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Lighting Control-Technology and Applications

This Page Intentionally Left Blank

Lighting

Control

–

Technology

Applications

Robert S. Simpson

and

CONTENTS

Focal Press An imprint of Elsevier Linacre House, Jordan Hill, Oxford OX2 8DP 200 Wheeler Road, Burlington MA 01803 First published 2003 Copyright© 2003, Robert S. Simpson. All rights reserved The right of Robert S. Simpson to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988 No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1T 4LP. Applications for the copyright holder’s written permission to reproduce any part of this publication should be addressed to the publisher

Permissions may be sought directly from Elsevier’s Science and Technology Rights Department in Oxford, UK: phone: (+44) (0) 1865 843830; fax: (+44) (0) 1865 853333; e-mail: [email protected]. You may also complete your request on-line via the Elsevier Science homepage (www.elsevier.com), by selecting ‘Customer Support’ and then ‘Obtaining Permissions’ British Library Cataloguing in Publication Data Simpson, Robert S. Lighting control : technology and applications 1.Electric lighting – Control I.Title 621.3’2 Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of Congress ISBN 0 240 51566 8

For information on all Focal Press publications visit our website at: www.focalpress.com

Printed and bound in Italy

iv

CONTENTS

Contents

Trade marks and disclaimer Acknowledgements Preface

vii viii ix

Part 1 Foundation chapters 1

Electricity and light

1

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11

Electricity Electrical units and components Electrical distribution Power factor Control of electric power Electromagnetic compatibility (EMC) Light The eye, how we see light Measurement of light Color Measurement of color

1 4 15 24 29 36 39 44 45 50 53

2

Lighting electronics

58

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8

Electronic principles The diode The transistor The thyristor, triac and GTO Analog and digital The integrated circuit and ASIC The microprocessor Programmable devices

58 63 68 81 85 93 102 110

Part 2 Lamps 3

Everyday lamps

112

3.1 3.2 3.3 3.4 3.5 3.6

Non-electric lighting The incandescent lamp Tungsten halogen lamps The fluorescent lamp Compact fluorescent lamps Special purpose fluorescent lamps

112 113 115 119 123 124

4

Arc lamps

130

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8

High intensity discharge lamps Mercury vapor lamps Sodium and high pressure sodium lamps Metal halide lamps Compact source metal halide lamps High pressure mercury vapor lamps Xenon arc lamps Arc lamp classification

130 130 133 136 138 139 140 141

5

Special purpose lamps

144

5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11

Induction lamps Flat lamps Neon lamps Electroluminescent lamps Light emitting diodes (LEDs) Lasers Ultra-violet lamps Infra-red lamps Flash tubes Fiber optics and lightguides Video displays as lightsources

144 148 149 151 153 159 162 163 164 166 171

Part 3 Lighting components 6

Electromagnetic components

172

6.1 6.2 6.3 6.4 6.5 6.6

172 186 192 204 207

6.7

Principles of transformers and inductors Transformers for lighting Ballasts for fluorescent lamps Ballasts for HID and arc lamps Ignitors and starters Lighting control by transformers and ballasts Power factor correction

7

Electronic components

215

7.1 7.2

Circuit elements Electronic ballasts for fluorescent lamps

215 219

210 211

v

CONTENTS

7.3 7.4 7.5

Electronic ballasts for HID and arc lamps Electronic transformers Lighting control by electronic transformers and ballasts

242 245 248

Part 4 Dimmers and control systems

12

Architectural lighting control systems 374

12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8

“Setting the scene” Manual versus automatic control Single channel control Small multi-channel control systems Large multi-channel control systems Switching systems Centralized versus distributed systems Emergency and safety

374 375 376 377 381 383 390 391

13

The merging of “architectural” and “entertainment” lighting control

396

8

Dimmers

250

8.1 8.2 8.3 8.4 8.5 8.6

Introduction to dimmers Non-electronic dimming Thyristor and triac dimmers Transistor dimmers Electromagnetic compatibility (EMC) New developments in electronic dimming

250 252 257 269 272 276

9

Control signals and protocols

280

13.2 13.3

9.1 9.2 9.3 9.4 9.5 9.6 9.7

Introduction Analog control Digital control Standard protocols for lighting control Networks and buses Computers in lighting control Cordless control

280 280 283 288 303 332 334

10

Why lighting control?

343

10.1 10.2 10.3 10.4 10.5

The practical role The esthetic role The energy management role Influence of legislation Lighting design

343 344 345 348 351

11

Stage and entertainment lighting control systems 353

11.1 11.2

Basis of stage lighting control Simple multichannel controls for entertainment Memory consoles Live versus automatic Control of moving lights Control of color Large scale entertainment lighting control

11.3 11.4 11.5 11.6 11.7

vi

353 354 356 362 363 370 372

13.1

User demands and the influence of designers Automatic lighting control in public shows and public areas Control of exterior lighting

396 398 400

14

Energy management and building control systems 402

14.1 14.2 14.3

Principles Sensors and timers Switching versus dimming, control algorithms Local versus central control Impact of lighting on HVAC Power quality Integrated versus separate lighting control Monitoring systems

14.4 14.5 14.6 14.7 14.8

402 403 412 417 417 417 418 418

Part 5 Applications 15

Architectural applications

420

15.1 15.2 15.3 15.4

The home Integrated home control systems The workplace Meeting rooms, conference centers, and auditoria Places of worship Museums, art galleries and libraries Visitor centers and exhibitions

420 428 431

15.5 15.6 15.7

436 447 449 454

CONTENTS

15.8 Hotels, hospitals and institutions 15.9 Restaurants, bars and pubs 15.10 Illuminated signs

459 463 464

16

Functional applications

469

16.1 16.2 16.3 16.4 16.5 16.6 16.7 16.8 16.9 16.10

Retail spaces Agriculture and horticulture Manufacturing processes Healthcare Simulation On water In the air On the road On railways Control rooms

469 472 473 474 475 476 481 490 505 508

17

Entertainment applications

17.1 17.2 17.3 17.4 17.5

510

Small stages Large stages Television Touring shows Outdoor shows, Son et Lumière, pyrotechnics 17.6 Stadia, arenas, sporting facilities 17.7 Theme parks 17.8 Entertainment within retail 17.9 Discotheques, dancefloors and clubs 17.10 Conclusion

510 514 520 524

Some suggestions for further reading Table of acronyms Index

547 550 552

526 534 539 542 544 546

Trade marks and disclaimer In order to ensure that this book can be of practical use, it has been necessary to mention many commercial products by name. The inclusion or omission of any particular company’s products does not imply any endorsement or comment by the publisher. Where summaries are given of manufacturers of different kinds of equipment, these are intended as examples only, no claim is made that such summaries represent any kind of comprehensive directory. This book is intended as a source of information only. Care has been taken to verify the accuracy of all information contained herein, but neither the author nor the publisher can take responsibility for the consequences of using the information, or for any errors or omissions. Example circuits, devices and techniques may be the subject of patent protection or patent application. Their publication in this book does not imply any license for their use. Any references to standards and protocols (whether public or proprietary) are intended to give readers an introduction to their nature and operation. There is no implication of any license to use them, and current standards specifications and details of any licensing associated with them must be obtained from the sponsoring body concerned. All trade marks are acknowledged. Where known they are identified in the text by TM or ®.

vii

CONTENTS

Acknowledgements

The author acknowledges the help given to him in preparing this book by many industry colleagues from around the world. As far as possible application illustrations and case histories are acknowledged within the text. Unacknowledged diagrams and photographs are mostly from Helvar and Electrosonic. Among the many individuals who have contributed information and help, special thanks are due to to the following, arranged in company alphabetical order: Individual independent consultants

Brian Legge Thomas Baenziger

ADB (aviation lighting) Ira Jackson AIM Aviation (Jecco) Ltd Rolf Startin Anytronics Ltd Bob Hall Artistic License Wayne Howell Art2Architecture Peter Fink Arup Acoustics Sam Wise Arup (Manchester office) John Waite Avolites Richard Salzedo British Library Michael Wildsmith Building Research Establishment Michael Perry Carr & Angier Paul Covell CCT Lighting David Manners Claude Lyons Jim McIlfatrick Color Kinetics Melissa Connor The Deep (Hull, UK) Dr David Gibson Delmatic Stephen Woodnutt Derungs Licht Claudio Roth DHA Lighting Design Adam Grater Dynalite Dannielle Furness ECS Philips Lighting Controls Chris Holder Electrosonic Ltd Yvonne Hegarty, Daniela Simonides ETC Fred Foster Firework Shop John Stapleton Fisher Marantz Stone Scott Hershman Rob Schoenbohm Focus Lighting Inc Paul Gregory Genlyte Controls Jason Moreno Helvar (Finland) Teijo Viljanen, Markku Nohiu Eeva Harjula Helvar (Germany) Ingo Sommer Helvar (UK) Alan Jackson, Trevor Forrest Austen Conway, Dr Scott Wade, Mel Collins Howard Industries Mike Dodds

viii

IBL Leviton Lighting Architects Group

Peter Saunders Breda Potter Jonathan Speirs Mark Major, Iain Ruxton LSI Projects Ltd Nick Mobsby Lumisphere Products Bob Myson Lutron Brian McKiernan MEM and MEM250 Richard Hunt, H. Milligan Osram Hans Jörg Schenkat, Verena Roemer P. Ducker Systems Ltd Richard Thomas Philips Marc Segers, Holger Moench Peter Woodward, John Rothery Pinniger & Partners Miles Pinniger Project Interational Richard Dixon Pulsar Light Cambridge Andy Graves, Paul Marden Pyrodigital Consultants Ken Nixon Quo Vadis Ltd Michael Stott Relco Daria Fossati repas AEG Dirk Buchholz Royal National Theatre Great Britain MikeAtkinson Schott Fibre Optics UK John Meadows Starfield Controls Wayne Morrow Strand Lighting Vic Gibbs, Ivan Myles Sutton Vane Associates Mark Sutton Vane Technical Marketing Ltd Andy Collier Teknoware Jari Tabell Thorlux Lighting Terry Fletcher Tunewell Transformers Derek Price, Glen McGovern Tridonic Stewart Langdown Kate Wilkins Lighting Design Kate Wilkins Vantage Controls Andy North Vari-Lite Europe Ltd Samantha Dean,Colin Brooker Waltzing Waters Douglas Tews, Michael Przystawik The Watt Stopper Inc Joy Cohen WRTL Exterior Lighting Tom Thurrell Wynne Willson Gottelier Tony Gottelier Wybron Inc Brandon James Young Electric Sign Co Graham Beland, Blake Gover In addition, grateful thanks to Paul Ashford for driving “Pagemaker”, Noel Packer (of Helvar UK) and Paul Ashford for drawing most of the diagrams and Maggie Thomas (of Electrosonic) for help thoughout the project. Finally, thanks to the staff at Focal Press for their support over three years.

CONTENTS

Preface

Artificial lighting is part of our daily lives; in the modern world there are few activities which take place without it. While there are many books on the subjects of light and lighting, there are few that cover the subject of lighting control. Those that do look at the subject from a limited perspective, for example that of stage lighting. Within the practical limits set by its size, this book is intended as a review of all methods of lighting control. It covers all the current technologies, and gives application examples from many aspects of our daily lives. It is intended for all those who already work in the lighting industry, for designers and consultants, and for the sophisticated end user. It is also intended as a training resource for those new to the industry. No significant prior technical knowledge is assumed. The book is written for the “intelligent layman”, and mathematics are kept to a minimum. In order to make it a complete resource Part 1 is in the nature of a “foundation course” to give the necessary background to those with limited (or no) knowledge of the basics of light, electricity and electronics. It can be skipped by engineers and technicians who already have this knowledge. Part 2 is a review of light sources. Unless you understand how a particular light source works, you cannot appreciate how to control it. Again much of the information in Part 2 may be well known to some readers, although there could be a few surprises arising from recent developments in traditional sources, and from the arrival of completely new sources.

Most light sources need some kind of “load interface”, for example a ballast or transformer, between them and the electricity supply. Part 3 reviews these “lighting components”, most of which are available in both electromagnetic and electronic form. In recent years there have been major advances in electronic lighting components, and they now form an essential part of many lighting control systems. Part 4 is the technical heart of the book, covering dimmers and control systems. The dimmer or dimming interface is now only part of lighting control. With modern systems often embracing thousands of lighting “channels” spread over a large building complex, a basic knowledge of network and computer technology becomes necessary. Lighting control is used for practical, esthetic and energy management reasons. Part 5 reviews how the technologies described in the first four parts of the book are applied in practice. The aim here has been to cite examples, without attempting to be exhaustive, representing best practice from different parts of the world. Lighting control has been part of my life for over 50 years, from operating slider dimmers for school plays, to being a member of a specifying team for the latest generation of digital dimmers. I hope that my enthusiasm for the subject is reflected in this book, and that even if you are an experienced lighting practioner, you will find something useful or unexpected within it. R.S.S.

ix

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Chapter 1

ELECTRICITY AND LIGHT

Electricity and light Part 1 – Foundation chapters The first two chapters are intended for those who have little or no knowledge of light, electricity and electronics. They include definitions and background technical information which is assumed in the succeeding chapters. Restrictions on space, and the use of only the simplest mathematics, mean that the introduction is cursory, but nonetheless it should help those new to the subject (and those of us who have forgotten our school physics and simple electronics, and just want a reminder!). Readers with a technical qualification in electronics or electrical engineering can safely omit the sections on electricity and electronics, but may still find the sections on light and color useful.

1.1 Electricity Both electricity and light are mysterious. We are well aware of their effect, but, if asked the question “what is electricity?” or “what is light?”, we have difficulty giving a convincing answer. We find it easiest to think in terms of a model, or models, which make it possible for us to understand how an electrical or optical device works. Provided the model, or “mind picture”, is consistent, this is a quite acceptable way to proceed – indeed it is the only way for the layman to get an insight into physical processes. In this chapter we describe the simplest models of electricity and light to the point where the methods of controlling them can be understood. The first model we need is that of the structure of matter. The atomic theory holds that the smallest unit of matter is the atom. In a compound there are several different kinds of atom, but in an element all the atoms are the same. All atoms are themselves made up of fundamental particles. Modern physics has produced an alarmingly long list of these, but for our purposes it is sufficient to know of only three: • the proton, a massive particle which carries a positive electric charge • the neutron which has the same mass as the proton, but has no charge • the electron which carries a negative electric charge equal and opposite to that of the proton.

The model of atomic structure which is sufficient for our purposes imagines that atoms have a planetary structure: a heavy nucleus consisting of a mixture of protons and neutrons, surrounded by orbiting electrons. The resulting atom is electrically neutral since the number of electrons in orbit exactly matches the number of protons. Figure 1.1 gives some examples of elements which are relevant to our subject. An important point to notice is that the electron orbits are not haphazard. The model shows that the electrons occupy “shells”, each one of which can only contain a defined number of electrons. As we move through the elements, from hydrogen the lightest, through to the heavy elements such as uranium, one unit of charge is added for each element. The shells get filled up in sequence, and this has the result that only some elements have a “complete” outer shell. It is the nature of the outer shell of electrons which determines many of the electrical and chemical properties of different elements. When an atom, for example a copper atom, has only a single electron in its outer shell, this electron can be easily dislodged – a so-called “free” electron. Such a free electron can be influenced by an electric charge. Electric charges have as their principal characteristic the fact that like charges repel each other, and unlike charges attract each other. An atom which has “lost” its free electron will itself aquire a

1

LIGHTING CONTROL – TECHNOLOGY AND APPLICATIONS

Figure 1.1 Simplified models of atomic structure. (a) Hydrogen, with one proton and one electron. (b) Carbon, showing two groups of electrons in elliptical orbits. (c) Simplified version of Carbon, showing positive charge of the nucleus balancing the negative charges of the electrons, and the K and L “shells”. (d) Neon, an insulator, with two full K and L “shells” of 2 and 8 electrons. (e) Silicon, a semiconductor, with a partially filled outer M shell having only four out of a possible maximum 18 electrons. (f) Copper, a conductor, with the three inner shells filled, but with the outer N shell having only one electron, out of a possible 32 maximum for this shell.

positive charge and is said to be positively ionized; it will then tend to attract a free electron to balance the charge again. However, free electrons are quite mobile in metals, and if an electric charge is applied across a length of metal, it results in a movement of free electrons; which we now identify as an electric current. Figure 1.2 shows the idea of an electron flow within a metal wire or conductor. The measure of the rate of flow of electric current is the Ampere, often abbreviated to Amp. For our mind picture of electricity it can be thought of as a flow of 6.26 × 10 18 (that is 6.26 multiplied by 1,000,000,000,000,000,000) electrons per second, although for reasons explained later this is not the actual definition of an Ampere.

2

The measure of electrical charge is the Coulomb. It is defined as the charge transferred when a current of one ampere flows for one second. Since the free electrons creating an electric current are themselves charged particles, the coulomb is equal to the total charge carried by 6.26 × 1018 electrons. Next for our model of electricity flow we need to have an idea of what can “push” the free electrons along the conductor. Figure 1.3 shows a simple electrical circuit where a battery is the source of electro-motive force or e.m.f. measured in Volts. When the first electrical discoveries were made, it was a convention that electric current flowed from the positive terminal of a battery to the negative terminal. This convention remains today, even though our “electron flow” model of electricity


Wire Conductor

POSITIVE CHARGE THIS END attracts electrons

-

+ Electron flow Conventional current flow

= Nucleus with full inner shells and free electron in outer shell

Figure 1.2 The idea of free electron flow creating an electric current. At the positive end of the conductor there is a deficiency of electrons, so the free electrons move to fill the space. Conversely at the negative end, there is a surplus of electrons “pushing”.

shows the free electrons flowing the other way. The more volts applied, the stronger the current. It is also

found that the conductor heats up while the current flows, and that this heating effect is proportional to the square of the current. The tendency for an electrical conductor to restrict current flow is called its resistance and is measured in Ohms. Table 1.4 shows how the Ohm is defined by the amount of heat produced by a current of one Ampere, and how in turn the Volt is defined as the Potential Difference across a resistance of one Ohm when it is carrying a current of one Ampere. The most useful and most easily remembered relationships which link the main electrical units together are: V = IR Ohm’s Law (Volts = Current in Amps × Resistance in Ohms) which can also be expressed: I=

V R

and

R=

V I

and the power dissipated in a resistance: W = I 2R

Figure 1.3 The concept of potential difference down the length of a uniform conductor. At any point in the wire, the potential difference is proportional to the length of wire. On the right a hydraulic analogy shows that water pressure drops in a pipe in a similar way. If the tap is closed, then the level in all the columns would rise to match the level in the tank.

3


(Power in Watts = Current in Amps squared Resistance in Ohms)

×

which Ohm’s Law allows us to more conveniently remember as: W = VA

(Power in Watts = Potential Difference in Volts × Current in Amps) Figure 1.3 shows the battery as a source of e.m.f. (measured in Volts) providing a potential difference (also measured in Volts) across the electrical conductor which we can think of as being some kind of heating element. Down the length of the conductor the potential drops in proportion to the length – if you take a Voltmeter to measure the voltage it gets lower along the length of the conductor. A simple hydraulic analogy is shown of water from a header tank losing pressure down the length of a pipe. In the real world the e.m.f of the battery measured on open circuit (nothing connected) only equals the p.d. across the load resistance when connected if the

battery itself has no internal resistance. If it does have internal resistance, then the actual voltage across the load will be lower. An automobile battery is an example of a battery with very low internal resistance; it can deliver a current of several hundred Amperes at 12 Volts to run a starter motor. A battery set for a portable compact disc player may also deliver 12V as an open circuit e.m.f., but its internal resistance limits the current available (and the actual p.d. at the load). See Figure 1.4.

1.2 Electrical units and components 1.2.1 Units Having introduced the concept of the flow of electricity, it is now necessary to be more rigorous about how it is measured. The way in which we measure things depends finally on having agreed reference standards, so that

Figure 1.4 A car battery has negligible internal resistance, so can deliver a full 12V to a lamp load. The same lamp load connected to a small 12V battery might show only half the current flowing through it because of the battery’s internal resistance. (The value shown here has been chosen for simplicity) Thus the p.d. across the load is much less then the e.m.f. of the open circuit battery. The idea that a source of power can itself limit the available current is important.

4


Quantity Length

Unit Meter

Symbol m

Mass Time

Kilogram Second

kg s

Plane angle

Radian

rad

Solid angle

Steradian

sr

Temperature

Degree Kelvin

K

Defined as The length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second. The mass of the prototype kilogram kept at Sèvres in France 9,192,631,770 periods of vibration of the Caesium 133 atom (formerly 1/86,400 of the mean solar day). The angle subtended at the center of a circle by an arc of equal length to the radius (r). The ratio of a circle's circumference to its diameter (which equals 2r) is S. Thus 360° is equivalent to 2S radians, and one radian = approximately 57.3°. The solid angle subtended at the center of a sphere by an area numerically equal to the square of its radius. Since the surface area of a sphere is 4Sr2, a sphere subtends 4S steradians. The temperature scale where each degree is numerically the same size as that on the Celsius scale (i.e. there are 100 degrees between the freezing and boiling points of water) but whose zero point is Absolute Zero (the lowest temperature possible where all molecular movement ceases) 0K = 273°C approximately.

Table 1.1 Fundamental units

there is consistency between different methods of arriving at the same result. For example the heat generated by a gas heater or an electric heater must be measured in the same units; or the power developed by an electric motor must be comparable to the power developed by a gasoline engine. It would be very confusing if we had different units for both, although in practice this can happen! For example most people like to talk in terms of horsepower (derived from “imperial” units of measurement) for the capability of a motor, whereas it is in fact easier to work in kilowatts (derived from metric units) when working as an engineer. Most electrical engineering is now based on SI (Système International d’Unités) units. These are based on the MKS or meter, kilogram, second system, whereby the primary units of length, mass and time are used as the basis of all other units. Thus, while we could define the flow of electric current in terms of the number of electrons passing a particular point per second, it is actually agreed to define it in terms of the mechanical force created by a current carrying conductor, because this way we can relate it back to the MKS primary units. Originally the meter was related to the dimensions of the earth, and represented by a

physical standard of platinum–iridium kept in Paris. Similarly the second was related to the mean solar day. However, the demand for greater accuracy in measurement has resulted in the meter being related to the velocity of light, and the second to the emission of radiation from a particular atom. Table 1.1 lists the fundamental SI or MKS units, and Table 1.3 shows how other commonly required Prefix

Symbol

Multiplier

Example

PicoNanoMicroMilliCentiDeciUnitDekaHektoKiloMegaGigaTera-

p n P m c d da h k M G T

10 -12 10 -9 10 -6 0.001 0.01 0.1 1 10 100 1000 10 6 10 9 1012

Picosecond Nanometer Microfarad Millisecond Centiliter Decibel Hectoliter Kilogram Megawatt Gigahertz Terabyte

Table 1.2 The common unit modifiers.

5


Quantity Frequency

Unit Hertz

Symbol Hz

Force

Newton

N

Energy (or Work)

Joule

J

Power (or the rate of doing work) Pressure (force per unit area)

Watt

W

Pascal

Pa

Defined as The number of repetitions of a regular occurrence within one second. Formerly cycles per second. The force which, when applied to a mass of one kilogram, gives it an acceleration of one meter per second per second. Force in N = Mass in kg u Acceleration in m/s 2 The work done when a force of one Newton acts for a distance of one meter in the direction of the force. Energy and heat are directly equivalent, so heat is also measured in Joules. (The SI avoids the use of the calorie; but a Kilogram-calorie, beloved of dieticians and the heat required to raise a kilogram of water by one degree Celsius, is found by experiment to equal 4187 Joules.) Work being done at the rate of one Joule per second. (The imperial based horsepower is 550 ft lb/sec, which converts to 746 watts.) When a force of one Newton is applied across an area of one square meter, the force is one Pascal. (One Bar is 10 5 Pa. Comparing with imperial measurement, one Pa is 1.45 u 10 -4 lb/sq. in.)

Table 1.3 Derived units (mechanical).

mechanical units are derived from them. Table 1.4 then shows the main electrical units, and here it can be seen that by relating the unit of electric current to physical force, and the unit of resistance to measurable heat energy, all the electrical units are “tied back” to the SI fundamental units. Many of the standard units are inconveniently big or small for certain types of measurement. They are modified by prefixes denoting powers of ten. Some of particular interest to lighting are frequencies such as kHz and MHz; and short wavelengths such as nanometers. Table 1.2 summarizes these prefixes.

of heat or pressure so that it becomes conducting. This will be discussed further in Chapter 3. Another category is the semi-conductor, the best known example of which is silicon. Pure silicon at room temperature is an insulator. But under certain conditions of heat or local impurities it is possible to detach a free electron from the incomplete M shell, at which point silicon becomes a conductor. Semiconductors are the basis of modern electronics, and are discussed further in Chapter 2.

1.2.2 Conductors and insulators

While we usually want a conductor to conduct electricity as well as possible, and an insulator not to conduct it at all, there are times that we need to limit the current flowing in a circuit. This is done with a resistor, which is simply a device which has a known resistance in ohms. Resistors take many forms (Figure 1.5.) At one extreme a heating element for a cooker or electric fire is a resistor, at the other modern electronic circuits use tiny resistors often with values in the kilohm or megohm range. The size of a resistor will depend on the way in which it is to be used. Because a resistor dissipates heat in direct proportion to its resistance in ohms, and in proportion to the square of the current going through it; it must be designed to get rid of this heat. A resistor in an

In Section 1.1 we identified copper and other metals as being good conductors of electricity, because of the availability of free electrons. Different metals have different conductivity, for example copper, silver and aluminum are good conductors; iron is a conductor, but is poor compared with copper. The opposite of a conductor is an insulator which in theory should not carry any electricity at all. In Figure 1.1 the gas Neon is shown as an insulator, because its electron shells are full and tightly bound to the nucleus. However, the perfect insulator does not exist. A very strong electric field can “break down” an insulator, or alternatively an insulating gas or compound can be ionized under certain conditions

6

1.2.3 Resistors


Quantity and symbol Electric Current I

Unit Ampere

Unit Symbol A

Electric Charge Q Electric Resistance R

Coulomb

C

Ohm

:

Electric Potential V

Volt

V

Electric Energy E

Joule

J

Electric Power P

Watt

W

Defined as One Ampere is the constant current which, if flowing in two infinitely long parallel conductors, of negligible cross-sectional area, in a vacuum and placed one meter apart, creates between them a force of 2 u 10-7 Newton per meter length. The quantity of electricity transported when a current of one Ampere flows for one second. The resistance of a conductor is one Ohm if a current of one Ampere flowing for one second generates one Joule of heat energy. When a resistance of one Ohm carries a current of one Ampere, the potential difference across the resistance is one Volt. Ohm's Law can be stated as V=IR The physicist James Joule determined that the heating effect in a wire conductor was proportional to the square of the current flowing I, to the resistance of the wire R, and to the time the current flows t, i.e. E vI2Rt (symbol v means “proportional to”) As shown above the definition of the Ohm is based on E = I2Rt when E is in Joules, I is in Amperes and t in seconds One Joule per second, so, from the basis of E above Power in Watts = I2R Applying Ohm's Law, the alternative Power in Watts = Current in Amperes u Potential Difference in Volts

Electric Capacitance C

Farad

F

Electrical Inductance L

Henry

H

Magnetic Flux )

Weber

Wb

Magnetic Flux Density B

Webers per square meter (or Tesla)

Wb/m2 (or T)

Or Watts = Amps u Volts is the more convenient and easiest form to remember. If the potential across the plates of a capacitor rises to one volt as a result of being charged with one coulomb, then its capacitance is one Farad. In practice this unit is too large for most purposes. The microfarad (PF), and nanofarad (nF) are common measures in electronics. A circuit (or coil) has an inductance of one Henry if an e.m.f. of one volt is induced across it when the current changes at the rate of one Ampere per second. Again this is a large unit and mH and PH are widely used. The magnetic flux which, linking a circuit of one turn, produces in it an e.m.f. of one volt as it is reduced to zero in one second at a uniform rate. But more strictly derived from the definition of flux density. A magnetic field has a flux density of one Weber per square meter if a conductor placed at right angles to the field and carrying a current of one Ampere, has a force of one Newton per meter acting on it.

Table 1.4 Electrical and magnetic units, showing how they are derived from the mechanical units in Tables 1.1 and 1.3

7


Figure 1.5 Examples of resistors. The circuit symbols for resistance, and some simple laws about using resistances in series and parallel. A variable resistor is a two terminal device; a potentiometer has a similar construction but is a three terminal device; a voltage is applied across the ends of the resistance element, and the moving contact has a varying potential difference (voltage) according to its position along the resistance.

Figure 1.6 Resistance varies with temperature. On the left a 100W tungsten lamp has the voltage presented to it changed by a variable resistor. Current through, and voltage across, the lamp are measured by A and V. The resistance can be calculated, and plotted against the voltage, and this shows that at proper operating temperature the filament has more than ten times the resistance it does when cold.

8


electronic circuit may only need to dissipate a few microwatts, or at most a watt or so. A resistor for use as a current limiter in a lighting or motor circuit may have to dissipate a kilowatt. There is a complication about resistance which is highly significant in lighting. In most materials it varies with temperature. Figure 1.6 shows a simple experiment in which a 100W light bulb is connected to a 230V mains supply via a suitable variable resistance to change the potential across the lamp. A voltmeter measures the volts across the lamp, and an ammeter measures the current through it. Using Ohm’s law it is possible to calculate the resistance for different voltages. The graph shows that when no current is flowing the lamp has a resistance of 50 ohms – but when the lamp filament is running at its rated operating temperature, its resistance has risen to over 500 ohms. The variation of resistance with temperature is defined by the temperature coefficient of resistance. The resistance of pure metals rises with temperature; whereas the resistance of carbon, silicon and insulating materials drops with temperature. Special alloys (such as Eureka, a 60/40 mixture of copper and nickel) have practically no change in resistance over a wide range of temperature and are used in applications where the resistance must remain constant.

I=

V R

1.2.4 Capacitors A capacitor (also formerly known as a condenser) consists of two conducting surfaces separated by an insulating layer called the dielectric. The plates (or electrodes) can be flat metal plates, and the insulator between them can be air; but as a practical component capacitors usually consist of thin plastic films on which layers of aluminum have been deposited, or are thin metal sheets with an electrochemically derived insulating layer – see Figure 1.7. An “empty” capacitor has no potential difference between its electrodes, but if an electric field is applied across them the capacitor “charges up”. The process is shown in Figure 1.8(a).

Figure 1.7 The principle of a capacitor (a), and (b) a possible construction using metallized film or paper.

9


At first the switch S is in a center-off position. There is no voltage across the capacitor C. If the switch is closed to terminal b, a battery is connected to the capacitor through a resistance R. Measurement of the current through the capacitor by ammeter A, and the voltage across it by Voltmeter V, gives the curves shown in Figure 1.8(b) with respect to time. The initial current is high, but as the voltage across the capacitor rises, the current drops to zero. The “charging time” is found to be limited by the resistor R. Double the resistor value, and the initial charging current halves, and the time to full charge is doubled. If the switch is returned to its center-off position, the voltmeter V will continue to show the full battery voltage across the capacitor – and in a perfect capacitor it will hold the charge indefinitely. Now if the switch is turned to the d position, the current and voltage will behave as shown in Figure 1.8(c) as the capacitor discharges. Figure 1.8(d) shows a hydraulic analogy of the operation of a capacitor. In the top cylindrical chamber there is a rubber diaphragm stretched across the cylinder (= the dielectric). In the bottom cylinder the piston can increase the pressure on one side of the diaphragm and move more fluid into the top chamber (= the battery applying an e.m.f.) If the force is removed from the piston, the diaphragm springs back and forces the fluid back again (= capacitor discharging). No fluid can get from one side of the diaphragm to the other under normal circumstances. However, if it is subjected to excessive pressure, it bursts. Exactly what happens when a capacitor is subjected to too high a voltage. Table 1.3 defines the ability to store charge in a way which can be expressed as a simple equation: Capacitance Ch arg e in Coulombs = in Farads Potential Difference in Volts

or

Q =C V

Practical capacitors used in lighting and electronics are usually measured in microfarads or mF. Their capacity can be related to their physical dimensions by the equation: C = ε0 ε r

10

A d

Figure 1.8 Charging and discharging a capacitor. Figure described in the main text.


where C is the capacitance in Farads, A is the area of the capacitor plates in square meters, and d is the distance between the plates in meters. ε0 is the permittivity of free space (or vacuum) a quantity which can be determined experimentally as having a value of about 8.85 × 10-12 F/m. It relates the electric force, or field strength across the capacitor plates measured in V/m to the electric flux density measured in Coulombs per square meter. (It also has a deeper significance, referred to in Section 1.6.) εr is the relative permittivity of the dielectric material. For a vacuum this is unity (one). For other materials it is higher, for example for paper it is 22.5. Plastic films have a relative permittivity of 4-6. Sometimes ε0εr is shortened to ε and is referred to as the dielectric constant of the material concerned. From an electrical power point of view a most important characteristic of a capacitor is that in any practical circuit containing even the smallest amount of resistance, it is not possible to change the voltage across a capacitor instantaneously. Figure 1.9 gives some more summary information about capacitors.

same happens with magnetic fields produced electrically – a force is produced which causes the affected items to repel or attract each other. The definition of the ampere given in Table 1.4 is based on the idea that any current carrying conductor creates a magnetic field, but also a conductor carrying a current in a magnetic field has a force exerted on it. An inductor consists of a coil of wire, usually, but not necessarily, wound on a core with

1.2.5 Inductors Electricity and magnetism are inextricably mixed. The fundamental discoveries of electromagnetism were: • when a conductor carries an electric current a magnetic field surrounds the length of the conductor. • if a conductor is moved in a stationary magnetic field, then an electric current is induced in it. Conversely if a magnetic field moves with respect to a stationary conductor, an electric current is induced in the conductor. This is the basis of electricity generation. • if a conductor carrying an electric current is placed across a magnetic field, a force is exerted on the conductor. This is the basis of electric motors. Any magnet has a “north” and “south” pole (named after the way small magnets align themselves with the earth’s magnetic field, as in a compass) and we are all familiar, from magnetic toys, with the way that like poles repel, and unlike poles attract. The

Figure 1.9 More about capacitors.

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Figure 1.10 A simple inductor, showing the magnetic field. The field is more intense when there is an iron core, and the iron magnetizes.

ferromagnetic (or iron-like) magnetic properties. The details of different kinds of inductors relevant to lighting control are given in later chapters, for now

it is sufficient to see what happens when an inductor is placed in a simple circuit – in the same way we considered capacitors in 1.2.4 above. Figure 1.10 shows a simple inductor, and the way a magnetic field is created by it. In Figure 1.11(a) a circuit is shown containing an iron cored inductor, arranged in a way that confines the magnetic field largely within the core. The circuit also contains a resistor R and two ammeters, one of which is “center-zero”. Closing the switch S connects the battery to both the resistor and the inductor, and results in the currents shown in Figure 1.11(b). The current through R jumps immediately to the value determined by Ohm’s law. But that through the inductor takes time to reach a steady level (which does indeed reach a value limited by the resistance of the coil – but only when the steady state is reached does Ohm’s law apply as usual). What slows the current rise? The current produces a magnetic flux in the core; but because this flux is changing, it itself introduces an e.m.f. back in the coil. Lenz’s Law states that when this happens, the e.m.f. is always in the opposite direction to that which is creating the flux. This effect limits how fast the current can rise in the coil – in an inductor it is not possible to change the current

Figure 1.11 Experimental circuit showing the effect of inductance in a DC circuit (a). The switch on and switch off currents are shown in (b). IL is the switch-on current through the inductor, and IR the switch-on current through the resistor. Ib is the switch-off current, which goes through the resistor in the opposite direction to IR and arises from the back e.m.f. of the inductor. The magnetic flux in the inductor core builds up during switch-on, and collapses at switchoff.

12


instantaneously (compare the capacitor – where it is not possible to change the voltage instantaneously). If switch S is now opened, an interesting thing happens. The original source of e.m.f. has gone, so the current drops in the coil – but this creates a change in flux again and, therefore, an e.m.f. The only way for its resulting current to go is through the resistance R – the center-zero ammeter now shows a decaying current in the opposite direction to that originally flowing through R. What happens if there is no resistance R in the circuit? Opening the switch S produces a rapid current change, therefore a rapid flux change. Therefore, in turn, a big “back e.m.f.” which in practical circuits can result in arcing (sparks!) at the switch. Even in the simplest circuits using inductive components precautions have to be taken to limit the effects of back e.m.f. 1.2.6 Magnetic units In the discussion on capacitors, the concept of permittivity was introduced to describe how different materials are affected by an electric field. A similar idea applies to inductors and magnetism, where permeability is the description of how materials are affected by magnetic fields. Since magnetic properties are fundamental to ballasts and transformers used in lighting control, some understanding of magnetic units is important.

We have described an electric current in a circuit as being due to the presence of electromotive force. In electromagnetism we can postulate that in a magnetic circuit a magnetic flux is created by the presence of magnetomotive force (or m.m.f.) caused by a current flowing through one or more turns in a coil. Since the m.m.f. is proportional both to the current and the number of turns, the unit of m.m.f. in the MKS system is the ampere-turn. Figure 1.12 shows a toroidal coil with T turns carrying a current I. The mean length of the magnetic circuit is l meters. The magnetizing force H is defined as the m.m.f. per unit length, so: H = IT/l

The strength of a magnetic field is termed its magnetic flux density B. This is measured in Webers per square meter, or Tesla, and is defined by relation to the ampere, see Table 1.4. To see how B and H are related we need to do a small thought experiment. Figure 1.13 shows a long thin conductor in a vacuum A. It is carrying current of one ampere in the direction of the paper. Let us suppose that the return path of this current is a long distance away, so any magnetic field the return path generates does not affect things. The magnetic flux created by the current is in the form of concentric circles. What is sometimes referred to as Maxwell’s corkscrew rule says that the direction of the flux is clockwise if the current is flowing away from you as in Figure1.13. In this figure just one line of flux is shown, at a distance of one meter from the conductor. Since the conductor (and its return partner) form one turn, the m.m.f. acting on the flux path is one ampere-turn. The length of the flux path (2πr where r is one meter) is 2π meters. Thus the magnetizing force acting at one meter radius is: H = 1 Amp

× 1 Turn /2π meters

H=1/2π

ampere-turn/meter

or:

Figure 1.12 Defining the magnetizing force H. Here a toroidal inductor has magnetic circuit length L meters, has a coil of T turns and is carrying a current I Amperes.

ampere turns/meter

Now let us imagine a second conductor A´ sited one meter away from A, and also carrying one

13


represented by the symbol μ0 and applies not only to vacuum but also to non-magnetic materials. The magnetic flux inside a coil such as that in Figure 1.12 or 1.10 is greatly increased if an iron or other magnetic core is introduced. Magnetic materials are defined by their relative permeability, μr such that: B= μrμ0H

Figure 1.13 Defining the magnetic flux density B. Imagine the conductor A as perpendicular to the paper, with current flowing into the paper. Maxwell’s corkscrew rule says the magnetic flux is in a clockwise direction.

ampere. The definition of magnetic flux density gives us the equation: Force on conductor in Newtons = Flux density in Wb/m2 X length in meters X current in amperes. So for A´ we have: Force per meter = B (Wb/m2) × 1 m

× 1A=B

Newtons

But we know the force acting under these circumstances, because our thought experiment has replicated the conditions which define the ampere (see Table 1.3). The force is 2 X 10-7 N. So now we can say that: the flux density B at one meter radius from a conductor carrying a current of one ampere is 2 × 10-7 Wb/m2. And further, if we compare the flux density at A´ to the magnetizing force at A´, we now have: B 2 × 10 −7 wb / m 2 = H 1 / 2 π ampere − turn / m

or

B = 4 π × 10 −7 H

The ratio B/H for the conditions we have defined is referred to as the permeability of free space. It is

14

While the relative permeability of air and other non magnetic materials is 1, that for special nickel– iron alloys can be as high as 100,000. If measurements are made on different materials of the effect of increasing magnetizing force H on the flux density B, a graph of the kind shown in Figure 1.14 is found. Here it is assumed that the experiments started with unmagnetized material. The increase in flux density follows the curve OAC, but beyond a certain value of H the value of B reaches a limit. The material is said then to be magnetically saturated. If H is now reduced to zero, the value of B does not go down to zero, but retains a remanent flux density OD (usually between 60 and 75% of the maximum). A reverse magnetizing force OE, known as the coercive force is required to return B to zero. If the reverse force is increased, point F can be reached where the material is now saturated in the other direction. If the cycle is continued, by reversing H again, the curve follows FGC. The complete loop resulting from a double reversal of magnetizing force is called the hysteresis loop. The “fatter” the curve, the more effort is needed to take the magnetic material through a flux reversal. 1.2.7 Inductance The unit of measurement of the ability of an inductor to slow down a rise in current, its inductance, is the Henry, defined in Table 1.4. For an air cored coil of the kind shown in Figure 1.12 the inductance is: • proportional to the square of the number of turns of wire T. • proportional to the cross sectional area of the core a.


same as if the flux varied linearly along FOC, and, therefore, that the inductance of the coil becomes: Induc tan ce L =

μ r × 4 π × 10 −7 × aT 2 Henrys l

1.3 Electrical distribution 1.3.1 Direct current sources

Figure 1.14 The relationship between flux density B and magnetizing force H is not linear, and displays hysteresis.

• inversely proportional to the length of the magnetic circuit l. In fact: Induc tan ce L =

4 π × 10 −7 × aT 2 Henrys l

When the air core is replaced by an iron or other magnetic core, the situation becomes very complicated. The magnetization curve Figure 1.14 shows that the variation of flux is not linearly proportional to the current (which causes the magnetizing force) – so a coil can have a lot of different inductance values depending on the range of current variation. However, our interest in inductance is primarily concerned with alternating current (described in Section 1.3.2) where the current changes direction repeatedly. If μr is the relative permeability of the core material corresponding to the maximum value of the flux (point C on Figure 1.14) and it is understood that the relevant use of the inductance involves repeated flux reversals (to point F) then it is assumed that the value of the inductance is the

In the concepts presented so far, only circuits using Direct Current or DC have been mentioned. With the exception of the changes in current or voltage introduced by capacitors or inductors, the circuits have been “steady state” with current flow conventionally seen as flowing from positive to negative. DC is essential for the operation of most electronic components, and is most familiar to us as being supplied by batteries. A battery is in fact a group of electrochemical cells whose action is to convert chemical energy to electrical energy. Primary cells or batteries are “use once” devices, exemplified by the common zinc/carbon dry battery. Secondary cells are rechargeable devices, where the chemical reactions leading to the generation of electricity can be reversed, and are exemplified by the standard automobile battery.

1.2v 300mAh battery used in cordless control

3.6V 4Ah rechargeable battery used in emergency light fitting

12V 11Ah sealed lead acid rechargeable battery

Figure 1.15 Examples of batteries. Battery capacity is given in ampere-hours, thus a 2Ah battery can sustain a current of 2A for one hour or 500mA for 4 hours.

15


Figure 1.15 shows some examples of batteries. The demands of portable electronic equipment, emergency power supplies and electric traction have resulted in considerable developments in battery technology.

DC is also derived by power conversion from AC, and this is described in Chapter 2. It can also be generated by DC generators or dynamos, but this is not now generally relevant to lighting.

Trigonometry In our treatment of electrical units and electricity, we are keeping mathematics to a minimum; using where possible simple algebraic formulae with each symbol clearly defined. In describing alternating current it is necessary to use simple trigonometry. For those who have forgotten the basic relationships, here they are: In the right angled triangle in Figure 1.16 the angle θ is identified, as are the three sides of the triangle: • the hypotenuse which is the longest side opposite the right angle, the length of which is identified here as H. • the adjacent side which is the side next to the angle θ. Identified here as having length A. • the opposite side, which is the side opposite to the angle θ, identified here as having length O.

Figure 1.17 A force F acts on an object at angle θ to the horizontal. If the line OF represents the magnitude and direction of the force; it can be resolved into horizontal and vertical components of forces H and V, represented by the lengths of OH and OV. Conversely, if there are two forces acting on an object, there is a resultant force whose magnitude and direction lies between them.

The following relationships apply: O2 + A2 = H2

(Pythagoras’s Theorem)

The Sine of the angle θ = O/H The Cosine of the angle θ = A/H The Tangent of the angle θ = O/A Sine, Cosine and Tangent are abbreviated to Sin, Cos and Tan in mathematical expressions. A mnemonic for remembering the above is:

Figure 1.16 Simple trigonometry. Defining the sine, cosine and tangent of angle θ.

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Old Always Other Hands Help Aviators


An important use of trigonometry is in “resolving” forces, and in defining the contribution of a vector quantity (a vector quantity is one which has both magnitude and direction) in a particular direction. Figure 1.17 shows the principle of resolving a force into two different components. A force of F Newtons is acting on an object at an angle θ. If we want to know the horizontal and vertical components of the force, we draw the rectangle shown, with the length of the diagonal corresponding to the magnitude of the force. The lengths of the sides of the rectangle now represent the horizontal and vertical components of the force, and it can be seen that:

F cos θ is the horizontal component, and F sin θ is the vertical component But there is an important concept here, which will be appreciated as we look further into how alternating current, described in the next section, behaves. That is that, if you have two vector components acting in two different directions, you have a resultant single vector – and while this is quite easy to appreciate when dealing with a force, it may not be so obvious when you are dealing with an electric current.

1.3.2 Alternating current The electricity which arrives in our home is Alternating Current or AC. The reason for this is that it is both easier to generate, and easier to distribute. To understand why this is so, it is necessary to understand how an alternator (or AC Generator) works. Figure 1.18 shows a rectangular loop conductor rotating in a magnetic field. A conductor cutting magnetic flux generates an e.m.f. but here the conductor is not always cutting the flux at the perpendicular. When the loop is vertical the conductors are parallel with the flux, and no e.m.f. is generated, when the loop is rotated 90° the loop is cutting across the flux and maximum e.m.f. is generated. The instantaneous e.m.f. is proportional to the sine of the angle at which the conductor cuts the magnetic flux. e.m.f. generated in one side of loop = Blv sin θ volts

Where B is the flux in Wb/m2, l is the length of one side of the loop, v is the velocity of the conductor through the flux and sin θ is the resolved component of that velocity perpendicular to the magnetic field. Without needing to worry too much about the details of this equation, it is clear that the more and longer conductors there are, and the faster the loop rotates, the more volts will be produced.

Figure 1.19 shows how a sine wave is produced by this action. It also shows another “mind picture” of how a rotating vector produces the sine wave. A vector quantity is one which has both magnitude and direction (for example speed is a scalar quantity, having magnitude only, whereas velocity is a vector quantity which must always be specified in both magnitude and direction). The magnitude in both cases can be km/hour or m/s or m.p.h. as appropriate. Notice here how the use of angular velocity ω, measured in radians per second simplifies the mathematics, since there are 2π radians in each revolution. A version of the device shown in Figure 1.18 could be used for generating AC by connecting pickup brushes to both ends of the conducting loop as shown in the diagram. (Indeed a very rough DC could be derived by having a “commutator” arrangement which switched the direction of current to the outgoing circuit every half revolution.) However “real” alternators of any size work the other way round. They have static conductor coils, which are then easily connected to the outside world without the need for any pick up brushes; and the moving magnetic field is produced by rotating electromagnets. These are powered by DC, traditionally generated by a dynamo on the same shaft as the main alternator, but now also derived by converting from AC.

17


Figure 1.18 Creating an alternating current. On the left a loop conductor being rotated in a magnetic field (shown in the position of rotation θ=90°). On the right it is shown that maximum e.m.f. is generated when the conductor cuts the flux at right angles (rotation angle 90°) and no e.m.f. is generated when the conductor is moving in the same direction as the flux lines (rotation angle 0°).

Figure 1.19 If the value of r sin θ is plotted at equal increments along an axis, a sine wave is the result, as shown on the left. On the right we see the idea of a rotating vector producing a sine wave. The horizontal axis can represent both the angle of rotation and time (if the angular velocity is known). If the vector length represents a current of maximum value IM the waveform is that of an alternating current.

18


Figure 1.20 The single phase alternator. The left hand diagram shows the principle with a single 2-pole magnet. The right hand diagram shows an 8-pole magnet system. In a real alternator the magnets are electromagnets fed by a DC supply.

Figure 1.20 shows the principle of an alternator. AC does not have to be sinusoidal (sine wave shaped) and many other waveforms exist, especially in electronics. But for power distribution it is the most efficient waveform, so alternators are designed to give a sinusoidal output. (To be technically correct it should be noted that the alternator construction implied by the two arrangements shown in Figure 1.20 would not give sinusoidal output without some refinement.) To make better use of the alternator frame, and to assist with power distribution, AC is often produced in three phase form. Here the alternator simultaneously produces three different sine waves, but each of these is timed to peak at different times. The idea is shown in Figure 1.21, and a real alternator is shown in Figure 1.22. In the USA, electricity is generated at 60Hz (cycles per second) in Europe it is generated at 50Hz.

1.3.3 AC circuits In Section 1.2 many of the basic rules about electricity were given in relation to DC circuits. Unfortunately they need some “tweaking” when applied to AC. First we need to know if Ohm’s law and the simple power calculation rules still apply. Clearly it is no use taking “average” current and voltage, since these are both zero if the waveform is symmetrical. The way forward is to say that the effective value of an alternating current is that which produces the same heating effect in a resistance as does a direct current of the same numerical value. We can do a comparison by “slicing up” an AC waveform. In Figure 1.23 we show n instants at which current is measured in a half cycle. The instantaneous heating effect at each instant is in2R. So we can say

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Figure 1.21 The principle of the 3-phase alternator. The top diagram shows how the three sets of stator coils are set at 120° (or 2π/3 radians) apart, the bottom diagram shows the relationship of the three waveforms.

In words, the current equals the square root of the mean of the squares of the current. The r.m.s. or root mean square values of current and voltage are used whenever AC supplies are specified. Ohm’s Law and the relationship Watts = Amps × Volts both work when applied to resistive loads. Some other aspects of the sine wave are shown in Figure 1.24. An interesting point arises with 3-phase AC. Usually each phase can be considered as an independent supply, requiring two wires. In most electrical systems one side of the supply is connected to earth or “ground”. It is referred to as the Neutral connection, and the other side is called the Live (or “Hot”) connection. You get an electric shock off the live connection, but not off the neutral connection (provided it is well bonded to earth) because the neutral is at the same potential as you are if you are standing on the ground. In a 3-phase system the three neutrals are obviously joined together if they are all to be at ground potential. If you look carefully at a 3 phase waveform as in Figure 1.21 you can see that at any instant the three currents add up to zero – for example if at any one moment there are two positive currents and one negative, the sum of the two positives

that the average heating effect over the whole half cycle is: 2

2

2

2

i 1 R + i 2 R + i 3 R + .......... ....... + i n R n

Now if we say that I amperes is the Direct Current through the same resistance R which gives the same heating effect as the average heating effect of the AC, we have: 2

I 2R =

2

2

2

i1 R + i2 R + i3 R + .......... ....... + in R n

Therefore: 2

I =

20

2

2

i 1 + i 2 + i 3 + .......... ....... + i n n

2

Figure 1.22 A power station generator under construction. This one is from Siemens AG, and is rated at 1,000 MVA, with output at 27 kV. It uses hydrogen cooling to achieve a comparatively small frame size. The rotor carrying the rotating electromagnets is being inserted into the stator assembly, the coils of which collect the induced alternating current.


Figure 1.23 Finding the heating effect of AC. Imagine the sine wave to be divided into many small sections, of current value i1 i2 i3 and so on to in. Each little chunk of current will make its own i2R heating contribution.

exactly equals the negative. The vector representation of currents helps make this clear. A 3-phase system with common neutral is referred to as a “star” or “Y” system, and its feature is that if the load on all phases is equal there is no neutral current. Power system design usually calls for balancing the load on the three phases in order to minimize or preferably eliminate neutral current. This is of great significance in lighting systems. While the r.m.s value of the voltage on a single phase in a 3-phase system is the same for each phase

Figure 1.24 Peak, r.m.s. and half wave average values of a sine wave. For all symmetrical waveforms the form factor is the ratio of r.m.s value to average value (1.11 for a sine wave) and the crest or peak factor is the ratio of the peak value to r.m.s. value (1.414 for a sine wave).

Figure 1.25 3-phase electricity. (a) shows the concept of three simultaneous rotating vectors, 120° apart. (b) shows a 3-phase alternator with Y output and Y connected loads; and (c) shows a 3-phase alternator with delta output (ǻ) and delta connected loads.

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(for example 230V in Europe and 115V in USA in the home) the voltage across phases is much higher (and more dangerous). The r.m.s voltage across two phases is √3 × the single phase voltage (about 398V when the single phase voltage is 230V). There is another way of distributing 3-phase AC, arising from the fact that the vector sum of the e.m.f.’s is zero. This is to do away with the “neutral” connection altogether, and connect the alternator windings in a “mesh” or “delta” format. Figure 1.25 shows the idea. Vector considerations of the current show that the current in each load line is √3 times the current in the phase winding of the alternator. The arrangement is particularly suitable for powering big electrical motors. It is also encountered in some electical distribution systems (for example in Norway and on board many ships) and this has some consequences for lighting control systems. 1.3.4 Transformers In any simple electrical system, the power delivered to the load is the product of the voltage and the current. If we want to transfer 10kW of power, we can do it in several ways:

The voltage conversion process is done by a transformer. A transformer extends the principle of the inductor described earlier. A typical construction is shown in Figure 1.26. A coil is wound on an iron core and is fed from an AC supply. A second coil is mounted on the same core. When the current changes in the first coil or primary winding, it produces a change in magnetic flux, and this flux change must cut the conductors of the second coil. A current is, therefore, induced in the secondary winding, through mutual inductance – the process where two coils share the same magnetic flux. In the description of the inductor in Section 1.2.5 its behavior was discussed in relation to the application or removal of a direct current. This showed that the current reached a steady value and the coil circuit obeyed Ohm’s law once the steady state was reached. If we were to apply DC to a transformer, the same thing would apply. The current induced in the secondary would be momentary, and all that would happen is that the transformer would cook due to the high current and low coil resistance of the primary.

• at 10 Volts, we would need a current of 1,000A • at 200 Volts, the current is a reasonable 50A • at 10,000 Volts, the current is only 1A High currents mean very big conductors if there is not to be significant heat loss in the conductor due to the conductor’s resistance. The heat effect is proportional to the square of the current, so a cable resistance of one ohm would result in 2,500W being dissipated by the 50A current, but only 1W by the 1A current. Thus for electrical distribution the power generated at the power station’s alternator at around 20,000 Volts (20kV) is “stepped up” for primary distribution to as much as 400kV. Local distribution may be done at 11kV or 33kV, requiring a “step down”. Final distribution to the home and office requires a further step down to 230V or 115V depending on where you live. Figure 1.26 Construction of a transformer.

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Figure 1.27 Example of a transformer used for lighting – a step-down transformer for low voltage tungsten halogen lamps. (Photo from Relco.)

But with AC the current, and, therefore, the magnetic flux, is constantly changing. There is a continuous back e.m.f. limiting the current in the primary, and the current in the secondary follows that of the primary. In a perfect transformer (and in fact most power transformers are quite efficient) all power put into the primary can be extracted from the secondary. For reasons explained in Section 1.4, transformer ratings are always given in VA (or VoltAmps). Figure 1.27 shows a 300VA transformer intended for use for lighting operating at 12V. This means that if it is fed with AC at 230V, the primary current is 300/230 = 1.3A. At the secondary, rated for 12V, there is a total of 300/12 = 25A available. The Ampere/Turns must be the same for the primary and secondary (clearly the transformer cannot actually create energy). Therefore:

In the example transformer, the 230V primary winding might have 400 turns, thus to give 12V at the secondary, the secondary winding needs only 21 turns. The turns ratio determines the voltage ratio. Good transformer design confines most of the magnetic flux close to the transformer core, so all of it is used to induce e.m.f.’s into the conductors. Power transformers working at 50 or 60Hz use special transformer iron alloy to provide the core. Being metal, this is a conductor – therefore the moving magnetic flux must induce electric current into it. The current so induced is called an eddy current because it circulates within the metal. Eddy currents cause heating, like any other current, and could be responsible for serious losses in transformers (and, of course, other electromagnetic devices, including alternators and motors). The problem is solved, if not entirely eliminated, by using a laminated construction for the transformer core. See Figure 1.28. The laminations are insulated from one another, and in this way any eddy current is confined to a single lamination. An example illustrates how effective the technique is. Suppose we substitute a single core piece with ten laminations: • the e.m.f. per lamination is only one tenth of that generated in the solid core.

