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ElectronicsElectronics
Principles & ApplicationsPrinciples & ApplicationsSixth EditionSixth Edition
Chapter 14Electronic Control
Devices and Circuits(student version)
©2003 Glencoe/McGraw-Hill
Charles A. Schuler
• The Silicon Controlled Rectifier
• Full-Wave Devices
• Feedback in Control Circuitry
• Troubleshooting
REVIEW
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 youto view that segment again, if you want to.
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.
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.
Gate
Anode
Cathode
Loa
dcu
rren
t
Time
Load…. or, notat all.
No gate pulses: ILoad = 0
0
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
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.
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
Solid state relays provide optical isolation.
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.
Illustration courtesy Powerex, Inc.
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
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
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.
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 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
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
REVIEW
• The Silicon Controlled Rectifier
• Full-Wave Devices
• Feedback in Control Circuitry
• Troubleshooting