Navy Electricity and Electronics Training Series

Re-Distributed by http://www.tpub.com NONRESIDENT TRAINING COURSE SEPTEMBER 1998 Navy Electricity and Electronics Trai...

Author: NAVEDTRA

98 downloads 1512 Views 45MB Size Report

This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Report copyright / DMCA form

DOWNLOAD PDF

Re-Distributed by http://www.tpub.com

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.



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

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.”

ii


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

.........................................................................................................................

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


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

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.

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.

viii


Student Comments Course Title:

NEETS Module 1 Introduction to Matter, Energy, and Direct Current

NAVEDTRA:

14173

Date:

We need some information about you: Rate/Rank and Name:

SSN:

Command/Unit

Street Address:

City:

State/FPO:

Zip

Your comments, suggestions, etc.:

Privacy Act Statement: Under authority of Title 5, USC 301, information regarding your military status is requested in processing your comments and in preparing a reply. This information will not be divulged without written authorization to anyone other than those within DOD for official use in determining performance.

NETPDTC 1550/41 (Rev 4-00)

ix



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.

1-2


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.

1-5


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.

1-6


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

1-7


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

1-8


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

1-9


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.

1-10


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.

1-11


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.

1-12


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.

1-13


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

1-14


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.

1-16


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.

1-17


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.

1-18


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.

1-19


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.

1-20


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?

1-21


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

1-22


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.

1-23


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

1-24


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.

1-25


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.

1-26


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.

1-27


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.

1-28


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.

1-29


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.

1-30


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.

1-32


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.

1-33


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).

1-35


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.

1-37


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

1-38


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?

1-39


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.

1-40


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

1-41


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

1-42


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.

1-43


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

1-45


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?

1-46


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).

1-47


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.

1-49


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.

1-50


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.

1-51


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.

1-52


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.

1-53


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.

1-54


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)

1-55


3. Heat (thermoelectricity)

1-56


4. Light (photoelectricity)

5. Chemical action (battery)

1-57


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.

1-58


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.

1-59


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.

1-60


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.

1-61


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)

1-62


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.

1-64


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.

1-65



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.

2-1


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.

2-3


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.

2-4


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.

2-5


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.

2-6


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.

2-7


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?

2-8


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.

2-9


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.

2-10


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.

2-13


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

2-14


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 (−).

2-15


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.

2-16


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?

2-17


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.)

2-18


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

2-19


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.

2-21


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.

2-22


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.

2-23


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.

2-24


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.

2-25


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.

2-26


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.

2-27


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.

2-29



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.

3-1


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.

3-2


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.

3-3


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.

3-4


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:

3-5


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.

3-6


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.

3-7


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.

3-8


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.

3-9


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.

3-10


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

3-11


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.

3-12


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:

3-13


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:

3-14


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.

3-15


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.

3-16


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?

3-17


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

3-18


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.

3-19


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?

3-20


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).

3-21


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.

3-22


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:

3-23


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).

3-24


Figure 3-18.—Solving for applied voltage in a series circuit.

Substituting:

3-25


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.

3-65


Figure 3-48.—Example parallel circuit.

Given:

Solution:

3-66


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.

3-67


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.

3-68


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.

3-69


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:

3-70


Solution:

Since R3 = R4 = R5 and the voltage across each branch is the same:

Solving for total resistance. Given:

3-71


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.

3-72


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.

3-73


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.

3-74


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:

3-75


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.

3-76


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.

3-77


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).

3-78


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:

3-79


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:

3-80


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:

3-81


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.

3-82


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:

3-83


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.

3-86


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.

3-87


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

3-88


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

3-90


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:

3-91


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.

3-92


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.

3-94


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?

3-95


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.

3-96


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.

3-97


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:

3-99


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.

3-100


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.

3-101


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.

3-102


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:

3-105


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.

3-106


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).

3-110


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.

3-111


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.

3-112


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.

3-113


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.

3-114


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.

3-115


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.

3-116


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.

3-117


HIGH EFFICIENCY in a circuit is achieved when the resistance of the load is high with respect to the resistance of the source.

3-118


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.

3-119


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).

3-120


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).

3-121


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.

3-122


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.

3-123


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.

3-124


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.

3-125


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.

3-126


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.

AI-1


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.

AI-3


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.

AI-4


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.



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?

THIS SPACE LEFT BLANK INTENTIONALLY.

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?


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.


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


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



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?



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?



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



Navy Electricity and Electronics Training Series Module 2—Introduction to Alternating Current and Transformers NAVEDTRA 14174






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



i



ii



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


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



iv



v



WARNING


CAUTION


NOTE


vi




assignment








•

COMPLETION TIME

•




vii





E-mail: Phone:




grading,

or

E-mail: Phone:







viii



NEETS Module 2 Introduction to Alternating Current and Transformers

NAVEDTRA:

14174

Date:


SSN:

Command/Unit

Street Address:

City:

State/FPO:

Zip



NETPDTC 1550/41 (Rev 4-00)

ix



CHAPTER 1

CONCEPTS OF ALTERNATING CURRENT LEARNING OBJECTIVES 8SRQFRPSOHWLRQRIWKLVFKDSWHU\RXZLOOEHDEOHWR 6WDWHWKHGLIIHUHQFHVEHWZHHQDFDQGGFYROWDJHDQGFXUUHQW 6WDWHWKHDGYDQWDJHVRIDFSRZHUWUDQVPLVVLRQRYHUGFSRZHUWUDQVPLVVLRQ 6WDWHWKHOHIWKDQGUXOHIRUDFRQGXFWRU 6WDWHWKHUHODWLRQVKLSEHWZHHQFXUUHQWDQGPDJQHWLVP 6WDWHWKHPHWKRGVE\ZKLFKDFSRZHUFDQEHJHQHUDWHG 6WDWHWKHUHODWLRQVKLSEHWZHHQIUHTXHQF\SHULRGWLPHDQGZDYHOHQJWK &RPSXWHSHDNWRSHDNLQVWDQWDQHRXVHIIHFWLYHDQGDYHUDJHYDOXHVRIYROWDJHDQGFXUUHQW &RPSXWHWKHSKDVHGLIIHUHQFHEHWZHHQVLQHZDYHV CONCEPTS OF ALTERNATING CURRENT $OORI\RXUVWXG\WKXVIDUKDVEHHQZLWKGLUHFWFXUUHQWGF WKDWLVFXUUHQWZKLFKGRHVQRWFKDQJH GLUHFWLRQ+RZHYHUDV\RXVDZLQPRGXOHDQGZLOOVHHODWHULQWKLVPRGXOHDFRLOURWDWLQJLQDPDJQHWLF ILHOGDFWXDOO\JHQHUDWHVDFXUUHQWZKLFKUHJXODUO\FKDQJHVGLUHFWLRQ7KLVFXUUHQWLVFDOOHG $/7(51$7,1*&855(17RUDF AC AND DC $OWHUQDWLQJFXUUHQWLVFXUUHQWZKLFKFRQVWDQWO\FKDQJHVLQDPSOLWXGHDQGZKLFKUHYHUVHVGLUHFWLRQDW UHJXODULQWHUYDOV0.05 ON L(D)—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.

2-3


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.

2-4


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.

2-5


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

2-6


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.

2-7


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?

2-8


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

2-9


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.

2-10


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.

2-11


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.

2-12


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.

2-14


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

2-15


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.

2-16


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.

2-17


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.

2-18


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.

2-19


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

2-20


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.

2-21


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

2-22


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.

2-24


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.

2-25


Figure 2-19.—MOSFET (depletion/enhancement type).

Figure 2-20.—MOSFET (enhancement type).

Figure 2-21.—MOSFET (depletion/enhancement type) equivalent circuit.

2-26


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).

2-27


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.

2-28


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.

2-29


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.

2-31


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.

2-33


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

2-34


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



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.

2-47


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.

2-48


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.

3-8


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

3-9


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

3-11


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.

3-13


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

3-14


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.

3-15


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.

3-16


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.

3-17


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.

3-19


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.

3-21


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.

3-22


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.

3-23


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?

3-25


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.

3-26


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.

3-27


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?

3-28


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:

3-29


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:

3-30


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.

3-31


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.

3-32


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?

3-33


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.

3-34


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.

3-35


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.

3-37


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.

3-39



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.

4-1


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:

4-9


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)

4-10

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

4-11


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.

4-13



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.

4-14


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.

5-30


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.


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



1


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

3


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

4


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

.

5


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?


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


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

14

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

15


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

16


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

17


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

18

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.


4.

19


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

20


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

21


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

22

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.

23

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

24


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

26



Navy Electricity and Electronics Training Series Module 23—Magnetic Recording NAVEDTRA 14195






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 Magnetic Recording 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 CTMC(SS) Milton Charles Georgo



i



ii



PAGE

1. Introduction to Magnetic Recording.........................................................................

1-1

2. Magnetic Tape ..........................................................................................................

2-1

3. Magnetic Tape Recorder Heads ...............................................................................

3-1

4. Magnetic Tape Recorder Transports ........................................................................

4-1

5. Magnetic Tape Recorder Record and Reproduce Electronics ..................................

5-1

6. Magnetic Tape Recorder Specifications...................................................................

6-1

7. Digital Magnetic Tape Recording ............................................................................

7-1

8. Magnetic Disk Recording.........................................................................................

8-1


AI-1

II. References .............................................................................................................. AII-1 INDEX

......................................................................................................................

iii

INDEX-1


CREDITS The figures listed below and included in this edition of NEETS, Module 23, Magnetic Recording, were provided by Datatape, Inc. Permission to use these illustrations is gratefully acknowledged.

SOURCE Datatape, Inc.

FIGURE 3-2, 3-3, 3-4, 5-3, 6-3, 6-4 A&B, 7-1, 7-4, 7-5, 7-6

iv



v



vi



WARNING


CAUTION


NOTE


vii




assignment








•

COMPLETION TIME

•




viii





E-mail: Phone:




grading,

or

E-mail: Phone:







ix


THIS PAGE LEFT BLANK INTENTIONALLY.

x



NEETS Module 23 Magnetic Recording

NAVEDTRA:

14195

Date:


SSN:

Command/Unit

Street Address:

City:

State/FPO:

Zip



NETPDTC 1550/41 (Rev 4-00)

xi



CHAPTER 1

INTRODUCTION TO MAGNETIC RECORDING LEARNING OBJECTIVES Learning objectives are stated at the beginning of each chapter. They serve as a preview of the information you are expected to learn in the chapter. The comprehension check questions placed within the text are based on the objectives. By successfully completing those questions and the associated NRTC, you show that you have met the objectives and have learned the information. The learning objectives for this chapter are listed below. After completing this chapter, you will be able to do the following: 1. Describe the history and purpose of magnetic recording. 2. State the prerequisites for magnetic recording. 3. Describe a magnetic recording head, how it’s constructed, and how it operates.

INTRODUCTION Have you ever wondered how a whole album of your favorite music got onto one of those little cassette tapes? Or, what about computer floppy disks; have you ever wondered how they can hold 180 or more pages of typed text? The answer to both of these questions is magnetic recording. Magnetic recording devices seldom get much attention until they fail to work. But without magnetic recording, recording your favorite television show on a video cassette recorder would be impossible, portable tape players wouldn’t exist, and you wouldn’t be able to get money from an automated bank teller machine at two o’clock in the morning. Now what about the Navy? Could it operate without magnetic recording? The answer is definitely no. Without it: • Computer programs and data would have to be stored on either paper cards or on rolls of paper tape. Both of these methods need a lot of storage space, and they take much longer to load into and out of the computer. • There wouldn’t be any movies to show or music to play on the ship’s entertainment system when the ship is at sea and is out of range for television and radio reception. • Intelligence-collection missions would be impossible since you couldn’t store the collected signals for later analysis. As you can see, magnetic recording plays a very important part both in our Navy life and in our civilian life.

1-1


HISTORY OF MAGNETIC RECORDERS In 1888, Oberlin Smith originated the idea of using permanent magnetic impressions to record sounds. Then in 1900, Vladeniar Poulsen brought Mr. Smith’s dream to reality. At the Paris Exposition, he demonstrated a Telegraphone. It was a device that recorded sounds onto a steel wire. Although everyone thought it was a great idea, they didn’t think it would succeed since you had to use an earphone to hear what was recorded. It wasn’t until 1925, when electronic amplifiers were developed, that magnetic recording started to receive the attention it deserved. The best magnetic recording is the one that produces an output signal identical to the input signal. It didn’t take long to realize that the magnetism generated during the recording process didn’t vary directly to the current which caused it. This is because there’s a step in the magnetism curve where it crosses the zero point and changes polarity. This step causes the output signal to be distorted when compared with the input signal. Figure 1-1 shows this step.

Figure 1-1.—Magnetic recording without bias voltage.

In 1907, Mr. Poulsen discovered a solution to this problem. He discovered dc bias. He found that if a fixed dc voltage were added to the input signal, it moved the input signal away from the step in the magnetism curve. This prevented the input signal from crossing the zero-point of the magnetism curve. The result is an output signal exactly like the input signal. Figure 1-2 shows this process.

1-2


Figure 1-2.—Magnetic recording with dc bias voltage.

Unfortunately, dc bias had its problems. Since only a small portion of the magnetism curve was straight enough to use, the output signal was weak compared with the natural hiss of the unmagnetized tape passing the playback head. This is commonly called poor signal-to-noise ratio (SNR). We’ll explain SNR in more detail later. From the beginning, the U.S. Naval Research Laboratories (NRL) saw great potential in magnetic recording. They were especially interested in using it to transmit telegraph signals at high speed. After electronic amplifiers were invented around 1925, W.L. Carlson and G.W. Carpenter at the NRL made the next important magnetic recording discovery. They found that adding an ac bias voltage to the input signal instead of a fixed dc bias voltage would • reproduce a stronger output signal • greatly improve the signal-to-noise ratio • greatly reduce the natural tape hiss that was so common with dc bias To make ac bias work, they used an ac frequency for the bias voltage that was well above what could be heard, and a level that placed the original input signal away from both steps in the magnetism curve. This resulted in two undistorted output signals that could be combined into one strong output. See figure 1-3.

1-3


Figure 1-3.—Magnetic recording with ac bias voltage.

Until 1935, all magnetic recording was on steel wire. Then, at the 1935 German Annual Radio Exposition in Berlin, Fritz Pfleumer demonstrated his Magnetophone. It used a cellulose acetate tape coated with soft iron powder. The Magnetophone and its "paper" tapes were used until 1947 when the 3M Company introduced the first plastic-based magnetic tape. In 1956, IBM introduced the next major contribution to magnetic recording—the hard disk drive. The disk was a 24-inch solid metal platter and stored 4.4 megabytes of information. Later, in 1963, IBM reduced the platter size and introduced a 14-inch hard disk drive. Until 1966, all hard disk drives were "fixed" drives. Their platters couldn't be removed. Then in 1966, IBM introduced the first removable-pack hard disk drive. It also used a 14-inch solid metal platter. In 1971, magnetic tape became popular again when the 3M Company introduced the first 1/4-inch magnetic tape cartridge and tape drive. In that same year, IBM invented the 8-inch floppy disk and disk drive. It used a flexible 8-inch platter of the same material as magnetic tape. Its main goal was to replace punched cards as a program-loading device. The next contribution to magnetic recording literally started the personal computer (PC) revolution. In 1980, a little-known company named Seagate Technology invented the 5-1/4-inch floppy disk drive. Without it, PCs as we know them today would not exist. From then on, it was all downhill. Magnetic tape became more sophisticated. Floppy disks and disk drives became smaller, while their capacities grew bigger. And hard disk capacities just went through the roof. All of the major hurdles affecting magnetic recording had been successfully cleared, and it was just a matter of refining both its methods and materials. Q-1. Why did the early inventors of magnetic recording find it necessary to add a fixed dc bias to the input signal? Q-2. How does dc bias added to the input signal correct the distortion in the output signal? 1-4


Q-3. Why does adding dc vice ac bias voltage to the input signal result in a poor signal-to-noise ratio (SNR)? Q-4. What are three advantages of adding an ac bias voltage to the input signal instead of adding a fixed dc bias voltage? Q-5. Why does using ac vice dc bias voltage result in a stronger output signal? PREREQUISITES FOR MAGNETIC RECORDING To perform magnetic recording, you need three things: 1. An input signal you wish to record. 2. A recording medium. (This is a recording surface that will hold the signal you wish to record.) 3. A magnetic head to convert the input signal into a magnetic field so it can be recorded. If any one of these are missing, magnetic recording cannot take place. Input Signal An input signal can come from a microphone, a radio receiver, or any other source that’s capable of producing a recordable signal. Some input signals can be recorded immediately, but some must be processed first. This processing is needed when an input signal is weak, or is out of the frequency response range of the recorder. Recording Medium A recording medium is any material that has the ability to become magnetized, in varying amounts, in small sections along its entire length. Some examples of this are magnetic tape and magnetic disks. These are thoroughly discussed in chapter 2 of this module. Magnetic Heads Magnetic heads are the heart of the magnetic recording process. They are the transducers that convert the electrical variations of your input signal into the magnetic variations that are stored on a recording medium. Without them, magnetic recording isn’t possible. Magnetic heads actually do three different things. They transfer, or record, the signal information onto the recording medium. They recover, or reproduce, the signal information from the recording medium. And they remove, or erase, the signal information from the recording medium. MAGNETIC HEAD CONSTRUCTION.—A magnetic head is a magnetic core wrapped with a coil of very thin wire (see figure 1-4). The core material is usually shaped like the letter C, and is made from either iron or ceramic-ferrite material. The number of turns of wire placed on the core depends on the purpose of that specific head. The gap in the core is called a head gap. It's here that magnetic recording actually takes place. We'll go into more detail of magnetic head construction in chapter 3.

1-5


Figure 1-4.—Magnetic field distribution around the head gap.

MAGNETIC HEAD OPERATION.—Whether you're recording on magnetic tapes or disks, all magnetic heads operate the same way. When an electric current passes through the coil of a magnetic head, magnetic field lines associated with the electric current follow paths through the core material. When the magnetic fields get to the head gap, some of them spread outside the core to form a fringing field. When a recording medium is passed through this fringing field, it is magnetized in relation to the electric current. This is called magnetic recording. Figure 1-4 illustrates this process. Q-6. What three things are required to perform magnetic recording? Q-7. What is the meaning of the term recording medium as it pertains to magnetic recording? Q-8. What are the three functions of the magnetic heads on a magnetic recording device?

SUMMARY This chapter briefly covered the historical development of magnetic recording principles and devices. The following is a summary of important points in the chapter. The BEST MAGNETIC RECORDING is one that produces an output signal that is identical to the input signal. However, a step in the magnetic curve causes the output signal to be distorted. In 1907, DC BIAS was added to the input signal to remove the distortion in the output signal. But the dc bias caused a weak output signal with a poor SNR. Around 1925, the NRL used AC BIAS to reproduce a stronger output signal and greatly improve the SNR. To perform magnetic recording, you need (1) an INPUT SIGNAL, (2) a RECORDING MEDIUM, and (3) a MAGNETIC HEAD. A RECORDING MEDIUM is any material that can become magnetized in varying amounts (such as magnetic tape and disks). MAGNETIC HEADS are used to (1) record the signal onto the recording medium, (2) reproduce the signal from the recording medium, and (3) erase the signal from the recording medium.

1-6


ANSWERS TO QUESTIONS Q1. THROUGH Q8. A-1. Because a step in the magnetism curve where it crosses the zero point and changes polarity causes the output signal to be distorted. See figure 1-1. A-2. The dc bias moves the input signal away from the step in the magnetism curve. This prevents the input signal from crossing the zero-point of the magnetism curve. See figure 1-2. A-3. With dc bias, the SNR is poor because only a small portion of the magnetism curve is straight enough to use, thus the output signal is weak compared with the natural tape hiss. A-4. a. Reproduces a stronger output signal. b. Greatly improves the SNR. c. Greatly reduces the natural tape hiss. A-5. Because an ac bias voltage of the proper frequency and level places the input signal away from both steps in the magnetism curve. The result is two undistorted output signals that are combined into one strong output. A-6. a. An input signal. b. A recording medium. c. A magnetic head. A-7. A recording medium is any material that has the ability to become magnetized, in varying amounts, in small sections along its entire length. A-8. a. Record the signal onto the recording medium. b. Reproduce the signal from the recording medium. c. Erase the signal from the recording medium.

1-7



CHAPTER 2

MAGNETIC TAPE LEARNING OBJECTIVES After completing this chapter, you’ll be able to do the following: 1. Describe the physical properties of magnetic tape in terms of: a. The Three Basic Materials Used To Make Magnetic Tape. 2. The function of the magnetic tape’s base material, oxide coating, and binder glue. 3. Describe the two types of magnetic recording tape. 4. Describe the following types of tape errors and their effects on magnetic tape recording: signal dropout, noise, skew, and level. 5. Describe the following causes of magnetic tape failure: normal wear, accidental damage, environmental damage, and winding errors. 6. Describe the purpose and makeup of tape reels and tape cartridges. 7. Describe the two methods for erasing magnetic tape, the characteristics of automatic and manual tape degaussers, and the procedures for degaussing magnetic tape. 8. Describe the proper procedures for handling, storing, and packaging magnetic tape, tape reels, and tape cartridges.

PHYSICAL PROPERTIES OF MAGNETIC TAPE The three basic materials used to make magnetic tape are (1) the base material, (2) the coating of magnetic oxide particles, and (3) the glue to bind the oxide particles onto the base material. See figure 2-1.

2-1


Figure 2-1.—Magnetic tape construction.

BASE MATERIAL The base material for magnetic tape is made of either plastic or metal. Plastic tape is used more than metal tape because it’s very flexible, it resists mildew and fungus, and it’s very stable at high temperatures and humidity. OXIDE COATING Oxide particles that can be magnetized are coated onto the base material. The most common magnetic particles used are either gamma ferric oxide or chromium dioxide. It’s very important that these magnetic particles are uniform in size. If they’re not, the tape’s surface will be abrasive and will reduce the life of the recorder’s magnetic heads. An ideal magnetic particle is needle-shaped. It’s actual size depends on the frequency of the signal to be recorded. Generally, long particles are used to record long wavelength signals (low-frequency signals), and short particles are used to record short wavelength signals (high-frequency signals). GLUE The glue used to bond the oxide particles to the base material is usually an organic resin. It must be strong enough to hold the oxide particles to the base material, yet be flexible enough not to peel or crack.

TYPES OF MAGNETIC RECORDING TAPE There are two basic types of magnetic recording tape in common use: analog and digital. Analog magnetic tape is used to record, store, and reproduce audio and instrumentation type signals. These signals are usually in a frequency band from very-low frequency (VLF) to 2.5 MHz. Digital magnetic tape is used to record, store, and reproduce computer programs and data. It’s base material thickness is about 50 percent thicker than analog magnetic tape. This allows the digital tape to withstand the more strenuous starts and stops associated with digital magnetic recorder search, read, and write functions.