Primary × Primary Secondary × Secondary turns amps = turns amps

But also: Primary × Primary Secondary × Secondary volts amps = volts amps

So: Secondary volts =

Secondary turns × Primary volts Primary turns

Figure 1.28 Electromagnetic devices working at 50/60Hz use iron alloy laminations to minimize the effect of eddy currents.

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Figure 1.29 The autotransformer has only a single tapped winding.

• the cross sectional area is also reduced to one tenth, so the resistance goes up ten times; this reduces the current by a further factor of ten. The current per eddy path is thus reduced to 1/100th of what it would be in the solid core. In fact the eddy current loss is proportional to the square of the thickness of the laminations, so within reason the aim is to use the thinnest laminations practicable. An important feature of most transformers is that primary and secondary windings are completely separate; the power transfer is achieved entirely by induction. For many applications this is essential for safety or signal isolation reasons. In an autotransformer only a single tapped winding is used (Figure 1.29). Transformers used at high frequencies work on the same principle but use a different construction, described in Chapters 6 and 7. The iron alloy used in low frequency transformers has pronounced hysteresis, making it unsuitable for high frequency operation.

In an AC circuit with a resistive load, the current in the load is exactly in phase or “in step” with the applied voltage. But this is not the case with capacitance and inductance. Figure 1.30 shows a sine wave of current flowing through a pure inductance. The instantaneous value of current is Imax sin ωt. Now the voltage across an inductance is given by the inductance multiplied by the rate at which the current is changing. Mathematically this can be stated as: e =− L

di dt

Where e is the voltage, L the inductance in Henrys, and di the change of current in small time dt. The negative sign arises because the induced e.m.f. is in the opposite direction to the applied current. The rate at which the current is changing at any instant corresponds to the instantaneous slope on our sine wave current curve. If we plot the value of this

1.4 Power factor 1.4.1 Reactance The description of the transformer has introduced the idea that some loads may behave differently to simple resistances when connected to an AC supply. If we consider what happens when a pure inductance (i.e. an inductor with no resistance and no capacitance) or a pure capacitance (a capacitor with no inductance and no resistance) is connected to an AC supply we find the voltage and current get “out of phase”.

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Figure 1.30 AC flowing through an inductance. The top sine wave is the current. The bottom curve is a plot of the gradient (or slope, or rate-of-change, whichever you find easiest to imagine) of the current sine wave. It turns out to be a cosine wave, which is simply another sine wave 90° (or π/2 radians) out of phase with the first one.


slope against time, we find we get another curve looking the same, but out of phase with the current curve. It is a cosine curve, which is exactly the same as a sine curve, but 90° (or π/2 radians) out of phase. It is at its maximum when the current is zero, and at zero when the current is at its maximum. This sounds counter-intuitive, but what is happening is that the external AC supply puts energy into the inductance, but the induced e.m.f. puts the energy back into the supply. A mechanical analogy is that of the pendulum – when the pendulum bob is stationary, as it is changing direction at its highest point, it has maximum potential energy (voltage) but no movement (current); and as it passes through its lowest point, it is moving fastest (highest current) but has no potential energy (voltage). What limits the current through a perfect inductor? Is there an equivalent of resistance? There is, it is called the inductive reactance with symbol XL. It is the ratio of r.m.s. voltage to r.m.s. current, and is, therefore, measured in ohms. We can derive the formula for reactance in several ways, but the easiest is using simple calculus. Start with the equation: i = I max sinωt

Which is the description of our current sine wave. Differentiate this with respect to time to obtain the slope of the curve at any time (remember this is the rate of change of current with time). di = ȦI max cos Ȧt dt

(This is a mathematical shorthand for describing the lower curve in Figure 1.30. It says that any point on the curve representing a particular value of di/dt, or rate of change of current, can be calculated by the instantaneous value of ωImax cos ωt.) Going back to the equation e = - L di/dt, we can substitute the general expression for rate of change with the curve value (ignoring the - sign): e = −L

di = ω LI max cos ω t dt

For any angle θ, Cosine θ = Sine (θ + 90°) so we can write: e = ωLImax sin( ωt + 90° )

But in magnitude terms we know that Imax sin (ωt + 90°) is the same as Imax sin ωt (but advanced by 90°) and this equals the instantaneous current i. So we have in magnitude: e = ωLi

We have defined the inductive reactance XL as e/i so we have: XL =

e ωLi = = ωL i i

Now ω is the angular velocity of the current vector in radians per second, but this is the same as 2πf, where f is the frequency of the supply. Even if you have not followed the mathematics completely, the end result X L = 2πfL

is interesting. It tells us that the reactance is not simply proportional to the inductance, but also to the frequency of the AC supply. The higher the frequency, the higher the reactance. The same kind of analysis can be applied to a pure capacitance. This time we start with the equation: i=C

dv dt

That is to say, the instantaneous current equals the capacitance multiplied by the rate of change of voltage dv/dt. This time it is easiest to start with the voltage sine wave, and create the the current waveform from it (by the same procedure of plotting the new curve of instantaneous dv/dt). We find that, whereas with inductance the current lags the voltage by 90°, with a capacitance the current leads the voltage by 90°, and that the capacative reactance XC is defined by:

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Xc =

1 1 ( ohms ) = ωC 2πfC

Again the reactance depends on frequency. This time, however, it is inversely proportional – the higher the frequency the lower the reactance. Our “mind picture” of what is happening with reactive loads such as capacitors and inductors must not get carried away. It is easy to think of a capacitor “carrying” current through itself, but actually charge is building up cyclically on either side and then falling away. With an inductor, current flows in, to be returned by the back e.m.f. Both devices are temporary stores of energy, and in their pure form do not take heat energy from the supply. The fact that in both cases the current is 90° out of phase with the voltage means that average VA product is zero. Pure inductive and capacitive loads are sometimes referred to as “wattless”. 1.4.2 Impedance In any real circuit there is no such thing as a pure inductor or a pure capacitor. Any such device, including any real-world connections to it, includes resistance. Depending on the frequency of operation, an inductor may include significant, in the sense of reactive, capacitance and a capacitor may exhibit some inductance. The combination of the reactance and resistance in a circuit is referred to as impedance, and is frequency dependent. It is represented by symbol Z. The rules for calculating impedance, and for calculating currents in an AC circuit are more complicated than for simple DC circuits. We could use several different mathematical treatments to explain them, but the simplest way to visualize what is happening is the vector method. Let us start with the problem of adding two AC voltages, which are not in phase, but separated by a phase difference of angle φ. What is the resultant voltage? In the vector model we imagine two rotating vectors, each generating a sinusoidal AC voltage. We draw a line OE1 representing the first voltage. The length of the line represents the peak voltage E1, and the instantaneous voltage we designate e1. Figure 1.31(a) shows the idea. (At this stage we are

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Figure 1.31 Adding two vector quantities (AC voltages) by the parallelogram method. (a) shows the principle for two rotating voltage vectors with phase difference φ. (b) shows an r.m.s. vector diagram, where V = 0.707E, and the vectors are stationary because time is not involved with r.m.s. values.

not bothered about the instantaneous phase situation of e1 but at any time e1 = E1 sin θ, where θ is the instantaneous phase angle.) Now we do the same for the other voltage, which we envisage as having a peak value of E2 and an instantaneous value of e2. The instantaneous sum of the two voltages can be expressed as: e = e1 + e 2 If we create a parallelogram from the two original vectors, we see that the diagonal of the parallelogram has an instantaneous value corresponding to e. Therefore we can say that the resultant voltage has a peak value E, corresponding to the length OE of the


We can generalize this approach for r.m.s. voltages and currents. Figure 1.31(b) shows the corresponding r.m.s. vector diagram (where r.m.s. values are 0.707 of peak) and the vectors are considered stationary. In practice an inductor is a device that has both inductive reactance and resistance, and can be considered as an inductance and resistance in series as shown in Figure 1.32(a). Figure 1.32(b) shows the vector relationship between the resistive voltage (which must be in phase with the current) and the pure inductive voltage, which lags 90° behind it. The line voltage V is the diagonal of the rectangle, and leads the current by φ°. The figure may be re-drawn as a “voltage triangle” as Figure 1.32(c). But we know in each case that the relationship between voltage and current is V = IR for a resistance, V = IX for a reactance and V = IZ for the circuit impedance. Pythagoras’ theorem gives us: (IZ)2 = (IR)2 + (IX)2 which can be restated as: Z2 = R2 + X2 or Z = √ (R2 + X2) From the above, the “voltage triangle” idea applies equally to impedance, as Figure 1.32(d). 1.4.3 Power factor

Figure 1.32 (a) shows a resistance and inductance in series. (b) shows the applied voltage V being made up of V1 across the resistance, in phase with the current; and V2 across the inductance, 90° ahead of the current. (c) shows the corresponding voltage triangle, from which we can derive the impedance triangle (d).

diagonal. Its phase relationship to the constituent voltages clearly depends on both their respective magnitudes and the phase relationship between them, and can be measured directly from the parallelogram model.

In a typical AC circuit with reactance present, a vector calculation of the resultant current and voltages done on the basis of Figure 1.32 will usually yield a VA product which is higher than the power in watts being delivered to the resistive load. The power factor in an AC circuit is defined as: watts r .m .s .Volts × r .m .s . Amps

or power factor =

watts VA

But watts = I2R and VA = I2Z.

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Table 1.5 Impedance formulae and power factors.

So power factor = R/Z, and from the impedance triangle we can see that: Power factor = cos φ where φ is the phase angle difference between current and voltage. When the current follows, or lags, the voltage, there is said to be a lagging power factor – as is the case with inductive loads. A leading power factor applies for capacitive loads. Table 1.5 shows the impedance and power factor for the common circuit elements. Any circuit involving inductance, especially electric motors and electromagnetic fluorescent lamp ballasts, will have a poor power factor of, typically, around 0.5. Why “poor”? Electric utilities companies don’t like low power factors because it means that their generation and distribution plant must be rated for the maximum VA product taken, but their customers only pay for the watts actually used. In practice inductive loads, with lagging power factor,

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are the problem – and the problem is solved by applying power factor correction using capacitors to restore the power factor to near unity. This is an important technique for most kinds of discharge lighting. Today the term “power factor” is used to cover all “wattless” activity. As will be discussed later, many modern power control systems introduce harmonics onto the power line, or, looked at another way, they switch the power on and off very rapidly. The power company only gets paid for the “on” time, and cannot divert the resource during the “off” time which is measured in milliseconds. So they don’t like harmonics, any more than they like their fundamental sinusoidal current and voltage being out of phase. These factors are highly significant in lighting. When cos φ is quoted on a piece of electrical equipment (typically a fluorescent lighting ballast) it is defining power factor in the traditional way described above, related to a standard power frequency of 50 or 60Hz. The alternative power


factor symbol is λ which is the ratio of watts to all wattless components however they arise.

mechanically power assisted, and have arrangements for arc quenching.

1.5 Control of electrical power

Where there is to be a repeated switching action at comparatively high frequency (for example in animating an electric sign) the ordinary switch is no longer appropriate. Before the advent of electronic switches the best that could be done was the use of motor diven switches with “brushgear” contacts or of mercury switches where connection was made through a “liquid contact” of mercury. Today all high speed switching is done by power electronic components such as the thyristor or power transistor, described in Chapter 2.

The control of electric power is divided into two: • at its simplest to be able to connect power to a device (whether it be a giant electric motor, an electric kettle or a bedside lamp) and, just as important, the ability to disconnect it. • at a more sophisticated level, the ability to regulate a power flow. In this case we may want to adjust the speed of the motor, adjust the temperature of the water in the kettle, or the brightness of the lamp. In this section we review some of the control elements which are relevant to lighting. 1.5.1 Switches The humble light switch is the most familiar electrical control device. It is no more than a device which makes a physical break in the electrical circuit. However: • The switch contacts must not only be able to carry the running current, they may have to absorb an “inrush” current on switching on (and, indeed, must be able to withstand a short circuit current matching the rating of the fuse or circuit breaker protecting it) and, more importantly, suffer some arcing when switching off. This is particularly the case for DC, and also for inductive AC loads like fluorescent lamps and motors. • The switch contact must have negligible resistance. If it gets corroded by arcing, its resistance goes up, I2R heating sets in, and its resistance goes up still further. Eventually it fails, but it may have caused a fire in the meanwhile. For this reason switch contacts are usually made with precious metal alloys, and are constructed with a “wiping” action to assist maintaining a clean connection. A spring “snap action” is also included to make a fast circuit break. • High power switches used in electrical distribution have their contacts immersed in an insulating oil; or are mounted in an inert or otherwise controlled atmosphere. Any “open air” switches are

1.5.2 Fuses and circuit breakers All electrical circuits require protection against faults which might either damage equipment or injure people. Fault protection is divided into different types: Short circuit protection. This protects against short circuits. For example a stage lighting dimmer needs good short circuit protection if a badly wired temporary cable creates a “short”. Short circuits can result in very high currents, so must be cleared quickly.

Figure 1.33 Power factor correction capacitors, as used in fluorescent and high intensity discharge lamp circuits.

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Overload protection. This is to protect against conditions where the actual load is greater than that for which the circuit or control device is designed. For example where someone puts a 3kW lamp load onto a 2kW dimmer, or where a fault like a “shorted turn” develops in a transformer, which could lead to a fire if not detected. Life safety protection. This is a form of protection against lethal electric shock. In this case the equipment may be functioning correctly, but someone may have inadvertently touched a “live” part of it. The best known protection device is the fuse, and this is based on the simple principle of having a deliberately weak link in a circuit which is destroyed under fault conditions. A simple fuse is a single strand of thin tinned copper wire. When a particular current is exceeded, the I2R heating effect melts the wire, and the circuit is broken. When the wire breaks an arc develops between the two pieces of wire, so the circuit is not actually broken until the arc is extinguished through further destruction of the wire. The simple fuse is, therefore, an imprecise device, especially as it changes its characteristics with age due to oxidation of the wire. Typically a wire fuse blows at about 200% rated carrying capacity. It is therefore acceptable for short circuit protection, but not for overload protection. Fuses on their own do not provide protection against electric shock. The high rupturing capacity or HRC fuse is a more satisfactory device. For power circuits it is housed in a strong ceramic body to withstand the thermal and mechanical stresses arising from both normal running and fault conditions. The body is filled with granulated quartz, and the fuse element is silver. When the fuse blows the arc is immediately quenched because the silver and quartz together yield a high resistance compound. HRC fuses are precision devices. They can run at 100% rated current capacity continuously, but in some versions can be relied upon to blow at only 120% rated current. When HRC fuses are subjected to short circuit, they have a “clearing time” consisting of the time taken to melt (“pre-arcing” time) and the time taken for the arc to extinguish (“arcing time”). Their

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5x20mm glass fuse rating 100mA - 10A Breaking capacity 35A@250V

5x20mm sand filled ceramic body fuse rating 100mA - 10A Breaking capacity 1500A@250V Panel mounted fuse holder for 5x20mm fuses; used in electronic and electrical equipment Consumer HRC fuse ceramic body rating 5A - 45A Breaking capacity 16.5kA@250V General purpose European style fuse ratings 1A - 63A in different sizes Breaking capacity up to 50kA@500V Industrial HRC fuse rating 2A - 200A in different sizes Breaking capacity up to 80kA@660V

Industrial HRC fuse carrier

Figure 1.34 Examples of fuses and their carriers.

characteristic is shown in Figure 1.35. It is obviously important that the “cut-off” current which is let through is lower than that which can be withstood by the equipment being protected. Usually the assymetric cut-off current is appreciably less than the prospective r.m.s. short circuit current. For


example a 60A fuse encountering a prospective short circuit of 50,000A should in practice cut off at 7,000A. All distribution systems need protection devices which have discrimination. Thus when protection is cascaded so, for example, one big fuse is used to protect a distribution panel with several lower rated fuses, it is important that the correct fuse blows when there is a fault. Thus the bigger fuse must have a higher cut-off current than the smaller one, otherwise a failure in a sub-circuit will blow the master fuse instead of the circuit fuse. (The same argument applies to circuit breakers.) When overcurrents involve fusing times of less than 10ms on a 50Hz supply, a different rating is used. This is based on the let-through energy measured as I2t, where I is the current in Amps and the time in seconds. In order to have correct discrimination, a family of fuses should have separated I2t, as shown in Figure 1.36. Small fuses are widely used for the protection of electronic equipment, both in the high voltage supply side and on the low voltage side. They represent the lowest initial-cost form of protection, but, of course, they are destroyed when they blow. In most cases a fuse failure in such equipment is a signal that something is seriously amiss.

Figure 1.35 The clearing time of a fuse.

Figure 1.36 Fuse discrimination measured by I2t in a family of fuses.

In power circuits fuses are used in primary protection, for example at the power intake to a house, and in some circuit protection (for example in the UK where the 13A portable appliance plug has a ceramic bodied fuse in it). Fuses are also widely used for the short circuit protection of electric motors. However, the majority of electrical distribution for lighting is based on circuit breakers. A circuit breaker is an automatic switch which automatically switches itself off when there is an overload. Circuit breakers use two different principles of operation. A thermal circuit breaker uses the heat of the current passing through it to heat a bi-metallic strip. Bi-metallic strips bend when heated due to the different rates of thermal expansion of each metal (the same action as in a thermostat). The strip triggers a spring release mechanism when it has heated up sufficiently. Thermal circuit breakers are useful for long term overload protection, but are imprecise because their tripping characteristic can depend on the load being carried and on ambient temperature. Magnetic circuit breakers rely on electromagnetic action. A coil carrying the fault current exerts a force on a plunger to trigger the release. This action can be very fast, resulting in nuisance tripping on loads which have an inrush current. This can be avoided by damping the plunger action hydraulically.

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Figure 1.37 The miniature Circuit Breaker or MCB. Diagram from MEM Circuit Protection and Control (Delta Electrical Ltd.)

In lighting systems local cicuit protection is usually by miniature circuit breakers or MCBs. These have a combined thermal and magnetic action to deal with both short circuits and long term overload. An electrical distribution panel will usually have a group of MCBs connected to a bus bar, a solid copper or aluminum conductor which in turn derives its supply from a larger circuit breaker. The larger breaker will usually be what is known as a moulded case circuit breaker or MCCB. It is of a much heavier construction than the MCB and is intended for interrupting higher fault currents. Some MCCBs have adjustments permitting fine adjustment of the overload and short circuit trip currents, and trip times. Distribution panels may be single phase or three phase. Most work on 3-phase + neutral supplies, so the neutral conductors for all the loads are connected to a common bus bar, and protection is only single pole in the “live” conductor. There is always a requirement to “balance” 3-phase loads, i.e. to ensure that each phase takes the same load and neutral current is minimized. However in some countries like the UK there is also a safety requirement that limits the proximity of circuits on different phases (in the home, for example).

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Sometimes “isolators” are fitted to electrical equipment. These are physically similar to MCBs or MCCBs, but have no trip element. They do, however, isolate both the live and neutral connection. The Residual Current Device or RCD is the device used for interrupting circuits for reasons of safety. It results in the RCB and the RCBO, or Residual Current Breaker, and Residual Current Breaker with Overload Protection. The principle of the RCD is that in any circuit the current flowing in the two wires (i.e. “live” and “neutral” in an AC system) should be equal. If it is not, then it is assumed that some current has “gone missing” – maybe through someone’s body. The RCD has current sensing coils in both legs of the supply, if it detects an imbalance, it trips. The principle is shown in Figure 1.39. The current which the human body can withstand without severe physical effect or even death is small, so individual RCDs are usually set to a current of around 30mA. There is a problem about RCDs in lighting control. Most systems have some unavoidable current imbalance, often a “leakage to ground”. (RCDs used to be called earth leakage trips by some.) This is quite often to do with the suppression of RFI (radio interference), or it may have to do with signalling circuits, or even the nature of the lamp load. While it is not a problem for single circuits,

Figure 1.38 Examples of Moulded Case Circuit Breakers or MCCBs. Some of these devices allow the user to set the tripping parameters. Photo from MEM.


As an aside it has to be said that designers of power control equipment based in the 115V areas of the world sometimes under-estimate quite how “stiff” a European 230V supply can be. Conversely European designers can be mean in their provision of the high current connections needed for 115V, and sometimes don’t allow for the much wider percentage voltage swings met in practice. 1.5.3 Relays and contactors

Figure 1.39 Principle of the Residual Current Device or RCD.

since it is easy to set the RCD to a safe value while still permitting the required out-of-balance current; it does become a problem in multi-channel systems. These may require RCDs on each channel or small group of channels. If RCDs are to be used in cascade the back-up RCD must clearly have a much higher trip current than that of the subcircuit RCDs. The RCBO combines a thermal overload trip with an RCD, so it can be used for complete circuit protection. Power supplies can differ greatly, and this can have an influence on the effectiveness of protection systems especially where electronic power control devices are being used. Supplies are popularly characterized by their stiffness. A stiff supply is one with a low source impedance – typically one near the step-down transformer, and most likely to be found in large buildings with their own sub-station. With stiff supplies the prospective short circuit currents are much higher, whereas a “soggy” supply, usually involving long supply cables, cannot deliver high fault currents. Stiff supplies are to be preferred because the supply voltage does not change with load and the performance of protection equipment is more predictable. Soggy supplies can result in significant volt drop as more load is added – the “brown out” phenomenon.

In the home it is quite satisfactory to have a switch which is directly in the power line to control a lamp. But wherever there is a requirement to control high power, to provide multiple control points, or have remote control, there is a need for something different. A relay is the simplest remote switching device. Figure 1.41 shows that it consists of a switch contact which is actuated by an electromagnet. A small current through the coil causes the core to magnetize and attract the switch actuator. Removing the current causes demagnetization and the opening of the switch. The important principle is that the control signal can be completely isolated from the power it is controlling and usually, but not necessarily, is of much lower power than the power being controlled. Figure 1.42 shows some examples of relays. Some points worth noting are:

Figure 1.40 The “Memshield” MCB shown here can be converted to a Residual Current Breaker with Overload Protection or RCBO by the addition of auxiliary module.

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Figure 1.41 The principle of the relay.

• relays can have AC or DC coils. Obviously the DC coil relays must have a high resistance because once the coil is magnetized the control circuit is only resistive. In an AC relay the coil is an inductive load, so the aim is the lowest practicable impedance to limit heating in the coil. • relays are not limited to single contacts. They can have multiple contacts in “Normally Open”, “Normally Closed” and “Changeover” configurations. • special relays are available for switching very low currents and high frequency signals (e.g. data signals, audio and video signals). Their construction is quite different to those used for power switching. • most DC relays use a magnetic core with low remanence to ensure quick demagnetization when current is removed. Some relays have high remanance, or are polarized with a permanent magnet, to provide bistable operation – i.e. even if the control power is removed, the relay stays operated. This usually involves the use of two coils, one to operate the relay, the other to release it. • in lighting control relays are usually being controlled by some electronic circuit. This means that they are typically operated with 24V DC coils. When the power is removed from the coil, the back e.m.f. may be sufficient to destroy the control electronics, so relay coils are usually fitted with suppression diodes. The diode (see Section 2.2) can

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carry current only one way, so in normal operation carries no current. The current from the back e.m.f. being in the opposite direction to the main control current is then fed back into the coil where it dissipates as heat. See Figure 1.43. • AC relay coils are not so bad, but again can produce a voltage “spike” which can both create interference, and damage the control circuit. With AC the direction of the current is indeterminate, so a “snubber” RC (resistor-capacitor) network is used. An alternative is a device called a voltage dependent resistor which normally has a high resistance, but goes low resistance when it is subjected to a voltage above a certain threshold. • it is not only the relay coil which can cause voltage spikes. Clearly the switching contact will usually be producing even bigger ones. Assuming that the load being controlled is AC, then an RC Reed relay, used for switching low level control signals. 5V DC coil, 500mA 50V single pole contact.

24V DC coil, single pole 20A 230V AC contact, used for circuit isolation in a professional automatic dimmer.

24V DC coil, 1A 100V dual changeover contacts. Used for control signal isolation in show control equipment.

Plug in general purpose power relay. Twin changeover contacts 10A 230V AC. Choice of AC (24230V) or DC (12-24V) coils.

Figure 1.42 Examples of relays.


network across the contact can be used for suppression. This is good practice in any large system, and wherever sensitive electronic equipment is installed; but it has to be admitted that in many installations no such snubbing network is installed. Big relays, switching high currents are, for some reason, referred to as contactors. These usually, but not necessarily, have AC coils working at line voltage. They are often used as part of lighting control systems for primary power switching. They are also the basis of conventional electric motor control (outside the scope of this book, but often requiring three phase operation with a switch from star to delta connection of the motor windings as the motor runs up to full speed). 1.5.4 Power control Power control for lighting is reviewed in detail in Parts 3 and 4 of this book, so here a simple summary of the possibilities will suffice. Clearly any power control system must vary the current passing through the “load”. At this stage it is easiest to think only in terms of a resistive load like a tungsten filament lamp.

Figure 1.43 The suppression diode reduces the high voltage back e.m.f. from relay coils. An RC network is often used to suppress interference from relay switching contacts.

Figure 1.44 Examples of contractors suitable for lighting control from MEM Circuit Protection and Control.

Figure 1.45 summarizes the possibilities for AC. A variable impedance can be introduced into the circuit to vary the current. At its simplest this could be a variable resistance, but this is wasteful and another possibility is a variable inductance. A means of providing a variable voltage could be provided. For example an autotransformer with a sliding contact tapping. A high speed switching device could be imagined which worked so fast that it could vary the electric power simply by switching it on and off. We would not see the effect of this because the thermal inertia of the lamp filament would keep it glowing even when the power is disconnected. This method, unlikely as it may seem, is the basis of most dimming systems today. The AC waveform is “chopped” to deliver as much or as little power as needed. The load receives continuous power at full on, no power at off and 100 or 120 (depending on

35


Figure 1.45 Control of AC power.

where you live) bursts of power per second of a variable duration for any setting in between. An electronic power converter could be used. There might be an advantage in having a device which could not only regulate the current, but could in fact change the power regime. For example a 230V 50Hz supply on the input might be changed to a variable 0-24V 50Hz supply on the output; or a 115V 60Hz supply on the input converted to a controlled output at 100V 30kHz. Electronic power conversion is now an important part of lighting control.

or even when an old automobile passes near by. The fact that such interference is actually much less noticeable today than it was some years ago is largely due to concerted international efforts to reduce or eliminate its effects. The huge proliferation in electrical and electronic devices has led to a realization that all such equipment must be “compatible”, in that the safe and effective operation of one device must not itself lead to the unsafe or unreliable operation of another. The term electromagnetic compatibility, often abbreviated to EMC is used to describe this hopedfor harmonious relationship. The compatibility must be of various kinds: • any electrical device must not itself emit interference (in practice above a certain minimum level) • notwithstanding this, any electrical device must itself be immune to interference (again, in practice, below a certain maximum level) • the emission and immunity must apply both to conducted and radiated interference. Conducted interference is fairly easy to understand. This is any interference which comes up the power line, or comes into to equipment by virtue of a wired control connection. The description of inductance given in earlier sections shows that it can create back e.m.f.s which can result in high voltages leading to switch arcing and other undesirable effects. An “unsuppressed” inductive load can, on switch-off, create a voltage “spike” with

1.6 Electromagnetic compatibility (EMC) Most of us are familiar with the effects of electrical interference. Maybe your computer has “crashed” after a lightning strike, or when you unplugged another electrical device on the same circuit. Or you have noticed your radio or TV has a buzz or crackle when certain other electrical devices are operating,

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Figure 1.46 The concept of the electromagnetic wave. It consists of simultaneous electric and magnetic fields exhibiting a transverse wave motion. The electric field E is orthogonal (at right angles) to the magnetic field B. Here the graph x axis is time, the y axis is the momentary value of E and the z axis the momentary value of B.


In words, the velocity of the wave motion is related to the relative permeability and the relative permittivity of the medium it is passing through (see Sections 1.2.4 and 1.2.6). Here the power -½ is the same as the reciprocal of the square root -1/√x. In the special case of free space or vacuum, the velocity is denoted by c, and: c = (μ0ε0)-½

Figure 1.47 Transverse wave motion, and the definition

a peak which might be more than double the normal line voltage peak. No wonder your computer objects. Conducted interference is not necessarily due to disturbance on the power line. Interference can be induced into control cables, even when there is no direct electrical connection. Radiated interference arises from the indiscriminate generation of radio waves. It might be wondered what radio waves have to do with switches and dimmers, but, in fact any electrical device is involved in some way or another with electromagnetic radiation – a posh name for radio waves (and other kinds of radiation). When the fundamental discoveries about electricity and magnetism were made in the nineteenth century the experimentally observed results were codified into a number of mathematical equations. Around 1865 James Clerk Maxwell compared Gauss’ theorems as applied to electrostatics (electric fields, like those found in capacitors) and magnetic fields, with Faraday’s and Lenz’s laws of electromagnetic induction and Ampère’s law for magnetomotive force. He realized that the presence of both an electric field and a magnetic field would result in a wave motion. The electromagnetic wave which he predicted was demonstrated experimentally by Heinrich Hertz about 20 years later. Maxwell showed that the velocity v of the wave motion in would be: v = (μμ0εε0)-½

Substituting the actual values of μ0 and ε0 gives value for c of around 299,790 km/second (about 186,000 miles per second) which had already been determined experimentally as the velocity of light. Maxwell realized, therefore, that his as yet undemonstrated electromagnetic waves were probably related to light. Any wave motion has the relationship: v = fλ where v is the velocity, f the frequency (number of oscillations per second) and λ is the wavelength, or the distance between successive “in phase” points on the wave – see Figure 1.47. Radio waves are generally defined as that part of the electromagnetic spectrum where the frequency runs from 1–10 12 Hz, and wavelengths from kilometers to fractions of a millimeter. AM Radio works around 1MHz, FM radio around 90MHz, UHF Television around 600MHz, and digital mobile phones in the GHz region. If lighting control is normally concerned with controlling power at 50 or 60Hz, why does this have relevance to equipment operating at much higher frequencies? Most electrical waveforms, especially those occurring in electronic control equipment, are not sinusoidal. However, Fourier showed that any complex waveform could be synthesized by adding together a number of different sine waves. A harmonic of a fundamental sine wave is one which has twice, three times or any multiple of the fundamental frequency. Figure 1.48 shows what happens when harmonics are added to a fundamental.

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Figure 1.48 The harmonics of a fundamental wave motion. Only the second and third are shown, but the principle applies to any multiple of the fundamental. Complex waveforms can be analyzed into the sum of a large number of sine waveforms. Square and sawtooth waveforms are widely used as current and voltage waveforms in electronics and are rich in harmonics.

Fourier’s bad news was that when there is a steeply rising waveform, as might happen when a sine wave voltage is switched at its peak, there can be a huge number of harmonics generated. Digital computers and microprocessors depend for their operation on square waves where current is being switched on and off as fast as possible. Fourier says that a real square wave would have an infinite number of harmonics. Any electrical or electronic equipment can produce electromagnetic radiation by virtue of the fact that the components and conductors within it produce electric and magnetic fields. If the waveform is complex, the resulting radiation could be high frequency even if the fundamental frequency of operation is low (as in AC power systems). The problem is to design the equipment so that the resulting electromagnetic radiation is minimized. Many components can be designed so that stray fields are minimized. In transformer design the aim is always to keep the magnetic field within the core material. Electric fields can be contained by electrostatic shielding, which in simple terms is no more than a conducting surface, for example a metal can or a metallic conducting paint on a plastic surface. The way in which cables are constructed is

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critical; twisted pair signal cables can be constructed so that fields from the two conductors cancel out and radiation is minimized. Most power control equipment requires RFI suppression components to be included in their design. Radio Frequency Interference suppression usually involves the use of small capacitors and inductors which have a low or high impedance at the high frequencies concerned. These can be used either to “short out” the high frequencies, for example by capacitor coupling to ground (earth) or reduce high frequency currents by inductive impedance, for example by using ferrite rings round the cable. The lumps you see in cables connecting a personal computer to peripheral equipment are an instance of this technique. Conducted interference signals can be measured in volts, so standards for limiting them, or for defining immunity to them can be laid down in terms of the voltage at different frequencies. Radiated signals can be defined both by their electric field strength and by their magnetic field strength. It is more usual to use the electric field strength E, measured in volts per meter (V/m, or its multiples, like μV/m) since the voltage developed in an antenna (aerial) is directly related to E, and can be measured


directly. However, magnetic field strength H, measured in amperes per meter (A/m) is one of the signals measured in EMC tests. The full requirements for EMC cover not only electromagnetic phenomena, but also magnetic fields, inrush currents, electrostatic discharge and other quantities. EMC is of great importance in lighting control equipment, and is referred to in the appropriate sections of this book. In particular a review of the main legal requirements is given in Section 8.5.

1.7 Light 1.7.1 The nature of light In some ways light is more mysterious than electricity. While a simple introduction to electricity allows us to use a single model or mind picture, light needs different models depending on the aspect of interest. Isaac Newton proposed that light existed as “corpuscles”. These would be radiated in straight lines by luminous bodies, and would act on the eye’s retina. Straight line optics work well for some aspects of optical design, especially when the components are large, like mirrors and lenses. So the idea was not without merit. Huygens, backed by experimental results from Young and others, demonstrated that light must have a wave nature. Observed effects like diffraction could not be explained any other way. The wavelengths were found to be very short. At the time there was a problem in describing what the waves travelled in – we can understand waves as ripples on water, or pressure waves in air, but how did the waves travel through a vacuum? It was proposed that an all pervading substance called the “ether” was the carrying medium. The wave theory received a big boost, and the problem of the ether disappeared, when James Clerk Maxwell proposed that light was a form of electromagnetic radiation as described in Section 1.6. Electromagnetic radiation is seen as having a very wide spectrum, and visible light occupies only a small part of it as shown in Figure 1.49.

Figure 1.49 The electromagnetic spectrum.

However Einstein, Planck and others showed that the photo-electric effect, where light falling on certain substances could cause an electric current to flow, could only be explained by a 20th century variation of the corpuscular theory. This new version was proposed as the quantum theory of radiation, because it requires that energy only travels in defined packets or quanta. The quantum of light was, and is, called a photon. Modern physics brings together both the quantum and wave theories by stating that every moving

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particle of mass has associated with it a wave whose length is given by: wavelength = h/mu where h is Planck’s constant (6.626 × 10-34 Joule seconds) m is the mass of the particle, and u is the velocity of the particle. The equation comes with the not altogether helpful rider that you cannot actually determine all the properties of a particle simultaneously. Heisenberg’s uncertainty principle tells us that if you know where something is, then you cannot know its precise mass – or if you know its mass, you don’t know where it is. If you try and apply the equation above to everyday objects, you obtain meaningless results; but when applied on the atomic scale things begin to make sense. A dual “mind picture” or model of light is needed: • for most practical applications, the wave nature of light, expressed as part of the electromagnetic spectrum, is the simplest model. • for some aspects of the behavior of light, the quantum or photon model is needed, on the understanding that the photon itself has a defined wavelength. The photon model becomes more comprehensible when the model of the atom described in Section 1.1 is recalled. What happens if an electron jumps from one shell, or orbit, to another? The idea that such a move can only happen between one defined energy level and another fits well with the quantum concept. The idea emerges that: • when an electron drops from a high energy level to a lower one, a photon is emitted. • similarly a quantum of energy can be absorbed, and when this happens an electron moves from a lower orbit to a higher one. Sometimes referred to as moving from a normal orbit to an excited orbit. The energy of a photon is defined as: E2 - E1 = hν where: E2 is the energy associated with the excited orbit,

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E1 is the energy associated with the normal orbit, h is Planck’s constant, ν (Greek letter nu, NOT our letter V) is the frequency of the radiation emitted as the electron moves from Level 2 to Level 1. E2 - E1 is conveniently measured in electron-volts. An electron-volt (eV) is the work required to move an electron through a potential difference of one volt. It is equal to 1.603 × 10-19 Joules. In lighting we are concerned with the wavelength of the emitted light, since this will determine whether or not the radiation is visible. Since we know that: c = νλ where c is the velocity of light (approximately 3 × 108 m/sec) λ is the wavelength, and ν the frequency; we can, by putting in the value of Planck’s constant, re-write the photon energy formula in the form: Wavelength λ in nm = 1239.76/energy in eV An example relevant to lighting is the fact that electrons surrounding mercury atoms (when in vapor form) can be made to emit photons with energy 4.89 eV. The formula shows us that this results in radiation of wavelength 254nm, well in the ultra-violet part of the spectrum. 1.7.2 Wavelength conversion Concepts which are needed to understand how the lightsources described in Part 2 work include the ideas of wavelength conversion and blackbody radiation. The energy needed to raise the energy level of electrons can be imparted in several ways. One significant way is from heat. Heat within an object is rapid motion of its atoms; the energy from this motion can change the energy levels of the outer electrons. Another way is by the application of an electric field; and yet another is by bombardment with electromagnetic radiation. An example of this last case in action is the fluorescent lamp (described in more detail in Chapter 3.) In this lamp the invisible 254 nm radiation from


example is sodium. Low pressure sodium vapor emits a characteristic monochromatic yellow light (actually two “lines” very close together at 589.0 and 589.6 nm) and this can be seen if salt is dropped on a flame, or in the low pressure sodium street lamp. Individual atoms have characteristic spectra. In order to get useful light for illumination purposes, it is necessary to have a source giving multiple lines, or, ideally, a continous spectrum. 1.7.3 Blackbody radiation Figure 1.50 Concept of transitions between different energy states. On the left, transitions between normal and different excited states. On the right, transitions between different excited states only.

mercury is directed at a phosphor, which is excited by the radiation. The excited electrons in the phosphor then drop back to a lower level; but the drop is not as great as the 4.89 eV of the original photon. It is more like 2.3 eV, resulting in visible radiation around 545nm. (Note that this is just an example – phosphors emit a band of radiation over a range of wavelengths.) This is an example of “wavelength conversion” whereby an efficient method of producing an invisible electromagnetic radiation (in this case UV) is used to stimulate the production of visible radiation, via, in this case, a phosphor. The outer electrons of atoms can have several excited states. Figure 1.50 shows in simplifed form what can happen. Electrons can be raised from their normal state to one of several excited states. But in addition there can be transitions between excited states. The result of this is that any atom has a characteristic absorption spectrum and emission spectrum. Each transition, whether between different excited levels, or between an excited level and normal level, is associated with a given frequency and corresponding wavelength (approximately 254 nm in the case of the mercury example already given). In lighting our interest is in those transitions which result in visible radiation. Reference is often made to spectral lines. This is because, when light from a source is analyzed using a spectroscope, it is seen as a series of colored lines. A well known

A metal heated to a high temperature glows; it emits visible radiation. A blackbody radiator is one whose intensity and spectral properties are dependent solely on its temperature. In fact the blackbody radiator is also a blackbody absorber – in that it absorbs all incident radiation of all wavelengths. It does not reflect any incident radiation. The blackbody is an ideal concept; it radiates more total power and more power at any given wavelength than any other lightsource operating at the same temperature. Figure 1.51 shows blackbody radiation curves for a variety of temperatures. From the curves it can be seen that a blackbody needs to be at a temperature of above 2,000K to give out useful quantities of visible radiation. Its important attribute is that the output spectrum is continuous. Note that for any given temperature there is a radiation peak at a particular wavelength, which gets shorter as the temperature rises. Also, as the temperature rises, the total amount of radiation increases. Real sources of radiation can never achieve the performance of a blackbody; but the blackbody is used as a performance reference. The spectral emissivity of a radiator is a wavelength dependent quantity which is the ratio of the output of the radiator at a particular wavelength, to the output of a blackbody at the same wavelength. Figure 1.52 shows the comparative performance of the perfect blackbody and tungsten (used in filament lamps) at 3,000K. The color temperature of a radiator (for example a tungsten filament) is the temperature at which a blackbody would have to be for its output to match

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control, and for which it is useful to have a basic understanding. While the blackbody absorbs all radiation falling on it, all real surfaces reflect some or most incident radiation. Some definitions are: Specular reflection is where light is reflected in the manner of a mirror. Specular reflectance is the proportion of light reflected expressed as a percentage of the incident light. Specular reflection obeys the Laws of Reflection illustrated in Figure 1.53. Specular reflectance can range from around 7% for a normal sheet of glass (which transmits most light) to 95% for a surface aluminized mirror. Polished stainless steel and chromium plate have reflectances of around 60%. Diffuse reflection arises when the surface is rough. In this case the light is scattered in many directions. A special case is the perfect diffuse reflector which reflects any incident light equally in all directions. A matt white painted surface is an example of a near perfect diffuse reflector. Most

Figure 1.51 Blackbody radiation curves at different temperatures.

as closely as possible that of the chosen radiator. The concept works well for any continuous spectrum source, for example an incandescent filament or sunlight, but breaks down when applied to sources with discontinuous spectra. Examples of color temperatures are: Typical tungsten filament lamp: Noon sunlight: Cloudy sky; average daylight:

2,850K 4,900K 6,700K

1.7.4 The behavior of light The subject of optics is beyond the scope of this book, but nonetheless there are aspects of the behavior of light which are of importance in lighting

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Figure 1.52 Comparison of the radiation from a blackbody at 3,000K, and the corresponding performance of tungsten.


surfaces exhibit a combination of diffuse and specular reflection. Refraction describes what happens when light passes from one transparent medium to another; for example when going from air into or out of glass, or from either into or out of water. Figure 1.54 shows light passing through a transparent glass prism. Light which does not arrive normal (at right angles) to the surface is bent or refracted. The following apply: • The angle of incidence (i in the figure) is the angle which the incoming ray of light makes with normal; similarly the angle of refraction (r) is the refracted ray in the new medium. Assuming the ray is going from a less dense medium to a denser one, r is less than i. • Refractive index μ is defined by

μ = Sin i / Sin r • The refractive index applies to different pairs of media, but the refractive index vacuum/glass is almost exactly the same as that of air/glass; so in practice it is common to refer to the refractive index of a material referred to air. • The wave theory of light shows that in fact the refractive index is the ratio of the velocity of light in Figure 1.54 Light being refracted through a prism (a). The dispersion of light into its different colors by a prism (b). The critical angle at the glass/air surface within a prism (c).

Figure 1.53 The Laws of Specular Reflection (Snell’s Laws). The incident ray, reflected ray and the line at right angles (normal) to the surface are all in the same plane; and the angle of incidence equals the angle of reflection.

a material to the velocity of light in a vacuum (or air, where the slowing down of light is negligible). • Within most materials there is a slight difference in the velocity of light for different wavelengths. This is why a prism can be used to split light into its different wavelengths or colors, because the bending effect is different for each color. • Transparent materials have a critical angle. From Figure 1.54 it can be seen that the emerging ray diverges from normal (i.e. gets closer to the prism surface). The internal ray can finally be at an angle (the critical angle) where theoretically the emerging ray would correspond to the plane of the glass surface. Beyond this angle the light is internally

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reflected, with the exit glass/air surface acting as a mirror. In binoculars and camera viewfinders prisms are used to reflect light. Glass and certain plastic materials such as acrylic and polycarbonate sheets are made to be transparent; but may also be made translucent. Here either surface treatment or the nature of the material itself causes multiple scattering of the light. In luminaire manufacture such material is used as a diffuser to increase the apparent size of a light source (and reduce its point brightness). Typical luminaires make use of reflection, refraction and diffusion in order to make the best use of the light from a particular source. Figure 1.55 shows examples. The wave theory of light predicts the possibility of interference. If two wave motions of identical wavelength meet, then their individual amplitudes can be added together. This means that if the wave motions are in phase they reinforce each other, but if they are out of phase, they cancel each other out.

ceiling tile luminaire with prismatic panel (refraction)

ceiling luminaire using both reflection (center) and diffusion (either side round tubes)

luminaire using aluminum reflectors

An anti-reflection coating is a practical manifestation of interference. Such coatings are used to minimize specular reflections in showcase glass, or to increase light transmission through lenses. The principle is shown in Figure 1.56. This shows the specular reflection off the first surface being cancelled out by a second wave exactly half a wavelength out of phase. The second wave has been refracted into the thin film, reflected off its other internal surface, and refracted out again – the film thickness being such that by the time it emerges, it is out of phase with the initial reflection. A thin film of different thickness could augment the reflection instead of destroying it. Multi-layer coatings are used to improve transmittance or reflectance. For example a 4-layer coating on a surface coated mirror (designed to increase reflectance) can raise reflectance above 99%; conversely triple coated lenses for cameras have a reflectance of less than 1%. The same technique can be used to create interference filters. By using multiple thin films (each layer of a different refractive index, and different thickness) it is possible to create a filter. A high pass optical filter passes high frequency (short wavelength) light, and reflects long wavelength light. A low pass optical filter does the opposite. In the lighting field such filters are important. They make available hot and cold mirrors, as found in some reflector spotlights and projection devices. A cold mirror reflects the full visible spectrum, but transmits infra-red radiation, which in the example spotlight application, would go out the back of the reflector instead of on to the illuminated object. Dichroic filters are efficient filters with a very sharp transition between transmission and reflection • for example having a transmittance band of 400– 500nm blue, and reflectance band 500–700nm yellow.

1.8 The eye, how we see light Figure 1.55 A luminaire using a fluorescent lamp may well make use of a combination of reflection, refraction and diffusion to make the most effective use of the light from the tube.

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The human eye is an incredible device – but the eye on its own is only a light collecting mechanism, it requires the brain to make sense of what is received.


Figure 1.56 Principle of the anti-reflection coating.

Figure 1.57 is a much simplified diagram of the eye, and identifies: • the cornea, the transparent outer layer which, with the tear layer, protects the eye, but also provides the first refractive element of the eye. • the iris, which regulates the amount of light entering the eye, by changing diameter. It does this in the range of about 2-8mm, representing a range of 16:1. This alone is not sufficient to cover the huge range of illumination encountered. Our eyes also have to “adapt” to the lighting condition, but this takes time, whereas the iris acts instantaneously. • the lens, which focuses light onto the retina. By use of the ciliary muscle the lens can change focal length, depending on whether it is looking at a near or distant object. • the retina, which receives the imaged light. It contains the receptors for light and produces the electrical signals for the brain to process. • the fovea, more or less at the center of the retina. This is the part of the eye by which we see most detail, but represents a very small part of our overall field of view. • the optic nerve. This communicates the information received by the retina to the brain. Because it occupies a small part of the retina, we have a “blind spot” at this point.

The retina is found to have two different kinds of light receptors, identified as rods and cones. Rods provide us with scotopic or night vision, and are very sensitive to light. There are no rods in the fovea. All rods contain the same photopigment, so they see in “monochrome”. Photopic or day vision is done by the cones. Of six million of these, more than one million are located at the center of the fovea, representing only a 1° field of view. All these have an individual connection to the brain. The remainder are spread across the retina, and, like the rods, report to the brain in groups. The cones are in three classes, each with a different photopigment. Figure 1.58(a) shows the spectral response of the eye as a whole, and here it is clear that the scotopic and photopic responses peak at different wavelengths. Figure 1.58(b) shows that for the three different kinds of cone there is a significantly different response. It seems that, between them, the cones deliver to the brain achromatic (non-color) information and separate hue information (not unlike a color TV).

1.9 Measurement of light In the same way that we needed objective methods of measuring electrical quantities, there is a need

Figure 1.57 The eye.

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(a)

(b)

Figure 1.58 Spectral response of the eye.

for the objective measurement of light. In fact the measurements can still be based on the MKS system, but it is necessary to introduce one more fundamental unit, and the SI system uses the Candela as that unit. A major problem about measuring light is the practical necessity of relating the measurement to

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human vision. As shown in Figure 1.58 the human eye has a different response to different wavelengths of light. By experiment it was found that the wavelength of maximum luminous efficiency, that is the wavelength which produces the maximum response in the eye for the minimum power input, is approximately 555nm. At all other wavelengths our eyes have a lower photopic spectral luminous efficiency V(λ), where λ is the wavelength. If V(λ) is unity (1) at 555nm, then it is found to be about 0.5 at 510 and 610nm, and as little as 0.00006 at 385 and 760nm. The Candela, the SI unit of luminous intensity is today linked to the human eye’s peak response, and is tied back to the power radiated in Watts; its official definition is given in Table 1.6. (Note that Table 1.6 also gives the definitions of total power radiated, independent of the eye’s response.) Our appreciation of the brightness of a light source is defined by its luminance, expressed in Cd/ m2. The definition is given in Table 1.6 – the bit about orthogonal projection is telling us that you get a different brightness if you view the source at an angle. Luminance figures are very important in lighting design and in the specification of electronic displays. Table 1.7 gives some examples of luminance. The lumen is the unit of luminous flux. This is a unit which many lay people have heard of (especially from salesmen who probably have no idea what it means) but which in fact is one of the more difficult units to understand fully. At 555nm, the lumen is fairly easy – since it is tied directly to the candela (see the definition in Table 1.6) – a one candela point source emits 4π lumens. At 555nm it is easy to calculate the number of lumens which can be derived from a given power (or Watt) input. But what happens at other wavelengths? The efficacy of a light source is defined in two ways. The luminous efficacy of radiant flux is measured in Lumens/Watt, and is the ratio of the luminous flux in lumens to the total radiant flux in Watts. More useful in the lighting business is the luminous efficacy of a source of light, also measured in Lumens/Watt, and the ratio of the luminous flux in lumens to the total lamp power input.


Quantity and symbol Luminous Intensity I

Unit

Luminance (or "Photometric brightness") L Luminous Flux F

Candela per square meter (also known as the "nit") Lumen

Cd/m2

Illuminance E (also Illumination) Radiant Intensity Ie Radiance

Lux

lx

Watts per steradian Watts per steradian per square meter Watts

W/sr

Radiant Flux )e Irradiance E

Candela

Watts per square meter

Unit Symbol Cd

lm

W/(sr.m2) W W/m2

Defined as The luminous intensity of a 555.016nm (or 540 u 1012 Hz) source which has a radiant intensity in a given direction of 1/683 Watts per steradian, when measured in that direction. (Formerly defined as 1/60 of the intensity of a square centimeter of a blackbody at the temperature of solidification of platinum.) The intensity of a source in a given direction, divided by its orthogonally projected area in that direction. An isotropic (one which emits radiation equally in all directions) point source of intensity one Candela produces a total luminous flux of 4S lumens. But see also main text which describes how, when related to input power, lumens are wavelength dependent. The concentration of luminous flux falling on a surface. One Lux is is one Lumen per square meter. Radiant power emitted by a point source in a given direction. The radiant intensity of a source in a given direction, divided by its orthogonally projected area in that direction. Radiant power of a source at all wavelengths Radiant power incident on a surface. (Compare Illuminance)

Table 1.6 The measurement of light. Radiant intensity, radiant flux, radiance and irradiance are physical units based on power and apply to all wavelengths. Luminous intensity, luminous flux, luminance and illuminance are luminous units, which are wavelength dependent and based on the human eye’s response. They are tied back to physical units through the Candela.

Everyday examples of the latter are around 14 lm/W for an ordinary tungsten lamp and 85 lm/W for a fluorescent lamp. To derive the luminous efficacy of a source which is emitting many wavelengths, it is necessary to add the contribution at each wavelength. For photopic vision the formula is:

770

Lumens = 683 Σ PλVλΔλ 380

where the Σ sign is telling us to “sum for all wavelengths between 380 and 770nm”; and:

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Source

Ft. lambert

Nit

482,000,000 Sun at meridian 1,750,000 Sun at horizon 600 – 2,300 Sky, typical 2,600 Fluorescent lamp 150 TV screen (CRT) 14-18 Motion picture screen

1,700,000,000 6,000,000 2,000 – 7,800 8,800 500 48-62

Table 1.7 Examples of luminance.

Pλ is the spectral power in watts of the source at wavelength λ. Vλ is the photopic luminous efficiency at wavelength λ. Δλ is the interval over which the spectral power is measured. To be really thorough these “photopic” lumens should be added to a separate calculation for “scotopic” lumens. In this case the initial multiplier is 1,700, the wavelength range is 300 to 770nm, and there is a quite different set of values for scotopic luminous efficiency. In practice scotopic vision is only of importance at very low light levels, so is not usually considered when discussing electric lighting. There is a maximum theoretical efficacy for an “ideal” white light source. This is a source which radiates light at constant power over the complete visible part of the spectrum, but does not radiate power outside it. The maximum efficacy is approximately 220 lm/W. An important measurement in lighting design is illuminance – this is a measure of how much light is falling on a surface per unit area. Today the accepted measure is the Lux or Lumen per square meter. However, some industries still use measurements based on imperial units where the illuminance is measured in Footcandles (or lumens per square foot) and luminance is measured in FootLamberts (where a surface emitting one lumen per square foot is said to have a luminance of one FtL). Table 1.8 shows the relationship between the imperial and metric units. Figure 1.59 may help visualize the units. It shows a sphere of radius r, where we know that the surface area of the sphere is

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4πr2. At the center of the sphere is a point source of one candela, which we know is emitting 4π lumens. Let us assume the inside of the sphere is totally absorbing, so the inside surface is ONLY illuminated by the point source, and not by any cross-reflection. If the radius of the sphere is one meter, then the flux falling on one square meter of its surface (= r2) must be one lumen, and the illuminance on that square meter one lumen/square meter or one lux. Another way of looking at it is that the flux density is 1 lm per steradian, which applies whatever the radius of the sphere. Thus if the sphere radius is only one foot, the illuminance on the inside surface will be one lumen per square foot or one footcandle. It would be very tedious if, every time we wanted to measure luminous flux or illuminance, we had to do it at many different wavelengths. Fortunately this is not necessary. Most light measurements are now made using solid state detectors based on silicon. The principle is that the absorption of a photon creates a free electron to contribute to an electric current. By having a detector of a known area, it is possible to measure the incident illuminance. Detectors respond to radiation in the visible spectrum (and, if required, either side of it). The detector is usually “corrected” so that its response closely matches the Photopic Luminous Efficiency characteristic of the eye. This means that the complete instrument or Photometer can directly read out in Lux, see example in Figure 1.60.

Quantity Luminance Illuminance To convert Nit to Ft Lambert Ft Lambert to Nit Lux to Ft Candle Ft Candle to Lux

Metric

Imperial

Nit or Cd/m2 Lux or lm/m2

Ft Lambert Ft Candle or lm/ft2 Multiply by 0.2919 3.426 0.0929 10.76

Table 1.8 Relation between metric and imperial units.


Figure 1.59 Derivation of units of illuminance.

A Luminance Photometer is similar, but it includes an optical system, like a telescope, which focuses a source onto the detector. The Commission Internationale de l’Éclairage or CIE is the body which lays down how lighting measurements are made. Their publications not only define how instruments are calibrated with reference to the SI standards, they also include the agreed luminous efficiency curves. One problem characteristic of illumination is glare. This does not lend itself to easy measurement because its effects are subjective. In practice it is a significant factor in lighting design, so various efforts have been made to codify its effect. We see things not only by their brightness, but also by their contrast relative to their surroundings. We can easily see black lettering on a white background, but making out white lettering on a white background is difficult if not impossible. Generally the higher the illumination, the better we see things; but if luminance levels vary greatly, for example when driving in the dark and having to contend with oncoming headlights, a high luminance relative to the surroundings causes our perception to break down. Discomfort glare is when the lighting situation is uncomfortable, disability glare is when the lighting situation means that it has become impossible to do

the intended task. Usually both apply at once. For example low winter sun streaming into an office can be both uncomfortable and make carrying out a normal office task impossible. Glare can arise in all lighting schemes. A person sitting in a low ceilinged office may have a well illuminated desk from an overhead luminaire, but also be distracted by the glare from an overhead luminaire some distance in front of him. Specular reflections in VDU screens arising from sources of relatively high brightness can make the screens impossible to use. Various methods are used to quantify the effects of glare, to assist lighting designers eliminate its effects. They are all related to defining the highest levels of luminance for a given source size, which can be tolerated within a given level of illumination for a particular activity. The USA use the Visual Comfort Probability (VCP) system, the UK the Glare Index system and the CIE has introduced the Unified Glare Rating (UGR). Glare is relevant in lighting control, because its effects can sometimes negate the intended results of a control scheme.

Figure 1.60 Example of a photometer used for making illuminance measurements.

