Download The silicon controlled rectifier

Electronics
Principles & Applications
Seventh Edition
Charles A. Schuler
Chapter 14
Electronic Control
Devices and Circuits
(student version)
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INTRODUCTION
• The Silicon Controlled Rectifier
• Full-Wave Devices
• Feedback in Control Circuitry
• Troubleshooting
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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.
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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.
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This two-transistor circuit is
stable in either of two states.
Q1
Q2
On state
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Off state
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A four-layer structure replaces
the two-transistor circuit.
}
{
p
Q1
n
Q2
Q2
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Q1
p
n
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The silicon controlled rectifier (SCR)
Anode
Anode
Anode
p
Q1
n
Gate
Gate
Gate
n
Q2
Cathode
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p
Cathode
Cathode
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The SCR can be turned on at its gate terminal.
With a dc
source, the
SCR stays
on after
it is gated.
Anode
Load
Load
current
Gate
Cathode
Gate pulse
occurs here
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Time
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With an ac
source, the
SCR turns
off at the
zero-crossing.
Anode
Load
Gate
Cathode
on
Load
current
off
Gate pulse
occurs here
Time
Turns off here
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The gate can
be pulsed for
each positive
alternation.
Load
Cathode
Load
current
Gate
Anode
Time
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Load
Gate
Anode
Cathode
Load
current
The average
load current
can be
decreased
by gating
the SCR later.
Time
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…. and later.
Load
Cathode
Load
current
Gate
Anode
Time
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…. or, not
at all.
Load
Load
current
Gate
Anode
Cathode
No gate pulses: ILoad = 0
0
Time
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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
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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.
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A full-wave rectifier allows
use of both alternations.
Load
Gate
Bridge rectifier
Load current
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Load
Two SCRs can provide
full-wave control.
Gated early for full power
Gated later for low power
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The TRIAC provides control of both ac alternations.
Main terminal 2
Load
Gate
Main terminal 1
Gated early
Gated late
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Solid state relays provide optical isolation.
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DIAC volt-ampere characteristic curves
+I
VP+
-V
+V
VP-
-I
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A popular diac-triac control circuit
Load
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Decreasing
R will gate
the TRIAC
earlier.
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Illustration courtesy Powerex, Inc.
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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 fullwave control is the __________. triac
The diode often used to trigger triacs is the
_________.
diac
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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
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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.
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Servomechanisms use negative feedback
to control velocity and/or position.
VREF
Velocity
set
Negative feedback
Error
Amplifier
Tachometer
Motor
If the mechanical load changes, causing the velocity
to change, the error amplifier will respond by
adjusting the motor drive to reduce the change.
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VREF
Error
Amplifier
Motor
Position
set
VREF
Feedback
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Gearbox
In this servo,
gears drive a
variable resistor
to provide
position feedback.
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Suppose, at time t1, a servo is
commanded to a new position.
Position
This is the ideal response
Time
t1
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Position
This is the critically damped response
Time
t1
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Position
This is the overdamped response
Time
t1
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Position
This is the underdamped response
Time
t1
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Position
Tuning a servomechanism involves
adjusting the loop gain and the phase
to achieve the best response.
Time
Which response is the best?
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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.
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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.
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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
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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
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REVIEW
• The Silicon Controlled Rectifier
• Full-Wave Devices
• Feedback in Control Circuitry
• Troubleshooting
McGraw-Hill
© 2008 The McGraw-Hill Companies Inc. All rights reserved.