D-6 LEARNING GUIDE D-6 ANALYZE ELECTRONIC CIRCUITS

Transcription

1 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM Level 3 Line D: Apply Circuit Concepts D-6 LEARNING GUIDE D-6 ANALYZE ELECTRONIC CIRCUITS

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3 Foreword The Industry Training Authority (ITA) is pleased to release this major update of learning resources to support the delivery of the BC Electrician Apprenticeship Program. It was made possible by the dedicated efforts of the Electrical Articulation Committee of BC (EAC). The EAC is a working group of electrical instructors from institutions across the province and is one of the key stakeholder groups that supports and strengthens industry training in BC. It was the driving force behind the update of the Electrician Apprenticeship Program Learning Guides, supplying the specialized expertise required to incorporate technological, procedural and industry-driven changes. The EAC plays an important role in the province s post-secondary public institutions. As discipline specialists the committee s members share information and engage in discussions of curriculum matters, particularly those affecting student mobility. ITA would also like to acknowledge the Construction Industry Training Organization (CITO) which provides direction for improving industry training in the construction sector. CITO is responsible for organizing industry and instructor representatives within BC to consult and provide changes related to the BC Construction Electrician Training Program. We are grateful to EAC for their contributions to the ongoing development of BC Construction Electrician Training Program Learning Guides (materials whose ownership and copyright are maintained by the Province of British Columbia through ITA). Industry Training Authority January 2011 Disclaimer The materials in these Learning Guides are for use by students and instructional staff and have been compiled from sources believed to be reliable and to represent best current opinions on these subjects. These manuals are intended to serve as a starting point for good practices and may not specify all minimum legal standards. No warranty, guarantee or representation is made by the British Columbia Electrical Articulation Committee, the British Columbia Industry Training Authority or the Queen s Printer of British Columbia as to the accuracy or sufficiency of the information contained in these publications. These manuals are intended to provide basic guidelines for electrical trade practices. Do not assume, therefore, that all necessary warnings and safety precautionary measures are contained in this module and that other or additional measures may not be required.

4 Acknowledgements and Copyright Copyright 2011 Industry Training Authority All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or digital, without written permission from Industry Training Authority (ITA). Reproducing passages from this publication by photographic, electrostatic, mechanical, or digital means without permission is an infringement of copyright law. The issuing/publishing body is: Crown Publications, Queen s Printer, Ministry of Citizens Services The Industry Training Authority of British Columbia would like to acknowledge the Electrical Articulation Committee and Open School BC, the Ministry of Education, as well as the following individuals and organizations for their contributions in updating the Electrician Apprenticeship Program Learning Guides: Electrical Articulation Committee (EAC) Curriculum Subcommittee Peter Poeschek (Thompson Rivers University) Ken Holland (Camosun College) Alain Lavoie (College of New Caledonia) Don Gillingham (North Island University) Jim Gamble (Okanagan College) John Todrick (University of the Fraser Valley) Ted Simmons (British Columbia Institute of Technology) Members of the Curriculum Subcommittee have assumed roles as writers, reviewers, and subject matter experts throughout the development and revision of materials for the Electrician Apprenticeship Program Open School BC Open School BC provided project management and design expertise in updating the Electrician Apprenticeship Program print materials: Adrian Hill, Project Manager Eleanor Liddy, Director/Supervisor Beverly Carstensen, Dennis Evans, Laurie Lozoway, Production Technician (print layout, graphics) Christine Ramkeesoon, Graphics Media Coordinator Keith Learmonth, Editor Margaret Kernaghan, Graphic Artist Publishing Services, Queen s Printer Sherry Brown, Director of QP Publishing Services Intellectual Property Program Ilona Ugro, Copyright Officer, Ministry of Citizens Services, Province of British Columbia To order copies of any of the Electrician Apprenticeship Program Learning Guide, please contact us: Crown Publications, Queen s Printer PO Box 9452 Stn Prov Govt 563 Superior Street 2nd Flr Victoria, BC V8W 9V7 Phone: Toll Free: Fax: crownpub@gov.bc.ca Website: Version 1 Corrected, July 2016 Corrected, March 2016 Revised, December 2014 Corrected, January 2014 New, October CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

5 LEVEL 3, LEARNING GUIDE D-6: ANALYZE ELECTRONIC CIRCUITS Learning Objectives Learning Task 1: Describe the features of the silicon-controlled rectifier (SCR) Self-Test Learning Task 2: Describe the basic action of the SCR Self-Test Learning Task 3: Describe SCR triggering circuits for AC phase control Self-Test Learning Task 4: Describe the features of the Triac Self-Test Learning Task 5: Describe the features of specialty thyristors Self-Test Learning Task 6: Describe the application of thyristors Self-Test Learning Task 7: Describe the operation of three-phase AC rectifier circuits Self-Test Learning Task 8: Determine values for rectified power supplies Self-Test Answer Key CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 5

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7 Learning Objectives D-6 Learning Objectives The learner will be able to describe the operating principles of thyristers. The learner will be able to analyze electronic circuits that utilize thyristers. Activities Read and study the topics of Learning Guide D6: Analyze Electronic Circuits Complete Self-Tests 1 through 8. Check your answers with the Answer Key provided at the end of this Learning Guide. Resources All the resources you need are in this guide. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 7