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1.10 Color 1.10.1 Introduction Sections 1.8 and 1.9 above have warned us that the eye does not work in an obvious way. Different wavelengths of light are seen with a different sensitivity; and the eye sees the visible spectrum with three different receptors. Worse still, our perception of color varies according to the field of view (i.e. the angle subtended at our eye by the object.) This means that any measurement of color must take into account how the eye works, and cannot be based on simple power spectrum measurements alone. In practice the situation is made much worse because the apparent color of an object will depend not only on its own properties, but on the spectral distribution of the source. As an extreme example an object which is seen as blue in daylight looks black or gray when illuminated by a low pressure sodium lamp (with its monochromatic yellow light). This section describes some color phenomena, and gives definitions of color attributes. Section 1.11 describes how color is measured. 1.10.2 Primary and secondary colors The way the eye/brain combination sees color has the interesting result that the simultaneous presentation of two colors makes the brain think it is seeing a third. For example if separate red and green spotlights are both directed at the same surface, the eye sees the surface as yellow. Figure 1.61 shows how three primary colors, red, green and blue can be mixed: red + green = yellow green + blue = cyan blue + red = magenta red + green + blue = white The two-color combinations so derived are called the secondary colors, they each represent “white minus a primary color”:

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yellow = white - blue cyan = white - red magenta = white - green This phenomenon is the basis of all aspects of color manipulation. Additive color mixing is where primary colors are mixed to achieve a desired color. There are many everyday examples related to lighting and display: • color television depends wholly on additive color mixing. The TV screen consists of a large number of small dots which, at a normal viewing distance, the eye integrates into a single image. The dots are in red, green and blue triads. Each small zone on the screen has its own color value determined by the mix of red, green and blue. • a stage backcloth can be illuminated by cyclorama floodlights in sets of three, each set red, green and blue. By setting different proportions of each, any color (within the gamut of the primaries) can be achieved. • lamps can be “tuned” to have an appropriate output. Thus, for example, a fluorescent lamp might be deemed to be too “cold”. This can be corrected by adding a phosphor which emits in the red part of the spectrum. The red will not be seen separately, but only as a “warming” of the white. Color mixing can also be subtractive. In additive mixing, light of different colors is added (to “black”) with the result that the perceived light is brighter than its constituents. Subtractive color mixing works the other way, it takes light away from white, so the result is darker than its constituents. The mixing of color paints, and the color printing process are examples of subtractive color mixing. The use of color filters is another subtractive process. A theatre spotlight with a tungsten filament lamp gives “white” light; a color filter removes all colors other than the wanted color. A problem can arise when filters are applied to light sources with uneven spectral output. Putting a red filter round a fluorescent tube may only go to show how little red there is in the light it is emitting.


Standards, the Optical Society of America, British Standards and DIN have developed alternative systems to Munsell, but use similar principles. This type of system is practical for many real world tasks; but does not relate color directly to physically measurable quantities. However, it is possible to relate Munsell (and similar) color descriptions to their equivalent CIE Chromaticity value, described in Section 1.11. Figure 1.61 Primary and secondary colors.

1.10.4 Color temperature

1.10.3 Aspects of color, the Munsell system

Color temperature was defined in Section 1.7.3. Strictly the term applies only to light sources with continuous spectra, and which lie on the blackbody radiation curve or planckian locus described in Section 1.11 below. However it is convenient to use it for other sources, and in this case it is referred to as the correlated color temperature or CCT. The CCT is the temperature of a blackbody whose chromaticity (defined in Section 1.11 below) most closely matches that of the source concerned. Computer programs and graphs are available which give “isotemperature” lines on the chromaticity chart, so if the chromaticity of the source is known, it is possible to derive its CCT. However, the further away from the blackbody locus the source lies, the less meaningful the derived CCT. Lamp manufacturers refer to lamps being “warm” or “cold” according to their CCT. For example:

An early attempt to quantify color was made in 1915 by the American teacher, Albert Munsell. It is still in use because in practice it provides an easy method for architects and designers to compare colors. In the Munsell system, color is defined by three attributes: Hue: this is simply defining the perceived color as being near to a principal color like red, green, blue, yellow or to a mixture of any two. The Munsell system defines five principal and five intermediate hues, but allows 100 hue steps in all. Value (also brightness): This expresses whether the surface is reflecting (emitting) more or less light. It can also be considered as the “whiteness” of the color, and is measured on a scale of 0 (black) to 10 (white). Chroma (or Saturation): is a measure of the “colorfulness” or intensity of the color. Munsell allows 20 (or more) steps from neutral through to highly saturated. The Munsell system is represented both by a three-dimensional “color atlas”, and by a collection of carefully standardized color chips. The ideas behind the Munsell system are illustrated by Figure 1.62, but note that this is for illustrative purposes only and cannot show the actual colors. Color matching using Munsell color chips can only be done with a standardized source (preferably daylight) but, of course, such chips can also be used to judge the performance of different light sources. Other institutions, including The Inter-Society Color Council working with the National Bureau of

CCT less than 3,300K is “warm” (example, tungsten lamp) CCT 3,300–5,300K is “intermediate” (example, cool white fluorescent lamp) CCT greater than 5,300K is “cold” (example “daylight” fluorescent and some metal halide lamps) The effect of color temperature is quite dramatic. If you are in a room lit by tungsten lighting, a small fluorescent fixture fitted with a “daylight” lamp looks very blue; conversely, if you are in a daylit space, an ordinary lamp looks somewhat orange. The effect is particularly notable in photography and television. A film intended for outdoor use is usually “balanced”

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52

MIRED filter corresponds to a change of 1,800K. But it is easy to work out that a +100 MIRED filter applied to a 10,000K source, makes a 5,000K change in color temperature.

1/

2/

4/

Chroma 6/ 8/

10/ 12/

14/

9/ 8/ 7/ 6/ Value

for 5,500K; that is to say that it will give faithful reproduction of colors if the scene is, in fact, illuminated by daylight, or its artificial equivalent with the same color temperature. If the film is used on a studio set illuminated with tungsten lighting at 2,800–3,200K, everything comes out very orange. While films balanced for studio tungsten lighting (at around 3,200K) are available, it is often the case that film and TV lighting cameramen must make use of filters to modify the color temperature of studio luminaires. Figure 1.51 showed the characteristics of blackbody radiation. Mathematically the graphs illustrate: • the Stefan–Boltzmann Law which states that the amount of radiation emitted by a blackbody is proportional to the fourth power of the temperature (so the amount of radiation increases enormously with temperature). • the Wien Displacement Law which states that the wavelength of peak emission is inversely proportional to the temperature. The latter leads to the observation that perceived equal changes in color are more accurately expressed by a measure of the reciprocal of the color temperature (i.e. 1/T). In order to have manageable units, the reciprocal megakelvin is used. This is simply 1,000,000 divided by the color temperature. The same unit is also known as the micro reciprocal degree or MIRED. Thus, for example, a source of 4,000K has a MIRED value of 250. If you just specified a color temperature correction filter as, for example, “blue” to correct a low temperature source, you would not be able to predict easily what its effect would be, since it would be quite different on each source. But if you specify a filter by its MIRED value, it is easy to calculate exactly what its effect will be – regardless of the color temperature of the source to be modified. As an example; suppose the available source is 5,000K (=200 MIRED) and it is required to apply a filter to convert to 3,200K (=312 MIRED), the filter has to be +112 MIRED. A filter to go from 3,200K to 5,000K would correspondingly, have to be a -112 MIRED filter. For this particular color temperature change, a 112

5/ 4/ 3/ 2/ 1/

Figure 1.62 The Munsell system of color classification. The diagram shows the main hue classification and the way in which value and chroma apply to a single hue.


CIE Color Rendering Index Ra >90 8090 6080 4060

2040

Typical Application

intervals, in practice 5nm. The most commonly used sources are:

Where accurate color matching is required; e.g. print inspection Where good color judgement is required, or where appearance is important; e.g. retail display Where moderate color rendering is required; e.g. commercial premises Where color rendering is not important but where a marked distortion of color is unacceptable; e.g. warehouses Where color rendering is of no importance, and a marked distortion of color is acceptable; e.g. some road lighting

Standard Illuminant A: Representing tungsten at a color temperature of 2,856K. Standard Illuminant B: Representing noon sunlight at a color temperature of around 4,870K (now superseded by Standard Illuminant D50) Standard Illuminant C: Representing average daylight (cloudy sky) with an average color temperature of 6,700K (now superseded by Standard Illuminant D65)

Table 1.9 The CIE Color Rendering Index.

1.10.5 Color rendering There is a need for different kinds of lamp to be classified according to their color rendering abilities. Clearly the monochromatic low pressure sodium yellow lamp has non-existent color rendering ability; and a tungsten filament lamp has quite good color rendering. The most common method of classifying color rendering is that of the CIE, the Color Rendering Index or CRI, designated Ra. The method uses sample surfaces of eight or fourteen Munsell test colors (described in 1.10.3 above.) Each surface is illuminated in turn by a standard reference source, and then by the source under test. At each stage the spectral reflectance (i.e. the reflectance at different wavelengths) of the test color is measured. The reference source is deemed to have a CRI of 100, and the test source is rated against it for each part of the spectrum. The problem here is, clearly, “what is the reference source?” The CIE have defined several in terms of their spectral power distribution. To be of any use in practice this has to be done to quite fine

Spectral power distribution tables are also available for other “daylight” illuminants, such as D55 and D75. Table 1.9 gives examples of CRI. Clearly the fact that different reference sources can be used to determine CRI means that a user must know which reference has been used.

1.11 Measurement of color 1.11.1 CIE Chromaticity Diagram The achievement of a desired color by mixing primary or secondary colors leads to the idea that any color can be described numerically as three tristimulus values. However, simply relating color to, for example, three spectral lines corresponding to optimized red, green and blue is not sufficient. The reasons can be summarized as: • such an arrangement cannot easily describe spectral colors (other than the three primary colors chosen). This is intuitive, since additive color mixing is always “lightening” the illuminated object. A simple analysis shows that any system based on this idea would need “negative” color values; which might be an acceptable mathematical trick (and can, in fact, be used in electronic analogs of color) but is not satisfactory for real world work. • the gamut of colors which can be described by three “real” primaries is limited. This can be seen by drawing an imaginary color triangle as in Figure 1.63. Points R G B represent the primaries, and any

53


point within the triangle represents colors which can be produced by mixing them. Point W in the middle represents the white light (equal energy of each of Red, Green and Blue). Colors which might exist outside the triangle (including pure spectral colors) cannot either be created by the chosen primaries, nor be described purely with reference to them. The CIE got round this problem with their Chromaticity Diagram, originally produced in 1931. The idea here is to postulate three imaginary primary colors denoted X, Y and Z. Any color is then defined by the proportions it contains of each, the values for any color being denoted by the chromaticity coordinates, designated x, y and z. Because it is always the case that: x+y+z=1 any color is, in fact, defined by its x and y coordinates on the diagram. Figure 1.64 shows the basis of the diagram. It starts with the spectrum locus which is the horseshoe shaped curve. On this curve appear the pure spectral colors, and the principal wavelengths are identified in the figure. The line across the bottom of the horseshoe represents the magentas and purples (which are not spectral colors, but mixtures of blue and red). The outer lines of the diagram represent saturated colors.

Figure 1.63 Simple color triangle

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Figure 1.64 The CIE chromaticity diagram showing the spectrum locus, and how an individual color is defined.

To help understand the diagram, it is shown in various forms. Figure 1.65 shows it in color. Here it can be seen that hue is dependent on the position relative to the spectrum locus, and saturation is dependent on how near the edge of the diagram the color is. Superimposed on it are two color triangles. Points R, G and B correspond approximately to the phosphors used in cathode ray tubes (CRTs) to produce a color TV picture. The triangle RGB defines the limit of the colors which can be reproduced. As a matter of interest earlier CRTs used more widely spaced primaries, theoretically giving a wider color gamut. However, today’s phosphors have been chosen as the best compromise in respect of overall image brightness and color range. Points R’, G’ and B’ correspond approximately to the triangle available in a theatrical luminaire used for color wash effects. In this case the color filters used are dichroic. Figure 1.66 shows the diagram with the planckian, or blackbody, locus superimposed. This line represents the appearance of a blackbody at increasing color temperatures. It clearly shows how


Figure 1.65 The CIE chromaticity diagram in color. Obviously this is an approximation, limited by the printing process. See main text for description of the triangles.

Figure 1.67 Some examples of isotemperature lines shown on the CIE chromaticity diagram. The number against them is in MIRED or recripocal MK.

“red” tungsten filament lighting is when compared with other “hotter” sources. Figure 1.67 shows examples of isotemperature lines, used to derive correlated color temperatures (see Section 1.10.4). 1.11.2 Luminance, metamers, color spaces It would be correctly argued that the introduction of the CIE chromaticity diagram in 1.11.1 has oversimplified matters. The diagram as it stands is twodimensional. While it correctly deals with hue and saturation, it does not properly deal with brightness or luminance. A more rigorous understanding of tristimulus values is needed for this. The aim is to define the light from a source, or from a reflective surface, with tristimulus values which uniquely describe its strength and color. The chromaticity co-ordinates x, y, and z are defined by:

Figure 1.66 The CIE chromaticity diagram showing the planckian, or blackbody, locus.

x = X/(X + Y + Z) y = Y/(X + Y + Z) z = Z/(X + Y + Z)

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Figure 1.68 The CIE primary color matching functions. The figure is an approximation. CIE issue two different sets or figures depending on field of view. Each set is given for a range of standard illuminants. Note here that the colors used to show x , y and z are simply to distinguish them in the figure.

X, Y and Z are the amounts of the imaginary primary colors needed to match a specified color. They are defined by equations which in words can be summed up, as: X is the sum for all wavelengths between 380nm and 780nm of the measured spectral energy, multiplied by the spectral tristimulus value x. Similarly for Y and Z. The values of, x, y and z for different wavelengths are shown graphically in Figure 1.68 assuming an equal power spectrum (i.e. the maximum power at all wavelengths is equal). The CIE issues a table

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giving precise values at 5nm intervals. In practical applications (for example in the film studio) X, Y and Z can be measured by portable instruments like the one shown in Figure 1.69. This does the necessary calculations to show the results in various ways in addition to tristimulus values – for example the x and y co-ordinates on the Chromaticity Diagram, with a separate reading for brightness. Color matching is bedevilled by the fact that it depends not only on the source, but also on the characteristics of the object being illuminated. Two visually indistinguishable lightsources with the same spectral compositions are said to have a spectral match. But two observed lights may appear to be visually indistinguishable despite having quite different spectral compositions. Such a color match is called metameric, and is conditional. For example two items illuminated by a tungsten lamp may appear to have an identical color; however, when illuminated by daylight they appear to have different colors by virtue of their different spectral reflectances. This phenomenon is of great importance in lighting design. For example, a stage or interior lighting designer can only do his or her job with the actual dress or furnishing fabrics to be used. “Rehearsal” dresses might look quite different in color terms under the final intended lighting, even

Figure 1.69 A portable tri-stimulus color meter from Minolta. This one shows luminance, the x and y color coordinates, and also temperature.


Figure 1.70 Adding a third axis to the chromaticity diagram allows luminance to be included.

if the “rehearsal” and “real” fabrics looked the same under daylight. The y function in Figure 1.68 corresponds to the photopic response of the eye. Thus the Y value is a measure of the brightness (luminance) of the source or illuminated object. A third axis can be added to the chromaticity diagram as shown in Figure 1.70. Here lighter colors appear above the point representing their chromaticity at a height representing their lightness. The Y axis “touches down” at the point representing the chromaticity coordinates of the illuminant. Figure 1.70 illustrates the concept of color space, a three dimensional representation of both color and luminance. Its problem is that it does not correlate all that well with perceived color differences, in that regions of apparent small color difference use up a lot of space, and some areas of major perceived difference are all “bunched up”. To get over this, in 1976 the CIE introduced other color spaces known as CIELAB and CIELUV. In both systems L* (lightness index) and either the a*,b* or u*,v* chromaticness indices (the way in which the perceived color appears more or less chromatic) are related to X, Y and Z by a set of equations. In industry the two systems co-exist because CIELAB has been found most suitable for

describing the color of objects, while CIELUV is used for emissive devices like TV screens. Figure 1.71 shows in diagramatic form the CIELAB color space. Notice that black now has a defined place on the map, at the origin. The L* , a* and b* axes can be considered as representing “white”, “blue-yellow” and “green-red” respectively. The alternative CIELUV is similar (L* is the same for both) but uses a different set of transform equations. In many practical systems (especially those concerned with electronic display and printing) somewhat simpler color models are used. These are normalized so that, for example, measurement of red, green and blue primaries (for display) or cyan, magenta, yellow secondaries (for printing) is weighted. This is so that each can be scaled from 0–1 or 0–100%, yet still sensibly yield a full color gamut on a simple incremental basis. The weighting is tied back to the CIE color spaces. Color, and the many different attempts to describe it, is a complex subject. This brief review can do no more than introduce it, and give a first insight as to why lighting in practice often gives surprising results. The reading list includes books that give more comprehensive coverage.

Figure 1.71 The CIELAB color space. The color of the point P is described by three quantities. Lightness L* measured on the vertical axis; Chroma C*ab measured as a*, b* co-ordinates, and Hue, measured as the angle h.

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Chapter 2


Lighting 2.1 Electronic principles 2.1.1 Introduction In this chapter the intention is to give sufficient background information to understand the basics of electronics as applied to lighting control. We build on the ideas of Chapter 1. Electronics differ from electrical engineering mainly in the power levels and frequencies involved. Traditional electrical engineering and electromechanical systems involve items such as motors, generators, switches, relays, transformers etc. all operating at something similar to mains frequency alternating current. Electronic engineering, on the other hand, is normally characterized by having no moving parts (other than items such as isolating switches) although sometimes the electronic device is itself connected to a “traditional” electromechanical unit such as a relay or motor. Most electronics operate at low voltages, and with comparatively low currents. They may also operate at high frequencies. For example a microprocessor may work from a 3.3V supply, consume only a few milliamps, and operate at a frequency of many MHz. In lighting control we encounter power electronics which is where electronic techniques are used to handle significant electrical power. The principle is usually that all the “clever stuff”, for example logic and control, is done at low power, i.e. in the normal electronic environment, but that this is connected to various electronic power devices which translate the intelligence into brute power. The range of power electronics is very wide. At the low end a small inverter to operate a fluorescent lamp from a battery may only be controlling a few watts. At the high end the motor controllers used on modern trains are electronic inverters handling more than a Megawatt. In lighting control we do not usually get into the Megawatt range for a single control channel; light-

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electronics ing electronics is mainly concerned with power control in the range 5W–10kW (for a single channel). Complete systems may, indeed, handle Megawatts, but always as multi-channel systems. 2.1.2 Some basic electronic operations Electronics are highly complex, but can be thought of as being based on a “building block” principle. However, in the same way that all electric motors work on similar principles, yet a 1MW train motor is significantly different in construction to a sewing machine motor; so the electronic building blocks, such as electronic switches, may start from the same principles, yet require very different designs to realize for different applications. Without, yet, considering how we are going to either create or use the devices concerned, let us postulate some items which could be useful. For example: The electronic switch. Section 1.5.3 introduced the idea of the relay, where a small signal current is used to control a bigger current. In the relay the action is electromechanical – there is still a physical means of breaking the circuit. In the electronic switch the principle is similar to that of the relay, except that the switching element is solid state, it has no moving parts, and no contact gap. Comparing the electronic switch to its relay equivalent might well raise the following questions: How is the control signal isolated from the power it is controlling? Typically transformers are used to provide signal isolation. Opto-electronic components are also used. Is it possible to have an electronic switch with no resistance? Obviously if there is a voltage drop across the switching element, heat will develop within it. There always is a drop, but in the best devices it is small. Most power devices need mounting on a heat sink to take away the excess heat. Is the “off” position of the switch really off? where safety is concerned any leakage could be dan-

LIGHTING ELECTRONICS

(a)

High current Load

Low current signal

Electronic switch

Large AC signal through loud speaker

(b)

Amplifier

Small A.C. Signal

Figure 2.1 (a) shows the concept of the electronic switch, where a low voltage, low current signal is used to instruct the electronic switch to apply full power to the load. Although shown for DC switching, the principle also applies to AC. (b) shows an amplifier, where a small varying voltage is amplified to a larger voltage.

gerous. Most systems have a separate isolating switch which is used when complete isolation is required. Does the switch work faster than its electromechanical equivalent? Yes, and this is its main advantage. It can switch millions of times without wearing out, and it can switch very fast indeed. The amplifier. The concept of an electronic am-

plifier is similar to the electronic switch. In the switch a small signal of fixed magnitude “lets through” a current which is only limited by the circuit impedance. In Figure 2.1(a) the switched current through the load will be determined by the voltage V, and by the total resistance represented by the load, the switch itself, and the internal resistance of the battery supply. (If it was an AC circuit we would be talking about impedances instead of resistances.) In Figure 2.1(b), the electronic switch is replaced by an amplifier. In this device the current let through is not constant, but varies in proportion to the control signal. No input signal, no output. 20% input signal, 20% output current, 100% input signal, 100% planned output current. Amplifiers are normally used with varying or alternating currents, the most obvious manifestation being audio equipment where very small signals derived from radio detectors, phonograph pick-ups or CD decoders are amplified so we can hear them when converted back to acoustic energy by a transducer, for example a headphone or a loudspeaker. The rectifier. There is often a need to turn AC into DC. Most electronic equipment (or at least its control and logic side) works from DC, so it is either battery powered, or is powered from a step-down transformer whose AC output is converted to DC. The start of the conversion process is by the rectifier. This can be thought of as a special kind of switch, which does not require any signal to operate, but only lets current through in one direction. Figure 2.2 shows the action. The output of a simple rectifier is DC, but is the result of half wave rectification. In practice it needs cleaning up. A full wave rectifier can be constructed in several ways, but the most common method is the bridge rectifier. Figure 2.3 shows how four rectifying elements are used. The positive going part of the AC waveform goes through one rectifier, then the load, and out through a second rectifier. The other two rectifiers provide a blocking action to ensure that the current only goes through the load in one direction. When the negative going part of the AC waveform arrives, the rectifier pairs exchange roles. Rectified AC has a high ripple content. In order to make it suitable for circuits needing a DC supply,

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Anode

Anode

Cathode

Cathode

Rectifier

Rectifier

Load

Load

Load

AC Supply

AC supply

Waveform at Current flow positive half cycle Anode

Waveform at anode

Waveform at cathode

Negative half cycle Waveform at Cathode Waveform at load

Figure 2.3 The most common full wave rectifier, the bridge rectifier. Figure 2.2 The half wave rectifier. Pulses of DC flow through the load.

it must be smoothed, and the simplest way of doing this is to add a capacitor. The capacitor value is chosen to minimize the ripple, but precautions have to be taken to ensure that the initial charging current does not exceed the rating of the rectifier. Figure 2.4 shows the idea. The oscillator. Electricity as described so far has come in two flavors, DC and AC. We have also admitted that there may be electrical signals used for carrying information or control instructions in the form of continually varying voltages. In some circuits there may be a need for a high frequency AC signal. This might be needed either because it was intended to operate an electrical device at a much high frequency than normal mains frequency, or because a precise means of timing was required by virtue of the accuracy of the signal. For this purpose we need some kind of oscillator. Electronic oscillators played an important part in the development of radio, producing sinusoidal waveforms. Today they still play an important role in electronics, but techniques have changed considerably.

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2.1.3 Resonance, tuned circuits and filters In Chapter 1 we encountered capacitors and inductors in isolation. Now we consider what happens when the two meet. Figure 2.5 shows an AC circuit with Addition of smoothing capacitor

Load

Waveform at load

Ripple

Original Smoothed

Figure 2.4 Using a capacitor to “smooth” the output of a sine wave rectifier.


R

L

C

I

The nett result of the two reactances, since they are in direct opposition, is OB-OC, and is shown as OD. The supply voltage V must be the vector sum of OA and OD, shown as OE on the diagram. A little bit of Pythagoras shows: OE2 = OA2 + OD2 = OA2 + (OB-OC)2

V

If we convert the vectors into the corresponding electrical quantities: V2 = (RI)2 + (ωLI - I / ωC)2

B

Factor out the current I to give: Voltage Vector

V = I √ [R2 + (ωL - 1 / ωC)2]

D

E

Restate as: I = V / √ [R2 + (ωL - 1 / ωC)2]

O

I

I

A

But I = V / Z where Z is the circuit impedance. Therefore in the series circuit shown, the combined

Current Vector

1

C

Q=

resistance, capacitance and inductance. If V and I are the r.m.s. values of the voltage and current respectively, we can say that the voltage across the resistance R is RI volts, shown as OA in the vector diagram, and is in phase with the circuit current I. We know that the voltage across the inductance is ωLI (i.e. the inductive reactance multiplied by the current) and that it is 90° in front of the current, and shown as OB in the diagram. Similarly the voltage across the capacitor is I/ωC but lagging the current by 90°, and shown as OC in the diagram. For the purposes of the diagram OB is considered greater than OC.

| 100

R Low Q | 10

Current I/I R

Figure 2.5 At the top an AC circuit containing resistance, inductance and capacitance in series. Below is the corresponding vector diagram. OD is the voltage across L and C combined. OA is the voltage across R and is in phase with current I.

High Q

Z2 L

0.5

0 0.5

1.0

Frequency f/f0

1.5

Figure 2.6 Current in a series resonant circuit. Maximum current possible is that due to the resistance only IR. The Q of a resonant circuit can be shown to equal the ratio of inductive reactance at resonance to the resistance. In the diagram f0 is the resonant frequency

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R C L I

fo

Figure 2.7 In a parallel resonant circuit resonance is achieved when φ equals zero; impedance is then at a maximum, and current at a minimum.

reactance arising from the inductance and capacitance is ωL - 1/ ωC, or inductive reactance minus the capacitive reactance An interesting thing happens when we juggle the values of L, C, and ω (i.e. the frequency) such that the two reactances cancel out. The current in the circuit is now limited only by the resistance, and it is possible for the voltages in the circuit to be many times the nominal supply voltage. When this condition arises the circuit is in resonance. The resonant frequency of the circuit is given by the condition that the two reactances cancel out, i.e.:

ωL - 1/ωC = 0 or ω2 = 1/LC but ω , the angular frequency, = 2πf where f is the frequency, so the resonant frequency is: f = 1/2π√(LC) The current carried by a circuit such as that of Figure 2.5 varies with frequency in the manner shown by Figure 2.6. The narrowness of the current peak and the ratio of the maximum to minimum current represent the Q of the circuit.

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If a similar analysis is carried out on a circuit where the inductance is in parallel with the capacitance, an interesting result is found. Again there is a resonant condition, and the formula for the resonant frequency turns out to be exactly the same. However, whereas in the series resonant circuit, the total impedance is at a minimum at resonant frequency; in the parallel resonant circuit the impedance is at a maximum at the resonant frequency. See Figure 2.7. The resonance phenomenon is widely used in electronics. Tuned circuits are used in radio – for example a circuit containing an inductance and a variable capacitor is used to “tune in” to a particular frequency – if the circuit has a “high Q”, it will accept the signal at the required frequency, and ignore signals either side of it. LC combinations can also be used as the basis of oscillators. While an LC combination on its own cannot maintain a continuous electrical output because of the inevitable resistance in the circuit; it can be the determinant of an oscillator frequency. A transistor is used to pulse energy into the circuit, but the transistor itself is triggered by the resonance. However, it is difficult to make frequency-accurate oscillators this way, and most oscillators used in lighting control, for example as the master timing element for a microprocessor, are based on quartz crystal oscillators. These use the natural mechanical resonance of the crystal as their basis. Finally LC combinations can form the basis of filters. In fact some filtering can be done using only RC combinations, for example simple low pass and high pass filters – but LC filters are more specific and effective for particular frequencies. They can use both series and parallel arrangements – for example a parallel arrangement can block a particular frequency, or a series arrangement can be used to divert or short-circuit unwanted signals. Such filters are of great importance in lighting control and power electronics. 2.1.4 Active and passive components In electronic circuits components are broadly categorized as being active or passive. Passive devices are those which require no energy to operate, other than


the applied voltage or “signal”. They generally present an impedance, and are represented by inductors, resistors and capacitors. Active devices are those which change the nature of an electrical signal or waveform, and allow the realization of the building blocks described above. They require an external energy source; examples are the transistor and the thyristor. The diode is a special case; although it is usually a passive component, certain types of diode or diode circuit make it an active component.

2.2 The diode 2.2.1 Background The diode is a two terminal device which provides the function of the rectifier described in 2.1.2 above. In the early days of electronics low current, high frequency diodes, as used in radio sets, were constructed as vacuum tubes (valves to the Brits) or as point contact crystal diodes (used in “crystal sets”). Medium power, low frequency diodes, as used in battery chargers, were made using selenium or copper oxide rectifiers; and high power diodes, suitable for providing DC for traction purposes (e.g. the London Underground) used mercury arc rectifiers. However, for all but highly specialist or replacement purposes, these have all been superseded by the semiconductor junction diode. It is this component, first realized in germanium, but now mainly in silicon, which is of importance to lighting electronics. 2.2.2 Semiconductors Most active electronic components start with extremely pure crystalline silicon, produced by growing a big crystal of silicon from a seed crystal, and then zone refining it. This is a local heating process where the heating is progressively moved down the length of the crystal; it has the effect of making all the impurities move down to one end. The impure end is cut off, and the remainder sliced up into wafers for further treatment. Figure 1.1 showed that the silicon atom has four electrons in its outer orbit. On its own, this is an un-

Si +4

Si +4

Si +4 Spare Electron

Si +4

+5 P

Si +4

Si +4

Si +4

Impurity Atom

Si +4

Valency Bond

Figure 2.8 An impurity within a silicon crystal lattice can provide an extra electron for conduction.

stable state and stable crystalline silicon arises from a sharing of the outer valency electrons. Covalent bonds between neighboring atoms result in each atom having eight electrons in its outer orbit, which is a stable state. Electrical charge neutrality is maintained because the additional electrons are only “borrowed”, and there is no nett change in electrical charge. Crystalline silicon at room temperature is an insulator. If extra energy is given to a silicon atom, for example in the form of heat, it is possible for an electron to break away from its valency bond, and become a free electron able to conduct electricity. However, the atom left behind now has a positive charge. This is referred to as a hole, and the movement of holes, arising from free electrons jumping from one atom to another in succession, is another form of conduction – in the opposite direction to the electron flow. Silicon (a Group IV element in the chemical periodic table; Group IV also includes carbon and germanium) is referred to as tetravalent because of its four valency electrons. Group III elements, such as aluminum, gallium and indium, are trivalent with three valency electrons, and Goup V elements, such as phosphorus and arsenic, are pentavalent with five valency electrons. Silicon’s ability to conduct electricity is changed dramatically if it has a few atoms of another element

63


dispersed within it. By deliberately adding impurities, it is possible to create two different kinds of conducting silicon. n type silicon is silicon doped with a very small amount a Group V element. n stands for “negative” where conduction is predominantly by negative electrons. Figure 2.8 shows that the effect of a pentavalent impurity is to provide a free electron which can wander round the crystal lattice. p type silicon is achieved by doping with a trivalent impurity, creating additional holes which provide “positive” conduction. In both types of conducting silicon electrical neutrality is maintained overall. In the one case a lot of fixed positively charged nuclei are balanced by the same number of wandering negatively charged electrons, and in the other a lot of fixed negatively charged nuclei are balanced by the corresponding number of holes. In any real silicon conduction is by both electrons and holes. However, in n type silicon the majority carrier is the electron, with the hole being the minority carrier. In p type silicon the situation is reversed.

P-type

n-type

+++ +++ +++

- - - - - - -

++ - + - ++ - + - ++ - + - ++ ++ ++ -

+ - + - + - -

(a)

(b)

(c)

Depletion Layer

p

n

Holes

Electrons

2.2.3 The semiconductor junction diode An interesting thing happens when a connection is made between n silicon and p silicon – a so-called pn junction. At the junction, free electrons on the n side are enticed into occupying holes on the p side, and vice versa. This has the effect of creating a depletion layer where there are no mobile current carriers. A barrier potential difference is set up, this is about 560mV for silicon. What happens if we try and pass a current through such a junction? Figure 2.9 shows that if the positive terminal of a battery is applied to the p side, the holes are pushed back across the junction; in fact provided the external voltage exceeds 560mV the depletion layer is eliminated and current can flow. On the other hand, if the battery connection is reversed, the migration effect at the junction is enhanced, and in effect the depletion layer gets wider, preventing current flow. Figure 2.10 shows the diode characteristic, where forward and reverse currents are plotted against ap-

64

+

- >0.6V

(d)

Current Flow

(e)

-

+

No Current Flow

Figure 2.9 The p-n junction. (a) shows the separate p and n type silicon, each with its mobile carriers. (b) shows that when they are placed together the carriers at the junction migrate across it. This has the effect of creating a depletion layer which has no carriers (c). If a battery of greater than 560mV is connected to forward bias the junction, the depletion layer is broken down and current flows (d). But if the battery is connected the other way round, the depeletion layer is made wider (e) and no current flows.


Current

Reverse Voltage -40

0.5

1

Forward + Voltage

Reverse Breakdown

Figure 2.10 The silicon diode characteristic curve. The negative voltage scale is condensed; the actual reverse breakdown voltage depends on the diode construction. It might be 40V as shown, or several hundred volts.

plied voltages. From this we can derive a number of important parameters. Most of these parameters are just as important for transistors and thyristors, and are highly relevant to the practical realization of lighting control electronics. Forward voltage drop. This arises because of a combination of the depletion layer voltage and the additional resistance of the junction material and connection to it. In any power component the aim is to keep this voltage to an absolute minimum. In operation any voltage drop at the device will result in a V2/R heating effect. Reverse leakage current. A p-n junction is not a perfect rectifying device. There is always a small reverse leakage current arising from several causes. As the temperature rises minority carriers can diffuse into the depletion region. In a real diode there are practical problems at the physical edges of the junction where surface contaminants can cause unwanted conduction. Finally the main cause is intrinsic conduction within the depletion layer; this is the conduction by silicon’s own free electrons. This is highly temperature dependent, since it is the thermal energy which creates the free electrons – the current can double for every 10°C rise in temperature. In

power electronics the aim is always to minimise the leakage current. Reverse breakdown voltage. Figure 2.10 shows that when the applied reverse voltage exceeds a certain value, the current rises very fast – a so-called avalanche effect. The effect of the electric field is to increase the velocity of the mobile carriers; when this exceeds the thermal drift velocity they have enough energy to knock otherwise stable valency bond electrons into conduction, creating hole-electron pairs. These pairs can then themselves create further pairs, leading to the avalanche. Any real semiconductor device is rated to ensure that it does not break down. Real world ratings are derived from the above characteristics of the device; they are usually given in graphical form, because most are temperature dependent and/or inter-dependent. Examples are: Maximum reverse voltage. This must be considerably less than the breakdown voltage. It is shown as three different values all related to temperature; one is a continuous value, which the device can withstand indefinitely. The second is peak repetitive voltage, higher than the continuous voltage, but limited as to the percentage of time it can be applied. For example it might correspond to the peak value of an AC waveform. The third is an absolute maximum voltage which can only be allowed occasionally in the life of the device, for example a transient voltage surge. Maximum current. Again this is specified three ways. Maximum continuous current is that which can be carried continuously at a specified temperature. Peak repetitive current applies for a limited time. A combination of these two ratings is used to derive the achievable performance in an AC circuit (for example a bridge rectifier) and the result is then expressed as an “average” or “r.m.s.” rating. The current waveform has a significant bearing on the achievable result. The absolute maximum rating is for occasional events, and, in the case of power diodes (and power transistors and thyristors) is used for determining the correct method of protection against short circuit and overload.

65


Power dissipation. A diode or other power electronic device dissipates heat according to the current being carried and the voltage drop across the device. For example in a bridge rectifier, the current has to go through two diodes in each half of the sine wave cycle. The total drop might be around 1.1V, and if the bridge is carrying 5A r.m.s, the total dissipation will be 5.5W. The heat must go somewhere, and power devices dissipating more than a very few watts are normally mounted on a heatsink to dissipate the heat. Thermal resistance. In respect of a diode rectifier, the junction to case thermal resistance is a measure of the difference between the junction temperature and the external temperature of the casing. It is measured in °C/W. Thus, for example, if the junction temperature is 120°C, the case temperature is 100°C, and the device is dissipating 5W, it has a thermal resistance of 4 °C/W. This information is needed when designing heatsinks, or determining the actual ambient temperature at which it is safe to run a device. The junction diode is available in a huge range of ratings and packages – from small high frequency, low voltage diodes carrying milliamperes to giant power devices carrying thousands of amps at high voltage. Some examples are shown in Figure 2.11. 2.2.4 The zener diode In addition to the standard diode mainly used as a rectifier, there are a number of special purpose diodes. The first of these is the zener diode. This name has now been given to all kinds of voltage reference diodes, although, strictly, it should apply only to diodes showing the zener effect. Voltage regulation diodes work as normal diodes in the forward direction, but have a precisely defined breakdown voltage in the reverse direction. i.e. they do not carry current in the reverse direction until a particular voltage is reached, whereupon they conduct. Varying the concentration of n and p carriers in the junction varies the width of the depletion layer. Heavy doping results in a narrow depletion layer, and this results in a very high electric field across the

66

a) small signal diode

c) four diodes in same package to make a bridge rectifier

b) individual low power diode used in electronic ballast or transformer

d) stud mounted power diode

Figure 2.11 Examples of silicon diodes and their packaging.

junction. The zener effect is when this field is so great that it gives electrons sufficiently high energy to break away from their valency bonds and become conducting electrons. It happens below about 5V. If the p and n layers are lightly doped, the depletion layer is wide, and the critical field for the zener effect cannot be reached. However, the diode still suffers from the avalanche effect. Figure 2.12 shows the difference between the avalanche and zener effects in reverse biased junctions. Voltage reference diodes are diodes made with precisely known avalanche breakdown or zener voltages. If they are placed across a supply, they will “clamp” the voltage. Such diodes are made in the range 1–300V. The low voltage diodes are based on zener effect only. Those between 5 and 7 volts on a combination of zener and avalanche effect, and those above 7V on avalanche effect. The zener voltage decreases with temperature, while the avalanche voltage increases with temperature. Voltage regulation diodes around 5.6V combining both effects have negligible variation with temperature since the effects cancel out, and are, therefore, particularly useful in providing a stable voltage reference.


Current Symbol for Zener diode

Ca5V Voltage Avalanche

Zener

Zener slope is more gentle

Figure 2.12 The zener characteristic is more gentle than the avalanche. The symbol for a voltage reference diode is the same, whichever mechanism is used.

Zener diodes are used in power supplies to ensure correct output voltages. For example in an automobile the output of the alternator is rectified to DC, but then regulated by a zener diode to ensure that the battery does not receive an excessive charging voltage. Clearly if a zener diode “clamps” a supply, the excess energy has to go somewhere. Zener diodes are, therefore, often power semiconductors, designed to dissipate heat. A special application of the voltage regulation principle is in voltage surge suppression. If two zener diodes are placed back to back across an AC supply, no current passes through the diodes if the peak voltage of the supply does not exceed the diode rating. If a voltage “spike” comes along, then the diode pair conducts – one half conventionally, the other by breakdown, and the load is protected. Clearly the pair must be designed to absorb the energy of the spike. 2.2.5 The light emitting diode Chapter 1 introduced the idea that if an electron changes energy level, then a quantum of energy is absorbed if the new level is higher, or is emitted if the new level is lower. At a semiconductor junction, electron/hole recombination results in a quantum of

energy being released as an electron drops from its conduction band energy level to the valence band. In silicon this mechanism does not produce any visible light; however in the compound semiconductor Gallium Arsenide (GaAs) it does. The wavelength of light produced varies according to the dopant used. Nearly pure GaAs emits in the near infra-red region, but doping with phosphorous, zinc oxide or nitrogen can produce visible red, green and yellow radiation. Light emitting diodes or LEDs are important in lighting control for several reasons. • The infra-red LED is used as the light source in opto-couplers. It is also used as the radiator in some cordless remote control systems. • The conventional colored LED is widely used as an indicator lamp in control panels and mimic diagrams and within indicating switches and pushbuttons. • LEDs of more complex construction than the simple junction diode are now available to give a wide range of colors with respectable conversion efficiency. They are becoming valid sources of light to take their place alongside conventional lightsources. 2.2.6 The photo-diode In the same way that a semiconductor junction can emit radiation (i.e. an electric current is converted into visible light) so the reverse is possible. The absorption of a photon can raise an electron to the conduction band, so light can be converted to an electric current. Silicon p-n diodes respond to visible and infrared radiation, and can be used as light detectors and as the basis of light measuring instruments. When modulated light is to be used (for example in infrared remote control systems) the speed of response of the simple diode is not fast enough due to capacitance effects, so in this case a p-i-n or PIN construction is used. Here the simple p-n junction is separated by a thin layer of intrinsic conduction (pure or very lightly doped) silicon. Photodiodes normally work in the reversed biased mode, the reverse current in excess of leakage current being directly proportional to the incident radiation. However, if they are operated in forward

67


individual LED in clear housing for general purpose indication

7 bar LED indicator used for channel number indication in dimmers and moving lights

2.3 The transistor 2.3.1 The Bipolar Transistor The Bipolar Junction Transistor has two different constructions, p-n-p and n-p-n. The principle is that of placing two semiconductor junctions together as The p-n-p Transistor

high power cluster of LEDs used in signs and video displays

photo transistor fitted in light level sensor unit

p-type

Emitter E

photo thyristor used to provide isolation in professional dimmers

conducting mode an e.m.f. of about 0.5V is developed, creating a solar cell. This alerts us to the fact that photo-sensitivity is an inherent property of silicon semiconductor devices, so unless it is a required function, all silicon electronic devices must be protected from light (or other electromagnetic radiation) An opto coupler is created when an LED is packaged next to a photodiode (or, more often, a phototransistor or photothyristor; however, the principle is similar) within a lightproof housing. In this case an electrical signal fed to the LED will be replicated as another electrical signal coming from the photo-detector. This method gives total electrical isolation between input and output. It is widely used in lighting control to separate low voltage control signals from high power/high voltage circuits. Figure 2.13 shows some opto-electronic components.

68

p-type

Base B

E

Figure 2.13 Examples of opto-electronic components.

n-type

C

Collector C

E

C

Symbols

B

B

The n-p-n Transistor

n-type

p-type

Emitter E

n-type

Base B

E

C

B

Collector C

E

Symbols

C

B

Figure 2.14 The bipolar junction transistor. The circuit symbols with the circle are used for discrete components. Those without the circle are used when the transistor is built in to an integrated circuit, usually with many other transistors.


Narrow emitter-base depletion layer due to forward bias

Wide base-collector depletion layer due to reverse bias

Emitter

Base

Collector

Hole Flow n-type

p-type

IE

>0.6v For Conduction

p-type

IB

IC

VBE +

-

VCB +

-

Figure 2.15 The operation of the p-n-p transistor showing the biasing arrangement. In an n-p-n transistor all voltages and current flows are reversed, and main current is carried by electrons instead of holes.

shown in Figure 2.14. Both types of bipolar transistor work in a similar manner. The reason they are called bipolar transistors is that they depend on both positive and negative majority carriers for conduction – holes in the p type silicon and electrons in the n type silicon. The bipolar transistor is usually just referred to as a transistor, whereas other types are always referred to with some additional description. It has three electrodes, the emitter, the base, and the collector. Its importance lies in the mechanism of current gain, whereby a small current in the base results in a big current in the collector. Figure 2.15 shows how a p-n-p transistor is biased. (The description which now follows also applies to n-p-n, but then the biasing arrangements would be reversed in polarity, and all references to holes would become references to electrons, and viceversa.) In the diagram batteries are shown as the power supplies for simplicity, and the direction of current is shown conventionally as flowing from positive to negative. Back in the mid – 19th Century, Kirchhoff set out laws about what happens when different electric currents meet at a point; and his first

law states that if several conductors meet at a point, then the total current flowing towards that point is the same as that flowing away from it. It is intuitive that this is so, since if it was not, the meeting point would accumulate an electric charge – and apart from no such charge being measured in practice, it is not easy to visualize how such a charge could be held. Applying the law to the transistor circuit, we have: IE = IB + IC i.e. the current flowing through the emitter connection equals the sum of the currents through the base and collector. The transistor is special because IB is very small compared with IC, and in use the effect is that small changes in IB make big changes in IC. The effect of forward biasing the emitter-base junction is to narrow its depletion layer; whereas the reverse biasing of the collector-base junction is to widen its depletion layer. The description of the p-n diode showed that if VBE exceeds 0.6V, the depletion layer between emitter and base reduces to zero width. The positive potential on the emitter repels the surplus positive holes

69


PNP Transistor

IC

IC

Current carrying limit

Imax

IB

50

VCE VBE

Power dissipation limit

IE

25 %Duty Cycle

75 100

Breakdown voltage limit

Secondary breakdown limit IC mA

Vmax

gm at P is slope of graph at P

Figure 2.17 The safe operating area for a transistor. Note that it changes with duty cycle.

IC

P

5

VBE 0 550

600

650

mV

VBE

hFE = IC /IB is the large signal or DC current gain; important for switching applications. hfe = 'IC/'IB is the small signal or AC current gain; where ' signifies a small change; important for audio amplifiers etc. gm = 'IC/'VBE is the transistor mutual conductance (measured in Siemens or mho, reciprocal ohms) relating small changes in base-emitter voltage to collector current. VCEO(SUS) is the maximum collector-emitter voltage which can be sustained without breakdown, measured with open circuit base. (Breakdown voltages are also specified with other base biasing conditions.) ICEO is the collector-emitter leakage current when there is no base current. It is highly temperature dependent. VBE(SAT) the base-emitter saturation voltage, and VCE(SAT) the collector-emitter saturation voltage are the respective voltage drops at the point when increasing the base current gives no further increase in collector current. The conditions under which they are measured must be specified. PD is the maximum power dissipation of the device (at a specified temperature) in principle the product of VCE and IC. But in fact transistors must operate in their safe operating area see Figure 2.17.

Figure 2.16 Some parameters relating to the bipolar transistor. The diagrams relate to p-n-p transistors.

70

VCE

which are attracted by the negative base region. The base-collector junction is reverse biased by the larger potential VCB – so the collector is negative with respect to the base. The widened depletion layer effectively prevents holes in the collector crossing to the base, but does not stop the flow of holes in the other direction, since they are attracted by the heavily negatively charged collector. Once conduction has started, the great majority of the holes starting from the emitter make it through to the collector, but a small proportion combine with free electrons in the base region, to form a small base current. The fundamental property of the transistor is the static current gain, or common emitter forward current transfer ratio designated hFE where: hFE = IC /IB Typical transistor gains are in the range 100–300. From a “mind picture” point of view, you can think of a small base current being “amplified” to a large collector current; or a small base current controlling the larger collector current flow. Actually it is the base emitter bias voltage which determines the collector current, and hFE which in turn determines the base current. Figure 2.16 lists some of the common parameters relating to a transistor. It is important to understand that many parameters are inter-dependent and temperature dependent. Figure 2.17 shows the idea of a the safe operating area for a transistor. For any particular tempera-


Lamp Load

+12V I C=120mA

I B=1mA ON 0.75V

OFF

hfe =120

0V

Figure 2.18 The transistor as a switch. The 1mA control signal switches 120mA through the n-p-n transistor.

ture the device will break down due to excessive voltage Vmax or excessive current Imax. There would normally be a straight line maximum power dissipation curve, but in a transistor there can be an uneven distribution of current, arising from temperature variations across the device and irregularities in construction. This can lead to secondary breakdown arising from local thermal runaway; so the safe operating area is less than expected. Many applications of transistors in power circuits require the transistor to operate without carrying current continuously; its duty cycle is less than 100%. In this case the safe operating area is increased as shown in the figure. From the description of how a transistor works, we can see that it could be used to make some of the building blocks referred to in 2.1.2. Figure 2.18 shows the transistor being used as a switch, where a very small current is used to switch a large current on and off. In practice the low power switch shown in the circuit would most likely itself be another, smaller, transistor being driven by some kind of logic circuit – possibly being driven very fast to provide fast on/ off switching. In practice if we want to use a transistor as a switch, we want the voltage across it to be the full line voltage when it is “off”, and to be as little as possible when “on” – since any remaining voltage will result in heat being dissipated within the transis-

tor. To achieve the minimum voltage requires the transistor to be driven to saturation which is the point at which an increase in base current produces no more increase in collector current. Figure 2.19 shows a transistor being used as a current regulator. Here the base bias voltage is simply being varied between the point at which the transistor is just not conducting, to the point at which it is passing maximum current. The problem here is that the transistor is now operating as a variable resistance – it is not in saturation. Figure 2.20 shows the transistor being used as a voltage amplifier. Here the idea is to produce an output which is a magnified but perfect copy of the the original input signal. The input is applied to the base of the transistor, and here shown as a 0.05V swing, superimposed on the base bias voltage (which must be there, otherwise there would be no conduction at all.) The output is shown as a varying voltage at the collector, arising from the change in collector current. The collector is biased at well below the maximum collector voltage to allow a voltage swing to develop. For a typical “discrete” transistor the 0.05V input voltage swing might produce a 7.5V swing in the collector voltage – corresponding to an amplifier voltage gain of 150. Notice that the output voltage is out of phase with the input, the higher the collector current, the lower the collector voltage.

Lamp Load

Rotary 0.8V Potentiometer

+12V

IC Varies with change in VBE

0.65 0.6V

VBE 0V

Figure 2.19 Replacing the input switch with a rotary potentiometer gives a current regulator. A resistor network ensures that the potentiometer only operates in the active range 0.6–0.8V.

71


VCC

IC

VRC=ICRC

RC

VC VOUT=VCC-ICRC VIN=VBE

IE

Input 0.07

VBE=

VIN

VBE 0.65

mon to both input and output. RB1 and RB2 set the bias voltage at the base, and are chosen to carry enough current such that the base signal current is insignificant compared to it, and therefore does not itself affect the bias voltage. CIN and COUT are capacitors which block the DC bias, but have a low AC impedance at the frequencies of the signal being amplified, they simply isolate the wanted signals from the amplifier. RE biases the emitter (which in turn implies an increase in the needed base bias). This is done so that if there is an unwanted rise in collector current due to high temperature, the voltage drop across RE increases, and has the effect of reducing the base-emitter voltage and, therefore, the collector current. CE is needed as a decoupling capacitor to pass the amplified AC current, which would otherwise be limited by RE.

Base Bias

IC

+VCC

RC C OUT

RB1

0

Time

C IN V OUT

Output 12

V IN

CE

RB2 RE

V

CC

0V

VC 6

0

Collector Bias

VC=

VOUT

IC Collector current with no input

Figure 2.20 The transistor as a voltage amplifier. Note that all values are examples only. Vb

2.3.2 Amplifiers 2.3.2.1 Class A and Class B amplifiers Figure 2.21 shows the components needed to surround a transistor to make it work as a Class A audio amplifier stage in a common emitter configuration. This simply means that the emitter terminal is com-

72

V IN Figure 2.21 The Class A amplifier. The transfer characteristic is linear, because the transistor is biased to ensure this is the case.


The Class A amplifier has low distortion; that is to say the output voltage waveform closely matches the input waveform; but it is inefficient because current must flow at all times. The transistor is biased so that the relationship between the base input signal and collector output is linear. The inefficiency does not matter in small signal work, or in low power audio amplifiers; but it matters a lot in high power amplifiers and power conversion equipment. A Class B amplifier works with minimum quiescent current (by biasing transistors to cut-off) and, as a result, can have high efficiency. Such amplifiers work in push-pull mode, where one transistor operates in the positive half cycles, and another one in the negative. Figure 2.22 shows the principle of the Class B amplifier. The output transistors are a complementary pair, one being p-n-p, and the other n-p-n. The variable resistor VR sets the bias condition for both

Nett input

(V IN- B VOUT) V IN

A

B

V OUT = A (V IN

-

BV OUT)

-BV OUT

Figure 2.23 The concept of negative feedback applied to an amplifier.

output transistors so that for zero input signal, no collector current flows. The diodes D1 and D2 provide compensation for the variation of VBE with temperature. Note how the output waveform is distorted, arising from the crossover distortion arising from using both transistors on the non linear part of their characteristic curve. 2.3.2.2 Negative feedback; Classes C and D

+ VCC TR1

VR D1 D2

C OUT

V IN

TR2 0V

IC

I OUT

In real audio amplifiers a hybrid Class AB approach is used, not as efficient as Class B, but much more efficient than Class A. Real power amplifiers also make use of negative feedback to stabilize the gain and reduce distortion. Figure 2.23 shows an amplifier with open loop gain (the gain with no feedback) A. A proportion B of the amplifier output is subtracted from the input, therefore the actual input voltage becomes VIN - BVOUT; so now the actual output voltage becomes A(VIN - BVOUT ). System gain is the actual output voltage divided by the original input voltage, and can be stated as:

VB2 VB1

V IN

Figure 2.22 The Class B amplifier. The transfer characteristic is non linear, because both transistors operate both in their linear regions and in their non-linear cut-off regions.

Overall Gain = A/(1 + AB) This expression is known as the closed loop gain. It has an interesting result. If A is 100, and B is 0.1, the overall gain is 9.1; if A is 200 and B remains 0.1, the gain only changes to 9.5. Practical power amplifers are built with A very large, in which case the approximation; Closed loop gain = 1/B

73


Signal IN

High speed switch

Output power transistors

Filter V OUT

Output waveform before filter

Minimum output

Maximum signal output

Intermediate output

Figure 2.24 The concept of the Class D amplifier.

applies; i.e the gain is independent of the actual gain of the underlying amplifier, and depends only on the feedback ratio. Feedback also reduces distortion. If an amplifier has distortion N% in its open loop state, negative feedback reduces this to: Distortion = N/(1 +AB) % However, this only applies if the distortion does not, itself, affect amplifier gain. Unfortunately crossover distortion falls into this category. Efficient Class C amplifiers are used in radio frequency applications. In this case the transistor is biased so far “off”, that only the peaks of the input waveform are amplified. Tuned loads restore the input waveform. Of more relevance to power control is the Class D amplifier. In Class D the transistor(s) act only as a switch, i.e they are on (saturated condition with minimum volt drop) or off. To work as an amplifier this requires that the switching frequency is far higher than the frequency being amplified. One possibility is to use a square wave of constant frequency with variable mark to space ratio. For example the full

+VCC

RB

width wave represents 100%, and the narrowest “spike” represents zero. Figure 2.24 shows the idea. The high speed switch generates a train of pulses of varying width, depending on the strength of the input signal. These are amplified by the output power transistors, and the result is filtered to eliminate the high frequency components and restore the original waveform. 2.3.2.3 The emitter follower There are a number of other amplifier terms which can be encountered in lighting control electronics, and a few of them are summarized here. The emitter follower is used as a buffer amplifier in signal distribution. Instead of being in the common emitter configuration used in the circuits described so far, the transistor is used in the common collector configuration shown in Figure 2.25. In such a circuit the output follows the input in phase and amplitude. That means that, as a voltage amplifier, the emitter follower (so called because the emitter voltage follows the base input voltage) has unity gain, so it might well be asked what use it is. Its virtue lies in the fact that it has a very low output impedance. Conversely it has a high input impedance. In practice this means that it does not load, and thus distort, its source; but itself can tolerate a wide range of loading. 2.3.2.4 The Darlington pair In power electronics conventional transistors suffer from the fact that at high currents they have low gain. This means they need large base currents, and, as a C

B

TR1

1

~

D1

Input high Z

D2

OUTPUT RB2

Low Z

Figure 2.25 The emitter follower.

TR2

~

RE 0V

74

R1

R2 E

Figure 2.26 The Darlington pair.


ts tf IC

90%

10% td

tr OFF

ON Turn on time

Turn off time

Figure 2.27 Turn-on and turn-off times in a transistor. td is a delay in start of collector current due to collector and emitter depletion capacitance; tr is the rise time, determined by junction capacitances and the carrier transit time in the base region. ts is the storage time, the time needed for excess charges in base and collector regions to recombine as the transistor comes out of saturation. tf is the fall time, determined by junction capacitance and negative base current.

result, complex base drive circuits. The Darlington pair shown in Figure 2.26 goes some way to solving the problem. TR1 is used as the base driver for TR2 . The practical circuit includes resistors R1 and R2 to prevent the transistors amplifying their own leakage current. Diode D2 protects the output transistor from reverse voltages. Diode D1 removes the charge carriers stored in the base of the output transistor, and thus speeds up its turn-off. Turn-off and turn-on times in switching transistors are critical, since they result in slow operation and heat dissipation. They are caused by charge storage and capacitance effects, as shown in Figure 2.27. While “power Darlingtons” can be made up from discrete components, they are usually supplied as a single semiconductor structure.

identical in charactersistic and in variations with temperature (achieved in practice by them both being in the same integrated circuit and encapsulation). The fact that all changes due to temperature etc are equal, and that emitter current is shared in R3 (the “tail”) means that any changes affect both transistors equally. The long tail pair amplifies the difference between the two inputs V1 and V2 . If V1 = V2 then it is clear from the figure that Vout is zero. As an example suppose V1 = 0.12mV and V2 = 0mV, and that the measured output Vout = 60mV. Then the amplifier is said to have a differential gain of: Ad = 60/0.12 = 500 Now suppose there is an unwanted noise signal of 1mV which affects both inputs simultaneously. Such a signal is referred to as being common mode. Now it is the case that V1 = 1.12mV and V2 = 1mV. It might be that now the output, instead of being 60mV has risen to 62mV. The common mode gain, which refers only to the extra 1mV signal is: Ac = (62 – 60)/1 = 2 The measurement of an amplifier’s ability to amplify differential signals while rejecting common mode signals is referred to as its common mode rejection ratio (or CMRR) and equals Ad/Ac. In the example its value is 250. In integrated circuits much higher values are achieved, so great that it is inconvenient to represent them as simple numbers. A typical “op-amp” (to be described in Section 2.6) has a CMRR of over 30,000 – a figure so big that it is easier to use a logarithmic measure, the dB; for example +VCC R1

VOUT TR1

TR2

V1

2.3.2.5. The long tail pair; differential amplifiers The long tail pair or differential amplifier is at the heart of DC amplifiers and many AC control signal amplifiers. The bipolar transistor version is shown in Figure 2.28. The two transistors TR1 and TR2 are

R2

IN

V2 R3 0

Figure 2.28 The long tail pair.

