clarity but with slightly reduced printing speeds. These printers can print up to 300 cps (characters per second). Laser Printers Laser printers direct a beam of light through a rotating disk containing the full range of print characters. The appropriate character image is directed onto photographic paper, which is then put through a toner, developed, and used to make additional copies. The print resulting from this process consists of sharp, clean images that are easy on the eyes. These printers can print up to 20,000 plus lines per minute, or 26,666 cps (characters per second). KEYBOARDS (INPUT) A keyboard is nothing more than an array of switches called keyswitches. Keyboards are designed to input a code to the computer when a keyswitch is depressed. Each keyswitch, or key, on the keyboard is assigned a particular code value; and it is usually imprinted with a legend to identify its function. Figure 2-26 shows a keyboard combined with a crt on a microcomputer.
Figure 2-26.—Keyboard combined with a crt and microcomputer.
The primary purpose of a keyboard is to enter or input alphanumeric (numbers, letters, and special characters) character codes. The major grouping of keyswitches on a keyboard will be in one of the two styles of a typewriter keyboard arrangement (QWERTY or DVORAK). The typewriter keyswitches are arranged in 4 rows of 10 or more switches. The keyboard arrangement shown in figure 2-27 is QWERTY. The rows are usually offset to the row above to make it easier to reach all the keys when typing. The tops of the individual keyswitches are sculptured to conform to the shape of the human finger.
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Figure 2-27.—Keyboard layout.
Other groupings of keyswitches are used for special purposes, such as number entry (calculator) keypads, special function switches (F1-F12), and cursor control keys. The special function switches allow an operator to use the special functions designed in the software. For example, in a word processing program, you can use them to spell check a document, search for a particular portion of text, move text from one place to another, and to print hard copies of a document. These are but a few of the functions allowed; however, as you become more familiar with computers you will learn them all. The cursor control key allows you to move to different locations on the screen. The design of keyboards varies from device to device and is dependent on the requirements of the system in which the keyboards are installed. Keyboards are generally used with nontactical computer systems. However, the newer tactical display system consoles have optional keyboards for data entry. A keyboard may be built into the display device, or it may be a separate component connected only by a communication cable. DISPLAY DEVICES Display devices are the crts and other displays that are part of computer terminals, computer consoles, and microcomputers. They are designed to project, show, exhibit, or display softcopy information (alphanumerics or graphic symbology). The information displayed on a display device screen is not permanent. That is where the term softcopy comes from. The information is available for viewing only as long as it is on the display screen. Two types of display devices used with personal/microcomputers are the raster scan crt's and the flat panel displays. Raster Scan Crts Raster scan crts (tv scan video monitors or display monitors) are used extensively in the display of alphanumeric data and graphics. They are used primarily in nontactical display applications such as SNAP II user terminals and desktop computers.
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The raster is a series of horizontal lines crossing the face of the crt screen (fig. 2-28). Each horizontal line is made up of one trace of the electron beam from left to right. The raster starts at the top left corner of the crt screen. As each horizontal line is completed, the blanked electron beam is rapidly returned or retraced to the left of the screen.
Figure 2-28.—Raster or TV scan.
Vertical deflection moves the beam down, and the horizontal sweep repeats. When the vertical sweep reaches the bottom line of the raster, a vertical blanked retrace returns the sweep to the starting position of the raster, and the process is repeated. Each completed raster scan is referred to as a field; two fields make up a frame. The display rate of fields and frames determines the amount of flicker in the display that is perceived by the human eye. Each field is made up of approximately 525 horizontal lines. The actual number of horizontal lines varies from device to device. A frame consists of the interlaced lines of two fields. The horizontal lines of the two fields are interlaced to smooth out the display. A display rate of 30 frames per second produces a smooth, flicker-free raster and corresponding display on the screen. PICTURE ELEMENTS.—The actual display of data results from the use of picture elements. A picture element is a variable dot of light derived from video signals input to the display monitor. The picture elements, often called pixels or pels, are contained in the horizontal scan lines crossing the face of the crt screen. The horizontal and vertical sweeps are continuous and repetitive in nature. Pictures with alphanumeric characters and graphics can be created and displayed by varying the intensity or brightness of the picture element dots. This is done in conjunction with the phosphor coating on the face of the crt. The number of picture elements in each horizontal line varies from device to device. The actual number of picture elements is dependent on the frequency bandwidth of the video monitor, the number of characters to be displayed on a line, and the physical size of the screen. 2-29
Each picture element is addressable by a row and column address. Picture elements are numbered from left to right on each horizontal line (column number). Each horizontal line has a row number. Picture elements, at a minimum, will have off (blanked) or on (full intensity) states. Many display devices have the capability to display picture elements at varying degrees of intensity for the display of graphics. Characters are assembled on the screen in much the same way as a dot-matrix print head prints a character. It takes several horizontal lines and picture elements on each line to create a character. Figure 2-29 shows the generation of the character A, 7 picture elements wide and 9 horizontal lines high. The character is built using what is, in effect, a 7 by 9 dot matrix. The picture elements used to build the character would be at full intensity; the remaining picture elements in the matrix would be blanked. If dark characters on a lighted screen were desired, then the character picture elements would be blanked and the remainder displayed at full intensity.
Figure 2-29.—A 7 by 9 picture element character.
Approximately 640 picture elements per horizontal line are required for the display of an 80 character line. Therefore, you can expect 140,000 picture elements on a raster scan display screen (80 alphanumeric characters per line and 25 lines). HORIZONTAL AND VERTICAL RESOLUTION.—Horizontal resolution is defined in terms of the number of picture elements that can be displayed on the horizontal line without overlapping or running into each other. It is often stated in terms of lines of resolution. In other words, a monitor with a horizontal resolution of 1,000 lines can display 1,000 vertical lines using 1,000 picture elements per line. Vertical resolution depends on the number of horizontal scan lines used by the particular display raster. Generally, the greater the number of scan lines, the easier it is to resolve a horizontal line of display. This characteristic remains true up to a point, called the merge point, where the variation between the lines cannot be detected by the human eye. DISPLAYING DATA ON RASTER SCAN SCREENS.—Raster scan displays are repetitive in nature. The raster frame is displayed approximately 30 times a second. The basic video monitor does nothing more than display the video signals it receives. If no video signals are received, then all the picture elements remain blanked, and the screen is blank in each frame. For data to be displayed accurately, each and every frame must blank and unblank the same picture elements. 2-30
The digital logic that drives video monitors is designed to take advantage of the repetitive nature of frames. There can only be a fixed number of picture elements on the screen of a display; therefore, the contents of the display screen are organized into a data unit called a page. The page contains the status of every picture element on the display screen. The page is usually stored in some form of random-access memory, RAM chips being the most common. The contents of page memory, or, as it is sometimes called, video memory, are continually scanned by the video generation logic and used to develop the video signals for the picture element display. The picture element locations in page memory are read in time to develop the video signals for the picture element display on the horizontal lines. If the display is to be changed, the contents of page memory must be changed. The display on the screen changes as new data is stored in page memory. Two addressing methods are used with page memory. Unformatted Displays.—Displays that reference page memory by picture element address are called unformatted or fully populated displays. These displays are more commonly used for graphics rather than alphanumeric characters. Formatted Displays.—Often displays are organized by character position and line number. These displays are known as formatted displays. This display method is used with devices displaying alphanumeric characters only or those with an alternate graphic capability. The video generation logic of these types of displays scans the entire page memory, as before, to generate the display picture elements. The difference is in the way the new data is written into the page memory. Individual picture element addresses are not used. Character addresses are used to reference page memory. The screen is organized into character lines. Each line is made up of a fixed number of character positions or columns. A fixed number of character lines can be displayed. A common arrangement found on display screens is twenty five 80-character lines, or 2,000 characters. The character set that can be displayed on a device's formatted screen is stored in ROMs or PROMS. That is, the dot-matrix (picture element) patterns for each individual character to be displayed are stored. Different character sets may be displayed by simply replacing the appropriate ROM or PROM chips with new chips containing different character patterns. Upon receipt of a character code and a row and column address, the device logic reads the picture element pattern (dot matrix) from the ROM and writes the pattern into the appropriate character position in the page memory. The desired character is then displayed at the correct position. Other display devices store the codes in page memory and convert the codes to picture element dots when scanning memory to refresh or redisplay the characters on the screen. The use of formatted displays greatly simplifies the programming requirements for the display of alphanumeric data. Flat Panel Displays A number of display methods are in use that are designed to reduce the depth of the crt display caused by the length of the tube. These devices are collectively known as flat panel displays. Three types of flat panel displays commonly in use with computer systems are liquid crystal displays (LCDs), gas plasma displays (GPDs), and electroluminescent displays (ELDs). The screens of these flat panel displays are made up of pairs of electrodes. Each pair of electrodes is used to generate one picture element.
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The liquid crystal display differs from the gas plasma and electroluminescent displays in that it does not generate its own light for the picture elements. The LCD requires an external light source, often called a backlight, for computer applications. The liquid crystal material between the charged electrodes becomes translucent when voltage is applied and allows the backlight to shine through as a picture element. In the gas plasma and electroluminescent displays, the picture element light is generated by ionizing a gas (neon or neon argon) between the charged electrodes (gas plasma display) or by stimulating a luminescent material in the same manner (electroluminescent display). In either case, the picture element only emits light when the electrodes have voltage applied to them. One of the advantages of flat panel displays is that smaller voltages are required for their operation than for a crt. Gas plasma displays use approximately 200 volts to charge the electrodes, and electroluminescent displays require only 20 volts. The picture elements in these displays are addressed by the row and column method. Displays with as many as 737,280 picture elements (960 rows by 768 columns) have been developed. The picture elements on flat panel displays are not lighted continually. This would require a large amount of power and generate excessive heat. A sequential scan similar to a crt raster is used. Once again a page memory is required. The picture element electrodes are on and off as the scan sequentially addresses page memory. Those picture elements that are to display a dot are momentarily turned on and off starting with the first picture element in the top row, or line, and ending with the last picture element on the bottom row. The picture elements are turned on and off at a high enough frequency that the human eye cannot detect the flicker of the off-on-off cycle. The sequential scan used to light the picture elements is continuous and repetitive. Once again, the page memory must be changed to change the display. Flat panel displays may be formatted or unformatted in the same manner as crt displays. Q-35. What is the purpose of any magnetic tape unit? Q-36. What are the major differences between magnetic tape units? Q-37. Why is direct accessing of data a big advantage over the sequential accessing of data? Q-38. What is a floppy disk? Q-39. What are the three most common sizes of floppy disks? Q-40. What output device expresses coded characters as hard copy (paper documents)? Q-41. What four types of printers are commonly used with personal computers? Q-42. What is the primary purpose of a keyboard? Q-43. Raster scan or tv scan video monitors are used extensively for what purpose? Q-44. How many fields make up a frame? Q-45. A field is approximately how many horizontal lines?
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Q-46. What are picture elements often called? Q-47. Vertical resolution depends on what? Q-48. Flat panel displays are designed to reduce what problem of a crt display? Q-49. What does the liquid crystal display require for computer applications?
SUMMARY Now that you have finished chapter 2, you should be feeling more at ease with digital computers. You should realize by now that they are not so hard to understand, once you have the terminology down. The information that follows summarizes the important points of this chapter. The CENTRAL PROCESSING UNIT is the brain of the computer. We generally refer to it as the cpu or mainframe. The CONTROL SECTION directs the flow of traffic (operations) and data, and maintains order within the computer. The ARITHMETIC-LOGIC SECTION performs all arithmetic operations-adding, subtracting, multiplying, and dividing. It also tests various conditions during processing and takes action based on the result. INTERNAL STORAGE is sometimes referred to as primary storage, main storage, or main memory (because its functions are similar to our own human memory). It stores the programs and data. MAGNETIC CORE STORAGE is made up of tiny doughnut-shaped rings made of ferrite (iron) that are strung on a grid of very thin wires. SEMICONDUCTOR STORAGE consists of hundreds of thousands of tiny electronic circuits etched on a silicon chip. BUBBLE STORAGE is made of semiconductor material in the form of a very thin crystal. READ-ONLY MEMORY (ROM) allows us to permanently store programs that will not be lost even when the computer is powered down. RANDOM-ACCESS MEMORY (RAM) is read/ write memory. It is the working memory, rather like a blackboard, that you can scribble down notes, read them, and rub them out when you are finished with them. SECONDARY STORAGE is the memory outside the main body of the computer (cpu) where we store programs and data for future use. MAGNETIC TAPE is a sequential access storage device. MAGNETIC DISK is a direct access storage device. INPUT/OUTPUT DEVICES are the means by which the computer communicates with the outside world. These include magnetic tape units, magnetic disk drive units, floppy disk drive units, printers (daisy-wheel, dot-matrix, ink jet, and laser), and display devices (raster scan crt and flat panel).
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ANSWERS TO QUESTIONS Q1. AND Q49. A-1. The central processing unit. A-2. Three. A-3. Control section, internal storage section, and arithmetic-logic section. A-4. A telephone exchange. A-5. Transfer, arithmetic, logic, and control. A-6. Logic. A-7. Internal storage. A-8. Loading. A-9. Tiny doughnut-shaped rings made of ferrite iron. A-10. Hundreds of thousands of tiny electronic circuits etched on a silicon chip. A-11. Integrated circuits. A-12. All data in memory is lost when the power source is removed. A-13. Nonvolatile (magnetic core storage and bubble memory are examples). A-14. A very thin crystal made of semiconductor material. A-15. By passing a current through a control circuit imprinted on top of the crystal. A-16. The data is still present after being read. A-17. Read-only memory (ROM). A-18. Only the manufacturer. A-19. No. A-20. Read/write memory. A-21. By giving the computer the address of the location where the data is stored or is to be stored. A-22. Already programmed by the manufacturer or in a blank state. A-23. If a mistake is made and entered, it cannot be corrected or erased. A-24. Erasable programmable read-only memory. A-25. With a burst of ultra-violet light. A-26. Largely because of their direct access capabilities. A-27. In a number of invisible concentric circles called tracks. A-28. A disk address. 2-34
A-29. By the bits per inch of track and the tracks per inch of surface. A-30. By cylinder or sector. A-31. Recording density. A-32. The usable recording (reading/writing) surface or usable storage area. A-33. The tracks in which the data is stored are assigned to channels that form circular bands around the drum. A-34. Sectors. A-35. To write data on or read data from a magnetic tape. A-36. The speed at which the tape is moved past the read/write head and the density of the recorded information. A-37. It gives us fast, immediate access to specific data without having to examine each and every record from the beginning. A-38. A thin, flexible platter coated with magnetic material so characters can be recorded. A-39. 8 inch, 5 1/4 inch, and 3 1/2 inch. A-40. Printers. A-41. Daisy-wheel, dot-matrix, ink jet, and laser. A-42. To enter or input alphanumeric character codes. A-43. The display of alphanumeric data and graphics. A-44. Two. A-45. 525. A-46. Pixels or pels. A-47. The number of horizontal scan lines used. A-48. Reduce the depth of the crt display caused by the length of the tube. A-49. An external light source, called a backlight.
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CHAPTER 3
SOFTWARE LEARNING OBJECTIVES Upon completion of this chapter, you will be able to do the following: 1. Recognize and compare the different types and functions of operating systems. 2. Identify the types of utilities and explain their functions. 3. Describe the different types and functions of programming languages. 4. Explain the steps necessary to develop a program and describe the tools used. 5. Compare and describe the types and functions of applications packages.
INTRODUCTION Up to now we have been discussing computer OPERATIONAL CONCEPTS and HARDWARE (the computer and its peripheral devices), and how these devices work and communicate with each other. What about this thing called SOFTWARE? Do we really need it? We most certainly do! Software plays a major role in computer data processing. For example, without software, the computer could not perform simple addition. It's the software that makes everything happen. Or putting it another way, software brings the computer to life. You already know it takes a program to make the computer function. You load an operating system into the computer to manage the computer's resources and operations. You give job information to the operating system to tell it what you want the computer to do. You may tell it to assemble or compile a COBOL program. You may tell it to run the payroll or print inventory reports. You may tell it to copy a tape using a utility program. You may tell it to print the data from a disk file, also using a utility program. You may tell it to test a program. This job information may be entered through the console or read into the computer from tape or disk. It also may be entered by the programmer or user from a remote computer terminal. The operating system receives and processes the job information and executes the programs according to that job information. Software can be defined as all the stored programs and routines (operating aids) needed to fully use the capabilities of a computer. Generally speaking, we say, "If it is not hardware then it must be software."
OPERATING SYSTEMS The operating system is the heart of any computer system. Through it, everything else is done. Basically, operating systems are designed to provide the operator with the most efficient way of executing many user programs.
