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Chapter 17
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The SCR is a switching device for highvoltage and high-current operations.
Like an ordinary diode, an SCR conducts in
one direction
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In these applications, the SCR gate circuit is used to monitor a situation
and trigger the SCR to turn on some portion of the circuit:
Battery-charging regulator
Temperature controller circuit
Emergency-lighting system
Higher power versions used in controlling high-voltage where timing is
critical:
X-ray beam timing controllers
High power frequency generation
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Three terminals
◦ Anode (Positive)
◦ Cathode (Negative)
◦ Gate (Control)
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To switch an SCR on:
• Forward bias the anodecathode terminals (VF),
and
• Apply sufficient gate
voltage (Vgate) and gate
current (IGT)
Once an SCR is switched on, it
remains latched on, even when
the gate signal is removed.
Holding current (IH): The minimum
required current from anode to
cathode
Reverse breakdown voltage: The
maximum reverse bias voltage for the
SCR
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To switch an SCR off:
• Remove the power source
the anode and cathode
terminals
OR
• Reverse bias the anode
and cathode terminals
An SCR cannot be switched off by
simply removing the gate voltage.
Commutation circuitry can be
used for satisfying either of the
conditions for switching off an
SCR.
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Three terminals
Much larger than transistors
High power versions may bolt in
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Used to output voltage
◦ Allow conduction when energized
◦ Used to control circuit on-time
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Two main approaches
◦ Self commutation
◦ Forced commutation
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Command to gate of SCR is dropped when
time is reached
◦ No conduction past the next 0V crossing
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Make contactor energized
Capacitor charged
Break contactor off
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Make contactor gate signal
dropped
Break contactor fired
◦ Capacitor discharges through
Break contactor
◦ Raises potential on output of Make
contactor
 Conduction through Make contactor
stops
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The gate can be set to trigger an SCR at any point in the AC cycle.
In this example, the SCR
triggers as soon as the
AC input crosses 0 V.
Therefore it acts like a
half-wave rectifier.
In this example, the SCR
triggers later – at the 90
point on the positive halfcycle.
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Insulated Gate Bipolar Transistor
◦ Replacing older SCR, operate like normal transistors
◦ High frequency, high power applications
 Ex- 50 kW Generator, (560 V, 89 A)
◦ Part time conduction
 Negative bias applied to G
 Conducts till bias removed, shuts off immediately
IGBT
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Check DC supply
Gate driver
SCRs
◦ May need be checked under pressure
◦ Checked similar to diodes
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IGBTs
◦ Checked similar to JFETs
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The Shockley diode conducts once
the breakover voltage is reached. It
only conducts in one direction.
The Shockley diode must be
forward biased. Once the forward
voltage reaches the breakover
level, it will conduct. Like an SCR,
it only conducts in one direction.
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Breakover component
◦ Conducts when voltage
exceeds set level
◦ Breakover voltage the
same magnitude for
positive and negative

Conducts either
direction
Applications:
• Trigger circuit for the Triac
• Proximity sensor circuit
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Operates like SCR
◦ Conducts only when gate
biased

More versatile
◦ Gate polarity can be positive
or negative
◦ Can conduct current both
directions / polarities
Application:
AC power control circuits
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Fig 17.32
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The phototransistor is a lightcontrolled transistor. The current
through the collector and emitter
circuits is controlled by the light
input at the base.
The collector current is the product
of the transistor current gain (hfe)
and the light induced base current
(Iλ).
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Used to transmit electrical signals using light
◦ Isolate high voltage from lower voltage
◦ Isolate high noise from sensitive components
Photodiode
PhotoDarlington
Photo-SCR
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