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VGS

+ Gate G p

VIN n

Source S

p

Figure 2.29 A center-tapped transformer producing a balanced signal.

90dB. (The dB or deciBel is described in Section 2.5.) A differential amplifier is fed in one of two ways. A “single ended” input could be applied as V1, whereas V2 could be tied to 0V. Alternatively a balanced input could be provided to both inputs simultaneously. Similarly at the output there is a choice of an in-phase (from TR2), or out-of-phase (TR1) output signal, or a balanced signal. A balanced signal is one where the same signal is fed on two wires, but each half is exactly out of phase. A “traditional” way of achieving a balanced signal is to use a center-tapped transformer as in Figure 2.29. The term common mode referring to an unwanted signal affecting two input wires simultaneously is of considerable significance in lighting control, especially with reference to EMC and digital control signals. 2.3.3 The Unipolar Transistor 2.3.3.1 The JFET Unipolar transistors depend for conduction on only one type of carrier. They are also called field effect transistors or FETs. This is because the current flow is determined by electric field – in practice just a voltage. In a bipolar transistor the gate current can be significant, whereas in a unipolar transistor it is negligible. Unipolar transistors come in several varieties. Figure 2.30 shows the standard biasing arrangement for an n-channel junction field effect transis-

76

Drain D

+

VDS

Depletion regions

S

G

D

p n channel p Substrate

JFET

+ D

G

G

D

p Channel

+

S

S n Channel

+

C

B

C B

n-p-n

E

p-n-p

E

+

BIPOLAR

Figure 2.30 The n-channel JFET. The concept (top), the principle of construction (center), and the comparison of JFET circuit symbols with those of the bipolar transistor (bottom).

tor, or n-channel JFET. A p-channel JFET, would, like its bipolar counterpart, simply have n and p regions reversed, and the biasing reversed. Figure 2.30


also shows the symbols for JFETs and compares them with those for bipolar transistors. The channel is the conduction channel between the source and drain electrodes. The gate electrode is shown as being either side of the channel. Being of the opposite polarity to the channel, it sets up a depletion region at its junctions with the channel. As the source–drain voltage is increased, the depletion region gets bigger, until a point is reached when both halves meet – the pinch-off voltage. At this point the current through the channel is limited to those electrons which can sweep through the depletion layer – so there is no further increase in current with increase in source-drain voltage. (Other than when a gatechannel breakdown occurs.) Figure 2.31 shows the characteristic of the JFET for different levels of bias. 2.3.3.2 The MOSFET The fact that the FET works by the influence of an electric field raises the possibility that there need be no actual conduction path from the gate electrode. The Insulated Gate Field Effect Transistor (IGFET) uses this idea; in practice its manifestation being the Metal-Oxide Silicon Field Effect Transistor (MOSFET). In the manufacture of semiconductor devices, there are a number of processes used. The process starts with the wafer of pure silicon (Section 2.2.2) ID

VDS PINCH IDSS

VGS=0 -1V -2V -4V

Gate to channel breakdown

VDS

Figure 2.31 The JFET characteristic showing variation of drain current with drain to source voltage for different levels of gate bias. The dotted line shows the pinch-off voltage.

but this can then be treated in several different ways: Oxide growth. If the silicon is exposed to oxygen or steam it oxidizes, producing a layer of Silicon Dioxide (SiO2). This is an insulating and protecting layer. It can be grown and removed (by acid etching) at various stages of production. Epitaxial growth. This is growing more silicon crystal on the existing base. It is done by exposing the silicon to Silicon Tetrachloride vapor (SiCl4) which disocciates when it meets the hot silicon. If the vapor also includes an impurity (for example Phosphorus Trichloride) then an n or p type silicon is grown. Diffusion. The introduction of impurities as part of epitaxial growth is really only applicable to large areas. If small areas are to be treated, then the impurities enter the silicon by diffusion. The silicon is held near its melting point so the impurity atoms from the vapor can easily enter the crystal lattice. Areas which are not to be diffused are protected by an SiO2 layer, which itself is put down using a photolithographic process. Metal formation. In this part of the process metal is evaporated or sputtered on to the silicon to provide interconnection between parts of the semi conductor device, or to provide connection to the outside world. Again photolithography is used to create the protective layer needed to ensure the metal only goes where it is needed. From this brief description, the construction of the JFET shown in Figure 2.30 can be appreciated. So can the fact that SiO2 , being an insulator, allows the idea of a gate electrode which acts through an insulating layer to be realized. The MOSFET is, in reality, made in two ways, shown in Figure 2.32. This identifies two kinds of MOSFET. The depletion mode version, with a narrow channel, works in the same way as the JFET, but has a much higher input impedance. In the alternative (and, in practice, standard) enhancement mode version there is no continuous n-channel; if there is no bias on the gate, no current flows. Forward bias must be increased to a threshold voltage before conduction starts. At the threshold voltage an inversion layer is created under the gate electrode and (in the figure example) an nchannel is created.

77


S

G

D

Si02

G

p Substrate

Channel S

Sub S

Sub G

DRAIN CURRENT

D

n

D

Si02 n

D

n

G

p

Sub S

Sub

Figure 2.32 The top figure shows the construction of the depletion mode MOSFET and its symbol. The bottom diagram shows the enhancement mode MOSFET. Note how the symbols clearly show the absence of direct connections.

MOSFETs have a number of advantages over bipolar transistors: • Because they do not have minority carriers (i.e. all the current is carried by electrons or holes, but not by a mixture) there is no minority carrier storage effect, which delays switching times (see Figure 2.27). MOSFETs are therefore faster than bipolar transistors. • MOSFETs are voltage controlled, with negligible gate current. Their gain is much higher than that of bipolar transistors. • MOSFETs do not suffer from secondary breakdown effects; their safe operating area can be extended to be solely power limited. • This is because as the temperature increases, the bulk resistivity increases, so there is no thermal runaway. This has the subsidiary benefit that devices can be connected in parallel to increase current capacity,

78

GATE TO SOURCE VOLTAGE

Figure 2.33 Example transfer characteristics of an n-channel enhancement mode power MOSFET. The linear relationship between gate voltage and drain current make them ideal amplifiers. T is the junction temperature.

without the need for any kind of current sharing components. All these considerations mean that the MOSFET is the preferred device for power electronics. Figure 2.33 shows their transfer characteristic. The linear characteristic and high mutual conductance make MOSFETs excellent amplifiers, and their fast switching speed makes them good switches. The high gate impedance of MOSFET devices makes them susceptible to damage from electrostatic discharge. Some devices are fitted with internal protection, using back-to-back zener diodes. But in most cases it is necessary to ensure that they cannot be subjected to static electricity. 2.3.3.3 Power MOSFETs The construction of the standard MOSFET shown in Figure 2.32 results in a thin “horizontal” conductive channel with limited current carrying ability. It also has a low breakdown voltage, largely dependent on the thickness of the insulating SiO2 layer. While the construction is suitable for small signal work, power MOSFETs use a different construction shown in Figure 2.34. The inversion layer, and current flow, is


“vertical”. Because the conducting channel is formed within both p and n diffusions, devices of this kind are also referred to as being double diffused or DMOS devices. The figure also shows that this construction of MOSFET provides a drain-source p-n junction diode between the n epitaxial layer and the p layer. If this diode is forward biased (i.e the transistor is reverse biased) it conducts. Some circuit designs take advantage of this parasitic diode, especially for the diversion of reverse voltage surges. One of the most important MOSFET parameters is RDS(ON) this is the drain-source resistance when the device is switched fully on. In any high current application a high value of RDS(ON) results in inefficiency and a lot of heat. A number of different strategies are used to limit this internal resistance. Each construction can have some additional benefit, for example higher frequency operation, or the use of less silicon in its manufacture. In practice MOSFETs intended for power control are made up of thousands of MOSFET cells, all operating in parallel. The concept works because the positive temperature coefficient of resistance of the conducting channels ensures equal current sharing.

G

n channel

S

p+

n+

n epitaxy

n+ substrate

D Conduction by electrons

Inversion layer

D

G S

Figure 2.34 In power MOSFETs the conduction channel is vertical. The symbols + or - applied to n and p mean heavy or light doping with impurities. The figure shows an n channel MOSFET with conduction by electrons. Note that conventional current flow is in opposite direction to electron flow.

Source metallization

Silicon gate

Si02

P

P n epitaxy + n

y ax pit e n +

n

D

Silicon gate

Drain metallization

Parasitic diode p-n junction D

Figure 2.35 Power MOSFETs are made with a cellular structure. Examples are TMOS® (originally from Motorola, who no longer participate in this market) that uses rectangular cells (left), and the HEXFET® construction from International Rectifier with hexagonal cells (right.)

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2.3.4 The Insulated Gate Bipolar Transistor

G S

p+

An objective, which has only been realized practically in the last few years, has been to create a device which combines the virtues of the MOSFET, such as negligible input current drive and a wide safe operating area, with those of the bipolar transistor – in particular that of low saturation voltage. Such a device is known as an insulated gate bipolar transis-

n+

-

n epitaxy

p

p

Construction

D

Inversion layer

Conduction by electrons

Figure 2.36 CoolMOS® from Infineon (formerly Siemens) uses a different diffusion pattern to reduce the sourcedrain resistance. RDS(ON) rises linearly with blocking voltage, not exponentially as is the case with conventional MOSFET.

The gate connection is made of poly-crystalline silicon instead of metal; this is possible because the gate operates at high impedance anyway. Manufacturers use different cell geometries, and different cell sizes and spacing to achieve their claimed best result, but the principle is the same for all of them. Figure 2.35 illustrates typical cell construction. A problem with MOSFETs is that at high forward voltages the epitaxial layer must be thick. RDS(ON) rises exponentially with blocking voltage, which has limited the application of MOSFETs. A recent development, exemplifed by Infineon’s CoolMOS® shown in Figure 2.36, is the introduction of vertical p stripes into the epitaxial drift region. This has the effect of providing blocking not only in the vertical direction, but also horizontally, which in turn allows the layer to be made thinner. The construction is claimed to make the relationship between RDS(ON) and blocking voltage linear instead of exponential, resulting much smaller devices for a given power handling. MOSFETs made this way are claimed to offer competition to the IGBT described in the next section.

80

G

E

n+ substrate

p+

n+

n epitaxy

p+ substrate

D Conduction by electrons Conduction by holes Symbol

C G E C

Equivalent Circuit

G

E

Figure 2.37 The IGBT. The figure emphasises the point that conduction is by both majority and minority carriers. The equivalent circuit shows that the device is in principle a p-n-p transistor driven by an n-channel MOSFET, but that the construction introduces other parasitic devices.


tor or IGBT. The idea is similar to that of the Darlington pair, where one device is used to control another – in this case a MOSFET input transistor driving a bipolar transistor. The construction, symbol and equivalent circuit is shown in Figure 2.37, drawn to emphasize that conduction is bipolar. In practice the manufacture of IGBTs is a similar process to that of making MOSFETs. One manufacturer points out that simply by changing the starting materials and varying some process steps, it is possible to make IGBTs with the same photolithographic mask set as is used for making power MOSFETs. The IGBT is fast to switch on, but slower to switch off, with a current “tail”, due to minority carrier storage in the epitaxial layer.

At the individual component level there are exotic high frequency transistors used in RF applications, and special low noise transistors which are well outside the scope of this book. A useful variant encountered in lighting control is the photo-transistor, a close relative of the photodiode described in Section 2.2.6. By having a transistor as part of the diode circuit it becomes easier to set minimum reponse levels – for example the phototransistor can be biased so that it does not conduct at low light levels.

2.4 The thyristor, triac and GTO 2.4.1 The thyristor 2.4.4.1 Basic construction

2.3.5 Other transistors It is clear that transistors exist in many forms. Each of the major types can be made in several different ways to match a particular application. Many small and medium power devices include additional circuitry for protection or more sophisticated control. Such circuitry can be added by the manufacturing process – for example by diffusing on more conducting and insulating layers to make additional active components.

small signal transistor used in control circuits

MOSFET power transistor used in electronic ballast

Figure 2.38 Examples of transistors used in lighting control.

The MOSFET and IGBT have made big inroads into the power control business, especially where high frequency is concerned. However, the workhorse of electronic power control is the thyristor. The full name of the main variant is the reverse blocking triode thyristor. It is particularly significant in lighting control as the basis of electronic dimmers. The thyristor was invented at General Electric (USA) in 1957, some ten years after the invention of the transistor at Bell Laboratories. It was originally called the Silicon Controlled Rectifier, and the acronym SCR is still widely used. Figure 2.39 shows that the thyristor is a four layer device, with three electrodes anode, cathode and gate. Its symbol gives a clue as to how it behaves. In the reverse current connection (positive voltage applied to the cathode) no current flows, exactly as would be the case for a conventional diode. If the reverse voltage is greatly increased, then avalanche breakdown can occur, again exactly as for a diode. If the thyristor is forward biased, but no gate signal is present, no current passes under normal operation. If the forward blocking voltage is exceeded, the device will, however, go into conduction. The thyristor will also conduct if there is a positive gate current. Once the thyristor goes in to forward conduction, it continues to conduct, even without a gate signal, unless one of the following happens:

81


Anode

Anode

A

p n

G

Gate

Gate

p n

K

Cathode Cathode

Two transistor Model

A p

n

p

n

p

K n

current flow between anode and cathode is limited to the transistor leakage current. If a current IG is introduced at the gate, and if the gain in the basecollector loop of T1 and T2 exceeds unity, the loop current is maintained regeneratively, driving both transistors into saturation. At this point the thyristor is said to be latched. The gains of the transistors are current dependent, so once the gate current has started the regenerative action, the anode current increases sufficiently that the gate current IG can be removed without the transistors coming out of saturation. Thyristors are bipolar devices. They have the advantage of very low on-state voltage. However, minority charge carriers must be removed before the thyristor can block an applied voltage, so switching times are long compared with, for example, MOSFET transistors. This means that in practice thyristors are used at mains supply frequencies.

G

A

2.4.4.2 Ratings

T2

Figure 2.41 shows the characteristics of a thyristor. All thyristor parameters are temperature dependent, so, as with transistors and diodes, device data is given as a set of graphs showing how the device behaves at different temperatures. Section 2.2.3 introduced a number of parameters relevant to power control devices, a few more are introduced here. The rate of change of voltage or current, denoted by the differential calculus symbols dV/dt and dI/dt

G

T1

IG K

Figure 2.39 The thyristor. Construction, symbol and twotransistor model.

• either the current through the device drops below a minimum holding current. • or the voltage across the device is changed in polarity, when it will block the reverse voltage. Notice that, unlike a transistor, the gate has no effective proportional control of the current. If the gate signal is above a minimum, the thyristor goes to full conduction, with current limited only by the circuit impedance. Figure 2.39 also shows that the thyristor can be thought of as a pair of transistors, one n-p-n and the other p-n-p. They are connected such that the base current from one transistor is derived from the collector current of the other. If there is no gate current,

82

40A thyristor used in professional dimmer encapsulated module containing two thyristors triac used in consumer dimmer

heavy duty triac used in professional equipment Figure 2.40 Examples of thyristor and triac construction.


Iforward

ON

IL

IG>0

IH

Vreverse

IG=0 Vforward

Avalanche breakdown

Vbreakover

Ireverse

Figure 2.41 Thyristor characteristics.

respectively can be important. dV/dt, measured in Volts/Second, or, more usefully, V/μs, is relevant, since a thyristor can be forced into conduction by high dV/dt. This arises because p-n junctions have a capacitance, and with a change of voltage a capacative charging current flows: iC = C × dV/dt If dV/dt is high, the charging current may be high enough to trigger the thyristor on. When a thyristor is turned on, the whole active area cannot be turned on simultaneously. The area nearest the gate connection turns on first and conduction “spreads” across the device. If the anode to cathode current rises too quickly, only part of the thyristor may be conducting, and this causes localized heating, possibly followed by device failure. So an excessive rate of forward current change dIf /dt can damage a thyristor. On the other hand, provided gate ratings are not exceeded, a high rate of change of gate current dIG /dt helps a thyristor to turn on quickly. The protection of a thyristor by fuse or circuit breaker is determined by its I2t capability, a measure of its ability to let through energy (see also Section 1.5.2). For reasons which will become clear, the figure is usually quoted with reference to the duration of one half cycle of alternating current (i.e. 10ms for 50Hz or 8.3ms for 60Hz). For example, if a thyristor is quoted as having an I2t of 26A2 seconds on the

8.3ms basis (a typical figure for an 8A thyristor), then it could let through an average √3132 = 56A for a half cycle of 60Hz mains. This leads to another important characteristic which is the peak forward surge current. This is the maximum peak current that the device can stand as a single non repetitive event, and again is usually quoted with reference to a half cycle of mains current. For example the 8A device referred to in the previous paragraph might be able to stand a peak of 80A within a half cycle of 60Hz (provided that, over the whole half cycle, the I2t rating was not exceeded.) This is an important parameter for lighting control, because tungsten lamps have a low cold resistance, resulting in inrush currents which can be 10–14 times running current. While the major ratings of the thyristor are related to the anode-cathode current path, the gate has its own set of ratings in respect of maximum and minimum gate voltage and current; maximum and average gate power etc. 2.4.4.3 Control of AC power The most commonly used arrangement for thyristor AC power control is the back-to-back pair shown in Figure 2.42. If neither thyristor has any gate signal, then the pair block any current from going through the load. If both thyristors have a permanent gate “on” signal, then the load will receive maximum current – the current being limited by circuit impedance and the nature of the load itself, and only very slightly by

AC Supply

Firing control Circuit

Load e.g. Lamp

Figure 2.42 AC power control using a pair of thyristors.

83


V

Voltage across thyristor

Supply voltage

Current through thyristor

I

Trigger angle G

Conduction angle

Normalized voltage power fraction

Figure 2.43 Conduction angle in AC power control by thyristor.

1.4 1.2

Peak

1.0 RMS

0.8 0.6

2.4.4.4 The triac Average

0.4 0.2

0

20

40

60

80 100 120 140 160 180 Conduction angle

Figure 2.44 Variation of peak, r.m.s. and average voltage with firing angle for AC power control by a thyristor pair. The voltage scale is normalized to r.m.s. sine wave voltage, so to get an actual voltage, multiply by 230 for European supplies and 115 for USA supplies. The red line shows power output for a resistive load.

84

the thyristors (which have a forward voltage drop of 1 – 2V). One thyristor carries the positive going half cycle, and the other carries the negative going. If the gate signal is discontinuous, and is applied at some point in the half cycle, the appropriate thyristor starts by blocking current, but then switches on. The thyristor continues to conduct for the remainder of the half cycle, but then, as the polarity of the AC waveform changes, self commutates, i.e. it ceases to conduct because of the polarity reversal. The second thyristor similarly controls the other half cycle. Assuming that both thyristors are symmetrically controlled, it becomes possible to continuously vary the power to the load by changing the point in the AC cycle at which the gate “on” signal is given. This firing point is defined by the delay or trigger angle δ and the conduction angle α as shown in Figure 2.43. The “angles” relate to the rotating vector described in Figure 1.19. Figure 2.44 shows how peak, r.m.s. and average voltage varies with firing angle. It also shows how power output varies for a fixed resistive load. Note, however, that, a tungsten lamp does not have a fixed resistance, so a tungsten lighting power curve would be a little different. Nonetheless the fact remains that the majority of the power change happens between conduction angles 30° and 150°. Details of thyristor power control and firing circuits are given in Section 8.3.

The triac, also known as the bidirectional triode thyristor or the TRIode AC semiconductor switch is a five layer device as shown in Figure 2.45. It can be considered equivalent to a back-to-back thyristor pair in a single package. Because it can conduct current in both directions, reference to an anode or cathode would be meaningless, so the main terminals are designated MT1 and MT2. The triac can be triggered by current flowing into (+) or out of (-) the gate terminal. This results in there being four possible operating quadrants for the triac as shown in Figure 2.46. In practice quadrants 1 and 3 are the most sensitive. Quadrant 4 requires a higher


2.4.4.5 The GTO and other special thyristors MT1 n

Gate

MT1 n

p

Gate

n p n

MT2 MT2

Figure 2.45 The triac.

gate current than the others, so is not used. Many practical circuits operate with quadrants 2 and 3, since then a negative going trigger pulse is used for both halves of the AC sine wave. While triacs are convenient low power control devices, they are not as robust as the equivalent thyristor pair. There can be problems in recovering blocking capability, especially at higher temperatures or with high dV/dt.

+

2

+

MT2+

1

The conventional thyristor and the triac are widely used for lighting control and their application is discussed in more detail in Chapter 8. There are a number of other “special” thyristors or thyristor-like devices, which have application in other areas. A brief summary of some of them is as follows: • The gate turn-off thyristor or GTO has a gate arrangement whereby positive current into the gate switches the thyristor on, and negative current taken from the gate switches it off. In the two transistor model, the arrangement can be thought of as being one where taking current from the gate breaks the regenerative cycle. GTOs have good forward blocking, but relatively poor reverse blocking capability. • The photothyristor uses light to trigger the thyristor – in effect a p-n junction is used as a photodiode to provide the gate current. Such thyristors are ideal for high power work in electrically noisy environments. At the other extreme small photothyristors have taken over from transformers as control isolation components in medium power applications (including dimmers.) • The insulated gate controlled thyristor like its transistor counterpart, has reduced gate drive requirements while maintaining low forward volt drop at high voltages.

2.5 Analog and digital 2.5.1 Analog

IG

-

-

IG

G-

G+

IG 3

+

MT2-

+

IG

Figure 2.46 The operating quadrants of the triac.

4

It is already clear that, for lighting control, electronics can do two things for us. Power electronics can be used for the direct control of electric power, generally by using high speed switching techniques. In turn the power electronic devices can be controlled by very low power electronic control devices or systems. These help make practical systems by, for example, allowing long distance remote control, or by bringing some “intelligence” to the control. A dictionary definition of “analogy” is the “agreement or correspondence between two things otherwise different”. In analog electronic control the

85


correspondence is between an electrical quantity, usually current or voltage, and some other physical quantity. An example already cited is that of an audio amplifier; where the chain is: • a transducer (in this case a microphone) which turns sound pressure level into a varying electrical voltage. The voltage is thus the analog of the sound. • an amplifier which increases the electrical signal. In this case it is clear that the amplification process must be one which does not distort the signal. • another transducer (in this case a loudspeaker) which turns the amplified electrical signal back into an audible sound. The power output of the amplifier can be controlled by a simple variable resistance at the input, which directly limits the strength of the signal reaching the amplifier. But a more sophisticated method would be to have a voltage controlled amplifier or VCA at the input. Such a device would then allow long distance remote control of the sound level, without the need to take the sound signal itself to the level regulating control. The small DC signal controlling the amplifier is an example of an analog control signal (representing the sound volume) controlling another analog signal (representing the sound signal itself.) In lighting control we can use the same idea; where a small remote control potentiometer carrying a tiny current, might produce a voltage variation of 0–10V representing the desired lighting level. This is then applied to an electronic dimmer, sited at a place most convenient for electrical power distribution, which does the actual power control using components like thyristors or IGBTs. 2.5.2 Digital The analog signal is continuously variable between limits set only by the nature of the apparatus concerned. Our 0–10V signal can, if the equipment is of sufficient precision, be set to any fractional voltage required, for example 5.467V. The nature of a digital signal, on the other hand, is that it can only adopt discrete values. For example, the 0–10V signal might only be available in integral voltages, 0, 1, 2, 3...V etc.

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Despite the marketing man’s hype of all things digital, we actually live in an analog world. Our senses of sight, sound, smell and touch work on a continuous basis. In any practical digital system the divisions between the discrete levels must be so small that we cannot detect them. For example, on today’s CD (compact disc) 65,536 discrete sound levels are recorded between zero and maximum output in order to fool our ears that the sound pressure variation is continuous. In practice all digital electronic equipment works by using only TWO discrete levels. This has a number of advantages. In any form of signal processing, the analog signal can suffer in many ways: • electrical noise can be added to the signal, thus degrading its quality (resulting in noisy sound or flickering lights). • the signal can suffer from distortion. The description of transistors showed that their transfer characteristics are not always linear, so this is easy to understand. • the signal can suffer lack of precision due to the processing device. For example the transistor control circuit for a dimmer might give a different output at different temperatures (since all transistor characteristics vary a lot with temperature.) However, if only two signal levels are used, let us say 0V and 5V (OFF and ON; or 0 and 1), there is little likelihood they will be confused. If the detector of the ON signal is set to look for anything over, say 4V as ON, and anything else as OFF, it means that the cable carrying the signal can have a lot of noise on it without impairing the information carried. Distortion and precision (in level) are not an issue, because we are only looking for two quite discrete states. Many of the principles of digital electronics were understood in the 1920s and 30s; and early computers based on vacuum tubes were based on them. However it was the advent of the high frequency transistor and the integrated circuit which made digital electronics practical. The transistor’s ability to block a voltage on the one hand, and then to go to a saturated switched-on condition on the other, is the key to achieving the 0 and 1 conditions required by digital signals.


2.5.3 Binary “Digital” is derived from digit, our fingers or toes. It implies a method of expressing numbers or counting to base 10. Our normal numbering system is referred to as being of decimal notation, where each “digit” in a number can assume only one of ten values 0–9. Although not wholly impossible, it would be difficult to make a precision electronic device able to adopt ten different states. With a transistor this might involve working the transistor in inefficient intermediate states between off and saturation, resulting in the dissipation of a lot of heat. To eliminate this kind of complication, digital electronics uses the binary notation for counting, where each digit can assume only one of two values 0 or 1. Binary digits are referred to as bits. Table 2.1 shows the relationship between binary and digital notation. It also introduces the idea of resolution. For some tasks low resolution is quite sufficient; for example many lighting control systems are based on 8-bit resolution (256 separate levels.) However, if moving lights are digitally controlled, and are working over a 180° arc, 8-bit resolution results in noticeable “jumps” as the device moves; after all, each step is nearly a degree. Therefore a higher resolution is needed. In the CD referred to earlier 16-bit resolution is used; a new generation of audio disc (based on DVD technology) is using 24-bit. Digital signals can be presented in two ways. Parallel data presents all the bits simultaneously on the requisite number of separate signal cables. Computers usually have a parallel bus architecture, for example 8, 16, 32 or 64-bit, requiring the corresponding number of connecting wires between each part of the computer. Data is transferred between the different parts of the computer in small chunks referred to as bytes. In small computer and microprocessor work, 8-bit bytes are used and references to memory capacity in bytes assumes 8-bit bytes. In a computer, however, the architecture could use a 32-bit parallel bus for either single 32-bit bytes, or for four 8-bit bytes. Parallel operation is fine for use within the confines of a computer. It requires very precise timing, so the bus structure includes a clock line. This car-

ries a clock signal which defines the moment at which the main bus should be examined for data. It is not practical to extend parallel signal buses for any distance, because the timing between different bus lines can vary so much. For this reason any longer distance communication is done using serial data signals. Here the bits are transmitted in sequence, with some identifying signal to show the start of each byte. Groups of bytes are referred to as words. It is now clear that all digital electronics is based on numbers. If we need to process any information, or measure a physical quantity, or control any device, we must do it by numbers. Examples of code jargon that is frequently encountered in electronics are as follows. Binary Coded Decimal or BCD is a coding technique sometimes used for input and output devices. Here any number is stored as decimal, with one 8-bit byte allocated to each decimal digit. Only the first ten binary numbers, 0000 to 1001 are used. This is obviously wasteful for computing, but is convenient for some applications. For example illuminated number displays, like those used on petrol pumps or train indicators, might use BCD for each decimal

Decimal and binary equivalents 0 0000 1 0001 2 0010 3 0011 4 0100 5 0101 6 0110 7 0111 8 1000 9 1001 10 A 1010 11 B 1011 12 C 1100 13 D 1101 14 E 1110 15 F 1111

Each power of 2 adds one bit 1 bit = 21 2 2 bits = 22 4 3 bits = 23 8 4 bits = 24 16 5 bits = 25 32 6 bits = 26 64 7 bits = 27 128 8 bits = 28 256 9 bits = 29 512 10 bits = 210 1024 For example “10 bit resolution” can represent 1024 different levels

Table 2.1 The relationship between binary and decimal notation. The letters A-F against decimal 10-15 are the hexadecimal equivalent (see text).

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digit section of the display. Hexadecimal Code. Here all binary numbers 00000000 through 11111111 (0–256 Decimal) can be represented by a two digit number to base 16 (0– FF hexadecimal). Decimal 0–9 are designated as normal, decimal 10–16 are designated by letters A–F. Hexadecimal or “hex” is used by computer programmers as a shorthand for describing 8-bit bytes. It is much easier to recognize, and less prone to transcription error, than the equivalent binary string of 1s and 0s. ASCII Code (American Standard Code for Information Interchange). In its original form this represents: • the digits 0–9 • the letters A–Z upper case • the letters a–z lower case • punctuation marks • some symbols (e.g. +, $, %, @ etc). • 32 commands and identifying symbols. The symbols include items like STX or “start of text”. The commands include printer commands like CR or “carriage return”. All fit into only 7 bits (128 different characters.) So, as an example, the letter “Z” (upper case) has the code 1011010. (This binary number is also equivalent to decimal 90 and hexadecimal 5A.) ASCII code has been extended in both standardized and non-standard ways to cope with different alphabets and frequently used symbols. 2.5.4 Analog to Digital Conversion. ADCs and DACs Because the real world is analog many digital systems need a method whereby an analog signal is turned into its digital equivalent and vice-versa. For example a digital audio system must, at some point, create a signal suitable for connecting to a loudspeaker; or a light sensor detecting light levels may be required to convert its analog output signal into a digital signal suitable for computer processing. The devices required are called analog to digital converters (ADCs) and digital to analog converters (DACs). The ADC’s job is to turn a continuously varying

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analog waveform into a series of numbers. The process is illustrated in Figure 2.47. The idea is to end up with a set of numbers which, if plotted as a graph against time, would reconstruct the original analog waveform. The ADC process is to sample the waveform at frequent intervals, and convert the measured signal level (usually voltage) to the nearest digital value which can be stored; a process called quantization. It is clear from the figure that if only a few samples are taken per second, and if only a few values are allowed (e.g. only 4-bit quantization), and then a graph is plotted from these figures, the result is a “staircase” version of the original. This may be satisfactory for some applications, but is usually unacceptable. The performance of the quantization process is determined by two factors. The sampling frequency and the quantizing level. The quantizing level is the same as bit resolution described in 2.5.3. The requirement will depend on the application – many bits for critical applications like audio, less for lighting and video. The sampling frequency determines the highest frequency that can be sampled. It is intuitive that if one wants to sample an audio frequency of, say 9kHz, it will be necessary to use a sampling frequency which is higher than 9kHz – but how much higher? Successive digital bytes OUT

Analog In

1 0 1 1 0 0 1 0

ADC

1 1 0 1 0 1 1 0

1 1 0 1 1 1 0 0

t

t 1 0 1 1 0 0 1 0

1 0 0 1 1 0 1 1

1 1 0 1 0 1 1 0

1 0 0 1 1 0 1 1

1 1 0 1 1 1 0 0

DAC

Successive digital bytes IN

Figure 2.47 Principle of the ADC and DAC.

Analog Out


Signal IN analog

Read clock

Reference voltage

R 1 R 255 to 8 encoder

Potential divider

2

R 254 R 255

Digital out 8 bit

Differential comparators

Figure 2.48 One of the methods of creating an ADC. This is the “flash” converter.

Nyquist’s theorem shows that the the Nyquist frequency, the highest frequency which can be accurately sampled, is half the sampling frequency. Therefore a sampling frequency of at least 18kHz would be needed to convert the 9kHz audio. In fact, in the CD system, a sampling frequency of 44.1 kHz is used to achieve an audio bandwidth of 20kHz. In a lighting control system, where one might be using an analog device such as a slider or sensor, as input, the sampling frequency could be comparatively low. In a video system requiring digital video, the sampling frequencies are much higher (e.g. 13.5MHz). An ADC is a complex device which can only be realized practically as an integrated circuit. An example which is easy to understand (but uses the most circuitry) is the flash converter shown in Figure 2.48. The principle is that the incoming voltage is compared to a known voltage. An 8-bit (256 level) converter has a reference voltage source which is fed to

a chain of 256 resistors. Therefore the voltage at the end of each resistor is known. The device then contains no less than 255 differential comparators. These are not unlike the differential amplifier described in Section 2.3.2.5. They have two inputs, and in this case are designed only to give an output if the sampled voltage exceeds the reference voltage. As can be seen from the figure, the sampled voltage is fed to one input of all the comparators. The other input of each comparator is connected to one of the 255 reference voltages. The result is that, if a continuously varying waveform is fed to the device, there is a continuously varying pattern of “on” signals appearing at the outputs of the comparators. In fact each comparator is fitted with a switch controlled by a clock signal, so that the outputs are only “read” at the fixed sampling frequency. It is then a comparatively simple matter to convert the pattern of 255 comparator outputs to an 8-bit digital signal. The digital output is normally stored in some kind of output register, so it can be extracted when required by the device which is processing the data. The flash converter has been described because the concept is simple. “Slower” ADCs use “sample and hold” techniques (and much less circuitry) where the incoming voltage is momentarily sampled and “held” in a capacitor. The “held” voltage is then measured by using a single comparator and measuring the time taken for the capacitor to discharge to a reference voltage. The counter that does the timing produces a digital signal as output. DACs work on similar principles, but in reverse. For example by having available a number of binary weighted currents which are switched into a common load according to the digital signal. The selected currents are summed, and together represent the analog of the digital signal, Figure 2.49. This abbreviated description of DACs and ADCs is not intended to be definitive, but rather to illustrate how circuit building blocks can be used to meet a particular function. Both DACs and ADCs are widely used in electronics, but use significantly different constructions according to the bandwidth and accuracy required. Their use in lighting control is one of the less demanding applications.

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R R

R

R

R

R

R

R

OUT ANALOG

2R

2R 2R

0

1

2R 2R

0

0

2R 2R 2R 2R

1

1 0

Transistor switches operate as 8 bit signal

importance in lighting. Some light sources cannot be continuously controlled; for example attempting to vary the light output of an LED by simply varying the applied voltage does not work very well. However, LEDs can be switched on and off very fast. Human eyesight has the attribute of persistence of vision whereby, above Modulating waveform

AM

1

Resistor network gives binary weighting to current flow

Reference voltage

Figure 2.49 One of the methods of creating a DAC.

Constant frequency carrier

2.5.5 Modulation The word modulation in an electronic context refers to the means by which source information is modified or encoded in order to match the means of transmission or storage. An easy example to understand is amplitude modulation as used by AM radio. An AM radio signal may be transmitted by an electromagnetic wave of, for example, 600kHz. Clearly a continuous transmission imparts no information, but by modulating the 600kHz carrier, information can be conveyed. In AM radio this is done by the simple expedient of varying the strength or amplitude of the carrier by the analog of the audio signal to be transmitted. The problem with AM is that the signal strength must vary in order to send the information. More efficient use of transmission power, and a better signal, can result from methods whereby the signal strength is constant. One way of achieving this is by frequency modulation or FM. It is intuitive that, for such a system to work, the carrier mean frequency must be much higher than the modulating frequencies. Another possibility is to use some form of pulse modulation. One frequently used example is pulse width modulation or PWM. This is used in Class D amplifiers (Section 2.3.2.2). Again the pulse frequency must be much higher than the frequency of the signal to be transmitted. PWM is of considerable

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Frequency varies as moulating signal

FM

Constant amplitude carrier

Original waveform PAM

t Pulse amplitude corresponds to instantaneous value of original waveform, sampled at interval t

1

0

- Measure amplitude at intervals - Convert to binary code - Send as serial digital signal

PCM

0

0

1

1

0

1

0

t 1st sample

Figure 2.50 Methods of modulation.

1

0

1 1

2nd sample


a certain frequency, our eye/brain combination integrates successive light stimuli. This is the basis on which film and television works, whereby, for example, 30 frames are presented each second and we see a continuous image variation. The same principle can be used to, apparently, vary the intensity of light. Clearly if the pulse frequency is too low, we just see successive flashes of light. At intermediate frequencies we see flicker which is extremely uncomfortable. The subjective effect of flicker is not only dependent on frequency, but also on other factors such as ambient light level, contrast etc, but once the switching frequency is high enough, we simply see an apparently steady source of illumination. For lighting control purposes the effective use of PWM depends on the source being used, but would usually need a minimum frequency of 120Hz. An intermediate system of modulation is pulse amplitude modulation or PAM where the source analog waveform is sampled at discrete intervals. Instead of transmitting the complete analog waveform, sample pulses only are transmitted. In fact, such a method is rarely used, but it does form the basis of the commonly used pulse code modulation or PCM. In PCM the magnitude of each pulse is converted into a binary number – exactly the process already described for the ADC. Once we are in the digital domain, all kinds of modulation systems become possible; for example delta modulation uses only the differences between successive amplitude values in order to reduce the required bandwidth. Highly sophisticated methods are used to eliminate errors, and ingenious methods used to increase the data throughput of any transmission channel. But most digital modulation systems start with PCM, whatever method is then used to attach the resulting signal to the carrier medium.

Volt). Even more confusing, it seems to turn up in unlikely places. Most people are aware of it as a measure of sound, but, less obviously, it turns up as a measure of radiated signal strength and as a measure of performance of data cables, both of which are relevant to lighting control. Sound levels may also be relevant, especially if control equipment is fitted with noisy fans, or emits a buzz. The decibel or dB is a measurement of ratios. It was originally applied to sound where, because our hearing is logarithmic, it was easier to express changes in sound levels using the logarithm of the ratio of two powers. It is best to start with the acoustic definition. The Bel was the original unit, where: B = log10 (P2/P1) and P1 is the reference power P2 is the new power. log10 is the common (base 10) logarithm, where numbers are expressed as their equivalent as the power of 10. In practice the Bel was found to be too big for sound intensity (defined as the energy passing through one square meter normal to the direction of propagation) so the unit used is the decibel where: Sound Intensity in dB = 10 log10 (P2/P1) Here the power is in Watts/m2, and the reference power P1 is taken as the threshold of hearing, the lowest sound intensity which a healthy young human being can detect. It is usually taken as 10-12 W. Sound intensity is a difficult quantity to measure, so it is more usual to measure sound pressure level or SPL. But this is proportional to the square root of the intensity, so the equation becomes: SPL in dB = 20 log (ρ/ρr)

2.5.6 The decibel A unit frequently encountered in electronics and audio is the decibel. It is often wrongly treated as though it was a physical quantity, which can be measured in physical terms related to fundamental units (like a

Where ρ is the pressure and ρr is the reference pressure corresponding to the threshold of hearing – taken to be 20μPa. Table 2.2 gives some examples of sound pressure levels. The log10 of one is zero (i.e. 100 equals 1) so when ρ = ρr = 20μPa, we have a reference SPL of

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SPL in dB 0 20 40 50 60 70 80 90 100 110 120 140

Actual Example SP 20PPa Threshold of hearing 200PPa Background in recording studio. Quiet countryside. 2000PPa Whisper at 2m Average suburban area 6310PPa Conversation at 1m General office 0.02Pa Restaurant, store 0.063Pa Radio/TV in the home 0.2Pa Busy street 0.63Pa Heavy truck at 6m Symphony orchestra fff 2Pa Disco 6.31Pa Pneumatic drill at 1m 20Pa Modern aircraft at takeoff 200Pa Military jet at take off at 30m Threshold of pain

Table 2.2 Sound pressure levels.

0dB. It is now easy to see why the logarithmic method of measurement is convenient. The 140dB range of sounds covers our normal experience, with quite easy reference points over a reasonable scale – especially when you consider that we can only just detect a 2dB change in SPL. However 140dB represents a range of sound pressures with the ratio 10,000,000:1. Note that without a reference level, a dB figure cannot give us an absolute value for a quantity. As with the human eye, the human ear does not hear all frequencies equally. The phon is the unit of loudness. At 1kHz one phon is equivalent to 1dB SPL; so if SPLs are only measured at 1kHz, the loudness in phons is the same as SPL in dB; but at all other frequencies they are different. The phon is, therefore, analagous to the lumen, in that its value depends on a human attribute (sensitivity to wavelength of sound, instead of wavelength of light). The fact that our ear response to sound varies with frequency means that using linear measurements of sound is misleading. Noise and sound pressure level measurements are weighted to be meaningful. The

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most commonly used weighting curve is the A weighting curve shown in Figure 2.51. Measurements made using it are referred to as dBA. Where noise is an issue for lighting control equipment (e.g. in ballasts) it is usual to see the noise figure quoted in dBA. Early designers of audio amplifiers realized that when analog electric currents and voltages were used to represent sound, the dB notation was useful. In this way the effect of, for example, a 6dB change in signal level, gives a good idea of the aural effect, in a way which a statement of increase in voltage or power would not. When dB are applied to electrical quantities the relationship only holds good if the circuit impedance is the same for both values. Also, as with the acoustic dB, the relationship is Power ratio in dB = 10 log10 (P2/P1) where P1 is the reference power; but, because both current and voltage are proportional to the square root of the power, voltage or current ratios are defined by: Voltage ratio in dB = 20 log (V2/V1) Current ratio in dB = 20 log (I2/I1) A Weighting dB +10 0 -10 -20

-30 -40 -50 -60 10

20 30

50

100

500 1000

5000 10000 Hz

Figure 2.51 The A weighting curve for acoustic measurements.


The dB has been found to be useful for electrical and electromagnetic quantities other than those directly related to sound. So it is common to see figures for the attenuation of all kinds of signals down a cable being referred to in dB. Likewise measurements of electric field strength are conveniently referred to dB. “Electrical dB” must, like their acoustic counterparts, have a reference level. Some of these are shown in Table 2.3.

2.6 The integrated circuit and ASIC 2.6.1 Introduction Many electronic circuits are made up of discrete components. Often, in order to get a workable circuit, the circuits become quite complex. This is because a realworld circuit needs additional components; for example to compensate for variations in temperature, to eliminate the effects of voltage spikes, or to stabilize a power supply. When high powers are involved it is generally necessary to use discrete components, but when a circuit is only required to operate at low power, the idea of an integrated circuit becomes realizable.

Notation dBV dBmV dBPV/m dBW

dBm

Reference Level 1V (for voltage or current a 6dB change is approximately u2, and a 10dB change u3) 1mV 1PV/m (electric field strength) 1 W (for power a 3dB change is approximately u2, and a 10dB change u10) 1 milliwatt (in audio circuits dBm is usually referred to 600 ohms. At this impedance the equivalent Voltage 0dB is 0.775V)

Table 2.3 Some “Electrical dB” with comments.

The description of the MOSFET in Section 2.3.3.3 indicated how this might be possible. The MOSFET power transistor consists of hundreds or thousands of individual transistors all operating in parallel. But a similar technique can be used to create individual transistors, diodes, zener diodes, resistors and capacitors together with a circuit interconnection pattern. Within integrated circuits, capacitors and resistors of defined value are difficult to make. For this reason many applications of ICs require external passive components for setting precise operating parameters. Also there are many cases where it is easier to make an apparently more complex circuit, using many transistors and few or no passive components, than it is to make a simpler circuit using fewer transistors with passive components. Early integrated circuits were comparatively simple, providing a simple “building block” with the equivalent of only a few transistors. Such circuits are still an essential part of everyday electronics but, as production techniques have improved, the possible component density within integrated circuits has reached a previously unimaginable level. The largest integrated circuits consist of millions of transistors. A broad classification of integrated circuits, or ICs, is as linear and digital ICs. Linear ICs are mainly amplifiers, and can be considered analog devices. Digital ICs consist mainly of gates of different complexity, where the signal paths of the circuit assume only one of two binary values, high or low (also expressed as 1 or 0). Most references to ICs are to monolithic ICs, where the IC is made on a single chip of silicon. The chip is then packaged in a hermetically sealed plastic or ceramic package which includes the lead out connections. For some applications much larger hybrid ICs are made. These consist of an assembly of several chips and other components mounted on a ceramic substrate which is then sealed as a whole. Hybrid ICs are widely used in applications requiring both low level control signals and power signals; for example medium power audio amplifiers and engine management electronics. They also have a place in lighting control.

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+ Supply typical +15 V

Inverting input Output

+ Non-inverting input High resistance typical >2M :

Low resistance typical 75 : Supply typical -15 V

Open loop gain typical 200,000

Figure 2.52 The operational amplifier or op-amp.

2.6.2 Linear integrated circuits 2.6.2.1 Operational amplifiers Depending on the application, linear ICs are based on either bipolar or unipolar transistors. The best known example of the linear IC is the Operational Amplifier or, more familiarly, op-amp. The name comes from its original discrete component ancestor, used to describe amplifiers carrying out mathematical operations in analog computers. The op-amp is a high gain differential amplifier. Figure 2.52 shows the symbol, and it is easily identified as the basis of the comparator in Figure 2.48 and the output amplifier in Figure 2.49. An op-amp should have an output of 0V when both its inputs are at equal voltages, but in real world op-amps there is usually an input offset voltage to achieve 0V. Some op-amps have an input null facility to ensure that the 0V output condition is given when the inputs are equal. Most op-amp circuits use negative feedback. As explained in Section 2.3.2.2 this makes the gain of the resulting circuit independent of the open loop gain of the amplifier, which, in the case of op-amps, is very high. Figure 2.53 shows how an op-amp can be used as an inverting amplifier (where the output is of opposite polarity to the input) and as a non-inverting amplifier. In the case of the inverting amplifier having its

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(+) input set at 0V, feedback causes the circuit to stabilise with the (-) input being at 0V too. The circuit action is as if there is a direct path between the (-) input to the 0V rail. The (-) input of the op-amp is said to act as a virtual earth (ground). Apart from straightforward signal amplification, op-amps form the basis of many useful circuit elements. A few examples are: • as a voltage adder. In Figure 2.49 an op-amp produces an output equal to the negative of the sum of the input voltages in a virtual earth circuit. • as a subtractor, or straightforward differential amplifier, where the output is the difference between the two inputs. • as a voltage follower (similar to the emitter follower) where the voltage gain is unity. This is used as a signal buffer, with very high input resistance, and low output resistance. Its advantage over a transistor follower is that the output voltage is exactly the same as the input, whereas there is always an offset with a simple transistor. • as the basis of a constant current circuit. Here the op-amp controls the base current to a transistor. RF RA

VIN

VOUT

RB

+

voltage gain

0V

RF RA

Inverting amplifier

RF

VOUT VIN

RB

+ voltage gain RA

R A RF RA

0V Non-inverting amplifier

Figure 2.53 Operational amplifier circuits. In this case the power connections have been omitted to simplify the diagram.


15V RE

R

Constant current circuit

+ R

RL

0V

IOUT constant even if RL changes

15V R

Inverting Schmitt trigger

VIN

VOUT

VT

+ RF

R

When VIN=VT

Positive feedback

0V

2.6.2.2 Phased locked loop

First order active filter using non inverting op-amp RF VIN

VOUT R

+ C

0V

RA

R

threshold voltage. Such circuits are essential in digital applications where an input signal may be degraded or noisy, and where it is essential to achieve a definite “on” signal when a threshold is exceeded (and a similar “off” signal as the input reduces). Schmitt triggers can, of course, be made from discrete transistors, but the op-amp version can be designed with great precision in respect of threshold voltages and hysteresis – in this case the difference between the on and off triggering voltages. Schmitt triggers use positive feedback in the op-amp circuit to achieve the snap action. • as the basis of active filters. Section 2.1.3 described how reactive components could be made the basis of filters. In small signal work, high pass, low pass and band pass filters are often needed, and opamps can be used as the basis of active filters. These can be based on simple resistance-capacitance networks to select the frequency pass bands. They use selective negative feedback to eliminate the unwanted frequencies.

Another common linear IC is the phase locked loop, or PLL. This device is sometimes called the electronic flywheel, because it maintains an accurately timed output even if the input has some jitter or is occasionally discontinuous. It is particularly useful for maintaining accurate clock signals in digital systems. In serial data systems it is not practical to send

-3dB frequency fc then -6dB per octave

1 2 SfcC

INPUT Nominal f

Phase Detector

Amplifier + filter (damping)

Divide by N

VCO

Figure 2.54 Some applications of the op-amp.

Negative feedback ensures that the emitter current is constant, which in turn ensures that the current flowing in the collector load is constant – even if the load resistance changes. • as the basis of Schmitt trigger circuits. The Schmitt trigger is a device which produces a fast “switch” output when an input signal exceeds a set

OUTPUT F locked

OUTPUT NxF

Figure 2.55 The phase locked loop or PLL.

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a separate clock signal, so it may be necessary to derive a clock signal from the transmitted data. The PLL can do this. At its heart is a voltage controlled oscillator or VCO; this device is an electronic oscillator whose frequency is determined by a voltage control signal. The control voltage in this case is derived from a phase detector. This compares the phase of an incoming signal with that of the oscillator. The complete PLL provides a device which ensures that the output is at the same frequency and same phase as the input should be. The output is at a constant amplitude regardless of the input amplitude. By using an arrangement whereby the VCO operates at a multiple of the input frequency the flywheel effect is increased, so the oscillator continues to provide an output even if there are discontinuities at the input. 2.6.2.3 Signal switches Many control applications require devices which can route control signals. In audio-visual applications these could be audio or video signals (either digital or analog) and in lighting control systems it could be the routing of serial control signals or analog control signals. The idea that a transistor can be used to do this, as opposed to a relay, has already been introduced. The most common arrangement is the CMOS transG Negative gate signal p-channel MOSFET VIN

VOUT

mission gate. CMOS construction is described in the next section. Each transmission gate consists of a complementary pair of MOSFET transistors which together constitute a bi-directional switch. Control is by complementary gate signals applied to the two transistor gates. Note here the need to distinguish between the “gate” electrode of a transistor, and the function of a circuit block as a logic “gate”. It should be clear from the context which is being referred to. Such switches are usually supplied as integrated circuits containing many such transmission gates. These may be independent but, more usually, the circuit is a multiplexer where either many input signals can be routed to a single output, or a single input can be routed to multiple outputs. Such devices form the basis of electronic patch panels. The name derives from old telephone practice where exchange operators would manually “patch” one circuit to another by inserting cords with jack plugs into a routing panel. With integrated circuits of this kind important attributes are the fan out and the propagation delay. Fan out describes the number of connections which can be made at the output. Ideally it should be possible to connect as many as required, but this obviously depends on the loading. For this reason circuit elements take advantage of the MOSFET properties of high input impedance (meaning that they present a small load to the signal source) and low output impedance (meaning that they can feed many high impedance loads). In practice the input impedance is limited by capacitance, meaning that the loading is heavier than would otherwise be expected. Clearly the problem gets worse as the signal frequency increases. Capacitance is also primarily responsible for a slight delay in any signal going through the gate, so propagation delay can become an issue when a signal has to pass through several of them.

Symbol n-channel MOSFET G

Positive gate signal

Figure 2.56 The CMOS transmission gate makes a bidirectional switch. Here simplified symbols are used for the MOSFETs. See also Figure 2.66.

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2.6.3 Digital integrated circuits 2.6.3.1 Logic gates An extended description of binary logic operations is beyond the scope of this book. Only a simple example is described here, but from the example it


Name and Logic Symbol

NOT _ A

AND AxB

NAND (= NOT AND)

___ AxB

OR A+B

NOR (= NOT OR)

___ A+B

EX-OR (= Exclusive OR)

A B

EX-NOR (= Exclusive NOR)

____ AB

Circuit Symbol

Truth table A 0 1 A B 0 0 0 1 1 0 1 1 A B 0 0 0 1 1 0 1 1 A B 0 0 0 1 1 0 1 1 A B 0 0 0 1 1 0 1 1 A B 0 0 0 1 1 0 1 1 A B 0 0 0 1 1 0 1 1

Z 1 0 Z 0 0 0 1 Z 1 1 1 0 Z 0 1 1 1 Z 1 0 0 0 Z 0 1 1 0 Z 1 0 0 1

A S B

C0

Figure 2.58 The half-adder.

for the most common gates are shown in Figure 2.57. This shows that, for example, an AND Gate with two inputs has a 0 output if any of the inputs are 0; and ONLY has a 1 ouput if both inputs are 1. Using a combination of gates it is possible to carry out both simple and complex mathematical operations. The idea can be understood with reference to the commonest arithmetical operation, that of adding two numbers. Figure 2.58 shows a half adder which can add two inputs A and B. The “Sum” output S is determined by the Exclusive OR gate, which means that it will be 1 if A or B = 1; but will be zero if both A and B are 1 (or both 0). The parallel AND gate will give a “Carry” ouput CO only if both A and B are 1.

A B

Ci

S

Figure 2.57 The common logic gates shown for two inputs A and B. Z is the output.

should be possible to understand how much more complex arrangements could be built up. Logic gates obey very simple rules. All inputs and outputs can only adopt one of two values 0 or 1. The different type of gate, for example AND Gate, OR Gate etc. have different rules which can be expressed in a truth table. The truth tables and symbols

C0

Figure 2.59 The full adder.

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+V

S 1

0

1

J Clock In

Q

J

0

1

Q

J

Q

J

C

C

C

C

K

K

K

K

y2

1

R

Q

y4

y8

y16

Outputs

Q

Figure 2.62 Linking JK flip-flops together to make a

Figure 2.60 The SR flip-flop or bistable.

This arrangement is called a half adder because it cannot deal with a carried input from a previous stage of addition. The logic arrangement gets more complicated for a full adder. Figure 2.59 shows that one way to achieve it is by using two Exclusive-OR gates and four Not-AND or NAND gates. The logical expressions which describe the operation of gates are called Boolean Algebra. Each expression is quite simple, but highly complex operations can be carried out by using multiple gates. Integrated circuits are available that provide the common gate functions. Often there are several gates on one chip, or one chip provides a complete arithmetic function. Thus one can obtain a quad 2-input AND IC, which means that it contains four 2-input AND gates; or a 4-bit adder with carry. The manufacturing process used for such chips tends to favor the production of one kind of gate over another. It is easier to construct an IC with a large number of gates of a single kind than it is to construct one with gates of different J Q

4-bit counter/divider. By setting J = K = 1, each flip flop “toggles” on successive clock pulses. The arrangement can be used to divide down, or count the clock pulses. kinds. By using Boolean Algebra it can be shown that the half adder of Figure 2.58 can be made using five NAND gates. While this is more gates than the two-gate arrangement, it is actually easier to make as an IC. Note that gates are not confined to two inputs. A three-input AND gate will only give a 1 output if all three inputs are 1. A four input OR Gate will give a 1 output if any of its four inputs are 1. A special case arises when there is an element of feedback in the logic. This leads to monostable, bistable and astable elements. Of particular significance is the bistable element, often referred to as a flip-flop. Flip-flops can be made using a number of different gate combinations; an example that is easy to understand is the SR flip-flop shown in Figure 2.60 The Set-Reset flip flop can be considered as a four terminal device, with inputs S and R, and outputs Q and Not-Q. In the example using NAND gates, S and

Change only on falling clock pulse

Parallel Outputs

Serial data IN

Clock Q

D

D

D

D

C

C

C

C

K J=K=0

Q

1

J=0 K=1

Q

0

J=K=1

Serial Data Out

No Change

J=1 K=0

Clock

Outputs change to opposite sense

Figure 2.61 The JK flip-flop.

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Q

Figure 2.63 A shift register made up from D flip-flops.