8 BC Trades Modules We want your feedback! Please go the BC Trades Modules website to enter comments about specific section(s) that require correction or modification. All submissions will be reviewed and considered for inclusion in the next revision. SAFETY ADVISORY Be advised that references to the Workers Compensation Board of British Columbia safety regulations contained within these materials do not/may not reflect the most recent Occupational Health and Safety Regulation. The current Standards and Regulation in BC can be obtained at the following website: Please note that it is always the responsibility of any person using these materials to inform him/herself about the Occupational Health and Safety Regulation pertaining to his/her area of work. Industry Training Authority January CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

9 Learning Task 1: Describe the features of the silicon-controlled rectifier The silicon-controlled rectifier (SCR) is the oldest member of the thyristor family. It is a semiconductor device that permits current to flow through it in one direction only, just like a diode. But unlike a diode, which has two terminals, the SCR has three. The third terminal, called the gate, controls the point in time at which the SCR conducts. This feature gives it the name controlled rectifier. A very small amount of power (the trigger pulse) at the gate terminal allows the SCR to switch and control a large amount of load power. SCRs can handle much more load power, both current and voltage, than transistors. Unlike the base of a transistor, which requires a continuous current in the tens or even hundreds of milliamperes range in some cases, SCRs use only a fraction of this control power, and for only a very brief period of time. SCRs are also different from transistors in that they are strictly bistable devices. They operate as either an open or a closed switch. But unlike a mechanical switch, the SCR is a solid-state device with no moving parts to wear out. SCRs are, therefore, not subject to corrosion, pitting, arcing or contact bounce. SCRs have current ratings that range from a fraction of an ampere to several thousands of amperes. SCR symbol and leads The SCR has three terminals. They are the anode (A), the cathode (K) and the gate (G). Like diodes and transistors, they vary in size, shape and configuration, depending on the application and loading requirements. A typical SCR is shown in Figure 1, with its schematic symbol to the right of the diagram. Figure 1 Typical SCR and its symbol The anode and cathode have to carry load current but the gate carries only a small current. Because of its smaller size, you can often quite easily identify the gate lead or terminal. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 9

10 Learning Task 1 D-6 Two different package types are shown in Figure 2: a stud type and a tab type. Figure 2 also shows that the SCR is a four-layer device made of P-type and N-type materials and has three junctions: The outer P-type material attaches to the anode terminal. The outer N-type material attaches to the cathode terminal. The inner P-type material goes to the gate terminal. The symbol for the SCR is similar to a diode, with the addition of a gate lead. Figure 2 SCR types and construction Typical ratings The initial selection of an SCR begins with the current rating. The larger the current rating, the larger the physical size of the SCR. The stud-mounted types generally have higher current ratings. Anode forward current is the maximum continuous current that can flow from cathode to anode. If the current exceeds this level, the device can become too hot and destroy the SCR. SCRs are available with current-carrying capacities from as low as 0.25 A to as high as 4000 A. Heat sinks are frequently used when installing SCRs in high-current applications. Other specifications and ratings used with SCRs include reverse breakdown voltage, forward breakover voltage, gate trigger voltage, gate trigger current and holding current. 10 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

11 Learning Task 1 D-6 Following are some of the more significant and relevant SCR parameters found in the manufacturers manuals (and packing slip information, as shown in Figure 3): I T (rms) V RRM V DRM I H V GT I GT dv/dt Operating temperature I T(rms) is the maximum continuous rms current rating. If this current is exceeded the temperature of the SCR will likely rise to a level that may damage or destroy it. V RRM is the maximum peak repetitive reverse-blocking voltage. If this voltage is reached or exceeded the SCR conducts in the reverse direction and will be permanently damaged. V DRM is the maximum peak repetitive forward-blocking voltage. If this voltage is reached or exceeded the SCR will turn on without a gate signal. This is not a desirable feature, as the gate control is then redundant. I H is the holding current, and is the minimum current required to sustain conduction. If the magnitude of the main anode-cathode current falls below this current, without a gate signal, the SCR blocks. V GT is the gate voltage needed to produce the gate current. I GT is the gate current needed to trigger the SCR into conduction. dv/dt is the rate of voltage rise that will turn the SCR on without a gate signal. The junction temperature range over which the SCR may be operated is the operating temperature. Temperatures commonly range from 40 C to +125 C. NTE5529 Silicon Controlled Rectifier V DRM V Ι GT - 30 ma Max G K Ι T(rms) - 25 A V GT - 2 V Max Ι H - 50 ma Min TO48 Replaces: ECG5529, SK6629 A Figure 3 Supplier s information accompanying an SCR CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 11

12 Learning Task 1 D-6 It is important not to exceed the gate parameters of SCRs. More SCRs are damaged by creating excessive gate power dissipation, or too much reverse gate voltage, than with any other variable. Common case styles Based on their current and voltage ratings, SCRs are packaged in various case styles and sizes. The packages are identified as TO-220, TO-48, TO-3 and so on. The manufacturers manuals will illustrate the package styles, identify the anode, cathode and gate leads, and give the dimensions. Representative types are shown in Figure 4. Figure 4 Common SCR case styles Now do Self-Test 1 and check your answers. 12 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