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An operating system is a collection of many programs used by the computer to manage its own resources and operations. These programs control the execution of other programs. They schedule, assign resources, monitor, and control the work of the computer. There are several types. TYPES OF OPERATING SYSTEMS Operating systems are designed to provide various operating modes. Some systems can only do one task at a time, while others can perform several at a time. Some systems allow only one person to use the system, and others allow multiple users. Single user/single tasking operating systems are the simplest and most common on microcomputers. CP/M®-801, CP/M-86®1, and MS-DOS®2 are examples. Single user/multitasking operating systems allow you to do more than one task as long as the tasks don't use the same type of resources. For example, you can print one job while you run another, as long as the second job does not require the printer. Examples are Concurrent CP/M® -86 3, Concurrent® DOS 3, and MSDOS®; (3.0 and above). Multiuser/multitasking operating systems let more than one user access the same resources at the same time. This is especially useful for sharing common data. These are only feasible on processors (the functional unit in a computer that interprets and executes instructions) of 16 bits or more and with large memories. UNIX 4 is an example. There are also multiprocessor systems, shared resource systems. This means each user (or operator) has a dedicated microprocessor (cpu), which shares common resources (disks, printers, etc.). 1. CP/M and CP/M-86 are registered trademarks of Digital Research Inc. 2. MS-DOS is a registered trademark of Microsoft Corporation. 3. Concurrent CP/M and Concurrent DOS are trademarks of Digital Research Inc. 4. UNIX is a trademark of AT&T. COMPATIBILITY WITH APPLICATIONS SOFTWARE To use an applications program, it must be compatible with the operating system. Therefore, the availability of application software for a particular operating system is critical. Because of this, several operating systems have become the most popular. For 8-bit microcomputers, CP/M (Control Program for Microprocessors) is widely used because many hardware manufacturers have adopted it. MS-DOS (MicroSoft Disk Operating System) designed from CP/M dominates in lower performance 16-bit systems. UNIX, an operating system for larger computers, is being used on the more powerful 16-bit and 32-bit microcomputers. Other operating systems are offered by microcomputer manufacturers. To overcome the applications software compatibility problem, some software comes in several versions so it can be run under several different operating systems. The point to remember is that not all applications software will run on all systems. You have to check to see that compatibility exists. You need the right version. OPERATING SYSTEM FUNCTIONS To give you a better idea of what you can expect to see on your microcomputer display screen, we will show a few fundamental disk operating system commands and messages. Again, the functions of each operating system are about the same, but each may use a different command to do about the same thing. For example, try not to get confused because CP/M uses the command PIP (peripheral interchange program) to copy a file, while MS-DOS uses the command COPY. Remember, the first thing you need to do is boot (initial program load) the system. There are many ways this can be done. Here is an example. When you turn on the power, a prompt may appear on the 3-2
screen. You then insert the operating system floppy disk into the drive A. Type a B (for boot) and press the RETURN key. The operating system will load from the disk. If you are using a system set up for automatic booting, you won't have to type the B. The system automatically loads the operating system when you insert the disk that contains it. Some systems will then ask for date and time. Enter these. You will next see a prompt, usually A> (or A:). The system is ready and drive A is assigned as your primary drive. One thing you might want to do is to display the disk directory to see what is on the disk. To do this, enter DIR following the A>. This will list your files. COMMAND CONFIGUR DATDBL DATDBL FINANCE MASTER
.COM .COM .BAK .DOC .BAS .DOC
It may also give you file size and the date and time of the file. Let's take an example. Let's say you are to copy the file "MASTER.DOC" from the floppy disk in drive A to the floppy disk in drive B and then delete the file on the floppy disk in drive A. You have just displayed the directory of the floppy disk in drive A. Check to see that the file you want is on the floppy disk in drive A. It is. You then insert the floppy disk on which you want the copy into drive B. Be sure it is formatted with the track and sector information so it is ready to receive data. Also, be sure the disk is not write-protected. On a 5-1/4 inch floppy disk that means the write protect notch is uncovered. Following the A> type COPY MASTER.DOC B: and press RETURN. The system will copy the file and give it the same name. Next you might want to display the directory on drive B to see that the file was copied. You can do this by entering DIR B: following the A> prompt. To delete the file on the floppy disk in drive A, type DEL MASTER.DOC following the A> prompt on the screen and press RETURN. You probably noticed each entry in the directory is followed by three characters. These are called extensions, and we use them to tell us the type of file we are working with. For example, .BAK .BAS .TMP .DOC .BIN
Means backup file. Means BASIC source program. Means temporary file. Means ASCII document file. Means binary file, and so on.
Other typical built-in operating system commands you can use might include:
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RENAME DISKCOPY FORMAT TIME
to change the name of a file to copy a whole floppy disk to initialize a floppy disk, get it ready to receive data and programs from the system to display or set the time
You will learn to use these and many other system commands as you operate a specific computer. We won't go into any more detail here. You will have documentation and reference manuals for the specific version of the operating system you will be using. Q-1. What is the heart of any computer system? Q-2. Which types of operating systems are the simplest and most common on microcomputers? Q-3. What types of operating systems let more than one user access the same resources at the same time? Q-4. Why is the availability of applications software for a particular operating system critical? Q-5. How is the applications software compatibility problem overcome?
UTILITY PROGRAMS Now that you have learned about operating systems, let's go into another type of program, utilities. In addition to the utility commands (like diskcopy and rename), which are built into the operating system, you will probably have some independent utility programs. These are standard programs that run under control of the operating system just like your applications programs. They are called utilities because they perform general types of functions that have little relationship to the content of the data. Utility programs eliminate the need for programmers to write new programs when all they want to do is copy, print, or sort a data file. Although a new program is not needed, we do have to tell the program what we want it to do. We do this by providing information about files, data fields, and the process to be used. For example, a sort program arranges data records in a specified order. You will have to tell the sort program what fields to sort on and whether to sort in ascending or descending sequence. Let's examine two types of utility programs to give you some idea of how a utility program works. The first will be sort-merge and the second the report program generator (RPG). SORT-MERGE PROGRAMS Sorting is the term given to arranging data records in a predefined sequence or order. Merging is the combining of two or more ordered files into one file. For example, we normally think of putting a list of people's names in alphabetical order arranging them in sequence by last name. We arrange those with the same last name in order by first name. If we do this ourselves, we know the alphabetic sequence B comes after A, C after B, and so on, and it is easy to arrange the list, even if it is a time consuming job. On a computer, the sequence of characters is also defined. It is called the collating sequence. Every coding system has a collating sequence. The capability of a computer to compare two values and determine which is greater (B is greater than A, C is greater than B, and so on) makes sorting possible. What about numbers and special 3-4
characters? They are also part of the collating sequence. In EBCDIC, (EBCDIC is a computer code that is discussed in detail in chapter 4) special characters, such as #, $, &, and *, come in front of the alphabetic characters, and numbers follow. When you sort records in the defined sequence, they are in ascending sequence. Most sort programs also allow you to sort in reverse order. This is called descending sequence. In EBCDIC, it is 9-0, Z-A, then special characters. To sort a data file, you must tell the sort program what data field or fields to sort on. These fields are called sort (or sorting) keys. In our example, the last name is the major sort key and the first name is the minor sort key. Sorting is needed in many applications. For example, for mailing we need addresses in ZIP Code order; personnel records may be kept in service number order; and inventory records may be kept in stock number order. We could go on and on. Because many of our files are large, sorting is very time consuming, and it is one of the processes most used on computer systems. As a user, you will become very familiar with this process. Sort-merge programs usually have phases. First they initialize: read the parameters, produce the program code for the sort, allocate the memory space, and set up other functions. The sort-merge program then reads in as many input data records as the memory space allocated can hold, arranges (sorts) them in sequence, and writes them out to an intermediate sort-work file. It continues reading input, sorting and writing intermediate sort-work files until all the input is processed. It then merges (combines) the ordered intermediate sort-work files to produce one output file in the sequence specified. The merging process can be accomplished with less memory than the sort process since the intermediate sort work files are all in the same sequence. Records from each work file can be read, the sort keys compared based on the collating sequence and sort parameters, and records written to the output file maintaining the specified sequence. REPORT PROGRAM GENERATORS Report program generators (RPG) are used to generate programs to print detail and summary reports of data files. Figure 3-1 is an example of a printed report. RPGs were designed to save programming time. Rather than writing procedural steps in a language like BASIC or COBOL, the RPG programmer writes the printed report requirements on specially designed forms.
Figure 3-1.—Printed report using a report program generator (RPG) program.
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Included in the requirements are an input file description, the report heading information lines, the input data record fields, the calculations to be done, and the data fields to be printed and summarized. The RPG program takes this information and generates a program for the specific problem. You then run that program with the specified input data file to produce the printed report. The input data file must be in the sequence in which you want the report to summarize the data. In our example (fig. 3-1), we summarized requisitions based on unit identification codes (UIC). We first sorted the input data file on the field that contains the UIC. We provided specifications to the RPG program to tell it to accumulate totals from the detail (individual) data records until the UIC changed. We then told it to print the total number of requisitions and total cost for that UIC. We did not have it print each detail record, although we could have. The UIC is called the control field. Each time the control field changes, there is a control break. Each time there is a control break, the program prints the summary information. After all records are read and processed, it prints a summary line (TOTALS) for all UICs. You can also use RPGs to generate a program to update data files. Q-6. What programs eliminate the need for programmers to write new programs when all they want to do is copy, print, or sort a data file? Q-7. How do we tell a utility program what we want it to do? Q-8. What is the term given to arranging data records in a predefined sequence or order? Q-9. To sort a data file, what must you tell the sort program? Q-10. What are report program generators used for?
PROGRAMMING LANGUAGES Programmers must use a language that can be understood by the computer. Several methods can achieve human-computer communication. For example, let us assume the computer only understands French and the programmer speaks English. The question arises: How are we to communicate with the computer? One approach is for the programmer to code the instructions with the help of a translating dictionary before giving them to the processor. This would be fine so far as the computer is concerned; however, it would be very awkward for the programmer. Another approach is a compromise between the programmer and computer. The programmer first writes instructions in a code that is easier to relate to English. This code is not the computer's language; therefore, the computer does not understand the orders. The programmer solves this problem by giving the computer another program, one that enables it to translate the instruction codes into its own language. This translation program, for example, would be equivalent to an English-to-French dictionary, leaving the translating job to be done by the computer. The third and most desirable approach from an individual's standpoint is for the computer to accept and interpret instructions written in everyday English terms. Each of these approaches has its place in the evolution of programming languages and is used in computers today. MACHINE LANGUAGES With early computers, the programmer had to translate instructions into the machine language form that the computers understood. This language was a string of numbers that represented the instruction code and operand address(es).
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In addition to remembering dozens of code numbers for the instructions in the computer's instruction set, the programmer also had to keep track of the storage locations of data and instructions. This process was very time consuming, quite expensive, and often resulted in errors. Correcting errors or making modifications to these programs was a very tedious process. SYMBOLIC LANGUAGES In the early 1950s, mnemonic instruction codes and symbolic addresses were developed. This improved the program preparation process by substituting letter symbols (mnemonic codes) for basic machine language instruction codes. Each computer has mnemonic codes, although the symbols vary among the different makes and models of computers. The computer still uses machine language in actual processing, but it translates the symbolic language into machine language equivalent. Symbolic languages have many advantages over machine language coding. Less time is required to write a program. Detail is reduced. Fewer errors are made. Errors which are made are easier to find, and programs are easier to modify. PROCEDURE-ORIENTED LANGUAGES The development of mnemonic techniques and macroinstructions led to the development of procedure-oriented languages. Macroinstructions allow the programmer to write a single instruction that is equivalent to a specified sequence of machine instructions. These procedure-oriented languages are oriented toward a specific class of processing problems. A class of similar problems is isolated, and a language is developed to process these types of applications. Several languages have been designed to process problems of a scientific-mathematical nature and others that emphasize file processing. Procedure-oriented languages were developed to allow a programmer to work in a language that is close to English or mathematical notation. This improves overall efficiency and simplifies the communications process between the programmer and the computer. These languages have allowed us to be more concerned with the problems to be solved rather than with the details of computer operation. For example: COBOL (COmmon Business Oriented Language) was developed for business applications. It uses statements of everyday English and is good for handling large data files. FORTRAN (FORmula TRANslator) was developed for mathematical work. It is used by engineers, scientists, statisticians, and others where mathematical operations are most important. BASIC (Beginner's All-Purpose Symbolic Instruction Code) was designed as a teaching language to help beginning programmers write programs. Therefore, it is a general-purpose, introductory language that is fairly easy to learn and to use. With the increase in the use of microcomputers, BASIC has regained popularity and is available on most microcomputer systems. Other languages gaining in popularity are PASCAL and Ada. PASCAL is being used by many colleges and universities to teach programming because it is fairly easy to learn; yet is a more powerful language than BASIC. Although PASCAL is not yet a standardized language, it is still used rather extensively on microcomputers. It has greater programming capabilities on small computers than are possible with BASIC. Ada's development was initiated by the U.S. Department of Defense (DOD). Ada is a modern general-purpose language designed with the professional programmer in mind and has many unique features to aid in the implementation of large scale applications and real-time systems. Because Ada is so strongly supported by the DOD and other advocates, it will become an important language like those
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previously mentioned. Its primary disadvantage relates to its size and complexity, which will require considerable adjustment on the part of most programmers. The most familiar of the procedure-oriented languages are BASIC and FORTRAN for scientific or mathematical problems, and COBOL for file processing. Programs written in procedure-oriented languages, unlike those in symbolic languages, may be used with a number of different computer makes and models. This feature greatly reduces reprogramming expenses when changing from one computer system to another. Other advantages to procedure-oriented languages are (1) they are easier to learn than symbolic languages; (2) they require less time to write; (3) they provide better documentation; and (4) they are easier to maintain. However, there are some disadvantages of procedure-oriented languages. They require more space in memory, and they process data at a slower rate than symbolic languages. Q-11. With early computers, the programmer had to translate instructions into what type of language form? Q-12. When were mnemonic instruction codes and symbolic addresses developed? Q-13. What led to the development of procedure oriented languages? Q-14. What computer language was developed for mathematical work? Q-15. What are two disadvantages of procedure oriented languages?
PROGRAMMING Programming is, simply, the process of planning the computer solution to a problem. Thus, by writing: 1. Take the reciprocal of the resistance of all resistors (expressed in ohms); 2. Sum the values obtained in step 1; 3. Take the reciprocal of the sum derived in step 2. A generalized process or program for finding the total resistance of a parallel resistance circuit has now been derived. To progress from this example to preparing a program for a computer is not difficult. However, one basic characteristic of the computer must be kept in mind. It cannot think. It can only follow certain commands, and these commands must be correctly expressed and must cover all possibilities. Thus, if a program is to be useful in a computer, it must be broken down into specifically defined operations or steps. Then the instructions, along with other data necessary for performing these operations or steps, must be communicated to the computer in the form of a language or code that is acceptable to the machine. In broad terms, the computer follows certain steps in executing a program. It must first read the instructions (sequentially unless otherwise programmed), and then in accordance with these instructions, it executes the following procedures: 1. Locates the parameters (constants) and such other data as may be necessary for problem solution 2. Transfers the parameters and data to the point of manipulation 3-8
3. Manipulates the parameters and data in accordance with certain rules of logic 4. Stores the results of such manipulations in a specific location 5. Provides the operator (user) with a useful output Even in a program of elementary character such as the one above, this would involve breaking each of the steps down into a series of machine operations. Then these instructions, parameters, and the data necessary for problem solution must be translated into a language or code that the computer can accept. Next, we'll provide an introduction to the problem solving concepts and flow charting necessary to develop a program. OVERVIEW OF PROGRAMMING Before learning to program in any language, it is helpful to establish some context for the productive part of the entire programming effort. This context comprises the understanding and agreement that there are four fundamental and discrete steps involved in solving a problem on a computer. The four steps are as follows: 1. State, analyze, and define the problem. 2. Develop the program logic and prepare a program flowchart or decision table. 3. Code the program, prepare the code in machine readable form, prepare test data, and perform debug and test runs. 4. Complete the documentation and prepare operator procedures for implementation and production. Figure 3-2 depicts the evolution of a program. Programming can be complicated, and advance preparation is required before you can actually start to write or code the program. The first two steps, problem understanding/definition and flowcharting, fall into the advance planning phase of programming. It is important at this point to develop correct habits and procedures, since this will prevent later difficulties in program preparation.
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Figure 3-2.—Evolution of a program.
Whether you are working with a systems analyst, a customer, or solving a problem of your own, it is extremely important that you have a thorough understanding of the problem. Every aspect of the problem must be defined: • What is the problem? • What information (or data) is needed? • Where and how will the information be obtained? • What is the desired output? Starting with only a portion of the information, or an incomplete definition, will result in having to constantly alter what has been done to accommodate the additional facts as they become available. It is 3-10
easier and more efficient to begin programming after all of the necessary information is understood. Once you have a thorough understanding of the problem, the next step is flowcharting. FLOWCHARTING Flowcharting is one method of pictorially representing a procedural (step-by-step) solution to a problem before you actually start to write the computer instructions required to produce the desired results. Flowcharts use different shaped symbols connected by one-way arrows to represent operations, data flow, equipment, and so forth. There are two types of flowcharts, system (data) flowcharts and programming flowcharts. A system (data) flowchart defines the major phases of the processing, as well as the various data media used. It shows the relationship of numerous jobs that make up an entire system. In the system (data) flowchart, an entire program run or phase is always represented by a single processing symbol, together with the input/output symbols showing the path of data through a problem solution. For example:
The second type of flowchart, and the one we will talk about in this section is the programming flowchart. It is constructed by the programmer to represent the sequence of operations the computer is to perform to solve a specific problem. It graphically describes what is to take place in the program. It displays specific operations and decisions, and their sequence within the program. For example:
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Tools of Flowcharting Next we will take a look at the tools used in flowcharting. These tools are the fundamental symbols, graphic symbols, flowcharting template, and the flowcharting worksheet. FUNDAMENTAL SYMBOLS.To construct a flowchart, you must know the symbols and their related meanings. They are standard for the military, as directed by Department of the Navy Automated Data Systems Documentation Standards, SECNAVINST 5233. Symbols are used to represent functions. These fundamental functions are processing, decision, input/output, terminal, flow lines, and connector symbol. All flowcharts may be initially constructed using only these fundamental symbols as a rough outline to work from. Each symbol corresponds to one of the functions of a computer and specifies the instruction(s) to be performed by the computer. The contents of these symbols are called statements. Samples of these fundamental symbols, definitions, examples, and explanations of their uses are shown in figure 3-3.
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Figure 3-3.—Fundamental flowcharting symbols.
GRAPHIC SYMBOLS.Within a flowchart, graphic symbols are used to specify arithmetic operations and relational conditions. The following are commonly-used arithmetic and relational symbols.
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+ * / ± = > < ! " # YES or Y NO or N TRUE or T FALSE or F
plus, add minus, subtract multiply divide plus or minus equal to greater than less than greater than or equal to less than or equal to not equal Yes No True False
FLOWCHARTING TEMPLATE.To aid in drawing the flowcharting symbols, you may use a flowcharting template. Figure 3-4 shows a template containing the standard symbol cutouts. A template is usually made of plastic with the symbols cut out to allow tracing the outline.
Figure 3-4.—Flowchart template.
FLOWCHART WORKSHEET.The flowchart worksheet is a means of standardizing documentation. It provides space for drawing programming flowcharts and contains an area for identification of the job, including application, procedure, date, and page numbers (fig. 3-5). You may find it helpful when you develop flowcharts. If you don't have this form available, a plain piece of paper will do.
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Figure 3-5.—Flowchart worksheet.
Constructing a Flowchart There is no "best way" to construct a flowchart. There is no way to standardize problem solution. Flowcharting and programming techniques are often unique and conform to the individual's own methods or direction of problem solution. This section will show an example of developing a programming flowchart. It is not the intent to say this is the best way; rather, it is one way to do it.