2-2


Digital magnetic tape is also held to much stricter quality control standards. It’s important not to have any blemishes or coating flaws on the tape’s surface. Because, if you lost one digital data bit, your computer program or data would be bad. In contrast, losing one microsecond of an analog signal is not nearly as critical. Q-1. Magnetic tape is made of what three basic materials? Q-2. Why is plastic magnetic tape used more than metal tape? Q-3. Which of the two types of magnetic tape is used to record audio and instrumentation type signals in the VLF to 2.5MHz frequency range? Q-4. What type of magnetic tape is used to record computer programs and data, and what are the additional thickness and quality standards for this type of tape?

TAPE ERRORS AND THEIR EFFECTS Four types of tape errors that will degrade the performance of a magnetic recording system are signal dropout, noise, skew, and level (signal amplitude changes). DROPOUT ERRORS Signal dropout is the most common and the most serious type of tape error. It’s a temporary, sharp drop (50% or more) in signal strength caused by either contaminates on the magnetic tape or by missing oxide coating on part of the tape. During recording and playback, the oxide particles on the tape can flake off and stick to the recorder’s guides, rollers, and heads. After collecting for awhile, the oxide deposits (now oxide lumps) break loose and stick to the magnetic tape. As the tape with the lumps passes over the head, the lumps get between the tape and the head and lift the tape away from the head. This causes the signal dropouts. Although oxide lumps cause most signal dropouts, remember that any contaminate (such as dust, lint or oil) that gets between the tape and the head can cause signal dropouts. NOISE ERRORS Noise errors are unwanted signals that appear when no signal should appear. They’re usually caused by a cut or a scratch on the magnetic tape. It’s the lack of oxide particles at the cut or the scratch that causes the noise error. SKEW ERRORS Skew errors only occur on multi-track magnetic tape recorders. The term skew describes the time differences that occur between individual tracks of a single magnetic head when the multi-track tape isn’t properly aligned with the magnetic head. There are two types of skew errors: fixed and dynamic. Fixed skew happens when properly aligned magnetic tape passes an improperly aligned magnetic head. Dynamic skew happens when misaligned tape passes a properly aligned head. This type of skew is usually caused by one or more of the following: • A misaligned or worn-out tape transport system. • A stretched or warped magnetic tape.

2-3


• A magnetic tape that is improperly wound on a reel. LEVEL ERRORS Magnetic tape is manufactured to have a specified output signal level (plus or minus some degree of error). Level errors happen when the actual output signal level either drops or rises to a level outside the expected range. For example, if a magnetic tape is rated for 10 volts ( +/−10%), any output signal level below 9 volts or above 11 volts is a level error. Level errors are caused by an uneven oxide coating on the magnetic tape. This can come from either the original manufacturing process or from normal wear and tear. Some causes of level errors are permanent and cannot be removed by any means. For example, a crease in the tape, a hole in the oxide, or a damaged edge. Other causes of level errors are removable and may be cleaned off the tape. For example, oxide flakes or clumps, metallic particles, or dirt are removable. Q-5. What are four types of tape errors that can degrade a magnetic recording system’s performance? Q-6. What are signal dropouts, and what are two tape defects that can cause signal dropouts? Q-7. What is the most common and most serious type of signal dropout? Q-8. You see a build-up of dust and lint on the take-up reel of a tape recorder. This can cause which of the four types of tape errors? Q-9. What type of tape error causes noise to appear on the tape when no signal should appear? What causes this type of tape error? Q-10. The multi-track tape recorder in your computer system has a fixed skew error. What does this mean and what is the probable cause? Q-11. Some tapes you are using may have level errors. What does this mean and what is the cause?

CAUSES OF MAGNETIC TAPE FAILURE Tape failure happens when a magnetic tape’s performance degrades to a point where it’s no longer usable. The exact point where failure occurs will vary, depending on the type of tape and how it is used. There are four main causes for tape failure: 1. Normal wear (natural causes) 2. Accidental damage 3. Environmental damage 4. Winding errors NORMAL WEAR Normal wear occurs because the tape must come in contact with fixed surfaces, such as a recorder’s magnetic heads, rollers, and guides. Over time, this repeated contact with the fixed surfaces causes excessive dropout errors and makes the tape unusable. 2-4


ACCIDENTAL DAMAGE Accidental tape damage that causes tape failure is any damage that wouldn’t normally occur under ideal operating and handling conditions. It can be caused by either a human operator or the tape recorder itself. Accidental tape damage caused by human operators can range from accidentally dropping a reel of magnetic tape to improperly threading a magnetic tape recorder. Accidental tape damage caused by recording equipment can occur if the recorder is poorly designed or if the tape transport mechanism is adjusted improperly. ENVIRONMENTAL DAMAGE The negative effect of environmental extremes on tape can also cause tape failure. Magnetic tape is very flexible and can be used in a wide range of environmental conditions. It’s designed for use in a temperature range of about 2 to 130 degrees Fahrenheit (−20 to 55 degrees Celsius), and in a relative humidity range of about 10 to 95%. Of course, these numbers are the extreme. Ideally, magnetic tape should be used and stored at a temperature of about 60 to 80º F (room temperature), and in a relative humidity of about 40 to 60%. Large changes from the ideal relative humidity cause tape to expand or contract and thus affect the uniformity of a tape's oxide coating. High relative humidity causes the tape to stretch and increases the tape's friction. The increased friction causes increased head wear, head clog by oxide particles, and head-to-tape sticking. Low relative humidity encourages oxide shedding and increases static build-up on tape surfaces, causing the tape to collect airborne contaminants. The effects of exceeding the ideal temperature and humidity ranges described above can cause the following environmental damage to magnetic tape: tape deformation, oxide shedding, head-to-tape sticking, layer-to-layer sticking, dirt build-up, and excessive tape and head wear. Tape Deformation Magnetic tapes are wound onto tape reels with tension applied. This tension causes great layer-tolayer pressure within the reel pack. Changes in temperature and humidity can cause the backing material to expand or contract, creating even more pressure. All of this pressure causes the tape to become deformed or warped. Oxide Shedding At temperatures above 130º F, a tape's oxide coating tends to become soft. At temperatures below 2º F, the oxide coating tends to be brittle. In both cases, the oxide coating will shed, flake off, or otherwise become separated from the base material. These free pieces of oxide will then stick to parts of the tape transport, to the magnetic heads, or back onto the tape and cause dropout or level errors. Head-to-Tape Sticking At higher temperatures, the tape binder glue can soften to the point where it will stick to the recorder's magnetic head. This head-to-tape sticking causes jerky tape motion. Layer-to-Layer Adhesion When reels of magnetic tape are stored at higher temperatures, the tape's binder glue may melt and cause the layers of tape to stick to one another. In very severe cases, layer-to-layer adhesion can separate the oxide coating from the base material and completely destroy a tape.

2-5


Dirt Build-up Dirt build-up happens when the relative humidity level is less than 10%. The low humidity causes static electricity that attracts dirt and dust which builds up on the magnetic tape and other parts of the magnetic tape recorder. Excessive Tape and Head Wear When the relative humidity is more than 95%, the high humidity causes increased friction as the tape passes over the heads. This, in turn, causes excessive tape and head wear. Q-12. What is tape failure? Q-13. What are four main causes of tape failure? Q-14. How does normal wear cause tape failure? Q-15. Accidental damage to magnetic tape is normally caused by the tape recorder itself or by human operators of the recorder. What are three frequent causes of such accidental damage? Q-16. Environmental damage to magnetic tape can occur when the tape is stored in an area that exceeds what ideal temperature and humidity ranges? Q-17. What six types of environmental damage can occur to tapes in storage when the ideal temperature and humidity ranges are exceeded? Q-18. After using a tape that was stored in an area where temperatures exceeded 130º F you notice pieces of oxide sticking to the recorder's tape-transport mechanism, to its magnetic heads, and onto the tape. What is the probable cause of these symptoms? Q-19. Your activity stores its magnetic tape in an area where the temperature is 100º F. What two types of environmental damage could occur that would make these tapes unusable? Q-20. When the relative humidity is below 10%, what happens to magnetic tape and parts of a tape recorder that could cause environmental damage? Q-21. How does relative humidity over 95% cause excessive tape and head wear? WINDING ERRORS Winding errors are another cause of tape failure. They happen when improper winding practices create an excessive or uneven force as the tape is being wound onto a tape reel. The form taken by the tape after it is wound onto the reel is called the tape pack. Winding errors can cause a deformed tape pack that will prevent good head-to-tape contact. In most cases, a deformed tape pack can be fixed simply by rewinding it onto another reel at the proper tension and at the right temperature and humidity. The four most common types of tape pack deformation are: 1. Cinching 2. Pack-slip 3. Spoking

2-6


4. Windowing Cinching Cinching happens when a tape reel is stopped too quickly. The sudden stop causes the outer layers of magnetic tape to continue to spin after the inner layers have stopped. This causes any loosely wound tape within the pack to unwind and pile up. Figure 2-2 shows an example of a cinched tape pack (note the complete foldover of one tape strand).

Figure 2-2.—Example of cinched tape pack.

Pack Slip Pack slip happens when the tape is loosely wound on the reel and is exposed to excessive vibration or too much heat. This causes the tape to shift (side-to-side), causing steps in the tape pack. When a tape reel with pack slip is used, the magnetic tape will unwind unevenly and rub against the sides of the tape reel or the recorder’s tape guides. This can damage the magnetic tape and cause oxide shedding. Figure 2-3 shows an example of pack slip.

Figure 2-3.—Example of pack slip.

2-7


Spoking Spoking happens when magnetic tape is wound onto the tape reel with the tension increasing toward the end of the winding. The higher tension on the outside of the tape pack causes the inner pack to buckle and deform. Spoking is also caused by the uneven pressures created when a tape is wound on a reel that has a distorted hub, or when the tape is wound over a small particle that is deposited on the hub. Figure 2-4 shows a spoked tape pack.

Figure 2-4.—Example of spoked tape pack.

Windowing Windows are voids or see-through air gaps in the tape winding. They happen when magnetic tape is loosely wound onto a tape reel, and especially when the loosely wound reel is later exposed to extreme heat or humidity. Figure 2-5 shows a windowed tape pack.

2-8


Figure 2-5.—Example of windowed tape pack.

Q-22. Tape winding errors can cause a deformed tape pack. What are four common types of tape pack deformation? Q-23. After rewinding a tape onto its supply reel, you examine the tape pack and notice pile-ups of tape resembling the example in figure 2-2. What causes this condition? Q-24. You notice steps in the tape pack such as those in figure 2-3. What causes this and how does it damage the magnetic tape? Q-25. A tape pack is buckled and deformed as shown in figure 2-4. What are three possible causes for this condition? Q-26. A tape pack has gaps in the tape winding as shown in figure 2-5. What causes this condition?

TAPE REELS AND TAPE CARTRIDGES There are two types of magnetic tape carriers: tape reels and tape cartridges. Both types can be used for either analog or digital recording. Tape cartridges are normally used only for digital recording. TAPE REELS Tape reels are used on magnetic recorders that use a manually loaded tape supply reel and a separate take-up reel. A reel’s purpose is to protect the magnetic tape from damage and contamination. It can be made of plastic, metal, or glass. A reel has two parts, the hub and the flanges. A tape reel is designed to hold magnetic tape on its hub without letting the magnetic tape touch the sides of the flanges. Contrary to popular belief, the flanges are not designed to guide the magnetic tape onto the tape reel.

2-9


TAPE CARTRIDGES Tape cartridges hold a spool of magnetic tape in the same way as tape reels, except that the inside of the cartridge contains both the supply reel and the take-up reel. Unlike tape reels which must be manually loaded into a recorder, when you insert a tape cartridge into a recorder, it’s automatically loaded and ready to use. Figure 2-6 shows two typical tape cartridges.

Figure 2-6.—Typical tape cartridges.

Q-27. When winding a tape onto a plastic or metal reel, should the tape ever touch the reel’s flanges?

TAPE ERASING AND DEGAUSSING One advantage of magnetic tape is that you can erase what you’ve previously recorded, and record on the same tape again and again. The erasing is done by demagnetizing the magnetic tape. You demagnetize a magnetic tape by exposing it to a gradually decreasing ac (alternating current) magnetic field. There are two ways to do this: (1) with an erase head that’s mounted on the magnetic recorder, or (2) with a separate tape degausser. ERASE HEADS A magnetic recorder’s erase head erases magnetic tape by saturating it with an ac signal that’s higher in frequency than the frequency range of the recorder itself. This method of erasing a tape works well in some cases, but it’s not the best way because: • It’s slow; the tape must be run through the recorder to be erased. 2-10


• If the erase head is not completely demagnetized, it may not do a complete erasure. • Some recorders do not have erase heads installed. MAGNETIC TAPE DEGAUSSERS By far, the best way to erase a magnetic tape is to use a separate magnetic tape degausser. There are two types of degaussers: automatic and manual. Automatic Tape Degausser Automatic degaussers erase magnetic tape by automatically moving the whole tape reel or cartridge slowly and steadily in and out of an intense ac magnetic field. This type of degausser erases a tape very well. Some automatic degaussers are made specifically for tape reels, and some are made for both tape reels and tape cartridges. Figure 2-7 shows a typical automatic degausser.

Figure 2-7.—Automatic tape degausser.

Manual Tape Degausser Both manual and automatic tape degaussers use the same electronic principles for erasing magnetic tape. However, the manual version is much more portable. It’s small, hand-held, and much less expensive. Figure 2-8 shows a typical manual degausser.

2-11


Figure 2-8.—Manual tape degausser.

To erase tapes with a manual degausser: 1. Place the tape reel or cartridge to be erased on a flat surface. 2. Hold the degausser very close to the magnetic tape and turn it on. 3. Slowly rotate the degausser in circles around the tape reel or cartridge for a few seconds. 4. Then slowly move it away until you’re about 12 to 14 inches away from the tape reel or tape cartridge. 5. Turn off the degausser. Q-28. What are two disadvantages of using a recorder’s erase head to erase data recorded on a magnetic tape? Q-29. What method for erasing magnetic tape is much more effective and reliable than using a recorder’s erase head?

HANDLING, STORING, AND PACKAGING MAGNETIC TAPE Today’s magnetic tape coatings can store recorded signals for years. The data recorded is a permanent record that won’t fade or weaken with age. And, it’ll remain unchanged until it’s altered by another magnetic field or until the tape coating deteriorates.

2-12


When magnetic tape recordings are ruined, the cause is usually poor handling, improper storage, or shipping damage. If you want your tape recordings to last a long time, you need to know how to properly handle, store, and ship magnetic tape. HANDLING MAGNETIC TAPE A magnetic tape reel or cartridge should always be in one of two places, either mounted on a tape recorder or in its storage container. When you handle magnetic tape, follow these rules: • DO use extreme care when handling magnetic tape. Careless handling can damage magnetic tape, tape reels, and tape cartridges. Always hold a tape reel by the hub, NEVER by the flanges, and NEVER handle or touch the working tape surface. • DO NOT let the magnetic tape trail on the floor. Even though the end of the tape may not have data stored on it, it can pick up dirt and dust that ends up on the recorder. • DO clean your hands before handling magnetic tape. You can contaminate magnetic tape with dirt and oils from dirty hands. • DO mount tape reels and cartridges properly. Improperly seated tape reels can cause unnecessary wear and tear on the magnetic tape. • DO replace any warped take-up reels, as they can damage magnetic tape. • DO keep the magnetic recorder and its take-up reel clean. Magnetic tape can pick up dirt and dust from the recorder itself. • DO NOT use the top of a magnetic recorder as a work area. This can expose the magnetic tape to dirt, excessive heat, and stray magnetic fields. • DO NOT allow eating, drinking, or smoking in areas where magnetic tape or devices are exposed. STORING MAGNETIC TAPE Most magnetic tape reels and cartridges spend a lot of time in storage. It’s very important that you protect the stored tape from physical damage and the damaging effects of contamination and temperature and humidity extremes. If you don’t, damage to the tape pack such as oxide shedding, layer-to-layer sticking, and tape deformation can happen. To protect magnetic tape from damage during storage, follow these rules: • DO make sure that magnetic tape is wound properly on the reel hub and at the proper tension. • DO always store tape reels vertically. DO NOT lay them on their side. • DO maintain a proper environment. Keep the storage area clean, and at a 60 to 80F degree temperature and a 40 to 60% relative humidity. • DO NOT store magnetic tapes near any equipment that generates stray magnetic fields. • DO handle all tape reels and cartridges as gently as possible. • DO NOT eat, drink, or smoke in a magnetic tape storage area.

2-13


PACKAGING MAGNETIC TAPE FOR SHIPPING There may be times when you are asked to package magnetic tape reels or tape cartridges for shipment. If you want the tape to arrive in good condition, you must pack it properly to protect it from damage. The packaging you use must protect the tape reels or cartridges from impact, vibration, and temperature and humidity changes. Here are some simple rules to follow: • DO always package tape reels so that they’re supported by their hub. This prevents any pressure on the reel’s flanges that might flex the flanges against the tape pack. Figure 2-9 shows a shipping box that supports the tape reel by the hub.

Figure 2-9.—Reel box that supports reel by the hub.

• DO always use reel bands where available. Reel bands are for placement around the outside edges of the reel flanges to help prevent the flanges from flexing and damaging the tape. • DO always ship magnetic reels in a container designed so its normal positioning is with the reels in a vertical position. This will prevent the tape pack from shifting and damaging the edges of the magnetic tape. • DO always package tape cartridges in their shipping cases. Tape cartridges are more durable than tape reels, but they still need to be protected during shipment. Q-30. When magnetic tapes are ruined, what three factors are normally the cause? Q-31. What is the correct way to hold a magnetic tape reel? Q-32. The take-up reel on your recorder is warped. What should you do to/with the reel? Q-33. If magnetic tape is stored in areas with temperature and humidity extremes, what are three types of tape damage that may occur? Q-34. List four rules you should follow when storing magnetic tape to protect it from damage. Q-35. When packaging tape reels or cartridges for shipping, what are four rules you should follow to protect the tape reels from impact and vibration? 2-14


SUMMARY Now that you’ve finished chapter 2, you should be able to (1) describe the physical properties of magnetic tape, (2) recognize the four most common magnetic tape errors, (3) recognize the four causes of tape failure, (4) describe the two methods for erasing magnetic tape, and (5) use the proper procedures for handling, storing, and packaging magnetic tape, tape reels, and tape cartridges. The following is a summary of the important points in this chapter. The three BASIC MATERIALS used to make magnetic tape are the (1) base material, (2) the oxide particles, and (3) the binder glue. ANALOG and DIGITAL are the two basic types of magnetic tape in common use. BLEMISHES OR COATING FLAWS ON DIGITAL TAPE can easily ruin the data or the computer program stored on the tape. SIGNAL DROPOUT, NOISE, SKEW, AND LEVEL are four types of tape errors. Dropout errors are the most common. OXIDE LUMPS accumulated on the tape cause most dropout errors. Other causes are dust or lint on the tape, or missing oxide coating on part of the tape. MAGNETIC TAPE FAILURE has four main causes: (1) normal wear, (2) accidental damage, (3) environmental damage, and (4) winding errors. IDEAL TEMPERATURE AND HUMIDITY RANGES for using and storing magnetic tape are 60 to 80º F and 40 to 60% relative humidity. ENVIRONMENTAL TAPE DAMAGE caused by excessive temperature or humidity includes the following: (1) tape deformation, (2) oxide shedding, (3) head-to-tape sticking, (4) layer-to-layer sticking, (5) dirt buildup, and (6) excessive tape and head wear. WINDING ERRORS can cause tape pack deformation. The four most common types are: (1) cinching, (2) pack slip, (3) spoking, and (4) windowing. The TWO PARTS OF A TAPE REEL are the hub and the flanges. The tape should be wound on the hub. No part of the tape should be touching the flange sides. ERASE HEADS AND TAPE DEGAUSSERS are two methods for erasing tape. Degaussers are the fastest and the most reliable. Rules for HANDLING MAGNETIC TAPE are (1) always hold the reel by the hub, not the flanges, (2) never touch the working tape surface, (3) replace warped or damaged reels, and (4) mount reels and cartridges properly. Rules for STORING MAGNETIC TAPE are (1) wind tape properly on the reel hub, (2) store tapes vertically, (3) keep storage area clean and at proper temperature and humidity levels, and (4) store tapes away from equipment that generates stray magnetic fields. Rules for PACKAGING TAPE FOR SHIPPING are (1) support reels by their hubs, (2) use reel bands, (3) pack reels in containers vertically, and (4) keep tape cartridges in their shipping cases.

2-15


ANSWERS TO QUESTIONS Q1. THROUGH Q35. A-1. a. Base material. b. Coating of magnetic oxide particles. c. Glue that bonds the particles to the base. A-2.

Plastic tape is used more than metal because it’s more flexible, resists mildew and fungus, and is very stable at high temperatures and humidity.

A-3.

Analog magnetic tape.

A-4.

Digital magnetic tape is for computer programs and data. Its base material is about 50% thicker. The tape’s surface must not have blemishes or coating flaws because losing even one digital data bit could ruin the recorded computer program or data.

A-5.

Signal dropout, noise, skew, and level. Dropout is the most common.

A-6.

Dropouts are temporary, sharp drops (50% or more) in signal strength. They’re caused by contaminates that lift the tape away from the magnetic head, or when magnetic oxide coating is missing on part of the tape.

A-7.

Oxide particles that get onto the magnetic tape.

A-8.

Signal dropout errors and level errors. The dust and lint on the reel will eventually get onto the tape where it can get between the tape and the recorder’s heads.

A-9.

Noise error is usually caused by a cut or a scratch on the magnetic tape.

A-10.

Skew means there are time differences between the individual tracks of a multi-track recorder’s magnetic head. It happens when the tape isn’t properly aligned with the head. Fixed skew happens when the tape passes over an improperly aligned magnetic head.

A-11.

The actual output signal level of the tape exceeds the manufacturer’s specified range for the output signal level (+ / − 10%). It’s caused by an uneven oxide coating on the tape due to worn tape or defective manufacture.

A-12.

Tape’s performance degrades to a point where it’s no longer usable.

A-13.

Normal wear, accidental damage, environmental damage, and winding errors.

A-14.

Repeated contact with a recorder’s fixed surfaces such as magnetic heads, tape rollers, and tape guides.

A-15. a. Improperly adjusted tape transport mechanism. b. Dropping a reel of tape. c. Improperly threading tape.

2-16


A-16.

Ideally, use and store tape at 60 to 80º F and at 40 to 60% relative humidity.

A-17.