13 Learning Task 1 D-6 Self-Test 1 1. Draw the symbol for an SCR, identifying the cathode, anode and gate leads. 2. How is an SCR normally turned on? 3. How does the power handling ability of the SCR compare to that of a transistor? 4. How does the quantity of control power needed at the gate of an SCR compare to that required for the base of a transistor controlling the same load? 5. Name two advantages that an SCR has over a mechanical switch. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 13

14 Learning Task 1 D-6 6. In a stud-type SCR, how does the gate terminal compare in physical size to the anode and cathode terminals? 7. SCRs are available with current ratings in the range of: a. 1 to 10 A b. 10 to 100 A c. 100 to 1000 A d to A 8. In general, how does the physical size of an SCR relate to its current rating? 9. What is likely to happen if the SCR s current rating is exceeded? 10. What happens if the SCR s reverse-blocking voltage is exceeded? 14 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

15 Learning Task 1 D What happens if the SCR s forward-blocking voltage is exceeded? 12. What terminal is commonly tied to the heat sink in the SCR? 13. An SCR is a thyristor, but a thyristor is not necessarily an SCR. a. True b. False 14. What happens to an SCR if the current drops below the holding current? Go to the Answer Key at the end of the Learning Guide to check your answers. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 15

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17 Learning Task 2: Describe the basic action of the SCR A mechanical analogy to the operation of an SCR is the sluice gate shown in Figure 1. The pin controls the latch on the sluice gate. There is a fine spring that will return the gate to the closed position as long as there is no water or only a tiny flow. If the pin is pulled outward, the latch on the gate is released and the water flows. The pin then plays no further part; once the pin is released the gate will be kept open by the water pressure. If the water pressure drops to a small trickle, the spring has enough tension to pull the door closed again, and once again it will latch closed. The water volume may rebuild, but, as before, the pin must now be withdrawn again to allow the sluice gate to open. However, if the pressure build-up behind the gate is extremely high, it will force the sluice gate open without the pin being released. Finally, we assume that the sluice gate will open inward only since the door jamb will prevent it from opening outward, just as in a common house door. But again, if the pressure in the opposite direction is sufficiently high, it will force the door to open in the opposite direction, destroying the door jamb and hinges in the process. Latch pin Figure 1 Mechanical analogy to an SCR SCR characteristics In the circuit in Figure 2(a), if the anode-to-cathode voltage V is increased to a sufficient magnitude, the SCR will turn on. This voltage is called forward breakover voltage (identified as BV F in Figure 2[b]). The voltage across the SCR then drops, and the current is limited only by the series resistance R. Once the SCR is turned on, it will remain on as long as the anode current is not reduced below the minimum necessary to maintain the on condition. This current (IH) is called the holding current. In practice, the SCR is turned on by a much lower voltage than its forward breakover voltage, because there is a current in the gate circuit. This gate current would be equivalent to the latch pin in our mechanical analogy. If the polarity of the supply is reversed, and the voltage increased sufficiently in the reverse direction, the SCR will experience avalanche breakdown, as shown in Figure 2(b). The SCR would then be destroyed, just as a diode would be destroyed if the voltage exceeded its peak inverse voltage (PIV) limit. The equivalent in our mechanical analogy would be a high-pressure water flow in the reverse direction. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 17

18 Learning Task 2 D-6 Figure 2 SCR characteristics As Figure 2(b) shows, the SCR then behaves like a switch in the forward direction but, unlike a mechanical switch, there will be some voltage across it as it conducts. This is exactly like the small voltage across a diode while it is conducting. The forward characteristics of the SCR are shown in Figure 2(b) with the gate open. Since the gate is normally open once the SCR is triggered anyway, the only difference is that in practice the value of BV F required to turn the SCR on is less. This is because there is a gate current in place. Electrical equivalent The SCR, as its name implies, is a rectifier, but one that can be controlled. An SCR behaves like a diode in series with a switch (Figure 3). Figure 3 SCR symbol and diode-switch analogy The switch is an integral part of the PNPN junction that forms the SCR. This switch closes when an electrical signal is applied to the gate terminal of the SCR. If the anode terminal of the device in Figure 3 is positive and the cathode terminal is negative, the diode is forward-biased and the device acts like a closed switch. If the polarity of the terminals is reversed, the diode is reverse-biased, and the device acts like an open switch. Notice that the SCR resembles the symbol for an ordinary diode, but has the additional third terminal the gate. 18 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

19 Learning Task 2 D-6 Operation in a DC circuit In order to conduct, the switch in Figure 3 must be closed and the diode must be forwardbiased. The switch is closed by a signal to the gate terminal. If the diode is forward-biased, the SCR is ready to conduct. Therefore, the SCR is unidirectional, meaning that it conducts in one direction only. SCRs, like many solid-state devices, have their own distinct terms: Firing or triggering an SCR means turning it on. Commutating an SCR means turning it off. Triggering the SCR To close the switch, a small positive potential must be applied to the gate with reference to the cathode. One way of doing this is with the circuit in Figure 4. Figure 4 Basic SCR trigger circuit A 1 V battery is connected across the gate-cathode terminals through a push-button. When this push-button is open, no current flows from anode to cathode, and there is no voltage across resistor R. Closing the push-button puts the gate at a positive potential with respect to the cathode. The gate is now 1 V more positive than the cathode. When this happens, the SCR conducts like an ordinary diode. And since the positive side of the 12 V battery is connected to the anode, the SCR diode is forward-biased and a current flows through it. The voltage across R then is 12 V, less the forward voltage drop of the diode. This is approximately 1 V when conducting rated current, leaving a voltage across R of about 11 V (12 V 1 V). Generally, the SCR will fire when the gate is made about 1 V positive with respect to the cathode. Something interesting now happens. If the push-button is opened, removing the positive potential from the gate, the SCR continues to conduct. Once turned on, SCRs do not need the gate-cathode potential to maintain conduction. They keep themselves conducting internally. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 19