R are normally high (1) and initially Q and Q are respectively 1 and 0. If R goes low (0) the device momentarily goes unstable, but settles when the gates’ logic truth tables are again satisfied resulting in Q now being 0, and Q being 1. The device stays in this state, even when R reverts to being 1. It can be seen that successive low (0) inputs on S or R cause the device to successively adopt one or other of the stable states. In practice flip-flops are used as the basis of registers and counters. Their bistable nature allows them to store numbers. However, when a number of them are used together, there can be timing difficulties, so practical flip-flops are more complex. An example is the JK flip-flop shown in Figure 2.61. Internally this consists of two sets of flip-flops, but externally can be considered as a clock controlled bistable. The idea is that the flip-flop can only change state internally on the rising clock-pulse, and whether it does so is determined by the state of the J and K inputs at the time. Once the change has been made, it does not matter that the J and K states then change again. However no output change (if there is to be one) is given at Q until the falling edge of the clock pulse. Whenever Q changes Q changes to be opposite to it. While the JK flip-flop introduces a delay, the delay is precisely determined by the clock. Thus when numbers are transferred between one set of flip-flops and another, it is always done with precision, with no danger that the temporary instability of an individual flip-flop as it changes over could affect the process. Figure 2.62 shows how JK flip-flops can be joined together to make a counter. Within integrated circuits it is possible to use simpler flip-flops called D or Data flip-flops. Here the output changes on the rising edge of the clock pulse (i.e. as the clock goes high.) It only changes if there has been a change in the data input since the last clock pulse. A common requirement is the shift register shown made up from D flip-flops in Figure 2.63. Here data shifts one step along the chain each time there is a clock pulse. It is clear that such a register could be used in several ways, for example:

• to convert a serial data stream into a parallel stream. In this case an 8 bit word could be clocked in to the register, and the parallel ouputs read once the whole word was in place. • to delay a serial data stream by a set time. In this case the output would be taken from the serial output shown, but the number of flip flops used would correspond to the number of clock cycles of delay required. 2.6.3.2 Logic gate construction In the above section the function of gates was described in logic terms, without reference to how the gate is actually made. It is quite possible to work the other way and, for example, to describe the electronic action of a circuit using two transistors, and to show that such a circuit is bistable. As already intimated, the actual logic gates created in integrated circuits are partly determined by the manufacturing process involved. Figure 2.64 shows that there is potentially a wide choice - and in fact the diagram has been simplified to show only the main families. The three main technologies are: • TTL or Transistor Transistor Logic. • CMOS or Complementary Metal Oxide Silicon. • ECL or Emitter Coupled Logic. TTL was the first logic family. It is fast, but is quite critical in its power requirements in that the supply must be kept within very close limits. For many applications it has given way to CMOS, as the

Unipolar FET

n - MOS

Bipolar

CMOS

Bi-CMOS Hybrid

Saturated

UnSaturated

TTL

ECL

Figure 2.64 The technologies available to make digital integrated circuits.

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INPUT A

Q1

+ VCC

Q3

Q2

INPUT

VDD

A B

Output Z = A B

B Output Z = A B Q4 0

0V

Figure 2.67 The NAND gate executed in CMOS.

Figure 2.65 The circuit of a TTL NAND gate.

latter’s performance has improved. Many ICs originally offered in TTL are now offered as “TTL compatible” CMOS. An example of a TTL gate is shown in Figure 2.65. This is a NAND gate, such that output is 1 unless both inputs are 1. Q1 is a transistor with two emitters. If both are set high (1) Q2 is switched on, resulting in Q3 being forced off, and Q4 on. The output is then low (0) fulfilling the NAND condition. If either input is low, Q1 conducts, the base of Q2 goes low, switching it off. The states of Q3 and Q4 are reversed, making the output high (1). The double transistor output arrangement is used to ensure that rise and fall times of the output are minimized – clearly the NAND condition itself is already satisfied earlier in the circuit. The complete circuit helps make the point that practical circuits are more comG

Polysilicon Gate D

S

n

G S

n

SiO2

p

p n well

p

n channel MOST

P Channel MOST

Figure 2.66 The structure of CMOS transistors. Although a common substrate is used, the use of “wells” allows both types of MOSFET to be made together. In this figure the drains are shown linked, as they would be in the CMOS inverter shown in Figure 2.68.

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plex than might be expected. CMOS is by far the most widely used technology for general purpose application. It can work at a wide range of voltages; from very low, making it suitable for battery powered equipment, to relatively high, making it suitable for signals in electrically noisy environments. CMOS gets its name from the fact that both nchannel and p-channel FETs are made on the same substrate. There are alternative ways in which this can be done, but the most common is the use of wells. For example n-well CMOS is made by starting with a p substrate and creating on it a number of n areas or wells. Within each well area a p channel FET is made; whereas on the ordinary substrate n channel FETs are made in the normal way. See Figure 2.66. A NAND gate based on CMOS has the outline circuit as Figure 2.67. This shows two n channel transistors in series, tending to pull the output down to 0, and two p channel transistors in parallel tending to pull the output up to 1. If both inputs are 1, the two series transistors switch on and connect the output to ground (low or 0). The other transistors will be off because their gate-source voltage is zero. In all other cases the output is high. At least one of the series transistors will be switched off, and at least one of the parallel transistors will be switching the output to VDD (high or 1). A simple CMOS element is the NOT gate or inverter. This can be achieved with a single pair of complementary transistors. In Figure 2.68 if the input is low (0) the effect is the p channel transistor switches on raising the output high (1). When the input goes high, the switching arrangement is reversed. Now the


+VDD

S p channel VIN A

VOUT

D D n channel

A

S

0V

Figure 2.68 The CMOS inverter or NOT gate.

p channel transistor switches off because its gate and source are both at VDD, and the n channel transistor switches on bringing the output low. This simple element forms the basis of a wide range of computer ICs because joining two inverters together creates a bistable. The principle is shown in Figure 2.69. In the complete D type flip-flop (or latch) additional transmission gates are shown. One provides the feedback connection to give the bistable action. The other lets in the new data. The transmission gates are clocked in antiphase, so the feedback loop is only disconnected when new data is clocked in. ECL is the fastest logic family. It operates bipolar transistors in an unsaturated regime to achieve very fast switching times, but requires high power. It is used in fast computers, but is not likely to be encountered in lighting control applications. The principal circuit element used is the long tail pair described in Section 2.3.2.5. 2.6.4 Application Specific Integrated Circuits Integrated circuits have evolved from circuits carrying a few gates and simple amplifiers to LSI and VLSI (Large Scale and Very Large Scale Integration) construction. A huge range of standard ICs is available; from gate assemblies to memories and complete computer devices (which are both reviewed in the next section). Many electronic designs can be realized using

“off-the-shelf” ICs in combination. However the result may not be cost-effective, or may represent a compromise. When large quantities, or special performance, are required, it may be better to use an ASIC or Application Specific Integrated Circuit. ASICs can be made in many forms. They can be digital, analog, mixed signal or hybrid. They can use different forms of construction depending on whether they are logic devices, signal processing devices or devices for power control. Section 2.3.3.2 gave a brief overview of the processes used in the manufacture of transistors. To the processes described there must be added ion implantation as a means of introducing the p and n impurities. This method is more complex than diffusion, but can give better distribution of the dopant which becomes very important at the the miniscule dimensions of transistors within an IC. Ions are simply atoms of an element with either an electron missing (positively charged ion – for example B+, a positively charged Boron atom for producing p silicon) or an additional electron, a negative ion. Ion implantation consists of placing the silicon wafer in a vacuum chamber and directing a beam of ions at it. Since the ions are charged, they can be attracted by an opposite charge. The beam can be controlled, and the dosage measured by measuring the ion beam current. Feedback

0 or 1

0 1 1 0 Principle of bi-stable Gate switches feedback loop

C Clock C

Bistable D Data IN

Gate switches data

Q OUT Q

Figure 2.69 The D flip-flop constructed from CMOS inverters.

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The production of ICs is, therefore, highly complex. Typically between 10 and 14 major process steps are involved in their production, each with many supporting steps, and each requiring very high repeat accuracy. Commodity ICs are cheap only because they are produced in huge quantities, and because one silicon wafer can carry maybe hundreds or thousands of identical chips. ASICs are produced as custom ICs and semicustom ICs. Custom ICs are designed from scratch and require a full knowledge of the semiconductor manufacturing process. Custom ICs of any complexity are extremely expensive in terms of their NRE or Non-Recurring Engineering charges, and in terms of development time. For this reason most ASICs are semi-custom. In this case the user does not have to know in detail how the device works. The manufacturer offers the user a set of cell libraries and a CAD (Computer Aided Design) program to enable him to design the ASIC round the standard cells. The “cells” are commonly used circuit elements. these could be as simple as a single transistor – but actually are more complex items, for example complete gates, bistables, multiplexers and input/output circuits. IC production involves the use of many different masks in the lithographic process, and the idea of semi-custom ASIC production is that several stages of manufacture are common to many different designs. The custom element of the design can often be restricted to the masks needed to create metalization layers providing the interconnection between circuit elements. Cell based designs have the cells in rows with channels between them. It is in the channels that the interconnections are made. For logic applications the customizing is made simpler in two ways. Most logic applications simply require standard gates; so in this case a gate array is produced. Large numbers of standard gates are produced in the lower level of the silicon, with the channel interconnections requiring only a few process steps. The idea is taken further by including through connections or vias in the gate construction. The idea is a miniature version of how multi-layer printed circuit boards are made. When this is done, there is no

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need to leave space for the interconnection channels, the resulting channel-less construction sometimes being referred to as a sea of gates. The interconnections are now on the top layers of the IC and are the only customized part of it. Where complex logic is required ASICs have, to some extent, lost out to the programmable devices described in Section 2.8. However, they are still very effective in high volume products of medium complexity, and are likely to be found in some lighting control products.

2.7 The microprocessor 2.7.1 Computers The idea of computers is not new. The first electrical or electronic computers or calculators were designed to carry out a fixed task, and were in some case successors to earlier purely mechanical devices – for example the simple cash register. The significant development, which occurred mainly in the 1940s, was the idea that electronics could not only make calculation much faster, but would allow the introduction of the programmable computer. An early office calculator is an example of a fixed program device. In it the data, the sums of money being entered, could be variable, but the processes applied to the data were fixed; for example simply adding up money totals. In a programmable calculator or computer the operations to be carried out on the data are themselves changeable or programmable; so the device not only works with stored data, it works under stored program control. So, for example, the data might be treated differently according to different input parameters. In a period of only a few years computers have become commonplace to the point where it is unlikely that anyone reading this book does not have access to one. For this reason it is easiest to start with describing what, until recently, would have been considered a very powerful machine. Today’s laptop computer has considerably more computing power than an office main frame computer of 20 years ago.


Monitor

Graphics controller I/O controller

Hard disk drive

Floppy disk drive

CD-ROM drive

DVD drive

HDD controller

FDD controller

CD drive controller

DVD drive controller

CONTROL BUS

DATA BUS

ADDRESS BUS

Microprocessor CPU

Address bus register

Internal bus Serial Parallel I/O ports

Program Stack counter pointer

Keyboard controller

Instruction and status registers

Data bus register

A A A A A A A A A A A A A A A A A A

Keyboard

A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A

Mouse

A A A A A A A A A A A A A A A A A A A A A A A A A A

Accumulator

General register

Control

Clock

mouse

Figure 2.70 Block diagram showing the principal components of a typical personal computer system.

Figure 2.70 is a simplified block diagram of a typical personal computer system. Within a computer most data is transferred by a parallel bus, since this gives a higher speed, and is practical over the short distances involved. Usually the data is carried over more than one bus, typically: • a control bus used by the central control to give control commands to the peripheral devices. • a data bus which carries data relevant to the application. This could be data being read off a hard disc drive, for loading in to the Random Access Memory or RAM. • an address bus. This carries the addresses of memory locations, much like a postal address. If the central control places an address (for example a location in the RAM) on the address bus, then the data currently on the data bus gets read in to it. The keyboard and mouse are the main methods by which the user communicates with the computer. These create alphanumeric, symbolic and positional data. Some applications benefit from additional or alternative input devices; for example a joystick or a

graphics tablet. For many applications, especially in the fields of automation and even lighting control, it is necessary for the computer to be able to communicate with other external devices, and it does this through the Input-Output or I/O Port. In a personal computer the I/O port typically provides two serial ports and a parallel port. The parallel port is usually used by a printer. The serial ports are usually EIA32 (see Section 9.3) although for special purposes other ports, for example the IEE1394 Firewire® high speed digital video port, might be fitted. Many computers are also fitted with USB (Universal Serial Bus) ports that provide much higher data transfer rates than the “traditional” serial ports. All activities within the computer are closely regulated by the central clock. Every time data is transferred, or an address read, it is always done synchronized to a clock cycle. This way it is known that a byte of data on the data bus has been transferred only to the address on the address bus, and has not been cut in half! But the outside world is not synchronized to the

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clock. So the I/O port is fitted with a Universal Asynchronous Receiver Transmitter or UART. The UART is a special type of data register. It receives data from the outside world and stores it until it receives the instruction to put it on to the data bus. Similarly for outgoing data it receives data from the data bus, and sends it out at the correct speed for the external serial data link. The computer is fitted with several memory devices of varying capacity and speed of access. The ones identified in the figure are: ROM or Read Only Memory. This is an IC with permanent data which cannot be changed. It is used to store the program which starts up the computer when it is switched on, and various other housekeeping routines. The start-up routine is sometimes referred to as the bootstrap or booting routine (derived from the expression to “pull oneself up by one’s own bootstraps”). RAM, already referred to, is the computer’s working memory. It is a collection of memory chips that allow data to be transferred very quickly. The RAM will normally hold both the current application program and its associated data – but if it is not big enough, data must also be exchanged with the hard disk. In the 1960s a main frame office computer was considered powerful if it had 8 kilobytes of RAM, resulting in an enormous amount of data shuffling between the RAM and external tape stores. Today even a modest personal computer may have 128 Megabytes of RAM or more. Hard disk drive. This device stores large quantities of data; for example all the different applications programs currently needed by the user, and all their related data files. Data is stored magnetically on one or more magnetically coated discs, using the same principles as magnetic recording tape but with much higher storage densities. 20 years ago a hard disc drive storing a few MB was very expensive; today a drive storing several GB is standard. Access to data on the hard disk drive is not as fast as it is to RAM, but is still respectably quick, whereas access to data on any kind of external drive is comparatively slow.The hard disk drive is a permament part of the computer and, except in special applications such as video servers, is not usually exchangeable since the

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storage medium is an integral part of its construction. The Floppy Disk Drive on the other hand is intended for data exchange, allowing computer files made on one machine to be used by another, and allowing data to be backed up so it can be recovered in the event of computer failure. Typical floppy disks have a capacity of 1.44MB. While in the early days of personal computers a floppy disk holding a few hundred kB was considered adequate, it is now the case that floppy disks are suitable for simple alphanumeric data, but inadequate for applications involving graphics and mixed media. Many types of high capacity drive exist, based on either magnetic tape or on some form of optically read disc. The CD-ROM drive is currently the most common large capacity device for data exchange. The CD-ROM itself works on the same principles as the audio Compact Disc (CD) but with additonal error correction algorithms to ensure the integrity of the data. Originally only available in ROM form, it is now available as a writeable medium, either write once for data which must be protected or re-writeable. The CD drive has a capacity of 650MB per disc and will remain a standard for some time, but is already being challenged by the DVD or Digital Versatile Disc. This takes the basic idea of the CD further by storing the data at a density unachievable when the CD was invented. In its first form DVD has a typical storage capacity of around 5GB, but multi-layer and double sided versions are forecast to increase this greatly. Most DVD drives fitted to personal computers also read CDs. In order that one can see what is going on in the computer there needs to be some kind of display. In personal computers this is provided by a monitor. Traditionally these have been based on a variant of the Cathode Ray Tube or CRT as used in TV sets. Portable computers, and, increasingly high end desk top computers, use flat screen displays usually based on Liquid Crystal Displays or LCDs. The display is served by a special memory device or graphics controller. This is a high speed RAM devoted to supporting the display. The display itself is capable of displaying a defined number of pixels or picture elements, for example an XGA display


shows 1024 × 768 = 786,432. In a high specification computer each such pixel needs 24-bits of data to specify its color and brightness (8-bits for each of red, green and blue). The computer itself, referred to as the Central Processing Unit or CPU, is centered round the arithmetic/logic unit or ALU that performs the calculations. It is supported by a number of registers and temporary memories. The number and size of these is determined by the computing power needed, but always includes items like the program counter which keeps track of where the computer is in the program routine it is supposed to be carrying out, the instruction register, which holds the current instruction for the ALU, and the accumulator which carries the data being currently operated on by the ALU. Today the entire CPU of a typical personal computer is carried in a single microprocessor that embodies the ALU and all its associated control and register devices. Early personal computers needed a chip set at their heart since it was not possible to get all the central functions on to a single chip. 2.7.2 Computer programs The complexity of a computer arises from the incredible speed it operates at, and the huge amount of data it manipulates. However, every single operation is very simple, consisting as it does of moving binary data from one store to another, and carrying out a simple mathematical operation while doing so. A computer program is a series of instructions executed in a set order. Often the instruction set changes as a result of either the result of a calculation being to tell the CPU to jump to a different part of the program, or of an interrupt, an external signal which takes priority. Each instruction is itself no more than a set of binary numbers which itself specifies a simple action, for example: “Load the accumulator with contents of memory address BB29” (here the address is given in hexadecimal, since this is much easier to write, and less prone to transcription error, than the actual binary address which would be expressed as two 8-bit bytes 10111011 00101001). Or another example: “Subtract 34 from the contents of the accumulator”.

When a computer program has been written as a series of binary or hexadecimal instructions, it is said to have been written in machine code. In low cost microcontrollers with limited memory capacity, or where a high speed of program execution is needed, it may be necessary to write the program this way. However, for most applications this would be tedious. The next step up from machine code is an assembler language. This simplifies the programming process by using mnemonics and labels. Thus the mnemonic DEC might mean “decrement the contents of the accumulator by 1”, or the label KEYC, identify the memory address of the C character on a keyboard. Both machine code instructions and assembler languages are specific to particular microprocessors. Again, assembler language can be appropriate for programming microcontrollers, but is still tedious for sophisticated applications. There is a need for a high level language. This takes the mnemonic idea much further by allowing programming commands to be entered in text form. For example a program line might consist of: 44 LET Lumens = Effic*Watts Here the number 44 is simply the line number in the program. A new variable called “Lumens” is being defined by the instruction LET, and itself is the product of two variables which have already been defined. If they have not been defined the program would stop. In this case the existing variables are “Effic”, the efficacy of a class of lamp, and “Watts” the power rating of a particular lamp. When a high level program is written, the result is source code. This includes not only the high level language instructions, but also plain text remarks which explain what each instruction is intended to do. This makes de-bugging, the removal of program errors or “bugs” easier, and is essential when a team of programmers is at work, or when programs have to be changed some time after their first issue. If a program has been created using a high level language, there is a need for an interpreter. When the program runs, the interpreter looks at each

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program line and converts it to the equivalent machine code instruction. For some applications running programs with an interpreter is satisfactory, but clearly it is an inefficient way of doing things. This is particularly the case when some simple command is repeated many times. So when it comes to running complex programs the procedure is to use a compiler. A compiler is a computer program that takes the high level source code and converts it directly to machine code. It does this as a separate operation. Once a program has been compiled, the final program installed by the user is a machine code program, and thus highly efficient. However, the development of compiled programs is a complex task, since the programmer doesn’t know whether his program works or not until it is compiled, and if it then fails to work, he can have some trouble working out what the problem is. Actual compilers, for example a compiler to convert a program written in the high level language C++ into a form suitable for running on an Intel Pentium™ microprocessor, have sophisticated tools to help the programmer analyze what is happening. The microcontrollers referred to in Section 2.7.4 might only run a single program. However, personal computers run several programs concurrently. These are in three categories: • The ROM based programs which get the computer under way, usually including the Basic Input Output System or BIOS and the booting routines. Programs which are carried in ROM (and its programmable variants) are referred to as firmware, distinguished from hardware, the physical construction of the computer device, and software, the changeable applications and operating programs not permanently resident. • The Operating System which determines how the computer carries out general tasks, particularly how it operates its memories, displays and Disc Operating System or DOS. Currently the best known operating system is Microsoft’s Windows®. But there are many others, some of which are more suitable for networked applications or real time tasks. • The Applications Program. For most users this is the only one that matters, because this is the one car-

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rying out the specific task, whether it be word processing or doing a lighting design. 2.7.3 Memories One of the keys to the success of microprocessors has been the incredible pace of development in semiconductor memory technology. There is now a wide range of memory types, each with its own acronym. The main ones are summarized in Table 2.4. Semiconductor memories are defined as volatile or non-volatile. A volatile memory is one which loses all its data when the power to it is disconnected. A non-volatile memory retains its data even when power is disconnected. Memory chips are available in a wide range of sizes, from 256 bits up to 256 Megabits (and beyond.) They come in different configurations to match different bus arrangements, so, for example, a 16,384 bit memory might be organized to store 2,048 8-bit bytes. The concept is shown in Figure 2.71. This greatly simplified diagram shows a 4 bit address bus able to address 16 different memory locations. When a location is addressed, it is connected to the 8-bit data bus. In a ROM all that can happen is

Acronym RAM DRAM SRAM S-DRAM VRAM ROM PROM EPROM EEPROM Flash E2 FIFO

Full Name Random Access Memory Dynamic Random Access Memory Static Random Access Memory Synchronous Dynamic Random Access Memory Video Random Access Memory Read Only Memory Programmable Read Only Memory Erasable Programmable Read Only Memory Electrically Erasable Programmable Read Only Memory see text abbreviation for EE as in EEPROM = E2PROM First In First Out

Table 2.4 The acronyms used to describe the most common types of semiconductor memory.

Memory locations

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

ADDRESS DECODER

Address bus


Data bus

Figure 2.71 Principle of the semiconductor memory.

that the data in the memory location can be read on to the bus. In a RAM, data can be read from the location, but can also be written into it, over-writing any previous data. RAMs operate in a read-write cycle, being either read-enabled or write enabled. In the former case its data is put on the data bus, in the latter it takes data from the data bus. One way to achieve a ROM is to have a single transistor at each cross point in the diagram, as shown in Figure 2.72. When the address decoder makes a memory location line go high, the transistor switches on and puts a signal on the data output line. Clearly if all the transistors in one location switch on the only output that can be given is 11111111. In a mask programmed ROM the manufacturing process includes a step whereby designated transistors are disabled, so each memory location can give any required pattern of 0 and 1. Mask programmed ROMs are expensive to set up, but cheap to manufacture in large quantities. In a PROM the same principle is used, but this time the user can decide which transistors are to be disabled. This is done by selectively applying a high voltage to the collector of transistors which are to be disabled. This then blows a fusible link in the emitter connection. The standard PROM is referred to as a one time programmable or OTP device. Both the masked ROM and the PROM are clearly non-volatile devices, but once data has been written in, it cannot be changed. During the development of

a product the ability to change data is essential; for example while debugging firmware. Even when products are fully developed, the ability to change the firmware to introduce new features can be important. The EPROM allows this. In this case the transistor is a MOSFET with a difference. It has a “floating gate” which can be charged up by applying a reverse voltage to the drain. The reverse voltage pulse is sufficiently big to give some of the electrons carrying the current enough energy to pass through the SiO2 layer and to charge up the gate. The negative charge is then “trapped” and will stay there for years. In Figure 2.73, G1 is the normal gate connected to the memory location line, G2 is the floating gate. If G2 is negatively charged it has the effect of increasing the gate G1 threshold voltage from around 1V to 5V. This effectively means that the transistor is OFF, and stores a 0. However, the charge on the floating gate can be erased by exposing the transistor to intense ultra-violet radiation; allowing the EPROM to be re-programmed. PROM and EPROM programmers are standard devices designed to transfer data into non volatile memory as firmware. For short and medium production runs it is quite practical to program devices as part of product production, and both PROMs and particularly EPROMs are widely used in lighting control equipment. Data bus Address +V

1

0

1

1

0

0

1

1

Figure 2.72 The operation of a ROM or EPROM, showing how a transistor is used to indicate a 0 or 1 at a particular memory location.

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S

D

G1 G2

n

n

SiO2

P

S

D

G1 G2

n

n

SiO2

p

Figure 2.73 Construction of the eraseable transistor memory cell. The top diagram shows the EPROM transistor, and the bottom diagram shows the EEPROM transistor.

Both devices require programming to be done as an off-line process, which may not be convenient. The EEPROM allows the programming to be done in situ. In this case the layer of SiO2 between the second gate and the drain is made very thin, less than 10nm compared with around 40nm for normal gate. The charging of the gate is now achieved without the need for such a high energy. When the programming voltage is applied to the drain, quantum mechanical tunneling (taking advantage of the fact that, like photons, electrons have a wave nature) gets the electrons through the insulator. Electrical erasure is achieved by applying a pulse of opposite polarity to the drain, a procedure which is obviously much quicker than taking the chip out of the equipment to flood it with UV. “Traditional” EPROM and EEPROM need quite long writing cycles, and are not intended for frequent

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re-writing. In the last few years great advances have been made in programmable memories, and this has led to the introduction of Flash memories. In principle these are similar to EEPROM, but are intended for frequent re-writing, and have a comparatively fast write cycle. Within computers standard RAM is volatile, and only carries data when the power is on. It is much cheaper to make and is much faster than any kind of EPROM, because more memory cells can be packed on to a smaller area of silicon. There are many varieties of RAM, the main ones being: DRAM or dynamic RAM. Here the 0s and 1s are stored as capacitive charges, which affect the gating of a transistor. However, these charges can leak away, so DRAM has to be continually refreshed. A typical 16 Mb DRAM may need 1024 refresh cycles every 16 milliseconds. There are a number of different architectures for DRAM, but a typical DRAM memory cell uses two transistors. SRAM or static RAM. This has the advantage that it does not need the refresh activity. It is based on flip-flops, described in Section 2.6.3.1, so a typical memory cell needs four transistors. The circuitry is more complex within the SRAM, but the external circuitry is made much simpler. In many applications requiring non-volatile memory, battery or capacitor backed static RAM is used where EPROM is not appropriate – where, for example, the memory contents changes as part of the equipment operation and the need is to guard against power failure and protect the current contents of the memory. If capacitors are used, they are very high capacitance, of the order of 1F, but only rated at 2.5 or 5V. However, this technique is giving way to the use flash memory. VRAM is an example of a special kind of RAM, referred to as a dual port RAM. VRAM itself is video RAM, and is the kind of memory used in graphics controllers to support the display screen. The need for dual port operation arises because the data coming in to the memory is regulated by the computer clock, so comes in at the convenience of the CPU. However, the data going out, representing what is to be seen on the display screen, needs to go out at timings determined by the display itself. Thus the memory must be of a kind where writing and


reading can take place simultaneously, and with different timing. 2.7.4 Micro-controllers Microprocessors have now developed so far that there is a great difference in capability between the extremes of the market. A lot of modern electronic products, including all sophisticated lighting control devices, are based on microprocessors. However, the microprocessors used are not often the same kind used in computers. They are more usually microcontrollers. Microcontrollers do not need all the paraphernalia associated with a full personal computer, but do need to be optimized for the task in hand. A typical microcontroller includes, all on one chip: • a CPU (of limited capacity compared with a personal computer’s CPU). • clock circuitry (but usually an external crystal is used to provide the clock master timing). Typical microcontrollers operate at a clock frequency of 1– 20MHz (compared with 500MHz to 3GHz for a personal computer). • provision for program memory. This is loaded by the user, using OTP, EPROM or flash programming. Typical program capacity is 4–16KB. Many smaller microcontrollers are made with mask programmed ROM once product development is complete. • RAM, typically in the range 128 bytes to 2kB. • ability to address further external memory, up to 64KB. • parallel I/O port, typically 32 bit wide. • serial I/O port(s) providing UART like facilities • provision for, typically, 4–32 interrupt sources. In a lighting control device an interrupt could come from the zero crossing of the AC sinewave, or as a signal from a presence sensor. • (sometimes) 4–8 analog I/O ports. • one or more timers. One of these is usually a watchdog timer which issues a signal if processor activity ceases for any reason. Programming of microcontrollers can be done using high level languages with suitable compilers, and is assisted by various forms of emulator. This

can be a software model of the microcontroller running on a larger computer, or can be an in-circuit emulator, which allows an actual microcontroller to be used while at the same time allowing the programmer to see what is happening within it. Such programming often requires the examination of the contents of individual registers and data streams. All microprocessors have an instruction set which cover their basic operation. For each operation there is an operation code or opcode. Microprocessors used in personal computers have quite a large instruction set which simplifies programming, but uses up processing power and slows down operation. The microprocessors used in microcontrollers are often examples of RISC Processors, or Reduced Instruction Set Computers.

2.7.5 Programmable Logic Controllers Computers, microprocessors and microcontrollers live in a world of their own, where everything happens very fast at low voltage and low current. But in the real world there is a need to interface with other devices. Within a product a microcontroller might be required to control a power device – for example an electronic dimmer might use a small microcontroller, in turn interfaced by suitable isolating components, to a power device such as a thyristor or IGBT. Within a system, as opposed to an individual

Figure 2.74 An example of a programmable logic controller.

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product, there is often a requirement to carry out logical sequences, but in a difficult electrical environment. For this purpose the Programmable Logic Controller, or PLC, has been developed. These devices vary in complexity, but generally embody the following features: • a central processor module, usually based on a microcontroller; but in the bigger models based on a PC core. • industrial modular construction; suitable for an industrial environment. • all solid-state with non-volatile program memory. • input and output modules giving a high level of isolation, and a range of power handling capabilities. PLCs are used in industry for the control of all kinds of industrial processes. They are also used for lighting control, usually when combined with the control of other items.

2.8 Programmable devices 2.8.1 FPGAs The different concepts and technologies which have individually given us the microprocessor, the standard logic chip and the programmable memory can be used in other ways to produce various kinds of programmable ICs. For many applications such devices can be used in place of ASICs. Because they are “off the shelf” and are well supported by high level programming tools, they give product designers a quicker way to market, and lower overall costs, even if the cost of the IC itself is higher than its ASIC equivalent. The devices work by providing the user with a number of logic gates, input and output lines and, in the case of the larger devices, a multiple bus structure. The biggest devices have sections that are optimized for use as local memory, or even as complete microprocessors. The way in which the various elements can be connected together represents the “programmable” aspect of the device. In the simpler de-

110

vices standard EPROM one time programming, the blowing a fuse to make a disconnection, is used; but this may be accompanied by the use of programmable anti-fuses to make a connection. The technique here is to create vias between two conducting layers separated by a very thin insulating layer. The insulation can be locally destroyed by the application of a strong enough electric field. In the more advanced devices, transistors are used to determine the signal routing. The transistors are arranged to operate collectively as a static RAM which is loaded on power-up from a non volatile memory (usually a set of flash memory on the same chip). Different manufacturers use different processes and architectures to achieve what they consider to be the optimum combination of capacity, flexibility and speed. Many different acronyms have been used to describe programmable devices. Most powerful ones are now collectively referred to as FPGAs or Field Programmable Gate Arrays. While the basis of the FPGA is quite simple and easily understood, the huge number of combinations possible means that the only practical way to program a big one is using computer aided design. The manufacturers provide the necessary programming tools, that also ensure that design rules are followed, and the timing of signals taking different routes through the array can be accurately predicted. Otherwise there could be the problem that data which should be “in sync” actually emerges from the FPGA out of synchronization. Small programmable devices offer a limited number of useable gates, and are similar in cost to EPROMs. Large programmable devices offer several hundred thousand gates, but may cost hundreds of dollars. FPGAs are used in logic intensive operations requiring high speed real time operation. An example of practical use is in digital video and graphics image processing. So, for example, big LED video screens make extensive use of FPGAs to convert the incoming digital video signal into the huge number of individual control signals needed to provide PWM control to vary the brightness of each individual LED.


2.8.2 DSPs The Digital Signal Processor is a special kind of microprocessor optimized to the real time processing of signals which until recently were analog. Many analog circuit elements, especially filters, have a digital counterpart, whereby the digitized signal is operated on mathematically to achieve the same effect. For this reason one of the most widespread uses of DSPs is in professional (and high end consumer) audio equipment. Obviously, if this idea is to work, the processes must take place at frequencies very much higher than those being treated. DSPs are also used in such diverse applications as motor control and image processing. DSPs are designed to carry out repetitive tasks with maximum efficiency. Many applications require the same calculation (with varying input data) to be carried out at very high speed, so some of the features likely to be included are: • the ability to deal with both fixed point and floating point calculations, in order to ensure that maximum possible accuracy is maintained, while at the same time not getting hung up on out of range results. • additional arithmetic hardware. This is likely to include one or more hardware multipliers to ensure

that multiplications can be carried out in a single clock cycle. The conventional ALU has to carry out many cycles to achieve multiplication. Much if not most DSP work involves successive multiplication (but this is not exclusive to DSP, FPGAs can also be programmed to carry out hardware multiplication). • additional bit manipulation hardware, for example the barrel shifter used to instantly shift bit arrays to left or right. This might be used to ensure that the results of floating point calculations are conformed for the next process (e.g. an audio signal might have been processed using 40-bit floating point calculations to minimize added distortion, but need conforming to a 16-bit output to match CDs). • additional buses, for example a “results data bus”, to allow simultaneous complex operations to be executed in a single clock cycle. This chapter has presented a very brief review of the basis of modern electronics relevant to lighting control. It is amazing that the behavior of different kinds of slighly impure silicon joined together can give us everything from an electronic switch, capable of switching a Megawatt, to a single memory chip able to hold the contents of a book – but that is now the case. Modern lighting control is taking selective advantage of electronic developments to create new products and new control concepts.

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Chapter 3


Everyday

lamps

Part 2 – Lamps The next three chapters describe the different kinds of lamps or “light sources” likely to be encountered. It is increasingly the case that the lightsource determines the method of control, so a good understanding of how lightsources work is essential for understanding lighting control design. Here sources are divided into “everyday lamps” of the kind that most of us are already familiar with; arc lamps which until recently were only used in industrial applications but which are now encroaching on many commercial applications, and special purpose lamps.

3.1 Non electric lighting In his book Lighting Historic Buildings (see Reading List) Derek Phillips reminds us that our forbears managed without electric light, yet were often very skilled at producing surprising and functional lighting effects. Architects understood how daylight worked, and designed their buildings to use daylight effectively. Today the effective use of daylight is once again considered important, on the grounds of both energy efficiency and creating pleasant places to live and work. Lighting based on flame, technically pyroluminescence, has been the only source of “artificial” light available to the human race until recently. Light from flames arises from the high temperature chemical reaction between oxygen and the “fuel”; energy is released from the excited atoms and molecules, and some of it is in the form of light. The amount depends both on the temperature and the fuel being burnt. An acetylene flame does not give much light compared with the input energy – as little as 0.2 lm/W. By comparison, the old style flashbulb (which works on the basis of combustion) using zirconium as a “fuel”, gives 56 lm/W. Fires were found to be inefficient sources of light, and the idea of using a concentrated fuel for lighting purposes resulted in the candle and the oil lamp. The arrival of gas lighting in the 19th Century

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brought entirely new concepts to lighting. The most obvious is the idea that the fuel for lighting is supplied from a distance; in practice both much more efficient and safer than carrying a fuel reservoir next to the lamp. Less obvious are two other points: • the invention of the gas mantle. This device, placed at the point of gas ignition, creates an intense white light; much brighter than the simple gas flame. The mantle, coated with rare earth salts, exhibits fluorescence, and converts some of the thermal radiation from the gas flame into visible radiation. The mechanism is similar to, but not the same as, that of the fluorescent lamp. • gas lighting permitted remote control of lighting. Admittedly pilot lights had to be used to ensure that any gas always ignited; but in principle many of the ideas we have about lighting control today were understood in the age of gas. Light from electricity was demonstrated in the early 19th Century, just as gaslight was becoming established. However its early manifestation was the carbon arc lamp. This source of illumination went on to be widely used where high brightness sources are required – for example searchlights, film projectors and film set lighting; but was impractical for “everyday” lighting. The carbon arc is created by placing two carbon electrodes together, passing a high current between them, and drawing them slightly apart. Light is radiated by various mechanisms. Principal is incan-

EVERYDAY LAMPS

descence, arising from the high temperature of the electrodes, but in addition there is luminescence from the vaporized electrode material and its combination with air.

3.2 The incandescent lamp. Practical artificial electric lighting had to wait until the late 1870s, when Joseph Swan in England and Thomas Edison in the USA invented the enclosed incandescent lamp. This solved the problem of high temperature material catching fire in the atmosphere. The incandescent lamp, or, more correctly, the incandescent filament lamp was, until comparatively recently, the main electric light source. For many people, especially in the home, it still is. Its great flexibility, variety of possible designs, pleasing light and ease of control mean that it will continue to play an important part in the lighting world. Incandescent lamps consist of a filament mounted either in a vacuum, or in an inert gas (one which does not chemically react with the filament). Early electric lamps used a carbon filament, but this was both fragile and inefficient. Tungsten is now the main filament material because it is easily worked, has a low vapor pressure and has a high melting point. The melting point of other metals is generally too low to produce useful incandescence while solid. Light is produced by passing an electric current through the tungsten filament and using the i2R heating effect to raise the filament temperature sufficiently to produce incandescence. While tungsten’s behavior when heated is similar to that of a blackbody (in that it produces a continuous, but not uniform spectrum of light) its total radiation is considerably less than that of a blackbody at the same temperature (see Figure 1.52); and anyway only a fraction of the total radiation of a blackbody can be in the visible spectrum. At its melting point of 3,655K tungsten has a luminous efficacy of around 53 lm/W. Practical tungsten lamps have efficacies in the range 8–30 lm/W depending on their construction and the filament operating temperature. A 100W general service lamp operates with a filament temperature of 2,700K, giving 12.5 lm/W. Photoflood

lamps may operate at 3,400K, giving around 30 lm/ W, but with a very short life. The tungsten filament fails mainly due to evaporation. The same energy which is exciting the tungsten to emit light can also impart sufficient velocity to some molecules that they escape – and generally appear as a blackening on the bulb wall. Evaporation can be reduced by having other molecules, in Optional silica diffuser coating

Glass envelope

Filament Molybdenum support

Nickel lead wires

Dumet wire

Glass pinch seal Cement Fuse in glass sleeve Screw base contact Exhaust tube Glass insulator

Coiled filament

Solder contact

Coiled coil filament

Figure 3.1 Construction of the general service tungsten filament incandescent lamp. Most use the coiled coil filament and have a gas filling. Low wattage lamps use a simple coiled filament in plain vacuum. Dumet wire is a compound iron/nickel/copper wire suitable for making a gas tight connection through glass.

113


the form of a gas, present to exert a pressure to prevent the escape. The choice of gas is a compromise. If its thermal conductivity is too high, it simply conducts heat away from the filament to the lamp wall, reducing the light output. It needs to have as high a molecular weight as possible to do its job. It must not ionize under normal conditions within the lamp. In practice the economic gas filler choice for most lamps is argon, with some nitrogen to reduce the tendency to ionization. Such ionization is particularly likely to happen when the filament breaks, resulting in a high arc current and quite possibly a shattered lamp. For this reason general service lamps are fitted with a fuse in the glass filament support. In order to reduce its overall length, and to minimise the need for supports (which conduct heat away from the filament) the tungsten filament is coiled. A filament which might need to be 90cm (3 ft) long can be tightly coiled to have a length of 5cm (2 in). This coiling also helps reduce the conduction of heat away from the filament by the filler gas. Still greater efficacy is achieved for medium power lamps by using a coiled coil construction. Figure 3.3 shows the relationship between nominal voltage, lamp life and light output for incandescent lamps. From this it is clear that life can be greatly extended by “under running” the lamp, but the light output drops by a much greater percentage than the voltage drop. For purely decorative applications this may not matter – but for any application where the cost per lumen is important, in terms both of power used and lamp replacement costs, it is better to run lamps at rated voltage. General service tungsten filament lamps are designed for a nominal life of 1,000 hours. Rough service, and “long life” lamps are rated for 2,000 hours or more, but have significantly lower efficacies. The resistance of tungsten varies greatly with temperature (see Figure 1.6). The cold resistance of a lamp can be around 14 times lower than the resistance at rated voltage, resulting in a high inrush current for the first few cycles after switching on. It takes between one tenth and half a second for the current to drop to the running value. In practice circuit impedance, and the impedance of the supply, reduces

114

Figure 3.2 The higher the molecular weight of the gas filling, the longer the life. The lamps illustrated above, which are incorporated in the Lumisphere X-24 Low voltage low voltage festoon system use a Xenon filling. Whilst the filling is too expensive for “everyday” domestic type lighting, it is justifiable for use in inaccessible places where extremely long life is essential. The life of these 24V 8.5W lamps is 20,000 hours and the glass lamp is mounted within a protective polycarbonate globe; the whole unit having an IP 68 protective rating.

the actual peak current; however control equipment, especially dimmers, should be designed to withstand the full inrush current. Most general purpose tungsten filament lamps are designed to run directly on the available supply – whether it be 12V in an automobile, 115V in the USA, 100V in Japan or 230V in Europe. The higher voltage lamps must use a thinner filament for a given power rating which has two disadvantages; the obvious one of fragility, and the less obvious one that the filament structure is less compact, and, therefore, less suitable for optical focusing. Tungsten filament lamps running on line voltage are easily and effectively dimmed. When used for decorative schemes, dimming can considerably prolong lamp life, not only by virtue of the “underrunning” of the lamp, but also because the “soft start” that can be imparted by automatic dimming reduces thermal shock. The quasi blackbody radiation characteristic of tungsten filaments results in a color temperature change as lamps are dimmed. In most practical applications this is not a problem, since the resulting “warmer” light seems subjectively more appropriate at lower light levels.

EVERYDAY LAMPS

180

300

160 200

140 Percent 120 Current 100 Power Lumens 80

100 Percent Life 80

Current

60

er Pow

40

60 ens Lum

20 40

50

60

70

40

Lif e

80

90

100

110

20 120

130

0

Percent nominal voltage

Figure 3.3 The relationship between light output, current, input power, lamp life and operating voltage for a typical tungsten lamp. The discontinuity in the life curve is due to a change of scale in the graph below 100%.

One problem can arise in systems using thyristor dimmers. These “chop” the mains power supply, and the resulting steeply rising waveform can cause a corresponding mechanical vibration in the filament. An audible “lamp sing” can be the result, particularly if the lamp and luminaire combination has a corresponding resonant frequency. The problem is worse on high current lamps, and as a result is more noticeable in 115V systems than it is in 230V systems. The only practicable cure is substantial filtering (discussed in Section 8.3). Infra-red lamps are a special variant of the tungsten lamp. All incandescent lamps emit a lot of infra-red anyway; so in appropriate ratings they represent an efficient source of heat. The infra-red lamp, made in ratings from around 150W to 5,000W, is especially convenient for some industrial, commercial and consumer applications, especially where heat must be directed or is required rapidly (e.g. in paint curing plants etc). Infra-red lamps have a long life, because at the temperature they are running, the tungsten vapor pressure is low.

3.3 Tungsten halogen lamps A heated tungsten filament radiates energy over a continuous spectrum in the range 300nm to 2,000nm, but the proportion radiated in the visible spectrum (say 400nm to 700nm) is dependent on the filament temperature. The nearer the filament is run to the melting point of tungsten, the greater the efficacy but the higher the rate of evaporation. The question arises, is there any way to minimize or even reverse the evaporation process? The answer lies in the idea of a regenerative cycle, where any tungsten that does evaporate from the filament is somehow returned to it. Tungsten halogen lamps have a proportion of halogen vapor in their gas filling. Halogens make up the highly reactive series of elements which include chlorine, bromine, iodine and fluorine. In the lamp the halogen reacts with the tungsten vapor to form a tungsten halide. This is gaseous, provided the temperature remains above 250°C. When the halide circulates to the filament area, the heat of the filament

115


Incandescent lamp

Relative energy

Daylight for comparison

IR UV

VISIBLE 400

500

600

700

800

900

1000

1100

I.R. Lamp

Figure 3.4 The spectral power distribution curve of an incandescent lamp shows a preponderance of infra-red. Infra-red lamps, such as the example from Osram shown, are made to produce minimum visible radiation, and to have a long life – in this case 5,000 hours.

breaks the halide down, and the tungsten is deposited back on the filament. This process allows the filament to be run a higher temperature, typically 3,000K, and gives a longer life, typically 2,000 hours at rated voltage. Indefinite life is not achievable, mainly because it cannot be assumed that the re-deposited tungsten will obligingly always land on the weakest parts of the filament. Tungsten halogen lamps are very compact because, in order to maintain the 250°C vapor temperature, the filament must be near the outside wall. This construction has the added advantage that filament cooling by gas convection is reduced or eliminated, and the small overall volume allows the use of the more expensive, but more efficient (because heavier) Krypton as a filling gas. The high temperature, and the corrosive effects of the halogen, mean that ordinary glass cannot be used. The envelope is, therefore, made of fused quartz. This presents some difficulty in making the external electrical connection since any disparity in thermal characteristics between the quartz and the lead-in wires causes the envelope to break.

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Molybdenum meets the requirement, but has the problem that at above 350°C it oxidizes easily, resulting in a mechanical stress at the external connection. Luminaire design must take this into account, and usually involves some kind of heatsink at the lampholder. The tungsten halogen re-generation cycle only operates at filament temperatures above 2,000K. This clearly has some implications when lamps are dimmed; it could be inferred that lamp life might actually be reduced when the lamp is dimmed. In practice this appears not to be the case, presumably because the rate of filament evaporation is greatly reduced anyway at the lower temperatures. Some experts recommend that, in installations where lamps are maintained at a low light level for long periods, they are periodically run at full output for a short time to activate the regeneration cycle. Certainly this has the effect of removing any tungsten blackening on the bulb wall. In an installation like a hotel public area, where the lighting might be bright at breakfast time and dim in late evening, any possible problem is eliminated automatically by virtue of the daily programmed cycle. The higher filament temperature, and the use of quartz as an envelope, results in the tungsten halogen lamp radiating a significant proportion of ultraQuartz envelope

>250K

Wall zone Formation zone

3,000K

Filament zone

Tungsten filament (W)

W 2I A A WI2 w 2 I A WI2 A I2

Figure 3.5 Simplified diagram of the tungsten halogen cycle. Here Iodine (I) is shown as the halogen, but this is only an example. The inert gas Argon (A) acts as catalyst, but it is known that impurities in the tungsten and in the filling vapor also play a part in the cycle.

EVERYDAY LAMPS

double ended for flood light low voltage, pinbase

low voltage reflector spotlight

mains voltage decorative

mains voltage reflector spotlight

Figure 3.6 Examples of tungsten halogen lamps. Photos from Osram.

violet. For most applications this is undesirable. It could be a safety hazard, and it definitely hastens the bleaching of pigments. For this reason most tungsten halogen luminaires or lamp assemblies are fitted with a clear UV filter. A comparatively recent development is the “doping” of quartz to render it UV absorbing so some tungsten halogen lamps are available which do not need filters. Mains voltage (110–230V) tungsten halogen lamps have traditionally used linear filaments and have had a double-ended construction. This makes them particularly suitable for floodlight applications, in a wide range of ratings typically 300W–1,000W, but available down to 60W, and up to 2,000W and more. This construction is not suitable for spotlights, so compact-filament single-ended halogen spotlight lamps operating at 12V or 24V are used for this purpose. Ratings are in the range 20–75W. The low voltage operation requires a transformer, which must be sited near the lamp in order to minimize the length of the high current cable run. Common practice for multi-lamp installations is to use one transformer for several lamps. Recent developments in filament manufacture have resulted in a new range of mains voltage compact spotlight lamps. These are not of the same dimensions as their low voltage counterparts, but, nonetheless, allow the creation of compact luminaires without the need for transformers.

For many architectural applications the lamps are supplied with an integral reflector. There can then be some confusion as to the merits of different kinds of reflector. “Cool” reflectors have a dichroic coating to reduce the amount of infra-red radiated by the lamp; but this means that the excess heat goes out the back of the lamp. This may be undesirable since it could over-heat the luminaire or the built-in transformer. Alternative “hot” (actually aluminum) reflectors reflect all wavelengths so the infra red accompanies the visible radiation. This identifies a problem area with lamps. Superficially similar lamps can have different beam angles, wattages and reflectors. A good lighting design can be spoilt by poor maintenance when the wrong lamp from a lamp family is fitted as a replacement. The most recent development in tungsten halogen lamps is the introduction of the halogen infrared lamp. This is a little more “bulbous” than the usual lamp, and has an axial filament with the return lead running down the center of the filament to produce a single ended lamp. The inside of the bulb wall has a thin film coating designed to transmit all visible light, but to reflect all infra-red back onto the filament. This technique can result in an increase in efficacy of as much as 37%. For example a 60W lamp in

Aluminum reflector reflects IR resulting in lower luminaire temperature

Dichrioc reflector transmits IR resulting in lower beam temperature IR

Visible IR

Visible

Figure 3.7 Tungsten halogen reflector lamps are available with two different kinds of reflector. It is important that the correct replacement is used.

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Construction Conventional incandescent Standard tungsten halogen Halogen-IR

16000

14000

Lumens 930

75

1030

60

1110

Life 2000 hours 2500 hours

Efficacy 10.9 lm/W 13.8 lm/W

3000 hours

18.5 lm/W

Table 3.1 Comparison of ca 1,000 lumen PAR38 lamps using different filament technologies. Acknowledgement to Osram Sylvania (reporting to the Lightfair® 2000 Annual Lamp Review).

12000

a conventional tungsten lamp in a number of different physical sizes, of which the most widely available is the PAR38. This is supplied in a range of wattages and beam angles; with either conventional filaments or tungsten halogen inserts. It is interesting to compare the performance of different versions, as in Table 3.1. In the case of the halogen-IR version the lamp designers have a choice of pushing for more lumens/ W at the expense of life, but the table shows a dramatic increase on both fronts.

10000

Spot

8000 Candelas

Watts 85

7000 6000 Wide spot

5000 4000

Floods

3000

IR reflected back to filament

2000 Wide floods

1000

90

60

30

0 Degrees

30

60

90

Figure 3.8 The luminous intensity distribution from variants of the same power reflector lamp. The graphs here are representative of the performance of different 12V 50W lamps. The result of fitting incorrect replacements can be to ruin the lighting design.

“standard” tungsten halogen might give 16.5 lm/W, whereas its halogen-IR equivalent can give as much as 24.2 lm/W. The new technology has shifted the balance of lumen output, wattage and lamp life. An example here is the popular PAR (Parabolic Aluminized Reflector) lamp. This lamp was originally produced as

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Optical coating

Visible light transmitted

Figure 3.9 The principle of the halogen-IR lamp (above) and an example MR16 lamp from Philips using the technology (below). MR stands for Multifaceted Reflector, and its most popular size is MR16.

EVERYDAY LAMPS

3.4 The fluorescent lamp The fluorescent lamp is the most familiar of the large class of lamps referred to as discharge lamps. In these lamps light is created by an electrical discharge within a gas or vapor. The fact that an electric or electromagnetic field within a gas can create light is manifest in the natural world both in lightning and in the “Northern Lights” (Aurora Borealis). In all cases the light creating mechanism relies on the electrical or electromagnetic power input being used to raise the energy levels of electrons. As they fall back to their normal level, they emit quanta of radiation. Most discharge lamps have a significantly higher efficacy than tungsten lamps, and many, but by no means all, have significantly longer lives. The construction of the standard fluorescent lamp is shown in Figure 3.10. Standard straight tubular lamps are designated by their nominal length, which includes the thickness of standard lamp holders, and by their diameter. The diameter is specified as a “T number” which is the diameter measured in eighths of an inch (never mind the metric system! but an eighth of an inch is just over 3mm). So a T12 tube is 1.5in (38mm) diameter, a T8 (currently the most common) is 1in (26mm) diameter and a T5 is five eighths of an inch (16mm) diameter. The smallest diameter tubes generally available are T2 (7mm), but these are classed “special purpose” in this book. At each end of the tube are electrodes, referred to as cathodes. In fact in an alternating current circuit each electrode alternates as cathode and anode, but the emphasis on the name cathode identifies the prime function of emitting electrons. In the normal hot cathode fluorescent lamp (cold cathode lamps are discussed in Section 3.6) each electrode is a tungsten filament of special construction coated with alkaline earth oxides. When heated to around 1,100°C the cathodes emit copious electrons. An anode can attract these electrons, creating a current between anode and cathode. In the tube itself there is a gas filling. The active constituent is mercury vapor at low pressure (around 1.07 Pa or 0.00016 lb/in2) with the addition of Argon (or a mixture of inert gases) at around 200 Pa to help the initial discharge. When a fluorescent lamp

is in its running condition, with an AC supply connected to either end, there is an arc discharge along the length of the tube, and the mercury vapor ionizes. The combination of the ionization itself, and the excitation of the mercury atoms produces electromagnetic radiation at a number of wavelengths – the principal of which are 254, 313, 365, 405, 546 and 578nm. The majority of these wavelengths are outside the visible spectrum and are ultra-violet. So the fluorescent tube uses a two-stage process; the inside wall of the tube is coated with a phosphor. Phosphors are inorganic crystalline compounds with a small proportion of metal, known as the activator. An example is calcium halophosphate with antimony as the activator. Phosphor science is an advanced form of cookery, and much research has been done to identify the phosphors giving the highest efficacy and best color rendering. Modern fluorescent lamps use a mixture of phosphors. The ultra-violet radiation causes the phosphor to fluoresce. The phosphor’s outer electrons are Tube

Length

Nominal Power

Tube Current

T12 T12 T12 T12 T12 T8 T8 T8 T8 T8 T5 T5 T5 T5 T5 T5 T5 T5

2,400mm 1,800mm 1,500mm 1,200mm 590mm 1,800mm 1,500mm 1,200mm 590mm 438mm 1,449mm 1,449mm 1,149mm 1,149mm 849mm 849mm 549mm 549mm

100W 85W 65W 40W 20W 70W 58W 36W 18W 15W 80W 35W 54W 28W 39W 21W 24W 14W

950mA 800mA 670mA 430mA 370mA 700mA 670mA 430mA 370mA 310mA 540mA 170mA 450mA 170mA 325mA 165mA 295mA 165mA

Table 3.2 Examples of standard linear fluorescent tubes in the range 550mm–2,400mm (1.8ft–8ft).

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Lamp cap

Double or triple coiled filament with oxide coating

Glass tube

Electrode shield

Bi-pin connector

Mercury vapor + inert gas

Coating on inside wall

Glass/metal seal

Figure 3.10 The standard fluorescent lamp.

stimulated to a higher energy level by the ultraviolet, but due to loss of heat energy, the quantum emitted when the electron drops back to a lower level has a visible, longer wavelength than that of the ultraviolet radiation. Fluorescence is the light emitted while the radiation is present, phosphorescence is light which continues to be emitted after the radiation is removed. The phosphors used in fluorescent lamps exhibit little phosphorescence, but it is sufficient to maintain a subjectively even light output as the current through the tube varies due to the AC supply. The cheaper halophosphate phosphors have a CRI of around 56. Halophosphate performance degrades comparatively quickly with time. Tubes with superior color rendering (CRI 80–85) and much better lumen maintenance use triphosphor technology. Intermediate tubes use a mixture of triphosphor and halophosphate, Figure 3.11. Triphosphors work on the basis of 3-color mixing (as opposed to a continuous spectrum). Three different rare earth phosphors are used; each one having an emission band which has a narrow peak. The peaks are around 610nm (red) 545nm (green) and 450nm (blue) close to the CIE tristimulus values. For CRI better than 85 multi-band phosphors are used, but the better color rendering (CRI > 90) comes at the expense of efficacy. Besides being classified by their color rendering abilities, fluorescent tubes are classified by their correlated color temperature. The standard whites are as follows: warm white CCT 3,000K white CCT 3,500K cool white CCT 4,000K daylight CCT 6,500K

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The spectrum achieved by some typical fluorescent lamps is shown in Figure 3.12. Fluorescent lamps can convert about 20% of their input energy to visible light, and have an efficacy of between 25 and 80 lm/W, depending on arc length, phosphors and power supply frequency. The way in which energy is used in a typical lamp is shown in Figure 3.14. The fluorescent lamp is a constant voltage device. For a given tube length, the voltage across the arc remains the same, regardless of the current. After the initial arc has been struck, more and more atoms ionize and, without something to prevent it, the current would build up until the circuit fuse failed or the tube exploded. Discharge lamps need, therefore, a device to limit the current through them, and this is referred to as a Sn 02 Conducting layer AL2 03 Barrier layer Halo phosphate Triphosphor blend

Figure 3.11 Fluorescent tubes may have up to four internal coatings. A transparent conducting layer to aid starting, a transparent barrier layer to prevent contamination of the phosphors by the glass, a halophosphate layer and a rare earth triphosphor layer.