Tape deformation, oxide shedding, head-to-tape sticking, layer-to-layer sticking, dirt build-up, and excessive tape and head wear.

A-18.

Oxide shedding. At temperatures above 130º F, oxide coating becomes soft and sheds.

A-19.

Head-to-tape sticking and layer-to-layer adhesion.

A-20.

Dirt build-up caused by static electricity.

A-21.

High humidity causes increased friction as the tape passes over the heads.

A-22.

Cinching, pack slip, spoking, and windowing.

A-23.

The tape is stopped too quickly when winding or rewinding.

A-24.

Pack slip. It's caused by loosely wound tape on a reel that is exposed to excessive vibration or heat. The vibration or heat causes the tape to shift, causing steps in the tape pack. The uneven tape will then rub against the reel's sides and the recorder's tape guides.

A-25. a. Reel has a distorted hub, b. tape wound over small particle deposited on hub, and c. tape wound on reel with tension increasing toward end of winding. A-26.

Tape is loosely wound on reel.

A-27.

No. The reel is designed to hold the tape on its hub without letting the tape touch the sides of the flanges.

A-28.

Using an erase head is slow, and it may not completely erase the tape.

A-29.

Using a magnetic tape degausser.

A-30.

Poor handling, improper storage, or shipping damage.

A-31.

Always hold reel by the hub, never by the flanges. Never touch the working tape surface.

A-32.

Always replace a warped reel.

A-33.

Oxide shedding, layer-to-layer sticking, and tape deformation.

A-34. a. Make sure the tape is wound properly on the reel hub, b. store tapes vertically, c. keep storage area at right temperature and humidity, d. store away from equipment that generates stray magnetic fields.

2-17


A-35. a. Package reels so they’re supported by their hub, b. use reel bands, c. package reels in vertical position, d. package tape cartridges in their shipping cases.

2-18


CHAPTER 3

MAGNETIC TAPE RECORDER HEADS LEARNING OBJECTIVES After completing this chapter, you'll be able to do the following: 1. Describe the construction, function, and placement of magnetic tape recorder record, reproduce, and erase heads. 2. Describe the preventive maintenance requirements for magnetic tape recorder heads.

MAGNETIC TAPE RECORDER HEADS Magnetic tape recorder heads are the heart of magnetic tape recording, because it's the magnetic heads (as we'll call them in this chapter) that actually: 1. Record signal or data information onto magnetic tape 2. Reproduce (play back) signal or data information from magnetic tape 3. Erase any signal or data off of magnetic tape To do these things, a magnetic tape recorder can have up to three different heads installed: one head for recording, one for reproducing, and one for erasing. Some magnetic tape recorders will use the same head for both recording and reproducing. Figure 3-1 shows a typical multitrack magnetic head.

Figure 3-1.—Typical multitrack magnetic tape recorder head.

3-1


MAGNETIC HEAD CONSTRUCTION Magnetic head construction is basically the same for all magnetic heads. They're all made up of a magnetic core wrapped with a coil of very thin wire. But, there's where the similarity ends. From here on, each magnetic head is built to perform a specific job. Will the head be used on a single track recorder? Will it be used on a multitrack recorder? Will it be a record head or a reproduce head? Or, will it be an erase head? What frequency will it be recording and/or reproducing? The answers to these questions will determine the final construction of a magnetic head. Figure 3-2 shows the construction of a typical multitrack magnetic head. Magnetic cores are wound with very thin wire, cemented together, and placed inside a half-bracket. A tip piece is then placed on top of the ferrite core, and the two half-brackets are assembled together. It's during this final assembly process that the headgap and the resulting frequency response of the magnetic head are determined. After some final contouring to give the magnetic head its curved face, it's ready for use.

Figure 3-2.—Multitrack tape recorder head construction.

3-2


Record and Reproduce Heads Record and reproduce heads convert and transfer electrical signals onto and off of magnetic tape. The maximum frequency these heads can transfer depends on the size of the headgap and the speed of the magnetic tape (we'll discuss speed in the next chapter). Most record and reproduce heads are in one of these three general bandwidth categories: 1. Narrowband—100 Hz to 100 kHz 2. Intermediate band—100 Hz to 500 kHz 3. Wideband—400 Hz to 2 mHz The only physical difference between a record head and a reproduce head is in the number of turns of wire on the core. A reproduce head will have more turns than a record head. This is because reproduce heads must be able to recover low-level signals from magnetic tape. The extra turns of wire allow the reproduce head to output the highest level possible and at a good signal-to-noise level. Record heads are always placed before reproduce heads on magnetic tape recorders. This allows you to monitor signals that you're recording. Figure 3-3 shows the placement of record and reproduce heads. Figure 3-4 shows some of the typical track arrangements used.

Figure 3-3.—Placement of magnetic tape recorder heads.

3-3


Figure 3-4.—Magnetic head track placement.

Erase Heads Erase heads transfer a signal to the magnetic tape that causes the magnetic particles to assume a neutralized or erased state. To do this, a high current signal that is 3 to 5 times higher in frequency than the maximum frequency response of the record and reproduce heads is used. In some audio recorders, a simple direct current (dc) voltage is used. Erase heads are always placed before the record and the reproduce heads on tape recorders. This allows you to erase the magnetic tape before it's recorded on. Figure 3-3 shows the placement of erase heads. Q-1. Magnetic tape recorders can have up to three different heads installed. What are the three functions performed by a recorder's heads? Q-2. The way a magnetic head will be used determines how it is constructed. Name three factors that determine the final construction of a magnetic head. Q-3. What two specifications determine the maximum frequency that a recorder's record and reproduce heads will be able to transfer? Q-4. Most record and reproduce heads are in one of what three bandwidth categories? Q-5. Why are record heads always placed before reproduce heads on recorders? Q-6. A recorder's erase head is always placed in what sequence on the record/reproduce track?

3-4


MAGNETIC HEAD MAINTENANCE It's very important to regularly maintain magnetic heads. If you do, you'll greatly reduce the chance of getting a poor recording or playback. Regular preventive maintenance will also increase the life of the magnetic heads. There are two things you must do to properly maintain magnetic heads: (1) keep them clean, and (2) keep them demagnetized. Cleaning Magnetic Heads Through use, magnetic heads pick up dirt, dust, lint, and oxide particles from the magnetic tape. These particles collect on the magnetic head and, if left unchecked, could cause signal dropout errors that degrade the quality of recording and playback. To keep magnetic heads clean, regularly clean them with a cotton-tipped applicator soaked in either isopropyl alcohol or in a magnetic head cleaner recommended by the recorder's manufacturer. A good rule of thumb is to clean the heads each time you change a tape reel or cartridge. Demagnetizing Magnetic Heads Magnetic heads can become magnetized from many sources. It could happen • during ac power losses, • during testing or alignment, • because of stray magnetic fields, • from normal use. No matter the cause, magnetized magnetic heads degrade the quality of the magnetic recording or playback. To demagnetize magnetic heads, you'll use a hand-held degausser. It could be like the one shown in figure 3-5, or like the manual degausser shown in the previous chapter. No matter how they look, they all generate an ac magnetic field that demagnetizes the metal parts of a magnetic head.

3-5


Figure 3-5.—Hand-held head degausser.

The procedure for demagnetizing a magnetic head is similar to the procedure for degaussing a magnetic tape. Here are the basic steps: 1. Remove the tape (reel or cartridge) from the magnetic recorder. 2. Holding the degausser an arm's length away from the magnetic head, energize the degausser. 3. Slowly bring the degausser closer and closer to the magnetic head. Don't touch the head with the degausser. 4. Move the degausser back and forth across the head for 15 to 30 seconds. Figure 3-6 shows how this looks. 5. Slowly move the degausser away from the magnetic head. When the degausser is an arm's length away, de-energize it.

3-6


Figure 3-6.—Demagnetizing magnetic heads with a degausser.

That's all there is to it. It's hard to determine exactly how often magnetic heads should be de-magnetized. Manufacturer's recommendations vary from every 8 to 25 hours of operation. To be safe, check the equipment's technical manual. Q-7. What two preventive maintenance actions must you do regularly to increase magnetic head life and to ensure good tape recording and playback? Q-8. How should you clean your recorder's magnetic heads? Q-9. What are four sources that can cause magnetic heads to become magnetized? Q-10. What type of equipment should you use to demagnetize your recorder's magnetic heads? Q-11. How often should you demagnetize a recorder's magnetic heads?

SUMMARY Now that you've finished chapter 3, you should be able to (1) describe the construction of magnetic tape recorder heads; (2) describe the purpose and placement of record, reproduce, and erase heads; and (3) describe the preventive maintenance requirements for tape recorder heads. The following is a summary of important points in this chapter: Magnetic tape recorders have up to THREE MAGNETIC HEADS to perform the erase, record, or reproduce function. Three factors that determine the CONSTRUCTION OF A MAGNETIC HEAD are the (1) type of head, (2) frequencies it will record, reproduce, or erase, and (3) use on a single or multitrack recorder. 3-7


Most tape recorder heads are designed for ONE OF THREE BANDWIDTHS: (1) narrowband, (2) intermediate band, or (3) wideband. A recorder's magnetic heads are in the following SEQUENCE on its record/reproduce track: (1) erase, (2) record, and (3) reproduce. Two important PREVENTIVE MAINTENANCE requirements for magnetic heads are cleaning and demagnetizing.

ANSWER TO QUESTIONS Q1. THROUGH Q11. A-1. Record, reproduce, and erase. A-2. a. Type of head (record, reproduce, or erase). b. Frequencies it will record or reproduce. c. Whether it will be used on a single or multitrack recorder. A-3. a. Size of the headgap. b. Speed of the magnetic tape. A-4. a. Narrowband—100 Hz to 100 kHz. b. Intermediate band—100 Hz to 500 kHz. c. Wideband—400 Hz to 2 mHz. A-5. Allow you to monitor the signals you're recording. A-6. First, before the record and reproduce heads. A-7. a. Keep the heads clean. b. Keep the heads demagnetized. A-8. With a cotton-tipped applicator soaked in either isopropyl alcohol or a head cleaner recommended by the recorder's manufacturer.

3-8


A-9. a. During ac power losses. b. During testing. c. Because of stray magnetic fields. d. From normal use. A-10. A hand-held degausser like the manual degaussers used for degaussing magnetic tape. A-11. Every 8 to 25 hours depending on the manufacturer's recommendations.

3-9



CHAPTER 4

MAGNETIC TAPE RECORDER TRANSPORTS LEARNING OBJECTIVES After completing this chapter, you'll be able to do the following: 1. Describe the function and components of a basic magnetic tape transport system. 2. Describe the operating characteristics and parts of the three most common tape reeling systems. 3. Describe the physical characteristics of the two basic tape reeling configurations, co-planar and co-axial. 4. Describe the characteristics of open-loop drive and closed-loop drive tape transport configurations and the three most common closed-loop designs. 5. Describe the capstan speed control function of a tape transport system and the relationship of the six basic parts of a typical capstan speed control unit. 6. Explain why, and describe how, magnetic tape transports must be cleaned and degaussed.

INTRODUCTION Magnetic tape recorder transports are precisely built assemblies that move the magnetic tape across the magnetic heads and hold and protect the tape. Figure 4-1 shows a basic tape transport assembly. Tape transports have four basic parts:

Figure 4-1.—Basic tape transport assembly.

4-1


1. A tape reeling system that, with the aid of tape guides, physically moves the tape across the magnetic heads. 2. A tape speed control system that monitors and controls the movement of the magnetic tape. 3. An electronic subsystem that activates the reeling device to move the magnetic tape. 4. A basic enclosure that holds and protects the reels or cartridges of magnetic tape. This chapter describes these basic parts, tells how they work, and shows diagrams of the more common ones.

TAPE REELING SYSTEMS A basic magnetic recorder tape reeling system (figure 4-1) has one supply reel and one take-up reel. Its job is to move the magnetic tape from one reel to the other. When this happens, four things occur: 1. The supply reel feeds out magnetic tape at a constant tension. 2. The tape passes the magnetic heads in a straight line. 3. The take-up reel accepts the magnetic tape at a constant tension. 4. Both the supply and take-up reels start and stop smoothly while maintaining the proper tape tension. These four things must happen, or the magnetic tape could be damaged. Three of the most commonly used tape reeling systems are (1) take-up control, (2) two-motor reeling, and (3) tape buffering. TAKE-UP CONTROL REELING SYSTEMS This system uses a motorized take-up reel which pulls the magnetic tape off of a free-spooling supply reel. It maintains tape tension by using mechanical drag on the supply reel. As you might guess, this method has its disadvantages. It only works in one direction, and the tape tension doesn't remain constant throughout the reel. As the supply reel gives out tape, the tape tension varies. Uneven tape tension can cause stretched tape, poorly wound tape reels, and tape damage during starts and stops. TWO-MOTOR REELING SYSTEMS To overcome the problems of take-up control reeling systems, designers added a motor to the supply reel. By using two motors, the magnetic tape direction can be forward or reverse. Two-motor reeling configurations usually use dc (direct current) motors, instead of ac (alternating current) motors, because dc motors run smoother and are easier to control. To help control tape tension, a small hold-back voltage is added to the motor for the supply reel. Unfortunately, two-motor reeling systems do not properly control tape tension during starts and stops. Something called tape buffering must be added.

4-2


TAPE BUFFERING REELING SYSTEMS Controlling a recorder's tape tension during starts and stops is a big problem. Tape buffering overcomes this problem by regulating the tape reel speed and by protecting against changes in tape tension. Every manufacturer of high-quality, high performance magnetic tape recorders uses some sort of tape buffering. It's especially important in magnetic recorders that operate at many different speeds, where precise tape tension must be maintained. Figure 4-2 shows the relationship between the tape reeling system and the tape buffering system. As you can see, the speed at which a tape reel will give up or take up magnetic tape is controlled by its respective speed control servo. Feedback from the supply and take-up buffers tells the servo to speed up or slow down.

Figure 4-2.—Tape buffering arrangement.

There are two basic types of reeling system buffers: (1) spring-tension, and (2) vacuum-column. 1. Spring-tension buffering systems use an electro-mechanical device to sense changes in tape tension. These changes are feedback that the speed control servo needs to adjust the speed of the tape reels. Figures 4-3 and 4-4 show two of the more common arrangements for spring-tension buffers.

4-3


Figure 4-3.—Mechanical arm spring-tension tape buffering.

Figure 4-4.—Mechanical arm spring-tension tape buffering.

2. Vacuum-column buffering systems operate like the spring-tension systems. They also regulate the speed control servos that control tape reel speed. But, as shown in figure 4-5, the vacuum-column buffer system uses a vacuum chamber instead of a spring to hold a length of magnetic tape as slack during tape recorder starts and stops. An electronic sensor in the vacuum chamber helps to control how much tape is in the buffer.

4-4


Figure 4-5.—Vacuum-column tape buffering system.

TAPE GUIDES Another job of a tape reeling system is to make sure the magnetic tape is protected from damage during operation. To do this, tape reeling systems use tape guides. Tape guides come in two designs, fixed and rotary. Both of these are shown in figure 4-6. Each type of tape guide has its drawbacks. Fixed tape guides produce a lot more friction, and rotary tape guides are more likely to cause errors because of their moving parts.

Figure 4-6.—Typical fixed and rotary tape guides.

4-5


Tape guides are strategically placed in a tape reeling system to make sure the magnetic tape is kept straight with respect to the supply and take-up reels and the magnetic heads. Some magnetic recorders use only fixed tape guides, some use rotary tape guides, and some use a combination of the two. TAPE REELING CONFIGURATIONS There are two basic tape reeling configurations: (1) co-planar, and (2) co-axial. Both of these describe the physical relationship between the supply reel and the take-up reel. The co-planar, which is used more often than the co-axial, has the supply reel and the take-up reel side by side. Figure 4-7 shows this configuration.

Figure 4-7.—Co-planar tape reeling configuration.

The co-axial configuration is used when physical space is limited. It places the supply and take-up reels on top of each other. Figure 4-8 shows this configuration.

4-6


Figure 4-8.—Co-axial tape reeling configuration.

Q-1. What are the four basic parts of a magnetic tape recorder's tape transport system? Q-2. What are the three most commonly used tape reeling systems? Q-3. What are two disadvantages of the take-up control reeling system? Q-4. What are two advantages of a two-motor reeling system over a take-up control reeling system? Q-5. What type of reeling system best controls a tape recorder's tape tension during starts and stops? Q-6. What are the two basic types of tape buffering reeling systems? Q-7. How do the tape guides on a tape reeling system protect the tape from damage during operation?

TAPE TRANSPORT CONFIGURATIONS There are two types of tape transport configurations: (1) open-loop capstan drive, and (2) closedloop capstan drive. The following paragraphs describe each of these. OPEN-LOOP CAPSTAN DRIVE This is probably the simplest tape transport configuration. Figure 4-9 shows how the magnetic tape is pulled off of the supply reel, taken across the magnetic heads, and wound onto the take-up reel. The tape is pulled by sandwiching it between a single capstan and a pinch roller. As the capstan turns, the friction between it and the pinch roller pulls the tape across the magnetic heads. The magnetic tape is held against the magnetic heads by using tape guides.

4-7


Figure 4-9.—Open-loop capstan drive tape transport.

The open-loop drive transport configuration has two major drawbacks: 1. It can only work in one direction. It can pull the tape, but it can't push it across the magnetic heads. 2. Tape tension and head-to-tape contact can vary. If the capstan motor hesitates or speeds up, the tape tension will vary. CLOSED-LOOP CAPSTAN DRIVE Closed-loop capstan drive tape transports were designed to overcome the drawbacks of the openloop drive design. They use more than one capstan and/or pinch roller to clamp the magnetic tape in the area around the magnetic heads. This keeps tape tension constant and improves the quality of the recording or the playback. Figure 4-10 shows the basic arrangement of the closed-loop capstan drive.

Figure 4-10.—Closed loop capstan drive tape transport.

4-8


The three most common closed-loop capstan drive designs are (1) differential velocity capstans, (2) dual-motors dual capstans, and (3) peripheral drive capstans. Differential Velocity Capstans Figure 4-11 shows a differential velocity capstan. In this design, the take-up capstan is made a little larger than the supply capstan. This causes the take-up capstan to pull the tape away from the heads slightly faster than the supply capstan feeds the tape to the heads. The result is a constant tape tension in the area around the magnetic heads.

Figure 4-11.—Differential velocity capstan drive.

Both capstans are turned by a single motor which is coupled to the capstan pulleys by a belt. This arrangement is very efficient in one direction, but, unfortunately, differential velocity capstans don't work in reverse. If you reversed the tape direction, a negative tension would occur, and the tape would bunch up in the area around the magnetic heads. Dual-Motors Dual Capstans Figure 4-12 shows a dual-motor dual capstan drive. In this design, each capstan is driven by its own motor. Tape tension is maintained by slowing down one of the motors. When reverse tape motion is needed, the opposite motor is slowed down.

4-9


Figure 4-12.—Dual-motors dual capstans drive system.

Peripheral Drive Capstans In this design, the magnetic tape is moved by a capstan placed directly against the tape reel or tape pack. Figure 4-13 shows two different peripheral drive capstan arrangements. The first arrangement, figure 4-13A, shows a single capstan design. In this method, two tightly wound tape reels, without flanges, are pushed against the capstan. As the capstan turns, it forces the tape reels to turn in the appropriate direction. Magnetic tape tension is maintained by using either spring loading or servo control.

Figure 4-13A.—Peripheral drive capstans.

The second arrangement, figure 4-13B, uses two capstans. In this method, the two tightly wound tape reels, without flanges, are pressed directly against the capstans. Tension in the magnetic head area is maintained by controlling the speed of the individual capstans.

4-10


Figure 4-13B.—Peripheral drive capstans.

CAPSTAN SPEED CONTROL Capstan speed control is an important part of the magnetic tape transport system. It makes sure the capstan is turning (1) at the right speed and (2) at a constant speed. This is important because errors in speed control can cause poor recordings and playbacks. Capstans are turned either by a motor only, or by a motor, belt, and pulley arrangement. In either case, it's the motor that the capstan speed control function acts upon to do its job. A capstan speed control function typically consists of six basic parts. Figure 4-14 shows these six parts and how they're related. Each of the parts is described below.

Figure 4-14.—Six parts of the capstan speed control function.

PRECISION FREQUENCY SOURCE This part of the capstan speed control provides a reference frequency that the speed select network and the comparison network use to drive the capstan motor. The precision frequency source is usually a very-high-frequency crystal with an accuracy of at least .001 percent.

4-11


SPEED SELECT NETWORK This network selects the desired operating tape speed. It takes the reference frequency from the precision frequency source and (depending on the desired operating tape speed) generates another specific reference signal that the comparison network uses to control the speed of the capstan. Table 4-1 is a list of the speed control reference signal frequencies for the various operating tape speeds.

Table 4-1.—Typical speed control reference signal frequencies

Operating Tape Speed (inches per second) 15/16 1 7/8 3 3/4 7 1/2 15 30 60 120 240

Speed Control Frequency (kilohertz) 1.5625 3.125 6.25 12.5 25 50 100 200 400

CAPSTAN SPEED MONITOR This circuit monitors the true capstan motor speed. It sends the true speed to the comparison network circuit. Most capstan speed monitor circuits are made using a photo-optical tachometer that's directly attached to the shaft of the capstan motor. Figure 4-15 shows this.

Figure 4-15.—Capstan speed monitor using a photo-optical tachometer.

4-12


COMPARISON NETWORK This network takes the input signals from the speed select network and the capstan speed monitor, compares the two signals, and decides if the capstan is at the right speed. If not, it tells the speed correction circuit. Sometimes, a third input signal, which comes from the magnetic tape itself, is supplied to the comparison network. It's called a servo control from tape signal. Tape recordings made on a specific recorder are sometimes shipped off for further analysis and played back on a different recorder. To help compensate for speed errors in the tape transport systems of the two recorders, the precision reference frequency of the originating recorder is recorded onto a track of the magnetic tape. During playback, this reference signal is also fed to the recorder's comparison network and is used to correct speed errors. SPEED CORRECTION CIRCUIT This circuit takes speed correction signals from the comparison network and tells the capstan motor drive circuit to either speed up or slow down the capstan motor. CAPSTAN MOTOR DRIVE CIRCUIT This circuit takes the speed-up or slow-down signals from the speed correction circuit and actually speeds up or slows down the capstan motor.

MAGNETIC TAPE TRANSPORT MAINTENANCE If you want good recordings and playbacks, you must keep magnetic tape transports clean and demagnetized. The following paragraphs describe preventive maintenance procedures for magnetic tape transport systems. MAGNETIC TAPE TRANSPORT CLEANING You can clean most magnetic tape transports with isopropyl alcohol, cotton swabs, and lint-free cloths. (Caution: Cotton swabs are not lint free, so use them only in places you can't get to with the lintfree cloths.) Figure 4-16 shows a technician cleaning a capstan. Here are some other points to remember:

4-13


Figure 4-16.—Cleaning the capstan on a magnetic tape transport system.