20 Learning Task 2 D-6 Commutating the SCR Once an SCR has been turned on, the only way to turn it off is to reduce the anode-cathode current through the SCR to a very low value, or to interrupt this current. One way you can do this is to add a normally closed push-button in series with the SCR, as shown in Figure 5. Turns SCR off + 12 V Turns SCR on + 1 V R Figure 5 Triggering and commutating the SCR To turn off the SCR after it has been turned on, you can press the stop push-button, which breaks the anode-cathode circuit. This reduces the current through the SCR to zero. As you saw before, it is not even necessary to reduce the anode-cathode current to zero: the current through the SCR must just be reduced below a certain low level the holding current (I H ). Holding current for an SCR with a current rating of several tens of amperes is in the order of milliamperes, so we are talking here of low current levels. Operation in an AC circuit To learn how the SCR behaves in an AC circuit, replace the 12 V battery used in the previous circuits with a 12 V rms AC supply, as shown in Figure 6. A K G AC supply + 12 V rms R 1 V Figure 6 Half-wave rectification If the gate circuit is open, the SCR will now act like an open switch: When it is forward-biased to the AC supply it blocks because it needs triggering. When it is reverse-biased to the AC supply it blocks like a common diode. Therefore, no voltage will appear across the load resistor R. 20 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

21 Learning Task 2 D-6 Half-wave rectification If the gate circuit is held closed, the SCR will now act like a closed switch over the positive halfcycle because it has the trigger input. But in the negative half-cycle, it will block like a diode. Therefore, the voltage that appears across the load is as shown in Figure 7, which is the same as a half-wave rectifier. 0 Figure 7 Voltage across the load resistor R, with push-button closed If the push-button has been closed and is then opened at any point during the positive halfcycle, the SCR will continue to conduct for the duration of the half-cycle. Recall that once triggered, an SCR will continue to conduct for as long as the anode is positive with respect to the cathode and the holding current is in place. Once that half-cycle ends, it becomes an open circuit again and blocks. To conduct again, it requires the positive half-cycle and the pushbutton to be reclosed. Phase control For the circuit in Figure 6, with the push-button held closed, commutation of the SCR occurs automatically, and the result is half-wave rectification. This is exactly what you would achieve if you were to use a single diode. If this were all the SCR could do, it would have no advantage over the diode. But with the appropriate gate control circuitry, an SCR can be triggered into conduction at any time during the positive half-cycle. And since commutation will always happen automatically at 180, it means the voltage appearing across the load resistor R can be adjusted from a complete half-cycle down to zero. This is what is meant by phase control (Figure 8). (a) Full power (b) High power (c) Medium power (d) Low power (e) Zero power Figure 8 Phase control with an SCR CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 21

22 Learning Task 2 D-6 In Figure 8: Diagram (a) illustrates maximum load voltage, due to triggering occurring at 0, which is at the very beginning of the cycle. In (b), triggering occurs at about 45 and the voltage wave that appears across the load resistor R will be as shown. This results in reduced load voltage. Diagrams (c), (d) and (e) show the consequences of triggering the SCR progressively later in the cycle. This results in progressively lower load voltages across R, until finally the voltage goes to zero, as shown in (e). Conduction and firing angles If the SCR is triggered at, say, 20 into the positive half-cycle (also called alternation), it will conduct for the remainder of that half-cycle, namely 160 ( = 160 ). Similarly, if the firing (or triggering) angle is at 45, it will conduct for 135 ( ). It follows that the firing angle plus the conduction angle will add up to 180 every time. This is shown in Figure 9. Figure 9 Conduction and firing angles The conduction angle may be defined as the number of electrical degrees over which the SCR conducts every half-cycle or alternation. The larger this angle, the higher the voltage across the load resistor, and the more power developed in the load. One common application of SCR phase control is for controlling the speed of a DC motor. The speed of the motor will increase and decrease as the magnitude of the voltage across the armature increases and decreases. This is described more fully later. 22 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

23 Learning Task 2 D-6 Although you would normally use a voltmeter to measure the average value of DC, you could calculate this value using the following formula (for half-wave rectification): V = E pk where: ( 1+ cos α) 2π V α E pk = average DC voltage = firing angle = peak of the applied AC Example calculation If the AC voltage is 120 V and the firing angle is 90, then: V = E pk ( 1+ cos α) 2π ( 1+ cos 90 ) = 170 V 2π ( 170 V 1 + = 0 ) 2π = 27 V Full-wave rectification Full-wave rectification can be achieved by replacing the diodes with SCRs in the single-phase, full-wave rectifier using the centre-tap transformer. It acts like an ordinary full-wave diode rectifier as long as the gate is triggered at the beginning of each cycle. But it is now a controlled full-wave rectifier also. This is because, with appropriate trigger circuitry, the gates of the SCRs can be pulsed simultaneously at any point in the half-cycle. The output voltage that appears across the load resistor for 60 triggering is illustrated in Figure 10. Triggering could be anywhere between 0 and 180. This enables us to vary the average DC voltage across the load from V pk (where V pk is the peak voltage across one half of the transformer) down to zero. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 23