EVERYDAY LAMPS

Warm white halophosphate

Daylight multi-band Relative energy

Relative energy

Relative energy

Warm white tri-phosphor

Wavelength

Wavelength

Wavelength

Figure 3.12 Examples of the spectra achieved by typical fluorescent lamps. From Osram. Spectra are diagrammatic only.

ballast. Ballasts are of two principal kinds, electromagnetic (described in Section 6.3) and electronic (Section 7.2). In this section we only discuss the simplest type of electromagnetic ballast to help describe how a fluorescent tube works in practice. Clearly the use of resistance to limit current would be wasteful, so inductive reactance in the form of a choke is the most common way of limiting current. The simplest and most common form of fluorescent lamp circuit, using switched start is shown in Figure 3.15. When power is applied, current flows through the choke, one cathode, the starter, and then through the other cathode. In this configuration the cathodes incandesce, and are hot enough to emit sufficient electrons for the discharge. Phosphor drops back in stages one of which emits visible quantum

UV stimulates phosphor to higher levels

100% electrical energy in 3%

Emits UV quantum

Excited mercury atoms electron drops in level

After a short time the starter switch opens automatically. This causes a high voltage to appear across the tube – a combination of the line voltage and the inductive spike from the choke – and the arc strikes. If, for any reason, it fails to be maintained, the starter switch closes again and has another go. Assuming, however the arc is successfully initiated, an AC arc is maintained in the fluorescent tube. Notice that: • once the starter has gone open circuit, there is no heating current through the cathode filaments. The cathodes continue to be maintained at the correct temperature for electron emission by a combination of the ion bombardment and the arc current. • the simple inductive ballast circuit has a poor power factor. The simplest way of correcting this is by a parallel capacitor of the appropriate reactance.

Hg

UV 53%

Heat 44%

Fluorescence

Heat 79%

Phosphor coating

Figure 3.13 The mechanism of fluorescence within a fluorescent lamp.

Visible 21%

Infra red 37%

Dissipated heat 42%

Figure 3.14 The conversion of energy within a fluorescent lamp.

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Choke (electromagnetic ballast) AC Supply

Starter

Figure 3.15 The simple switch start circuit for a fluorescent lamp.

Alternative power factor correction circuits are given in Chapter 6. • the starter switch has a small capacitor across its terminals. This reduces radio frequency interference and helps prolong the starting pulse. The most common starter switch is the glow starter shown in Figure 3.16. It consists of a small gas filled bulb in which there is a bi-metallic electrode and a fixed electrode (or two bimetallic electrodes). Initially these are apart. When power is applied there is no voltage drop across the fluorescent tube, so the full line voltage appears at the starter, causing a discharge between the electrodes. The heat from the discharge bends the bimetallic element(s) so the electrodes touch, and cathode heating begins. But when they touch, the discharge stops, so the heating stops. The electrodes move apart creating the starting pulse. Once the tube is running, there is not sufficient voltage at the starter electrodes to recreate the glow discharge. There are other types of starters. The thermal starter uses a heating element to operate a bimetallic strip. It draws a continuous current, and is slow to operate – but does give reliable starting under cold conditions. Recently electronic starters have been introduced. There are also circuits which work on different principles. Control of the light output of the tube is achieved by controlling the current through it, but below a certain current level the discharge becomes unstable. Dimming of fluorescent tubes is possible with most types, but only by using circuitry which maintains the correct cathode temperature for electron emission. In anticipation of later chapters:

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• the “old” T12 tubes are satisfactorily dimmed using electromagnetic ballasts and a separate cathode heating supply. • T8 and T5 tubes can only be satisfactorily dimmed using electronic ballasts. • fluorescent dimming is fine for architectural “scene setting” and energy management. It is not recommended for auditorium dimming down to blackout. Below a certain level the performance of different tubes varies, and anyway the lowest achievable level is far above blackout, resulting in the need for a sudden drop in light as blackout is achieved. • the best specified performance of fluorescent dimming is to 1% light level. But in practice this is only achieved with specified tubes at specified operating temperatures. It is much better to plan on 5% or more (but only after checking the tubes are suitable). • the long 6ft (1800mm) and 8ft (2400mm) tubes are not good candidates for dimming. Fluorescent lamps have long lives, typically 6,000 to 15,000 hours. However, life is affected by operating conditions. Frequent switch starting and unsuitable cathode heating circuitry can reduce life. The performance of fluorescent lamps is greatly enhanced by the use of electronic ballasts in place of the traditional electromagnetic ballast. Electronics are also making possible new lamp concepts. The T5 lamps listed in Table 3.2 can only be operated with electronic ballasts. T12 and T8 lamps of the same length are generally interchangeable, and, over a period of many years the T8 replaced the T12 for most applications. The narrower T8 tubes use a mixture of Krypton and

Fixed contact

Bi-metallic contact

Glass envelope low pressure inert gas with ionizer

Protective can

Twist socket contacts

Figure 3.16 The glow starter is the most common fluorescent lamp starter.

EVERYDAY LAMPS

Figure 3.17 Circular and U-shaped lamps are derived from their linear counterparts. While for some luminaire designs they are being superseded by compact fluorescent lamps, the new T5 circular lamps are proving popular. Photos from Osram.

Argon as the inert gas filling. The use of Krypton lowers the losses at the electrodes, and results in a lamp needing less power for a given light output. The T5 lamps listed in Table 3.2 take fluorescent lamp development further and represent a completely new generation. Their narrowness opens up new possibilities in luminaire design. Some points to notice about the T5 tubes are as follows: • their lengths are slightly shorter than their T12 and T8 “equivalents”. • for most lengths two different regimes are offered. For example the 1,149mm length tube is available at 54W (ca 5,500 lumens) or 28W (ca 2,700 lumens). This allows luminaire designers to achieve the right balance of light output and efficiency for any given design. • efficacy is of the order of 100 lm/W. • the conductive coating and the barrier layer applied to the inside of the tube prior to the application of multi-band phosphors play a significant part in the improved performance. This last point needs some explanation. The conductive layer assists the initial ionization in the tube. It is also designed to reflect UV, so any UV which has not been absorbed by the phosphors, gets refelected back into the tube as opposed to being absorbed by the glass wall. The barrier layer protects the phosphors from sodium in the glass which would otherwise shorten phosphor life. Mercury vapor is a poison, so anything which reduces its presence in the atmosphere is to be welcomed. The T5 tube uses only 10% of the mercury than its equivalent T12 of 25 years ago. Just as significant though, is the fact that if a given number of

lumens is produced by either incandescent lamps or modern fluorescent lamps, the incandescent lamps are actually responsible for more mercury vapor in the atmosphere than the fluorescent (even if the fluorescent tube’s mercury is allowed to escape into the air rather than being re-cycled). This is because much more power is needed and most power stations discharge significant amounts of mercury into the atmosphere. The high efficacy of fluorescent lamps resulted in many attempts to create different shaped lamps to allow greater flexibility in luminaire design. The most common shapes are the 20/40/65W T12 “U” shaped lamp, and circular lamps (approx 200mm, 300mm and 400mm diameter for 20/30/40W) based on 30mm diameter (T 9.5!) tubes.

3.5 Compact fluorescent lamps The long life and high efficacy of fluorescent lamps make them attractive for many architectural applications. However, their practical execution as long thin sources is not ideal for all applications. The compact fluorescent lamp uses a tube of T4 or T5 size, but is folded into two, four or even six “fingers” to make a high brightness lamp of short overall length. Such lamps were originally introduced with the idea of replacing GLS (General Lighting Service) tungsten filament lamps, but the range has now been greatly extended so that the largest of them are used in modular luminaires - replacing the straight 600mm (2ft) tube. Compact fluorescent lamps designed for the direct replacement of GLS lamps are two terminal devices, and have an internal electronic ballast. Table 3.3 gives an indication of the input power needed to achieve a given number of lumens using either incandescent or compact fluorescent lamps. Lamps designed for new luminaires are four terminal devices; and these in turn are divided into those which have an internal starter, and those which do not. Internal starter compact fluorescent lamps are used with suitable electromagnetic ballasts; those without a starter may be suitable for electromagnetic ballast and external starter, or may only be suitable for electronic ballasts.

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It is accepted that in luminaires using low power lamps (say below 20–30W total) power factor correction is not usually applied. This is acceptable where only a few lamps are involved. However in schemes using large numbers of lamps, provision for correction must be made. If electromagnetic ballasts are used, the correction can be by central capacitors. Modern electronic ballasts include power factor correction. Compact fluorescent lamps work well when they are installed in luminaires which are designed round their characteristics, and when the designer or user has chosen a lamp of the correct color. Unfortunately there are many cases where they have been used indiscriminately – usually in a misguided attempt at “economy”. Truly grotesque examples of their use, for example by replacing tungsten lamps in candelabra in churches and similar sensitive applications with “daylight” compact fluorescent lamps, are all too often to be seen. Many compact fluorescent lamps are unsuitable for dimming, including most, but not all, of those intended as simple replacements for incandescnt lamps and those with built-in starters. Compact fluorescent lamps use a mixture of old and new filling and phosphor technologies, so if dimming is required it is essential to check that the lamps are suitable for it.

Approximate lamp output in lumens 100 240 400 600 900 1,200 1,500 1,800 3,000

Incandescent lamp load 15W 25W 40W 60W 75W 100W 120W 150W 200W

Compact fluorescent lamp load 3W 5W 7W 11W 15W 20W 23W 28W 42W

Table 3.3 A guide to the equivalence of incandescent lamps and their compact fluorescent equivalents. Note that the appearance of the compact lamp may be significantly different unless chosen to match incandescent. This means choosing lamps with the correct phosphor blend and a CCT of around 2,700K.

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Conventional base

Electronic ballast

Folded tube

Figure 3.18 Construction of a compact fluorescent lamp intended as a replacement for incandescent lamps. Figure from Osram.

The best way to dim those that are suitable is to use luminaires fitted with controllable electronic ballasts. The same restrictions concerning the minimum achievable light levels apply as to the standard tubular fluorescent lamp.

3.6 Special purpose fluorescent lamps 3.6.1 Variations on a theme. There are a lot of “special” types of fluorescent lamps designed to meet the needs of particular applications and industrial processes. A few of them are listed here. When the fluorescent lamp was first invented, and phosphor science was in its infancy, it was expected that its main application would be as a source of colored light. In practice fluorescent lamps using single color phosphors, red, green and blue are available, but are not widely stocked. Another way of achieving a single color lamp is to sleeve a white lamp with a color filter. However, this can only be done by comparing the filter characteristic with the spectral distribution of the lamp. Tri-phosphor lamps emit at specific wavelengths, and the result of using filters can be disappointing if the filter does not match the peak wavelength. Manufacturers use different trade names for lamps with enhanced or diminished radiation at particular wavelengths. Tubes with enhanced red and blue are

EVERYDAY LAMPS

4 pin lamps for external control gear suitable for dimming

18-80w

225mm-570mm long

10-26w

87mm-150mm long Built in electronic ballast Not suitable for dimming

8-21w

5-15w

46-70mm dia

16-21w

100-120mm dia

Figure 3.19 Examples from Osram of the many variants of compact fluorescent lamps.

used for lighting plants and aquariums. The enhanced wavelengths promote photo-biological action. Lamps with specially chosen red phosphors are used for the illumination of meat and delicatessen products in shops and supermarkets. Lamps with a somewhat yellow cast, which do not emit any UV radiation at all, are used in factories where UV lithography is the main process (e.g. chip manufacturing and other “clean room” processes).

Ultra violet radiation is divided into three bands: UV-A 400nm – 315nm UV-B 315nm – 280nm UV-C 280nm – 100nm Lamps for sun-tan beds and solaria are available in various grades. The “weakest” radiate only at wavelengths 350nm and longer; they are considered safe for long exposure and for maintaining a tan. The middle range lamps radiate from 315nm upwards, creating new pigment, but again, considered safe for long exposure. The “strongest” sun-tan lamps have a more “sun like” UV spectrum containing some UVB, and must only be used for limited periods. Lamps for sun-tan beds and solaria are T12. While short (590mm 40W) lamps are available, most of the market is served by long lamps, for example 80W 1,500mm, 100W 1,800mm, 140W 1,500mm and 160W 1,800mm. The lamps operate at comparatively high current (1A for the 100W and 1.5A for the 140W.) Black glass fluorescent lamps are fitted with a black filter to eliminate almost entirely the visible radiation. They radiate in the range 350–400nm with a strong peak at around 365nm. They are used in processes based on fluorescence, ranging from the detection of cracks in metal to the detection of forged bank notes. They are also used for special effect lighting in the theater and other entertainment applications requiring the use of “black light”. In such applications there may be a call to provide dimming control to allow the black light to be treated like any other light source.

3.6.2 Ultra violet lamps Obviously fluorescent lamps without the phosphor could be used as a source of UV radiation. However, short wavelength UV is dangerous, and anyway would be severely attenuated by the glass. A range of UV tubes is available for different applications.

Figure 3.20 Encapsulite manufacture a range of color sleeves for fluorescent lamps, some of which also provide protection in the case of tube breakage. Here they are used in the Anytronics “Anycolour” fluorescent luminaire.

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Figure 3.21 Examples of UV “fluorescent” lamps, from Osram. “Blacklight” lamp used in entertainment applications (above). Compact lamp for insect attraction (below).

Insects are attracted by near UV. Both compact and linear lamps are available which emit light in the 350–400nm range with some visible violet radiation. When fitted in an insect killer, such lamps attract insects onto an electric “zapper” which kills them. The same lamps are used for other applications like the polymerization (“curing”) of plastics and adhesives, and in diazo copiers. Both compact and linear lamps are available which produce UV-C, with a peak at 253.7nm. Wavelengths of this order are germicidal and of comparatively high photon energy, so these lamps are used for: • the disinfection of water, for example drinking water and swimming pool water. • the disinfection and de-odorizing of air, in air conditioning plants. • the disinfection of surfaces in the food and pharaceutical industries. • the erasure of EPROMs. Linear UV lamps of all kinds tend to use electromagnetic ballasts, whereas the compact lamps are more likely to use electronic ballasts.

light distribution were achieved using the reflector fluorescent lamp. In these lamps a reflective layer is deposited over 200°–330° of the tube prior to the deposition of the phosphor. Such a construction is useful for situations like cabinet lighting and the edge lighting of signs where it may not be possible to fit a proper optical reflector. It is also useful in situations where the top of a horizontally mounted fluorescent tube may gather dust and be inaccessible for cleaning. The reflection is not total, and some light gets through the reflective layer; but the great majority is directed out of the 30°–160° gap remaining. There is a slight loss of overall lumens. The tubes are otherwise conventional and are suitable for dimming with suitable control gear. 3.6.4 Instant start lamps It can come as surprise that some hot cathode fluorescent lamps come in versions with only a single connection pin at each end. From the description of the simple starter circuit in Section 3.4 it is clear that, once an arc has been struck and sufficient current is flowing, the cathodes maintain thermionic emission and a high temperature from the ion bombardment

Glass Reflecting layer Phosphor

Normal

3.6.3 Reflector fluorescent lamps The arrival of the T5 family of lamps has simplified the optical arrangement for some applications, allowing slim, compact, luminaires with good light distribution to be produced. Prior to the adoption of long T5 lamps, and still a valid construction, narrow fittings with a unilateral

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Reflector Approx 30

Typical light distribution pattern

Figure 3.22 Reflector fluorescent tubes have an internal reflective coating over part of the tube. The Philips TL-D has an especially narrow aperture of only 30°.

EVERYDAY LAMPS

Figure 3.23 An example of an instant start hot cathode fluorescent lamp, used in non-pressurized luminaires in semi-hazardous atmospheres. Notice the single pin connection and the conducting strip used to aid starting. The lamps are T12 and are available in nominal lengths 440, 600, 1,200 and 1,500mm. Photo from Philips.

alone, and do not require any continuous heating current. Is it possible to do without the starter? With appropriately designed tubes the answer is “yes”. The starting circuit described in Section 3.4 is known as pre-heat starting. Other starting regimes for hot cathode fluorescent lamps will be described in the chapters dealing with ballasts, but here we can anticipate one type of starting known as instant start or cold start. In this case there is no pre-heating, and starting relies on field emission from the cathodes, requiring a higher starting voltage across the tube. Lamps of this kind are sometimes specified for use in increased safety damp-proof and explosion proof luminaires. The possibility of local heating and arcing from the individual cathode circuits due to poor connection or faulty ballast is eliminated. Instant start lamps need correct conditions for starting. The initial glow discharge can be between an electrode and an outside groundplane before the main arc strikes. It has been found that tubes should have a silicone, non wetting, finish to give a high surface resistance, and should be within 10mm of a grounded metal surface – usually the luminaire itself. An alternative arrangement is to fit the tube with a thin metal strip, connecting both of the metal end caps, which should be grounded. Cold start lamps are not suitable for dimming.

For example in a T8 hot cathode fluorescent tube the optimum electric field for UV generation is around 1 V/cm, so a typical volt drop down the arc is around 100V. But in addition to the arc volt drop, there is a volt drop at the cathode, the voltage required to create ion and electron flow. This drop is referred to as the cathode fall. In a hot cathode lamp thermionic emission of electrons arising from the heating of the cathode to 1,100°C, and the use of an oxide coating providing a low work function (meaning that not so much effort is needed to peel the electrons away) results in a cathode fall of around 10–12V. However, a discharge can still take place in a tube fitted with electrodes which have no heating arrangement. Such lamps are called cold cathode lamps, and in this case the cathode fall can be as high as 50V. This means that the tube must operate at a higher voltage. Cold cathode fluorescent tubes are not as efficient as their hot cathode equivalent for “standard” lengths. For long lengths the electrode loss contribution gets proportionately less. Cold cathode lamps have a long life, because they do not suffer from cathode emission degradation, and are not affected by repeated starting. For these reasons cold cathode fluorescent has a number of specialist applications:

Voltage 500 Cold cathode 400 300 200

Within a discharge tube there is a substantially linear voltage drop down the length of the arc discharge.

mA 50

Hot cathode

A 0m 50

100 0

3.6.5 Cold cathode fluorescent lamps

A 0m 10

500 1000 1500 2000 2500 Arc length mm T8 Tube

Figure 3.24 The operating voltage of hot and cold cathode fluorescent lamps as a function of arc length.

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3.6.6 T2 and miniature fluorescent lamps 25mm

20mm

Fluorescent lamps are normally associated with relatively large physical dimensions and wide diameter tubes. However, there is a significant industry devoted to producing miniature fluorescent lamps. The largest of these are the hot cathode 7mm diameter (T2) linear lamps in 6–13W ratings and lengths of 220–520mm (8.6–20 in). These lamps are, typically, available with the same colorimetry as mainstream T5 lamps, and can, therefore, complement them in lighting schemes requiring unusual luminaire designs. In common with the T5 lamp, these miniature hot cathode lamps require electronic ballasts. The smaller sizes of miniature fluorescent lamps are made in a wide range of tube diameters, including 6.5, 4.8, 4.1, 3.8, 3.0, 2.6 and 2.0mm. Linear tubes are in the range 38–420mm, and nominal ratings from less than 1W to around 6W. These lamps are cold cathode, and are used mainly for instrumentation and backlighting, for example as backlights for LCD screens in portable computers.

Figure 3.25 Construction and typical operating circuit of a cold cathode lamp.

• where a long run of light is needed, without an obvious break. For example cove lighting and other specialist architectural applications. • where custom shapes are needed. • where repeated flashing of the lamp is required. A lot of sign lighting is done with cold cathode; most “neon” sign lamps are not neon at all, but cold cathode fluorescent. One of the reasons that cold cathode lamps are useful for this purpose is that they operate over a wide temperature range, and always start. The light output of hot cathode lamps drops dramatically below 0°C, and at low temperatures they can be difficult or impossible to start. Long cold cathode lamps need relatively high voltages to operate (see Figure 3.24) so are operated from transformer type ballasts or their electronic equivalent. They are suitable for dimming. The cathodes in cold cathode lamps are in the form of a hollow cylinder, as shown in Figure 3.25.

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7mm hot cathode (Osram)

3.8mm cold cathode (LCD Lighting Inc.)

Figure 3.26 Examples of miniature fluorescent lamps.

EVERYDAY LAMPS

Often these miniature lamps are made in customized forms with multiple bends to give even backlighting. They all work with electronic ballasts; such ballasts are quite specialized, since a typical specification can be as demanding as: • input voltage 12V DC • lamp starting voltage 2,000V • lamp running voltage 480V rms • lamp current 5mA Users of laptop computers might be surprised to learn that such high voltages are present in their machines! For instrumentation use the ballasts may be supplied as dimming ballasts, in which case PWM control is normally used, although some simple ballasts give limited control by varying the input current to the ballast. While cold cathode lamps work down to quite low temperatures, their visible light output then goes down significantly. For some specialist applications miniature cold cathode lamps are provided with heater coils loosely wound round the exterior of the tube. But for most applications it is sufficient to wait for the infra-red, which is still generated at low temperatures, to warm up the lamp. 3.6.7 Long life fluorescent lamps The life of fluorescent lamps is determined by a number of factors. The most significant are: • loss of lumen output due to phosphor degradation. • lack of electron emission arising from cathode failure in hot cathode lamps. In the case of hot cathode lamps useful life is greatly extended by the use of the correct electronic ballast. Cold cathode lamps generally have a longer life than their hot cathode counterparts (e.g. 20,000 hours rated instead of 10,000). Some specialist manufacturers produce long life fluorescent lamps by combining a number of techniques:

Figure 3.27 The “Thermo-LL” lamp from Auralight AB combines a 36,000 hour life with enhanced low temperature performance (to -20°C) by virtue of its glass enclosed construction.

• first and foremost by extra attention to eliminating phosphor contamination during lamp manufacture. • secondly by using a more complex (and expensive) cathode construction designed to maintain electron emission, and to minimise the effect of lamp starting (which otherwise tends to damage the cathode by the stress of field emission). • choice of phosphor. Such lamps may have a rated life of around 36,000 hours instead of the more usual 12,000 hours or so. In this case “life” might be defined as the time taken for lumen output to drop to 70% of initial lumens, and based on a 3-hour switching cycle. Tri-phosphor lamps have a shorter useful life, so long life versions of these might give, for example, 25,000 hours. 3.6.8 Electrodeless fluorescent lamps The extra cost of long life lamps can be recouped on the first lamp change if luminaires are difficult to access – the cost of replacement can easily outweigh the additional cost of long life lamps. Besides cold cathode and long life hot cathode lamps there is a range of electrodeless fluorescent lamps with very long lifetimes. These are described in Chapter 5 under the heading of “Induction Lamps”.

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Chapter 4


Arc

lamps

4.1 High intensity discharge lamps

4.2 Mercury vapor lamps

The carbon arc was the first commercially useful high intensity source of light powered by electricity. It consists of two carbon rods which are brought together and then drawn slightly apart while carrying an electric current. An arc is formed between the two and visible radiation results both from the incandescense of the electrodes and the flame luminescence resulting from the vaporizing of the electrodes. Carbon arc lamps operate in the atmosphere and, as a result, have a short burning time and require continuous attention. Nonetheless they were widely used until comparatively recently in film projection, film production and searchlights. Now they have been replaced by xenon arc and metal halide lamps according to application. All other practical arc lamps operate in a controlled atmosphere within some kind of arc tube. The fluorescent lamp already described in Chapter 3 is one kind of arc or discharge lamp, in its case characterized by a long arc length, low pressure of operation within the arc tube, and a low tube wall temperature. The arc lamps described in this chapter have comparatively short arcs and operate under more stringent conditions than the fluorescent lamp. They are generally characterized by high luminous efficacy and/or high point brightness. The main family of lamps such as mercury HID (High Intensity Discharge) and high pressure sodium lamps are widely used for industrial, public area and street lighting. Metal halide lamps can have a pleasing color performance and are made in many different formats, making them suitable for architectural lighting and even for projection. Xenon lamps have a very short arc length and a near continuous spectrum, making them ideal for projection.

The Mercury Vapor HID lamp was the first HID lamp to be widely used commercially. Electromagnetic radiation is created from a discharge within mercury vapor, but the regime is different than that found in the normal fluorescent lamp: • when running, the pressure within the lamp is in the range 200–400kPa (compared with only 1Pa in the fluorescent lamp, and equivalent to 2–4 times atmospheric pressure). • it is not possible to achieve the mercury vapor discharge in a cold lamp. For this reason the lamp also includes argon, and the initial arc is struck as an argon arc. The energy from this discharge vapor-

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Figure 4.1 The carbon arc lamp, in this case built in to a stage follow spot. Photo from Strand Lighting/Brian Legge.

ARC LAMPS

reflector

elliptical

globe

ply is not great enough to initiate the argon ionization. Provision has to be made to start the discharge with an auxiliary electrode. • once running, the arc produces a much greater proportion of visible light than the discharge given in a fluorescent lamp. Main wavelengths are 405, 436, 546, 577 and 579nm. Together these give a bluish-white light, but it is almost totally deficient in red. • for this reason most mercury vapor HID lamps also include a phosphor layer. The mercury vapor still produces a lot of ultra-violet, and the phosphor can be used to improve the color rendering and give higher overall efficacy.

Standard coating

Relative power

Outer glass soda-lime 125W

450

550

650

Main electrodes

650

Starting electrode

Wavelength in nm

Support

Nitrogen filling

Quartz arc tube

Relative power

Warm coating

450

550

Wavelength in nm

Mercury Argon filling

Figure 4.2 Examples of Mercury Vapor High Intensity Discharge Lamps above; and the spectrum of the lamps, below. Illustrations from Osram.

izes the mercury to get the main discharge going. • metal oxide electrodes are used to provide electron emission. They are not separately heated, and rely on the arc bombardment energy to maintain the correct temperature. • the field provided by the standard electricity sup-

Resistor

Figure 4.3 The construction of the mercury vapor HID lamp.

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Lamp L

Ballast

N Power Factor Correction (PFC) Figure 4.4 Electrical circuit for the mercury vapor HID lamp.

A typical construction for a mercury vapor HID lamp is shown in Figure 4.3. The high pressure and temperature needed for the arc requires the use of a compact arc tube, usually made of quartz. This in turn requires molybdenum ribbon connections (for the same reason as the tungsten halogen lamp.) The arc tube itself is housed in an outer envelope for a mixture of practical and safety reasons. The outer envelope is usually made of borosilicate glass. It ensures a more practical outside wall temperature, provides an interior surface on which the phosphors can be deposited, and filters out undesirable ultra-violet wavelengths. The outer envelope itself is usually filled with nitrogen. This prevents oxidation of internal parts and ensures a stable temperature regime for the arc tube. An auxiliary electrode is fitted to aid starting of the lamp. It is fed through a high resistance, and creates a local glow discharge to start the ionizing of the argon. The starting period is characterized by a low voltage across the lamp and a current somewhat higher than the eventual running current. As the lamp warms up an increasing amount of the current is taken by the mercury vapor. It takes several minutes for the vapor pressure to stabilize. If power is disconnected, the lamp will not relight if power is restored quickly. It must cool down until the vapor pressure has dropped to the point where an arc can be established with the available voltage. The cooling down period is similar to the warm up time – a few minutes.

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Mercury vapor HID lamps have an efficacy in the range 30–60 lm/W and a life of 12,000 to 24,000 hours. Ratings are from 50W to 2,000W, but the most popular sizes are in the 80W to 400W range. They are favored for outdoor and some industrial use because of their comparative simplicity, but the quality of light is not so good as other sources because the light is based on comparatively few spectral lines. The circuit of a typical lamp is shown in Figure 4.4. It is similar to that of the fluorescent lamp, but has no starter. There are limited possibilities for controlling HID lamps – if the current is reduced too much the arc becomes unstable. Some limited control of light output is provided by the use of tapped chokes. Note that the simple circuit of Figure 4.4 applies when the line voltage is sufficient. Chapter 6 describes different types of HID ballast used for various lamp types and line voltages. A variant of the mercury vapor HID lamp is the self ballasted lamp. This was introduced to allow luminaires fitted with high wattage tungsten lamps to operate at higher efficiency without the need to fit ballasts. In these lamps the arc current is limited by a resistance – but the resistance used is a tungsten filament within the outer envelope, and itself acts as an incandescent lamp. The idea has the merit of simplicity, and the incandescent filament helps with color rendering at the red end of the spectrum. However, the resulting lamp does not have a particularly high efficacy – it is barely double that of a tungsten lamp. This is because the arc only strikes at a threshold voltage, so current through the filament is not continuous – it therefore takes a disproportionate amount of power. High pressure mercury vapor lamps are also supplied as sources of UV-A and UV-B. In this case, as in that of the low pressure “fluorescent” tube, the outer envelope is black Woods Glass, which blocks the visible radiation, and lets through the long wave ultra violet. These lamps are used in similar applications to the linear lamps, but where a point source is more appropriate.

ARC LAMPS

4.3 Sodium and high pressure sodium lamps 4.3.1 Low pressure sodium lamp

2

mw per m per 5 nm per 1000 lux

The low pressure sodium lamp generates light from a discharge in sodium vapor at the low pressure of around 0.7–1Pa. “Sodium lighting” is familiar because of its almost monochromatic nature, actually two yellow wavelengths very close together at 589.0 and 589.6nm not far from the eye’s peak sensitivity. Terrible for color rendering, but great on efficacy at between 150 and 200 lm/W. The sodium “D lines” can also be generated if you drop some salt into a flame, when sodium’s characteristic yellow radiation can again be seen. Low pressure sodium lamps are made in ratings from 18–180W, and have a life of between 16,000 and 23,000 hours. The 180W lamp gives around 33,000 lumens. Low pressure sodium lamps need a long arc for low current density and are constructed either double ended, or, more commonly, single ended. The single ended construction requires a U shaped tube. In both cases the tube is made of sodium resistant glass and is mounted inside an evacuated outer

2400 2000 1600 1200 800 400

SOX 55 W, SOX-E 36

300

Outer jacket

Arc tube support Neon and sodium filling

ITO IR reflective layer

Some designs have dimples for sodium condensation

High vacuum Inner arc tube

Coated tungsten cathode

Sodium resistant layer

Lamp cap

Figure 4.6 The construction of the low pressure sodium lamp.

envelope. This is necessary both for safety and to ensure that the main tube wall maintains a temperature of 260°C. Sodium resistant glass is damaged by common impurities, so the actual construction of the arc tube is “two ply”, where a thin coating of sodium resistant glass is applied to the inside of the discharge tube. The outer envelope has a thin internal coating of indium-tin oxide which reflects the IR radiation Lamp

L PFC 400

500

600

700

Leakage reactance auto-transformer

800

Wavelength in nm

Figure 4.5 A low pressure sodium discharge lamp, SOX, from Philips (above) and the spectrum of the lamp (below). This series of lamps is between 300 and 1,100mm long, with ratings between 35W and 180W.

N

Figure 4.7 Electrical circuit for the low pressure sodium lamp.

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Coated elliptical 1000W 400mm Mercury free 80W 150mm

Double ended 400W 200mm Clear 250W 250mm Relative power

Standard

450

Relative power

back to the lamp, helping to maintain the optimum tube wall temperature. As with mercury vapor HID, the sodium lamp needs a starting gas. In this case it is 99% neon and 1% Argon at a pressure of around 1,000Pa. Sodium street lights are noticeable for their dull red neon discharge as they warm up. Warming up time is quite long, at around 10 minutes; however, the hot reignition performance is good. Low pressure sodium lamps need around 400– 500V to start. Because of the high starting voltage the ballast needs to be a combined transformer and current limiting reactance; the ballasts used are described in Chapter 6. The lamp’s main application is in street lighting and, like mercury vapor lamps, the need and opportunities for variable output control are limited.

550

650

550

650

Wavelength in nm

Mercury free

450

Wavelength in nm

Figure 4.8 Examples of High Pressure Sodium Discharge Lamps (above) and the spectrum of the lamps (below). Illustrations from Osram.

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4.3.2 High pressure sodium lamp The high pressure sodium lamp (HPS) operates at a pressure of 5–10kPa. Although uneven, with some pronounced peaks, the spectrum is nearly continuous, resulting in a pleasantly warm light. Curiously the strong sodium D lines at 589nm, which we associate with “sodium” are not present, because at this pressure the D line light is self-absorbed by the gas. The main spectra either side of the absorption dip can be regarded as a broadening of the original lines. HPS lamps have negligible radiation in the ultraviolet, but around 25% of input power radiation in the infra-red. The high efficiency versions can radiate as much as 25% visible radiation. The balance is lost in electrode losses and non-radiated losses in the arc column. The lamp construction is shown in Figure 4.9. At high pressure sodium is highly corrosive, and neither glass nor silica can be used. The arc tube is made of a ceramic, polycrystalline alumina, which is a translucent material transmitting about 90% of the light. The alumina is difficult to work, and cracks easily, making connection to the tungsten electrodes difficult. A ceramic plug is fitted into each end of the arc tube, through which a niobium connection is made. The plug is able to bond with both the alumina and the niobium.

ARC LAMPS

Once again the arc tube is itself mounted in an evacuated outer envelope. Sometimes the outer envelope has a diffusing internal surface to create a larger, lower point brightness, source. Lamps are available in the range 35W to 1,000W, and efficacy is between 45 and 150 lm/W depending on size; efficacy goes up with power rating, levelling out above 400W. A higher sodium pressure improves the color rendering, but decreases efficacy. A higher xenon pressure improves efficacy, but makes starting more difficult in the larger lamps. Life is between 10,000 and 24,000 hours; unfortunately the shorter life applies to the lamps with the best color rendering. The starting of a high pressure sodium lamp is complex. First the starting gas is a mixture of xenon and a sodium-mercury amalgam. The xenon at 20kPa gets the discharge going, and the mercury vapor (60kPa when warmed up) helps raise the pressure. Warm-up time is about 10 minutes, and re-strike time about one minute. Arc tube support and current lead-in

Hard glass envelope

Coated tungsten electrode

Vacuum

Ceramic arc tube

Sodium amalgam and xenon filling

Glass seal Alumina plug

Niobium wire Lamp cap

Figure 4.9 The construction of the high pressure sodium lamp.

L

Ballast Lamp

PFC

Ignitor

N

Figure 4.10 Electrical circuit for the high pressure sodium lamp.

Second, the starting voltage required is much higher than can practically be provided by transformer action. Starting is achieved by using a component called a superimposed pulse ignitor. This device produces a series of high voltage pulses, in the range 2.5kV to 4kV depending on lamp size. Color rendering ranges from R a = 25 for the “standard” HPS, through 60 for intermediate lamps up to 85 for the highest pressure “improved color” versions. The comparatively good color rendering of the improved color HPS lamp means that it is used for architectural applications, and there can be a demand to dim the light, both for energy management and practical/esthetic reasons. It is possible to achieve limited dimming but: • lamps should always be run up to full first. • the range is limited (typically 50% is the very best that can be achieved, and 60% is a practical minimum). • even at the 60% level, color rendering is affected, with significant changes to lamp color. • dimming can shorten lamp life. A recent development is the mercury-free sodium-xenon HPS lamp. In this lamp the xenon pressure is around 10 times higher than in the normal HPS lamp, and no mercury vapor is used. The arc tubes are significantly longer and thinner than the standard type. Starting is more difficult and in practice this type of lamp is only offered in low ratings (35–80W) using an electronic ballast with builtin igniter. This arrangement does allow the current through the lamp to be precisely controlled, both in

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30 0m m

72W compact with ceramic discharge tube

20 0m m

2000W tubular

11 0m

m

1200W PAR 64 reflector

250W single ended for theatre luminaires

250W compact source

50 0m m

180mm

80 m m

400W elliptical suitable for open fitting

400W tubular double ended

40 0m m

10 0m m

respect of absolute current and current waveform. As a result of this such lamps can be offered with a selectable color temperature.

18000W 44mm arc length

Figure 4.11 Examples of Metal Halide Discharge Lamps (photos from Osram). The mm dimension is the approximate overall length.

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4.4 Metal halide lamps Metal halide lamps can be considered as a variant of the high pressure mercury vapor lamp. In addition to mercury vapor and argon, these lamps contain metal halides. The halides can be a mixture of rare earth halides, usually iodides, or a mixture of sodium and scandium iodide. The mercury vapor radiation is now augmented by that of the metals, which use a regenerative cycle not unlike the one used in tungsten halogen lamps. Once the lamp has achieved operating temperature, the metal halides vaporize. However if the halide vapor reaches the high temperature core of the discharge, it dissociates into the halogen and respective metal, and the metal radiates its associated spectrum. The separated halogen and metal molecules move by diffusion and convection to the cooler parts of the tube, especially the walls, where they recombine, for the cycle to start again. Metal halide lamps have efficacies in the range 75–125 lm/W. Color rendering is significantly better than mercury vapor, and can be tailored by the choice of halides. Life is in the range 6,000 to 20,000 hours, and ratings range from 35W to 20,000W. There are various different approaches to starting metal halide lamps: • all small lamps use high voltage pulse start. • in Europe it has been the practice to use superimposed pulse start ignitors on the larger lamps for many years (3–7kV pulses, depending on lamp type). Recently this approach has gained ground in the USA as well. • however, auxiliary electrode start is also possible on the higher rated lamps when suitable ballasts are used. • but in this case the reactive nature of the filling can lead to electrolysis in the quartz envelope between the main and auxiliary electrode. For this reason some lamps include an arrangement (operated by a bi-metallic switch) to disconnect the starting electrode.

ARC LAMPS

The standard circuit for commercial metal halide lamps in the 35W–1,000W range, when using superimposed pulse ignition, is the same as that for HPS lamps, as shown in Figure 4.10. Metal halide lamps operate at a significantly higher pressure than the normal mercury lamp, in the range 1,000–1,500kPa. They also emit considerable quantities of UV. For both reasons they may only be operated in specially designed luminaires, or be offered in special versions, which filter out the UV, and which can contain the lamp fragments in the case of catastrophic lamp failure. Some lamps using the conventional double envelope construction use UV-stopping quartz, obviating the need for external filtration (while still requiring a shatter proof cover). There are also lamps designed for use in open luminaires, but in this case they have a “third jacket” within the outer envelope to provide the necessary protection. Lamps designed for open luminaires have special bases to prevent the “standard” lamp being fitted by mistake. The range of possibilities afforded by different lamp constructions and halide dosing has resulted in the metal halide lamp being made available in a huge range of formats. Figure 4.11 gives an idea of this range. Some points to note are: • CCT is in the range 3,600–4,200K for the general use lamps. Rare earth dosed lamps achieve a higher CCT in the range 3,800–5,600K

Relative intensity

3

2

1

0

400

600

800

1000

1200

1400

1600

1800

2000

2200

Wavelength in nm

Figure 4.12 The spectral radiant intensity distribution of an Osram HMI 4,000W lamp closely matches that of the sun (gray line). It is an example of a metal halide lamp dosed with rare earths and results in a high Ra of 95. “Standard” metal halide lamps have Ra = ~ 70.

Hard glass envelope

Transparent protective shroud

Metal halide filling

Quartz arc tube

Figure 4.13 Construction of the general service metal halide lamp suitable for open luminaires.

• CRI (Ra) is around 70 for the general use lamps, but can rise as high as 95 with rare earth dosed lamps, at the expense of lamp life. • some lamps use an internally phosphor-coated outer envelope. This converts UV to visible radiation. The technique provides a more diffuse (lower point brightness) source and gives a “warmer” light, but does not significantly improve efficacy. • “single element” dosing of metal halide lamps can be used to create moderately high efficacy color lamps, considerably more efficient than using color filters if the color is acceptable. Indium yields a blue light, and thallium a green light. • metal halide lamps are also available in UV versions for industrial applications. • most metal halide lamps use a quartz (fused silica) arc tube; however some lower rated lamps now use ceramic arc tubes (as used by HPS.) This allows a higher operating temperature and results in improved color rendering. Lamps using ceramic tubes are referred to as CDM lamps, Ceramic Discharge Metal halide. They are now widely used in architectural applications because they are available in low power ratings (e.g. 20W, 35W and 70W).

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Metal halide (and other) arc lamps may have some restriction in respect of burning angle. Lamps designed for vertical burning fail if operated horizontally because the arc bows upwards due to convection currents. This both overheats the top side of the arc tube and disturbs the halide cycle. Lamps designed for horizontal burning have shaped or assymetrical arc tubes. Some of the shaped arc tube designs for general use lighting achieve the highest efficacies. The remarks concerning dimming are basically the same as for HPS, with the additional problem that as current is reduced, the color temperature actually goes up, resulting in a more “blueish” light. This is because the metals providing the improved red in the lamp are the first to condense out. However the need for some degree of level control is now appreciated, and both improvements in the lamps and the availability of controllable electronic ballasts mean that dimming of these lamps (over a limited range) is becoming a practical proposition. Metal halide lamps have a warm-up time of between one and four minutes. Most general use lamps are not suitable for hot re-strike and must be allowed to cool for a period of two to ten minutes before restrike is attempted. Some lamps (including all the “compact” types referred to in the next section) are designed for hot restrike. However, such restrike requires a much higher superimposed pulse than normal, in the range 20–60kV depending on the lamp type.

4.5 Compact source metal halide lamps Metal halide lamps are made with a range of arc lengths. At one extreme a 1,500W double ended lamp designed for floodlighting has an arc length of 190mm, at the other extreme the latest generation of projection lamps has an arc length of 1.3mm. Lamps intended for general use have arc lengths of around 40mm for a 250W lamp to 140mm for a 2,000W lamp. Lamps with medium or short arc lengths (compared with the general use type) are referred to as compact source metal halide lamps. The

138

“medium arc” lamps used for applications such as film set lighting have arc lengths from 6mm for 400W lamp up to 44mm for an 18,000W lamp. Typical compact source lamps have arc lengths around 4mm for a 400W lamp up to 15mm for a 2,500W lamp. The short arc allows the lamp to be used in focusing optical systems. Lamps are often supplied with integral ellipsoidal reflectors, and applications are as diverse as: • theatrical luminaires (principally moving light fixtures, with mechanical shutter dimming) • video, data and graphics electronic image projectors (but see also Section 4.6) • fiber optic illumination • film and television lighting • automobile lighting • architectural and display feature lighting Most of these applications demand good color rendering, and much experimentation has gone into the choice of halide mixture to achieve it. Both rare earth iodides and bromides are used. Significant problems with the compact source metal halide lamps are achieving long lamp life, and maintaining color rendering through life. There can be a tendency for different life performance from the different halides, resulting in a color shift during life. This problem is being addressed, and some of the newer projection lamps combine reasonable life with maintained color performance. The higher color temperature needed for these applications is achieved by the use of different halides, particular that of tin. Lamps based on tin halide are good at maintaining color temperature when the lamp is dimmed. Medium arc lamps intended for film set lighting have a CCT of 6,000K. Most other compact source lamps are available with CCTs in the range 4,800–6,500K; but those intended for video and data projection are in the range 7,000– 8,500K. Lamp life depends on the lamp power and arc length; it ranges from around 750 hours to 10,000 hours or more. Some projector lamps (e.g. 400W) are designed for “boosted” operation, whereby they can be operated at up to 50% above nominal running current, but with a sacrifice in lamp life of 50%.

ARC LAMPS

In general compact source lamps use electronic ballasts with integral ignitors, some of which allow limited control. They are able to re-strike instantly when hot. When used in theatrical applications, for example in moving light luminaires, dimming of the metal halide light is achieved using an iris shutter or similar mechanical arrangement. This varies the light output from the lamp, which itself remains full on at all times the luminaire is in use.

4.6 High pressure mercury vapor lamps for projection Compact source metal halide lamps are commonly used as the light source for video and data projectors, but currently the preference is to use xenon arc for the larger projectors, and high pressure mercury vapor arc lamps for small and medium sized projectors. This is because the arc length required is small, typically around 1mm, and it is difficult to get a reasonable lamp life with metal halide. In electronic projectors the object being illuminated is very small, and efficient light collection requires a source of the order of 1 Gcd/m2. The technology used is popularly called UHP (a Philips trademark) Ultra High Performance or Pressure. UHP and its equivalents from other manufacturers is actually a pure mercury vapor lamp; but the lamp is operated at very high pressure (around 20 MPa, about 200 times atmospheric pressure.) The

high pressure yields a continuous spectrum with reasonable red performance, and also gives a high arc voltage (≈80V) despite the short arc length. Color performance remains constant throughout the life of the lamp. UHP lamps are currently made in the range 100– 200W where they have the desirable characteristic of long service life (around 8–10,000 hours for the 100W version.) They achieve this partly due to the use of a small dose of bromine that maintains the tungsten arc electrodes. UHP lamps require dedicated electronic ballasts to provide a special waveform to the lamp that ensures a stable arc that does not “jump”. Such jumping would otherwise shorten lamp life and be visible as a flicker. The ballast also provides the 20kV needed for starting the lamp. Recent developments have reduced the starting voltage to 5kV. Normally 20kV is required to extract electrons from tungsten, but they can also be produced by photo-emission if low wavelength ( 1,000 cd/m2) displays may be simplified by the introduction of new flat lamp technology.

The microwave lamp has a long life; in practice the life is limited by the life of the magnetron which generates the microwave. For obvious economic reasons, the makers have chosen to use magnetrons operating at 2.45GHz, the same as those used in microwave ovens. The sulfur microwave lamp as exemplified in a lamp by Fusion Systems has a 28cm diameter spherical bulb rotating at 400 rpm. It operates at an effiDischarge Phosphor coating

Top glass

Spacer

The illumination of Liquid Crystal Displays (LCDs) backlit transparencies and similar items demands a system which keeps the overall combination of lightsource and light modulator as thin as possible. Some applications can allow the use of multiple fluorescent tubes (with hot or cold cathode depending on the size of the item) but this usually results in unevenness in the illumination. Many applications of this kind use edge lighting with lightguides (see Section 5.10) to achieve a thin display. Clearly if some kind of “thin flat” lamp could be developed, which had a uniform light output across its surface, with a Lambertian light distribution, results could be improved. Such flat lamps are now being developed. They are based on UV fluorescence, like normal fluorescent lamps, but use only a Xenon discharge without any mercury vapor. Two examples are described here. The Planon™ lamp, developed by Osram, is only 9mm thick, but delivers a luminance of 6,000 Nit. Current sizes are 10 inch and 18 inch diagonal, but the manufacturers indicate that sizes up to 30 inch diagonal (illuminated area 24 × 18 inches, 610mm×457mm) are practical. Dielectric barrier

Xe fill

Anode

Frame Bottom glass

Cathode Anode

Dielectric coating

Reflecting layer

Figure 5.10 Principle of the Osram Planon™ flat lamp.

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Cathode Shaped discharge Anode

Figure 5.11 Electrode pattern in the Planon™ lamp.

SPECIAL PURPOSE LAMPS

Color temperature is 6,500–7,000K and life is expected to be 100,000 hours. Depending on the particular lamp and control unit, dimming of the lamp is possible in the ranges 20%–100% and 50%–100%. Hitachi Lighting Equipment in Japan have developed flat lamps for the backlighting of small LCD displays, such as those used in camera viewfinders, vehicle instrumentation etc. The lamps are in sizes of 0.5 inch to 5.2 inch diagonal. They use a xenon discharge in an Argon-Neon-Xenon fill mixture, with pulsed excitation. Because the lamps are small, there are only two electrodes, one either side of the lamp. Driving is from high voltage pulses (800–1,200V) with a typical cycle of 1μs on, 60μs off. Figure 5.12 The Osram Planon™ lamp built in to a prototype luminaire.

Pure xenon discharges have great advantages over combined rare gas/mercury vapor. Mercury vapor is toxic, results in a temperature dependent light output and significantly shortens the phosphor life. Xenon on its own eliminates these problems, but the physics of the discharge are complex, especially if reasonable efficiency is required. The lamp consists of two glass substrates, separated by a glass frame and by spacers. The bottom substrate is thick-film printed with a reflecting layer and with parallel lines of anode and cathode electrodes. The electrodes are encased in a dielectric barrier which limits the discharge current. The top substrate is coated with a tri-band phosphor. The regime under which the dielectric barrier discharges operate is covered by Osram’s patent, and achieves a UV conversion efficiency of around 60%. Instead of applying continuous high frequency energy to the electrodes, the Planon system applies discontinuous excitation pulses, the timing of which is directly related to the kinetic properties of the gas. Gas pressure within the Planon lamp is around 13kPa. Figures 5.10 and 5.11 show the principle of the Planon lamp. Because the multiple discharges are shaped, the resulting light is not completely uniform, so a front diffuser is used. The resulting lamp is slightly thicker than its edgelight counterpart, but significantly thinner than a multi lamp arrangement.

5.3 Neon lamps 5.3.1. The neon glow lamp The neon glow lamp is a low efficacy (0.3 lm/W) low power source used mainly for indication. Some larger types have been made for decorative and warning purposes, but most are in the range of less than one tenth of a Watt up to 2W. Their most widespread use is as mains power indicators. They are convenient for this, because their

Figure 5.13 The neon glow lamp used as a mains indicator. Basic lamp (above); built in to panel mounted indicators (below). Photos from MEM 250.

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nature allows them to be connected directly to mains voltages. Thus a glowing neon indicator is a sure sign that dangerous voltages are about! The standard indicator consists of a small glass envelope, similar to a small torch bulb in size and with or without a conventional lamp cap. The envelope has two electrodes, and has a neon gas fill. Below a starting voltage the lamp acts as an insulator, but once at the critical starting voltage, the glow starts. In common with other gas discharge lamps current would rise uncontrollably unless limited, and with glow lamps a resistor of the appropriate value is either fitted externally, or fitted within the lamp base. If a neon indicator was operated on DC the glow would be seen on the negative electrode; with AC both electrodes glow. 5.3.2 Neon tube lamps When people refer to “neon” lighting they do not have the neon indicator in mind. They are much more likely to think of Las Vegas. The neon lamp is associated with signage, advertising and color. While, strictly speaking, the term “neon lamp” should apply only to lamps containing neon gas, the word has Pinch seal electrode

Alternative ring seal electrode

Electrode insulating boot

Coated tube

Plasma Glass joint

Electrode

Figure 5.14 Neon lamp construction. Figure derived from information supplied by Masonlite.

150

become generic for colored and shaped tubular lamps. As pointed out in Section 3.6.5 most lamps referred to as “neon” are in fact cold cathode fluorescent. A more useful demarcation could be by the method of manufacture. Hot cathode fluorescent lamps, and mainstream cold cathode lamps, are only made on fully automated machinery working a continuous production process. However cold cathode lamps intended for signage and similar applications are made in a batch process. Typically: • one process makes the glass tubing; which is cut into standard lengths. • a second process (carried out by another company) cleans the glass and applies any required phosphor coating. A side process prepares the short lengths of tubing that will be fitted to the ends of the tube and which carry the electrodes. • and finally the tube may be shaped, or spliced to another tube, have the electrodes fitted and be evacuated and dosed with the required filling. This will be done at the sign manufacturers’ or specialist lamp fabricators’ premises. In principle only a few companies supply the basic components, but many companies do the final customized assembly. There is now a bewildering choice in the “neon” market, and it requires some knowledge of the processes involved to get the required lamp appearance. There are four main variables: The type of glass used. Most tubes use a clear glass; but for more saturated colors it can be better to use a colored glass tube. The gas fill. Most use argon/mercury vapor; but neon is also used as the basis of red and orange tubes. The gas pressure used can also affect the color. Pressure is in the range 600–1,600Pa. The phosphor coating. Some tubes (e.g. neon in a clear or red glass tube) do not use a phosphor. Most do. Another technique is “double coating”, where instead of using colored glass, the inside of the clear tube is coated with a color pigment, and then by a phosphor. The tube diameter. “Neon” tubes are supplied in a variety of diameters; between different manufacturers it is possible to obtain tube diameters in millimeter increments from 6–25mm. The majority


Clear glass

Colored glass

Clear or colored glass phosphor coat

Clear glass double coat pigment + phosphor

Neon clear, red Argon clear, blue Argon clear coated, blue Argon green glass coated, green Argon yellow glass coated, yellow

depending on the filling and the tube diameter. Narrower tubes have a high volt drop, and neon filled tubes have a higher drop than argon/mercury vapor tubes. Thus voltages significantly higher than “mains” voltage are needed to operate these lamps. Most neon tubes are fed by conventional transformers with outputs from just below 1kV (optimistically referred to as “low voltage!”) for short tubes, up to 10kV or even 15kV for long tubes; but electronic transformers are also used. The operating voltage is limited by regulations in many countries. More information on neon control gear is given in Chapters 6 and 7.

Argon clear coated, green

5.4 Electroluminescent lamps

Argon clear double coated, gold

5.4.1 Introduction

Neon clear coated, red

Figure 5.15 Examples of the colors available for neon lamps. Derived from information supplied by Masonlite.

of work is done using tubes in the 10–20mm range. Narrow tubes are bright but have a short life. In all a neon supply company may well offer a choice of over 50 colors in a wide choice of tube diameters. In the case of white tubes, results similar to those achieved by normal fluorescent lamps can be obtained, since the same tri-phosphor technique can be used. While the prime manufacturer of tube parts may offer the tubes in standard lengths, the final assembler can make up longer tubes if required. Most tubes use a “fold back” arrangement for the electrodes to facilitate uninterrupted runs of light (see Figure 3.25). The light output and to some extent the color of the tube depends on the running current. Each tube diameter has an optimum running current, and typical currents are 25, 30, 50 and 100mA. 100mA is only used on 20 and 25mm tubes. The operating circuit is as Figure 3.25, but tubes are often operated in series. The required voltage will then depend on the overall tube length. The cathode fall for neon tubes is between 100 and 150 volts at each electrode; and the volt drop per meter length of tube varies between 450V and 1,300V

In the discharge lamps already described an electric field causes an intermediate medium (usually a gas) to emit mainly ultra-violet radiation. The ultra violet then stimulates a phosphor to emit visible light. A question that can be asked is “is it possible for an electric field to create light directly?” The answer is “yes”, and the phenomenon is known as electroluminescence. It is manifested in two principal ways. One is in the Light Emitting Diode, to be described in Section 5.5, and the other is where an electric field is applied directly across a phosphor. In this case the field needs to be very strong, which in practical terms means that the phosphor layer must be thin. Most practical electroluminescent lamps are based on the use of AC, and one way of looking at the electroluminescent lamp is to think of it as a capacitor where a phosphor is mixed in to the dielectric, and where one electrode is transparent. 5.4.2 Practical construction The construction principle of the electroluminescent lamp is shown in Figure 5.16. A conducting substrate is covered with a reflective layer. On top of this is the phosphor layer (actually phosphor dispersed in some carrier). The top electrode is a transparent conducting layer, and then there is an outer protective layer.

151


Transparent protective layer Transparent conducting layer

Light

The principle is realised in two main ways. A rigid lamp is based on: • a steel substrate. • enamel with additive to improve reflection and dielectric constant as the reflective layer. • phosphor in transparent enamel as the light emitting layer. • tin oxide as the transparent conducting layer. • transparent vitreous enamel as the protective layer. This results in a thin rugged lamp suitable for back lighting and similar applications, but somewhat lacking in flexibility in all senses of the word. Today the more common electroluminescent lamp is based on plastic films, and the layers now become: • an aluminum foil or metallized plastic film substrate. • barium titanate in a plastic binder as the reflector layer. • phosphor as the light emitting layer (in the same binder as used in the reflector layer). • transparent plastic foil coated in indium oxide as the transparent conducting layer. Then the whole assembly is sandwiched between layers of transparent plastic film for protection. The plastic electroluminescent lamp is made in sheet or continuous roll form. In sheets it can, within reason, be cut to special shapes. In the roll form it is possible to get continuous lengths of 500ft (152m) or more. Such continuous light sources are used both

Phosphor layer Reflective layer AC Supply Conducting substrate

Figure 5.16 Principle of electroluminescent lamps.

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Figure 5.17 Electroluminescent sheet, here incorporated into an aircraft cockpit floodlight. Photo courtesy Ultra Electronics.

for special effect, and for the safety marking of exit routes, stair treads etc. Electroluminescent lamps operate in the voltage range 100–200V AC, and at frequencies 50–3,000Hz. In principle the higher the voltage, and the higher the frequency, the greater the light output. Unfortunately, the higher the frequency the shorter the life. The lamp does not usually suffer catastrophic failure, so useful life is defined as the point at which light output declines to a specified level. Apart from the electrical conditions, life is determined by phosphor deterioration in the presence of impurities (especially water vapor) so the manufacturing process is crucial. The color range of practical electroluminescent lamps is limited. The most suitable phosphors are based on zinc sulfides, with different activating elements. The most efficient lamps give a blue-green light; but blue, yellow-green, orange, white and red are also available. Electroluminescent lamps are not high brightness devices, their value is in their thinness and suitability for applications that cannot be met easily by more conventional sources. Electroluminescent sheets, for example, can be less than 1mm thick. Typical initial luminance is 70Cd/m2 for blue-green and 50Cd/m2 for other colors when operated at 115V 400Hz (this is a representative example, not related to any particular manufacturer). Electroluminescent lamps are “cold” lightsources, operating at around 1–2μA per mm2. A typical dissipation is 6mW/cm2 meaning that the lamp temperature never rises more than a few degress above ambient temperature. The operating ambient temperature range can be as wide as -40°C to in excess of 100°C. Some vendors offer equipment that varies the operating frequency in order to achieve a more uniform


Figure 5.19 Electroluminescent wire. Photo from Anytronics Ltd.