• DO always remove the magnetic tape from the transport before cleaning it. • DO apply the cleaner onto the lint free cloth or cotton swab; DON'T apply it directly onto the tape transport. • DO pay extra attention to the flanged parts of tape guides. It's here that oxide particles collect the most. • DON'T use the same lint-free cloth or cotton swab to clean many parts of the tape transport. Switch cloths and swabs often. If you don't, you may transfer dirt and oxide particles from one part of the tape transport to another. MAGNETIC TAPE TRANSPORT DEMAGNETIZING With use, tape transport parts become magnetized. It's hard to say exactly what will happen if the magnetic tape passes a magnetized part of the tape transport before the tape is recorded on. The effects can range from just a little more noise on the tape to a complete tape saturation. Either way, magnetized tape transport parts can ruin magnetic recordings. To prevent this, you must periodically demagnetize the tape transport. The procedures for doing this are identical to those listed in chapter 2 for demagnetizing magnetic heads. You'll even use the same manual hand-held degausser you saw in figure 2-8 of chapter 2. Figure 4-17 shows a technician demagnetizing a tape guide.

4-14


Figure 4-17.—Demagnetizing a tape guide with a hand-held degausser.

Q-8. There are two types of tape transport configurations, open-loop capstan drive and closed-loop capstan drive. What are two major disadvantages of open-loop capstan drive tape transports? Q-9. How do closed-loop capstan drive tape transports overcome the disadvantages of the open-loop drive design? Q-10. What are the three most common closed-loop capstan drive designs? Q-11. How do tape transports with differential velocity capstans maintain a constant tape tension in the area around the magnetic heads? Q-12. How do dual-motor dual capstan drives maintain a constant tape tension while operating in either a forward or reverse direction? Q-13. What are the two critical functions of the capstan speed control part of a magnetic tape transport system? Q-14. Which part of the capstan speed control function monitors the true capstan motor speed? Q-15. Sometimes it's necessary, but why should you avoid using cotton swabs when cleaning a magnetic tape transport? Q-16. When cleaning the parts of a tape transport, why should you switch lint-free cloths and swabs often? Q-17. What equipment should you use to de-magnetize a magnetic tape transport?

4-15


SUMMARY Now that you've finished chapter 4, you should be able to describe magnetic tape transport systems in terms of their operating characteristics, parts, and preventive maintenance requirements. The following is a summary of the important points in this chapter. A MAGNETIC TAPE RECORDER TRANSPORT has four basic parts: (1) tape reeling system, (2) tape speed control system, (3) electronic subsystem, and (4) basic enclosure. The TAPE REELING SYSTEM must move the tape in a straight line at a constant tension, and it must start and stop smoothly while maintaining the proper tension. Three of the MOST COMMON REELING SYSTEMS are (1) take-up control, (2) two-motor reeling, and (3) tape buffering. The two types of TAPE TRANSPORT CONFIGURATIONS are (1) open-loop capstan drive and (2) closed-loop capstan drive. The open-loop type works in only one direction, and the tape tension can vary. The closed-loop type keeps the tape tension constant. Three types of CLOSED-LOOP CAPSTAN DRIVES are (1) differential velocity capstans, (2) dual-motors dual capstans, and (3) peripheral drive capstans. The CAPSTAN SPEED CONTROL component of a tape transport keeps the capstan turning at the correct operating speed and at a constant speed. It has these six parts: (1) precision frequency source, (2) speed select network, (3) capstan speed motor, (4) comparison network, (5) speed correction circuit, and (6) capstan motor drive circuit. You should CLEAN magnetic tape transports with isopropyl alcohol, cotton swabs, and lint free cloths and DEMAGNETIZE them using a hand-held degausser.

ANSWERS TO QUESTIONS Q1. THROUGH Q17. A1. a. Tape reeling system. b. Tape speed control system. c. Electronic subsystem. d. Basic enclosure. A2. a. Take-up control. b. Two-motor reeling. c. Tape buffering.

4-16


A3. a. It only works in one direction. b. The tape tension varies as the supply reel unwinds, which can cause damage during starts and stops.

A4. The two-motor configuration runs in both directions and a holdback voltage helps control tape tension, but it does not properly control tape tension during starts and stops. A5. A tape buffering reeling system. A6. a. Spring-tension buffering systems. b. Vacuum-column buffering systems. A7. They keep the tape straight with respect to both the supply and take-up reels and the magnetic heads. A8. a. Only operates in one direction. b. The tape tension and head-to-tape contact can vary. A9. Closed loop capstan drive transports use more than one capstan to clamp the tape in the area around the magnetic head. A10. a. Differential velocity capstans. b. Dual motors dual capstans. c. Peripheral drive capstans. A11. The supply capstan is slightly larger than the take-up capstan. This causes the take-up capstan to pull the tape slightly faster than the supply capstan feeds the tape. A12. Each capstan is driven by its own motor. It maintains tape tension by slowing down one of the motors. When the tape motion is reversed, the opposite motor is slowed down. A13. Makes sure the capstan turns at the right speed and at a constant speed. A14. Capstan speed monitor. A15. Cotton swabs are not lint free. A16. You may transfer dirt or oxide particles from one part of the tape transport to another. A17. A hand-held degausser.

4-17



CHAPTER 5

MAGNETIC TAPE RECORDER RECORD AND REPRODUCE ELECTRONICS LEARNING OBJECTIVES After completing this chapter, you’ll be able to do the following: 1. State the two types of record and reproduce electronics used on magnetic tape recorders. 2. Describe the purpose and function of direct record electronics and the four main parts of a recorder’s direct record component. 3. Describe the purpose and function of direct reproduce electronics and the three main parts of a recorder’s direct reproduce component. 4. Describe the purpose and function of frequency modulation (FM) record electronics and the three main parts of a recorder’s FM record component. 5. Describe the purpose and function of FM reproduce electronics and the four main parts of a recorder’s FM record component.

RECORD AND REPRODUCE ELECTRONICS There are two ways to record and reproduce analog signals. The first way is direct record. It’s also called amplitude modulation (AM) electronics. The second way is frequency modulation (FM). Even though direct record and reproduce circuits are much different from FM record and reproduce electronics, they both share the same two very important jobs. They both must: 1. Take an input signal, process it as needed, and then send it to the record magnetic head for reproduction. 2. Take the reproduced signal from the reproduce magnetic head, process it as needed, and output it for listening or analysis. DIRECT RECORD ELECTRONICS Direct record electronics record input signals onto magnetic media just as the signals appeared at the recorder’s input. The only processing an input signal receives is the adding of a bias signal. The added bias signal makes sure the signal stays away from the steps of the magnetism curve. Figure 5-1 shows a basic block diagram of a recorder’s direct record electronics.

5-1


Figure 5-1.—Direct record electronics.

Direct record electronics has four main parts: 1. Input pre-amplifier circuit. This circuit takes the input signal, amplifies it, and sends it to the summing network. It also matches the impedance between the source of the input signal and the magnetic tape recorder. 2. Bias source. This circuit generates the high-frequency bias signal and sends it to the summing network. Normally, the frequency of the bias signal will be five to ten times higher than the highest frequency the tape recorder can record. 3. Summing network. This network takes the input signal and the bias signal, mixes them, and sends the resulting signal to the head driver circuit. 4. Head driver circuit. This circuit takes the signal from the summing network, amplifies it, and sends it to the record head for recording. DIRECT REPRODUCE ELECTRONICS Direct reproduce electronics amplify the very weak input signals from the reproduce head, and send them out for listening or analysis, as needed. Figure 5-2 shows a basic block diagram of direct reproduce electronics.

Figure 5-2.—Direct reproduce electronics.

Direct reproduce electronics consists of three main parts:

5-2


1. Pre-amplifier circuit. This circuit takes the very weak reproduced signal from the reproduce head and (a) amplifies the signal, (b) removes any bias signal that was used during the recording process, and (c) sends the signal to the equalization and phase correction circuit. 2. Equalization and phase correction circuit. This circuit takes the pre-amplified signal and fixes any frequency response problems that the reproduce magnetic head may have caused. To better understand this, look at the voltage versus frequency response graph in figure 5-3. The top of the graph shows the input signal that comes from the pre-amplifier and the bottom shows the equalization signal generated by the equalization circuit. In the top part of the graph, note how the output voltage level varies as the frequency of the signal varies. This isn’t good. A good output voltage level is one that remains constant as the frequency changes. The equalization signal corrects this problem. Notice that when the input signal and the equalization signal are combined they cancel each other out. This allows a nice flat (voltage versus frequency) output signal to go to the output amplifier circuit.

Figure 5-3.—Equalization process.

3. Output amplifier circuit. This circuit takes the signal from the equalization and phase correction circuit and amplifies it for output. It also matches the magnetic recorder’s impedance to the output device that is used for listening or recording. FM RECORD ELECTRONICS FM record electronics process signals to be recorded differently than direct record electronics. Instead of recording the input signal just as it appears at the recorder’s input, FM record electronics use the input signal to vary (modulate) the carrier frequency of a record oscillator. The frequency modulated output signal of the record oscillator then becomes the signal that’s actually recorded onto the magnetic media. Figure 5-4 shows a block diagram of the FM record electronics.

5-3


Figure 5-4.—FM record electronics.

FM record electronics consist of three main parts: 1. Input pre-amplifier circuit. This circuit does two things: (a) it serves as an impedance matcher between the signal source and the magnetic recorder, and (b) it pre-amplifies the input signal. 2. Record oscillator circuit. This circuit generates a carrier signal onto which the input signal will be modulated. The input signal is used to vary (frequency modulate) the carrier signal. This is how the input signal gets frequency modulated onto the carrier signal. The output of this circuit is the frequency-modulated carrier signal. The center frequency of the carrier depends on two things: (a) the bandwidth of the signal you’re recording, and (b) the media onto which you’re recording. For magnetic tape, the carrier frequency can be as low as 1.688 kHz for an operating tape speed of 1-7/8 inches per second, and as high as 900 kHz for 120 inches per second. 3. Head driver circuit. This circuit takes the frequency-modulated output from the record oscillator circuit, amplifies it, and sends it to the magnetic head for recording. The output level of this circuit is set to be just below the magnetic saturation point of the magnetic media. FM REPRODUCE ELECTRONICS The FM reproduce electronics work just like direct reproduce electronics, with one exception. FM reproduce electronics must first demodulate the original input signal from the carrier frequency before the intelligence can be sent to the output device for listening or analysis. Figure 5-5 shows a block diagram of the FM reproduce electronics.

Figure 5-5.—FM reproduce electronics.

FM reproduce electronics consist of four main parts: 1. Pre-amplifier circuit. This circuit takes the frequency modulated carrier frequency from the reproduce head and amplifies it. 2. Limiter/demodulator circuit. This circuit takes the output of the preamplifier, stabilizes the amplitude level, and demodulates the signal intelligence from the carrier frequency.

5-4


3. Low-pass filter circuit. This circuit takes the signal intelligence from the limiter/demodulator circuit and cleans up any noise or left over carrier signal. 4. Output amplifier circuit. This circuit takes the output from the low-pass filter and amplifies it for output. It also matches the impedance of the magnetic recorder to the output device. Q-1. What two types of record and reproduce electronics are used by magnetic tape recorders? Q-2. The head driver circuit in a tape recorder’s direct record electronics component (figure 5-1) performs what function? Q-3. The equalization and phase correction circuit in a tape recorder’s direct reproduce electronics (figure 5-2) performs what function? Q-4. How do FM record electronics differ from AM (direct record) electronics? Q-5. The head driver circuit of a tape recorder’s FM record electronics (figure 5-4) performs what function? Q-6. What is the major difference between direct reproduce electronics and FM reproduce electronics?

SUMMARY Now that you’ve finished chapter 5, you should be able to (1) state the two types of record and reproduce electronics used on magnetic tape recorders and (2) describe the function and main parts of direct record and reproduce electronics and FM record and reproduce electronics. The following is a summary of important points in this chapter: DIRECT RECORD (AM) and FREQUENCY MODULATION (FM) are the two types of record and reproduce electronics used by magnetic tape recorders. The four main parts of DIRECT RECORD ELECTRONICS are the (1) input pre-amplifier circuit, (2) bias source, (3) summing network, and (4) head driver circuit. The three main parts of DIRECT REPRODUCE ELECTRONICS are the (1) pre-amplifier circuit, (2) equalization and phase correction circuit, and (3) output amplifier circuit. FM RECORD ELECTRONICS record a frequency modulated signal onto the magnetic tape. It has three main parts: (1) input pre-amplifier circuit, (2) record oscillator circuit, and (3) head driver circuit. FM REPRODUCE ELECTRONICS must demodulate the original input signal from the carrier signal. It has four main parts: (1) preamplifier circuit, (2) limiter and demodulator circuit, (3) low-pass filter circuit, and (4) output amplifier circuit.

5-5


ANSWERS TO QUESTIONS Q1. THROUGH Q6.

A1. a. Direct record (AM). b. Frequency modulation (FM). A2. It takes the signal from the summing network, amplifies it, and sends it to the record head for recording. A3. It generates an equalization signal which corrects any frequency response problems in the input signal from the pre-amplifier circuit. A4. Instead of recording the signal just as it appears at the recorder’s input, FM record electronics records a frequency-modulated carrier signal from a record oscillator (figure 5-4) onto the magnetic tape. A5. It amplifies the frequency-modulated output from the record oscillator and sends it to the record head. A6. FM record electronics must use a limiter and demodulator circuit (figure 5-5) to demodulate the signal intelligence from the carrier frequency.

5-6


CHAPTER 6

MAGNETIC TAPE RECORDING SPECIFICATIONS LEARNING OBJECTIVES After completing this chapter, you’ll be able to do the following: 1. Define the seven most common magnetic tape recording specifications. 2. Describe a magnetic tape recorder’s signal-to-noise ratio (SNR) specification, how it’s measured, and why a high SNR is important. 3. Describe a tape recorder/reproducer’s frequency-response specification, how it’s measured, and the three factors that can limit or degrade a recorder’s frequency response. 4. Describe a tape recorder’s harmonic-distortion specification, how it’s measured, and how a recorder produces harmonic distortion. 5. Describe a recorder’s phase-response specification, how it’s measured, and why good phase response is important. 6. Describe a recorder’s flutter specification, how it’s measured, and why minimal flutter is important. 7. Describe a recorder’s time-base error (TBE) specification, how it’s measured, and why minimal TBE is important. 8. Describe a multi-track magnetic tape recorder’s skew specification, how it’s measured, and why minimal skew is important.

INTRODUCTION Have you ever gone to a store to buy a magnetic tape recorder? Were you able to decide which of the displayed models was the good one to buy? Or, did you instead end up confused when the salesperson started spouting words like SNR, flutter, and bandwidth. If so, you weren’t alone. This chapter (1) defines the seven most common magnetic tape recording specifications, (2) describes their effect on the magnetic recording process, and (3) tells how to measure each specification. The remaining paragraphs in this chapter describe each of the following magnetic tape recorder specifications: 1. Signal-to-noise ratio 2. Frequency response 3. Harmonic distortion 4. Phase response

6-1


5. Flutter 6. Time-base error 7. Skew

SIGNAL-TO-NOISE RATIO Signal-to-noise ratio (SNR) is the first magnetic tape recorder specification we’ll describe. It’s one of the most important specifications of a magnetic tape recorder. SIGNAL-TO-NOISE RATIO DEFINITION The SNR is the ratio of the normal signal level to the magnetic tape recorder’s own noise level. It’s measured in decibels (dB). In other words, the higher the SNR of a magnetic tape recorder, the wider the range of input signals it can properly record and reproduce. The noise part of the signal-to-noise ratio is generated in the magnetic tape recorder itself. Although noise can be generated by almost any part of the magnetic tape recorder, it’s usually generated by either the magnetic heads or the magnetic tape. SIGNAL-TO-NOISE RATIO MEASUREMENT You can measure the SNR with a vacuum tube voltmeter (VTVM) and a signal generator. The equipment set up for measuring the SNR is shown in figure 6-1. After equipment setup, measure the SNR as follows:

Figure 6-1.—Test equipment setup for measuring signal-to-noise ratio.

1. Set the signal generator to inject a test signal into the tape recorder. The technical manual for the tape recorder you’re testing will tell you how to set up the signal generator. 2. While recording and reproducing, set the output level of the tape recorder’s reproduce electronics to a level that displays 0-dB reference on the VTVM. 3. Disconnect the signal generator. The voltage displayed on the VTVM will drop from 0-dB to some negative dB level. This level is the magnetic tape recorder’s SNR.

6-2


There are two things you should know when reading SNR specifications in technical manuals, equipment brochures, etc. First, the SNR is stated in three ways. You’ll see it as (1) root-mean-square (RMS) signal-to-RMS noise, (2) peak-to-peak signal-to-RMS noise, or (3) peak signal-to-RMS noise. If the SNR specification doesn’t state which way it was measured, you could be mislead. For example, a 25-dB RMS SNR is equal to a 34-dB peak-to-peak signal-to-RMS noise ratio, or a 28-dB peak signal-to-RMS noise ratio. Second, all SNR specifications should include the record level that was used. Since the SNR varies directly to the record level, you could be mislead by a SNR that doesn’t include the record level of the test signal used when the SNR was measured.

FREQUENCY RESPONSE The frequency-response specification of a magnetic tape recorder is sometimes called the bandwidth. A typical frequency-response specification might read within + / − 3 db from 100 Hz to 100 kHz at 60 ips. This means the magnetic tape recorder is capable of recording all frequencies between 100 Hz and 100 kHz at 60 inches per second (ips) without varying the output amplitude more than 3 dB. FREQUENCY-RESPONSE DEFINITION Frequency response is the amplitude variation with frequency over a specified bandwidth. Let’s convert this to plain English. The frequency-response specification of a magnetic tape recorder tells you the range of frequencies the recorder can effectively record and reproduce. What exactly does the word effectively mean? That’s hard to say because frequency response varies from recorder to recorder, and from manufacturer to manufacturer. But a good rule of thumb is that an effective frequency-response specification tells the lowest and highest frequencies that the recorder can record and reproduce with no more than + / − 3-dB difference in output amplitude. FREQUENCY-RESPONSE MEASUREMENT The equipment setup for measuring the frequency response of a magnetic tape recorder is the same as for measuring the signal-to-noise ratio. It’s shown in figure 6-1. After equipment setup, measure a recorder’s frequency response as follows: 1. Set the signal generator to output a test signal. The technical manual for the tape recorder will tell you how. 2. Set the recorder’s reproduce electronics output level to a 0-dB reference on the VTVM. 3. While recording at a set speed, vary the frequency of the signal generator from the lowest to highest frequency you’re checking. Make sure that the output level of the signal generator stays the same. 4. As you sweep through the frequencies, look at the VTVM. You’ll see the amplitude rise and fall as you vary the output frequency of the signal generator. As you approach the lowest and the highest frequencies that the magnetic tape recorder can effectively record, you’ll see the VTVM drop to less than − 3 dB. This determines the lower and upper limits of the frequency-response specification for that magnetic tape recorder.

6-3


FREQUENCY-RESPONSE LIMITING FACTORS Four factors that can limit or degrade the frequency response of magnetic tape recorders are: 1. A too-high or too-low bias signal level setting for the record head. 2. An improper reproduce head. 3. An improper tape transport speed. 4. A poor magnetic tape-to-head contact. The magnetic record head transforms the electrical signal into a magnetic field for recording onto magnetic tape. If the bias signal level is set to high, you might erase the higher frequencies. If it’s too low, you’ll get excessive tape distortion. The reproduce head transforms the magnetic field from the magnetic tape back into an electrical signal. As explained in chapters 3 and 5, the head gap of a recorder’s reproduce head and the operating speed of the magnetic tape transport determine the wavelength of the reproduce head. The wavelength determines the center frequency of a recorder’s frequency-response specification. Once you pass this center frequency, both below and above, the output voltage level of the recorder’s reproduce head will decrease. Figure 6-2 shows this. This is why the equalization circuits described in chapter 5, figure 5-3, are used.

Figure 6-2.—Frequency response of a reproduce head.

Poor tape-to-head contact can seriously degrade the record and reproduce process. Magnetic heads are designed to reduce tape-to-head gap as much as possible. A tape-to-head gap is extremely degrading at the higher frequencies. Figure 6-3 shows this. Note how a .1-mil gap causes only a small loss at 10 kHz. But, at 1 MHz, it causes a 46-dB loss! You must maintain tape-to-head contact. Keeping the magnetic tape recorder heads and tape transport clean is the best way to do this.

6-4


Figure 6-3.—Effects of poor tape-to-head contact.

Q-1. Two tape recorders have signal-to-noise ratios (SNRs) of 25-dB RMS and 35-dB RMS respectively. Which of the SNRs can record and reproduce the widest range of input signals and why? Q-2. You plan to measure your tape recorder’s SNR. What test equipment will you need? Q-3. Technical manuals for tape recorders can state the SNR in what three different ways? Q-4. The frequency-response specification of your tape recorder reads within +/− 3 dB from 150 Hz to 150 kHz at 60 ips. What does this mean? Q-5. While measuring frequency response, as the signal generator approaches the lowest and highest frequency the recorder can effectively record, the VTVM reading drops to less than − 3 dB. What does this indicate? Q-6. List four factors that can degrade the frequency response of magnetic tape recorders.

HARMONIC DISTORTION A magnetic tape recorder’s harmonic-distortion specification is very important. It usually determines where the record level of a recorder’s electronics should be set. The record level is also used to determine the signal-to-noise ratio and frequency-response specifications. A typical harmonic-distortion specification might read "1% third harmonic of a 100-kHz signal at 60 ips." This means that the magnetic tape recorder has 1% third-harmonic distortion of a 100-kHz signal at 60 ips.

6-5


HARMONIC-DISTORTION DEFINITION Harmonic distortion is the production of harmonic frequencies by an electronic system when a signal is applied at the input. When an input signal goes through nonlinear electronic circuitry, the output signal will include some harmonic distortion (or unwanted frequencies). If you analyzed this distortion, you’d see that a pattern exists. A pattern, whereby the frequency of each unwanted frequency is a multiple (×1, ×2, ×3, etc.) of the center frequency of the input signal. There are two types of harmonic distortion: even-order and odd-order. If the frequencies of the distortion are 2, 4, 6, etc., times the center frequency, it’s even-order harmonics. If the frequencies of the distortion are 3, 5, 7, etc., times the center frequency, it’s odd-order harmonics. Odd-order harmonics are normally caused by the magnetic tape itself. Even-order harmonics are normally caused by (1) permanently magnetized magnetic heads, (2) faulty circuits, or (3) asymmetrical or unbalanced bias signals. As you might guess, even-order harmonics can be reduced by doing the right maintenance and periodic performance tests. The primary harmonic distortion in magnetic tape recorder systems is third-order harmonics. If the level of third-order harmonics in a recorder increases, the level of distortion will also increase (figures 6-4A and B show this relationship). Two things that determine the level of third-order harmonics in a recorder are (1) the signal bias level, and (2) the record level. Figure 6-4A shows how third-order harmonic distortion decreases as the signal bias level increases. Figure 6-4B shows how the third harmonic increases gradually at first and then abruptly as the record level increases. That’s why the third harmonic is used to determine the normal record level.