24 Learning Task 2 D-6 Trigger circuit 60 AC supply Load Trigger pulse Output load voltage Trigger circuit Figure 10 Full-wave rectifier control using two SCRs Full-wave rectification can also be achieved using the circuit shown in Figure 11. Here only one SCR is used, and the voltage across the load is varied by means of the circuitry controlling the gate pulse. As in the previous circuit, the gate can trigger the SCR into conduction at any desirable point in the half-cycle. The load voltage in Figure 11 shows triggering at about 45. Figure 11 Full-wave rectifier control using one SCR We have not yet discussed the circuitry of the gate firing circuit, which you will examine later. It is enough to know here that shifting of the gate pulse is achieved by varying the resistance of a rheostat. This, in turn, fires the gate into conduction at a desired point on the wave, and is commonly called phase control. 24 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

25 Learning Task 2 D-6 Ohmmeter testing of SCRs Functional testing of SCRs usually requires special thyristor test equipment that can supply specified gate current and minimum holding current. Testing with an ohmmeter is not recommended for high-current SCRs, but you can use an ohmmeter to check low-power SCRs for operation, and for opens and shorts. Use the following procedure. Stage 1 Verify ohmmeter polarity You must find or verify the polarity of the ohmmeter leads by connecting the leads of the meter across a polarized device such as a diode or a DC milliammeter. This is shown in Figure 12. Figure 12 Checking the leads of the ohmmeter The diode and the DC milliammeter show continuity when the polarity of the ohmmeter terminal is the same as the polarity of the diode or ammeter. Also, the pointer of the ammeter will move upscale when the polarities match: If the polarity of the ohmmeter leads is opposite to that of the ammeter, the pointer of the ammeter will try to move to the left of the scale. If opposite to the diode polarity, the ohmmeter will indicate a high resistance. Stage 2 Test the SCR Once the ohmmeter polarity is identified, proceed with the following steps. Step 1: Connect the negative lead of the meter to the cathode terminal and the positive lead to the anode terminal of the SCR (Figure 13). Figure 13 Testing an SCR with an ohmmeter Step 2: Connect a jumper wire, shown in the form of a switch, between the anode and gate terminals. With the switch in the open position the ohmmeter should indicate a very high resistance. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 25

26 Learning Task 2 D-6 Step 3: Step 4: Short the gate to the anode (close the switch). This makes the gate positive with respect to the cathode and should trigger the SCR into a conducting mode. The ohmmeter should now indicate continuity. If the switch is now reopened, removing the gate-anode short, the same reading should still show on the ohmmeter. This is the hallmark of the SCR: once triggered into conduction it will keep itself conducting even though the gate signal has been removed. Reverse the meter leads to the SCR anode and cathode terminals. With the switch in the open position the ohmmeter should indicate a very high resistance. Close the switch. The ohmmeter resistance should still read a very high resistance. The ohmmeter must be capable of supplying sufficient gate current to trigger the SCR into conduction. And it must supply sufficient holding current in the anode-cathode circuit to maintain conduction after the jumper is removed. This could, in some instances, exceed the source current capability of the ohmmeter. To maximize the available current, the selector switch of the ohmmeter may be required to be on the R 1 scale. Now do Self-Test 2 and check your answers. 26 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

27 Learning Task 2 D-6 Self-Test 2 1. To turn on the SCR, what must the polarity of the gate be with respect to the cathode? 2. Once an SCR has been turned on, how can it be turned off? 3. What words are used to describe the action of turning an SCR on? 4. Why is it not required to maintain the gate signal once the SCR is on? 5. Normally, in order for an SCR to conduct the anode must be with respect to the cathode, and the cathode must be with respect to the gate. 6. Which of the following ranges of current would approximate the typical holding current magnitude in an SCR? a. microamperes b. milliamperes c. amperes CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 27

28 Learning Task 2 D-6 7. When an SCR is conducting, how many volts (to the nearest round number) would appear across the terminals A K? 8. Why must you connect a resistor (load) in series with an SCR? 9. An ohmmeter is used to test a correctly working SCR, as illustrated in Figure 1. With the switch open, the ohmmeter should indicate a (high or low) resistance. If the switch was then closed, the ohmmeter should indicate a (high or low) resistance. Figure If the SCR in Figure 1 tested correctly with the switch closed, what should the ohmmeter indicate if the switch is reopened and the test leads are still connected to the anodecathode terminals? (high or low) 28 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

29 Learning Task 2 D For the circuit shown in Figure 2, what will be the voltage across the load, if the gate circuit is to remain open? + 12 V rms 1 V R Figure If the gate circuit is held closed in this circuit, draw the shape of the voltage wave that will appear across the load over two AC cycles. 13. If an SCR is triggered at 10 into the cycle, for how many degrees will it conduct? 14. What is meant by the term phase control with reference to an SCR? CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 29