Figure 5.18 Extruded electroluminescent lamp available in long runs; suitable for exit route and stair tread marking. Photos from Electroluminex Lighting Corporation.

light output through life. This is done by detecting capacitance changes as the lamp ages, and changing the frequency accordingly. The useful life of electroluminescent lamps varies widely according to design and application, from as little as 1,000 hours to 10,000 hours and more. Electroluminescent lamps can be dimmed, simply by reducing the RMS AC voltage. However, the relationship between voltage and light output is not linear, and zero light output occurs at a relatively high voltage. A practical problem in any system relying on pre-set voltages to achieve pre-set light levels is that as the lamps age, light output declines for a given voltage. 5.4.3 Other constructions The principle of the electroluminescent lamp does allow of other constructions. One such is the electroluminescent wire. This is based on a copper wire with a phosphor coat. Spiral electrode wires are wrapped round the phosphor, and the whole is encased in a tough plastic jacket. The resulting “wire of light” lends itself to decorative effects. Unfortunately the life of the “lamp” is comparatively short,

with a time to half brightness of less than 1,000 hours. The principle of electroluminescence is used in the AC Thin Film ElectroLuminescent display (ACTFEL). In this case the phosphor layer is deposited as a thin film instead of being within a powder. Phosphors and electrodes are arranged in columns and rows to allow pixel addressing, and the resultant device is a rugged high resolution display that competes with LCD (liquid crystal display) for some applications.

5.5 Light emitting diodes (LEDs) 5.5.1 Introduction Section 2.2.5 introduced the LED as a special variant of the p-n junction. Electron-hole recombination can result in the emission of photons, the wavelength Hole and electron combine to give light quantum

Conventional current flow

Electron flow

Figure 5.20 Principle of the LED.

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Top contact GaP window AlIn GaP Active layer

+

Substrate Bottom contact

5.5.2 New generation LEDs +

P-GaN Active layers AlGaN and InGaN n-GaN GaN Buffer layer Sapphire substrate

Figure 5.21 High efficiency LED structures. Red AlInGaP made by epitaxial crystal growth (above). Blue/Green InGaN made by chemical vapor deposition (below). The colors in the diagram are used for clarification.

of which depends on the energy gap between the conduction band and the valence band of the electron. λ = hc/Eg where λ is the wavelength, h is Planck’s constant, c is the velocity of light, and Eg is the energy gap. As an example the case of gallium arsenide (GaAs) gives the following. Eg is 1.49eV, where one eV (electron volt) is equal to 1.602 × 10 -19 Joules. h = 6.6262 × 10 -34 Js, and c = 2.99793 × 10 8 m/sec. Putting these values into the equation gives λ as 8.322 × 10 -7 metres, or 832nm, a wavelength in the infra-red. LEDs used in electronics for power indication, simple numerical displays etc were for many years based only on modified GaAs. GaAsP (Gallium ArsenidePhosphide) produces visible wavelengths in the range 590–660nm, depending on the proportion of phosphide to arsenide. When GaPN (GalliumPhosphideNitride) is introduced, a green of 570nm is produced. While the first generation of LEDs were useful

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products, they were comparatively inefficient, and could not be considered as proper light sources. Their spectral characteristics were also limited, being confined to red and orange, with an inefficient and yellowy green.

The situation changed dramatically when alternative combinations were introduced. In the late 1980s a new generation of high output LEDs began to appear based on AluminumIndiumGalliumPhosphide. They produce light in the range 590–630nm (amber – red). At the same time blue LEDs based on SiC (silicon carbide) appeared. While the red end of the spectrum was now served by a high efficiency device, the green and blue were still deficient. The available green was inefficient and Gold wire bond LED chip Reflector cup

Cathode lead

Plastic lens

Epoxy dome lens

Anode lead

Silicone encapsulent

Cathode lead

InGaN semiconductor flip chip

Gold wire Heatsink slug

Solder connection

Silicon sub-mount chip with ESD protection

Figure 5.22 LED packaging. Traditional “indicator” LEDs simply mount the chip in an epoxy package (above). High light output LEDs have more efficient optics, and include a generous heatsink slug. This in turn transfers heat to the printed circuit on which it is mounted, which itself may have an aluminum core to assist with heat dissipation. Diagram courtesy Lumileds Lighting.


Figure 5.23 Examples of LED lighting “engines”. The “coin” from Osram (left) and a white light unit from Insta (right).

too yellow. The blue was hopelessly inefficient as a lightsource, although suitable for indication. In 1993 Shuji Nakumura at Nichia Chemical announced the development of a blue LED based on GalliumNitride, closely followed by an IndiumGalliumNitride “family” which could cover the wavelength range 450–525nm (blue – green). Now the stage was set for LEDs to take their place as lightsources for many applications. The construction of high efficiency LEDs is complex because they require multiple layers. The AlInGaP family is made using epitaxial crystal growth. An epitaxially grown AlGaInP/GaP layer is sandwiched between p type and n type GaP layers. The InGaN family is made by chemical vapor deposition of the successive layers onto a sapphire substrate. Cree make hybrid devices where GaN or InGaN are built on to an SiC substrate. Each wafer can be the basis of many thousands

Figure 5.24 LEDs mounted in strips and then placed in a plastic tube can emulate neon lighting. Here an example marketed by Lumenyte International Corporation is shown applied to an Applebee’s outlet.

of LEDs, since the individual LED is as small as 0.25mm × 0.25mm. The LED chip is packaged in various different ways according to power and application, Figure 5.22 shows the principle of the commonest forms of packaging. Individual LEDs are low power devices, operating in the voltage range 1.5 to 4V. Earlier generation LEDs operated at around 20mA with a power dissipation of not more than 0.1W; however the newer generation operate at higher currents, for example 100mA, and dissipate between 0.65 and 1.2W. While it is confidently expected that LEDs will, in due course, have a role in general lighting, their present roles are in applications that benefit from their properties of high brightness and long life. LEDs are “candela” as opposed to “lumen” sources. In its basic form the LED is a highly directional source with a peak intensity in the range 100–1,200mcd. An array of LEDs, as used, for example, in outdoor TV screens, can thus achieve a very high brightness in the range 1,000–7,000 Nit, albeit by using hundreds of thousands of LEDs in a single display. Practical LEDs incorporate lensing to spread the light, and are specified in terms of: Peak brightness. Viewing cone angle, defined as twice the angle at which the intensity has dropped to one half the peak value. Dominant wavelength; for example 626nm red, 615nm red-orange, 605nm orange, 590nm amber, 525nm green, 505nm blue-green and 470nm blue. Spectral half bandwidth, defined as the wavelength range over which the output is not less than half the dominant wavelength output. Typically the “red” range of LEDs has a half bandwidth of 15nm, and the “blue” range 30nm. Manufacturers of LED based luminaires are currently a little coy about luminous flux, because there is so little of it! An individual LED has an output around one lumen; or up to 10 lumens or so in high brightness red devices. So the cost per lumen is very high compared with other sources. Efficacy has been low, as little as 0.01 lm/W for early blue devices and 0.6lm/W for early red devices. Current devices intended as light sources achieve efficacies 13–25 lm/ W for red and green, and 3–5 lm/W for blue.

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The manufacturing process of LEDs results in a considerable “spread” of characteristics between individual LEDs. In order to make practical products it is necessary to sort the production in respect of the major characteristics to achieve a uniform result in multi-LED devices. In critical applications like giant video screens the basic sorting alone is not sufficient, and individual LEDs require calibrating in order to ensure uniform change in intensity under varying drive conditions. Some wild claims are made about the life of LEDs. From some product literature you would think it is infinite, but it is not. Some of the LED products are so new that reliable life data is not yet available. Unless there is a packaging failure due to overtemperature operation, the LED does not fail catastrophically like a filament lamp, but degrades over life. Useful life is generally accepted to be the “half life”, when intensity has dropped to half the initial value. While operating temperature does not greatly affect light output, it does affect life. Any practical luminaire must be designed to ensure that LEDs are not operated above recommended ambient temperatures. Present indications are that single color LEDs have useful lives in the range 50,000–100,000 hours. 5.5.3 White LEDs and other developments LEDs are potentially excellent sources of near monochromatic light. For decorative and signage purposes they can be mounted in strips within colored plastic tube to emulate neon lighting, as shown in Figure 5.24. Applications taking advantage of the LEDs long life, such as emergency exit signs, are not usually color critical, so can use monochromatic LEDs or a mixture of two colors. When white or any intermediate color is needed, it is possible to achieve it by 3-color mixing using 470, 525 and 626nm primaries. Figure 5.25 shows examples of 3-color luminaires that can be programmed to give any required color output lying within the color triangle determined by the dominant wavelengths of the actual LEDs used. However, the use of three different kinds of LED

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Figure 5.25 Color lighting achieved by 3-color mixing in LED luminaires. The Pulsar ChromaBank™ (above) and the Color Kinetics ColorBlast™ (below)

to achieve white is not really an attractive proposition to luminaire manufacturers. This has led to the introduction of the “white” LED, a hybrid device using photoluminescence. The principle is that a blue (around 470nm) LED is coupled to a yellow phosphor to produce a blueish white. Devices using this structure have shown efficacies of 10 lm/W, Ra around 85 and a CCT of between 6,500 and 8,500K. Newer devices may give as much as 18 lm/W or more.

Figure 5.26 Color Kinetics Inc offer the “Board Family” of color mixing LED engines for OEM use. The different shaped engines permit the production of a wide range of luminaires.


Clearly once this kind of performance can be achieved over a range of wavelengths (and always assuming a sensible price) LEDs will take their place alongside the more conventional sources in general lighting applications. LEDs can be dimmed. Varying the drive current directly affects intensity, but it also affects the dominant wavelength which decreases as drive current is increased. In non critical applications drive current variation can be used to achieve dimming down to around 10%, below which differences in individual devices show up. The preferred method of controlling the output of LEDs is by using pulse width modulation. This ensures that the color output remains constant. PWM can dim LEDs down to below 0.05% of maximum output; essential for applications like big video screens, and important for the emerging architectural and entertainment applications of LED. Figure 5.27 The relative intensity vs wavelength of different single color LEDs (above) and that of the white LED (below). From information provided by Lumileds Lighting.

The present problem with the white LED is life. In common with other phosphor based devices, useful life is limited by the phosphor life. In 1999 the Lighting Research Center at Rensselaer Polytechnic University carried out tests on the then available white LEDs, and found that useful life was around 10,000 hours, but this seems to have improved subsequently. Ultimately an alternative white LED architecture, using AlInGaP semiconductor wavelength conversion layers instead of phosphor may yield a longer lived device. LED research has revealed that the potential efficacy of LEDs is much higher than that achieved in practice. Theoretical internal efficacies range from 75 to 500 lm/W, but the problem is getting the radiation out of the device; much of it gets internally reflected and ultimately absorbed. Agilent, a partner with Philips in Lumileds, have raised the extraction efficiency by abandoning the usual parallel sided LED construction and using instead chamfered sides, such that the device is a truncated pyramid. Individual devices using this construction have achieved over 100 lm/W at 610nm, and produced outputs of 60 lm.

Figure 5.28 LED sources are now being widely used for introducing color into architectural lighting. This is the Fruitmarket Gallery in Edinburgh, where the color lighting is provided by Color Kinetics LED luminaires. These are DMX compatible, so a DMX sequencer is used for control. Photo from the lighting designer, Peter Fink, of Art2Architecture.

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5.5.4 LEP and OLED Cathode (calcium)

The semiconductors and phosphors described in this book have, so far, been exclusively inorganic. This means that they are based on elements we instinctively think of as being mineral and unlifelike, like silicon, zinc and sulfur. There are now organic semiconductors, meaning semiconductors based on carbon chemistry, which covers both life forms and materials like plastics. The idea that one could make a light source directly out of plastic is obviously attractive since it holds out the idea of simplicity of manufacture and great flexibility in application. The work going on in the field of organic light emitters is primarily aimed at the display market. Emissive displays of exceptional thinness and reasonable efficiency are promised. Manufacturing costs should ultimately be much lower than the corresponding backlit LCD equivalent. However, there is no reason why the same technology can not be used in bulk as a light source; possibly providing a more versatile and lower cost alternative to inorganic electroluminescent sources. One of the attractive aspects of organic electroluminescence is that most of the production processes can be carried out in solution – much easier than the complex high temperature processes involved with inorganic semiconductors.

Cathode Electron transport layer Light emission layer Hole transport layer Hole injection layer Anode Glass substrate

Figure 5.29 Structure of a molecular OLED. Diagram adapted from information provided by Cambridge Display Technology.

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Light emitting layer PPV semiconducting polymer Conducting polymer layer PEDOT + PSS Anode IT0

Glass substrate

Figure 5.30 Structure of a LEP device. The calcium cathode injects electrons into the PPV semiconductor film. The anode, which uses ITO as the electrical connection and a conducting polymer to “match” the energy characteristics of the PPV, injects holes. Electron and hole capture in the PPV results in the formation of neutral excitons, bound excited states that decay by emitting a photon. PPV = poly(phenylene vinyline) ITO = Indium Tin Oxide PEDOT = poly(ethylenedioxy)thiophene PSS = poly (styrene sulphonic acid) Explanation and diagram based on information provided by Cambridge Display Technology.

Although the basic idea behind both is the same, organic light emitters are currently classified in two main types, the OLED (Organic Light Emitting Diode) and the LEP (Light Emitting Polymer). The chemical difference is that OLEDs are based on comparatively “small” molecules, whereas LEPs are based on polymer molecules with very large molecular weights (300,000 and more). While research and development of these devices is now worldwide, early and leading exponents of OLED were and are the Eastman Kodak Company; and of LEP the University of Cambridge, UK, and the spin-off company Cambridge Display Technology. Figures 5.29 and 5.30 show the construction of OLED and LEP devices. At present these are made onto glass substrates, because this material keeps out oxygen and water vapor, either of which destroy the device. It would clearly be much better if the devices could be made on a “plastic” substrate, but unfortunately presently available polymers are unsuitable.

25

10000

20

1000

15

100

10

10

5

1

0

1

2 3 Voltage

4

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Brightness Nit log scale

Effifacy Lm/w


0.1

Figure 5.31 Perfomance of a green LEP device. This shows that an efficacy of 20 lm/W can be achieved for an output of 100 nit. Higher brightness (1,000 nit) is achieved at the expense of efficacy (14.5 lm/W). Diagram from Cambridge Display Technology.

In order to show the different layers, the diagrams may give the impression that the devices are quite thick. They are, in fact, extremely thin. In a polymer device the light emitting layer is only 100nm thick. Figure 5.31 shows the performance of a green LEP device. The achieved 20 lm/W with promise of more to come certainly makes the device worthy of consideration as a light source. The problems that remain to be overcome are: • life. Lifetimes of more than 10,000 hours are already being achieved, but not on all colors. • color. For display applications satisfactory red, green and blue devices have been demonstrated or are in production. It is not yet clear whether an efficient white light device could be developed.

5.6 Lasers The laser (acronym for Light Amplification by Stimulated Emission of Radiation) is a device which produces an intense monochromatic beam of coherent light. Most light sources produce light of multiple wavelengths, in multiple bursts of radiation that are not in phase with each other and that are multidirectional if not omnidirectional. The laser in its purest form produces monochromatic (single wavelength, single color) light that is coherent, that is to say that all the light produced is in phase, or “in step”.

This results in an intense narrow beam of light which does not diverge. In high power lasers the energy carried by the laser beam is so great that it can be used for the precision cutting of sheet metal. Laser action arises when a large number of atoms or molecules are put into an “excited” state. When an “excited” atom drops from one energy state to another it emits a photon or burst of light energy. In a normal discharge tube like an arc lamp the process is random and the light incoherent. In a laser tube the energy is so great that a very large proportion of the molecules are in the excited state, and when one decays it can stimulate neighboring ones to do the same. This simultaneous lemming-like drop in energy results in a burst of coherent radiation. The laser differs from the normal discharge lamp in that the transition between energy levels is stimulated by radiation of the same wavelength as the emitted light. Some lasers are pulsed lasers, where the energy needed to get the high proportion of molecules into the high energy state is provided by, for example, a flash tube. Display lasers (those used for lighting effects in entertainment and presentation applications) need a continuous form of energy to ensure that they can give a continuous output. Lasers are constructed in many ways. For display work inert gas lasers are used with optical power outputs of typically 5–15W. They consist of a long ceramic tube which forms a resonant cavity for light. At either end there is a highly polished mirror, one of which is only partially coated, and it is from this end that the laser beam emerges. A DC arc is set up in the tube using a regulated power supply (typically taking 30–60Amps per phase from a 3-phase 400Volt supply). The plasma discharge is of sufficient energy to maintain continuous laser action. A continuous water flow of around 13 l/m is required to cool the laser tube. In terms of turning incoming electrical power into visible light the whole contraption is only about .005% efficient. A practical gas laser for display includes magnetic stabilization of the plasma discharge and a means of maintaining the correct gas fill pressure. It also includes a method for ensuring correct mirror alignment. The gas mixtures in the laser are chosen to

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Brass outer jacket contains magnet

Helical water flow diverter Ceramic plasma tube Brewster window

Mirror plate mounted on invar rods fixed to the outer jacket Tungsten discs contain plasma

Tube support disc transfers heat to tube wall

Figure 5.32 Spectra-Physics Inc’s Chroma® ion laser used for laser lightshows. Available in 5, 10, 12 and 15W versions.

ensure that there is a means of getting the lasing ions to the right energy in the first place. For example in the Helium-Neon laser (in practice realized as a low power device using RF as the exciter) helium is the energizing gas. One of its energy levels is one where it can only lose energy by collision (loss of kinetic energy.) However, this same level is one at which neon emits infra-red light. So in the He-Ne laser the helium is energized, transfers its energy by collision with neon; whereupon the excited neon emits photons. The photons bounce back and forth between the end mirrors stimulating any newly energized neon to create more photons. An equilibrium is reached when the radiation loss through internal absorption, and from the exit beam from the partially reflecting mirror, matches the gain from the lasing process. Laser technology has now advanced to the point that a single laser can produce multiple wavelengths, and it is this type of laser that is used for display. The beam emerging from the laser is less than 2mm diameter, and may have a divergence of less than .00065 radians. It emerges as polarized light from a Brewster window , constructed from crystalline quartz. A laser display system consists of one or more ion lasers (argon, krypton or mixed) mounted on an optical bench followed by an array of optical ele-

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ments to manipulate the beam. These include: Dichroic color separators, to separate the colors from a multiple wavelength laser. Shutters to “switch” the beam on and off. The most sophisticated of these are acoustic modulators

Figure 5.33 Coherent Inc’s Star II entertainment laser includes a motorized mirror system for automatically maintaining laser alignment. The mixed gas multi-line PL model produces a total of 3.5W of different wavelengths: 647nm Red 1.1W 568nm Yellow 0.05W 521–515 nm Green 0.8W 488–477nm Blue 0.8W 458nm Deep Blue 0.045W


Figure 5.34 Optical bench for the direction and control of laser beams for lightshows. Photo from Lobo Laser and Multimedia Systems, Aalen, Germany.

that can work at very high (video image) frequencies. An acoustic wave in a crystal can diffract light, and in a modulating system, the straight through light is either absorbed or used for another purpose, and the diffracted light (as much as 80% of the available light is diverted to the first order diffraction angle) is the light that is used in the display. Static beam diverter mirrors. Dynamic beam diverter mirrors. These are usually based on the principle of the mirror galvanometer. The mirror needed to divert a laser beam can be very small, so it is practical to support such a mirror on a thin torsion wire in a magnetic field. Varying the current though the wire varies the angle of the mirror. Light scattering devices. In rare cases a laser is used to produce a fixed light pattern. More usually it produces dynamic light patterns by the programmed operation of the electrooptical components. The programming is done by a computer with suitable external interfaces to the controlled devices. Because of its size, power requirements and need of water cooling, the big gas display laser can be difficult to site within a lighting scheme. For this reason the laser is sometimes sited remotely, with the light being taken to the optical elements by optical fiber. Gas lasers are big and fragile, and solid state lasers would obviously be more convenient. The idea of a solid laser is not new; the first lasers were based

on ruby. A ruby rod with highly polished ends, one with a full reflecting surface and the other with a partial reflecting surface is the basis of a pulsed laser. The rod is surrounded by a helical xenon flash tube which provides the pumping energy. Of more practical use is the semiconductor laser. This can be achieved by extending the principle of the LED. If a p-n junction is arranged such that the semiconductor is polished with two parallel faces at right angles to the junction plane, light created at the junction can reflect back and forth. Further, if the current through the junction is sufficient, the requisite population inversion can occur – this is the condition needed for laser action whereby there are more electrons at conduction band energy than there are at valence band. Small solid state lasers, laser diodes, have been available for many years, most operating in the infra-red. They form the basis of many industrial and consumer products (including laser printers and CD players.) Visible light laser diodes with sufficient output to be used as a light source are a more recent development. They are already being considered as the light source for some types of video and graphics image projection, either as a simple source of lumens, or the basis of a scanning beam system. In such cases

Figure 5.35 The Viper® Green (532nm) laser from Coherent Inc uses remotely mounted infa-red laser diodes. Their output is fed by optical fiber to pump the green solid state laser head. Outputs of 5–10W are available, and the system only needs around 600W of single phase power. Air cooling is sufficient for all but the 10W model which uses closed circuit water cooling.

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used. These have powers in the range 400W–5kW. • for some medical and solar simulation applications hybrid lamps are available. The high power metal halide lamps are tailored to specific applications. Just as in metal halide lamps intended for the visible spectrum, where particular metals give a required color, other metals enhance particular UV wavelengths. For example UV polymerization of plastics, and photo-resist applications like the manufacture of printed circuits need around 350nm, achieved with elements iron and cobalt. Figure 5.36 Laser diode. This one is a 5mW device from Nichia Chemical, it operates at 405nm (violet).

efficacy around 10 lm/W is expected. In due course they may also be suitable as sources for special luminaires, especially those based on fiber optics. Solid state lasers can be coupled to “frequency changing” devices. These are non linear crystals that have the property of doubling the input frequency. This means that efficient high power infra red lasers can have the output converted to a visible wavelength, and it is likely that this type of laser will displace the “traditional” ion laser in the entertainment lighting field.

5.7 Ultra-violet lamps Ultra-violet lamps are special versions of the fluorescent and high intensity discharge lamps already described. As already pointed out in Section 3.6.2, UV is classified in wavelength bands with the shorter wavelengths being more dangerous. As a summary: • low power UV lamps, used for sun-tanning, fluorescence inspection, stage effects, insect attraction etc are based on normal fluorescent lamp construction with ratings in the range 7W–125W. Versions are also available for UV-C, but these should only be used in their proper enclosure for a specific application (EPROM erasure, sterilisation etc.). • when something more like a point source is needed, black glass mercury vapor lamps are used (Section 4.2). Typical ratings are 125W and 250W. • when higher powers and specific wavelengths are required, special versions of metal halide lamps are

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Figure 5.37 A UV luminaire from Wildfire Inc of Los Angeles, suitable for entertainment and architectural applications. It uses a 400W metal halide lamp. The same company offers a range of fluorescent paints, the ones in the photo are visible under normal light and fluoresce under UV. There are other paints that are clear or “invisible” under normal light, but that fluoresce under UV.


But much better effects (and the almost complete concealment of any kind of visible source) can be achieved by using luminaires which can direct the UV in much the same way as a theater spotlight or floodlight. This avoids the unintentional illumination of other objects or people which could give rise to unwanted or inappropriate fluorescence. This type of luminaire uses either the black-glass mercury vapor lamp, or a 400W metal halide lamp and filter optimized for UV-A. The reflector and lens must also be suitable for UV. Control of the UV light is by shutter. Specialist companies supply a wide range of “invisible” fluorescent paints, which are neutral or white under visible radiation, but are brightly colored under UV.

5.8 Infra-red lamps

Figure 5.38 Example of the use of UV light in scene painting. “Daylight” scene conventionally lit (top). “Night time” scene revealed by UV lighting (below). Illustration from UV Light Technology Ltd. (UK) courtesy of the artist Jim Harper.

Metal halide lamps are also used in medical applications of UV, for curing skin conditions. The hybrid lamps, which combine incandescent and discharge sources, radiate in the range 300– 2,000nm. They are used both as “health” lamps, where the combination of UV and IR is considered beneficial, and as radiation sources for testing materials and equipment in respect of sunlight. As an example Osram suggest that an array of 16 of their 300W hybrid lamps arranged to cover an area of one square meter, and sited 50cm from it, simulates sunlight from a midsummer noon sun with an irradiance of 1kW/m2. It is clearly important that in any weathering test the radiation used has the same proportion of UV in it as experienced from real sunlight. Ultra-violet radiation can be refracted and reflected just like visible radiation. In theater and entertainment applications UV is often used as a “wash” light using simple fluourescent batten type fittings.

Infra-red radiation is the band of radiation between (and overlapping with) visible radiation and the very highest radio frequencies. Wavelengths range from 600nm to 1mm. The filament of a tungsten or tungsten halogen lamp behaves in a similar manner to a black body, and if heated to above 2,000K has peak radiation in the area of 1,000nm (see Figures 1.51 and 1.52). We can feel heat in various ways. If we are in still warm air, then we feel warmth by conduction as the air molecules conduct their movement to our skin. Conduction also applies if we touch a warm pipe or panel. But we also feel heat by radiation when we stand in front of a fire, or receive any source of IR. In this case our bodies absorb the radiation and the process creates heat – i.e greater molecular movement. Short wavelength IR is not absorbed by air over short distances. It also behaves like light in that it can be reflected and focused like light when non IR absorbent reflectors are used. It is, therefore, a useful source of directed heat. IR lamps are simply special purpose tungsten and tungsten halogen lamps. Some of them have considerable visible radiation, since their application benefits from its presence – for example “health” lamps and lamps used in animal husbandry. Others have

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filters to eliminate visible radiation, for example those used for night security surveillance by closed circuit TV by IR sensitive cameras. The attraction of IR as a source of heat is that it can provide localized heat instantaneously and locally. The applications already referred to use reflector type lamps (see also Figure 3.4). Those used for high power heating are special tungsten halogen lamps in quartz envelopes. Examples of use are: • as radiant heaters in entrances to shops, restaurants etc. in winter. Whereas warm air heaters simply lose the warm air to the cold outside, radiant IR heaters give heat that can be immediately felt. (But best to get them on before any ice forms, since the IR can be reflected by white ice!) • as heaters for many industrial processes, for example paint finishing and plastic forming.

Straight tube 20W for warning lights

2400 Joules photo flash U-Tube

Helical strobe tube 20W

Airport runway flash lamp 60 Joule Helical tube in PAR 56 envelope

Figure 5.39 Examples of flash tube construction. Based on information from Amglo Kemlite Laboratories Inc.

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• in food processing and cooking. Modern hotplates use special IR lamps as the source. Since most IR lamps are simply special versions of tungsten filament lamps, thay can easily be controlled using the same methods. Such control will normally be for functional reasons, but it can also be for special effect.

5.9 Flash tubes Flash tubes are used for a wide range of applications, for example: • photography • industrial stroboscopes for examining rotating machinery etc • aircraft warning lamps • obstruction warning lamps • laser pumping • entertainment stroboscopes They produce high intensity pulses of short duration, and are made in a wide range of sizes. They are based on long arc discharges in xenon. A flash tube consists of a borosilicate glass or quartz tube which, depending on the application, may be straight, U-shaped or helical. The tube is filled with xenon gas at a moderate pressure of around 60kPa. The main electrodes are at the extreme ends of the tube. Flashtubes are designed to operate at various voltages depending on length, from 250V to 3,500V or more. However, xenon gas is normally an insulator; to become a conductor it must be ionized. While some specialist tubes use a superimposed high voltage trigger pulse across the main electrodes, most flash tubes use a separate trigger electrode. This is in the form of a wire wrapped round the exterior of the tube. Small tubes use a conductive coating. When a high voltage (several kV) is applied to this electrode it ionizes the xenon which then conducts. Continuous conduction, if it was possible, would blow up the tube, and anyway would not produce the desired short pulse of light. The principle of the flashtube is shown in Figure 5.40. The voltage across the tube is derived from a capacitor or bank of capacitors. The capacitor must be suitable for discharging a high current in to a low impedance load. The


Flash tube

Trigger circuit

a.c in D.C rectifier

Storage capacitor

Trigger transformer

Figure 5.40 Principle of the electronic flash tube. The trigger coil ionizes the xenon gas in the tube. It then becomes conductive and discharges the energy stored in the capacitor. When the current drops below a threshold, conduction ceases, the gas reverts to being an insulator, and the capacitor re-charges.

trigger circuit is an oscillator producing an AC voltage impulse of 150–400V. This is fed through a stepup transformer to produce the high voltage required to ionize the xenon (4–20kV). The trigger circuit is arranged to operate either on a “one shot” basis, to give a single flash, or on a continuous basis, to give a series of flashes at a precisely defined interval. In photographic applications the emphasis is on producing infrequent individual flashes of maximum energy. In stroboscopic applications the requirement is for rapid repetitive flashes at frequencies from 1Hz to 500Hz. Specially designed tubes can operate at frequencies above 1kHz. A flash tube is rated according to average power input, that is to say the product of the energy per flash and the number of flashes per second. Ultimately the limit is set by the power that can be absorbed by the envelope, which is around 5W/cm2 in free air, with considerably higher ratings for forced air or liq-

Tube type Color temperature Life Flash duration Flash duration Energy per flash Tube rating average

Xenon flash 6,500K 100 million flashes 2050ms slow speed 10500Ps high speed typical 180mJ @ 20ms 15W

Table 5.2 Outline specification of a xenon flash tube used in portable stroboscopes.

Figure 5.41 A portable stroboscope for industrial applications. This one has a range of 30–14,000 flashes per minute (0.5–233 Hz). Photo of the Monarch Nova Strobe from Monarch Instrument.

uid cooled tubes. The life of a flash tube is determined by the number of flashes it gives in relation to the maximum flash energy (defined as the explosion energy which bursts the tube within a few flashes). A photographic flash tube might be designed to carry out a few thousand or tens of thousands of flashes, in which case it could be run at around 30% of maximum. A stroboscope tube, intended for millions of flashes, would only be run at 5% maximum. Electronic flash units for photography usually have their flash energy specified in watt-seconds; for example: • flash attached to a camera 100Ws. • self contained studio flash 250Ws. • big studio flash with separate power supply 6,000Ws.

Figure 5.42 Examples of entertainment strobes from Pulsar Light of Cambridge Ltd. The 20W “Jumbo” strobe uses a comparatively small U tube. The 1,500W “Demon” uses a linear tube.

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Flash tube nominal W

20 60 180 1,500

Flash Power at 16 flash/sec J

1.25 3.75 11.25 30

Flash Power at 4 flash/sec J

5 15 45 375

Table 5.3 Examples of flash tubes used in entertainment strobes. Derived from the Pulsar Light of Cambridge catalog.

Photographic flash tubes are not intended for continuous operation, and have a minimum time between flashes of 1–15 seconds. Of course the Watt-second is the same as the Joule, and industrial flash tubes are more usually quoted in J or mJ. Stroboscopes are devices for giving flashes of light at precisely known flash frequencies; they are used for observing moving and rotating machinery. When the machine or rotating device appears stationery, the flash frequency is the same as (or a sub-multiple of) the reciprocating or rotation frequency of the device under observation. Examples of flash tube specifications for portable industrial stroboscopes are given in Table 5.2. The flash duration is limited by the impedance of the flash tube, the capacitor value and the presence of any inductance in the circuit. Flash tubes used for aircraft and airport runway applications are rated for low frequency continuous operation. The frequency range is typically from one flash every 10 seconds to a maximum of two flashes per second. For example a 120W warning flash mounted in a PAR56 envelope might be rated for two 60J flashes/second with a life of more than seven million flashes. A different kind of stroboscope is the entertainment stroboscope. This is a low cost device used in discotheques and stage productions. Here the aim is to get the brightest possible flash for the minimum outlay. Flash frequency is limited to the range 0.5– 16Hz, or the stroboscope can be externally triggered. This allows the flashes to synchronize to the beat of music, for example. The flash power can be quite significant when a large number of high power entertainment strobes are used together. Table 5.3 gives some examples.

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5.10 Fiber optics and lightguides 5.10.1 Fiber optics Strictly speaking fiber optics and lightguides are not lightsources at all, but for many applications they have become the means of delivering light to where it is wanted; so an understanding of how they work is useful if one is to understand how to control their light output. The principle of the optical fiber is shown in Figure 5.43. It depends on the total internal reflection of light arising from using two materials of differing refractive index. A single optical fiber consists of a core surrounded by a cladding of a lower refractive index. The concentric fiber may then itself be placed in a protective outer sheath, or many fibers may share a common sheath. For light to get in to the fiber in the first place, it must be directed at one end. The fiber has an acceptance angle which is determined by the refractive index of the core and the cladding. Light incident outside the acceptance angle may either be reflected off the entry face, or fail to be totally internally reflected inside the core. In Figure 5.43 it can be seen from the internal reflection mechanism that the acceptance angle also determines the angle at which the light comes out the other end of the fiber. In principle optical fiber can be made from any transparent material, for example glass, plastic or even liquid. In practice optical fibers used for lightLight acceptance cone

Sheath (optional)

Cladding

Core

Figure 5.43 The principle of fiber optics.


ing are made either from glass or from plastic polymers such as PMMA (poly-methyl methacrylate). For lighting purposes, fiber is configured in one of four principal ways: • as a single large core plastic fiber. • as a bundle of large core plastic fibers. • as a bundle of small core plastic fibers. • as a bundle of glass fibers. The fibers within a glass bundle are thin, from as little as 50μm to 1mm. Plastic fibers are thicker; the large core fibers can be as much as 18mm diameter. Glass fiber optic components are normally fully factory made in the form of “harnesses”. This is necessary because the ends of the fiber need finishing and polishing. The result is a precision, long life but somewhat expensive product. Plastic fiber can be finished “in the field”, but for some applications its optical performance is not as good. Plastic fiber can discolor, and lasts about 10–15 years. It is not suitable for outdoor use. The performance of fiber optics as a light delivery method is measured by a number of factors, for example: Attenuation over a given length. This can either be expressed as a simple percentage, or as a relative power figure, such as dB/m. It is significant, even the best fiber optic illumination system will lose around 20% of the light over 5m.

Figure 5.44 Examples of the performance of fiber optics. Top, light transmission of Schott Spectraflex® glass fiber harnesses. Middle, the variation in optical attenuation with wavelength of single glass fibers (also from Schott). Bottom, variation with wavelength in a large core plastic fiber (derived from papers given by the LRC at Rensselaer Polytechnic Institute).

Figure 5.45 Fiber optic “harness” from Schott.

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Figure 5.46 Examples of illuminators. The SpectraStar uses a 150W metal halide lamp (Left) while the 100SH uses a halogen lamp (50–100W). Both illuminators from Schott.

Spectral response. The transmission of different wavelengths is not uniform (see Figure 5.43). If “white” light is put through a fiber, it may appear “colored”, for example a greenish tinge, at the output, especially if a long length is involved. Bending capability. Thin fibers are best for a short bending radius. Uniformity. Light attenuation is caused by both absorption and by light scattering (instead of the desired reflection.) Scattering is worse on bends. In a multiple fiber system not all the entry light gets into the fibers since, if they are circular, it is clear there must be gaps between them. This loss is referred to as packing fraction loss. In a glass fiber bundle there are various ways of organizing the fibers. For optical instruments they can be coherent, such that they can be used for imaging. For lighting they can be randomized or unrandomized. This is significant when one illuminator illuminates a big bundle of fibers that is then split into several smaller bundles, as happens with pre-prepared harnesses. If the fibers are unrandomized, and if the end illumination is uneven, one bundle may receive more light than another. A randomized arrangement means that each small bundle has its fibers distributed randomly in the big bundle – ensuring that all the small bundles receive the same amount of light. A fiber optic illumination system thus consists of three principal components. The fiber, fiber bundle or fiber harness itself, suitably finished at both entry and exit, an illuminator (that may be designed to

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illuminate more than one bundle) and, in many cases, exit optics. In addition to the basic components, there may be accessory components, for example a color wheel. The aim of a fiber optic illuminator is to collect as much light as possible from a conventional lamp, and direct it on to the entry end of the fiber or fiber bundle, in such a way that the light is all within the acceptance angle. Studies by the Lighting Research Center at the Rensselaer Polytechnic Institute, Troy, NY have found enormous differences in the effectiveness of different illuminators. One illuminator can get twice as much light into a fiber harness than another illuminator using the same lamp. Their work has shown that effective illuminators must be designed with low numerical aperture or NA. NA = n sin α where n is the refractive index of the material (in this case the core) and α is the most oblique angle of incidence used. For plastic fiber, in particular, low NA is essential for illuminating long fibers. On fiber lengths over 10m there can not only be an improvement of around 55% in overall light transmission, there is also an improvement in color performance. The transmission of different wavelengths in plastic is markedly different for different values of α. The most commonly used sources are tungsten halogen lamps and metal halide lamps. Tungsten

Figure 5.47 Examples of end optics. Adjustable and fixed lens fittings from Schott (above). Waterproof and bollard fittings from Philips (below).


Figure 5.48 Leaky, or side emitting, fiber optics make a good substitute for neon. These examples are from Lumenyte International Corporation. Top photo is of the Franklin County Courthouse, Columbus OH. Bottom photo of a pedestrian bridge in Santa Rosa, CA. Lighting design by Michael Hayden of Thinking Lighting.

halogen has the merit of simplicity, and is suitable for conventional dimming. Metal halide gives higher efficacy, longer life and higher color temperature (usually around 4,200K for this application). Suitable ballasts allow only limited dimming. Most illuminators have fan cooling, so noise can be an issue. Exit optics exist in many forms. In a plastic fiber based “star cloth”, the individual fiber ends are viewed directly. In sophisticated systems for display cases, the exit end of the fiber bundle is coupled to a miniature “luminaire” which itself can provide the normal functions of spotlighting or floodlighting, often by the use of additional lensing. A special form of fiber optics is the “leaky fiber”. Such fibers allow deliberate leakage of light by scat-

tering down their length. They are used as comparatively low brightness substitutes for neon and cold cathode tubing. The system efficacy of fiber optic illumination is low. When all losses are taken into account a tungsten halogen system might give 3 lm/W and a metal halide system 12 lm/W. However, for some applications fiber optics have considerable advantages. • ideal for wet and hazardous locations. The source can be safely mounted in its own suitable environment. The light has no electrical connections whatever. • the exit light is free of IR and UV (but for some fibers filters are needed at the entry to avoid damage to the fiber itself). • easy maintenance for multiple light points. Instead of having to change 50 or 1,000 lamps in a showcase or a starfield, only one conveniently placed lamp needs replacing. • esthetic advantages for some applications, arising from the absence of bulky luminaires. Coupled with this is the ability to get a source of illumination into a position that would be quite impractical with a conventional luminaire. Applications range from entertainment and display, through signage, swimming pools and fountains, to lighting for museums, galleries and showcases. Industrial and professional applications include those in medicine, dentistry, clean rooms, and manufacturing processes. 5.10.2 Light guides There are many lighting systems and luminaires that use a combination of reflection and refraction to achieve a particular distribution of light, and the general subject is outside the scope of this book. However, it is appropriate to include one other device that is a near relative of the optical fiber – but this time with significantly greater dimensions. The tubular light guide or light pipe (promoted in particular by 3M and by TIR Systems Ltd) is another device that works by internal reflection with controlled “leaking” of light. The principle is shown in Figure 5.49. The guide is an acrylic or polycarbonate tube of around 150mm diameter. Light

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Cross section

Endfeed light guide

Acrylic housing

Extractor

Input endcap

Extractor Reflected light ray

Mirror endcap

Emitted light ray 0

X Clear prismatic film

Emiting sector

Figure 5.49 Examples of 4in (100mm) and 6in (150mm) diameter tubular lightguides from TIR Systems Ltd. The bigger guide can be up to 40ft (13m) long with single end illumination, and with a 400W HID lamp gives 310 lumens/ft at this length.

Figure 5.50 TIR Ltd use LEDs as a source of illumination in their color changing light pipes.

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enters one end of the guide, and is normally internally reflected down its length. The far end is mirrored, so light is reflected back again (although for long lengths it is possible to have a lamp at both ends). The inside of the tube is lined with a clear optical film that has a microprismatic surface. This ensures a uniform and efficient spread of light down the tube. Part of the interior of the tube is fitted with an “extractor” film. This provides a diffuse reflecting surface. Part of the reflected light from the extractor is incident on the tube wall at an angle that allows it to escape. The combination of prismatic film and extractor film can be tailored to ensure that most light is emitted from a defined angle. A light pipe of this kind, illuminated from one end only, can give even lighting over a length of over 13m. Cylindrical light guides or pipes generally use metal halide lamps as the original source of lumens. However, they also use less conventional sources. 3M have used the Sulfur Lamp (see Section 5.1.5) as a source, because it makes good optical fit with the pipe, and has excellent spectral characteristics. For narrower diameter pipes used for interior decorative installations, LEDs can form the basis of color changing pipes. The use of special optical films for light control is not limited to cylindrical light guides. Rectangular guides are forming the basis of exceptionally thin


illuminated advertising signs. Instead of using multiple fluorescent tubes across the back of a color transparency, tubes are sited at the edge only, and light is distributed to the back of the transparency by optical film.

5.11 Video displays as lightsources It is easy to overlook the fact that there are sources of electrically generated light other than “lamps” as generally understood. For example, the cathode ray tube (CRT) has been a significant source of light in

the home for 60 years; although that is not its intended role. The surface brightness of CRTs, and now other emissive display devices like plasma display panels, is significant. In the appropriate surroundings they can form part of a lighting scheme, whether they are showing normal video images or are showing color washes for effect. Clearly this idea has only been used in the entertainment environment, but there is no reason why adventurous lighting designers should not create electronically shaped color. Work of this kind is likely to be based on massed LED, but the older technologies should not be forgotten.

Figure 5.51 CRT Video screens as a main source of light. Photo of the “Point After” dance club on the Carnival Destiny cruise liner from Wynne Wilson Gottelier.

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Chapter 6


Electromagnetic

components

Part 3 – Lighting components Today most lamps require some kind of “load interface” between themselves and the electricity supply – for example discharge lamps need a ballast, and low voltage halogen lamps need a transformer. The next two chapters look at the common interfaces, Chapter 6 deals with electromagnetic components, and Chapter 7 deals with electronic components. While electronic components are taking an increasing proportion of the market, electromagnetic components still have an important role to play.

6.1 Principles of transformers and inductors. 6.1.1 Introduction In this Chapter we are concerned with electromagnetic components that themselves are in the lamp circuit, and that operate at line voltage and frequency. In practice this means transformers and ballasts. The word ballast is the generic word for any device that limits the current through a discharge tube. The simplest ballasts are inductive reactors or chokes. More complex electromagnetic ballasts combine transformer and choke action. A complete ballast circuit usually includes a capacitor for power factor correction, but sometimes the capacitor is also used as a current limiting reactance. The main design aim with this kind of component is to minimize losses. In lighting components losses are not only wasteful, they can actually be dangerous by producing local hotspots which can result in fire. Legislation in both Europe and the USA is now requiring higher efficiencies in all kinds of lighting equipment, and this is helping to ensure a higher standard of component. (See also Sections 1.2.6, 1.2.7, 1.3.4 and 1.4.1 for definitions etc. that are assumed in this chapter).

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6.1.2 Transformer action A simplifed explanation of what happens in a transformer starts by looking at Figure 6.1 This shows two coils carrying current, coupled together. Coil 1 we can refer to as a primary coil, and Coil 2 as a secondary coil. In the description that follows, the symbols have the meanings ascribed to them in the figure. The voltage induced in a coil is proportional to the number of turns in the coil and to the rate of change of flux through it. In the SI system of units: e = −N

dφ dt

This tells us two things about transformers. First, that the voltages are proportional to the turns ratio; since, for the mutual flux that is shared between two coils, dφ/dt is the same for primary and secondary. Thus it is the case that: e1 N = 1 e2 N2

Second, that a high voltage can be achieved either by lots of turns, or by a faster rate of change of flux. This can be achieved by working at a higher frequency. Transformers are, in practice, remarkably efficient, so it is valid to start by thinking of a perfect transformer. This is one that has no leakage flux, so

ELECTROMAGNETIC COMPONENTS

Figure 6.1 Basic transformer action.

all flux is mutual, and has no other losses. In such a case, for the primary, the applied and induced voltages are equal, so: e1 = v1 = V1m sin ωt (equation 6.1) but also: e1 = N1 d(φm1 - φm2)/dt So: V1m sin ωt = N1 d(φm1 - φm2)/dt Which can be re-written (by integration) as: φm1 - φm2 = - V1m cosωt/N1ω The expression (φm1 - φm2) represents the mutual flux Φ, so its peak value can be written as: Φm= V1m /N1ω

Therefore the peak applied voltage V1m = ωΦm N1 Substituting this in Equation 6.1 gives us: e1 = ωΦm N1 sin ωt From this the r.m.s. voltages induced in primary and secondary can be written as: E1 = ωΦm N1 /√2 = 2πf Φm N1 / √2 = 4.44 f Φm N1 Similarly E2 = 4.44 f Φm N2 But the mutual flux is the product of flux density and core area, so it is also possible to write: E1 = 4.44 f BmAN1 and E2 = 4.44 f BmAN2

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This confirms to us that the induced voltage is proportional to core area, the frequency and to the maximum flux density. This shows that in principle high frequency transformers can be smaller for a given power rating than their low frequency counterparts. Notice that there is nothing in the transformer equations above that says a transformer must have an iron core. However at low frequencies it is simply not possible to construct an effective transformer without one. Only by using a core material of high permeability is it possible to achieve a sufficiently high flux density, and a high mutual flux with minimum leakage. 6.1.3 Core materials The core material used in a transformer or choke must exhibit the right magnetic properties at the operating frequencies to be used. The B/H curve shown in Figure 1.14 represents the magnetizing effect as DC is applied to an inductor. Corresponding curves for AC get progressively fatter as the frequency is increased, and in practice the simpler materials cannot be used at high frequencies because of hysteresis loss. The use of laminations minimizes eddy current losses, but, again, at high frequencies they become appreciable and a different approach must be used. The principal ferromagnetic materials used for transformer cores are summarized in Table 6.1. The most common material, sometimes referred to as transformer steel, is a silicon iron alloy. A special version of this is grain oriented steel. In such material the magnetic domains within it are aligned, and this has the effect of reducing the magnetizing force needed to achieve a particular flux. The alignment is achieved by cold rolling the steel strip. Standard transformer steel is fine for transformers operating at normal mains frequencies, but is not suitable for audio frequencies or for circuits that depend on saturation. Alloys of nickel and cobalt are used and in all cases the final magnetic properties of the material depend very much on how it is treated. This relates both to the rolling process to achieve lamination thickness, and to heat treatment and cooling cycles. Table 6.1 refers to “square loop” and “round loop” alloys. Round loop alloys have a

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traditional hysteresis loop like Figure 1.14. Square loop materials have an almost rectangular hysteresis curve like Figure 6.2. Such materials are characterized by a low magnetizing force to achieve saturation, high remanent flux density and low coercive force. For frequencies above the kilohertz range, eddy currents become a serious problem and it is necessary to use a core material that has a high resistivity. This can be achieved by using a powdered core, where, instead of separating the core into laminations, it is separated into particles. Both powdered iron and powdered alloys are used. Ferrites, shown at the bottom of Table 6.1, are non metallic ferromagnetic oxides of the form XO.Fe2O3 where X is a divalent metal, for example Iron (Fe), Zinc(Zn), Manganese(Mn), Nickel(Ni) or Magnesium(Mg). Actual ferrite core materials are usually a mixture of two or more types of ferrite crystals. Ferrites have a resistivity around 1013 greater than iron, and, as a result, eddy current losses are negligible. This allows operation at very high frequencies. The practical problems of ferrites are: • they are brittle, ceramic-like materials, difficult to work. • they have a relatively low saturation flux density. • they have a low Curie point. The behavior of the core materials described here is called ferromagnetic. At high temperatures they cease to be ferromagnetic, and the Curie point of a magnetic material is the temperature at which the change takes place. Ferrite Curie points can be so low that in a practical circuit design it is necessary to take account of operating temperature. 6.1.4 Magnetizing current When a transformer primary is fed an alternating voltage, and when there is no load on the secondary, it takes a very small current. Indeed, if it behaved as a perfect transformer, the primary of an unloaded transformer would behave as a perfect inductance, with no net current taken. What happens when a load is applied to secondary can be imagined as modifying this perfect inductive behavior. The mutual flux cuts through the secondary


and induces a voltage across it – if a load is connected, a current flows. As a load is taken from the secondary, the back e.m.f. of the primary is reduced, resulting in a net current flow. In the perfect transformer the mutual flux remains constant. For this to be the case the magnetizing force, and hence the m.m.f. expressed in Ampere Turns must also remain constant. So if the secondary ampere turns rises, the primary ampere turns reduce, correspondingly reducing the back e.m.f. No transformer is perfect, and it is interesting to examine the way the current varies. Figure 6.3 relates the induced voltage to the sinusoidal variation in flux. But the current needed to achieve the variation is NOT sinusoidal. This is because the core must be dragged round the hysteresis loop. The actual magnetizing current waveform will be dependent on the B/H loop for the material being used. Material 4% silicon iron alloy 34% silicon alloy, grain oriented 40% nickel iron alloy “square loop” 50% nickel iron alloy “round loop” 70-80% nickel, 3-5% molybdenum iron alloy 3-50% cobalt iron alloy Powdered iron Powdered Molybdenum nickel iron alloy Manganese zinc ferrite Nickel zinc ferrite

Figure 6.2 The rectangular or “square” hysteresis loop. Hc is the coercive force necessary to reduce B to zero.

Trade name (all )

Initial Permeability 400

Maximum Permeability 8,500

Maximum Flux Density 1.6T

Remanent Flux Density 1.1T

Typical Frequency 1.6 compared with 1.4 for a simple reactor) in order to assist lamp starting. This is achieved by cutting slots in the transformer core under the secondary. However, the lamp electrodes are affected by a high crest factor, and the lamp manufacturer does specify a limit. For high pressure sodium lamps, ballasts with better regulation than achieved by the CWA may be offered. One variant uses isolated secondary windings, with a separate winding for the capacitor. This achieves tight regulation, but at the expense of increased losses and greater weight. USA magnetic ballast manufacturers offer “kits” to luminaire manufacturers, with many options. In addition to the standard ballasts mentioned here, other options include multi-lamp ballasts (actually 2-lamp lead-lag configurations) and the series feeding of high pressure mercury lamps through current limiting transformers. As with the fluorescent lamp ballasts, HID ballasts are usually supplied with flying leads.

starting switch, and this is generally provided by the glow starter already described in Section 3.4. The glow starter does have some disadvantages; one being that its life may be no longer than that of the tube. The need for the glow to be established delays the application of the pre-heat current, and the breaking of the circuit is random timed. This means first, that the starting pulse is of indeterminate amplitude, depending on when in the half cycle the break is made, and second, that a start attempt may be made before the cathodes have reached operating temperature. In turn this means that if the starting voltage is too low or early, the tube does not strike, and needless damage may be done to the cathode coating, shortening the life of the tube. A solution to this problem is the use of an electronic starter. This device can do several things: • by being active as soon as power is applied, the starting delay is reduced, since there is no equivalent of waiting for the glow discharge to warm up the bimetallic strip. • the pre-heat time can be precisely timed (usually two seconds).

6.5 Ignitors and starters 6.5.1 Starters for fluorescent lamps Fluorescent lamps using the pre-heat start circuits of Figure 6.31 (a), (b) and (c) require an automatic

Figure 6.48 Examples of CWA (left) and reactor (right) “core and coil” ballasts offered on the USA market. Photo from Howard Industries.

Figure 6.49 Electronic starter circuit using the Power Innovations Fluoractor™. The diode bridge ensures the device works for both half cycles. When power is applied the gate voltage delivered by R1 causes the device to switch on, starting cathode pre-heat. Cathode current causes a voltage to be developed across D6 and D7 which starts to charge the timing capacitor. When peak voltage at TH2 gate is sufficient, TH2 fires and shuts off TH1 creating the starting pulse. In fact a series of pulses is produced until the lamp starts conducting. Once the lamp is conducting the input voltage drops to a point where it is insufficient to trigger TH1. Should the lamp fail to strike, a point is reached where the capacitor is fully charged, and at this point TH2 is held hard on, and, therefore TH1 is held off. Circuit © Power Innovations Ltd

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• the application of the starting pulse can be optimized in relation to the half cycle, so that the starting pulse is of the right energy to start the tube without damaging the cathodes. This also reduces electromagnetic interference. • if a tube fails to start after a set number of attempts (around 10) the luminaire is shut down. This avoids repeated start attempts on a failed tube which can result in ballast overheating. Electronic starters can be triac, thyristor or transistor based. The company Power Innovations have developed a special GTO device for the purpose called the Fluoractor® which combines in one package crowbar protection (and the ability to absorb choke energy when a lamp fails to strike) and thyristor switching with gate turn off control. A circuit showing its use is shown in Figure 6.49. Electronic starters are housed in the same housing as glow starters and are plug-in replacements. See Figure 6.50. While it is recommended that fluorescent lighting that is to be frequently switched (e.g. by occupancy sensors) use electronic ballasts, this cannot always be justified economically, especially if a retro-fit is involved. In such cases the electronic starter is a good investment, its small extra cost being quickly repaid by extended tube life. Its own life is at least that of the ballast. 6.5.2 Ignitors for HID lamps Ignitors for HID lamps are required to generate a high voltage pulse, or series of pulses, to initiate the discharge. In electromagnetic ballast HID circuits

Figure 6.50 An electronic starter for fluorescent lamps; it is a plug-in replacement for the conventional glow starter. Photo from Auralight AB.

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Figure 6.51 The common ignitor arrangements for HID lamps. Parallel ignitor (top), impulser ignitor (center) and superimposed pulse ignitor (bottom).

there are three main types of ignitor: • parallel • impulser • superimposed pulse The connection arrangements are shown in Figure 6.51. The figure also shows the nature of the starting pulse provided by each arrangement. The parallel starter works by placing a capacitor in series with the ballast inductor, switched into circuit by a thyristor. The high capacitor voltage resulting from the LC combination is then discharged into the lamp. But “high” voltage is relative, and this type of starter is only suitable for small metal halide lamps, and small low pressure sodium lamps using a reactor ballast in place of the more common transformer ballast. The tendency for small metal halide lamps to be operated from electronic ballasts (or to use superimposed pulse start) means that the parallel ignitor is the least used of the three techniques. The impulser ignitor uses a similar capacitor arrangement to generate the initial pulse, but greatly


Figure 6.53 Outline circuit of the superimposed pulse ignitor. Figure 6.52 Example of superimposed pulse ignitor. This one is suitable for HPS lamps of 100–400W and metal halide lamps of 70–400W.

increases the voltage by using part of the ballast winding as an autotransformer to boost the pulse to the kilovolts required. The advantages of this system is that the ignitor has virtually no losses when the lamp is running, and the lamp can be at some distance from the ballast. The disadvantage is that the ballast choke must have a suitable tapping and must be able to withstand the high starting voltage – both factors tending to increase the cost and complexity of the ballast. The superimposed pulse ignitor is the most widely used. Its disadvantage is that it is permanently in circuit, so does contribute to circuit losses. Such losses are around 1W for a 70W lamp ignitor, 2.5W for a 250W lamp ignitor and 5W for a 1,000W lamp ignitor. Its great advantage is that it is independent of the ballast construction, and the ballast is not subjected to the starting voltage. As can be seen from the figure, the superimposed pulse ignitor superimposes a number (between two and six) of pulses on each half of the sine wave. These are precisely timed to occur at specified phase angles, for example 60–90° on the positive going cycle and 240–270° on the negative going. The impulse amplitude is between 2kV and 5kV depending on lamp type, and typical pulse width is 1.5μS. Figure 6.53 shows the circuit of a typical ignitor. Here a SIDAC or equivalent device is used to discharge the capacitor. The SIDAC is not unlike a

diac in that it has an avalanche breakdown at a specified voltage, but the voltage, around 130V, is higher than that of a diac. This creates the necessary di/dt in the primary to create the high voltage impulse in the secondary. When an HID lamp reaches end of life, a condition can occur where the arc voltage increases as the discharge is established. This results in the lamp coming on, but quickly going out as the arc voltage exceeds the available instantaneous voltage at each half cycle. The result is a continuous attempt at ignition which is both subjectively irritating and stressful to the lamp circuit. For this reason most superimposed pulse and impulser ignitors are available with an automatic cut-out feature that shuts down the ignitor after a preset period (between 2 and 15 minutes depending on lamp type). 6.5.3 Hot re-strike ignitors; auxiliary switches Where it is possible, hot re-strike of HID lamps requires an ignitor voltage of tens of kilovolts. Xenon (DC) arc lamps require a similar voltage for striking. Ignitors for this duty require an alternative construction to achieve a high enough di/dt in the primary of the pulse circuit. Sometimes a spark gap is used for this purpose. A manually operated hot restrike ignitor circuit is shown in Figure 6.54. However, it is now the case that those applications requiring hot re-strike are also the ones requiring lightweight ballasts. The majority of such applications are now served by electronic ballasts.