Figure 6-4 A & B.—Effect of signal bias level and record level on harmonic-distortion level.

6-6


HARMONIC-DISTORTION MEASUREMENT Figure 6-5 shows a typical test equipment setup for measuring harmonic distortion. With this setup, the test signal from the signal generator is recorded and reproduced by the magnetic tape recorder at a normal record level. The amount of harmonic distortion is measured at the recorder’s output on the wave analyzer.

Figure 6-5.—Test equipment setup for measuring harmonic distortion.

The technical manual for the magnetic recorder you’re testing will tell you how to set up the test equipment. It’ll tell you to set up the wave analyzer to measure a specific frequency. This frequency will be one of the multiples (×1, ×2, ×3, etc.) of the frequency the signal generator is outputting. For example, let’s say the technical manual told you to set up the signal generator to input a 10-kHz test signal into the magnetic tape recorder. Since you want to measure third-order harmonics, the technical manual will tell you to set the wave analyzer to measure the amount of harmonic distortion at 30-kHz.

PHASE RESPONSE It used to be thought that the only important specifications of magnetic tape recorders were signal-to-noise ratio and frequency response. But now, with the need to record and reproduce more complex waveforms, such as telemetry and computer data, the phase-response specification becomes as important as frequency response. PHASE-RESPONSE DEFINITION Phase response is the expression of the variation of the phase shift with respect to frequency. A good magnetic tape recorder will have linearly increasing phase response as frequency increases. In simpler terms, good phase response shows that a magnetic recorder can reproduce a complex waveform (such as a square wave which has an infinite number of sine waves) without distorting it. Figure 6-6 shows both good and bad phase response.

6-7


Figure 6-6.—Pictures showing the effect of good and bad phase response on square-wave reproduction.

PHASE-RESPONSE MEASUREMENT You cannot directly measure phase response. The best way to check the phase response of a magnetic tape recorder is to record and reproduce a square wave and watch the output on an oscilloscope. If the output signal is symmetrical, like in figure 6-7, the recorder has good phase response.

Figure 6-7.—An example of good linear phase response.

Q-7. A recorder’s harmonic-distortion specification reads 2% third harmonic of a 100-kHz signal at 60 ips. What does this mean?

6-8


Q-8. What are three possible causes of even-order harmonics? Q-9. What number harmonic is the primary harmonic distortion in magnetic tape recorders? Q-10. When measuring harmonic distortion, you set the signal generator to input a 15-kHz test signal. To what frequency should you set the wave analyzer? Q-11. How should a tape recorder with good phase response reproduce a complex waveform, such as a square wave? Q-12. How could you check the phase response of a tape recorder?

FLUTTER The general audio and broadcast field coined the term flutter to describe what you’ll actually hear from the bad effects of this specification. FLUTTER DEFINITION Flutter is the result of non-uniform tape motion caused by variations in tape speed that produces frequency modulation of signals recorded onto magnetic tape. Flutter is usually expressed as a percent peak or a peak-to-peak value for instrumentation recorders and as a root-mean-square (RMS) value for audio recorders. It’s caused by magnetic tape transports. Low-frequency flutter (below 1000 Hz) is caused by the rotating parts of a tape transport such as: • Irregular magnetic tape supply or take-up reels. • Uneven or sticking guide rollers and pinch rollers. • Capstans. High-frequency flutter (above 1000 Hz) is caused by the fixed parts of a tape transport, such as fixed tape guides and magnetic heads. When the magnetic tape passes over a fixed tape guide or magnetic head, the transition from static to dynamic friction causes something called stiction. It’s this stiction that causes the variations in tape speed which, in turn, cause the flutter. As you might guess, it’s hard to prevent flutter. The only way to lessen flutter is through skilled engineering, machining, and design of magnetic tape recorders. FLUTTER MEASUREMENT There are many ways to measure flutter. Most are based on the fact that tape speed variations cause frequency modulation of a recorded tone. Figure 6-8 shows a typical setup for measuring the peak-to-peak value of flutter with a frequency-modulation (FM) demodulator and an oscilloscope. The technical manual for the magnetic tape recorder you’re testing will tell you how to set up the signal generator to output the test signal. After setting up the test equipment, follow these procedures:

6-9


Figure 6-8.—Test equipment setup for measuring flutter.

1. Record the test signal onto magnetic tape; then rewind the magnetic tape. This is necessary because you can’t measure flutter as you’re recording. Since the tape-speed variation past the record head is almost the same as past the reproduce head, the flutter level is too small to see. 2. After you rewind the tape, play it back. During playback, the output signal from the tape recorder goes through the FM demodulator to remove the original test signal. The waveform you now see on the oscilloscope is the actual flutter signal that was modulated onto the test signal. 3. Using the oscilloscope display, measure the peak-to-peak value of the flutter signal.

TIME-BASE ERROR The time-base error (TBE) specification of magnetic tape recorders is closely related to the flutter specification. In fact, the TBE is a direct measure of the effects of flutter on the stability of recorded data. TIME-BASE ERROR DEFINITION The TBE is the time-relationship error between two or more events recorded and reproduced from the same magnetic tape. It’s also defined as the displacement of a point on the magnetic tape from where it should have been, during a specific time interval. A typical TBE specification might read "+ / − 100 microseconds over a 10-millisecond time interval at a tape speed of 60 inches per second, referenced to a control tone." This means that the time-base error could cause a signal to jitter +/− 100 microseconds over a 10-millisecond period at a tape speed of 60 inches per second. TBE jitter introduces noise or unwanted frequency modulation (when using FM recording techniques) into the magnetic tape recording process. It can also cause a loss of accuracy in pulseduration modulation (PDM), pulse-coded modulation (PCM), or other magnetic recordings where precise timing relationships exist between two or more signals. TIME-BASE ERROR MEASUREMENT The simplest way to measure the TBE is with an oscilloscope. Figure 6-9 shows a typical test equipment setup for measuring TBE. After you set up the test equipment, measure the TBE as follows:

6-10


Figure 6-9.—Test equipment setup for measuring time-base error.

1. Set the signal generator to generate a test signal. The technical manual for the magnetic tape recorder you’re testing will tell you how. 2. Connect the test signal output from the signal generator to both the recorder’s input and the oscilloscope’s trigger (sync) input. 3. Connect the output of the tape recorder to the oscilloscope’s signal (vertical) input. 4. Record and reproduce the test signal. 5. Adjust the oscilloscope’s intensity control until you can see the TBE on the oscilloscope’s display. (Limit glare by using a hood on the oscilloscope’s display.)

SKEW This magnetic tape recording specification only applies to multi-tracked magnetic tape recorders. SKEW DEFINITION Skew is the inter-track fixed and dynamic displacement, or change in azimuth, encountered by different tracks across the width of the magnetic tape as it passes the magnetic heads. In other words, it’s the time difference between the tracks on a multi-tracked magnetic head. A typical skew specification might read "+/− 0.15 microseconds between adjacent tracks on the same head stack at 120 inches per second." This means that one of the tracks on a magnetic head could lead, or lag, the track next to it by as much as 0.15 microseconds at 120 ips. This specification applies to both fixed and dynamic skew. Fixed skew can be caused by • magnetic tape recorder electronics,

6-11


• gap scatter in the magnetic head stack, • azimuth alignment of the magnetic head stack, or • fixed difference in tension along the tape path You can minimize most fixed skew by adjusting the magnetic recorder’s electronics or by realigning the magnetic heads. Fixed skew errors usually do not show up when magnetic tapes are recorded and reproduced on the same tape recorder. Since fixed skew errors are additive, they’ll usually show up when you record on one magnetic tape recorder and then reproduce on another. Dynamic skew errors are caused by either the magnetic tape transport or the magnetic tape itself. If the tape transport guides are worn or sticking, the magnetic tape won’t properly pass over the magnetic heads. It’ll drift and pass the magnetic head at an angle (like a car skidding on an icy road). If the magnetic tape itself is warped or isn’t uniform across its width it, too, will cause dynamic skew. SKEW MEASUREMENT Skew is best measured with an oscilloscope. Figure 6-10 shows a typical test equipment setup for measuring skew. The technical manual for the magnetic tape recorder you’re testing will tell you how to set up the signal generator. After test equipment setup, measure the skew as follows:

Figure 6-10.—Test equipment setup for measuring skew.

6-12


1. Inject the test signal into a reference track and one other track of the multi-track magnetic tape recorder. (The reference track should be one of the two outside tracks of the magnetic head.) 2. Connect the output from the reference track to the sync input of the oscilloscope to trigger the horizontal sweep. 3. Connect the output from the other track to the vertical input of the oscilloscope. 4. While recording and reproducing the test signal, measure the fixed and dynamic skews which are displayed on the oscilloscope. Figure 6-10 shows how this looks.

Q-13. What causes flutter in a tape recorder’s output? Q-14. What causes low-frequency flutter (below 1000 Hz)? Q-15. What causes high-frequency flutter (above 1000 Hz)? Q-16. Your recorder’s TBE specification reads " +/− 80 microseconds over a 10 millisecond time interval at a tape speed of 60 ips, referenced to a control tone." What does this mean? Q-17. Why is it important to minimize TBE jitter in magnetic tape recordings where precise timing relationships exist between two or more signals? Q-18. The skew specification of your multi-tracked tape recorder reads " +/− 0.20 microseconds between adjacent tracks on the same head stack at 120 ips." What does this mean? Q-19. How can you minimize fixed skew? Q-20. When are fixed skew errors most likely to show up? Q-21. How do worn or sticking tape transport guides cause dynamic skew on a multi-track recorder?

SUMMARY Now that you’ve finished chapter 6, you should be able to describe the seven most common magnetic tape recording specifications and how to measure each specification. The following is a summary of important points in this chapter: The SIGNAL-TO-NOISE RATIO (SNR) is the ratio of the normal signal level to the tape recorder’s own noise level measured in dB. The higher a recorder’s SNR, the wider the range of signals it can record and reproduce. SNR IS STATED IN ONE OF THREE WAYS based on how it was measured. If you don’t know the way it was measured, you could be misled. A recorder’s FREQUENCY-RESPONSE specification is sometimes called its bandwidth. It tells the range of frequencies a recorder can effectively record and reproduce. Factors that can degrade a recorder’s frequency response are an improper bias level setting, reproduce head gap, or tape transport speed. Also, failure to clean the heads and the tape transport can cause poor tape-to-head contact.

6-13


HARMONIC DISTORTION is the production of unwanted harmonic frequencies when a signal is applied at the recorder’s input. The primary harmonic distortion in tape recorders is third order harmonics. It’s measured with a wave analyzer. You can reduce this distortion with proper preventive maintenance and periodic performance tests. Good PHASE RESPONSE means the recorder can reproduce complex waveforms such as square waves without distortion. The best way to check a recorder’s phase response is by recording and reproducing a square wave and checking the output on an oscilloscope. FLUTTER results from non-uniform tape motion caused by variations in tape speed. The tape speed variations are caused by design and machining deficiencies in the rotating and fixed parts of the tape transport. TIME-BASE ERROR (TBE) is the time-relationship error between two or more events recorded on and reproduced from the same magnetic tape. It causes TBE jitter, which introduces noise or loss of accuracy where precise timing relationships exist between two or more signals. SKEW is the time difference in microseconds between the tracks on a multi-tracked tape recorder. Fixed or dynamic skew can happen when one of the tracks on the multi-track head leads or lags the track next to it. Fixed skew errors only show up when you record on one recorder and reproduce on a different recorder. You can minimize fixed skew by adjusting the recorder’s electronics and aligning the heads. Dynamic skew errors are caused by worn or sticking tape transport guides or by warped magnetic tape.

ANSWERS TO QUESTIONS Q1. THROUGH Q21. A1. 35-dB RMS because the highest SNR can always record and reproduce the widest range of input signals. A2. A VTVM and a signal generator. (See figure 6-1.) A3. a. Root-mean-square (RMS) signal-to-RMS noise. b. Peak-to-peak signal-to-RMS noise. c. Peak signal-to-RMS noise. A4. The recorder can record all frequencies between 150 Hz and 150 kHz at 60 ips without varying the output amplitude more than 3 dB. A5. The upper and lower limits of the frequency response specification for that tape recorder. A6. a. A too-high or too-low bias signal level setting for the record head. b. An improper reproduce head gap. c. An improper tape transport speed. d. Poor tape-to-head contact.

6-14


A7. The recorder has 2% third-harmonic distortion of a 100-kHz signal at 60 ips. A8. a. Permanently magnetized heads. b. Faulty circuitry. c. Asymmetrical bias signal. A9. Third-order harmonic. A10. 45 kHz. A11. With no distortion. A12. Record and reproduce a square wave and see if the output on an oscilloscope is symmetrical. A13. Non-uniform tape motion caused by variations in tape speed. A14. Rotating parts of a tape transport, such as irregular tape reels, sticking guides and pinch rollers, and capstans. A15. Fixed parts of a tape transport, such as fixed tape guides and magnetic heads. A16. The TBE could cause a signal to jitter +/− 80 microseconds over a 10-millisecond period at a tape speed of 60 ips. A17. The jitter could cause noise and a loss of accuracy. A18. One of the tracks on a magnetic head could lead or lag the track next to it by as much as 0.20 microseconds at 120 ips. A19. Adjust the recorder’s electronics or realign the magnetic heads. A20. When you record on one tape recorder and then reproduce on a different recorder. A21. The tape drifts past the multi-track head at an angle.

6-15



CHAPTER 7

DIGITAL MAGNETIC TAPE RECORDING LEARNING OBJECTIVES After completing this chapter, you’ll be able to do the following: 1. Describe the characteristics of digital magnetic tape recording and the difference between analog and digital recording. 2. Describe each of the three formats for digital magnetic tape recording (serial, parallel, and serialparallel). 3. Define the following terms as they apply to digital magnetic tape recording: mark, space, bit-cell period, packing density, and bit-error rate (BER). 4. Describe the eight most common methods for encoding digital data onto magnetic tape. 5. Describe the characteristics and use of the following categories of digital magnetic tape recorders: (1) computer-compatible, (2) telemetry, and (3) instrumentation.

INTRODUCTION TO DIGITAL MAGNETIC TAPE RECORDING This chapter introduces you to digital magnetic tape recording. It describes (1) the three formats for digital magnetic tape recording, (2) the eight methods of encoding digital data onto magnetic tape, and (3) the configuration differences between the three types of digital tape recorders. Until now, you’ve learned about magnetic tape recording from an analog point-of-view. That is, the signal you record and reproduce is the actual analog input signal waveform. In digital magnetic tape recording, the signal you record and reproduce is, instead, a series of digital pulses. These pulses are called binary ones and zeros. These ones and zeros can represent one of three types of data: (1) data used by digital computers, (2) pulsed square-wave signals, or (3) digitized analog waveforms. The digital magnetic tape recording process stores data onto tape by magnetizing the tape to its saturation point in one of two possible polarities: positive (+) or negative (−). The saturation point of magnetic tape is the point where the magnetic tape is magnetized as much as it can be.

DIGITAL MAGNETIC TAPE RECORDING FORMATS There are three digital magnetic tape recording formats: serial, parallel, and serial-parallel. Each of these is described below. Figure 7-1 shows each of the three formats as they apply to recording an eight-bit binary data stream.

7-1


SERIAL DIGITAL MAGNETIC TAPE RECORDING FORMAT This is the simplest of the three digital magnetic tape recording formats. It’s usually used when recording instrumentation or telemetry data. In this format, the incoming data pulses are recorded onto a single recorder track of the magnetic tape in a single, continuous stream. Figure 7-1A shows how this looks.

Figure 7-1A.—Digital magnetic tape recording formats.

PARALLEL DIGITAL MAGNETIC TAPE RECORDING FORMAT In this format, the incoming data pulses come in on more than one input channel and are recorded side-by-side onto more than one tape track. The data pulses across the width of the magnetic tape are related to each other. Figure 7-1B shows how this looks. This format is usually used to store computer data.

Figure 7-1B.—Digital magnetic tape recording formats.

7-2


SERIAL-PARALLEL DIGITAL MAGNETIC TAPE RECORDING FORMAT This format is more complex. It takes a serial input stream of data pulses, breaks them up, and records them on more than one recorder track. When the tape is reproduced, the recorder recombines the broken-apart data into its original form. Figure 7-1C shows how this looks. The serial-parallel format is usually used in instrumentation recording when the input data rate is high.

Figure 7-1C.—Digital magnetic tape recording formats.

DIGITAL MAGNETIC TAPE RECORDING DEFINITIONS Before we describe the methods for encoding digital data onto magnetic tape, let’s define the following terms: Mark: The voltage state of a digital one (1) data bit. It’s also sometimes called true. Space: The voltage state of a digital zero (0) data bit. It’s also sometimes called false. Bit-cell period: The time occupied by a single digital bit. Packing density: The number of bits per fixed length of magnetic tape per track. There are three categories of packing density: 1. Low density—200 to 1,000 bits per inch (bpi). 2. Medium density—1,000 to 8,000 bpi. 3. High density—8,000 to 33,000 bpi. Bit-error rate: The number of bits within a finite series of bits that will be reproduced incorrectly. Q-1. In digital magnetic tape recording, the series of recorded digital pulses can represent what three types of data? Q-2. What three formats are used for digital magnetic tape recording? Q-3. What format of digital tape recording is normally used to store computer data?

7-3


Q-4. What format of digital tape recording takes a serial input stream of data pulses, breaks them up, and records them on more than one data track? Q-5. What format of digital tape recording is normally used to record instrumentation or telemetry data?

DIGITAL MAGNETIC TAPE RECORDING ENCODING METHODS This section describes how digital data is electrically encoded onto the magnetic tape. The following paragraphs describe the eight most common digital data encoding methods. 1. Return to bias (RB) 2. Return to zero (RZ) 3. Non-return to zero (NRZ) and these four variations of the NRZ method: a. Non-return-to-zero level (NRZ-L) b. Enhanced non-return-to-zero level (E-NRZ-L) c. Non-return-to-zero mark (NRZ-M) d. Non-return-to-zero space (NRZ-S) 4. Bi-phase level RETURN-TO-BIAS (RB) ENCODING The RB encoding method uses magnetic tape that is pre-set to one of the two polarities (+ or −). This pre-sets the magnetic tape to all zeros. Digital ones are then recorded onto the magnetic tape by magnetizing the tape in the opposite polarity. After each one pulse, the tape returns to its original bias condition. Figure 7-2 shows the magnetic tape preset to a negative bias condition. It also shows how the digital data word 0100110001 is stored onto the magnetic tape using the RB encoding method.

Figure 7-2.—Return-to-bias (RB) digital encoding method.

This method has a serious drawback: It requires an external clocking signal to read the zeros stored on the tape. 7-4


RETURN-TO-ZERO (RZ) ENCODING The RZ encoding method uses magnetic tape that is normally in a neutral condition (the tape is not biased positively or negatively). A digital one is recorded as a positive-going pulse: a digital zero is recorded as a negative-going pulse. The magnetic tape returns to its neutral state in between pulses. Figure 7-3 shows the magnetic tape in its neutral state. It also shows how the digital data word 0100110001 is stored onto the magnetic tape using return-to-zero encoding.

Figure 7-3.—Return-to-zero (RZ) digital encoding method.

NON-RETURN-TO-ZERO (NRZ) ENCODING The NRZ encoding method is, by far, the most widely used. It’s accurate, simple, and reliable. It does not return the magnetic tape to its neutral state in between pulses. The magnetic tape is always in saturation, either positively or negatively. The polarity of the saturating signal only changes when incoming data changes from a zero to a one and vice versa. Figure 7-4 shows how the digital data word 101100011010 is stored onto the magnetic tape using the NRZ encoding method.

Figure 7-4.—Non-return-to-zero (NRZ) digital encoding method.

7-5


There are four widely used variations to the basic NRZ encoding method. Each of these is described in the following paragraphs. Non-Return-To-Zero-Level (NRZ-L) Encoding In NRZ-L encoding, the polarity of the saturating signal changes only when the incoming signal changes from a one to a zero or from a zero to a one. Figure 7-4 also shows how the digital data word 101100011010 is stored onto the magnetic tape using the NRZ-L encoding method. Note that the NRZ-L method looks just like the NRZ method, except for the first input one data bit. This is because NRZ does not consider the first data bit to be a polarity change, where NRZ-L does. The NRZ-L encoding method isn’t normally used in higher density (over 20,000 bpi) digital magnetic recording. This encoding method is sometimes called the non-return-to-zero-change (NRZ-C) encoding method. Enhanced Non-Return-to-Zero-Level (E-NRZ-L) Encoding This encoding method takes the basic NRZ-L data and adds a parity bit to it after every seven incoming data bits. The polarity of the parity bit is such that the total number of ones in the eight-bit data word will be an odd count. Figure 7-5 shows how the digital data word 0100010 is stored onto the magnetic tape using the E-NRZ-L encoding method.

Figure 7-5.—Enhanced non-return-to-zero-level (E-NRZ-L) digital encoding method.

Before the parity bit is added, the original incoming data is compressed in time. This is done so that when the parity bit is added, the eight-bit data word takes up the same amount of time as the originalseven bit data word. When the tape is reproduced, the parity bit is taken out. This encoding method works very well in high density (up to 33,000 bpi) magnetic tape recording. And, it offers an extremely good bit-error rate of 1 error per 1 million bits. Non-Return-to-Zero-Mark (NRZ-M) Encoding The NRZ-M encoding method is probably the most widely used encoding method for 800-bpi digital magnetic tape recording. In this method, the polarity of the saturating signal changes when the incoming signal is a one. An incoming zero would not change the polarity of the saturating signal. NRZ-M offers better protection from error than straight NRZ. In NRZ-M, there’s a one-to-one relationship between incoming data and polarity changes. If one data bit is lost, only that one bit is lost.