30 Learning Task 2 D If the firing angle is large, the average DC output voltage across the load is (small/large). 16. The maximum number of degrees that the firing angle can have is. 17. When used with a single-phase bridge rectifier, how many SCRs are needed to give complete phase control for a load? 18. Name one common application of phase control using the SCR. Go to the Answer Key at the end of the Learning Guide to check your answers. 30 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

31 Learning Task 3: Describe SCR triggering circuits for AC phase control As mentioned earlier, a key advantage of an SCR over a diode rectifier is the ability to provide phase control of the AC waveform. There are two fundamental triggering (or gate control) circuits for obtaining phase control using SCRs. These are the: resistor triggering circuit resistor-capacitor triggering circuit Resistor triggering To understand how resistor (R) triggering works and why it is limited to 90 phase control, you must remember that an SCR will trigger when there is sufficient current flowing in the gatecathode circuit. Assume that for our particular SCR, we need 10 ma of current in the gate circuit to fire (trigger) the SCR into conduction. The circuit in Figure 1 shows what is happening. AC supply D 1 R 1 L o a d Figure 1 Resistor triggering circuit During the positive half-cycle, both the anode of the SCR and the gate are positive with respect to the cathode. Current flow through the gate will trigger the SCR into conduction and a current will flow in the load circuit. The function of the diode in the gate circuit is to prevent reverse voltage from being applied to the gate-cathode junction during the negative half-cycle. This is a PN junction with a low PIV rating and should not be exposed to more than about 10 V. Assume that the gate rheostat is variable from 0 to Ω. If the rheostat is set at its maximum value the gate current would never reach the 10 ma required for triggering. The maximum value of the available AC voltage at the gate is 120 V = 170 V. If we divide 170 V by Ω the current is 8.5 ma. This is less than the current needed to get the SCR to conduct. The SCR will be triggered if the resistance of the rheostat is reduced to Ω because 170 V Ω = 10 ma. This means that we must wait until the AC voltage reaches its maximum value before it will trigger, and the maximum voltage occurs at 90 in the cycle. If we now reduced the resistance of the rheostat to, say, 8000 Ω, what value of voltage would be needed to give us the required 10 ma to trigger the SCR? The answer is 80 V (10 ma 8000 Ω). CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 31

32 Learning Task 3 D-6 The maximum voltage of 170 V occurs at 90 ; the rms value of 120 V occurs at 45 ; and 80 V occurs at approximately 28. If the resistance is reduced below 8000 Ω it is clear that the 10 ma required for triggering the SCR into conduction occurs earlier than at 28. If the resistance is increased above 8000 Ω (but below Ω), the SCR will be triggered later than at 28. In principle, you should be able to fire the SCR right at the beginning of the alternation. In practice, the SCR will fire a few degrees into the cycle. Figure 2 shows the maximum and minimum output waveforms across the load for this type of triggering. You will appreciate that if the voltage magnitude up to 90º is insufficient to trigger the SCR, it will not trigger in the region between 90 and 180 either, because now the voltage is decreasing. This means the SCR can be triggered at between 0 to 90 only, using resistor triggering. It will then conduct for the remainder of the half-cycle. Figure 2 Resistor triggering waveforms Resistor-capacitor triggering With resistor-capacitor (RC) triggering you can get phase control from 0 to 180. The basic circuit that allows you to do this is shown in Figure 3. A variable resistor is connected as a rheostat that is, in series with the capacitor. The capacitor can be looked upon as the source voltage for the gate circuit. Because the capacitor is of fixed value, the rate at which the capacitor charges is determined by the resistance of the rheostat. The faster the capacitor charges, the faster the voltage across the capacitor rises. When the capacitor charges to the value of voltage required to trigger the SCR, conduction begins. Figure 3 shows the circuit that allows for full control from 0 to 180. On the positive half-cycle of the SCR anode voltage, the capacitor charges to the trigger voltage of the SCR in a time determined by the product of R and C, and the rising anode voltage. On the negative half-cycle, the top plate of the capacitor charges to the peak of the negative voltage cycle through diode D 1. This resets the capacitor for the next positive half-cycle. 32 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

33 Learning Task 3 D-6 Load D 1 R 1 D 2 AC supply C 1 Figure 3 Resistor-capacitor triggering circuit The diode D 2 in the gate circuit protects the gate-cathode junction when the AC cycle reverses. The PIV rating of the gate junction is low, and on voltage reversal, the SCR would be quickly destroyed without this diode. Figure 4 shows representative voltages that can appear across the load. You can fire the SCR anywhere from 0 to 180, in contrast to the resistor triggering, which limited the phase control from 0 to 90. Figure 4 Resistor-capacitor triggering waveforms Triggering angles are shown at (a) 0, (b) 45, (c) 90 and (d) 135. But remember, the SCR can be triggered at any angle lying between 0 and 180. And, as always, the SCR blocks on the negative half-cycle. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 33

