2. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Electronics Principles & Applications Seventh Edition Chapter 14 Electronic Control Devices and Circuits (student version) Charles A. Schuler McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved.

3. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. The Silicon Controlled Rectifier Full-Wave Devices Feedback in Control Circuitry Troubleshooting INTRODUCTION

4. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Dear Student: This presentation is arranged in segments. Each segment is preceded by a Concept Preview slide and is followed by a Concept Review slide. When you reach a Concept Review slide, you can return to the beginning of that segment by clicking on the Repeat Segment button. This will allow you to view that segment again, if you want to.

5. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Concept Preview An NPN-PNP latch can be either on or off. Once gated on, the latch cannot be gated off. Silicon controlled rectifiers (SCRs) are latches. An SCR is turned on by applying a pulse to its gate terminal. With a dc source, the SCR stays on after it is gated. With an ac source, the SCR turns off at the zero crossing. Load power is controlled by gating the SCR earlier or later during the ac cycle.

6. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. This two-transistor circuit is stable in either of two states. On state Off state Q1Q1 Q2Q2

7. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Q1Q1 Q2Q2 p p n n A four-layer structure replaces the two-transistor circuit. Q1Q1 } { Q2Q2

8. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. The silicon controlled rectifier (SCR) Q1Q1 Q2Q2 Anode Gate Cathode p p n n Anode Gate Cathode Anode Gate Cathode

9. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Gate Anode Cathode Load current Time Gate pulse occurs here Load With a dc source, the SCR stays on after it is gated. The SCR can be turned on at its gate terminal.

10. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Gate Anode Cathode Load current Time Gate pulse occurs here Load With an ac source, the SCR turns off at the zero-crossing. Turns off here on off

11. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Gate Anode Cathode Load current Time Load The gate can be pulsed for each positive alternation.

12. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Gate Anode Cathode Load current Time Load The average load current can be decreased by gating the SCR later.

13. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Gate Anode Cathode Load current Time Load …. and later.

14. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Gate Anode Cathode Load current Time Load …. or, not at all. No gate pulses: I Load = 0 0

15. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Concept Review An NPN-PNP latch can be either on or off. Once gated on, the latch cannot be gated off. Silicon controlled rectifiers (SCRs) are latches. An SCR is turned on by applying a pulse to its gate terminal. With a dc source, the SCR stays on after it is gated. With an ac source, the SCR turns off at the zero crossing. Load power is controlled by gating the SCR earlier or later during the ac cycle. Repeat Segment

16. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Concept Preview An SCR, in conjunction with a bridge rectifier, can provide full-wave control. Two SCRs can provide full-wave control without a bridge rectifier. A triac is equivalent to two SCRs. Solid state relays combine triacs and optoisolators in a single package. SCRs and triacs are both members of the thyristor family. A diac breaks down in both directions and is ideal for gating triacs.

17. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Load A full-wave rectifier allows use of both alternations. Bridge rectifier Load current Gate

18. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Load Two SCRs can provide full-wave control. Gated early for full power Gated later for low power

19. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Load Main terminal 1 Main terminal 2 Gate The TRIAC provides control of both ac alternations. Gated early Gated late

20. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Solid state relays provide optical isolation.

21. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. DIAC volt-ampere characteristic curves +I -I-I +V -V-V VP+VP+ VP-VP-

22. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Load A popular diac-triac control circuit Decreasing R will gate the TRIAC earlier.

23. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Illustration courtesy Powerex, Inc.

24. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Thyristor quiz The three terminals of an SCR are anode, cathode and _________. gate SCR turnoff in ac circuits occurs at the __________ crossing. zero In ac control, load power is increased by gating the SCR ______ in the cycle. earlier The device equivalent to two SCRs for full- wave control is the __________. triac The diode often used to trigger triacs is the _________. diac

25. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Concept Review An SCR, in conjunction with a bridge rectifier, can provide full-wave control. Two SCRs can provide full-wave control without a bridge rectifier. A triac is equivalent to two SCRs. Solid state relays combine triacs and optoisolators in a single package. SCRs and triacs are both members of the thyristor family. A diac breaks down in both directions and is ideal for gating triacs. Repeat Segment

26. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Concept Preview Servos use negative feedback to provide accurate velocity or positioning. A velocity servo compares tachometer voltage with a set voltage. A position serve compares potentiometer voltage with a set voltage. Servo response can be critically damped (most desired), overdamped, or underdamped. Servos are tuned for the best response by adjusting gain and phase.

27. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Servomechanisms use negative feedback to control velocity and/or position. Motor Tachometer Error Amplifier Negative feedback Velocity set V REF If the mechanical load changes, causing the velocity to change, the error amplifier will respond by adjusting the motor drive to reduce the change.

28. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Motor Error Amplifier Position set V REF Gearbox In this servo, gears drive a variable resistor to provide position feedback. Feedback

29. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Position Time t1t1 Suppose, at time t 1, a servo is commanded to a new position. This is the ideal response

30. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Position Time t1t1 This is the critically damped response

31. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Position Time t1t1 This is the overdamped response

32. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Position Time t1t1 This is the underdamped response

33. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Position Time Tuning a servomechanism involves adjusting the loop gain and the phase to achieve the best response. Which response is the best?

34. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Troubleshooting SCRs can fail by opening or shorting. A shorted SCR means full load power. An open SCR means no load power. A defective gate circuit can produce either full power or no power. TRIAC troubleshooting is much the same.

35. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Servo Troubleshooting Mechanical problems can be confused with electrical faults. Slippage and excessive mechanical play can cause various symptoms and should be eliminated before tuning or troubleshooting. The underdamped response is often caused by excessive gain. The overdamped response is often caused by insufficient gain.

36. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Servo quiz Servos use negative _________ to control position or speed. feedback When servo response is sluggish, the response is ___________. overdamped When servo response is oscillatory, the response is ___________. underdamped The overdamped response can be caused by _________ gain. insufficient The underdamped response can be caused by _________ gain. excessive

37. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Concept Review Servos use negative feedback to provide accurate velocity or positioning. A velocity servo compares tachometer voltage with a set voltage. A position serve compares potentiometer voltage with a set voltage. Servo response can be critically damped (most desired), overdamped, or underdamped. Servos are tuned for the best response by adjusting gain and phase. Repeat Segment

38. McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. REVIEW The Silicon Controlled Rectifier Full-Wave Devices Feedback in Control Circuitry Troubleshooting