7-6


Whereas, in straight NRZ, if one bit is lost, all of the bits that follow will be exactly the opposite in polarity from what they should be. Figure 7-4 also shows how the digital data word 101100011010 is stored onto the magnetic tape using the NRZ-M encoding method. Non-Return-to-Zero-Space (NRZ-S) Encoding The NRZ-S encoding method works just like NRZ-M encoding, with one exception. Instead of the saturating signal changing polarity when the incoming data signal is a one, it changes when the incoming data signal is a zero. BI-PHASE LEVEL ENCODING The bi-phase level encoding method records two logic levels for each incoming data bit. When an incoming data bit is a one, bi-phase level recording records a zero-one. When an incoming data bit is a zero, bi-phase level recording records a one-zero. This encoding method helps to overcome any low-frequency response problems that the magnetic tape recorder may have. Figure 7-6 shows how the digital data word 101000111001 is stored onto magnetic tape using the bi-phase encoding method.

Figure 7-6.—Bi-phase level digital encoding method.

Bi-phase encoding requires exactly twice the bandwidth of NRZ-L. That’s why it’s mostly used in medium-density digital magnetic tape recording. In fact, this encoding method is probably the most widely used encoding method for 1600-bpi digital magnetic tape recording.

DIGITAL MAGNETIC TAPE RECORDER USES As you already know, digital magnetic tape recorders are used to store and retrieve digital data. These recorders fall into one of three categories, (1) computer compatible, (2) telemetry, and (3) instrumentation. COMPUTER-COMPATIBLE DIGITAL TAPE RECORDERS Computer-compatible digital tape recorders store and retrieve computer programs and data. They’re usually multi-tracked tape recorders with at least two, and up to nine, tracks for data. They use either 1/4" or 1/2" magnetic tape on either reels or cartridges. TELEMETRY DIGITAL TAPE RECORDERS Telemetry digital magnetic tape recorders are more commonly called wideband recorders. They’re used for recording radar signals and other pulsed square-wave type signals with a bandwidth of 500 kHz to 2 MHz. They’re also multi-tracked tape recorders that have either 14 or 28 tracks for data. They use 1" magnetic tape on either aluminum or glass reels.

7-7


INSTRUMENTATION MAGNETIC TAPE RECORDERS Instrumentation digital magnetic tape recorders are used to record other special signals with a bandwidth of less than 500 kHz. They, too, are multi-tracked recorders, normally with 7 tracks for data. They use 1/2" magnetic tape on metal or glass reels. Q-6. Which of the eight methods for encoding digital data onto magnetic tape is most widely used because it’s accurate, simple, and reliable? Q-7. Which digital data tape encoding method presets the magnetic tape to all zeros and then records digital ones onto the tape? Q-8. Which digital data encoding method records a digital one as a positive pulse and a digital (zero) as a negative pulse and returns the tape to neutral between pulses? Q-9. Which method of digital data encoding does NOT return the tape to neutral between pulses but, instead, saturates the tape positively or negatively as the incoming data changes between zero and one? Q-10. What are the four widely used variations of the NRZ encoding method? Q-11. Which digital data encoding method helps overcome a tape recorder’s low-frequency response problems by recording two logic levels for each incoming data bit? Q-12. Digital magnetic tape recorders used to store and retrieve digital data fall into what three categories? Q-13. What category of digital tape recorder is used for recording pulsed square-wave signals with a bandwidth of 500 kHz to 2 MHz? Q-14. What category of digital tape recorder is used to record special signals with a bandwidth of less than 500 kHz?

SUMMARY Now that you’ve finished chapter 7, you should be able to describe (1) the characteristics of digital magnetic tape recording, (2) the three formats for digital magnetic tape recording, (3) the eight methods for encoding digital data onto magnetic tape, and (4) the characteristics and uses of the three types of digital magnetic tape recorders. The following is a summary of important points in this chapter: Digital magnetic tape recorders record a SERIES OF DIGITAL PULSES called binary ones and zeros. These digital pulses can represent (1) data used by digital computers, (2) pulsed square-wave signals, or (3) digitized analog waveforms. Three FORMATS FOR DIGITAL MAGNETIC TAPE RECORDING are serial, parallel, and serial-parallel. There are EIGHT COMMONLY USED METHODS FOR ENCODING digital data onto magnetic tape. The non-return-to-zero (NRZ) method and the four variations of the NRZ method are most commonly used.

7-8


THREE CATEGORIES OF DIGITAL MAGNETIC TAPE RECORDERS are (1) computercompatible, (2) telemetry, and (3) instrumentation.

ANSWERS TO QUESTIONS Q1. THROUGH Q14. A1. a. Data used by digital computers. b. Pulsed squarewave signals. c. Digitized analog waveforms. A2. (1) Serial, (2) parallel, and (3) serial-parallel. A3. Parallel digital magnetic tape recording. A4. Serial-parallel digital magnetic tape recording. A5. Serial digital magnetic tape recording. A6. Non-return-to-zero (NRZ) encoding. A7. Return-to-bias (RB) encoding. A8. Return-to-zero (RZ) encoding. A9. Non-return-to-zero (NRZ) encoding. A10. a. Non-return-to-zero level (NRZ-L). b. Enhanced non-return-to-zero level (E-NRZ-L). c. Non-return-to-zero mark (NRZ-M). d. Non-return-to-zero space (NRZ-S). A11. Bi-phase level encoding. A12. a. Computer-compatible digital tape recorders. b. Telemetry digital tape recorders. c. Instrumentation digital tape recorders. A13. Telemetry digital tape recorders. A14. Instrumentation digital tape recorders.

7-9



CHAPTER 8

MAGNETIC DISK RECORDING LEARNING OBJECTIVES After completing this chapter, you'll be able to do the following: 1. Describe how flexible (floppy) disks are constructed; how data is organized on them; how they are handled, stored, and shipped; and how they are erased. 2. Describe how fixed (hard) disks are constructed; how data is organized on them; how they are handled, stored, and shipped; and how they are erased. 3. Describe each of the following methods for recording (encoding) digital data onto magnetic disks: frequency-modulation encoding, modified frequency-modulation encoding, and run length-limited encoding. 4. Describe the characteristics of floppy disk drive transports and hard disk drive transports and describe the preventive maintenance requirements of each type. 5. Describe the following parts of the electronics component of a magnetic disk drive: control electronics, write/read electronics, and interface electronics. 6. Describe the five most common types of disk drive interface electronics. 7. Define the following magnetic disk recording specifications: seek time, latency period, access time, interleave factor, transfer rate, and recording density.

INTRODUCTION Magnetic disk recording was invented by International Business Machines (IBM) in 1956. It was developed to allow mainframe computers to store large amounts of computer programs and data. This new technology eventually led to what's now known as the computer revolution. This chapter introduces you to the following aspects of magnetic disk recording: •

Disk recording mediums

•

Disk recording methods

•

Disk drive transports

•

Disk drive electronics

•

Disk recording specifications

8-1


MAGNETIC DISK RECORDING MEDIUMS There are two types of disk recording mediums: flexible diskettes and fixed (hard) disks. The following paragraphs describe (1) how flexible and fixed disks are made; (2) how data is organized on them; (3) how to handle, store, and ship them; (4) and how to erase them. FLEXIBLE MAGNETIC RECORDING DISKETTES Flexible diskettes, or floppy disks as they're more commonly called, are inexpensive, flexible, and portable magnetic storage mediums. They have the following characteristics. Floppy Disk Construction Floppy disks are made of round plastic disks coated with magnetic oxide particles. The disks are enclosed in a plastic jacket which protects the magnetic recording surface from damage. Floppy disks come in three sizes: 8 inch, 5 1/4 inch, and 3 1/2 inch. Figure 8-1 shows each size. All disk sizes can either be single-sided or double-sided. Single-sided disks store data on only one side of the disk; double-sided disks store data on both sides.

Figure 8-1.—Floppy disk construction.

When floppy disks are manufactured, the magnetic oxide coating is applied to both sides. Each disk is then checked for errors. Disks certified as single-sided, are checked on only one side; disks certified as double-sided are checked on both sides. Floppy disks are also classified by how much data they can store. This is called a disk's density. There are three levels of floppy disk density: single-density, double-density, and high-density. Some of the more common types of floppy disks and their storage capacity are listed below:

8-2


TYPE OF FLOPPY DISK 5-1/4" double-sided, double-density

STORAGE CAPACITY 360,000 bytes 1,200,000 bytes

5-1/4" double-sided, high-density 3-1/2" double-sided, double-density

720,000 bytes 1,400,000 bytes

3-1/2" double-sided, high-density

Floppy Disk Data Organization Data is stored on a floppy disk in circular tracks. Figure 8-2 shows a circular track on a floppy disk. The total number of tracks on a floppy disk is permanently set by (1) the number of steps the disk drive's magnetic head stepper motor can make, and (2) whether the disk drive has a magnetic head for one or both surfaces of the floppy disk. These two things will also determine the type of floppy disk that's needed. Each type of disk is rated with a number that represents how many tracks per inch (TPI) it can hold. Some common track capacities are 40, 48, 80, and 96 TPI.

Figure 8-2.—Tracks and sectors of a magnetic disk.

Each track of a floppy disk is broken up into arcs called sectors. A disk is sectored just as you'd slice an apple pie. Figure 8-2 shows the sectors of a floppy disk. How many slices are made? That depends on who made the disk and in what host computer the disk is used. There are two methods for sectoring a floppy disk: 1.

Hard Sectoring: This method sectors the disk physically. The disk itself will have marks or sensor holes on it that the floppy disk drive hardware can detect. This method is seldom used today.

2.

Soft sectoring: This method sectors the disk logically. The computer software determines the sector size and placement, and then slices the disk into sectors by writing codes on the disk. This

8-3


is called formatting or initializing a floppy disk. During formatting, if the computer software locates a bad spot on the disk, it locks it out to prevent the bad spot from being used. Soft sectoring is by far the most popular method of sectoring a floppy disk. Once a floppy disk is formatted, the computer uses the disk's side number, a track number, and a sector number (together) as an address. It's this address that locates where on the disk the computer will store the data. Floppy Disk Handling, Storage, and Shipping Floppy disks hold a lot of data. Even disks with only a 360,000-byte storage capacity can hold 180 pages of data! That's why it's important to handle, store, and ship floppy disks properly. One hundred and eighty pages of data is a lot of data to retype just because of carelessness. Before we get into disk handling and storage procedures, let's first learn about head-to-disk contact. Do you remember reading in chapter 2 that the quality of magnetic tape recording is seriously degraded when dust, dirt, or other contaminates get between the magnetic head and the tape? Well, the same is true for magnetic disk recording. In fact, head-to-disk contact is extremely important with floppy disks. This is because floppy disk drives, unlike magnetic tape drives, spin at very high speeds — 300 to 600 revolutions-per-minute (RPM). If anything gets between the head and the recording surface, you can lose data, or even worse, you can damage the magnetic head and the disk's recording surface. Figure 8-3 shows the size relationship between a disk drive's magnetic head, the disk recording surface, and some common contaminants.

Figure 8-3.—Size relationship of distance between head and disk to contaminants.

You must handle, store, and ship floppy disks with great care if you want them to stay in good condition. Here's some specific precautions you should take: • DO always store 8" and 5-1/4" floppy disks in their envelopes when not in use. Dirt, dust, etc., can get on the recording surface through the magnetic head read/write access hole if you leave it exposed for any length of time.

8-4


• DO always write on a floppy disk label first, and then place the label on the disk. NEVER write directly on a floppy disk. If you absolutely must write on a disk, use a felt-tip marker. • DO hold floppy disks by their outside corners only. DO NOT bend them. And NEVER, NEVER paper clip them to anything, or anything to them. • DO always store floppy disks in an upright position. Laying them on their side can cause them to warp. • DO always keep floppy disks away from food, liquids, and cigarette smoke. All of these can easily damage floppy disks. • DO always ship floppy disks in appropriate shipping containers. When shipping only a few disks, use the specially designed cardboard shipping envelopes. If you must ship a large number of disks, make sure the box you use is sturdy enough to protect the disks from damage. A good rule of thumb is to use a shipping box that allows you to place 2 inches of packing material around the disks. • DO NOT touch any exposed recording surfaces. Something as simple as a fingerprint can destroy the data on a floppy disk. • DO NOT expose a floppy disk to magnetic fields. Telephones, magnetic copy holders, printers, and other electronic equipment generate magnetic fields that can destroy the data on a floppy disk. • DO NOT expose floppy disks to extreme heat or cold. Floppy disks will last longer if they're stored in an environment that stays around 70-80 degrees Fahrenheit and 30-60 percent relative humidity. Floppy Disk Erasing There are two ways to erase a floppy disk: (1) degauss it and then reformat it, or (2) just reformat it. The process for degaussing floppy disks is the same as for degaussing magnetic tape. Refer back to chapter 2 for the details on this. If the floppy disks were used to store classified, or unclassified but sensitive information, they can't be de-classified by erasing them. This is because, with the right equipment and software, the data that was on the disk can be reconstructed. Floppy disks are cheap and easy to replace. If you can't re-use the floppy disks to store other classified data, just destroy them, using the procedures in OPNAVINST 5510.1, DON Information and Personnel Security Program Regulation. Q1. Floppy disks are manufactured in what three sizes? Q2. What type of floppy disk is made to store data on both sides of the disk? Q3. What are the three levels of floppy disk density? Q4. What is the storage capacity of a 5-1/4" double-sided, high-density floppy disk? Q5. The floppy disks you are using have a rating of 96 TPI. What does this mean? Q6. The process of formatting a floppy disk is called what type of sectoring?

8-5


Q7. What three components determine the address that locates where on a floppy disk the computer will store the data? Q8. Why should you always store floppy disks in their envelopes? Q9. Why should you never place floppy disks near telephones or other electronic equipments that generate magnetic fields? Q10. What are the two ways to erase floppy disks? FIXED MAGNETIC RECORDING DISKS Fixed disks, or hard disks as they're more commonly called, are expensive, rigid, semi-portable, magnetic storage mediums. They have the following characteristics: Hard Disk Construction Most hard disks are made of aluminum platters coated on both sides with either iron oxide or thin-film metal magnetic coatings. The first type, iron oxide, is the most common (you can recognize this coating by its rust color). This is the same oxide coating that's used on magnetic tape. The second type of coating, thin-film metal, is the newer and better of the two. This coating is a microscopic layer of metal that's bonded to the aluminum platter. You can recognize it by its shiny silver color. Thin-film metal-coated hard disks are becoming more and more popular because they allow more data to be stored in less space. Hard disks can hold a lot of data, the smallest disk being 10,000,000 bytes, and the largest being about 2,500,000,000 bytes (and they're working on larger ones). Hard disk platters come in many sizes, ranging from 14" to 2". The most common sizes are 3-1/2", 5-1/4" and 14". The first two sizes are usually used with smaller personal computers. The 14" size is usually used with the larger mini and mainframe computers. Most hard disk drives use more than one hard disk platter to store data. These are called disk packs. Some hard disk drives use removable hard disk platters. These can use just one platter, or they can use disk packs containing many platters. Most of the multi-platter removable hard disk drives in use today use 14" hard disk platters. Figure 8-4 shows a hard disk-pack.

8-6


Figure 8-4.—Magnetic hard disk pack.

Hard Disk Data Organization Data is stored on a hard disk the same way it's stored on a floppy disk, in circular tracks. The total number of tracks on a hard disk is set, just like floppy disk, by (1) the number of steps the disk drive's magnetic head stepper motor can make, and (2) whether the disk drive has a magnetic head for one or both surfaces of the hard disk platter. A computer places data on a hard disk using one of two methods, either (1) the cylinder method, or (2) the sector method. The manufacturer of the hard disk drive decides which method to use. THE CYLINDER METHOD.—This method uses a cylinder as the basic reference for placing data on a hard disk. Look at figure 8-5 view A. This is a picture of a disk pack containing six hard disk platters. Notice that this particular disk drive uses only 10 out of the 12 available recording surfaces. If you imagine that you're looking down through the disk pack from above, the tracks with the same number on each of the 10 recording surfaces will line up. Put together, these tracks make up a cylinder. Each of these 10 tracks with the same number, one on each recording surface, can be read from and written to by one of the disk drive's 10 read/write magnetic heads that are positioned by the five access arms.

8-7


Figure 8-5.—Cylinder and sector method of organizing data on a hard disk pack.

So, to locate a place to store data using the cylinder method, a computer must specify the cylinder number, the recording surface number, and the record number. Figure 8-5 view A shows record number 1 stored on cylinder 25 of recording surface number 6. Special data is stored on each track to tell the computer where the start of a track is. THE SECTOR METHOD.—Although we talked about this method earlier under the heading "Floppy Disk Data Organization," we need to repeat it here as it also applies to hard disks. The sector method of organizing data on a hard disk is actually a variation of the cylinder method. As you already know, the sector method slices up a hard disk into pie-shaped slices (just like floppy disks). The total number of slices is set by the hard disk drive manufacturer. Figure 8-5 view B shows an example of the sector method. Unlike a floppy disk drive, which locates a place on the disk using the surface number, track number, and sector number, a hard disk drive locates a place on the disk by using the surface number, cylinder number, and sector number. This is true even if the hard disk has only one platter. That's because both surfaces of that one platter still form a cylinder. Hard Disk Handling, Storage, and Shipping Hard disks hold a lot more data than floppy disks; even the lowest capacity hard disk can hold 5,000 pages of data! That's why it's important to handle, store, and ship hard disks properly. If you think 180 pages of data is a lot to retype, just think of retyping 5,000 pages!

8-8


Hard disk drives spin at a very high speed of about 3600 RPM. It is extremely important that nothing gets between the head and the recording surface. If it does, you can lose data and you can damage both the magnetic head and the disk's recording surface. Most hard disk failures involve a head-crash. It's the worst thing that can happen to a hard disk. A head-crash is the result of the disk drive's magnetic heads crashing into the recording surface and grinding into the hard disk platter. Figure 8-6 shows a good hard disk platter and a bad hard disk platter that was the victim of a head-crash.

Figure 8-6.—Example of a hard disk crash.

You must handle, store, and ship hard disks with extreme care if you want them to stay in good condition. Here are some specific precautions you should take: • DO always store removable hard disks in their storage cases when not in use. Dirt, dust, etc., can get on the recording surface through the magnetic head read/write access hole if you leave it exposed for any length of time. • DO always handle hard disks with extreme care. DO NOT drop them. Even a small drop of 2" can warp a hard disk platter enough to cause a head crash. • DO always keep removable hard disks away from food, liquids, and cigarette smoke. All of these can easily cause damage. • DO always ship hard disks in their proper shipping containers. If you don't have the original shipping container, make sure the shipping box is sturdy and big enough to allow 2" of packing material around the disk. Save the original packing material for the hard disk just in case you need to ship it somewhere. • DO NOT touch any exposed recording surfaces. Something as simple as a fingerprint can cause a head crash and destroy a hard disk platter. • DO NOT expose hard disks to extreme heat or cold. Hard disks will last longer if they're stored in an environment that stays around 70-80 degrees Fahrenheit and 30-60 percent relative humidity. Hard Disk Erasing There are two ways to erase a hard disk: (1) degauss it and then reformat it, or (2) just reformat it. As you might guess, the first method can only be used for removable hard disk platters. The second method 8-9


(reformatting) is the most common. If you must degauss a removable hard disk, the process is the same as degaussing magnetic tape. Refer back to chapter 2 for the details on this. If the hard disks were used to store classified information or unclassified but sensitive information, you can't de-classify the hard disks by erasing them. This is because with the right equipment and software, the data that was on the disk can be reconstructed. If you can't re-use the hard disks to store other classified data, you must sanitize or destroy them, using the procedures in OPNAVINST 5510.1. Q11. What are the three most common sizes of hard disk platters? Q12. Computers use what two methods to place data on a hard disk? Q13. Which method for placing data on hard disks divides a hard disk into pie shaped slices? Q14. When computers use the cylinder method to store data on a hard disk pack, what three items make up the address that tells the computer where on a specific disk to store the data? Q15. What is the most common type of hard disk failure? Q16. Hard disks should be stored in an environment that stays within what relative humidity and temperature range? Q17. What is the most common method for erasing a hard disk?

RECORDING DIGITAL DATA ON MAGNETIC DISKS Digital data is stored on a magnetic disk using magnetic pulses. These pulses are generated by passing a frequency modulated (FM) current through the disk drive's magnetic head. This FM current generates a magnetic field that magnetizes the particles of the disk's recording surface directly under the magnetic head. The pulse can be one of two polarities, positive or negative. Digital data isn't just recorded onto a magnetic disk as-is. Instead, it's encoded onto the disk. Three of the most popular encoding methods are (1) frequency modulation (FM), (2) modified frequency modulation (MFM), and (3) run length limited (RLL). The following paragraphs describe each of these encoding methods. FREQUENCY MODULATION (FM) ENCODING The FM method of encoding digital data onto a disk uses two pulse periods to represent each bit of data (a pulse period is the time span of one pulse). The first pulse period always contains a clock pulse. The second pulse-period may, or may not, contain a data pulse. If the digital data is a "1," a data pulse will be present in the second pulse-period. But, if the digital data is a "0," then there's no pulse present. Figure 8-7 shows this. The clock pulse, which is always present, tells the disk drive's interface that the next pulse is a data pulse. It is used to compensate for changes in the disk's rotation speed.

8-10


Figure 8-7.—Frequency-modulation (FM) encoding.

The FM method of encoding is old, and isn't used much anymore. You'll only see it in some of the older single-sided, single-density floppy disk drives, and in some of the older military hard disk drives. MODIFIED FREQUENCY-MODULATION (MFM) ENCODING The MFM method of encoding digital data onto a disk is more popular because it is more efficient and more reliable than straight FM encoding. MFM encoding still uses two pulse periods, but uses a lot fewer pulses to store the digital data onto the disk. It does this in two ways: 1. It does away with the clock pulse that the FM method uses. 2. It stores a digital "1" by generating a no-pulse and a pulse in the two pulse periods. It stores a digital "0" as either a pulse and a no-pulse if the last bit was a "0," or as two no-pulses if the last bit was a "1." Figure 8-8 shows this.

Figure 8-8.—Modified frequency-modulation (MFM) encoding.

8-11


RUN LENGTH-LIMITED (RLL) ENCODING The RLL method of encoding digital data onto a disk is actually a refinement of the MFM encoding method. As its name implies, RLL limits the run length (distance) between pulses (also called flux reversals) on a hard disk. The basic theory of RLL encoding is that you can store more data in less space if you reduce the number of flux reversals (or pulses) that you must record. There are several versions of the RLL encoding method, the most popular version being the 2,7 RLL. This means that no fewer than 2 no-pulses and no more than 7 no-pulses can occur between pulses.