34 Learning Task 3 D-6 Paralleling SCRs for full AC load control In the phase-control circuit of Figure 3, the SCR conducts only during the positive half of the input voltage. This suffices for DC loads, but AC loads require bidirectional current, and control is needed in both the positive and negative halves of the cycle. This can be obtained by connecting two SCRs in a reverse back-to-back arrangement, as shown in Figure 5. Each of the SCRs has its own trigger circuit. Figure 5 SCR control of an AC load During the positive half-cycle, SCR 1 turns on when the gate is triggered. During the negative half-cycle, SCR 2 turns on when its gate is triggered. The trigger angles may be independent of each other, but in practice they are usually the same so that the firing angle is the same in both half-cycles. Full control is now attainable over the whole 360. If both SCRs are triggered at 90, the waveform of the voltage across the load in Figure 5 will be as shown in Figure 6. Figure 6 AC load waveform This method of varying AC voltage is common when a large amount of AC power is to be controlled. Where the amount of power to be controlled is smaller, the two SCRs are replaced by a device called the Triac, discussed in the next Learning Task. Now do Self-Test 3 and check your answers. 34 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

35 Learning Task 3 D-6 Self-Test 3 1. What are the two common forms of triggering used with SCRs? 2. If resistor triggering is used, the SCR can be fired between degrees up to degrees in the positive half-cycle. 3. If resistor-capacitor triggering is used, the SCR can be fired from degrees up to degrees in the positive half-cycle. 4. Regardless of the type of triggering circuit, the SCR will always block for at least degrees of the AC cycle. 5. What determines the rate at which the capacitor charges to the required SCR positive gate voltage in the resistor-capacitor phase-control circuit? (cont d) CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 35

36 Learning Task 3 D-6 6. Draw the shape of the voltage wave that would appear across the load over two AC cycles if the triggering occurs at 30, as shown in Figure 1. AC supply D 1 R 1 L o a d Figure 1 7. Draw the shape of the voltage wave that would appear across the load over two AC cycles if triggering were at 30, as shown in Figure 2. Figure 2 Go to the Answer Key at the end of the Learning Guide to check your answers. 36 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

37 Learning Task 4: Describe the features of the Triac Triac is the common name for the bidirectional triode thyristor. It is a bidirectional switching device, very similar in appearance to an SCR. Triacs operate like the reverse-parallel back-to-back operation of the two SCRs triggered from a common gate, described previously. With the Triac s ability to conduct in a forward and reverse direction, it is ideal for controlling power to AC loads. You may then ask: Why would AC loads use reverse-parallel SCRs? The answer is that, while Triacs are ideal for AC load control, they have relatively low current-handling capability compared to SCRs, and cannot substitute for SCRs in high-current applications. Triacs are ideal for light-dimming applications, domestic-range hot plates, small motors and other applications that do not need high currents. If we consider a 100 W/120 V incandescent lamp, the Triac can control the light output from zero to full brightness. (An SCR could control the light output from zero to half brightness only.) Symbol and leads Like the SCRs, Triacs vary in physical size, shape and configuration, depending on their application and load rating. The schematic symbol for a Triac is shown in Figure 1. Like the SCR, the Triac has three terminals: main terminal 1 (MT 1 or T 1 ) main terminal 2 (MT 2 or T 2 ) gate MT 1 Gate MT 2 MT 1 Gate G MT 1 MT 2 MT 2 Figure 1 Triacs and symbol Triac symbol T 1 and T 2 are the main terminals through which the current may flow in either direction. Terminal 1 is the reference terminal to which all the other voltages refer. Terminal 2 is normally the case, or metal mounting part, in the stud-mounted type. This attaches to the heat sink where desired, much like the anode of the SCR. The gate terminal in some Triacs can usually be identified by its small size. It is small because the gate is required to handle only a very small control current. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 37

38 Learning Task 4 D-6 Triac characteristics In the circuit in Figure 2(a), if the voltage across the main terminals of the Triac is increased to a sufficient magnitude, in either direction, the Triac will turn on without a gate signal. This voltage is called the breakover voltage and is identified as V F or V R in Figure 2(b). The voltage across the Triac then drops, and the current is limited only by the series resistance R L. Figure 2 Triac characteristics The operational characteristics of the Triac are very similar to those of the SCR, with one important difference. Whereas the SCR is a unidirectional switch, the Triac is bidirectional; therefore, its characteristic curves are the same in the two quadrants in Figure 2(b). The Triac then behaves like a regular switch but, as with diodes and SCRs, it will have a small voltage across it while conducting. Because of the Triac s bidirectional capability, the gate current normally triggers the Triac into conduction. Once the Triac has been triggered on by a gate signal, the gate has no further control. The Triac, like an SCR, needs a minimum holding current to sustain the on condition. When conducting rated current, the voltage drop across a Triac is 1V to 2 V. 38 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

39 Learning Task 4 D-6 Triac ratings Triac parameter ratings are much the same as those for SCRs. The initial selection begins with the current rating. Triacs are rated in terms of their maximum continuous rms current and are available in current ratings from 0.6 A to 45 A. If the rated current is exceeded, the device is subjected to excessive temperature rise and will likely fail. As with other thyristors and transistors, a manufacturer s data sheet will contain a lot of information, but some of this data is relevant only to a circuit designer or someone who specializes in the field of solid-state switching. Triacs are designed for operation over a much smaller frequency range than SCRs and are limited to switching frequencies up to about 400 Hz. SCRs can operate as static switches up to about Hz. The terms used with the Triac are very similar to those used for the SCR. (One exception is the term reverse breakdown voltage. Since the Triac will conduct in both directions, reverse breakdown voltage does not apply here.) Some of the more significant and relevant Triac parameters found in the manufacturers manuals and packing slip information are shown below: I T(rms) V RRM I H V GT I GT dv/dt Operating temperature The rms current is the maximum rms value of current that the Triac may safely carry. V RRM in a Triac is the breakover voltage that will trigger the Triac into conduction without a gate signal. A Triac, like an SCR, is rarely switched on by this means. In normal operation the gate pulse triggers the Triac. In the Triac, the holding current (I H ) is the minimum current that must be flowing between the main terminals to maintain conduction. V GT is the gate trigger voltage necessary to turn on the Triac. I GT is the gate trigger current required to turn on the Triac. dv/dt is the rate of voltage rise that will turn on the Triac without a gate signal. This is the junction temperature range over which the Triac may be operated. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 39

