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

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

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

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

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

6. 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

7. 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

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

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

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

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

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

13. 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

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

15. Solid state relays provide optical isolation.

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

17. DIAC Volt-ampere Characteristic Curves
Load
Decreasing
R will gate
the TRIAC
earlier.

18. Illustration courtesy Powerex, Inc.

19. 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

20. 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.

21. 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

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

23. This is the critically damped response
Position
Time t1

24. This is the overdamped response
Position
Time t1

25. This is the underdamped response
Position
Time t1

26. 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?

27. 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.

28. 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.

29. 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

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