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ElectronicsElectronics
Principles & ApplicationsPrinciples & ApplicationsFifth EditionFifth Edition
Chapter 14Electronic Control
Devices and Circuits
©1999 Glencoe/McGraw-Hill
Charles A. Schuler
• The Silicon-Controlled Rectifier• Full-Wave Devices• Feedback in Control Circuitry• Troubleshooting
INTRODUCTION
This two-transistor circuit isstable in either of two states.
On state Off state
Q1
Q2
Q1
Q2
p
p
n
n
A four-layer structure replacesthe two-transistor circuit.
Q1}{Q2
The silicon controlled rectifier (SCR)
Q1
Q2
Anode
Gate
Cathode
p
p
n
n
Anode
Gate
Cathode
Anode
Gate
Cathode
Gate
Anode
CathodeL
oad
curr
ent
TimeGate pulseoccurs here
LoadWith a dc source, the SCR stays
on after it is gated.
The SCR can be turned on at its gate terminal.
Gate
Anode
Cathode
Loa
dcu
rren
t
TimeGate pulseoccurs here
LoadWith an ac source, the SCR turnsoff at the
zero-crossing.
Turns off here
on
off
Gate
Anode
Cathode
Loa
dcu
rren
t
Time
LoadThe gate canbe pulsed foreach positivealternation.
Gate
Anode
Cathode
Loa
dcu
rren
t
Time
LoadThe averageload current
can bedecreasedby gating
the SCR later.
Gate
Anode
Cathode
Loa
dcu
rren
t
Time
Load…. and later.
Load
A full-wave rectifier allows use of both alternations.
Bridge rectifier
Load current
Gate
Load
Two SCRs can providefull-wave control.
Gated early for full power
Gated later for low power
Load
Main terminal 1
Main terminal 2
Gate
The TRIAC provides control of both ac alternations.
Gated early
Gated late
DIAC volt-ampere characteristic curves
+I
-I
+V-V
VP+
VP-
Load
A popular diac-triac control circuit
Decreasing R will gate the TRIAC
earlier.
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 gatingthe 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
Servomechanisms use negative feedback to control velocity and/or position.
Motor
Tachometer
ErrorAmplifier
Negative feedback
Velocityset
VREF
If the mechanical load changes, causing the velocityto change, the error amplifier will respond by
adjusting the motor drive to reduce the change.
Motor
ErrorAmplifier
Positionset
VREF
VREFGearbox
In this servo,gears drive a
variable resistor to provide
position feedback.
Feedback
Pos
itio
n
Time
t1
Suppose, at time t1, a servo is commanded to a new position.
This is the ideal response
Pos
itio
n
Time
t1
This is the critically damped response
Pos
itio
n
Time
t1
This is the overdamped response
Pos
itio
n
Time
t1
This is the underdamped response
Pos
itio
n
Time
Tuning a servomechanism involves adjusting the loop gain and the phase
to achieve the best response.
Which response is the best?
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
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.
Servo quiz
Servos use negative _________ to controlposition or speed. feedback
When servo response is sluggish, theresponse is ___________. overdamped
When servo response is oscillatory, theresponse is ___________. underdamped
The overdamped response can be causedby _________ gain. insufficient
The underdamped response can be causedby _________ gain. excessive
REVIEW
• The Silicon-Controlled Rectifier• Full-Wave Devices• Feedback in Control Circuitry• Troubleshooting