40 Learning Task 4 D-6 Figure 3 shows a typical manufacturer s data sheet. V RRM DC or Peak IT RMS Max Forward Current (Amperes) Volts 0.8 A 2.5 A 4 A 8 A 10 A 50 ECG5601 ECG5612 ECG5622 ECG ECG5640 ECG5650 ECG5602 ECG5613 ECG5623 ECG ECG5655 ECG5641 ECG5651 ECG5603 ECG5614 ECG5624 ECG ECG5656 ECG5642 ECG5652 ECG5605 ECG5629 ECG5608 ECG5638 ECG5616 ECG5626 ECG ECG5657 ECG5643 ECG5653 ECG5607 ECG5609 ECG5618 ECG5628 ECG5637 ECG ECG5610 ECG5620 I GT Min (ma) Quadrants I & III Quads I, II, III I GT Min (ma) Quadrants II & IV Quad IV * 75 V GT Max (V) I Surge Max (A) I Hold Min (ma) V on Max (V) V GM (V) ±5.0 ±5.0 ±5.0 ±5.0 ±5.0 ±10.0 ±5.0 ±10.0 ±5.0 ±5.0 ±10.0 ±5.0 P G Av (W) Operating Temperature T J C -40 to to to to to to to to to to to to +110 Off State dv/dt (Typ) V/µ sec Operating Quadrants I, II, III, IV I, II, III, IV I, II, III, IV I, III I, II, III, IV I, II, III, IV I, II, III, IV I, II, III, IV I, II, III, IV I, III I, II, III I, II, III, IV Figure 3 Typical data in specifications manual Courtesy of Philips ECG 40 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

41 Learning Task 4 D-6 Common case styles Figure 4 shows a representative selection of common Triac case styles. You can see that these case styles are very similar to those of the SCRs. Typical case styles are the TO-220, TO-48 and TO-3. The manufacturers manuals will illustrate the package styles, identify terminal leads and give the dimensions. As with the SCRs, the physical dimensions generally increase with an increase in current ratings. Figure 4 Triac case styles Triac operation The behaviour of the Triac is similar to the operation of two SCRs wired back-to-back and having a common trigger (Figure 5). Firing circuit AC load Figure 5 Full-wave phase control using SCRs The firing is arranged so that the upper SCR is triggered on the positive half-cycle and the lower SCR is triggered on the negative half-cycle. As well, the triggering occurs at the same relative CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 41

42 Learning Task 4 D-6 point in each half-cycle so that the waves are symmetrical both above and below the baseline. This method of varying AC voltage using two SCRs is actually used where a large amount of AC power has to be controlled. As long as the load current is not very large, the two back-to-back SCRs may be replaced by a Triac, which does the same job using a single device. Figure 6 shows the Triac controlling a load. Load Gate control circuit G MT 2 MT 1 Triac Figure 6 Full-wave phase control using a Triac A gate control circuit supplies the gate pulses to trigger the Triac into conduction at any desired angle. In almost every respect the Triac behaves like an SCR. There is a certain value of gate current needed to trigger it, and once it is triggered there is a minimum value of holding current required to sustain conduction. Representative waveforms that appear across the load are illustrated in Figure 7. Zero power High power Low power Full power Half power Figure 7 Phase control waveforms 42 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

43 Learning Task 4 D-6 Triac trigger control Like an SCR, Triac trigger control circuits may be resistor control or resistor-capacitor control. And as with the SCR, the resistor control has the limitation that the firing angle can be adjusted only from about 0 to 90 on each half-cycle. Phase control from 0 to 180 requires the resistorcapacitor Diac control circuit. This allows you to adjust the light output of a lamp from full brightness to the full off position, or a motor speed from high to low. (The Diac will be discussed in the next Learning Task.) Triac testing with an ohmmeter As with the SCR, you need special test instruments to do comprehensive tests on a Triac. However, an ohmmeter may be used to verify whether the device is short-circuited or opencircuited, using the following procedure. Step 1: Step 2: Step 3: Verify the polarity of the ohmmeter leads, if not already known. Use a polarized device like a DC ammeter or a diode, as outlined in Step 1 in Stage 2 Test the SCR (Learning Task 2). Connect the positive lead of the ohmmeter to MT 2 and the negative lead to MT 1, with the gate terminal open circuit. The meter should show no continuity and the Triac should behave like an open switch, as with a reverse-biased diode (Figure 8). Keeping the ohmmeter leads in the same place as in Step 2, connect a jumper lead between the gate and MT 2. The ohmmeter should now indicate continuity, as with a forward-biased diode (Figure 9). Ohmmeter RX1 + + MT 2 MT 1 Gate Figure 8 Step 2 Figure 9 Step 3 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 43