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Figure 6.54 Manually operated hot re-strike circuit using a spark gap.

There are some applications where the delay in reaching full light output from an HID lamp is unacceptable. For example a secure warehouse with HS lamps might require some immediate lighting when power is applied. This problem is dealt with by having an auxiliary tungsten halogen lamp that is switched on when power is applied to the luminaire, but which switches off after 3–5 minutes when the main lamp has reached full brightness. Automatic auxiliary switches are available for this purpose (Figure 6.55.)

6.6 Lighting control by transformers and ballasts 6.6.1 Summary Variable autotransformers and variable reactors can be used for dimming lighting, and these items are discussed in Section 8.2. Variable autotransformers, multi-tapped autotransformers, and multi-tapped electromagnetic HID ballasts are used to regulate the power available to lighting circuits or luminaires. Therefore they can form the basis of energy management schemes, although care must be taken to ensure that the lighting performance is not compromised to the point at which it is ineffective. These devices are discussed both in Section 14.3 and in the relevant applications sections.

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Figure 6.55 Auxiliary lamp switches to provide illumination during HID lamp run-up. The one on the left uses hybrid relay and solid state technology, the one on the right is triac based. Photo courtesy of Venture Lighting International.

6.6.2 Dimming of electromagnetic ballasts and transformers Generally the suitability for dimming of particular lamps, ballasts or transformers is covered in the sections dealing with the lamp or the ballast concerned, and is also covered, where relevant, in the applications chapters. Here “suitability for dimming” is taken to mean the connection of the device to conventional thyristor or triac dimmers. As a summary: • electromagnetic ballasts and transformers should not be connected to electronic dimmers unless approved by the transformer/ballast manufacturer. • approved low voltage transformers for incandescent (tungsten, tunsgten halogen) lighting, and high voltage transformers for cold cathode lighting are suitable for dimming. • some HID lamps with reactor or CWA ballasts are suitable for partial dimming. It should be noted that when phase cutting dimmers are used, there is a considerable increase in odd harmonics compared to the harmonics generated at 100% output . For fluorescent lamps today’s trend is towards the use of electronic ballasts when lighting level control is required. However, it is possible to dim T12 fluorescent lamps using electromagnetic ballasts, and the preferred circuit for doing so is shown in Figure 6.56. The circuit is a variation of the rapid start circuit. Instead of having a cathode heating winding on the


main ballast, there is a separate cathode heating transformer that maintains full cathode heating at all times the circuit is on. The main tube voltage is varied by the thyristor dimmer, the sharp rising edge of the thyristor switching helping to develop the strike voltage needed. Much lower light levels can be achieved if this is augmented, and one way of doing this is to use a tapped choke with a small capacitor (known to some as the “tickling” capacitor). In any circuit involving the dimming of electromagnetic ballasts it is essential that power factor correction components are installed prior to the dimmer. Connecting power factor correction capacitors to the output of dimmers is likely to destroy both the dimmer and the capacitor. Large installations using dimmed electromagnetic ballasts are candidates for central variable power factor correction (see Section 6.7.3). A possible exception to this rule is where the dimming device has a sine wave output. In practice this means either a variable autotransformer or a sine wave output electronic dimmer (see Section 8.6.1). Specialist suppliers offer systems of this kind for controlling conventional HID lamp circuits to achieve some energy savings when full light output is not required.

6.7 Power factor correction 6.7.1 Introduction All discharge lighting that uses a reactor ballast or leakage reactance transformer to limit the current through the lamp results in a poor power factor, typically as low as cos φ = 0.3–0.5. There are some fluorescent lamp circuits that use a series capacitor to provide a current limiting reactance, and these circuits can have an acceptable power factor. In these circuits the capacitors must have a precise value (within ± 4% or better) to ensure correct lamp current, and because high voltages are developed in these circuits, they need to have a voltage rating considerably higher than line voltage. Apart from this special case, all electromagnetic ballasted lamps above around 30W are fitted with

Figure 6.56 Circuit for the dimming of some types of fluorescent lamps using reactor ballasts. The capacitor is optional; it improves low light level performance.

parallel connected capacitors to provide power factor correction to 0.85 or better. The capacitors are normally fitted within the luminaire alongside the choke; however it is also possible to apply central correction to a large group of lamps. Power factor correction is also referred to as compensation. For electromagnetic ballasts the correction relates to cos φ also known as displacement power factor. With electronic ballasts that generate harmonics the correction requirement is more complicated and is described in the next chapter. 6.7.2 Power factor correction for individual fluorescent and HID lamps. In the standard fluorescent lamp circuit of Figure 6.30 the current i is out of phase with the supply voltage Vs. In Figure 6.57 the circuit is repeated, but this time showing the supply current i having two components, iL going through the lamp, and iC going through a parallel connected power factor correction capacitor. The vector diagram in Figure 6.57 shows as an example φ being 70° (power factor 0.34) for the uncorrected circuit, but only 25° (power factor 0.9) for the corrected circuit. Applying simple trigonometry to the diagram, and using the principles explained in Section 1.4, a formula can be derived for the capacitance in microfarads required to achieve a given power factor. This is also shown in Figure 6.57. The manufacturers of electromagnetic ballasts specify the capacitor to be used to achieve an

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overpressure pushes the layers apart leaving an insulated area, and the capacitor continues normally, with a minute reduction in capacitance as the result of the loss of some electrode area. Depending on the size and the requirements of the luminaire manufacturer, connection to the capacitor is by fixed solid conductor leads, push on terminals or other proprietary connection system. Mounting is by spring clip, molded-on plastic feet, or a stud on the end of the capacitor. Examples of construction are shown in Figure 1.33. The specification of a capacitor for power factor correction in luminaires covers the following: • Line voltage. • Test voltage (typically twice the nominal line voltage for 60 seconds). • Insulation (typically 2kV for 60 seconds for a 250V AC rated capacitor). • Operating frequency (normally only 50/60Hz). • Working temperature range (typical -25°C to 85°C). Lamp Type

Figure 6.57 Power factor correction in a fluorescent lamp circuit. The formula shown here gives the capacitance in microfarads.

acceptable power factor. Table 6.5 lists some examples, and shows clearly the big difference in line current that is involved. This in turn explains why power factor correction is usually applied near the lamp, since central correction would require the conductors to the lamps to be of almost double the current carrying capacity. The comparatively small capacitors needed for correction at the luminaire are normally cylindrical capacitors in metal or self extinguishing plastic cases. The capacitor itself is made from metallized polypropylene film that has self healing characteristics. If there is a voltage breakdown in the capacitor the metal layers around the breakdown channel evaporate due to the high temperature of the arc that is formed. Within microseconds the resulting

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TC 18W T 18W T8 T 36W T8 T 58W T8 T 85W T12 T 100W T12 T 140W T12 HM 80W HM 250W HS 50W HS 100W HS 150W HS 250W HS 400W HS 1000W HI 35W HI 70W HI 150W HI 400W

Lamp current A

Line current A

Capacitor PF

0.20 0.37 0.43 0.67 0.80 0.96 1.5 0.80 2.15 0.75 1.2 1.8 3.0 4.4 10.3 0.53 1.0 1.8 3.5

0.11 0.16 0.22 0.35 0.43 0.55 0.77 0.45 1.4 0.33 0.56 1.1 1.4 2.2 5.0 0.27 0.43 1.1 2.2

2 4 4 7 8 10 18 8 18 8 12 20 32 50 100 6 12 20 35

Table 6.5 Approximate capacitor values for achieving 0.9 power factor on common fluorescent and HID lamps on 230V 50Hz supply.


Figure 6.58 Simplified equivalent circuit of a real capacitor, with losses shown as a parallel resistance. The tangent of angle δ is the loss factor.

• Capacitance tolerance (typical ±10%). • Loss factor. The loss factor of a capacitor is a measure of its imperfection. All capacitors have losses analogous to those in inductors. There is a leakage loss since no solid insulator is perfect, and there is a dielectric heat loss arising from the continual change in current direction – analagous to hysteresis in a magnetic medium. Both together can be considered as a resistance in parallel with the “pure” capacitance, and the loss factor is an expression that relates the resistive to the capacitive impedance. Figure 6.58 shows how a real capacitor does not have the 90° phase shift of a “perfect” capacitor, but has one that is less by a small angle δ. The tangent of this angle is the loss factor, and in a typical lighting capacitor the value is around 0.00003. For clarity the

figure exaggerates δ which is very much less than one degree. There are some safety issues with capacitors. The first is that if power is disconnected from the luminaire, a capacitor may still have a substantial charge, presenting a potentially lethal shock hazard. Where the lamp circuit itself does not provide a discharge path it is usual to fit the capacitor with a resistance that ensures discharge. The resistors are typically 1MΩ for capacitors of less than 20μF and 470kΩ for 20–50μF. The idea is to ensure that the voltage drops to a safe level, e.g. 50V, within 60 seconds. While this arrangement is sufficient for permanently installed luminaires, it is not for luminaires that can be removed by a simple unplugging action (e.g. track mounted). Here the person removing the luminaire could be exposed to shock at the power connector. Under EN60598-1 such luminaires are required to have a discharge method that reduces the voltage to 34V within one second. The resistor required would be such that a lot of heat would be dissipated, and two solutions to the problem are: • either the use of a small reactor choke across the capacitor. This has low dissipation when the AC supply is present, but acts as a low resistance for the capacitor discharge. • or the use of an electronic switch that discharges the capacitor through a low resistance when the AC power is removed. This method has lower losses (16,000 Hours 5,000 starts >100,000 starts

Table 7.2 Life rating for representative T5 linear fluorescent lamps. (Actual figures achieved depend on particular lamp and ballast combination).

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Electronic ballasts are offered in a wide range of types and physical constructions. In the USA the electronic ballast was at first regarded as a straight substitute for its magnetic forbear, so many USA electronic ballasts are made in similar chunky can sizes to those of USA magnetic ballasts. In Europe the magnetic ballasts were already much smaller (especially in cross section) and, again, electronic ballasts for conventional fluorescent tubes initially followed the form factor of the magnetic components. With the arrival of the slim T5 lamp (which cannot be operated from a 50/60Hz magnetic ballast) a demand for slimmer ballasts emerged, since luminaire designers wanted to take advantage of the lamp’s slimness, and did not want a bulky ballast to take away some of the space gain arising from the thinner lamp. Several important parameters are specified for electronic ballasts. Some of these are international standards, some are manufacturer specific.

Figure 7.27 Examples of electronic ballasts for linear fluorescent tubes. For T8 tubes from Philips/Advance Transformer (USA), and for T5 tubes from Helvar (Finland.)

ELECTRONIC COMPONENTS

Transistors

Input filter Mains connection

Boost converter Main storage capacitor

Half bridge inverter

Output choke Lamp connection

Figure 7.28 A typical standard electronic ballast circuit board with pre-heat start showing the principal components.

Current crest factor should not exceed 1.7 (although in Japan higher crest factors are permitted). In practice meeting the requirements for EMC and harmonic reduction goes a long way to ensuring this requirement is met. Apart from the power quality issues raised by a high crest factor, lamp life can be shortened by it. High quality electronic ballasts usually have a current crest factor of around 1.4, i.e. the same as for a sine wave. Inrush current or I2t. There is a high inrush current on electronic ballasts arising from the charging up of the main DC storage capacitor. When a large group of ballasts is switched on together, the inrush can be great enough to trip the circuit breaker. The ballast data should give an I2t rating to allow a circuit breaker of the right characteristic to be selected. Case temperature or TC. As with magnetic ballasts there is a maximum temperature for the transformer windings to ensure long life. In addition many of the electronic components will also have limits on operating temperature. Ballasts are designed so that internal operating limits are not exceeded provided the case temperature does not exceed a specified case temperature TC. The ballast has a test point marked on it where the temperature should be measured. The ballast specification will indicate a maximum TC usually around 70–80°C. However the ballast life figure may be based on a somewhat lower figure, like 60°C at the test point. A 10°C difference in case temperature can double or halve the expected life.

Lamp starting temperature. A fluorescent lamp will only reliably start if the lamp tube is at a reasonable temperature. The ignition voltage varies considerably with the lamp temperature, being at a minimum at around 10°C for the older T12 tubes and around 45°C for T8 tubes. Either side of the minimum the ignition voltage goes up until a point is reached when the lamp will not strike at all. The starting temperature range will depend on both the lamp and the ballast. A typical temperature range is -15°C to +50°C, but some lamps only reliably start from 0°C or even higher. Leakage current. The Y capacitors in the input filter (see Figure 7.10) are the primary cause of leakage current. Typical ballasts have a leakage current in the range 0.2–0.4mA. Thus if a large number of

Figure 7.29 The business end of a ballast using the circuit of Figure 7.24. In this case the output choke winding is in two halves; and the two cathode windings are positioned between them.

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conditions, and to eliminate lamp flashing when a lamp fails. Fluorescent lamps usually fail through poor cathode emission. If one cathode fails before the other, then more current flows in one direction through the lamp than through the other. There is a partial rectification effect that results in lamp flashing and damage to the ballast because of the DC. Faulty lamps may also fail to start in the first case; this can result in very high circuit voltages (Figure 7.25) which must not be maintained for any length of time. Similarly a faulty lamp holder or lamp

+V

Driver

Converter Control

DC Detect

VCathode

VLamp

Bridge I

+V

0V

Ballast Control Supply Cycle

Figure 7.30 Block diagram showing the function of the ballast monitoring and control circuit.

ballasts are installed in a circuit with RCD protection, there will be a limit to the number that can be connected to a single RCD. Typically a maximum of 30–40 ballasts is recommended for a single 30mA RCD. DC operation. Since the electronic ballast starts by converting incoming AC to DC, it is reasonable to suppose that the ballast can work directly from DC. Most “professional” electronic ballasts can work from DC, and this feature is used for emergency lighting systems based on high voltage batteries. The ballast specification will give the range of AC and DC supply voltages that the ballast can be safely operated from. Any changeover from one supply to another must be on a “break before make” basis. Safety features. The introduction of electronics into the ballast opens up new control possibilities, especially when a controlled oscillator is being used to drive the half bridge. Professional ballasts are fitted with a control circuit that can be used for several purposes including regulating the lamp power. Its prime use, however, is to prevent potentially unsafe

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Ideal Light Output

Real Light Output

Flicker =

+

x 100%

Area Flicker Index = Area + Area

Figure 7.31 Flicker percentage and flicker index.


Rating AC line voltage range V Max AC V one hour DC line voltage range V Line frequency range Hz Line current mA Power factor Power loss W Lamp voltage V Lamp current mA Lamp frequency kHz Preheat KH current mA Running KH current mA Start time s O/C Volts t < te O/C Volts t > te Leakage current mA

3u18W T8 171264 320 200300 4961 240 0.98 8 55 290 30 480 100 2.0 230 600 0.35

Ballast 2u28W T5 171264 320 200300 4961 278 0.98 7 167 170 32 205 70 2.0 185 650 0.35

Type 1u35WT5 171264 320 180300 4961 165 0.97 3 205 170 26 210 80 2.0 160 550 0.35

2u36W T8 170300 320 180400 4961 190 0.99 6 102 320 25 570 170 1.8 340 550 0.19

1u70W T8 170300 320 180400 49-61 290 0.99 6.5 129 470 27 950 210 1.35 430 680 0.21

Table 7.3 Typical data for a representative range of rapid start electronic ballasts. In the table KH = cathode heating, and O/C = open circuit. te is the time thermionic emission starts.

connection could result in a high voltage condition. Finally the ballast itself may not operate correctly unless it receives a supply voltage within a specified range. A typical ballast control circuit not only controls the half bridge to operate the starting routine, but also senses the common fault conditions and puts the ballast into a “stand-by” mode if a fault is detected. “Stand by” turns the low side driver on, so the half bridge output is clamped to 0V. It also stops the boost converter, so the DC rail drops to the peak AC supply voltage (or the DC supply voltage). The control circuit might have the following inputs: • sensing of lamp voltage. If this rises above the tube starting voltage, or if a starting voltage does not drop within a few seconds, this denotes tube ignition failure, so stand-by must be invoked. • sensing of half bridge current (this can be done by putting a low value resistance in series with the low side transistor and measuring the voltage across it). If the current is too high, the controller can slightly increase the operating frequency. This will reduce lamp voltage (Figure 7.25).

• sensing of cathode voltage at “cold” end of lamp. If no voltage detected, lamp must be assumed missing or not connected, so stand-by should be invoked. On the other hand when a voltage is once again detected, the ballast can start up again. • sensing of significant DC at the blocking capacitor. This signifies rectification and imminent lamp failure. • sensing of the DC storage capacitor voltage. If this drops low, then stand-by is initiated. • sensing of open circuit filament. It is possible to realize the entire control circuit using discrete components, but more usually a mixture of discrete components and a microprocessor (or ASIC) is used. Figure 7.30 shows a block diagram of the control circuit operation. Cathode failure can lead to severe hotspots, and in the worst case to the lamp “exploding” at the hotspot. The problem is potentially sufficiently serious that for linear and CFL T5 lamps it is mandatory under the IEC standard to have automatic ballast shutdown when an irregular condition is detected. Ballast manufacturers are expected to identify which method of lamp failure detection

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between one make of ballast and another. Obviously parameters that are set by international standards (like lamp voltage) should not, but a parameter like the frequency can change according to the ballast design. 7.2.6 Multiple lamp operation From main ballast

Figure 7.32 Operation of tubes in series.

(assymetric power, assymetric voltage or open circuit filament) they are using. Flicker index/percentage. Flicker arises in discharge lamps because of asymmetry in the discharge. In a 50 or 60 Hz system the light output peaks at 100 or 120Hz. If each half cycle has identical light output, the eye is not disturbed by the variation within cycles. However, if one half cycle is brighter than another, flicker can become noticeable and objectionable. One method of measuring flicker is by the modulation difference between the two half cycles. If measured as a percentage, then some people can find a 1% flicker objectionable and most people find 2% objectionable. The USA uses flicker index as the metric, which compares the amount of light that exceeds the average (as opposed to simple amplitude comparison). Figure 7.31 shows how flicker index and flicker percentage are measured. Flicker index varies from 0– 1, with 0 indicating continuous light output without flicker. Electromagnetic ballasts have a flicker index in the range 0.01 to 0.1, the higher end of which, 0.07–0.1 is perceptible to some people. Electronic ballasts should not in principle suffer from flicker, but even they can have some asymmetry so can have a measurable flicker index below 0.01. A particular problem can be any form of electrical noise, not necessarily related to the line frequency, which can raise the flicker. Table 7.3 gives outline specifications of typical ballasts for linear fluorescent lamps. The values are examples – individual ballasts of a type have variations, and some parameters may vary considerably

232

Electronic ballasts are suitable for operating multiple lamps. Various different circuits are used. Just as for electromagnetic ballasts, series operation of lamps is possible. In instant start circuits starting of the second lamp is assured by the high voltage still present even after the first lamp has ignited. In preheat circuits it is necessary to provide an isolated source of cathode heating for the centre cathode pair, and a simplified circuit for doing this is shown in Figure 7.32. Professional ballasts usually use separate output stages for twin tube operation as in Figure 7.33; but where short tubes are being used, for example in a 4 × 18W ballast, each of the output drivers can drive a series pair. An example of a different circuit topology is shown in Figure 7.34. Here a current fed parallel resonant push-pull inverter is used. The resonance

Bridge Output

0V

Figure 7.33 Professional ballasts have twin output stages. Sometimes a capacitor links the two circuits to balance the light output, and to assist the starting of the second lamp.


12V Battery

R

Base Drive

C

Base Drive

+

CB C

Figure 7.35 Battery operated single transistor inverter used for low power fluorescent lamps.

+ VDC -

Figure 7.34 Multiple lamp circuit based on a push-pull inverter with isolated output and capacitor ballasting for each lamp.

arises from the capacitor and the primary inductance of the transformer. The current source is the separate inductor, which has the advantage of ensuring a sine wave output from the transformer. In turn this allows the use of ballast capacitors for each tube that can also develop the tube starting voltage. The circuit has the features that the output is isolated and that the failure or disconnection of any one tube does not affect the operation of the remainder. In all electronic ballast circuits the presence of high frequencies makes the realization of EMC standards difficult. The lamp leads themselves can be the source of radiated interference, and the means by which interference can enter the ballast leading to unstable operation. For this reason manufacturers recommend the shortest possible lamp leads, and, in particular, that the “hot” end leads should be as short as possible. Also that on no account should lamp leads run parallel with the ballast supply leads. These factors can determine the position of the ballast within the luminaire, particularly where multiple lamps are being used.

7.2.7 Ballasts for compact fluorescent lamps One of the earliest uses of electronics for lighting was the battery powered inverter for small (4–13W) linear fluorescent lamps as used in vehicle, camper and boat lighting, handlamps and emergency lights. In this application EMC is still an issue, but harmonic generation is generally not an issue. This allows very simple circuitry to be used; the same circuit at high voltages would impose very severe stress on the transistor. Figure 7.35 shows the outline circuit of a single transistor flyback inverter that can be used for this application. An auxiliary winding on the transformer provides feedback to the transistor base to maintain

Figure 7.36 Battery inverters use very few components.

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The monolithic IC in a small SO8 package. But its internal circuit is quite complex

5m

m

Collector

diac Diac Sec

OSC

Figure 7.37 Examples of compact fluorescent lamp ballasts. These are from Philips, Tridonic, Harvard Engineering and Hüco.

oscillation. The frequency of oscillation is determined by the transformer’s primary inductance, R and C. These also determine the mark-space ratio of the switching which must be 1:1 to avoid rectification developing in the lamp. The by-pass capacitor CB provides a low impedance return path for the high frequency. The high voltage needed to operate the lamp is determined by the transformer turns ratio. The transformer is of leakage reactance construction so there is a high open circuit voltage to start the lamp, but current is limited once the lamp is running. The cathodes can be heated by auxiliary windings which, due to the action of the leakage reactance transformer, will deliver more power at start up than when the lamp is running. In general the description “compact” when applied to fluorescent lamps refers to lamps that are single ended; requiring the tube to have been folded along its length one or more times. Some of these are of the same power as the normal tubular lamp, for example TC-L lamps of 36 and 55W rating, and although only half the length of the tubular equivalent are still quite long. They can therefore use electronic ballasts of similar characteristics and form factor to their T equivalents. In luminaires using the shorter lamps (either low power “2-finger” TC-E, or 4 and 6 finger lamps TCDE and TC-TE) the “long thin” ballast is not often

234

+ +

R

Vref

2Vref Source

A complete ballast circuit using two ICs

IC High Side

IC Low Side

Bridge and input filter

Optional additional components for frequency change for pre-heating

AC IN

The completely assembled ballast

goes in to a lamp like this Figure 7.38 An example of the use of small power integrated circuits as the basis of an electronic ballast built in to the lamp. Information courtesy ST Microelectronics.


convenient, so while the ballast circuitry remains the same or similar, the ballast form factor changes – examples are in Figure 7.37. Now many compact fluorescent lamps are available with built-in ballasts – the ballast becoming “disposable” like the lamp itself. Present international standards allow some relaxation in harmonic performance at the powers at which these lamps operate, so circuitry tends to be simpler. Nonetheless the circuitry must be robust and efficient, as well as being extremely compact. The present trend is to higher circuit integration whereby component count is reduced, but at the same time considerable sophistication can be incorporated. One of the problems at the low powers concerned is that the circuit can start having losses that are a significant proportion of overall circuit power. An example of the kind of integrated component now available is given by the VK05CFL driver from ST Microelectronics. This is a monolithic device that can operate as the complete high side or low side lamp driver in a half bridge circuit. It combines the functions of gate drive and power switching in one device. In order to minimize losses it uses an emitter switch as its power output stage. This is a hybrid arrangement using a low voltage MOSFET in cascade with a high voltage bipolar Darlington pair. The idea is to achieve the advantages of the low ON voltage drop and high OFF breakdown voltage of the bipolar, and the high switching frequency of the MOSFET. Figure 7.38 shows the integrated circuit, its internal block diagram, the circuit of a complete ballast, a completely assembled ballast, and a lamp using this kind of ballast circuit. The intention here is not to suggest that this is the only way of achieving a built-in lamp ballast, but to show the underlying sophistication needed. It can be seen that the circuit embodies: • feedback windings that not only provide oscillation, but also provide power for the IC. • diac initiation of the oscillator. Only one IC needs connection to the external RC network since the other IC will follow. • external capacitors to set oscillator frequency.

• an optional facility for timed pre-heat at a different frequency. 7.2.8 Controllable fluorescent lamp ballasts From Figure 7.30 it is clear that the ballast control circuit could do more. It is already required to ensure stable operation of the lamp at a specified power, so the obvious extension is for it to be able to vary the lamp power in response to an external signal. Controllable ballasts have provision for a low voltage input signal that is electrically isolated from the lamp circuit. The signal can vary the light intensity between set limits. Standards exist for both analog and digital control signals and these are discussed in more detail in Chapter 9. The achievable dimming range with fluorescent lamps is limited by the lamp-ballast combination. Smooth dimming to total extinction is, in practice, impossible. Applications requiring smooth dimming to extinction for esthetic effect, such as cinema auditoria, should not use fluorescent dimming. Some practical points now follow. The relationship between lamp power and light output is not linear. What matters to the user is the light output, so the ballast specification should give the percentage range of light output. For ballasts intended solely for use in energy management schemes, a control range 25–100% (light output) can be sufficient. Where they are to be used in architectural lighting schemes, a range of 10–100% is the minimum specification and 5–100% is preferable. The human eye adapts to low light levels, and 10% light output still seems quite bright. Where a subjectively low level is required, even 5% is too high. Some ballasts are specified for a wider range, such as 3% or even 1%. In practice it may be difficult to get even performance between multiple ballasts at these low levels. Some specifications are only valid at a particular lamp operating temperature. Lamp/ballast combinations should be evaluated under their intended operating conditions if the application is critical. Demand from the residential market is leading to the introduction of “1%” ballasts for selected compact lamps.

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(a)

Lamp L

R CR

C DC

ZL I

ZC V IN

Lamp Voltage VL

VIN R Z2 LCR ZL R

R R

VL

C

Lamp

L

(b)

IL

Lamp Current IL

VIN Z2 LCR ZL R

Figure 7.39 Simplified equivalent circuit of the output stage of an electronic ballast and a fluorescent lamp.

Fluorescent lamps operating at low light levels and high frequencies can develop acoustic resonance (described in Section 7.3.1). With fluorescent lamps the visible result is a series of swirling rings of light of varying intensity along the tube. One way of reducing this effect is to introduce a small amount of DC into the discharge. This can be done by connecting a high resistance between the high side of the DC blocking capacitor C in Figure 7.24 and the main DC rail. In principle there are various ways by which lamp power can be controlled. The obvious method is to simply reduce the voltage on the DC supply rail, by, for example changing the operation of the boost converter. While in principle this method works for a limited range of control, it would be difficult to implement for a wide range of control. Another method is to use pulse width modulation (PWM) of the high frequency output. This is

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practical, but with conventional fluorescent tubes the situation is complicated by the needs of cathode heating. If PWM is used, a separate circuit is needed to maintain cathode heating. PWM control is widely used for small cold cathode lamps. The practical method that is most widely used is to take advantage of the combination of lamp and circuit characteristics that allow light output to be changed simply by changing the frequency of operation. The details of circuit operation will depend on the circuit topology of the ballast (it must be repeated that this chapter is using examples of recognized practice, and that there are other circuits that can achieve similar results). Taking as an example the output stage of Figure 7.24, and treating the lamp as a resistance, a simplified circuit is Figure 7.39(a). Since the DC blocking capacitor CDC is very large compared with resonant capacitor CR , a further simplification gives Figure 7.39(b), where also a sinewave generator is shown replacing the rounded square wave that is the actual output of the bridge. Here it can be seen that as the frequency of the source changes, the relative voltages across L and C change. The inductance and capacitance represent an AC potential divider where the midpoint voltage changes with frequency; the higher the frequency, the lower the voltage across the capacitor (because at high frequencies the capacitor represents less of an impedance) and the lower the frequency, the higher the voltage (because at low frequencies the inductance represents less of an impedance). The voltage across the lamp is the same as that across the capacitor, so in principle the power delivered to the lamp is inversely proportional to the frequency. In fact the simple principle is complicated by the fact that the lamp resistance is not constant. Figure 7.40 shows an example of the Voltage/Current relationship for a fluorescent lamp, and its variation with operating temperature. Figure 7.39(b) shows the lamp current and lamp voltage for an instantaneous value of lamp resistance. Using these equations and the lamp data of Figure 7.40 it is possible to develop a graph that shows the relationship of lamp power to frequency. In practice the process is somewhat empirical since


0

10 C 0

Lamp Voltage

-15 C 0

22 C 0

35 C

10

100

Lamp current (log scale)

1000

Figure 7.40 The relationship of lamp voltage and lamp current for a T5 54W fluorescent lamp at different ambient temperatures.

initially the values of C and L are not known. One way to proceed is to choose a practical frequency range; then apply the R value corresponding to full output to the lowest frequency to be used; and the R value corresponding to the lowest output to the highest frequency. A practical capacitor C value can be chosen, and the corresponding L derived. The equations can then be solved for different values of R to derive the power/frequency relationship. In practice this shows that at the high frequency end a small change in frequency makes a significant difference to the power, as shown in Figure 7.41. Table 7.4 shows two examples of controllable ballast characteristics. Once again these are only representative examples. In this case the lamp parameters are related to the control voltage setting, assuming the use of the 1–10V analog control system. The dimming of fluorescent lamps brings potential problems in respect of lamp life. Clearly if the lamp power is reduced, then so is the energy that keeps the cathodes in a thermionic emissive state. It is evidently important to increase cathode heating at low light levels, since the lamp discharge itself will not provide enough energy to keep the cathodes at the right temperature. One method is to provide a separate cathode heating supply that is on at all times; but this is energy

inefficient at high light outputs. Indeed the lamp cathodes are under attack from two different mechanisms: • too much cathode heating leads to cathode loss by evaporation. • too little cathode heating results in cathode loss due to sputtering. This is where the discharge current is concentrated on cathode hotspots. The aim of controllable ballast design is to ensure that the cathodes receive as much additional heating as is necesssary for the particular lamp power level. While the output circuit of Figure 7.24 and others like it have the characteristic that cathode heating current is automatically increased as frequency increases, there is a need to optimize the arrangement, while at the same time keeping the ballast design simple. The concept is that of ensuring that the total energy at the cathode is correct, and the principle is that of extending the starting condition equations to steady state at different dimming levels. The most widely used metric relates to the current in the high current lead. The top part of Figure 7.42 shows how the currents flowing in the two leads of a cathode are different. The high current lead ILH is carrying both the main discharge current ID and the cathode heating current, whereas the low current lead ILL is carrying only the heating current. 100

10

1 10

Frequency kHz

100

Figure 7.41 Multiplying the values of VL and IL using the equations of Figure 7.39, and applying the varying values of R derived from Figure 7.40, gives a lamp power/frequency relationship of the kind shown here.

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Control voltage V

10

Line current mA Line power W Power factor Lamp voltage V Lamp current mA Cathode voltage V Lamp frequency kHz Light output %

170 245 39 58 0.98 0.98 100 111 320 455 2.4 1.6 40 43 96 96

8 152 35 0.98 103 270 2.7 44 84

6 225 53 0.98 115 395 2.0 50 87

117 27 0.98 111 180 3.6 51 62

4 167 39 0.98 130 230 3.1 66 60

87 20 0.98 123 95 4.4 55 36

2 108 25 0.94 160 85 4.5 71 27

55 69 12 14 0.92 0.85 130 170 29 20 4.7 4.7 56 72 10 6

1 48 64 11 13 0.87 0.83 120 165 14 14 4.6 4.7 57 72 4 4

Table 7.4 Relationship of control voltage to lamp operation in example lamp-ballast combinations. The red figures relate to 36W T8 lamps, and the black figures relate to 58W T8 lamps. Ballasts are Helvar HFC series.

The bottom part of Figure 7.42 shows the permitted parameters of operation for a T5 high output 54W lamp. Similar diagrams are provided by the lamp manufacturers for each lamp type. The green area is the area of permitted operation; the darker green hatched area is an area in which the discharge current alone is sufficient to maintain cathode temHeating Voltage

ILH ID ILL mA 700 ILH 600 500 400 300 200 100 0

mA ID

Figure 7.42 Limits of operation for a high output 54W T5 fluorescent lamp. See text for description. Figure derived from data provided by Philips and Osram.

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perature, and where additional cathode heating is optional. In the lighter green area, additional heating is essential. The red line represents one manufacturer’s recommendation for maintaining conditions for maximum tube life. At all times the current in the low current lead ILL must not exceed a value given by the lamp manufacturer (approximately 490mA for the example T5 HO 54W lamp). For low lighting levels, lamps are run at ID currents lower than that implied by the diagram. At the time of writing the IEC standard test procedures are fairly well defined for the equivalent of the green area of the diagram, but performance at low discharge current levels is still the subject of research. Actual performance in the low current area is, therefore, manufacturer specific and depends on the particular lamp/ballast combination. While current recommendations for cathode heating under dimming operation are based on the ILH parameter, research is showing that the interaction between the heating and discharge currents is complex. The two currents may be out of phase, and in most cases there is some discharge current through both lead wires. A new method of defining “target” optimum lamp operating conditions, which may help with the definition of permitted low current operation, is that of setting target values for the sum of the squares of the two lead currents, i.e. ILH2 + ILL2. The aim of this kind of rule is to have a method of measurement that is easy to apply, but which is itself based on both theoretical and experimental data.


7.2.9 Dimmable fluorescent lamp ballasts For all new installations the most practical method of controlling the light output of fluorescent lamps is by the use of controllable ballasts as described in the previous section. In general these ballasts require a separate control cable that operates at low voltage, which in turn means that the ballast must have some method of isolation for the control signal. It is possible to eliminate the separate control cable by superimposing a control signal on the mains supply. Such a technique is useful in retrofit situations where luminaires are replaced or upgraded with new ballasts, since there is then no need to install the separate control cable. Sometimes there is a requirement that fluorescent lamps be dimmed by standard triac or thyristor dimmers. One method is to use the electronic equivalent of the circuit shown in Figure 6.56, whereby there is a separate maintained live supply to provide cathode heating, and a variable supply for the dimming. This requires additional wiring, so not surprisingly there is a preference for a ballast that can work solely from the variable supply. This is technically possible, but there are two difficulties. In order to meet the requirement to provide sufficient cathode heating at low light levels, the circuit must maintain or increase cathode heating when the power actually reaching the ballast is decreasing. The fact that the incoming waveform is a phasecut sinewave running at 50/60Hz means that con-

Figure 7.43 The Lutron Hi-Lume™ series of electronic ballasts uses a maintained live feed to allow dimming from conventional dimmers. This ensures sufficient cathode heating to allow the attainment of low light levels (1% for some models).

Figure 7.44 On the USA market the Lutron Tu-Wire™ ballast is an example of ballasts that can be dimmed by conventional dimmers without the need for additional control or maintained live wires.

siderable harmonics are developed and it is difficult to design a ballast that meets the standards for harmonics. In the USA some ES (Edison Screw) cap compact fluorescent lamps with built in electronic ballast are offered as being suitable for dimming by triac dimmers. This is convenient since it means they can be used to replace tungsten lamps in table lamps and standard lamps that have built-in triac dimmers. The dimming range is from 5% at best, and maximum lamp power is 23W. In addition some manufacturers offer dimmable electronic ballasts for T8 linear lamps. In Europe thyristor dimmable electronic ballasts for T8 and TC-L fluorescent lamps have been available for many years; however in order to meet EMC standards, the ballasts each use two substantial filter chokes. One choke is fitted in the live feed and one in the neutral. While the combination of two chokes and an electronic ballast seems somewhat clumsy, the convenience of being able to use standard dimmers is helpful. This is particularly the case where standard multi-channel dimmers are being used with predominantly tungsten or transformer fed tungsten halogen lamp loads. For example a prestige conference room might have only one or two fluorescent lamp circuits, but several tungsten halogen circuits. It may

239


Input voltage Input current Input frequency Output voltage Output current Output frequency Short circuit current Power factor Neon tube 10mm Argon tube 10mm Neon tube 20mm Argon tube 20mm

Nominal 25mA ballast 230V 350mA 50/60Hz 8000V 23mA 25kHz 29mA 0.95 8m 11m 11m 15m

Nominal 50mA ballast 230V 350mA 50/60Hz 4000V 46mA 25kHz 58mA 0.95 5m 7m 7m 12m

Figure 7.45 A European fluorescent luminaire fitted with an electronic ballast suitable for dimming with a thyristor or triac dimmer. Notice the two line filter chokes.

well be easier to use standard dimming equipment throughout, and use dimmable ballasts for the few fluorescent lamps involved. The same argument applies in some entertainment and theater lighting schemes, where it is easier to connect fluorescent lamps direct to the outputs of the dimmer system than to make special control provision for them. 7.2.10 Electronic ballasts for cold cathode and neon lamps Electronic ballasts for cold cathode lamps are similar to ballasts for hot cathode lamps in that they use the half bridge topology, and have active power factor correction. The principal differences are that there is no requirement for cathode heating, and that the open circuit output voltage is significantly higher. The high output voltage is achieved using a transformer at the output, and like its 50/60Hz counterpart it can be of leakage reactance construction to limit the output current. Examples of cold cathode electronic ballasts are shown in Figure 7.46. The ballasts illustrated in Figure 7.46 are not suitable for dimming. It is possible to obtain controllable cold cathode ballasts using the 1–10V control signal, and these work in a similar manner to their hot cathode equivalent, but without the complication of cathode heating. An example is shown in Figure 7.47.

240

Figure 7.46 25mA and 50mA electronic ballasts for neon and cold cathode lighting from Tunewell Transformers Ltd. Tube lengths reduce by 0.5m for each pair of electrodes.

There is a type of neon or cold cathode ballast referred to as “low voltage”. In fact this is something of a misnomer, because the lamp tube itself still operates at high voltage. The ballast device has a high frequency output at around 24V. This is coupled to the lamp using a transformer “boot”. The arrangement has the advantage of confining the high voltage to the immediate vicinity of the electrodes, and therefore simplifies the wiring, especially in multi-lamp systems. This arrangement has an interesting feature. If the high frequency power to the boot is ramped up from zero to full, the discharge in the lamp moves

Figure 7.47 The Coldstar II ballast from AC/DC Lighting Systems Ltd allows control of cold cathode lighting from a 1–10V control signal, bringing it into line with other contollable fluorescent lamp ballasts.


Figure 7.48 The Smart Neon™ controller from Fluid Light Technologies Inc provides sequence control for four neon tubes; the output is 4 × 24V 36kHz, which is coupled to the tubes using transformer “boots”. Varying the power applied to the boot propagates the discharge through the tube.

down the tube. The speed of propagation is proportional to the rate at which the power is varied, so neon signs can appear to “write” in light. Controllers are available to sequence multiple tubes (see Figure 7.48). Electronic ballasting of neon/cold cathode tubes has another possibility, that of obtaining more than one color from a lamp. The excitation of different gases is affected both by the frequency of excitation, and the waveform. As examples neon is excited better by a short duty cycle, but steep pulse

Figure 7.49 Here a 15inch LCD display is being serviced (left). The inverter for the CCFL backlight is mounted on a separate printed circuit, shown enlarged on the right.

rise-time, and mercury vapor is better excited by a longer duty cycle and slower rise time. If a lamp tube is filled with a mixture of neon and mercury vapor, and if it has a phosphor coating, then it is possible to achieve a range of color. The neon will produce red, but the mercury vapor (which is actually producing ultra-violet) will produce a color that depends on the phosphor. By varying the frequency, rise time and mark-space ratio a tube can be made to change color, for example: • from blue to red • from green to yellow • from turquoise to mauve Small cold cathode lamps as described in Section 3.6.6 present a challenge to the ballast designer since in many applications power and space are scarce. Most ballasts of this kind are supplied as OEM (Original Equipment Manufacturer) items, and are often highly customized to meet the needs of the particular application. For example backlight inverters for LCD screens used in laptop computers, and dashboard and navigation display lighting systems used in automobiles. Figure 7.50 shows some examples. Despite the very low powers involved, the

Figure 7.50 Examples of inverters for cold cathode compact fluorescent lamps. These are the LXM1617 PanelMatch™ series from Microsemi, intended for LCD backlighting. Typical power ratings are in the range 2.5– 6W, operating from 3.3, 5 and 12V supplies.

241


starting voltages are considerable, even a 2.5W ballast may need to develop 1,500V from a supply of only a few volts. Lamp brightness can be controlled in various ways, including supply voltage adjustment and frequency control. However the technique that gives the widest dimming range, and the highest efficiency is PWM. Small cold cathode lamps are sometimes required to operate at very low temperatures. Those used in automotive and similar applications may include an arrangement to boost the output at low temperatures before switching over to PWM control. Driving the lamp continuously at 50% above the normal amplitude for a minute or so helps get the lamp up to full light emitting temperature. 7.2.11 Infra red modulation Notwithstanding its general efficiency, the fluorescent lamp radiates a considerable amount of infrared (Figure 3.14). When such a lamp is run from an electronic ballast, the lamp’s radiation is modulated by the high frequency supply and its harmonics. This does result in some practical problems. Most cordless remote controllers for TV sets and video recorders, and also some room controllers, use coded infra-red signals, and operate at frequencies in the range 33–40kHz and at 56kHz. They can be affected by fluorescent lamps running on electronic ballasts. Some efforts have been made to use lamp frequencies that do not interfere, but more significant progress has been made on the controller side, where new modulation schemes have made them less susceptible to interference. Another class of equipment that can be affected is cordless audio equipment; for example cordless headphones for TV and HiFi; and multi-channel simultaneous interpreting equipment, both of which also use modulated infra red. Typically such equipment uses a number of carriers in the 95–250kHz range. It can be affected by the IR from fluorescent lamps, especially those working at the higher frequencies required for dimming. Headphone manufacturers have got round the problem by going to much higher frequencies. Some

242

of the lower frequencies used in the simultaneous interpretation application may prove unuseable in the presence of some luminaire/ballast combinations.

7.3 Electronic ballasts for HID and arc lamps 7.3.1 Introduction Electronic ballasts for fluorescent lamps have been available for many years and it is now the case that for many applications it can be shown that their use has an overall economic benefit – quite apart from any consideration of light quality and energy usage. The situation with HID lamps is different. Electronic ballasts are only used in a minority of mainstream HID applications, although their use is now increasing. The economic advantage is not so clear cut, but as prices come down may become significant. In some applications, for example projection, film lighting and automobile headlamps, electronic ballasts are the only practical method of powering the lamp. At frequencies higher than normal mains supply frequencies, HID lamps suffer from an instability problem referred to as acoustic resonance. Under certain circumstances a near instantaneous change in lamp power causes a rapid fluctuation in plasma temperature. The direct relationship between gas temperature and pressure creates a corresponding pressure wave in the arc. Under these circumstances the arc tube can behave like an organ pipe, since the discharge is between two fixed electrodes. The resulting standing wave can distort the arc; this affects its color, inten-

Figure 7.51 Examples of electronic ballasts for HID lamps.


sity and electrical characteristics. It may also extinguish the arc if a critical frequency is hit. The acoustic resonance effect sets in at around 1kHz, and is operative at all the “convenient” frequencies used in fluorescent lamp ballast circuits. There are three possible solutions to the problem, but only one is widely used. One possibility is to apply a varying frequency to the lamp, so that there is not time for a resonance to build up. In practice this technique needs considerable knowledge of the particular lamp, and it may not be practical for generic HID lamps. The ballast runs at a frequency of a few tens of kHz, chosen to match a window in the lamp’s performance known to be where acoustic resonance is at a minimum. The basic operating frequency is then frequency modulated at a low frequency (for example 100Hz) to ensure that resonance cannot start. Another possibility is to use a frequency above the acoustic resonance range; but this must be as high as 350kHz, and power components are both lossy and expensive at this frequency. However, transistor technology does now allow designs using this technique to be considered. At such frequencies any ballasting components are very small, so in theory very compact ballasts can be made. The most widely used method is the use of square wave low frequency drive, using a frequency in the range 90–200Hz, and it is this method that is now considered in more detail.

the other does not itself introduce high frequency products. A switching time of around one microsecond proves satisfactory in practice. An outline circuit for an HID electronic ballast is shown in Figure 7.52. The front end is similar to a fluorescent lamp ballast, providing a DC supply and ensuring a good power factor. The lamp circuit is based on a bridge inverter with opposing transistors switching together to ensure a square wave through the load. A capacitor prevents DC going through the load, and an inductor limits the current through the load. The lamp is shunted by an LC filter that ensures that high frequencies bypass the lamp. HID ballasts benefit from being able to integrate the ignitor into the ballast, and one method is to use the suppression choke as the secondary of the ignition transformer. A significant advantage of electronic ballasts for HID is that they can regulate the power to the lamp, making the lamp performance substantially independent of line voltage variations. This regulation

Filter

Rectifier

PFC

Bridge Driver

7.3.2 Ballasts for standard HID lamps The smaller sizes of high pressure sodium and metal halide lamps up to 400W are now well served by a range of electronic ballast products from many manufacturers, all using the square wave drive principle. The idea here is that in a square wave the power is constant, since (during any half cycle) the voltage is constant, so fluctuations are not set up. Two things must then be assured: • first that no high frequency due to active power factor correction or regulation gets into the load. This can be dealt with by simple filtering. • second that the switching from one half cycle to

Ignitor

Choke Filter

Lamp

Figure 7.52 Possible bridge inverter circuit for HID electronic ballast.

243


AC In

Boost Choke

Lamp Choke

Filter

Lamp Voltage Sense Lamp

Sidac Ignitor

Lamp Current Sense

Boost Converter PFC Control 50KHz

Regulator PWM Driver 25KHz

Bridge 200Hz Driver

Figure 7.53 Alternative circuit placing the ballast inductor prior to the bridge, showing the principle of power regulation.

can also counter the effect of arc voltage changing with lamp life. In practical MOSFET circuits the regulation is easier to do with a separate transistor, and by placing the ballast choke prior to the bridge as in Figure 7.53. Power control by the bridge itself is not an option since a square wave output is required. In Figure 7.53 the regulating MOSFET is driven by a circuit that receives input signals from the lamp circuit that correspond to lamp current and lamp voltage. It can, therefore, not only regulate the lamp

0V

Half Bridge Drive

Frequency Control 25KHz - 200Hz

+V

C L

Current Feedback

Figure 7.54 A half bridge HID ballast circuit using high frequency start.

244

power under normal conditions, but also shut down the ballast when a lamp failure condition is detected. Power is regulated using PWM, typically at around 25–30kHz. While practical circuits for the larger lamps, and for lamps using conventional ignition circuits, use the full bridge inverter, it is possible to use half bridge inverters, especially if a different method of iginition is used. Figure 7.54 shows an alternative method of ignition suitable for small HID lamps. The circuit operates under two different frequency regimes. When power is first switched on the system operates at high frequency (above 20kHz). The frequency is chosen to match the resonant frequency of the LC combination formed by the choke and capacitor across the lamp. This develops a sufficently high voltage to strike the lamp (the operation is similar to that of the circuit used in fluorescent lamp ballasts). As soon as the lamp strikes, the voltage drops because the circuit goes out of resonance. Once the arc discharge is established the system switches over to low frequency operation. Because the starting voltage is sinusoidal, it is practical to site the lamp some distance from the ballast. The connecting cable must, of course, be suitable for the starting voltage, typically around 1,500V, and of


low capacitance. Distances of around 25m are practical. All electronic HID ballasts include various safety features, in particular high temperature shutdown, and shutdown for failed lamps. Electronic HID ballasts are not suitable for connecting to normal dimmers. However some ballasts use their regulation circuit as a means of providing lamp dimming, typically over the range 50–100% light output. Electronic ballasts with a remote control input similar to that provided in controllable fluorescent lamp ballasts are becoming available – but such ballasts can not compensate for color temperature shifts arising from lamp dimming. For higher power HID lamps a hybrid circuit can be used. Here the lamp is ballasted by two reactors in series; however the second reactor has a thyristor (or equivalent) regulator across it that varies the amount of current that goes through or by-passes the reactor. This arrangement can be used both to provide superior regulation, overcoming the main disadvantage of the simple reactor, and to vary light level. An example is shown in Figure 7.60. 7.3.3. Benefits of electronic ballasts The electronic HID ballast has small size and light weight compared to the simple electromagnetic equivalent. It has other advantages. There are some savings due to increased circuit efficiency. In percentage terms these are most significant for the low power (30–70W) lamps.

Figure 7.55 Ballasts for low power HID lamps using the principle of high frequency starting are available from Harvard Engineering. Lamps can be up to 25m away from the ballast.

Figure 7.56 Examples of portable ballasts from Powergems Ltd for high powered metal halide lamps used for film lighting. These include a 50–100% dimming facility.

Its excellent regulation means that not only is light output maintained when the line voltage varies, but, just as important, color temperature is also maintained. The light output through life is better maintained. This is probably due to the reduced crest factor arising from the use of square wave drive, and to a more benign starting regime. Some classes of HID lamps suffer from a considerable (~ 40%) drop in light output over life, and the use of electronic ballasts significantly improves the situation. Many OEM electronic ballasts are made for projectors, theatrical luminaires, fiber-optic illumination systems, automobile headlights, film studio lighting equipment and other specialist applications.

7.4 Electronic transformers 7.4.1 Introduction In the lighting world an “Electronic transformer” is a device for providing power to low voltage tungsten halogen lamps. They now command a large part of the market, primarily at low power, serving one to three lamps. In common with their electromagnetic cousins, electronic transformers are designed as SELV devices. In the electrical engineering world: LV or Low Voltage refers to circuits operating at 1,000V or below. ELV or Extra Low Voltage refers to circuits operating at less than 42V r.m.s. FELV or Functional Extra Low Voltage implies

245


Current Transformer

Output Transformer

Diac

Lamp AC In

Figure 7.57 Outline circuit of an electronic transformer.

that one side of the ELV supply may be grounded or otherwise referenced. SELV or Separated (also Segregated) Extra Low Voltage means that both legs of the ELV supply are “floating”. This is much the safest arrangement since it means that no other electrical circuit will create a current by accidental connection. If, for example, one side was grounded (to the same potential as neutral in a mains system) then any “live” wire touching any part of the ELV system would cause a current to flow. In practice EMC requirements mean that electronic transformers are not quite “pure” SELV

devices because of the presence of suppression capacitors. Because the final load is resistive, the electronic transformer is nowhere near as complex as the fluorescent lamp ballast, although the basic voltage conversion principle is the same. The simplifications arise because: • there is no requirement to pre-heat the lamp or develop high starting voltages. • the load is conventional, and does not need a current limiting choke. • there is no need for active power factor correction, because there is also no need for a high voltage rail that itself is higher than mains voltage. 7.4.2 Electronic transformer circuit The circuit for a typical electronic transformer for feeding nominal 12V tungsten halogen lamps is shown in Figure 7.57, and the photograph Figure 7.58 can be directly related to the circuit. The circuit has much in common with Figure 7.23. The incoming AC is full wave rectified, and it is this waveform that powers the half bridge. Thus the current and voltage taken from the supply are in phase, and in principle power factor and harmonics problems are reduced. Nonetheless a choke must be fitted to prevent high frequencies getting back up the supply, and there are also other EMC

Figure 7.58 An electronic transformer from Kaoyi Electronic Co. using the principles of Figure 7.57. This one is rated at 120W.

246


Osram Halotronic™ “Mouse” 70–105VA

IBL IP65 rated 35–105VA

IBL SM range 60–105VA

Relco 60VA

Figure 7.59 Examples of electronic transformers for low voltage tungsten halogen lighting.

components (capacitors) required. The circuit self oscillates, using a current transformer in the output to feed back the transistor base drive signal. Typically the transformer might have 1–2 turns in its primary, and 6 turns in each secondary. The bridge center point has an r.m.s. voltage approximately half that of the AC line voltage; thus for a 12V output the output transformer has a turns ratio of 10:1 on 230V supplies and 5:1 on 115V supplies. In the photograph the secondary has 10 turns, so the transformer has 100 turns on the primary. It is interesting to see that the secondary is wound with four wires in parallel; this is done to simplify the winding (since thick wires are difficult to wind on a small core) and to reduce skin effect losses. In common with their electromagnetic equivalents, electronic transformers are equipped with protective devices to guard against overloads. Since some of these add significantly to cost, it cannot be assumed that all the following will feature in any one transformer. Transient protection. Most electronic transformers have a voltage dependent resistor or VDR at the input. This device carries insignificant current unless a particular voltage is reached, and is used to clamp any voltage spikes on the supply line. Fuse protection. Some electronic transformers

are fitted with fuse protection; but most rely on a thin part of the printed circuit track at the input to blow as a “one time fuse”. This kind of protection is also used in fluorescent lighting ballasts, and would only operate if there was a catastrophic component failure within the ballast or transformer. Over-current protection. By detecting the current flowing in the low side driver, it is possible to shut down the transformer (or to limit the bridge output) in the event of the current exceeding the rated value. This means that suitably equipped electronic transformers are substantially short-circuit proof. Some work on the basis that once the cause of the over-current condition is removed, they start up again automatically. Others require that the power is switched off and then switched on again to restart the oscillator. Thermal protection. Similarly a temperature sensor, usually fixed to the output transformer, can be used to shut down the electronic transformer. Winding temperature limitations are the same as for electromagnetic transformers. Consideration of Figure 7.57 raises the question of what happens when the input voltage to the half bridge drops to zero, as it must do every half cycle. In principle the oscillator stops. In practice this is not a problem; however, it obviously cannot start again until there is sufficient input voltage to kick it into action again. Some electronic transformers have a small capacitor or other arrangement to provide sufficient energy to keep the oscillator going over what would otherwise be the voltage zero. It is a common requirement for electronic transformers to be connected to dimmers. It is essential to check with the manufacturer whether or not the electronic transformer is suitable for dimming. Connecting unsuitable transformers to dimmers can result in the destruction of the dimmer, or the transformer or both. In particular electronic transformers presenting any kind of capacitive load (due to the use of a holdover capacitor or similar) cause trouble on conventional triac or thyristor dimmers. Electronic transformers should include in their specification their suitability for dimming. Some are not suitable at all, some are suitable for trailing edge (transistor) dimming only, and some for leading edge

247


arises both because of the high frequency circuit operation limitations, and because of EMC problems.

7.5 Lighting control by electronic transformers and ballasts 7.5.1 Conventional lamps

Figure 7.60 The Philips “Dyna Vision” electronic ballast for 150W high pressure sodium lamps (left) provides 1– 10V analog control of light output (in practice power range 35%–100%; light level range 20%–100%). However, economic considerations mean that for 400W lamps it is at present better to use a hybrid electronic/electromagnetic system to achieve light level range 35%–100% for a power range 50%–100% (right.)

(triac/thyristor) dimming only. Many are suitable for both. When leading or trailing edge phase cutting dimmers are used with electronic transformers of the basic circuit of Figure 7.57, the whole system relies on the oscillator starting every half cycle. This leads to the obvious question of whether a separate dimmer is needed at all. The oscillator start circuit works in a similar way to the gate firing circuit used in simple dimmers (described in Section 8.3.3.1). By deliberately delaying the start, the output power can be regulated. Electronic transformers with a simple built-in dimming facility can be constructed by adding a variable resistor in series with the diac resistor. This is only suitable for local control with very short leads. The variable resistor is several Megohms and operates at mains voltage. Electronic transformers suitable for remote analog or digital control are becoming available. While it might seem attractive to construct such a device with a significant output power (so that the cost of the control element could be spread over a number of lamps) this is not practical because there are restrictions on output lead length. Manufacturers specify the maximum output lead length, usually

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