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By following this text example you should grasp the idea of solving problems through flowchart construction. As you gain experience and familiarity with a computer system, these ideas will serve as a foundation. To develop a flowchart, you must first know what problem you are to solve. It is then your job to study the problem definition and develop a flowchart to show the logic, steps, and sequence of steps the computer is to execute to solve the problem. As an example, suppose you have taken a short-term second mortgage on a new home, and you want to determine what your real costs will be, the amount of interest, the amount to be applied to principal, and the final payment at the end of the 3-year loan period. The first step is to be sure you understand the problem completelyWhat are the inputs and the outputs and what steps are needed to answer the questions? Even when you are specifying a problem of your own, you will find we don't usually think in small, detailed sequential steps. However, that is exactly how a computer operates, one step after another in a specified order. Therefore, it is necessary for you to think the problem solution through step by step. You might clarify the problem as shown by the Problem Definition in figure 3-6.
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Figure 3-6.—Problem definition and programming flowchart.
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After you have this level of narrative problem definition, you are ready to develop a flowchart showing the logic, steps, and sequence of steps you want the computer to execute to solve the problem. A programming flowchart of this problem is also shown in figure 3-6. Study both the problem definition and the flowchart to see their relationship and content. You now have a plan of what you want the computer to do. The next step is to code a program that can be translated by a computer into a set of instructions it can execute. This step is called program coding. PROGRAM CODING It is important to remember program coding is not the first step of programming. Too often we have a tendency to start coding too soon. As we just discussed, a great deal of planning and preparation must be done before sitting down to code the computer instructions to solve a problem. For the example amortization problem (fig. 3-6), we have analyzed the specifications in terms of (1) the output desired; (2) the operations and procedures required to produce the output; and (3) the input data needed. In conjunction with this analysis, we have developed a programming flowchart that outlines the procedures for taking the input data and processing it into usable output. You are now ready to code the instructions that will control the computer during processing. This requires that you know a programming language. All programming languages, FORTRAN, COBOL, BASIC and so on, are composed of instructions that enable the computer to process a particular application, or perform a particular function. Instructions The instruction is the fundamental element in program preparation. Like a sentence, an instruction consists of a subject and a predicate. However, the subject is usually not specifically mentioned; rather it is some implied part of the computer system directed to execute the command that is given. For example, the chief tells a sailor to "dump the trash." The sailor will interpret this instruction correctly even though the subject "you" is omitted. Similarly, if the computer is told to " ADD 1234," the control section may interpret this to mean that the arithmetic-logic section is to add the contents of address 1234 to the contents of the accumulator (a register in which the result of an operation is formed). In addition to an implied subject, every computer instruction has an explicit predicate consisting of at least two parts. The first part is referred to as the command, or operation; it answers the question "what?" It tells the computer what operation it is to perform; i.e., read, print, input. Each machine has a limited number of built-in operations that it is capable of executing. An operation code is used to communicate the programmer's intent to the computer. The second specific part of the predicate, known as the operand, names the object of the operation. In general, the operand answers the question "where?" Operands may indicate the following: 1. The location where data to be processed is found. 2. The location where the result of processing is to be stored. 3. The location where the next instruction to be executed is found. (When this type of operand is not specified, the instructions are executed in sequence.) The number of operands and the structure or format of the instructions vary from one computer to another. However, the operation always comes first in the instruction and is followed by the operand(s). The programmer must prepare instructions according to the format required by the language and the computer to be used. 3-18
Instruction Set The number of instructions in a computer's instruction set may range from less than 30 to more than 100. These instructions may be classified into categories by the action they perform such as input/output (I/O), data movement, arithmetic, logic, and transfer of control. Input/output instructions are used to communicate between I/O devices and the central processor. Data movement instructions are used for copying data from one storage location to another and for rearranging and changing of data elements in some prescribed manner. Arithmetic instructions permit addition, subtraction, multiplication, and division. They are common in all digital computers. Logic instructions allow comparison between variables, or between variables and constants. Transfer of control instructions are of two types, conditional and unconditional. Conditional transfer of control instructions are used to branch or change the sequence of program control, depending on the outcome of the comparison. If the outcome of a comparison is true, control is transferred to a specific statement number. If it proves false, processing continues sequentially through the program. Unconditional transfer of control instructions are used to change the sequence of program control to a specified program statement regardless of any condition. Coding a Program Regardless of the language used, there are strict rules the programmer must adhere to with regard to punctuation and statement structure when coding any program. Using the programming flowchart introduced earlier, we have now added a program coded in BASIC to show the relationship of the flowchart to the actual coded instructions (fig. 3-7). Don't worry about complete understanding, just look at the instructions with the flowchart to get an idea of what coded instructions look like.
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Figure 3-7.—Programming flowchart and coded program.
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You will have to have specific information about the computer you are to use. It will tell you how the language is implemented on that particular computer, in order to code a program. The computer manufacturers or software designer will provide these specifics in their user's manual. Get a copy of the user's manual and study it before you begin to code. The differences may seem minor to you, but they may prevent your program from running. Once coding is completed, the program must be debugged and tested before implementation. Debugging Errors caused by faulty logic and coding mistakes are referred to as "bugs." Finding and correcting these mistakes and errors that prevent the program from running and producing correct output is called "debugging." Rarely do complex programs run to completion on the first attempt. Often, time spent debugging and testing equals or exceeds the time spent in program coding. This is particularly true if insufficient time was spent on program definition and logic development. Some common mistakes which cause program bugs are mistakes in coding punctuation, incorrect operation codes, transposed characters, keying errors, and failure to provide a sequence of instructions (a program path) needed to process certain conditions. To reduce the number of errors, you will want to carefully check the coding sheets before they are keyed into the computer. This process is known as "desk-checking" and should include an examination for program completeness. Typical input data should be manually traced through the program processing paths to identify possible errors. In effect, you will be attempting to play the role of the computer. After you have desk checked the program for accuracy, the program is ready to be assembled or compiled. Assembly and compiler programs prepare your program (source program) to be executed by the computer, and they have error diagnostic features which detect certain types of mistakes in your program. These mistakes must be corrected. Even when an error-free pass of the program through the assembly or compiler program is accomplished, this does not mean your program is perfected. However, it usually means the program is ready for testing. Testing Once a program reaches the testing stage, generally, it has proved it will run and produce output. The purpose of testing is to determine that all data can be processed correctly and that the output is correct. The testing process involves processing input test data that will produce known results. The test data should include: (1) typical data, which will test the commonly used program paths; (2) unusual but valid data, which will test the program paths used to process exceptions; (3) incorrect, incomplete, or inappropriate data, which will test the program's error routines. If the program does not pass these tests, more testing is required. You will have to examine the errors and review the coding to make the coding corrections needed. When the program passes these tests, it is ready for computer implementation. Before computer implementation takes place, documentation must be completed. Documentation Documentation is a continuous process, beginning with the problem definition. Documentation involves collecting, organizing, storing, and otherwise maintaining a complete record of the programs and other documents associated with the data processing system. The Navy has established documentation standards to ensure completeness and uniformity for computer system information between commands and between civilian and Navy organizations. SECNAVINST 5233.1 establishes minimum documentation requirements.
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A documentation package should include: 1. A definition of the problem. Why was the program written? What were the objectives? Who requested the program, and who approved it? These are the types of questions that should be answered. 2. A description of the system. The system environment (hardware, software, and organization) in which the program functions should be described (including systems flowcharts). General systems specifications outlining the scope of the problem, the form and type of input data to be used, and the form and type of output required should be clearly defined. 3. A description of the program. Programming flowcharts, program listings, program controls, test data, test results, and storage dumps these and other documents that describe the program and give a historical record of problems and/or changes should be included. 4. Operator instructions. Items that should be included are computer switch settings, loading and unloading procedures, and starting, running, and termination procedures. Implementation After the documentation is complete, and the test output is correct, the program is ready for use. If a program is to replace a program in an existing system, it is generally wise to have a period of parallel processing; that is, the job application is processed both by the old program and by the new program. The purpose of this period is to verify processing accuracy and completeness. Q-16. What is programming? Q-17. In programming, how many steps are involved in solving a problem on a computer? Q-18. What is required before you can actually start to write or code a program? Q-19. In flowcharting, what method is used to represent different operations, data flow, equipment, and so forth? Q-20. What type of flowchart is constructed by the programmer to represent the sequence of operations the computer is to perform to solve a specific problem? Q-21. How many tools are used in flowcharting? Q-22. Is there a "best way" to construct a flowchart? Q-23. What controls the computer during processing? Q-24. What is the fundamental element in program preparation? Q-25. What type of instructions permit addition, subtraction, multiplication, and division? Q-26. Where is specific information about the computer you are to use contained? Q-27. How do we refer to errors caused by faulty logic and coding mistakes? Q-28. What is the purpose of testing a program?
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PACKAGED SOFTWARE Fortunately you don't have to write a program for every problem to be solved. Instead, you can use packaged or off-the-shelf programs that are designed for specific classes of applications. Everyday more and more packaged software (software written by the manufacturer, a software house, or central design agency [CDA]) becomes available for general use. It may be up to you to set up and process a job within the specifications of a packaged program. Let's look at four classes of packaged software you may work with: word processing, data management, spreadsheets, and graphics. WORD PROCESSING You can use word processing software for any function that involves text: letters, memos, forms, reports, and so on. At a minimum, it includes routines for creating, editing, storing, retrieving, and printing text. Under the word processing software control, you generally enter the text on the keyboard and it is printed on a display screen as shown in figure 3-8. At that point, you may store it on disk or tape, print it on a printer, or change (edit) it. Using the edit functions you can add or delete words, characters, lines, sentences, or paragraphs. You can rearrange text; for example, move a paragraph or block of information to another place in the same document or even move it to a different document. Word processing is particularly useful for text documents that are repetitive or that require a lot of revisions. It saves a lot of rekeying.
Figure 3-8.—Word processing example.
Other features and software often available with a word processing software package include: spelling checkers, mailing list programs, document compilation programs, and communications programs. Spelling checker software helps find misspelled words but not misused words. It scans the text matching each word against a dictionary of words. If the word is not found in the dictionary, the system flags the word. You check it. If it is misspelled, you can correct it. You will still have to proofread the document to see that everything was keyed and that the words are used correctly. 3-23
Mailing list programs are for maintaining name and address files. They often include a capability to individualize letters and reports by inserting names, words, or phrases to personalize them. Document compilation programs are useful when you have standard paragraphs of information that you need to combine in different ways for various purposes. For example, you may be answering inquiries or putting together contracts or proposals. Once you select the standard paragraphs you want, you add variable information. This saves both keying and proofreading time. Communications software and hardware enable you to transmit and receive text on your microcomputer. Many organizations use this capability for electronic mail. In a matter of minutes you can enter and transmit a memo to other commands or to personnel in other locations. You can transmit monthly reports, notices, or any documents prepared on the microcomputer. DATA MANAGEMENT Data management software allows you to enter data and then retrieve it in a variety of ways. You define your data fields and set up a display screen with prompts. You enter the data records according to the prompts. Figure 3-9, view A, shows an example. The system writes the records on a disk or tape. Once you have a file keyed and stored, you can retrieve records by a field or several fields or by searching the records for specific data. For example, if you wanted a list of all personnel who reported aboard before January 1988, you could tell the system to search the file and print selected fields of all records that meet that condition. You tell the system what fields to print (that is name, rate, SSN, date reported) and where (what print positions) to print them. At the same time, you can specify in what order you want the records printed. For example, figure 3-9, view B, shows the records printed in alphabetical order by last name. The software also provides routines so you can easily add, delete, and change records.
Figure 3-9A.—Data management example. PROMPTS (IN BOLD) AND DATA (IN ITALICS).
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Figure 3-9B.—Data management example. SAMPLE PRINTED REPORT (SORTED BY LAST NAME).
You can also generate reports by specifying what records to use, what fields to print, where to print the fields, and which data fields, if any, need to be combined. For example, your supply officer wants to know the value of the inventory. You can specify that the extended price is to be calculated by multiplying the item quantity by the unit price, and that the extended prices are to be totaled.
ITEM Swabs Brooms Foxtails TOTAL
INVENTORY VALUE QUANTITY UNIT PRICE 47 1.65 62 2.25 36 1.85
EXTENDED PRICE 77.55 139.50 66.60 283.65
You can also specify the information to be used in report and column headings. While the data management programs on micros are not as sophisticated as the data base management systems on mainframes and minis, they do provide an extremely useful capability in offices or aboard ship. SPREADSHEETS Spreadsheets are tables of rows and columns of numbers. Figure 3-10 shows an example. Spreadsheet processors allow you to set up a table of rows and columns and specify what calculations to perform on the columns. You enter values for the basic information into the appropriate rows and columns. Then the processor performs the calculations. In our example (fig. 3-10), we used a spreadsheet to project magnetic media costs. You enter the item descriptions, column headings, report title, and data for columns 1, 2, and 4, and the software calculates column 3 by adding columns 1 and 2. Then it multiplies column 3 times column 4 and puts the result in column 5. It also subtotals and totals the columns you specify; in this case, columns 1 through 3 and column 5.
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Figure 3-10.—Spreadsheet example.
GRAPHICS Graphics capability is available on many microcomputers. One use is to produce data displays, like bar charts, pie charts, and graphs. See figure 3-11, view A and view B. On some micros, you can do line drawings; on others you can create sophisticated engineering drawings. High resolution color graphics are also available for specialized applications.
Figure 3-11A.—Graphics examples. PIE CHART.
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Figure 3-11B.—Graphics examples. BAR CHART.
You cannot use all printers for graphics output. They must be capable of producing graphics and also be compatible with the software. Some character printers can be used for limited graphics. Dot-matrix printers and plotters work well for graphics output. Laser and ink jet printers are also good for both text and graphics. Q-29. What is packaged software? Q-30. What are some of the other features and software available with a word processing software package? Q-31. What software allows you to enter data and then retrieve it in a variety of ways? Q-32. What are spreadsheets? Q-33. Are all printers capable of handling graphics output?
SUMMARY Congratulations you have just finished chapter 3. By now you should be convinced that anyone, with a little study, can understand digital computers. You probably thought when you first started this NEETS MODULE that it would get more difficult as your studies progressed. Our objective was to show you that the more you learn, the easier the material is to understand. OPERATING SYSTEMS are a collection of many programs used by the computer to manage its own resources and operations and to perform commonly used functions like copy, print, and so on. UTILITY PROGRAMS perform such tasks as sorting, merging, and transferring (copying) data from one input/output device to another: card to tape, tape to tape, tape to disk, and so on. SORT-MERGE PROGRAMS arrange data records in a predefined sequence or order and are capable of combining two or more ordered files into one file. REPORT PROGRAM GENERATORS are used to generate programs to print detail and summary reports of data files. 3-27
PROGRAMMING LANGUAGES are the means by which human-computer communication is achieved. They are used to write the instructions to tell the computer what to do to solve a given problem. A MACHINE LANGUAGE uses a string of numbers that represent the instruction codes and operand addresses to tell the computer what to do. SYMBOLIC LANGUAGES improved the program preparation process by substituting letter symbols (mnemonic codes) for basic machine language instruction codes. A PROCEDURE ORIENTED LANGUAGE is a programming language oriented toward a specific class of processing problems. Examples are BASIC, COBOL, and FORTRAN. PROGRAMMING is the process of planning and coding the computer instructions to solve a problem. FLOWCHARTING is one method of pictorially representing a procedural (step-by-step) solution to a problem before you actually start to write the computer instructions required to produce the desired results. PACKAGED SOFTWARE is designed for specific classes of applications. Examples are word processing, spreadsheets, data management, and graphics. These off-the-shelf programs are written by the manufacturer, a software house, or a central design agency.
ANSWERS TO QUESTIONS Q1. THROUGH Q33. A-1. The operating system. A-2. Single user/single tasking. A-3. Multiuser/multitasking. A-4. Because, to use applications software, it must be compatible with the operating system. A-5. Some software comes in several versions so it can run under several different operating systems. A-6. Utility programs. A-7. By providing information about files, data fields, and the process to be used. A-8. Sorting. A-9. What data field or fields to sort on. A-10. To generate programs to print detail and summary reports of data files. A-11. Machine. A-12. In the early 1950's. A-13. The development of mnemonic techniques and macroinstructions. A-14. FORTRAN.
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A-15. They require more space in memory and they process data at a slower rate than symbolic languages. A-16. The process of planning the solution to a problem. A-17. Four. A-18. Advance preparation. A-19. Different shaped symbols. A-20. A programming flowchart. A-21. Four. A-22. No, there isn't a way to standardize problem solution. A-23. Coded instructions. A-24. The instruction. A-25. Arithmetic. A-26. In the computer manufacturers or software designers user's manual. A-27. Bugs. A-28. To determine that all data can be processed correctly and that the output is correct. A-29. Off-the-shelf programs designed for specific classes of applications. A-30. Spelling checkers, mailing list programs, document compilation programs, and communications programs. A-31. Data management. A-32. They are tables of rows and columns of numbers. A-33. No.
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CHAPTER 4
DATA REPRESENTATION AND COMMUNICATIONS LEARNING OBJECTIVES Upon completion of this chapter, you will be able to do the following: 1. Explain data and how it is represented. 2. Explain computer coding systems. 3. Define a parity bit and what it is used for. 4. Explain data storage concepts. 5. Describe three storage access methods. 6. Describe networks and data communications.
INTRODUCTION One of the major problems we face in using a digital computer is communicating with it. We must have one or more ways of getting data into the computer to be processed. You learned in chapter 2 that there are several types of input devices that read data into a computer. But how does one prepare the data to be used as input? How do we convert human-readable documents into a computer-readable form, and what type of input media do we use? If the data is to be used by another computer some distance away, how do we transmit it? Well, as you probably suspect, there are several ways to perform this conversion and transmission process, and that is the chapter of our discussion.
DATA Data is a general term used to describe raw facts. To put it simply, data is nothing more than a collection of related elements or items, that when properly coded into some type of input medium, can be processed by a computer. Data items might include your service number, your name, your paygrade, or any other fact. Until some meaning has been given to the data, nothing can really be determined about it; therefore, it remains data. When this data has been processed together with other facts, it then has meaning and it becomes information we can understand and properly use. DATA REPRESENTATION Data is represented by symbols. Symbols convey meaning only when understood. The symbol itself is not the information, but merely a representation of it. Symbol meaning is one of convention (fig. 4-1). Symbols may convey one meaning to you and me, another meaning to others, and no meaning at all to those that do not know their significance. Data must be reduced to a set of symbols that the computer can read and interpret before there can be any communication with the computer. The first computers were designed to manipulate numbers to solve arithmetic problems. But as you can see in figure 4-1, we create,
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use, and manipulate many other symbols to represent facts in the world in which we live. We are fortunate that early computer experts soon realized the need to manipulate nonnumerical symbols as well. Manipulating these symbols is possible if an identifying code or coded number is assigned to the symbol to be stored and processed. Thus, the letters in a name such as ALBERT or CAROL can be represented by different codes, as can all special characters, such as #, (,), &, $, @, and yes, even the comma. The data to be represented is called source data.