MAGNETIC DISK DRIVE TRANSPORTS Magnetic disk drive transports, like magnetic tape drive transports, move the magnetic disks across the magnetic heads and protect the disks from damage. The following paragraphs will (1) introduce you to the characteristics of both floppy and hard disk drive transports, and (2) describe their preventive maintenance requirements. FLOPPY DISK DRIVE TRANSPORTS Floppy disk drive transports contain the electromechanical parts that (1) rotate the floppy disk, (2) write data to it, and (3) read data from it. Figure 8-9 shows a typical floppy disk drive transport. Four of the drive transport's more important parts are the

Figure 8-9.—Typical floppy disk drive transport.

8-12


1. drive motor/spindle assembly, 2. head arm assembly, 3. actuator arm assembly, and 4. drive electronics circuit board. Drive Motor/Spindle Assembly The spindle in this assembly holds the floppy disk in place while it spins. The drive motor spins the spindle at 300 to 600 RPM, depending on the type of floppy disk drive. The following is a list of the types of floppy disk drives and the spinning speeds of their spindles.

FLOPPY DISK DRIVE TYPE

SPINNING SPEED

5-1/4" 360-KB storage

300 RPM

5-1/4" 1.2-MB storage

360 RPM

3-1/2" 720-KB storage

600 RPM

3-1/2" 1.44-MB storage

600 RPM

The spindle of a 5-1/4" disk drive is activated and released by a small arm that's mounted on the front of the disk drive. You must turn the small arm to lock and release the floppy disk. The spindle of a 3-1/2" disk drive is activated when the floppy disk is inserted into the disk drive. It's released by a push-button that's located on the front of the disk drive. When you push this button, the floppy disk is released and pops out of the disk drive. Head Arm Assembly This part of a floppy disk drive transport holds the magnetic read/write heads. There are four heads on a head arm assembly, two write heads and two read heads - one of each for each recording surface. The head arm assembly is attached to the actuator arm assembly. Actuator Arm Assembly The actuator arm assembly positions the magnetic heads over the recording surface of the floppy disk. It does this by using a special type of dc motor called a stepper motor. This motor, which can be moved in very small steps, allows the read/write heads to be moved from track to track as needed to write data onto and read data off of the floppy disk. Drive Electronics Circuit Board This circuit board contains the circuitry which (1) controls the electromechanical parts of the disk drive transport, (2) writes data to and reads data from the floppy disk, and (3) interfaces the floppy disk drive to the host computer.

8-13


HARD DISK DRIVE TRANSPORTS Hard disk drive transports contain the electromechanical parts that (1) rotate the hard disk platter, (2) write data to it, and (3) read data from it. There are two types of hard disk drive transports, fixed disk and cartridge disk. Fixed disk drive transports use non-removable hard disk platters. Cartridge-disk drive transports use removable hard disk platters that are built into protective cartridges. These two transports serve very different purposes, but they each contain the same basic parts. Figure 8-10 shows a typical hard disk drive transport. Four of the more important parts of a hard disk drive transport are the 1. drive motor/spindle assembly, 2. head arm assembly, 3. actuator arm assembly, and 4. drive electronics circuit board.

Figure 8-10.—Typical hard disk drive transport.

Drive Motor/Spindle Assembly This assembly holds and spins the hard disk pack. The spindle assembly holds the hard disk pack in place and the drive motor spins the spindle at 3600 RPM. On cartridge disk drives, the spindle is electronically disengaged to release the disk pack so it can be removed.

8-14


Head Arm Assembly This part of the hard disk drive transport holds the magnetic read/write heads. There is a separate head arm assembly for each of the hard disk platters in the disk pack. Each assembly has four magnetic heads, two write heads and two read heads-one pair of heads for each surface of the hard disk platter. The head arm assembly is attached to the actuator arm assembly. Actuator Arm Assembly This part of the hard disk drive transport positions the magnetic heads so they can write data to and read data from the correct track of the hard disk. It does this by using either a stepper motor or a voice coil servo. A stepper motor is a special type of dc motor which can be moved in very small steps to accurately position the magnetic heads. A voice coil servo by itself cannot move the magnetic heads from track to track. Instead, it must use special signals called servo signals to make sure it's positioning the heads where they should be. The servo signals are pre-recorded signals which are stored on either the same hard disk platter as the data or on a separate hard disk platter. The voice coil servo method of moving the magnetic heads to the correct track of the hard disk is also called embedded servo control. This type of control is becoming very popular because voice coil actuator assemblies can position the magnetic heads much quicker and more accurately than dc stepper motors. Drive Electronics Circuit Board This circuit board contains the circuitry which (1) controls the electromechanical parts of the hard disk drive transport, (2) writes data to and reads data from the hard disk, and (3) interfaces the hard disk drive to the host computer. Q18. What are the three most popular methods for encoding digital data onto magnetic disks? Q19. Older, single-sided, single-density floppy disk drives would probably use what method for encoding digital data onto the floppy disk? Q20. What method for encoding digital data enables you to store more data in less space by limiting the distance between pulses on a hard disk? Q21. What are the four most important parts of a floppy disk drive transport? Q22. The drive motor of a 3-1/2", 1.44-MB floppy disk drive spins the disk at what RPM? Q23. The head arm assembly of a floppy disk drive transport has how many read heads and how many write heads? Q24. What part of a floppy disk drive transport uses a dc stepper motor to position the magnetic heads over the recording surface of a floppy disk? Q25. What part of a floppy disk drive transport contains the circuitry which controls the electromechanical parts of the transport? Q26. Hard disk drive transports contain the electromechanical parts that perform what three functions? Q27. In the actuator arm assembly of a hard disk drive transport, what device can position the magnetic heads to the correct track of a hard disk more accurately than a dc stepper motor? 8-15


MAGNETIC DISK DRIVE PREVENTIVE MAINTENANCE Like magnetic tape recorders, if you want a magnetic disk drive to continue storing and retrieving data without errors, you must periodically perform preventive maintenance. Fortunately, disk drives require less maintenance than magnetic tape drives. The following paragraphs describe the preventive maintenance requirements for both floppy and hard disk drives. FLOPPY DISK DRIVE PREVENTIVE MAINTENANCE Of all of the magnetic disk drives in use today, floppy disk drives require the most maintenance. This is because they are not sealed units like most hard drives and because they use flimsy plastic disks that are coated with the same type of oxide as magnetic tape. It's this oxide that causes most of the problems you'll have with floppy disk drives. Just as with magnetic tape, the oxide coating wears off of the plastic backing and sticks (mainly) to the magnetic heads. This contamination causes dropout errors which have much graver consequences than with magnetic tape. It can cause a program to crash, or even worse — it can destroy your valuable data. To prevent this, you must periodically clean the floppy disk drive's magnetic heads. There are many kits available to do the job. A kit has a cleaning disk and a bottle of cleaning solution. A cleaning disk looks just like a regular disk, except that instead of an oxide-coated disk, it has a cloth or fiber cleaning disk inside the protective jacket. The instructions that come with the cleaning kit will lead you through the cleaning process. Here is an example of the cleaning procedures for a floppy disk drive's magnetic heads: 1. Pour some of the cleaning solution onto the cleaning disk through the access hole in the protective jacket. 2. Insert the cleaning disk into the disk drive. 3. Exercise the disk drive for at least 30 seconds. 4. Remove the disk from the disk drive. There are also some cleaning kits that use disposable cleaning disks. These kits will instruct you to clean the heads as follows: 1. Open the scaled envelope that contains a cleaning disk soaked in cleaning solution. 2. Insert the cleaning disk into the protective jacket provided. 3. Insert the cleaning disk (with protective jacket) into the disk drive. 4. Exercise the disk drive for 30 seconds. 5. Remove the disk from the disk drive. 6. Remove the cleaning disk from the protective jacket and throw it away. Now comes the question "How often must I clean the heads?" That's hard to say. It depends on the type of disk drive, the quality of the floppy disks you use, and how much you use the disk drive. On the average, you should clean a floppy disk drive • once a month if it gets heavy use, • once every 6 months if it gets moderate use, or 8-16


• once a year if it gets very little use. HARD DISK DRIVE PREVENTIVE MAINTENANCE Hard disk drives need little or no preventive maintenance. If it's a fixed hard disk drive, it doesn't need preventive maintenance because it's a sealed unit that you must not open for any reason. If it's a cartridge disk drive, the manufacturer will have a special cleaning disk with instructions for doing the preventive maintenance. The Navy uses some larger cartridge disk drives, such as the 14" disk pack drives, that require some other preventive maintenance. This could include the following: • Cleaning air filters. • Cleaning spindles, rails, and slides. • Cleaning and buffing read and write heads. The technical manual for the disk drive will guide you through this type of preventive maintenance. Q28. Why do floppy disk drives require more preventive maintenance than hard disk drives? Q29. A kit for cleaning floppy disk drives contains what two items? Q30. Approximately how often should you clean a floppy disk drive that gets heavy use? Q31. Cartridge hard disk drives with 14" disk packs may require what additional types of preventive maintenance?

MAGNETIC DISK DRIVE ELECTRONICS Magnetic disk drive electronics consist of three main parts: 1. Control electronics to control the electromechanical parts of a disk drive. 2. Write/read electronics to write data to and read data from a disk drive. 3. Interface electronics to interface the disk drive to the host computer. Some disk drives require a separate controller card. When this is true, some of the drive electronics are part of the disk drive itself, and some are part of the host computer's controller card. As different as disk drives can be (floppy, fixed, cartridge, etc.), their electronics is surprisingly similar. That's why the following paragraphs will only very basically describe these three main parts. CONTROL ELECTRONICS The main functions of a disk drive's control electronics are to: • Spin the disk at the proper speed. • Move the magnetic heads across the recording surface. • Tell write/read heads when to write data and when to read data.

8-17


WRITE/READ ELECTRONICS The write/read electronics consists of the write part and the read part. The write part takes incoming data from the interface electronics, formats it as needed, and writes it onto the disk. The read part reads the data off of the disk, formats it as needed, and sends it to the interface electronics for output to the host computer. The write/read electronics also performs the initial disk formatting function. INTERFACE ELECTRONICS Interface electronics do two things: 1. Receive control signals from the host computer that tells them to spin the disk, move the magnetic heads, write/read data, format a disk, etc. 2. Convert the incoming and outgoing data as needed. A disk drive is a serial device. This means the data stored on the disk is stored in a serial pulse-train format. But the data coming from the disk drive and going to the host computer needs to be in a parallel data format. The interface electronics converts the data from parallel to serial, and vice versa, as needed. There are many types of disk drive interfaces in use today. The five most common ones are the: 1. Naval Tactical Data System (NTDS) interface. 2. ST-506/412 interface. 3. Enhanced small device interface (ESDI). 4. Small computer systems interface (SCSI). 5. Integrated drive electronics (IDE). The following paragraphs describe each of these interfaces. Naval Tactical Data System (NTDS) Interface The NTDS interface is used by many naval electronic warfare systems. There are three versions of this interface: 1. NTDS FAST: A parallel interface that can transfer data at a rate of 250,000 32-bit words per second. 2. NTDS SLOW: A parallel interface that can transfer data at a rate of 41,667 32-bit words per second. 3. NTDS SERIAL: A serial interface that can transfer data at a rate of 10 million bits (Mbits) per second. ST-506/412 Interface The ST-506/412 interface was developed by Seagate Technology, Inc. It's often used in the hard disk drives installed in older IBM-compatible desktop computers that have a maximum capacity of 125 MB. It's also the interface used to control most floppy disk drives in use today.

8-18


This is one of the interfaces where most of the electronics is actually on a controller card mounted in the host computer. With this interface, the controller card does most of the work (moving the magnetic head, spinning the disk, etc.). The controller card also cleans any data coming from the disk drive by stripping off the formatting and control signals that were used to store the data onto the hard disk. A hard disk drive is connected to the controller card in the host computer via two ribbon cables (a 34-pin control cable and a 20-pin data cable). Floppy drives use only the 34-pin control cable to transfer both data and control signals. When this interface was originally developed in 1981, it's 5-Mbits per second transfer rate was considered too fast. They actually slowed it down by using a 6:1 interleave factor (we'll define this later) so it could operate with the computers being built at that time. With today's transfer rates pushing the envelope at 24 Mbits per second, you can see that it's now one of the slowest interfaces. Enhanced Small Device Interface (ESDI) The ESDI is an optimized version of the ST-506/412 interface. The main difference is that with ESDI, most of the disk drive's interface electronics is located in the disk drive itself, rather than on a controller card in the host computer. The result is a much faster transfer rate and more hard disk capacity. ESDIs have a transfer rate of up to 24 MB per second. And, they can handle disk drives with a maximum capacity of 1.2 GB (gigabytes). The ESDI uses the same interface cables as the ST-506/412 interface, but that's where the similarity ends. With ESDI drives, only the clean data is sent to the controller card in the host computer. All formatting and control signals are stripped off at the hard disk drive. Small Computer Systems Interface (SCSI) The SCSI (pronounced skuzzy) is very different from both the ST-506/412 and the ESDI. The SCSI is an 8-bit, parallel, high-level interface. High-level means that instead of a host computer asking for data by specifying a track, cylinder, and sector number, all it asks for is a logical sector number. The SCSI then translates the logical sector number into the actual disk location. The SCSI also has other improvements over the previous disk drive interfaces. For example, it can: • Transfer data at rates of up to 4 MB per second. • Handle hard disk drives of almost any size. • Disconnect itself from a host computer's bus while it processes requests. This frees-up the host computer to do other things. • Daisy-chain up to eight units off of one controller. The SCSI interface uses one 50-pin ribbon cable to connect the hard disk drive(s) to the controller card mounted in the host computer. Some computer manufacturers include the SCSI electronics in their motherboards and do away with a separate controller card altogether. This interface got its big break when Apple Computer Corporation used the SCSI as its hard disk drive interface in its MacIntosh computers.

8-19


Integrated Drive Electronics (IDE) The IDE is the newest interface available. It was developed as a result of trying, to find a cheaper way to build computer systems. It includes all of the controller card electronics in the hard disk drive itself, thus, the hard drive does all the work. The hard disk drive connects to the host computer's bus with a 40-pin ribbon cable. The ribbon cable connects directly to either a 40-pin connector on the host computer's motherboard or a 40 pin connector on a small interface card that plugs into the host computer's motherboard. This interface offers a transfer rate of up to 1 MB and can handle hard drives with a maximum capacity of 300 MB.

MAGNETIC DISK RECORDING SPECIFICATIONS Think back to the chapter 6 on "Magnetic Tape Recording Specifications." Do you remember how to measure and adjust them if needed? Well, magnetic disk recording specifications are a little different. They're set by the manufacturer and you can't change them. All you can do is measure them. The following paragraphs describe six of the most common specifications. SEEK TIME The seek time is the amount of time it takes for the magnetic head to position itself over a specific track of a magnetic disk. It's usually stated in milliseconds. LATENCY PERIOD The latency period is the amount of time it takes for a specific sector of a specific track to position itself under the magnetic head. It too, is usually stated in ms. ACCESS TIME The access time is the sum of the seek time and the latency period. It's the total amount of time in ms that it takes a disk drive to retrieve a sector of data from the magnetic disk. Access time is stated in one of the following three ways: 1. Track-to-track seek time: This is the amount of time it takes a disk drive to access data from a track next to the track it's presently over. 2. Average seek time: This is the amount of time it takes a disk drive to access data that's located one-third of the way across the magnetic disk. 3. Maximum seek time: This is the amount of time it takes a disk drive to access data from the last track of a magnetic disk when it's presently on the first track of the magnetic disk. INTERLEAVE FACTOR The interleave factor applies only to hard disk drives. They spin at 3600 RPM, a very fast speed compared to floppy disk drives which only spin at 300-600 RPM. Interleave indicates how many physical sectors are between sequentially numbered logical sectors on a hard disk. It's used when the magnetic heads and the control circuitry can't process the data fast enough to sequentially number the sectors on a hard disk platter. With interleave, the magnetic head is told to skip X number of sectors to get to the next one. For example, a hard disk with 17 sectors per track and no interleave is numbered 1, 2, 3, 4.... 17. The same hard disk with an interleave factor of 3 is numbered 1, 7, 13, 2, 8, 14, 3, 9, 15, 4, 10, 16, 5, 11, 17, 6, 8-20


12, and then back to 1. If you count every third sector, they're sequential. The most efficient hard disk drives have no interleave. TRANSFER RATE The transfer rate states how fast a disk drive and a disk drive controller (working together) can transfer data to the host computer. An example of a transfer rate specification is "2 Mbits/sec," or two million bits per second. The higher the number, the faster the data transfer rate. RECORDING DENSITY The recording density states how close together bits can be stored on the recording surface of a magnetic disk. It determines two things: (1) How close together the tracks on the disk will be, and (2) how close together the bits on each track will be. An example of a recording density specification is "12 Mbits/in2," or 12 million bits per square inch. Q32. The control electronics component of a floppy or hard disk drive performs what three main functions? Q33. The write/read electronics of a disk drive performs what three functions? Q34. The interface electronics of a disk drive performs what three functions? Q35. What type of interface electronics is used in many naval electronic warfare systems? Q36. What type of disk drive interface has most of the electronics on a controller card mounted in the host computer? Q37. The SCSI is a high level disk drive interface. What does this mean? Q38. What type of hard disk drive interface has all of the controller card electronics included in the disk drive itself?

SUMMARY Now that you've finished chapter 8, you should be able to describe the (1) characteristics of floppy and hard disks, (2) methods for encoding digital data onto magnetic disks, (3) disk drive transports and their preventive maintenance requirements, (4) parts of a disk drive's electronics component, and (5) common types of disk drive interface electronics. The following is a summary of important points in this chapter: FLOPPY DISKS are single-sided or double-sided plastic disks coated with oxide particles. The disks can be single-density, double-density, or high density. Data is stored on floppy disks in CIRCULAR TRACKS. The tracks are divided into arcs called SECTORS. HANDLE, SHIP, and STORE floppy disks carefully. Contaminates between the heads and the disk surface can cause serious damage.

8-21


FIXED (HARD) DISKS are aluminum platters coated on both sides with iron oxide or thin-film metal. Most hard disk drives use disk packs which are several disk platters stacked together. Some hard disk drives use removable disk platters. Either the CYLINDER OR SECTOR METHOD is used to place data on hard disks. HANDLE, STORE, AND SHIP HARD DISKS with extreme care. Contaminates on the heads or the disk surface can cause head-crash. ERASE HARD AND FLOPPY DISKS by reformatting or degaussing. Three popular METHODS FOR ENCODING DIGITAL DATA ONTO DISKS are frequency modulation, modified frequency modulation, and run length limited. FLOPPY DISK DRIVE TRANSPORTS contain the parts that (1) spin the floppy disk, (2) write data to the disk, and (3) read data from it. The DRIVE MOTOR/SPINDLE ASSEMBLY of a floppy disk drive transport holds and spins the floppy disk. The transport's HEAD ARM ASSEMBLY holds the read/write heads and its ACTUATOR ARM ASSEMBLY positions the heads over the disk's recording surface. HARD DISK DRIVE TRANSPORTS contain the parts that (1) rotate the hard disk platter, (2) write data to the disk, and (3) read data from the disk. The DRIVE MOTOR/SPINDLE ASSEMBLY of a hard disk drive transport holds the disk pack in place while the drive motor spins the spindle at 3600 RPM. The transport's HEAD ARM ASSEMBLY holds the read/write heads and its ACTUATOR ARM ASSEMBLY positions the heads over the correct track of the hard disk. FLOPPY DISK DRIVES REQUIRE PREVENTIVE MAINTENANCE at regular intervals because they are not sealed units and the disks use an oxide coating that wears off and sticks to the heads and other parts. HARD DISK DRIVES REQUIRE VERY LITTLE PREVENTIVE MAINTENANCE. Cartridge disk drives will have a special cleaning kit for doing the preventive maintenance. Magnetic DISK DRIVE ELECTRONICS consist of (1) control electronics to control the electromechanical parts of a disk drive, (2) write/read electronics to write data to and read data from a disk drive, and (3) interface electronics to interface the disk drive to the host computer. MAGNETIC DISK RECORDING SPECIFICATIONS are set by the manufacturer; all you can do is measure them. Six of the most common specifications are seek time, latency period, access time, interleave factor, transfer rate, and recording density.

ANSWERS TO QUESTIONS Q1. THROUGH Q38. A1. 8 inch, 5 1/4 inches, 3 1/2 inches. A2. Double-sided.

8-22


A3. a. Single-density, b. double-density, and c. high-density. A4. 1,200,000 bytes or 1.2 megabytes. A5. The disks can hold 96 tracks per inch. A6. Soft sectoring. A7. a. disk side number, b. track number, and c. sector number. A8. Dust and other contaminates can get on the recording surface through the read/write hole. A9. Magnetic fields can destroy the data on a disk. A10. a. Degauss the disk and then reformat it. b. Reformat the disk. A11. 3-1/2 inches, 5-1/4 inches, and 14 inches. A12. (1) Cylinder method and (2) sector method. A13. Sector method. A14. a. Cylinder number. b. Recording surface number. c. Record number. A15. Head-crash. A16. a. 30-60 percent relative humidity. b. 70-80 degrees Fahrenheit. A17. Reformat the disk.

8-23


A18. Frequency modulation (FM). a. Modified frequency modulation (MFM). b. Run length limited (RLL). A19. Frequency-modulation encoding. A20. Run length-limited (RLL) encoding. A21. a. Drive motor/spindle assembly. b. Head arm assembly. c. Actuator arm assembly. d. Drive electronics circuit board. A22. 600 RPM. A23. Two read heads and two write heads. A24. Actuator arm assembly. A25. Drive electronics circuit board. A26. a. Rotates the hard disk platters. b. Writes data to and reads data from the disk platters.

A27. Voice coil servo. A28. They are not sealed units, and they use flimsy plastic disks with an oxide coating that wears off and sticks to the heads. A29. a. A cloth or fiber cleaning disk. b. A bottle of cleaning solution. A30. Once a month.

8-24


A31. a. Cleaning with a special cleaning disk. b. Cleaning air filters. c. Cleaning spindles, rails, and slides. d. Cleaning and buffing read/write heads. A32. a. Spins the disk at the correct speed. b. Moves the heads across the recording surface. c. Tells the write/read heads when to write data and when to read it. A33. a. Formats and writes incoming data from the interface electronics onto the disk. b. Reads data off the disk, formats it, and sends it to the interface electronics for output. c. Performs the initial disk formatting. A34. a. Receives control signals from the host computer and sends them to the control electronics or write/read electronics. b. Receives data from the write/read electronics and outputs it to the host computer. c. Converts incoming and outgoing data from parallel to serial, and vice versa, if needed. A35. NTDS interface. A36. ST-506/412 interface. A37. The host computer asks for data by specifying a logical sector number. The SCSI translates the sector number into the actual disk location. A38. Integrated drive electronics (IDE).

