1. Principles & Applications
Electronics
Principles & Applications
Sixth Edition
Charles A. Schuler
Chapter 14
Electronic Control
Devices and Circuits
(student version)
© Glencoe/McGraw-Hill

2. REVIEW The Silicon Controlled Rectifier Full-Wave Devices
Feedback in Control Circuitry
Troubleshooting

3. 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.

4. 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.

5. This two-transistor circuit is stable in either of two states.Q1 Q2
On state
Off state

6. A four-layer structure replaces the two-transistor circuit.Q1 } { Q2 p Q1 n p n Q2

7. The Silicon Controlled Rectifier (SCR)
Anode
Anode
Anode p Q1 n p
Gate
Gate
Gate n Q2
Cathode
Cathode
Cathode

8. The SCR can be turned on at its gate terminal.
With a dc
source, the
SCR stays
on after
it is gated.
Load
Anode
Cathode
Gate
current
Load
Gate pulse
occurs here
Time

9. With an ac source, the SCR turns off at the zero-crossing.
Load
Anode
Cathode
Gate on
off
current
Load
Gate pulse
occurs here
Turns off here
Time

10. The gate can be pulsed for each positive alternation.
Load
Anode
Cathode
Gate
current
Load
Time

11. The average load current can be decreased by gating the SCR later.
Anode
Cathode
Gate
current
Load
Time

12. …. and later.
Load
Anode
Cathode
Gate
current
Load
Time

13. …. or, not at all. Load Anode Cathode Gate current Load
No gate pulses: ILoad = 0
Time

14. 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

15. 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.

16. A full-wave rectifier allows use of both alternations.
Load
Bridge rectifier
Load current
Gate

17. Two SCRs can provide full-wave control.
Load
Two SCRs can provide
full-wave control.
Gated early for full power
Gated later for low power

18. The TRIAC provides control of both ac alternations.
Main terminal 2
Load
Gate
Main terminal 1
Gated early
Gated late

19. Solid state relays provide optical isolation.

20. DIAC Volt-ampere Characteristic Curves
VP+ -V +V
VP- -I

21. A popular diac-triac control circuit
Load
Decreasing
R will gate
the TRIAC
earlier.

22. Illustration courtesy Powerex, Inc.

23. 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

24. 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

25. 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.

26. Servomechanisms use negative feedback
to control velocity and/or position.
VREF
Negative feedback
Tachometer
Error
Amplifier
Motor
Velocity
set
If the mechanical load changes, causing the velocity
to change, the error amplifier will respond by
adjusting the motor drive to reduce the change.

27. In this servo, gears drive a variable resistor to provide
VREF
Error
Amplifier
Motor
Position
set
Gearbox
VREF
In this servo,
gears drive a
variable resistor
to provide
position feedback.
Feedback

28. Suppose, at time t1, a servo is commanded to a new position.
This is the ideal response
Position
Time t1

29. This is the critically damped response
Position
Time t1

30. This is the overdamped response
Position
Time t1

31. This is the underdamped response
Position
Time t1

32. Which response is the best?
Tuning a servomechanism involves
adjusting the loop gain and the phase
to achieve the best response.
Position
Time
Which response is the best?

33. 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.

34. 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.

35. 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

36. 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

37. REVIEW The Silicon Controlled Rectifier Full-Wave Devices
Feedback in Control Circuitry
Troubleshooting