Figure 4-1.—Communications symbols.
SOURCE DATA Source data or raw data is typically written on some type of paper document, which we refer to as a source document. The data contained on the source document must be converted into a machine-readable form for processing either by direct or indirect means. The data may be entered directly into the computer in its original form; namely right from the source document on which it is recorded by way of magnetic ink characters, optically recognizable characters, or bar code recognition. Or the data on the documents may be entered indirectly on input media, such as punched cards, paper tape, magnetic tape, or magnetic disk. It may also be keyed directly into a computer from a keyboard. If you look at figure 4-2, you see a list of SERVMART items that have been typed on a preprinted form. To most people this is just another piece of paper; however, to the Storekeeper (SK) it is a source document to be used to provide input data to the computer. In this example, the SERVMART form deals with requisitioning supplies. The form could be sent to the data-entry department to be used as a source document. There the data-entry operator can key the data into or on whatever computer medium is to be used, according to a prescribed format. The data elements are numbered in the order they are to be keyed: (1) document identification, (2) stock number, (3) unit of issue, (4) quantity, and so on. You'll notice we need more than numbers, and that is where coding systems come into play.
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Figure 4-2.—SERVMART shopping list (source document).
Q-1. What is a general term used to describe raw facts? Q-2. How is data represented? Q-3. What were the first computers designed to manipulate in order to solve arithmetic problems? Q-4. By what two means can the data contained on a source document be converted into a machinereadable form for processing? Q-5. What are some of the types of input media on which data may be indirectly entered?
COMPUTER CODING SYSTEMS To represent numeric, alphabetic, and special characters in a computer's internal storage and on magnetic media, we must use some sort of coding system. In computers, the code is made up of fixed size groups of binary positions. Each binary position in a group is assigned a specific value; for example 8, 4, 2, or 1. In this way, every character can be represented by a combination of bits that is different from any other combination. In this section you will learn how the selected coding systems are used to represent data. The coding systems included are Extended Binary Coded Decimal Interchange Code (EBCDIC), and American Standard Code for Information Interchange (ASCII). EXTENDED BINARY CODED DECIMAL INTERCHANGE CODE (EBCDIC) Using an 8-bit code, it is possible to represent 256 different characters or bit combinations. This provides a unique code for each decimal value 0 through 9 (for a total of 10), each uppercase and lowercase letter (for a total of 52), and for a variety of special characters. In addition to four numeric bits, four zone bit positions are used in 8-bit code as illustrated in figure 4-3. Each group of the eight bits makes up one alphabetic, numeric, or special character and is called a byte.
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Figure 4-3.—Format for EBCDIC and ASCII codes.
When you look at figure 4-3, you will notice that the four rightmost bits in EBCDIC are assigned values of 8, 4, 2, and 1. The next four bits to the left are called the zone bits. The EBCDIC coding chart for uppercase and lowercase alphabetic characters and for the numeric digits 0 through 9 is shown in figure 4-4, with their hexadecimal equivalents. Hexadecimal is a number system used with some computer systems. It has a base of 16 (0-9 and A-F). A represents 10; B represents 11; C represents 12; D represents 13; E represents 14; and F represents 15. In EBCDIC, the bit pattern 1100 is the zone combination used for the alphabetic characters A through I, 1101 is used for the characters J through R, and 1110 is the zone combination used for characters S through Z. The bit pattern 1111 is the zone combination used when representing decimal digits. For example, the code 11000001 is equivalent to the letter A; the code 11110001 is equivalent to the decimal digit 1. Other zone combinations are used when forming special characters. Not all of the 256 combinations of 8-bit code have been assigned characters. Figure 4-5 illustrates how the characters DP-3 are represented using EBCDIC.
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Figure 4-4.—Eight-bit EBCDIC coding chart (including hexadecimal equivalents).
D 1100 0100
P 1101 0111
— 0110 0000
3 1111 0011
Figure 4-5.—DP-3 represented using 8-bit EBCDIC code.
Since one numeric character can be represented and stored using only four bits (8-4-2-1), using an 8-bit code allows the representation of two numeric characters (decimal digits) as illustrated in figure 4-6. Representing two numeric characters in one byte (eight bits) is referred to as packing or packed data. By packing data (numeric characters only) in this way, it allows us to conserve the amount of storage space required, and at the same time, increases processing speed.
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DECIMAL VALUE EBCDIC CODE BIT PLACE VALUES
9 1001 8421
2 0010 8421 BYTE 1
7 0111 8421
3 0011 8421 BYTE 1
Figure 4-6.—Packed data.
AMERICAN STANDARD CODE FOR INFORMATION INTERCHANGE (ASCII) Another 8-bit code, known as the American Standard Code for Information Interchange (ASCII) (pronounced ASS-KEY), was originally designed as a 7-bit code. Several computer manufacturers cooperated to develop this code for transmitting and processing data. The purpose was to standardize a binary code to give the computer user the capability of using several machines to process data regardless of the manufacturer: IBM, HONEYWELL, UNIVAC, BURROUGHS, and so on. However, since most computers are designed to handle (store and manipulate) 8-bit code, an 8-bit version of ASCII was developed. ASCII is commonly used in the transmission of data through data communications and is used almost exclusively to represent data internally in microcomputers. The concepts and advantages of ASCII are identical to those of EBCDIC. The important difference between the two coding systems lies in the 8-bit combinations assigned to represent the various alphabetic, numeric, and special characters. When using ASCII 8-bit code, you will notice the selection of bit patterns used in the positions differs from those used in EBCDIC. For example, let's look at the characters DP3 in both EBCDIC and ASCII to see how they compare.
Character EBCDIC ASCII
D 1100 0100 0100 0100
P 1101 0111 0101 0000
3 1111 0011 0011 0011
In ASCII, rather than breaking letters into three groups, uppercase letters are assigned codes beginning with hexadecimal value 41 and continuing sequentially through hexadecimal value 5A. Similarly, lowercase letters are assigned hexadecimal values of 61 through 7A. The decimal values 1 through 9 are assigned the zone code 0011 in ASCII rather that 1111 as in EBCDIC. Figure 4-7 is the ASCII coding chart showing uppercase and lowercase alphabetic characters and numeric digits 0 through 9.
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Figure 4-7.—Eight-bit ASCII coding chart (including hexadecimal equivalents).
At this point you should understand how coding systems are used to represent data in both EBCDIC and ASCII. Regardless of what coding system is used, each character will have an additional bit called a check bit or parity bit. PARITY BIT This additional check or parity bit in each storage location is used to detect errors in the circuitry. Therefore, a computer that uses an 8-bit code, such as EBCDIC or ASCII, will have a ninth bit for parity checking.
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The parity bit (also called a check bit, the C position in a code) provides an internal means for checking the validity, the correctness, of code construction. That is, the total number of bits in a character, including the parity bit, must always be odd or always be even, depending upon whether the particular computer system or device you are using is odd or even parity. Therefore, the coding is said to be in either odd or even parity code, and the test for bit count is called a parity check. Now, let's talk about bits and bytes, primary storage, and storage capacities; or, to put it another way, the capacity of a storage location. Sit back, keep your memory cycling, and we will explain the ways data may be stored and retrieved inside the computer. Q-6. What does the acronym EBCDIC stand for? Q-7. By using an 8-bit code, how many characters or bit combinations can be represented? Q-8. What is the base of a hexadecimal number system? Q-9. What term is used for the representation of two numeric characters stored in eight bits? Q-10. What does the acronym ASCII mean? Q-11. What was the purpose of several computer manufacturers cooperating to develop ASCII code for processing and transmitting data? Q-12. Are there any differences in the concepts and advantages of ASCII and EBCDIC? Q-13. How is the parity bit in each storage location used? Q-14. A computer or device that uses 8-bit ASCII or EBCDIC will use how many bits to store each character?
DATA STORAGE CONCEPTS You learned in chapter 2 that a computer's primary storage area is divided into four areas, each serving a specific purpose. The input storage area accepts and holds input data to be processed. The working storage area holds intermediate processing results. The output storage area holds the final processing results. The program storage area holds the processing instructions (the program). You also learned that these separate areas do not have built-in physical boundaries, rather the boundaries are determined by the individual programs being used. You also may recall in chapter 2, we talked about the different types of primary storage used in computers and how they differ from one another. Some were magnetic in nature, such as magnetic core storage; others were electronic, such as semiconductor and bubble storage. For purposes of simplicity, we have selected magnetic core storage to show you how data is represented and stored in the computer's primary memory. BITS AND BYTES A bit is a single binary digit. It represents the smallest unit of data just like the good old American penny). However, computers usually do not operate on single bits, rather they store and manipulate a fixed number of bits. Most often, the smallest unit or number of bits a computer works with is eight bits. These eight bits make up a byte. You just learned that both EBCDIC and ASCII codes use eight bits (excluding the parity bit), and that eight bits represent a single character, such as the letter A or the
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number 7. Thus, the computer can store and manipulate an individual byte (a single character) or a group of bytes (several characters, a word) at a time. These individual bytes, or groups of bytes, form the basic unit of memory. Primary storage capacities are usually specified in number of bytes. The symbol "K" is used whenever we refer to the size of memory, especially when the memory is quite large. The symbol K is equal to 1,024 units or positions of storage. Therefore, if a computer has 512K bytes (not bits) of primary storage, then it can hold 512 × 1,024 or 524,288 characters (bytes) of data in its memory. MAGNETIC CORE STORAGE In primary storage, many magnetic cores are strung together on a screen of wire to form what is called a core plane (fig. 4-8, view A). As you may know, each core can store one binary bit (0 or 1) of data. A core is magnetized by current flowing through the wires on which the core is strung. Hence, a core magnetized in one direction represents a binary 0, and when magnetized in the opposite direction, a binary 1. It is the direction that the core is magnetized that determines whether it contains a binary 0 or a binary 1 (refer to fig. 4-8, view B). These core planes look very much like small window screens and are arranged vertically to represent data as shown in figure 4-8, view C. In looking at this figure, you will notice that nine planes are needed to code in 8-bit EBCDIC. The ninth plane provides for a parity (check) bit. Figure 4-8, view C, shows DP-3 in EBCDIC code, even parity.
Figure 4-8A.—Core storage with DP-3 represented using 8-bit EBCD1C code.
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Figure 4-8B.—Core storage with DP-3 represented using 8-bit EBCD1C code.
Figure 4-8C.—Core storage with DP-3 represented using 8-bit EBCD1C code.
STORAGE CAPACITY AND ADDRESSES The storage capacity of an address is designed and built into the computer by the manufacturer. Over the years several different design approaches to partition primary storage have been used. With this in mind, let's take a look at some of the ways primary storage is partitioned into addresses. One way to design or organize the primary storage section is to store a fixed number of characters (bytes) at each address location. We can then reference these characters as a single entity called a word, as illustrated in figure 4-9, view A. The name CHARLIE (address location 400) or the amount he is paid, in this case $69.00 (address location 401), are each treated as a single word. Computers that are built to 4-10
retrieve, manipulate, and store a fixed number of characters in each address are said to be word-oriented, word-addressable machines, or fixed-word-length computers.
Figure 4-9A.—Fixed-word-length vs variable-word-length storage. FIXED-LENGTH WORDS, CONTAINING EIGHT CHARACTERS EACH, OCCUPYING TWO ADDRESS LOCATIONS (WORD ADDRESSABLE).
Another way to design the primary storage section is to store a single character, such as the letter L or the number 8, in each address location. An address is assigned to each location in storage. Computers designed in this way are said to be character-oriented or character addressable. We also call them variable-word-length computers. Therefore, the name CHARLIE (fig. 4-9, view B) now requires seven address locations (300 through 306), while amount paid ($69.00) occupies six address locations (307 through 312).
Figure 4-9B.—Fixed-word-length vs variable-word-length storage. VARIABLE-LENGTH WORDS (CHARACTER ADDRESSABLE).
Whether a computer addresses a group of bytes as a word or addresses each byte individually is a function of the circuitry. Both designs have advantages and disadvantages. Variable-word-length computers make the most efficient use of available storage space, since a character can be placed in every storage location. In a fixed-word-length computer, storage space may be wasted. For example, if the storage capacity in each address of a fixed-word-length computer is eight bytes, and some of the data elements to be stored contain only three or four characters, then many of the storage positions in each word are not being used.
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Fixed-word-length computers have faster calculating speeds. They can add two data words in a single operation. This is not so with character-addressable computers. Here, only one digit (byte) in each number can be added during a single machine operation. Thus, eight steps are required to complete the calculation. The larger mainframe computers (super-computers like the CRAY-1 and CYBER 205) use only fixed-word-length storage. Most microcomputers use the variable-word-length approach allowing them to operate on one character at a time. Somewhere in between these two extremes are the dozens of existing minicomputer and mainframe models that have what is called built-in flexibility. These flexible computers are byte-oriented but can operate in either a fixed- or variable-word-length mode through the use of proper program instructions. Let's take a look at how these flexible computers operate in a variable- and fixed-word-length environment. Working in a variable-word-length environment, each address holds one alphanumeric character as shown in figure 4-9, view B. Since a byte usually represents a single alphanumeric character, unless you are using packed decimal, a flexible computer is often said to be byte-addressable. Don't become confused; the terms character-addressable, character-oriented, and byte-addressable all have the same meaning. By using the appropriate program instructions, a programmer can retrieve a stored data element by identifying the address of the first character (say position 300 as in fig. 4-9, view B) and specifying the number of address locations to be included in the word. In this case there are seven, positions 300 through 306. When a flexible computer is working in a fixed-word-length environment, each address identifies a group of bytes that can be operated on as a unit. This processing method helps to achieve faster calculating speeds. A programmer can use program instructions to cause the computer to automatically retrieve, manipulate, and store, as a unit, a fixed word of say, two, four, or eight bytes of data in one machine operation by identifying the address of the first character of data. At the same time all remaining bytes are acted upon as a unit moving from left to right. Figure 4-10 illustrates the different word lengths possible with many byte-addressable computers. They are half-word (2 bytes), full-word (4 bytes), and double-word (8 bytes).
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Figure 4-10.—Word lengths used on flexible byte-addressable computers.
By now, you should have a good idea of how primary storage locations are identified by their storage addresses, how these addressable storage locations are used, and how the storage capacity at an address can vary depending on the design of the computer. Now, let's go one step further, to see how these bits and bytes are represented (coded) on some of the more common secondary storage media. SECONDARY STORAGE DATA ORGANIZATION Remember, secondary storage devices (also called auxiliary or mass storage devices) are those devices which are not part of the central processing unit (cpu). They include: external core; semiconductor, thin film, and bubble memories; punched cards; paper tape; and several different types of mass storage, such as magnetic tape, disk, and drum. You already know it takes a certain number of bits to make one byte (normally eight), and when bytes are grouped together at a single address they make up a word in the computer's memory. When data
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is recorded on some type of magnetic storage medium, such as disk or tape, it is normally organized by bits, characters (bytes), fields, records, and files (fig. 4-11). The following definitions should help you understand the relationship between bits, characters, bytes, words, fields, records, and files.
Figure 4-11.—Data organization.
BIT—The smallest unit of data; it represents one binary digit (0 or 1). CHARACTER (BYTE)—A group of related bits (usually eight) that make up a single characterletter, number, or special character. WORD—A group of related bytes that are treated as a single addressable unit or entity in memory. FIELD—One or more related characters that are treated as a unit of information. A field (also referred to as a data item) may be alphabetic, numeric, or alphanumeric, and may be either fixed or variable in length. For example, your social security number (SSN) is of a fixed length; that is, it's always 9 positions in length. Whereas, names are variable length because they may be from 2 to 25 positions in length. RECORD—A group of related fields, all pertaining to the same subject; a person, a thing, or an event. For example, your payroll record (LES statement) might include fields for your name, amount paid, taxes withheld, earned leave, and any allotments you might have. On the other hand, a supply inventory record might consist of fields containing stock number, the name of the item, its unit price, the quantity on hand, and its bin location. FILE—A collection of related records, such as the payroll or supply inventory records. Normally, all records within the file are in the same format. When processing data, we think in terms of data files. For example, to process a parts inventory, you would need the master parts inventory file and the file that contains up-to-date information on each part that has been issued. The master parts inventory file would have a record for every part in the inventory. The update file, parts issued file, would have a record for each part issued. You would use a program to read the records on the parts issued file and update the matching records on the master parts inventory file. Depending on whether the data is stored on magnetic tape or disk or in internal storage, the program would use different methods to access storage to obtain the data. In the next section you'll learn about storage access methods.
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Q-15. What area in the computer's primary storage area holds the processing instructions (the program)? Q-16. How are the boundaries determined for the separate areas of the computer's primary storage area? Q-17. What is a bit? Q-18. How many bits make up a byte? Q-19. Primary storage capacities are usually specified in what unit of measure? Q-20. How are core planes formed? Q-21. Where are core planes used? Q-22. Who designs and builds the storage capacity of an address into a computer? Q-23. What is another name for computers designed to be character-oriented or character-addressable? Q-24. Which computer has the faster calculating speeds, the variable-word-length or the fixed-word-length? Q-25. What is the normal organization of data recorded on magnetic storage media? Q-26. What is a file?
STORAGE ACCESS METHODS How data files are stored in secondary storage varies with the types of media and devices you are using. Data files may be stored on or in sequential-access storage, direct-access storage, or random-access storage. SEQUENTIAL-ACCESS STORAGE Punched cards, paper tape, and magnetic tape are examples of sequential-access storage media. When operating in a sequential environment, a particular record can be read only by first reading all the records that come before it in the file. When you store a file on tape, the 125th record cannot be read until the 124 records in front of it are read. The records are read in sequence. You cannot read just any record at random. This is also true when reading punched cards or paper tape. DIRECT-ACCESS STORAGE Direct-access storage allows you to access the 125th record without first having to read the 124 records in front of it. Magnetic disks and drums are examples of direct-access storage media. Data can be obtained quickly from anywhere on the media. However, the amount of time it takes to access a record is dependent to some extent on the mechanical process involved. It is usually necessary to scan some (but not all) of the preceding data.