8-25



APPENDIX I

GLOSSARY ABRASIVITY—The ability of the magnetic tape to wear the head. ADDITIVE—Any material in the coating of magnetic tape other than the oxide and binder resins. (Examples: plasticizers (to soften an otherwise hard or brittle binder), lubricants (to lower the coefficient of friction of an otherwise high-friction binder), fungicides (to prevent fungus growth), dispersants (to uniformly distribute the oxide particles), and dyes.) ALTERNATING CURRENT (ac) BIAS—(1) The alternating current, usually of a frequency several times higher than the highest input signal frequency, that is fed to a record head in addition to the input signal current. (2) Linearizes the recording process. (3) Is universally used in direct analog recording. AMPLITUDE/FREQUENCY RESPONSE—(See frequency response.) ANALOG RECORDING—A method of recording in which some characteristic of the record current, such as amplitude or frequency, is continuously varied in a manner similar to the variations of the original signal. AZIMUTH ALIGNMENT—The alignment of the recording and reproducing gaps so their center lines lie parallel with one another. Misalignment of the gaps causes a loss in output at short wavelengths. BACKINGS—(See base film.) BANDWIDTH—The frequency within which the performance of a recorder with respect to some characteristic (usually frequency response) falls within specified limits, or within which some performance characteristic (such as noise) is measured. BASE FILM—The plastic substrate material used in magnetic tape that supports the coating. BER—(See bit error rate.) BIAS-INDUCED NOISE (See noise.) BINARY—(1) Two values ("O" or "1") or states (ON or OFF). (2) The number systems used in computers. BINDER—A compound consisting of organic resins used to bond the oxide particles to the base material, the actual composition being considered proprietary information by each magnetic tape manufacturer. The binder is required to be flexible but still maintain the ability to resist flaking or shedding during extended wear passes. BIT—(1) The acronym binary digit. (2) The smallest unit of data, either "O" or "1". (3) One recorded information cell, as applied in magnetic recording.

AI-1


BIT DENSITY—(See packing density.) bit error rate (BER).—(1) The number of errors a specific magnetic tape may contain, as used in high-density recording. (2) Is expressed in errors per data bit, such as 1 in 106, or one error in one million data bits. BREAK ELONGATION—The relative elongation of a specimen of magnetic tape of base film at the instant of breaking when it has been stretched at a given rate. BROWN STAIN—(1) A thin discoloration of the head's top surface, usually a chemical reaction between the head's surface materials and the tape binder, the tape lubricant, or the head's bonding materials. (2) Its origin is not well understood, but is known to occur in low humidity. BUCKLING—(1) A deformation of the circular form of a tape pack. (2) Caused perhaps by a combination of improper winding tension, adverse storage conditions, and/or poor reel hub configuration. BUILD-UP—(1) A snowballing effect started by debris and tape magnetic particles embedded in the contamination. (2) The thickness of this build-up can cause an intense in head-to-tape separation, as well as an increase in the coefficient of friction. (3) Solvent cleaning of the head's top surface will usually remove the build-up. BULK-ERASED NOISE—See noise. BULK ERASER (DEGAUSSER)—An equipment for erasing a full reel of previously recorded signals on tape. BYTE—A group of bits (next to each other) that are considered a unit (example, an 8-bit byte). CERTIFIED TAPE—A tape that is electrically tested on a specified number of tracks and is certified by the supplier to have less than a certain total number of permanent errors. CERTIFIER—(1) An equipment that tests the ability of magnetic tape to record and reproduce. (2) Counts and charts each error on the tape, including the level and duration of dropouts. (3) In the certify mode, stops the tape at an error to allow for visual inspection of the tape to see if the cause of the error is correctable or permanent. CHICKEN TRACKS—(1) A line of small craters in the head's top surface running in the direction of tape motion. (2) Usually caused by a loose, small, hard particle moving with the tape over the head. CINCHING—(1) The tape folds resulting from longitudinal slippage between the layers of tape in a tape pack. (2) Caused by uneven tension when the roll is accelerated or decelerated. CLEAN ROOMS—The rooms of which their cleanliness is measured by the number of particles of a given size per cubic foot of room volume. (Examples (1) A class 100,000 clean room may have no more than 100,000 particles 0.5 fm or larger per cubic foot, and so on for class 10,000 and class 100 rooms. (2) A class 10,000 room may have no more than 65 5-fm particles per cubic foot, while class 100,000 may have no more than 700 5-fm particles per cubit foot.) CLEANER—(See winder/cleaner.) COATING—The magnetic layer of a magnetic tape consisting of oxide particles held in a binder that is applied to the base film. AI-2


COATING RESISTANCE—(1) The electrical resistance of the coating measured between two parallel electrodes spaced a known distance apart along the length of the tape. (2) Called resistivity on specification sheets. COATING THICKNESS—The thickness of the magnetic coating applied to the base film. COATING-TO-BACKING ADHESION—(See anchorage.) CONTAMINATION—(1) A thick, tacky (viscous) deposit on the head's top surface, which causes a large increase in the effective head-to-tape coefficient of friction. (2) May not be removable by solvent cleaning. CORE MATERIAL—(1) hard core material.—(a) Hard metal laminations bonded together to form the core, with a typical thickness of 0.005 to 0.004 inch. (Hard metal wears much more slowly than soft laminations.) (b) Hard solid metal, such as alphenol or sendust. (Wear rates are much lower than those of soft metal laminations.) (2) soft core material.—(a) Soft metal laminations bonded together to form the core, with a typical thickness of 0.0005 to 0.004 inch. (b) Usually, a high nickel/iron alloy, such as Hy Mu 800. (c) These materials have a relatively poor wear rate. CRACK—A narrow, deep break in the head's surface material. CREEP—The time-dependent strain at a constant stress (tape deformation). CROSSFEED—See crosstalk and write feedthrough. CROSSPLAY—The ability to interchange recordings between recorders while maintaining a given level of performance. CROSSTALK—(1) The magnetic coupling from one track to another in the tape's read/write head. (2) See also write feedthrough. CUPPING—(1) The curvature of a magnetic tape pack in the lateral direction. (2) May be caused by differences between the coefficients of thermal or hygroscopic expansion of coating and base film. db—See decibel. dc NOISE—(See noise.) DECIBEL (db)—A dimensionless unit for expressing the ratio of two powers or voltages or currents on a logarithmic scale. If A and B represent two voltages or currents, the ratio A/B corresponds to 20 log A/B decibels. One decibel represents a ratio of approximately 1.1 to 1 between A and B. Other values are as follows:

AI-3


RATIO 1 1.4 2 4 10 100 1000

DECIBEL 0 3 6 12 20 40 60

DEFECT—(1) An imperfection in the tape leading to a variation in output or a dropout. (2) The most common defects are surface projections of oxide agglomerates, embedded foreign matter, and redeposited wear products. DEGAUSSER—See bulk eraser. DELAY MODULATION—(See modified frequency modulation.) DIGITAL RECORDING—(1) A method of recording in which the information is first coded in a digital form. (2) Usually, a binary code is used, with recording taking place in two discrete values/polarities of residual flux. DIRECT RECORDING—An analog recording that records and reproduces data in the electrical form of its source. DISK—(1) A disk-drive storage device on which information is magnetically recorded and retrieved. (2) Can be either hard (rigid) or floppy (flexible). DISK DRIVE—A storage device for recording and retrieving data on hard or floppy disks. DISK PACK—A portable, interchangeable device that contains more than one hard-disk platter and is used in hard-disk drives. DISPERSION—The distribution of the oxide particles within a tape's binder. DISTORTION—(see harmonic distortion.). DRAG—The fractional tension differential across the contact area caused when the tape contacts some element in the tape path (such as the head, tape guides, tape bearings, or column walls). DROPOUT—(1) A temporary reduction in the output of a magnetic tape of more than a certain predetermined amount. (2) Expressed in terms of the percentage reduction or decibel loss. DROPOUT COUNT—The number of dropouts detected in a given length of magnetic tape. DURABILITY—The number of passes that can be made before a significant degradation of output occurs, divided by the corresponding number that can be made using a reference tape. DYNAMIC—(See tape skew.) DYNAMIC RANGE—(1) The bandwidth within which a satisfactory signal-to-noise ratio is obtained. (2) See also resolution.

AI-4


DYNAMIC SKEW—The change in skew caused by tape motion. DYNAMIC TAPE SKEW—(See tape skew.) EQUIPMENT NOISE—(See noise.) ERASURE—A process by which a signal recorded on a tape is removed and the tape made ready for rerecording. May be accomplished in two ways. (1) In ac erasure, the tape is demagnetized by an alternating magnetic field that is reduced in amplitude from an initially high value. May be accomplished by passing the tape over an erase head fed with highfrequency ac, or by placing the whole roll of tape in a decreasing ac field (bulk erasure). (2) In dc erasure, the tape is saturated by applying a primarily unidirectional field. May be accomplished by passing the tape over a head fed with dc or over a permanent magnet. Additional stages may be included in dc erasure to leave the tape in a more nearly unmagnetized condition. FERRITE—A powdered and compressed ferric-oxide material that has both magnetic properties and light resistance to current flow. FLOPPY DISK—(See disk.) FLUTTER—(See wow.) FLUX—A term, with reference to electrical or electromagnetic devices, that designates 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. FM—(See frequency modulation.) FREQUENCY MODULATION (FM)—(1) A flux reversal at the beginning of a cell time represents a clock bit. (2) A "1" bit is a flux reversal at the center of the cell time. (3) A "0" bit is an absence of a flux reversal. FREQUENCY RESPONSE—(1) The variation of sensitivity with signal frequency. (2) The frequency response of a tape is usually given in decibels relative to a referenced frequencyoutput level. (3) Also called amplitude/frequency response. GAMMA-FERRIC OXIDE—The common magnetic constituent of magnetic tapes in a dispersion of fine, needle-like particles within the coating. GAP EROSION—(1) The read or write gap increased in length and retreated below the head surface. (2) Usually due to deterioration of core material at the edges of the gap. GAP LOSS—(1) The loss in output due to the finite gap length of the reproduce head. (2) The loss increases as the wavelength decreases. GAP WIDTH—The dimension of the gap of a magnetic head measured in the direction perpendicular to the direction of the tape path. GAUSS—The metric unit of the magnetic flux density equal to 1 Mx/CM 2. HARD DISK—(See disk.)

AI-5


HARD FERRITE—A ferrite with a very low wear rate when compared with the soft metal laminations. HARD METAL LAMINATIONS—(See core material, hard.) HARD SOLID METAL—(see core material, hard.) HARMONIC DISTORTION—A signal non-linearity with harmonics of the fundamental in the output when the input signal is sinusoidal. HEAD CONTAMINATION—(See tape-to-head separation.) HEAD CONTOUR—(1) The complex shape of the contacting surface of a head as a result of manufacture, head lapping, or wear. (2) The contour of a head is always changing throughout the head's life and, in many cases, is responsible for retiring the head. HEAD CRASH—A term used for the damage to a hard disk caused by the physical contact made between the magnetic read/write heads and the surface of the hard-disk platter. HEAD STICK—(1) A common word for a large increase in head-to-tape friction caused by (a) a stick by-product exuded by conditions due to tape age, temperature/humidity, and head-totape pressure, and (b) very smooth tapes coupled with large area heads. (2) See also sticktion and stick-slip. HEAD-TO-TAPE CONTACT—The degree that a tape's magnetic coating approaches the surface of the record or reproduce head during normal operation. INTERLAYER TRANSFER—Any loose material, such as oxide, generated by tape wear or a head-stick condition which is transferred from the oxide to the back of the tape, or from the back side to the oxide when the tape is wound on a reel. INTERMODULATION DISTORTION—(1) A signal non-linearity with frequencies in the output equal to sums and differences of integral multiples of the component frequencies present in the input signal. (2) Harmonies are usually not included as part of the intermodulation distortion. INTERSYMBOL INTERFERENCE—(1) An interference resulting in a phase shift of the cell playback crossover point with respect to the data clock. (2) When a recording system has limited record resolution, a flux transition being recorded will extend beyond its cell boundaries, adding or subtracting from the flux in the adjacent bit cells of symbols. IRON OXIDE—(See gamma-ferric oxide.) LAMINATIONS—(see core material.) LATERAL DIRECTION—The direction across the width of the tape. LAYER-TO-LAYER ADHESION—The tendency for adjacent layers of tape in a roll to adhere to one another. LAYER-TO-LAYER TRANSFER—The magnetization of a layer of tape in a roll by the field from a nearby recorded layer, sometimes referred to as print through. LBE—(See lower band edge.)

AI-6


LINEARITY—The extent to which the magnitude of the reproduced output is directly proportional to the magnitude of the signal applied to the input of the recorder. LONG-TERM TAPE SPEED—(See tape speed.) LONGITUDINAL CURVATURE—Any deviation from the straightness of a length of tape. LOOSE DEBRIS—Any material that is very lightly bonded to the tape or the head's top surface, removable by tape motion. LOWER BAND EDGE (LBE)—The lower band edge of the recorder/reproducer response (usually at the −3-dB point). LUBRICANT—(See additive.) MAGNETIC INSTABILITY—(1) The property of a magnetic material that causes variations in the residual flux density of a tape to occur with temperature, time, and/or mechanical flexing. (2) A function of particle size, magnetizing field strength, and anisotropy. MAGNETIC MEDIA—A base film, coated with magnetic particles held in a binder. (The magnetic particles are usually needle-like, single-domain, gamma-ferric oxide.) MAGNETIZING FIELD STRENGTH—The instantaneous strength of the magnetic field applied to a sample of magnetic material. MFM—(See modified frequency modulation.) MODIFIED FREQUENCY MODULATION (MFM)—(1) A code that has a "1" and a "O" corresponding to the respective presence or absence of a transition in the center of the corresponding bit cell. (2) Additional transitions at the cell boundaries occur only between bit cells that contain consecutive "0" values. (3) Also called delay modulation. MODULATION NOISE—(See noise.) NOISE—Any unwanted electrical disturbances other than crosstalk or distortion components that occur at the output of the reproduce amplifier. (1) System noise is the total noise produced by the whole recording system, including the tape. (2) Equipment noise is produced by all the components of the system, with the exception of the tape. (3) Tape noise can be specifically ascribed to the tape. The following are typical sources of tape noise: (a) Bulkerased noise arises when a bulk-erased tape with the erase and record heads completely deenergized is reproduced. (b) Zero-modulation noise arises when an erased tape with the erase and record heads energized as they would be in normal operation, but with zero input signal, is reproduced. This noise is usually 3 to 4 dB higher than the bulk-erased noise. The difference between bulk-erased and zero-modulation noise is sometimes termed bias-induced noise. (c) Saturation noise arises when a uniformly saturated tape is reproduced. This is often some 15 dB higher than the bulk-erased noise and is associated with imperfect particle dispersion. (d) Dc noise arises when a tape that has been non-uniformly magnetized by energizing the record head with dc, either in the presence or the absence of bias, is reproduced. This noise has pronounced, long, wavelength components that can be as much as 20 dB higher than those obtained from a bulk-erased tape. At very high values of dc, the dc noise approaches the saturation noise. Dc noise is actually the low-frequency component of modulation noise. (e) Modulation noise is essentially a modulation of the desired signal by noise that is caused by non-uniform dispersion of elementary magnetic particles in the

AI-7


tape’s coating material. This noise, which occurs only when a recorded tape is reproduced, increases with the intensity of the reproduced signal. Dc noise is actually the low-frequency component of modulation noise. NOISE PULSE—(1) A short-duration false signal occurring during the reproduction of a tape. (2) A signal that is of a magnitude considerably in excess of the average peak value of the ordinary system noise. NOMINAL BIT TIME—(1) The average bit time of recording at continuous maximum-flux reversals. (2) Also called cell time. NON-RETURN-TO-ZERO (NRZ0)—The flux reversal for a "0"; no flux reversal for a "1." NON-RETURN-TO-ZERO INVERTED (NRZI)—The flux reversal for a "1"; no flux reversal for a "0." NON-RETURN-TO-ZERO (NRZ) RECORDING—(See digital recording.) OERSTED—A unit of magnetic field strength. OUTPUT—The magnitude of the reproduced signal voltage, usually measured at the output of the reproduced amplifier. OXIDE BUILD-UP—The accumulation of oxide or wear products as deposits on the surface of heads and guides. OXIDE LOADING—(1) A measure of the density with which oxide is packed into a coating. (2) The weight of the oxide per unit volume of the coating. OXIDE SHED—The loosening of particles of oxide from the tape coating during use. PACKING DENSITY—The amount of digital information recorded along the length of a tape, measured in bits per inch. PARTICLE ORIENTATION—The rotation of needle-like particles so that their longest dimensions tend to lie parallel to one another. PARTICLE SHAPE—The needle-like particles of gamma-ferric oxide used in conventional magnetic tape, with a dimensional ratio of about 6 to 1. PARTICLE SIZE—The physical dimensions of magnetic particles used in a magnetic tape. PERMANENT ELONGATION—The percentage of elongation remaining on a tape or a length of base film after a given load applied for a given time has been removed. PLASTICIZER—(See additive.) POLYESER—(1) An acronym for polyethylene glycol terephthalate. (2) The material most commonly used as a base film for precision magnetic tape. READ/WRITE ERASE HEAD—A three-gap head (read, write, erase) on one body. Sometimes the erase head is bolted to the read/write head. READ/WRITE HEAD—A two-gap head (read, write) on one body.

AI-8


REEL—The metal-, glass-, or plastic-flanged hub on which magnetic tape is wound. REFERENCE TAPE—A tape used as a reference against which the performances of other tapes are compared. REMANENCE—(1) The magnetic flux density that remains in a magnetic circuit after removal of applied magnetomotive force. (2) Is not necessarily equal to residual flux density. REMOVABLE FIXED DISK—A hard-disk device (usually) with only one hard-disk platter that's used to record and retrieve data. RESISTIVITY—(See coating resistance.) RESOLUTION (DYNAMIC RANGE)—The average peak-to-peak signal amplitude at the maximum flux reversal divided by the average peak-to-peak signal amplitude at the minimum flux reversal at the desired recording method. RETENTIVITY—The maximum value of residual flux density corresponding to saturation flux density. RZ RECORDING—(See digital recording.) SATURATION FLUX DENSITY—(1) The maximum intrinsic flux density possible in a sample of magnetic material. (2) The intrinsic flux density asymptotically approaches the saturation flux density as the magnetizing field strength increases. SATURATION NOISE—(See noise.) SCRATCH—A long, narrow, straight defect in the top surface of a head track or a tape. SENSITIVITY—The magnitude of the output when reproducing a tape recorded with a signal of given magnitude and frequency. SEPARATION LOSS—The loss in output that occurs when the surface of the coating of a magnetic tape fails to make perfect contact with the surface of the record or reproduce head. SHEDDING—The loss of oxide or other particles from the coating or backing of a tape, usually causing contamination of the tape transport and, by redeposit, of the tape itself. SHORT-TERM TAPE SPEED—(See tape speed.) SIGNAL-TO-NOISE RATIO—(1) The ratio of the power output of a given signal to the noise power in a given bandwidth. (2) Is usually measured by the corresponding root mean square signal and noise voltages appearing across a constant output resistance. SKEW—A deviation of a line connecting the average displacement of the read or write track gaps from a line perpendicular to the reference edge of the tape in the direction of tape motion. SKEW TAPE—(1) The continuous strings of "1" values written on a properly adjusted tape drive for the entire recoverable length of the tape. (2) An "all '1' pattern" on all tracks. (3) The write head, the write delays, and the tape drive adjusted to write with minimum physical skew and gap scatter.

AI-9


SOFT METAL LAMINATIONS—(See core material, soft.) SPOKING—A buckling in which the tape pack is deformed into a shape that approximates a polygon. SPOOL—(See reel.) SQUEAL—(See stick-slip.) STANDARD REFERENCE TAPE—A tape intended for daily calibration, the performance of which has been calibrated to the amplitude reference tape. STATIC—(See tape skew.) STICK-SLIP—(1) A low-speed phenomenon. (2) A relationship between tension, temperature, humidity, wrap angle, head material, tape binder, and elastic properties. (3) When detected audibly, it is a squeal. STICKTION—The tape's adhering to transport components, such as heads or guides. STIFFNESS—(1) The resistance to bending the tape. (2) A function of tape thickness. (3) A modulus of elasticity. SURFACE TREATMENT—Any process by which the surface smoothness of the tape coating is improved after it has been applied to the base film. SYSTEM NOISE—(See noise.) TAPE CONTAMINATION—(See tape-to-head separation.) TAPE NOISE—(See noise.) TAPE PACK—The form taken by the tape wound on a reel. TAPE SKEW—(1) The tape's deviation from following a linear path when transported across the heads. (2) The terms static and dynamic distinguish between the physically fixed components and the fluctuating components of total tape skew. TAPE SPEED—(1) The speed at which the tape is transported across the read/write head during normal recording or reproduction. (2) Long-term speed is averaged over a minimum of 15 inches of tape (in inches per second). (3) Short-term speed is the instantaneous (dynamic) tape speed (in inches per second). TAPE-TO-HEAD SEPARATION—(1) Separation.—The separation between a magnetic head and the magnetic tape caused by the (a) foil-bearing effect, (b) improper head contour, which generates standing waves in the tape, and (c) surface roughness of the tape. These conditions are interrelated and are greatly influenced by tape tension and tape compliancy. In a properly designed system, tape roughness is the limit of head-to-tape separation, usually

Navy Electricity and Electronics Training Series

Navy Electricity and Electronics Training Series (NEETS) (SuDoc D 207.217:EL 2 MOD.1-24 CD)

Electricity and Electronics Fundamentals

Teach Yourself Electricity and Electronics

Teach Yourself Electricity and Electronics

Electricity and Electronics for HVAC

Electricity and Electronics for HVAC

Teach yourself electricity and electronics

Electricity and Electronics for HVAC

Electricity and Electronics: Lab Manual

Teach Yourself Electricity & Electronics

Hydrogen Power Electricity Electrical Electronics

Tab Electronics Guide to Understanding Electricity and Electronics

Electricity and Electronics Fundamentals, Second Edition

Introductory Electronics (Telp series)

Introduction to Power Electronics (Essential Electronics Series)

ELECTRICITY

Electricity

Electricity and Electronics for Aerospace Vehicles (Northrop Institute of Technology series)

Electricity and Magnetism

Love, Hope, and Electricity

Electricity and Magnetism

Electricity and atomic physics

Static And Dynamic Electricity

FUEL CELL - Handbook - (Hydrogen Power Electricity Electrical Electronics)

Fuel Cell Handbook. Hydrogen Power Electricity Electrical Electronics

FUEL CELL - Handbook - (Hydrogen Power Electricity Electrical Electronics)

Electricity and Atomic Physics

Electricity and Magnetism

Electricity and magnetism

Electricity and Magnetism

Navy Electricity and Electronics Training Series