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RANDOM-ACCESS STORAGE Random-access storage media refers to magnetic core, semiconductor, thin film, and bubble storage. Here, a given item of data can be selected from anywhere in storage without having to scan any preceding items. And, the access time is independent of the storage location. Q-27. Punched cards, paper tape, and magnetic tape use what storage access method? Q-28. What kind of storage allows you to access the 125th record without having to read the 124 records in front of it? Q-29. Random-access storage media refers to what types of storage?
NETWORKS A network can be defined as any system composed of one or more computers and terminals; however, most are composed of multiple terminals and computers. In this section you will learn how this allows dissimilar computers to work together as a team. LOCAL AREA NETWORKS (LANs) In local area networks (LANs), various machines are linked together within a building or adjacent buildings. Figure 4-12 shows an example of a LAN. A LAN allows dissimilar machines to exchange information within one universal system. With the ability to communicate, the dissimilar machines act as a team. The information that exists in one system can be reused without being reentered via keyboard or disk into another separate system. A universal system for the integration and exchange of information is connected to all input devices. The entire system is usually housed within the same building or the same geographic area. A local area network is made up of a communications facility (for example, a coaxial cable, such as that used for cable television) and interface units creating a link for the computers and terminals to the communications facility. Two designs can be used: broadband or baseband.
Figure 4-12.—Local area network system.
A baseband communications channel uses the basic frequency band of radio waves and a coaxial cable. This coaxial cable has one channel, which is like a party line. Only two machines can use this cable
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at one time, even though many have the channel available, but there is no central switching unit to route traffic over the network. A more expensive channel, called a broadband communications channel, can handle more advanced applications. This includes transmission of voice as well as data and text. Because of the use of a controller to route traffic for a large number of simultaneous users, the users are able to share one of the many individual channels of the system. WIDE AREA NETWORKS Wide area networks provide for global connections and are sometimes referred to as global networks. Organizations are able to send information from city to city, across the nation, and to other countries throughout the world, through the expansion of local area networks into larger network configurations. Combinations of telephone lines, microwave radio links, and satellites are used by these larger telecommunications networks to send information. In 1965, the first successful communications satellite for business applications was launched. It was not the only try, it was preceded by many more primitive satellites. With the launching of larger and more complex satellites, the size and complexity of earth stations have been shrinking. Since satellite services' costs have been steadily decreasing, it is becoming more cost effective to employ them for business-type uses. MODEMS Since both signals and data can be transmitted and received through cables (communications lines), we refer to them as input/output channels. And when we transmit data directly to a computer over long distances, it becomes necessary to add two other devices, one at each end of the communications line. These devices are called modems (fig. 4-13). The word modem is an acronym for modulator/ demodulator (combines first syllable of each word). A modem converts the digital signal produced by your terminal or the computer to an audio signal suitable for transmission over the communications line. The modem at the other end of the line converts the audio signal back to a digital signal before it is supplied to the computers or your terminal. If this conversion were not carried out, the digital signal would degenerate during transmission and become garbled.
Figure 4-13.—Modem.
The physical link or medium that is used to carry (or transmit) data from one location to another is a communications channel. It allows remotely located input/output devices to communicate directly with the computer's central processing unit (cpu). Telephone lines (often referred to as land lines) are a frequently used type of communications channel. In a simple data communications system, terminals and other remote I/O devices are linked directly to one or more cpu's to allow users to enter data and programs and receive output information. Interface elements (those devices that serve to interconnect), such as modems are used to bridge and control the
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different data communications environments. Modems are used to permit the system to switch back and forth from computer digital data to analog signals that can be transmitted over communications lines. A modem never knows exactly when to expect data; therefore, it must be given some type of signal warning that data is about to be transmitted. This gives the modem time to get itself aligned and in synchronization with the incoming signal. Special characters, known as message characters, provide this warning and are placed in front of and behind the data to mark the beginning and ending of the message. Two methods are used: asynchronous and synchronous. With asynchronous transmission, each character of data must be surrounded by message characters. As a result, more total bits must be transmitted (transferred) than would be necessary if the synchronous method were used. With synchronous transmission, only a single set of start and stop message characters is needed per block of data, thus allowing more characters to be transmitted per second. As you can see, synchronous transmission is more efficient and faster. However, it has the disadvantage of requiring a more complex and expensive modem than does asynchronous transmission. You should be aware that whenever data is transferred between devices, it also involves an exchange of prearranged signals. This is known as handshaking. These signals, in combination with a prearranged pattern of message characters, define the rules for exchanging data over a communications line. The exact rules depend upon each individual computer manufacturer, the telephone company, and the related devices (the modems) that make up the computer system. Protocol is the term used for the specific set of rules that govern handshaking and message characters. In the system illustrated in figure 4-14, data to be sent to the main computer's cpu is entered through a remote online user terminal (far left). As the data is keyed, it is keyed in digital form and sent to a nearby modem to be converted into an analog signal suitable for transmission. This converted data is then transmitted over the telephone (or land) lines to another modem, that is located near the main computer system's cpu. The data, now in digital form, can be sent directly to the cpu for processing. The same route is followed when information is sent from the cpu back to the remote terminal.
Figure 4-14.—Modems used in network system.
Data communications and networks expand our use of computer technology by providing a means for computers and other machines to talk to each other.
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Q-30. Any system composed of one or more computers and terminals can be defined as what? Q-31. A network system allows dissimilar machines to do what within one universal system? Q-32. What does the make-up of a local area network consist of? Q-33. How many designs of local area networks are there that can be used? Q-34. What are the different designs of local area networks called? Q-35. What is a baseband communication channel like? Q-36. What do wide area networks provide for? Q-37. Where does the word modem come from? Q-38. What are interface elements? Q-39. How does a modem know when to expect data? Q-40. Whenever data is transferred between devices, it involves the exchange of prearranged signals; what is this process called?
SUMMARY Congratulations! You have just finished the last chapter in Introduction to Digital Computers. In this chapter you learned about many things that were mentioned in other chapters, without a detailed explanation. This was done intentionally, as some of the subjects would have been too difficult and hard to understand without background knowledge. Through your study of chapters 1, 2, and 3, you gained enough knowledge to understand chapter 4. This chapter should have answered a lot of questions for you and made certain subjects more clear. DATA is a general term used to describe raw facts like your service number, name, and paygrade. SOURCE DATA is raw data typically written on some type of paper document. DATA REPRESENTATION is accomplished by the use of symbols. The symbol itself is not the information, but merely a representation of it. Symbols convey meaning only when understood. In computers, symbols are represented by CODES. COMPUTER CODING SYSTEMS are used to represent numeric, alphabetic, and special characters in computer storage and on magnetic media. EXTENDED BINARY CODED DECIMAL INTERCHANGE CODE (EBCDIC) is an 8-bit code used in computers to represent numbers, letters, and special characters. AMERICAN STANDARD CODE FOR INFORMATION INTERCHANGE (ASCII) is another 8-bit code developed to standardize a binary code to give the computer user the capability of using several machines to process data regardless of the manufacturer. A PARITY (CHECK) BIT is used to detect errors in computer circuitry. MAGNETIC CORE STORAGE is used as primary storage in some computers. 4-19
PRIMARY STORAGE CAPACITY AND ADDRESSES are designed and built into the computer by the manufacturer. Computers may be WORD-ADDRESSABLE, CHARACTER-ADDRESSABLE, or FLEXIBLE. Data in SECONDARY STORAGE like disk or tape is normally organized by bits, characters (bytes), fields, records, and files. STORAGE ACCESS METHODS vary with the types of media and devices you are using. SEQUENTIAL-ACCESS STORAGE is associated with punched cards, paper tape, and magnetic tape. DIRECT-ACCESS STORAGE is obtained by using magnetic disks and drums. RANDOM-ACCESS STORAGE refers to magnetic core, semiconductor, thin film, and bubble storage. A NETWORK is any system composed of one or more computers and terminals; however, most are composed of multiple terminals and computers. LOCAL AREA NETWORKS (LANs) allow dissimilar machines to exchange information within one universal system within a building or small geographic area. WIDE AREA NETWORKS provide for global connections and are sometimes referred to as global networks. A MODEM converts the digital signal produced by your terminal or the computer to an audio signal suitable for transmission over a communications line. It also converts the audio signal back to a digital signal before it is supplied to your terminal or computer.
ANSWERS TO QUESTIONS Q1. THROUGH Q40. A-1. Data. A-2. By symbols. A-3. Numbers. A-4. By either direct or indirect means. A-5. Punched cards, paper tape, magnetic tape, or magnetic disk. A-6. Extended Binary Coded Decimal Interchange Code. A-7. 256. A-8. 16. A-9. Packing or packed data. A-10. American Standard Code for Information Interchange.
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A-11. To standardize a binary code to give the computer user the capability of using several machines to process data regardless of the manufacturer. A-12. No, they are identical. A-13. To detect errors in the circuitry. A-14. Nine. A-15. Program storage area. A-16. By the individual programs being used. A-17. A single binary digit. A-18. Eight. A-19. Number of bytes. A-20. Magnetic cores are strung together on a screen of wire. A-21. In primary storage. A-22. The manufacturer. A-23. Variable-word-length or byte-addressable. A-24. Fixed-word-length. A-25. By bits, characters (bytes), fields, records, and files. A-26. A collection of related records. A-27. Sequential-access. A-28. Direct-access storage. A-29. Magnetic core, semiconductor, thin film, and bubble. A-30. A network. A-31. Exchange information. A-32. A communications facility and interface units. A-33. Two. A-34. Broadband and baseband. A-35. A party line. A-36. Global connections. A-37. It is an acronym for modulator/demodulator. A-38. Those devices that serve to interconnect.
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A-39. It is given a signal warning that data is about to be transmitted. A-40. Handshaking.
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APPENDIX I
GLOSSARY ACCESS TIME—The amount of time between the time a request for data from a storage device is made and the time the data is delivered. ADA—A high-level programming language designed by the Department of Defense. ALPHAMERIC (ALPHANUMERIC) CHARACTER SET—The set of characters that includes letters, numbers, and special characters. ANALOG COMPUTER—A computer that solves problems using continuous data from physical quantities like voltage or temperature. APPLICATION (PROGRAMS) SOFTWARE—Programs written to solve user problems. ARITHMETIC-LOGIC UNIT—The part of the cpu that contains the logic capability and performs all the arithmetic functions (addition, subtraction, multiplication, and division). ARTIFICIAL INTELLIGENCE—The capability of a machine to perform human-like intelligence functions, such as learning, adapting, reasoning, and self-correction. ASCII (American Standard Code for Information Interchange)—A standardized 8-bit code (originally a 7-bit code) designed for transmitting and processing data. ASSEMBLER—A computer program that translates source programs written in assembly language into machine language (object) programs. ASSEMBLY LANGUAGE—A low-level, machine-oriented programming language in which each instruction (written as a mnemonic) translates into a single machine language (computer) instruction. AUTOMATIC DATA PROCESSING (ADP)—A general term used to define a system for automatically performing a series of data processing functions by means of machines using mechanical, electromechanical, and electronic circuitry. AUXILIARY EQUIPMENT—The peripheral equipment or devices that may or may not be in direct communication with the central processing unit of a computer. AUXILIARY STORAGE—See storage, secondary. BACKUP FILE—A copy of a program or data file to be used in the event something happens to the original. BASIC (Beginners All Purpose Symbolic Instruction Code)—A high-level, general-purpose programming language primarily used on microcomputers. See NAVEDTRA 10079, Introduction to Programming in BASIC. BAUD—A unit for measuring data transmission speed. For practical purposes, it is now used interchangeably with bits per second as the unit of measure of data flow.
AI-1
BINARY—Two values (0 or 1) or states (ON or OFF); the number system used in computers. BIT—An abbreviation for binary digit; the smallest unit of data, either a 0 or 1. BLOCKED RECORDS—One or more logical records grouped and treated as a unit (physical record or block) for input/output processing. BLOCKING FACTOR—The number of records stored in a record block. BOOT OR BOOTSTRAP—(1) A set of instructions that causes additional instructions to be loaded until the complete computer program is in storage. (2) A technique or device designed to bring itself into a desired state by means of its own action; e.g., a machine routine whose first few instructions are sufficient to bring the rest of itself into the computer from an input device. (3) That part of a computer program used to establish another version of the computer program. BUG—A mistake in a program. BYTE—A group of bits next to each other that is considered a unit; for example an 8-bit byte. CENTRAL PROCESSING UNIT (cpu)—The part of the computer hardware that directs the sequence of operations, interprets the coded instructions, performs arithmetic and logical operations, and initiates the proper commands to the computer circuits for execution. It controls the computer operation as directed by the program it is executing. CHARACTER—One symbol; for example, A, Z, a, z, 0, 1, 9, !, ". CHIP—A small piece of silicon impregnated with impurities in such a way as to form transistors, diodes, and resistors. Electrical paths are formed on the silicon by depositing thin layers of aluminum or gold. COBOL (COmmon Business Oriented Language)—A high-level programming language designed for business-type applications. COMPATIBLE SOFTWARE—Programs that can be run on more than one type of computer. These programs come in several different versions so they can be run under several different operating systems. COMPILER—A program that translates source programs written in a high-level programming language (for example COBOL or FORTRAN) into machine language. COMPUTER—A programmable electronic device that can store, retrieve, and process data. COMPUTER OPERATOR—The person who sets up and operates the computer system. COMPUTER PROGRAMMER—A person who designs, writes, tests, debugs, and documents programs. COMPUTER SYSTEM—The cpu (mainframe) with its console, input, and output devices, and secondary (auxiliary) storage devices. COMPUTER USERS—See users. COMPUTER WORD—See word, computer.
AI-2
CONSOLE—The unit of the computer used by the computer operator to communicate with and control the computer system. CONTROL SECTION—The part of the cpu that directs the flow of operations and data, maintains order in the computer, and initiates execution of the instructions. CRT (Cathode-Ray Tube) TERMINAL—A computer terminal that displays its output on a televisionlike screen that may be black and white or color. CURSOR—A pointer (a dot of light) on a crt screen to let you know the next position in which data will be entered. By depressing cursor control keys, the operator can move the cursor from line to line and from character to character. CUSTOM SOFTWARE—Programs designed and written to the specifications of a user or an organization. DATA—Facts represented by numbers, letters, or symbols to which meaning is or can be assigned. DATA BASE—A structured collection of data that can be extracted, organized, and manipulated by a program. DATA COMMUNICATIONS—The means by which data is transmitted electronically from one location to another over a communications channel. DATA ELEMENT—One item of information; the smallest unit of data that can be referenced. DATA FLOWCHART—See flowchart. DATA, REFERENCE—The source document identification. DATA REPRESENTATION—The symbols and codes used by computers to represent letters, numbers, and special characters. DEBUGGING—The process of finding errors (bugs) in a program or system and correcting them so that the program or system runs correctly. DENSITY, RECORDING—The number of bits, bytes, characters, or frames per linear inch on a recording medium, like tape or disk. DIGITAL COMPUTER—A computer that solves problems on discrete data using 0's and 1's (OFF and ON states) to represent data and operations. DIRECT ACCESS—A storage method that allows the computer to locate and read a particular record without having to search through an entire file. The computer is able to access data independent of its location. Magnetic disks, diskettes, and drums are considered direct access devices. DISK DRIVE—A direct-access storage device for recording and retrieving data on hard (rigid) disk or floppy disks (diskettes). DISK PACK—A mass storage device in which information is stored on one or both sides of a rigid disk that can be magnetized. The disk is rotated by a disk drive and information is stored and retrieved by one or more magnetically sensitive read/write heads.
AI-3
DISKETTE—A mass storage device in which information is stored on one or both sides of a flexible disk that can be magnetized. The diskette is rotated by a diskette drive and information is stored and retrieved by one or more magnetically sensitive read/write heads. Diskettes are also called floppy disks because the disk bends easily. DOWNTIME—The length of time the computer is not operating, either because of preventive maintenance (scheduled downtime) or a malfunction (nonscheduled downtime). EBCDIC (Extended Binary Coded Decimal Interchange Code)—An 8-bit coding system for representing uppercase and lowercase letters, numbers, and special characters. EPROM—The acronym for erasable programmable read-only memory. FIELD, DATA—An item of information in a data record. One or more related characters that are treated as a unit of information. FILE—A collection of related records; for example, a payroll file. Any collection of records holding similar data or transactions that are stored together to permit systematic access and modification. FILE, MASTER—The file that contains all the data records in up-to-date form. It is a main reference file of relatively more permanent information, which is usually updated periodically. FIRMWARE—A set of program instructions, a microprogram, permanently stored in read-only memory. FLAT PANEL DISPLAY—A display device that consists of a grid of electrodes in a flat, gas-filled panel. The image can persist for a long period of time without refresh. FLOPPY DISK—See diskette. FLOWCHART—A graphic representation of the processing steps (logic) of a program (a program flowchart) or the inputs, outputs, and processing steps of a system (a systems [data] flowchart). The graphic representation uses symbols to represent operations and directional lines to indicate sequence and direction of flow. FORMAT—The arrangement or layout of data in or on a data medium. FORTRAN (FORmula TRANslator)—A high-level programming language for scientific and mathematical applications. FULL-DUPLEX CHANNEL—A channel that provides for simultaneous transmission in both directions, such as the telephone. GENERAL-PURPOSE COMPUTER—A computer designed to operate on a program of instructions for the purpose of solving many different types of processing problems. GENERATIONS OF COMPUTERS—Historically, the distinctive types of computers from the 1940s to the present; the first generation was based on vacuum tubes, the second on transistors, the third and current features integrated circuits. Recent developments involve the use of VLSI (very large scale integration) and semiconductor memories. GRAPHICS—The use of pictorial means to present data in the form of plotted curves, graphs (bar, pie, line, and so on), or diagrams. These may be displayed on a crt or printed.
AI-4
HANDSHAKING—The process through which the rules for exchanging data over a communications line are defined for the two devices involved. HARD COPY—The term given to humanly readable printed output from a computer. HARDWARE—The visible, physical equipment of a system, including the computer (cpu) and related peripheral equipment; as distinguished from software. HEAD POSITIONING—Placing a read/write head over a specified track on a disk or drum. HEXADECIMAL—The number system with base 16 (0-9 and A-F). A represents 10; B represents 11; C represents 12; D represents 13; E represents 14; and F represents 15. Used in some computer systems. HIGH-LEVEL LANGUAGES—Programming languages that allow the programmer to write programs in English-like terms and symbols and mathematical notation, rather than the 0's and 1's used by the computer. These high-level programs must be translated into machine language before the computer can execute them. FORTRAN, Ada, COBOL, and BASIC are examples. HOST COMPUTER—The main or controlling computer in a distributed data processing network (ddp). HYBRID COMPUTER—A computer that combines the functions of both analog and digital computers. I/O—Input/Output. INPUT—The data entered into a computer system for processing. INPUT DEVICES—Devices for reading data and programs into the computer system for processing. INPUT/OUTPUT DEVICES—Secondary storage devices for writing and reading data. Magnetic tape drives, magnetic disk drives, and drums are examples. INTEGRATED CIRCUIT—A miniaturized chip in which semiconductor components and other such technology combine the functions of a number of conventional components (such as transistors, resistors, capacitors, and diodes). INTERNAL STORAGE (MEMORY)—See storage, primary. INTERRECORD/INTERBLOCK GAP—A blank section of recording surface separating each record or block of records on a magnetic data medium. K—An abbreviation for the value 1,024 which is 210. Often used to express the memory capacity of a computer. For example, a 512K computer has 524,288 bytes of memory. KEY-TO-DISK—A process, similar to key-to-tape, in which data is transmitted from a keyboard to magnetic disk. KEY-TO-TAPE—An operation in which data is transmitted from a keyboard to magnetic tape. LANGUAGE TRANSLATOR—A program that reads a source program and converts it into an object (machine language) program. Assemblers and compilers are examples. LOCAL-AREA NETWORK—A network that normally operates within a well-defined and generally self-enclosed area. The communication stations or terminals are usually linked by cable and are within 1,000 feet of each other. AI-5
LOGICAL RECORD—A record that includes all the data that belongs together as a unit regardless of the physical size or storage location of the data. M—A unit of measurement approximately equal to one million and used to express the capacity of a computer memory. 1M is about 1,000,000 units. Memory size is usually measured in words or bytes. MACHINE LANGUAGE—Machine instructions in binary bit patterns that the central processing unit can execute directly without additional interpretation or translation. MACHINE (TAKE-UP) REEL—A reel that remains on the tape drive and on which magnetic tape is wound during the processing of a tape. MAGNETIC MEDIA—Magnetic cards, tapes, disks, drums, cartridges, and cassettes used to record data or information. MAGNETIC TAPE—A mass storage device in which information is stored on a plastic tape coated with a magnetic film. The tape is wound on reels that are rotated by tape drives. Information is stored and retrieved sequentially by magnetically sensitive read/write heads. MAINFRAME COMPUTERS—This term is usually used to designate large-scale computer systems, although the precise definition of mainframe is the cpu and the control elements of any computer system. MAIN STORAGE (MEMORY)—See storage, primary. MASS STORAGE—Any external storage medium (magnetic tape, disk, drum, and so on) that supports and can be linked to the cpu's main memory in the computer. When the power is turned off, information in the mass storage is retained (not lost). MEDIUM—The material on which data and instructions are recorded, such as punched cards, paper tape, and all forms of magnetic media (tape, disk, drum, and so on). MEMORY—A device or section of the computer in which computer instructions and data can be stored for retrieval (synonymous with primary or internal storage). MICROCOMPUTERS—The smallest category of computers, usually with the entire central processing unit on a single chip. Unlike large-scale and minicomputer systems, they are designed to be used by one person at a time (hence the term, personal computer [PC]). MICROPROCESSOR—The semiconductor central processing unit (cpu) of a microcomputer that fits on a small silicon chip. The microprocessor is the central chip containing the control units of the computer. MICROSECOND—One millionth of a second. MILLISECOND—One thousandth of a second. MINICOMPUTERS—Midsize computers that are smaller than large-scale systems but with the same components. They are less expensive and have less strict environmental requirements. MODEM—Acronym for MOdulator-DEModulator. A device that converts data from digital-to-analog format for transmission on analog transmission lines and converts data in analog format to digital format for computer processing.
AI-6
MULTIPROCESSING—A computer processing mode that provides for simultaneous processing of two or more programs or routines by use of multiple cpu's. MULTIPROGRAMMING—A computer processing mode that provides for overlapping or interleaving the execution of two or more programs at the same time by a single processor. NANOSECOND—A billionth of a second. NETWORK—Computers and terminals linked together through a communications system to allow users at different locations to share data files, devices, and programs. ONLINE PROCESSING—Processing from terminals under the direct control of a computer. OPERATING SYSTEM—Software that controls the execution of programs. An operating system may provide services such as input/output control and data management. It may also provide job scheduling, memory allocation, and other general functions. It is usually loaded by a bootstrap program. OUTPUT—The results of computer processing. It may be data transferred to tape, disk, paper, and so on. PACKAGED SOFTWARE—Programs already written (and tested) to solve specific types of problems; usually designed by a central design agency (CDA) or purchased from a software firm or computer manufacturer. PACKED DECIMAL—In ASCII and EBCDIC, the representation of two digits stored in one eight-bit byte. PAPER TAPE—See punched tape. PARALLEL TRANSMISSION—A method of data transmission in which all bits of a particular character are transmitted simultaneously. PARITY BIT—A check bit; an extra bit added to a group of bits for use in detecting errors during data transfer. PARITY CHECK—An internal error checking method in which the binary digits in a character or word are added and the sum is checked against a single previously computed parity digit. The check tests whether the number of one bits in a character or word are odd or even, depending on the parity of the computer. PASSWORD—A protected word or string of characters that identifies or authenticates a user for access to a specific resource, such as a file or record. PERIPHERAL EQUIPMENT—Equipment used for data entry, storage, or retrieval, but which is not a part of the central processing unit. Peripherals include crt displays, terminals, printers, and mass storage (tape, disk, and drum) devices. PERSONAL COMPUTER (PC)—A computer, usually a microcomputer, that is more affordable than minicomputers or mainframes and is used by one person at a time. PICTURE ELEMENT—Synonym for pixel. See pixel. PIXEL—In computer graphics, the smallest element of a display surface that can be independently assigned color or intensity.
AI-7
PRIMARY STORAGE—See storage, primary. PRINTER—A device used with a computer to produce hard copy, printed output. PROGRAM—(1) Verb—The act of writing instructions for computer execution. (2) Noun—The set of instructions that tells the computer the steps to execute to automatically solve a problem. PROGRAM FLOWCHART—See flowchart. PROM—Acronym for programmable read-only memory. PUNCHED CARD—A card punched with hole patterns that represent data or program instructions. Punched cards can be read by an input device (card reader) to a computer. PUNCHED TAPE—A tape punched with hole patterns that represent data or program instructions. Tape can be read by an input device to a computer. RAM—Acronym for random-access memory. RANDOM ACCESS—A method of accessing data (or instructions) without having to scan any preceding information. Magnetic core, semiconductor, and bubble memories are considered random access storage devices. REAL-TIME PROCESSING—A computer processing method in which data about a particular event is entered directly into the computer as the event occurs and is immediately processed so it can influence future processing. RECORD—A group of related fields, all pertaining to the same subject. RECORD BLOCK—Several records blocked together. RECORD LENGTH—The number of characters in a record. REMOTE TERMINAL—A display terminal, such as a crt or other piece of equipment, which is not located with the computer but is connected by a communications line. In a typical online, real-time communications system, the remote device is usually a teletypewriter or a crt visual display unit. ROM—Acronym for read-only memory. ROTATIONAL DELAY—The time required for the read/write head to find a specified record on a disk, diskette, or drum once head positioning has occurred. SECONDARY STORAGE—See storage, secondary. SECTORS—The pie-shaped segments of a disk's recording surface. SEQUENTIAL ACCESS—A storage technique in which the stored items of information become available only in a one after the other sequence, whether or not all the information or only some of it is desired. Magnetic tape is an example. SOFT COPY—Output of a computer displayed on a display terminal or monitor (crt). It is nonpermanent. SOFTWARE—Programs, routines, codes, and other written information used to direct the operation of a computer; as distinguished from hardware. AI-8
SORT—The process of arranging data records in a predefined sequence by use of sort keys; for example, to sequence personnel records by social security number (the sort key). SOURCE DATA—The data in its initial state to be processed by a computer system. SOURCE DOCUMENT—The document that contains the initial (raw) data for computer processing. SOURCE PROGRAM—A computer program written in a language like COBOL, FORTRAN, or assembly language. It must be translated into an object program before it can be executed by a computer. SPECIAL-PURPOSE COMPUTER—A computer designed to perform one specific function such as a weather computer. STORAGE, PRIMARY (MAIN, INTERNAL)—The section of the cpu in which instructions and data are held. Also called main memory. STORAGE, SECONDARY (AUXILIARY, EXTERNAL)—Storage outside the cpu where programs and data are stored for future computer processing; for example tapes, disks, and punched cards. STORED PROGRAM—The set of instructions stored in computer memory for execution. SUBROUTINE LIBRARY—A set of standard and proven computer routines that are kept on file for use at any time. TELECOMMUNICATIONS—The transmission of data between computer systems and/or terminals in different locations. TELEPROCESSING—A method of data processing in which communication devices are used. TERMINAL—A device linked to the central processor for entering or receiving data and programs. TIME SHARING—A processing mode in which many users share the computer systems' resources through online terminals. Each user gets a slice of computer time. TRACK—(1) One of seven or nine, horizontal rows stretching the entire length of a magnetic tape and on which data can be recorded. (2) One of a series of concentric circles on the surface of a disk. (3) One of a series of circular bands on a drum. UNBLOCKED—Having a blocking factor of one logical record per block. USER PROGRAMS—Programs written to solve specific user problems, called applications software. USERS—(1) The people who use the output from computer processing. (2) The people who operate a computer for their own purposes. UTILITY PROGRAMS (UTILITIES)—Programs designed to perform often needed general functions. WIDE AREA NETWORK—A network that usually covers large geographical areas. Communications between stations or terminals usually occur using standard telephone lines or microwave relays. WORD, COMPUTER—A group of related bytes treated as a single addressable unit or entity in computer memory.
AI-9
APPENDIX II
REFERENCE LIST Casady, Mona J., and Sandburg, Dorothy C., Word/Information Processing, South-Western Publishing Co., Cincinnati, Ohio, 1985. Davis, Gordon B., Introduction to Electronic Computers, McGraw-Hill Book Co., New York, N.Y., 1971. Delco Electronics, Fundamentals of Digital Computers Course, General Motors Corp., Milwaukee, Wis., 1972. Fuori, William M., D’Arco, Anthony M.S., and Ovilia, Lawrence, Introduction To Computer Operations, 2nd edition, Prentice-Hall Inc., Englewood Cliffs, N.J., 1981. Jansen, John T., How Microprocessors Function, Dun-Donnelley Publishing Corp., New York, N.Y., 1983. Lavin, Albert J. III DSCS(SW), Data Systems Technician 3 & 2, Volume 2, NAVEDTRA 10231, NETPDTC, Pensacola, Fla., 1987. McGee, Fred H. III DPC, Introduction to Programming in BASIC, NAVEDTRA 10079-2, NETPDTC, Pensacola, Fla., 1983. Mims, Forrest M. III, Understanding Digital Computers, Tandy Corp. Co., Fort Worth, Tex., 2nd edition, 1987. Moore, Alvin W., et. al., Microprocessor Applications Manual, McGraw-Hill Book Co., New York, N.Y., 1975. Rau Bernie, ZENITH 100 Series Microcomputer Operations, NAVEDTRA 40801, Naval Oceanography Command Facility, Bay St. Louis, Miss., 1985. Shelly, Gary B., and Thomas J. Cashman, Introduction To Computers and Data Processing, Anaheim Publishing Co., Brea, Calif., 1980. U S Army Signal School, Digital Computers FM 11-72, Fort Gordon, Ga., 1977. Van Overberghe, Albert G., Jr. DPCS, Data Processing Technician 3, NAVEDTRA 10263, NETPDTC, Pensacola, Fla., 1987. Vermillion Associates, Computer Concepts For Small Business, Heath Company, Benton Harbor, Mich., 1978.
AII-1
MODULE 22 INDEX
Baseband communications channel, 4-16 BASIC, 3-7 Bits and bytes, 4-8, 4-13 Blocking factor, 2-15 Booting the system, 1-18 Bootstrap program, 2-4 Broadband communications channel, 4-17 Bubble storage, 2-6 Bytes, 4-8, 4-13
Computer generations, digital, 1-10 to 1-14 first generation, 1-10 fourth generation and beyond, 1-12 second generation, 1-10 third generation, 1-11 Computer, stored-program, 2-3 Computer, using a desktop, 1-17 to 1-21 operating system, 1-18 storage media handling and backup, 1-19 Computers, classification of, 1-3 to 1-9 accuracy of computers, 1-7 to 1-9 analog computers, 1-6 digital computers, 1-6 to 1-7 electromechanical computers, 1-4 electronic computers, 1-5 general-purpose computers, 1-6 mechanical computers, 1-3 special-purpose computers, 1-6 Computers, history of, 1-2 to 1-3 Control break, 3-6 Control field, 3-6 Control instructions, 2-3 Cursor control key, 2-28 Cylinder method, 2-11
C
D
Central processing unit (cpu), 2-1 to 2-4 arithmetic-logic section, 2-3 control section, 2-2 to 2-3 memory (internal storage) section, 2-4 Character-oriented/addressable computers, 4-11 Check bit, 4-7 COBOL, 3-7 Coding, program, 3-19 to 3-21 Coding systems, 4-3 to 4-8 Collating sequence, 3-4 Communications, data, 4-16 to 4-18 Communications software and hardware, 3-24 Compiler programs, 3-24 Computer coding systems, 4-3 to 4-7 ASCII, 4-6 to 4-7 EBCDIC, 4-3 to 4-6
Daisy-wheel printers, 2-25 Data flowcharts, 3-11 Data management software, 3-24 Data organization, 4-10 to 4-14 Data representation and communications, 4-1 to 4-22 computer coding systems, 4-3 to 4-7 American Standard Code for Information Interchange (ASCII), 4-6 to 4-7 extended binary coded decimal interchange code (EBCDIC), 4-3 to 4-6 parity bit, 4-7 to 4-8 data, 4-1 to 4-3 data representation, 4-1 to 4-2 source data, 4-2 to 4-3
A Access arms, 2-22 Accounting and recordkeeping, 1-15 Ada, 3-7 Alphanumeric, 2-27 American Standard Code for Information Interchange (ASCII), 4-6 to 4-7 Arithmetic instructions, 2-3 ASCII, 4-6 to 4-7 Asynchronous transmission, 4-18 Auxiliary storage, see storage, secondary B
INDEX-1
Data representation and communications— Continued data storage concepts, 4-8 to 4-15 bits and bytes, 4-8 magnetic core storage, 4-9 to 4-10 secondary storage data organization, 4-13 to 4-15 storage capacity and addresses, 4-10 to 4-13 networks, 4-16 to 4-19 local area networks (LANs), 4-16 to 4-17 modems, 4-17 to 4-18 wide area networks, 4-17 storage access methods, 4-15 to 4-16 direct-access storage, 4-15 random-access storage, 4-16 sequential-access storage, 4-15 Data storage concepts, 4-8 to 4-15 Debugging, 3-21 Desk-checking, 3-21 Direct-access storage, 2-9, 2-16, 4-15 Disk, cylinder method, 2-11 Disk pack, 2-22 Disk, sector method, 2-12 Disks, magnetic, 2-8 to 2-12 Displays, formatted, 2-31 Displays, unformatted, 2-31 Document compilation programs, 3-24 Documentation, 3-21 to 3-22 Dot-matrix printers, 2-25 Drum, magnetic, 2-16 to 2-18 E EBCDIC, 4-3 to 4-6 Erasable programmable read-only memory (EPROM), 2-8 Extended binary coded decimal interchange code (EBCDIC), 4-3 to 4-6 Extensions, 3-3 F Field, 4-14 File, 4-14
Firmware, 2-7 Fixed-word-length computers, 4-12 Flexible computers, 4-12 Floppy disk drive units (input/output), 2-23 Flowcharting, 3-11 to 3-18 FORTRAN, 3-7 Function switches, 2-28 G Generations, digital computers, 1-10 to 1-14 Glossary, AI-1 to AI-9 Graphics software, 3-26 to 3-27 H Handshaking, 4-18 Hardware, 2-1 to 2-35 central processing unit (cpu), 2-1 to 2-4 arithmetic-logic section, 2-3 control section, 2-2 to 2-3 memory (internal storage) section, 2-4 classifications of internal storage, 2-7 to 2-8 erasable programmable read-only memory (EPROM), 2-8 programmable read-only memory (PROM), 2-8 random-access memory (RAM), 2-8 read-only memory (ROM), 2-7 input/output devices (external), 2-18 to 2-33 display devices, 2-28 to 2-33 flat panel displays, 2-31 to 2-32 raster scan crts, 2-28 to 2-31 floppy disk drive units (input/output), 2-23 keyboards (input), 2-27 to 2-28 magnetic disk drive units (input/output), 2-22 to 2-23 magnetic tape units (input/output), 2-19 to 2-22 printers (output), 2-24 to 2-27 daisy-wheel printers, 2-25 dot-matrix printers, 2-25
INDEX-2
Hardware—Continued ink jet printers, 2-27 laser printers, 2-27 internal storage, types of, 2-4 to 2-7 bubble storage, 2-6 magnetic core storage, 2-5 semiconductor storage (the silicon chip), 2-6 secondary storage, 2-9 to 2-18 magnetic disk, 2-9 to 2-12 magnetic drum, 2-16 to 2-18 magnetic tape, 2-13 to 2-16 Hard-wired memory, 2-7 History, 1-2 to 1-3
L Large scale integration (LSI), 2-6 Laser printers, 2-27 Loading, 2-3 Local area networks (LANs), 4-16 to 4-17 Logic instructions, 2-3 M
I Ink jet printers, 2-27 Input/output devices (external), 2-18 to 2-33 display devices, 2-28 to 2-33 floppy disk drive units (input/output), 2-23 keyboards (input), 2-27 to 2-28 magnetic disk drive units (input/output), 2-22 to 2-23 magnetic tape units (input/output), 2-19 to 2-22 printers (output), 2-24 to 2-27 Instructions, arithmetic, 2-3 Instructions, control, 2-3 Instructions, logic, 2-3 Instructions, transfer, 2-3 Integrated circuits (ICs), 2-6 Internal storage, types of, 2-4 to 2-7 bubble storage, 2-6 magnetic core storage, 2-5 semiconductor storage (the silicon chip), 2-6 Interrecord (interblock) gap, 2-16 K Keyboards (input), 2-27 to 2-28
Machine languages, 3-6 to 3-7 Macroinstructions, 3-7 Magnetic core storage, 2-4 to 2-5, 4-9 to 4-10 Magnetic disk drive units (input/output), 2-22 to 2-23 Magnetic tape units (input/output), 2-19 to 2-22 Mailing list programs, 3-24 Memory, bubble, 2-6 Memory, nondestructive, 2-6 Memory, read/write, 2-8 Memory, see storage also Memory, volatile, 2-6 Miniaturized circuits, 1-11 Mnemonic instruction codes, 3-7 Modems, 4-17 to 4-18 Multiprocessor, shared resource systems, 3-2 Multiuser/multitasking operating systems, 3-2 N Networks, 4-16 to 4-19 local area networks (LANs), 4-16 to 4-17 wide area networks, 4-17 Nondestructive memory, 2-6 O Operating systems, 1-18, 3-1 to 3-4 Operational concepts, 1-1 to 1-25 classification of computers, 1-3 to 1-9 accuracy of computers, 1-7 to 1-9 analog computers, 1-6 digital computers, 1-6 electromechanical computers, 1-4 electronic computers, 1-5
INDEX-3
Operational concepts—Continued general-purpose computers, 1-6 mechanical computers, 1-3 special-purpose computers, 1-6 digital computer generations, 1-10 to 1-14 first generation, 1-10 fourth generation and beyond, 1-12 second generation, 1-10 third generation, 1-11 history of computers, 1-2 to 1-3 uses of a digital computer, 1-14 to 1-17 accounting and recordkeeping, 1-15 work center uses, (SNAP II), 1-15 word processing, 1-14 using a desktop computer, 1-17 to 1-21 operating system, 1-18 booting the system, 1-18 running an application program, 1-19 storage media handling and backup, 1-19 data backup, 1-21 exposure, 1-20 handling, 1-20 labeling, 1-21 storage, 1-20 P
Packaged software, 3-23 to 3-27 data management, 3-24 to 3-25 graphics, 3-26 to 3-27 spreadsheets, 3-25 to 3-26 word processing, 3-23 to 3-24 Parallel processing, 3-22 Parity bit, 4-7 Parity check, 4-7 PASCAL, 3-7 Password, 1-16 Pels, 2-29 Pixels, 2-29 Printers (output), 2-24 to 2-27 daisy-wheel printers, 2-25 dot-matrix printers, 2-25 ink jet printers, 2-27 laser printers, 2-27
Procedure-oriented languages, 3-7 to 3-8 Programmable read-only memory (PROM), 2-8 Programming, 3-8 to 3-22 coding, 3-18 to 3-22 debugging, 3-21 documentation, 3-21 to 3-22 flowcharting, 3-11 to 3-18 implementation, 3-22 overview of programming, 3-9 to 3-11 testing, 3-21 Programming languages, 3-6 to 3-8 Ada, 3-7 BASIC, 3-7 COBOL, 3-7 FORTRAN, 3-7 machine languages, 3-6 to 3-7 PASCAL, 3-7 procedure-oriented languages, 3-7 to 3-8 symbolic languages, 3-7 Prompt, 1-19 Protocol, 4-18 R
Random-access memory (RAM), 2-8 Raster scan crts, 2-28 to 2-31 Read-only memory (ROM), 2-7 Read/write memory, 2-8 Record, 4-14 Record block, 2-15 Recording density, tape, 2-14 Reference list, AII-l Report program generators, 3-5 to 3-6 ROM (read-only memory), 2-7 S Sector method, 2-12 Semiconductor storage (the silicon chip), 2-6 Single user/multitasking operating systems, 3-2 Single user/single tasking operating systems, 3-2 SNAP II, 1-15 Software, 3-1 to 3-29 operating systems, 3-1 to 3-4
INDEX-4
Software—Continued compatibility with applications software, 3-2 operating system functions, 3-2 to 3-4 types of operating systems, 3-2 packaged software, 3-23 to 3-27 data management, 3-24 to 3-25 graphics, 3-26 to 3-27 spreadsheets, 3-25 to 3-26 word processing, 3-23 to 3-24 programming, 3-8 to 3-22 flowcharting, 3-11 to 3-18 constructing a flowchart, 3-15 to 3-18 tools of flowcharting, 3-12 to 3-14 overview of programming, 3-9 to 3-11 program coding, 3-18 to 3-22 coding a program, 3-19 to 3-21 debugging, 3-21 documentation, 3-21 to 3-22 implementation, 3-22 instruction set, 3-19 instructions, 3-18 testing, 3-21 programming languages, 3-6 to 3-8 machine languages, 3-6 to 3-7 procedure-oriented languages, 3-7 to 3-8 symbolic languages, 3-7 utility programs, 3-4 to 3-6 report program generators, 3-5 to 3-6 sort-merge programs, 3-4 to 3-5 Sort-merge programs, 3-4 to 3-5 Source document, 4-2 to 4-3 Spelling checker, 3-23 Spreadsheets, 3-25 Storage access methods, 4-15 to 4-16 direct-access storage, 4-15 random-access storage, 4-16 sequential-access storage, 4-15 Storage, auxiliary, see storage, secondary Storage, internal, 2-4 to 2-7 addressing, 4-13 to 4-15
Storage, internal—Continued bubble storage, 2-6 erasable programmable read-only memory (EPROM), 2-8 magnetic core storage, 2-5 programmable read-only memory (PROM), 2-8 random-access memory (RAM), 2-8 read-only memory (ROM), 2-7 semiconductor storage, 2-6 Storage, secondary, 2-9 to 2-18 data organization, 4-10 to 4-15 magnetic disk, 2-9 to 2-12 magnetic drum, 2-16 to 2-18 magnetic tape, 2-13 to 2-16 Symbolic languages, 3-7 Synchronous transmission, 4-18 System commands, 3-3 System flowcharts, 3-11 T
Tape, magnetic, 2-13 to 2-16 Tape recording density, 2-14 Testing, 3-21 Transfer instructions, 2-3 Transistors, 1-10 U
Uses of a digital computer, 1-14 to 1-17 Utility programs, 3-4 to 3-6 report program generators, 3-5 to 3-6 sort-merge programs, 3-4 to 3-5 V
Vacuum tube, 1-10 Variable-word-length computers, 4-12 Very large scale integration (VLSI), 2-6 Volatile memory, 2-6 W Wide area networks, 4-17 Word, 4-14 Word-oriented/addressable computers, 4-11
INDEX-5
Word processing software, 3-23 to 3-24 Work center uses, (SNAP II), 1-15
INDEX-6
Assignment Questions
Information: The text pages that you are to study are provided at the beginning of the assignment questions.
ASSIGNMENT 1 Textbook assignment: Chapter 1, “Operational Concepts,” pages 1-1 through 1-25. ___________________________________________________________________________________ 1-6. Mechanical computers are what type of devices?
1-1. When was the first mechanical adding machine invented? 1. 2. 3. 4.
1. 2. 3. 4.
1264 1426 1462 1642
1-7. What determines the size of an analog computer?
1-2. What year did electronics enter the computer scene? 1. 2. 3. 4.
1. 2. 3. 4.
1918 1919 1920 1921
1-3. In modern digital computers, circuits that store information, perform arithmetic operations, and control the timing sequences are known as what? 1. 2. 3. 4.
1. 2. 3. 4.
Flip-flops Amplifiers Oscillators Multipliers
Gun fire control Data processing Ships steering Missile fire control
1-9. Compared to mechanical computers, electromechanical computers are different in which of the following ways?
1944 1946 1950 1951
1. 2. 3. 4.
1-5. The field of research that is developing computer systems which mimic human thought in a specific area and improve performance with experience and operation is what field of research? 1. 2. 3. 4.
Where it will be installed Number of operators using it Cost Number of functions it has to perform
1-8. What is the primary use of analog computers in the Navy?
1-4. When was the UNIVAC I developed? 1. 2. 3. 4.
Digital Electrical Analog Electromechanical
They cost more They are bigger They are less accurate They use electrical components to perform some of the calculations and to increase the accuracy
1-10. In early electronic computers, what was the weak link in electrical computations? 1. 2. 3. 4.
Human intelligence Artificial intelligence Animal intelligence Computer intelligence
1
Transistors Resistors Vacuum tubes Capacitors
1-17. A digital computer knows how to do its work by what means?
1-11. A computer that is designed to perform a specific operation is what kind of computer? 1. 2. 3. 4.
1. By a list of instructions called a program 2. By a list of instructions called a job sequence 3. By its hardware 4. By its peripheral equipment
All-purpose General-purpose Special-purpose Single-purpose
1-12. How are the instructions that control a computer applied to a special-purpose computer? 1. 2. 3. 4.
1-18. What is the most popular generic term for computer programs? 1. 2. 3. 4.
From a stored program From a keyboard From an input device From built-in instructions
1-19. First generation computers were characterized by what technology?
1-13. What is a drawback to the specialization of a special-purpose computer? 1. 2. 3. 4.
1. 2. 3. 4.
Low speed Lack of versatility Large size High cost
1. 2. 3. 4.
1. It can store and execute different programs in its internal storage 2. It is a much larger computer 3. It has a huge built-in program 4. It can operate faster than other computers
Machine COBOL BASIC Fortran
1-21. Computers of the second generation were characterized by what technology? 1. 2. 3. 4.
1-15. All analog computers are what type? Mechanical Electromechanical Special-purpose General-purpose
1-16. What are computers called that combine the functions of both analog and digital? 1. 2. 3. 4.
Transistors Resistors Vacuum tubes Printed circuits
1-20. What type of computer language was used with first generation computers?
1-14. What gives a general-purpose computer the ability to perform a wide variety of operations?
1. 2. 3. 4.
Hardware Software Wordprocessing Graphics
Analog-digital computers Mixed computers Duplexed computers Hybrid computers
2
Vacuum tubes Capacitors Transistors Resistors
1-27. Fourth generation technology has which of the following results for the computer industry?
1-22. The small, long lasting transistors used in second generation computers had which of the following effects?
1. Computers that are significantly smaller and lower in cost 2. Computers that are significantly larger and lower in cost 3. Computers that are significantly smaller and higher in cost 4. Computers that are significantly larger and higher in cost
1. They increased processing speeds and reliability 2. They decreased processing speeds and increased reliability 3. They increased processing speeds and decreased reliability 4. They decreased processing speeds and reliability
1-28. What does the acronym ROM stand for?
1-23. Internal processing speeds of second generation computers were measured at what speed? 1. 2. 3. 4.
1. 2. 3. 4.
Hundredths of a second Thousandths of a second Millionths of a second Billionths of a second
1-29. Which of the following will be one of the future challenges involving computer power?
1-24. Third generation computers are characterized by what technology? 1. 2. 3. 4.
1. How to properly and effectively use the computing power available 2. How to increase computer storage capacity 3. How to increase computer power 4. How to properly install the computers available
Capacitors Transistors Resistors Miniaturized circuits
1-25. A circuit and its components can be etched onto which of the following materials? 1. 2. 3. 4.
1-30. What term is used for programs such as assemblers, compilers, operating systems, and applications programs?
Plastic Silicon Gold Pressed fiber
1. 2. 3. 4.
1-26. The internal processing speeds of third generation computers are measured at what speed? 1. 2. 3. 4.
Run-on manual Read-only minutes Read-only memory Read-only manual
Hardware Peripheral devices Software Sub-systems
1-31. Which of the following is one of the more widespread uses of the computer in the Navy?
Hundredths of a second Thousandths of a second Millionths of a second Billionths of a second
1. 2. 3. 4.
3
Research Word processing Manufacturing Games
1-37. What type of classified use does SNAP II allow?
1-32. Computers have an advantage over typewriters in what area? 1. 2. 3. 4.
1. 2. 3. 4.
Cost Speed Reliability Correcting errors
1-38. What is a central set of programs called that manages execution of other programs and performs common functions like read, write, and print?
1-33. What does the acronym S-N-A-P stand for? 1. 2. 3. 4.
Shipboard navigational aid package Shipboard Navy applied program Shipboard non-tactical ADP program Shipboard nuclear active program
1. 2. 3. 4.
1-34. Which computer is used with the SNAP II system? 1. 2. 3. 4.
Managing system Execution system Operating system Word processing system
1-39. What is the function of a built-in program called a bootstrap loader?
UYK-4 UYK-7 UYK-20 AN/UYK-62 (V)
1. To load a word processor into the computer’s internal memory 2. To load an external operating system into the computer’s internal memory 3. To load a graphics program into the computer’s internal memory 4. To load a bootstrap program into the computer’s internal memory
1-35. Where are the user terminals for SNAP II placed on board ship? 1. 2. 3. 4.
Unclassified Confidential Secret Top Secret
Engineering spaces Work centers Supply spaces Electronics spaces
1-40. When an error message such as device error is shown on the display screen, which of the following problems could be the cause?
1-36. The work center supervisor can update which of the following items from a user terminal?
1. No floppy disk in drive A 2. Floppy disk inserted incorrectly in drive 3. Lock handle on drive A not lowered 4. Each of the above
1. COSAL, APL, EIC, and CSMP only 2. APL, EIC, SHIP’S FORCE WORK LIST, and CSMP only 3. COSAL, APL, SHIP’S FORCE WORK LIST, and CSMP only 4. COSAL, APL, EIC, SHIP’S FORCE WORK LIST, and CSMP
1-41. A display similar to this A> means what in computer terminology? 1. 2. 3. 4.
4
A device error No system A prompt Run again
1-47. What maximum number of disks should be stacked horizontally?
1-42. What does it mean when the computer displays a prompt on the screen?
1. 5 2. 10 3. 15 4. 20
1. The computer has made an error 2. There is no system in the computer 3. You need to stop putting information into the computer 4. You can tell the computer what to do next
1-48. What is perhaps the most common source of a magnetic field that can affect a floppy disk?
1-43. To tell the operating system what program to run, you should take which of the following actions following the operating system prompt A>? 1. 2. 3. 4.
1. 2. 3. 4.
Type help Reboot the computer Press the execute key Type the program name
1-49. In which of the following ways does smoke affect a computer? 1. 2. 3. 4.
1-44. Online HELP screens serve what purpose? 1. Display the contents of memory 2. Display the operating system directory 3. Tell the operator how to perform a given function 4. Stop computer processing so the operator can read the instruction manual
It damages the electronics It causes the monitor to fail It coats the keyboard It causes buildup on disks and disk drives
1-50. What, if anything, can happen to a floppy disk when it is exposed to direct sunlight or excessive heat? 1. It can become warped or distorted so it cannot be used 2. It can become sticky, which stops the drive 3. It can lose part of the data recorded on it 4. Nothing, it is not affected
1-45. Floppy disks provide which of the following functions? 1. Store data 2. Perform arithmetic operations 3. Provide alternate power to the computer 4. Check the accuracy of computer operations
1-51. Typically, floppy disks will operate only in what temperature range? 1. 2. 3. 4.
1-46. Touching the exposed area seen through the timing hole and the read/write slots on a floppy disk can do what, if anything, to the data in that area? 1. 2. 3. 4.
Crt’s Printer Telephone Disk drives
Ruin it Add to it Move it Nothing
5
40 to 120 degrees Fahrenheit 50 to 120 degrees Fahrenheit 60 to 120 degrees Fahrenheit 70 to 120 degrees Fahrenheit
1-55. What two media are commonly used for backup?
1-52. A floppy disk will accept what relative humidity range?
1. 2. 3. 4.
1. 5% to 60% 2. 10% to 70% 3. 10% to 80% 4. 10% to 90%
1-56. What is the most common method of creating a backup for a microcomputer?
1-53. When a pencil or ballpoint pen is used to write on the label after it is attached to the disk, what, if anything, can happen to a disk?
1. Copying the disk onto a magnetic tape 2. Copying the disk onto a paper tape 3. Copying the disk onto a punched card 4. Copying the disk onto another disk
1. Some of the data written on the label can be added to the disk 2. All of the data can be lost, but the disk can be used again 3. The disk can be destroyed 4. Nothing; there can be no effect 1-54. In the computer world, what method provides a means to ensure that any data lost can be recovered? 1. 2. 3. 4.
Paper tape and punched cards Magnetic tape and punched cards Disk and magnetic tape Disk and drum
Records Backup files Tracks Blocks
6
ASSIGNMENT 2 Textbook assignment: Chapter 2, “Hardware,” pages 2-1 through 2-35. ___________________________________________________________________________________ 2-6. What part of the computer dictates how and when each specific operation is to be performed?
2-1. The components or tools of a computer system can be grouped into what two categories? 1. 2. 3. 4.
1. 2. 3. 4.
Hardware and software Hardware and firmware Firmware and software Software and programs
2-7. Of the four major types of instructions, which one has the basic function of moving data from one location to another?
2-2. What section/unit is the brain of a computer system? 1. 2. 3. 4.
Control section Arithmetic-logic section Central processing unit Input unit
1. 2. 3. 4.
2-3. What section/unit is the computing center of a computer system? 1. 2. 3. 4.
Control Logic Arithmetic Transfer
2-8. To send commands to devices not under direct command of the control section, what type of instructions are used?
Arithmetic-logic section Central processing unit Control section Output unit
1. 2. 3. 4.
2-4. The central processing unit is made up of which of the following sections?
Control Logic Arithmetic Transfer
2-9. Operations like adding and multiplying are performed by what section?
1. Control and internal storage only 2. Central and arithmetic-logic only 3. Arithmetic-logic and internal storage only 4. Control, internal storage, and arithmetic-logic
1. 2. 3. 4.
2-5. When a program is so large and complex that it exceeds the memory capacity of a stored-program computer, where is the overflow stored? 1. 2. 3. 4.
Control section Arithmetic-logic section Input storage area Output storage area
Control-logic Storage-logic Arithmetic-logic Transfer-logic
2-10. When processing is taking place, data is transferred back and forth between what two sections? 1. 2. 3. 4.
Input storage area Output storage area Primary memory Auxiliary memory
7
Control and internal storage Internal storage and arithmetic-logic Control and arithmetic Arithmetic and output
2-16. Electronic circuits are placed on a silicon chip by what method?
2-11. The process by which instructions and data are read into a computer is called what? 1. 2. 3. 4.
1. 2. 3. 4.
Moving Storing Inputting Loading
2-17. Each of the individual electronic circuits on a silicon chip is called what?
2-12. An auxiliary (wired) memory is used in some computers to permanently store a small program that makes manual loading unnecessary. What is this program called? 1. 2. 3. 4.
1. 2. 3. 4.
Operating system Bootstrap Word processing Graphics
1. 2. 3. 4.
1. 2. 3. 4.
Binary Decimal Octal Hexadecimal
Bubble Magnetic core Semiconductor Capacitive
2-20. In bubble memory, where is the control circuit imprinted on the crystal of semiconductor material? 1. 2. 3. 4.
2-15. The state of each core in magnetic core storage is changed by what? 1. 2. 3. 4.
It is too slow It is expensive It is volatile It is unreliable
2-19. Using a very thin crystal made of semiconductor material, what type of memory can be created?
Aluminum Steel Tin Ferrite
2-14. Data is stored in computers in what form? 1. 2. 3. 4.
A memory cell A bit cell A byte cell A holding cell
2-18. Semiconductor storage has which of the following drawbacks?
2-13. The tiny doughnut-shaped rings used to make up magnetic core storage are made of what material? 1. 2. 3. 4.
Wired Drawn Etched Printed
The amount of magnetism The amount of current The direction of magnetism The direction of current
The side The bottom The middle The top
2-21. Who installs the programs in read-only memory? 1. 2. 3. 4.
8
The programmer The manufacturer The operator The dealer
2-28. Data is stored on all disks in a number of invisible concentric circles called what?
2-22. Programs that are tailored to certain needs and permanently installed in ROM by the manufacturer are called what? 1. 2. 3. 4.
1. 2. 3. 4.
Firmware Software Hardware Diskware
2-29. A floppy disk surface has what maximum number of tracks?
2-23. What kind of memory used inside computers has a read/write capability without any additional special equipment? 1. 2. 3. 4.
1. 2. 3. 4.
ROM RAM EPROM PROM
66 77 88 99
2-30. When data is written on a disk in the same area where data is already stored, the old data is affected in which of the following ways, if at all?
2-24. A special device is needed to burn the program into what type of memory? 1. 2. 3. 4.
Cracks Grooves Paths Tracks
1. 2. 3. 4.
ROM PROM ERAM RAM
It is moved to a new area It is mixed with the new data It is replaced It is not affected
2-31. How are records on a track separated? 2-25. EPROM can be erased by what method? 1. 2. 3. 4.
1. By a gap in which no data is recorded 2. By a gap in which the name of the record is recorded 3. By a gap in which the record is numbered 4. By a gap in which the operator's name is placed
With a current charge With a voltage change With a burst of ultra-violet light With a special program
2-26. To coat magnetic disks, what magnetizable recording material is used? 1. 2. 3. 4.
2-32. To increase the amount of data we can store on one track, what technique can be used?
Plastic Mylar® Aluminum oxide Iron oxide
1. 2. 3. 4.
2-27. What is the size range of the diameters of magnetic disks? 1. 2. 3. 4.
3 inches to 4 feet 4 inches to 3 feet 5 inches to 6 feet 6 inches to 5 feet
9
Records Files Disk address Blocking
2-38. By which of the following methods are magnetic tape units categorized?
2-33. Designers were able to increase the data density of a disk by increasing the number of tracks. What code name was given to this technology? 1. 2. 3. 4.
1. 2. 3. 4.
Computer Winchester Solid state Colt
2-39. What determines if a standard 1/2-inch tape will have either seven or nine tracks of data?
2-34. During reading and writing, which of the following changes are achieved by reducing the distance of the read/write heads over the disk surface?
1. The brand of tape 2. The read/write heads installed in the tape unit 3. The type of computer used 4. The speed at which the tape unit is run
1. Data density can be improved and storage capacity decreased 2. Data density is lessened and storage capacity increased 3. Data density can be improved and storage capacity increased 4. Data density is lessened and storage capacity decreased
2-40. For multitrack tapes, what is the range of common recording densities in bits/bytes per inch (bpi)? 1. 2. 3. 4.
2-35. To physically organize data on diskettes, what method is used? 1. 2. 3. 4.
Records Cylinder Files Sector
1. By the thickness of the tape and the capacity of internal storage 2. By the length of the tape and the speed of internal storage 3. By the width of the tape and the speed of internal storage 4. By the length of the tape and the capacity of internal storage
From 400 to 1,000 feet From 500 to 2,000 feet From 600 to 3,000 feet From 700 to 4,000 feet
2-37. Magnetic tapes can be packaged in which of the following ways? 1. 2. 3. 4.
From 200 to 6,250 bpi From 300 to 6,275 bpi From 400 to 6,300 bpi From 500 to 6,350 bpi
2-41. On magnetic tape, the size of a record that holds the data is restricted in what two ways?
2-36. The lengths of magnetic tapes used with computers have what range? 1. 2. 3. 4.
Type of packaging used for tape Size of tape Speed of tape Cost of tape
2-42. In computer terminology, what is called a file?
Open reel only Cartridge and cassette only Open reel and cartridge only Open reel, cartridge, and cassette
1. 2. 3. 4.
10
A collection of tapes A collection of disks A collection of records A collection of characters
2-48. What is the storage capacity range of magnetic drums in characters or bytes of data?
2-43. In order for data to be read from or written on a magnetic tape, the tape must do what? 1. 2. 3. 4.
1. From 20 million to more than 150,000 million 2. From 30 million to more than 150,000 million 3. From 40 million to more than 200,000 million 4. From 50 million to more than 200,000 million
Speed up Move at a predetermined speed Slow down Stop
2-44. Storing single records on a magnetic tape has which of the following disadvantages?
2-49. Input data may be in any one of how many forms?
1. It takes too long to record the data 2. It takes too long to recover the data 3. Too much of the recording surface is wasted 4. Too much of the recording surface is used
1. 2. 3. 4.
2-45. The magnetic drum is another example of what type of access storage device? 1. 2. 3. 4.
2-50. When data is input from a keyboard, a high average speed is how many characters per second?
Random Direct Multiple Single
1. 2. 3. 4.
2-46. What is the speed range of a magnetic drum? 1. 2. 3. 4.
Five Two Three Four
One to two Two to three Three to four Four to five
2-51. Output information is made available in how many forms?
300 to 3,000 rpm 400 to 4,000 rpm 500 to 5,000 rpm 600 to 6,000 rpm
1. 2. 3. 4.
2-47. When using a magnetic drum, what is rotational delay?
One Two Three Four
2-52. Magnetic tape stores data in what manner?
1. Time that occurs in coming up to speed 2. Time that occurs in slowing down 3. Time that occurs in reaching a desired record location 4. Time that occurs in changing a drum
1. 2. 3. 4.
11
Sequential Non-sequential Direct Random
2-58. The distance between the read/write head and the surface of a hard disk is called what?
2-53. The magnetic tape unit reads and writes data in channels or tracks along the length of the tape. How are these tracks referenced to each other? 1. 2. 3. 4.
1. 2. 3. 4.
Perpendicular Parallel Vertical Random
2-59. Floppy disks come in several sizes with diameters of what size range?
2-54. How does a two gap head allow for increased speed? 1. 2. 3. 4.
1. 2. 3. 4.
By checking before writing By using two gaps to write By checking while writing By using two gaps to check
500 and 1,000 bpi 600 and 1,200 bpi 700 and 1,500 bpi 800 and 1,600 bpi
1. 2. 3. 4.
2-56. The drive motor of a disk drive unit rotates the disk at a constant speed, normally how many revolutions per minute? 1. 2. 3. 4.
Dot-matrix Ink jet Daisy-wheel Laser
2-61. What is another name for the dot-matrix printer? 1. 2. 3. 4.
2,000 rpm 2,500 rpm 3,000 rpm 3,600 rpm
Hammer-matrix Pin-matrix Ink-matrix Wire-matrix
2-62. Dot-matrix printers have which of the following ranges of speeds in characters per second?
2-57. The usual range of rotational speed for floppy disks is what? 1. 2. 3. 4.
2 to 6 inches 3 to 8 inches 4 to 9 inches 5 to 10 inches
2-60. In the character-at-a-time impact printer class, which printer has the most professional-looking, pleasing-to-the-eye print?
2-55. What are the most common tape densities in bits/bytes per inch? 1. 2. 3. 4.
The flying height The disk height The head height The recording height
1. 2. 3. 4.
100 to 200 rpm 200 to 300 rpm 300 to 400 rpm 400 to 500 rpm
12
50 to 200 cps 60 to 350 cps 70 to 400 cps 80 to 450 cps
2-68. Each field of a raster scan crt is made up of approximately how many lines?
2-63. Ink jet printers have what maximum speed in characters per seconds? 1. 2. 3. 4.
1. 2. 3. 4.
300 cps 400 cps 500 cps 600 cps
2-69. In a video monitor, what do the frequency bandwidth, the number of characters to be displayed on a line, and the physical size of the screen determine?
2-64. Laser printers can print up to approximately what total number of characters per second? 1. 2. 3. 4.
20,666 cps 22,666 cps 24,666 cps 26,666 cps
1. 2. 3. 4.
2-65. What are the two styles of typewriter keyboard arrangements used with a computer? 1. 2. 3. 4.
Actual level of brightness Actual number of picture elements Actual speed of scan rate Actual number of vertical lines that can be displayed
2-70. A monitor that uses 1,000 picture elements per line with a horizontal resolution of 1,000 can display what total number of vertical lines?
QWERYT or DVORAK QWERTY or DVORAK ABCDEF or DVORAK ABCDEF or DVOARK
1. 10 2. 100 3. 1,000 4. 10,000
2-66. When working with display devices, what does the term soft-copy mean?
2-71. A raster frame is displayed approximately how many times a second?
1. The information displayed is not permanent 2. The information displayed is permanent 3. The information displayed has a soft glow 4. The information displayed has no glow
1. 5 2. 10 3. 20 4. 30 2-72. To reduce the depth of the crt caused by the length of the tube, what type of displays were designed?
2-67. On a raster scan crt, a raster is a series of what type of lines across the face of a crt? 1. 2. 3. 4.
525 550 575 600
1. 2. 3. 4.
Diagonal Vertical Horizontal Wavy
13
Wide panel Flat panel Narrow panel Short panel
2-74. The operation of an electroluminescent display requires what total number of volts?
2-73. Compared to the gas plasma and electroluminescent displays, a liquid crystal display differs in which of the following ways?
1. 5 2. 10 3. 15 4. 20
1. It does not use as many picture elements 2. It does not use a light for the picture elements 3. It does not generate its own light for the picture elements 4. It does not have a backlight
14
ASSIGNMENT 3 Textbook assignment: Chapter 3, “Software,” pages 3-1 through 3-29. ___________________________________________________________________________________ 3-5. To overcome the applications software compatibility problem, which of the following is done so the application can be run under several different operating systems?
3-1. What must you load into a computer to manage its resources and operations? 1. 2. 3. 4.
Bootstrap program Word processor Graphics program Operating system
1. Some software comes in several versions 2. Computers are designed to accept all applications software 3. Software comes in a universal version 4. Operating systems are changed to be compatible
3-2. What program controls the execution of other programs according to job information? 1. 2. 3. 4.
An operating system A bootstrap program A word processor A utility program
3-6. What is another term for "initial program load" the system?
3-3. The simplest and most commonly used operating systems on microcomputers are which of the following types? 1. 2. 3. 4.
1. 2. 3. 4.
Multiuser/single tasking Single user/single tasking Single user/multitasking Multiuser/multitasking
3-7. When the symbol A> is on the screen of a computer crt, it tells the operator/user which of the following information?
3-4. Which of the following programs must be compatible with the operating system in use? 1. 2. 3. 4.
Start Boot Kick Run
1. The system is not ready, and drive A is busy 2. The system is ready, and drive A is assigned as your secondary drive 3. The system is ready, and drive A is assigned as your primary drive 4. The system is activating, and no drive is available
CP/M-86 UNIX Applications MS-DOS
3-8. The three characters following each directory entry are called what?
THIS SPACE LEFT BLANK INTENTIONALLY.
1. 2. 3. 4.
15
Files Records Locators Extensions
3-14. Sort-merge programs usually have which of the following characteristics?
3-9. Commands built into the operating system that control actions, like diskcopy and rename, are what type of commands? 1. 2. 3. 4.
1. 2. 3. 4.
Independent Copy Spread Utility
3-15. What personnel or methods are used to generate programs to print detail and summary reports of data files?
3-10. To eliminate the need for programmers to write new programs when all they want to do is copy, print, or sort a data file, which of the following types of programs can be used? 1. 2. 3. 4.
1. 2. 3. 4.
Word processor Graphics Utility Spreadsheet
1. 2. 3. 4.
Sorting Merging Writing Shifting
1. 2. 3. 4.
Numerical sequence Collating sequence Random sequence Alphabetic sequence
Input information Output information Summary information Programming information
3-18. A computer language that is a string of numbers which represents the instruction code and operand addresses is what type?
3-13. To sort a data file, what must you tell the sort program? 1. 2. 3. 4.
Run time Programming time Operator time Printer time
3-17. Each time there is a control break, what does the program developed by the report program generator print?
3-12. On a computer, what is the sequence of characters called? 1. 2. 3. 4.
Programmers Operating systems Sort-merge programs Report program generators
3-16. What are report program generators designed to save?
3-11. What is the term given to arranging records in a predefined sequence or order? 1. 2. 3. 4.
Specific file length Specific run time Phases Names
1. 2. 3. 4.
How many characters are in the file How many records are in the file The length of the data file The data field or fields to sort on
Machine Printed Symbolic Procedure-oriented
3-19. Mnemonic instruction codes and symbolic addresses were developed early in what decade? 1. 2. 3. 4.
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1940s 1950s 1960s 1970s
3-25. What are the two most familiar of the procedure-oriented languages used for scientific or mathematical problems?
3-20. Compared to machine language coding, symbolic languages have which of the following advantages?
1. 2. 3. 4.
1. Detail is reduced 2. Fewer errors are made 3. Less time is required to write a program 4. All of the above
3-26. Compared with programs written in symbolic languages, programs written in procedure-oriented languages differ in which of the following ways?
3-21. An instruction that allows the programmer to write a single instruction which is equivalent to a specified sequence of machine instructions is what type of instruction? 1. 2. 3. 4.
1. They can only be used with small computers 2. They can only be used with large computers 3. They can only be used with the computer for which the program was written 4. They can be used with a number of different computer makes and models
Machine language instruction Graphic language instruction Macroinstruction Scientific instruction
3-22. What does the acronym COBOL stand for? 1. Computer ordered byte oriented language 2. Computer ordered business oriented language 3. Common business oriented language 4. Common business ordered language
3-27. Compared with symbolic languages, procedure-oriented languages have which of the following disadvantages? 1. They require more space in memory, and they process data at a slower rate 2. They require more space in memory, and they process data too fast for some printers 3. They require a special memory, and they process data at a slower rate 4. They require a special memory, and they process data too fast for some printers
3-23. PASCAL is being used by many colleges and universities to teach programming for which of the following reasons? 1. It is fairly easy to learn and more powerful than BASIC 2. It is hard to learn and weaker than BASIC 3. It is easy to learn and cheaper than BASIC 4. It is a shorter course and produces better programmers
3-28. Which of the following is a simple definition of programming? 1. The process of planning which computer system to use 2. The process of planning the computer solution to a problem 3. The process of planning the mathematical solution to a problem 4. The process of planning which computer program to use
3-24. The development of Ada was initiated by what organization? 1. 2. 3. 4.
PASCAL and FORTRAN PASCAL and COBOL COBOL and FORTRAN BASIC and FORTRAN
U. S. Navy U. S. Army U. S. Department of Defense U. S. Department of Transportation
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3-34. The method of pictorially representing a step-by-step solution to a problem before computer instructions are written to produce the desired results is called what?
3-29. Which of the following is NOT a basic characteristic of a computer? 1. It needs commands 2. It needs specifically defined operations 3. It can think 4. It can understand instructions only in an acceptable form
1. 2. 3. 4.
3-30. How many fundamental and discrete steps are involved in solving a problem on a computer? 1. 2. 3. 4.
3-35. What two types of flowcharts are there? 1. 2. 3. 4.
Five Two Three Four
System and programming System and data Processing and programming Processing and data
3-36. What are the four basic tools used in flowcharting?
3-31. In the advance planning phase of programming, what are the first two steps?
1. Advanced symbols, graphic symbols, flowcharting template, and flowcharting worksheet 2. Fundamental symbols, graphic symbols, flowcharting template, and flowcharting worksheet 3. Fundamental symbols, mathematical symbols, flowcharting symbols, and flowcharting worksheet 4. Fundamental symbols, advanced symbols, flowcharting template, and flowcharting worksheet
1. Program coding and machine readable coding preparation 2. Problem understanding/ definition and flowcharting 3. Test data preparation and test run performance 4. Documentation completion and operator procedures preparation 3-32. Which of the following is NOT part of defining every aspect of a problem?
QUESTION 3-37 IS TO BE JUDGED TRUE OR FALSE.
1. What information (or data) is needed 2. Where and how will the information be obtained 3. What is the desired output 4. What is the computation time
3-37. Fundamental symbols are standard for the military, as directed by Department of the Navy Automated Data Systems Documentation Standards, SECNAVINST 5233.1.
3-33. Once you have a thorough understanding of the problem, what is the next step in programming? 1. 2. 3. 4.
Flowcharting Constructing Documenting Debugging
1. True 2. False
Gathering information Coding the program Flowcharting Debugging
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3-43. What is the step called in which you code a program that can be translated by a computer into a set of instructions it can execute?
3-38. Within a flowchart, what type of symbols are used to specify arithmetic operations and relational conditions? 1. 2. 3. 4.
Fundamental symbols Graphic symbols Arithmetic symbols Arabic symbols
1. 2. 3. 4.
3-39. What is the graphic symbol for less than or equal to? 1. 2. 3. 4.
QUESTION 3-44 IS TO BE JUDGED TRUE OR FALSE.
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