The document discusses the Silicon Controlled Rectifier (SCR), which acts as a switch that is cheaper than a relay and can handle large power dissipation. The SCR has a four-layer construction like a conventional rectifier but includes a third gate terminal that controls the rectifier conduction. When a positive voltage is applied to the anode and gate, current will flow through the device, turning it on. Once on, it remains on even after the gate signal is removed, as long as the minimum holding current is maintained.

1. The four-layer construction in Silicon
Controlled Rectifier, or SCR
We ground both the cathode and the gate,
and apply a positive voltage to the anode,
no current will flow through this device
This device acts as a switch that is cheaper
than a relay and is able to handle the large
power dissipation that we are expecting
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2. SCR
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3. Simply a conventional rectifier controlled
by a gate signal.
The main circuit is a rectifier, however
the application of a forward voltage is not
enough for conduction.
A gate signal controls the rectifier
conduction.
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4. Silicon Controlled Rectifier (SCR)
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5.  The rectifier circuit (anode-cathode) a low
forward
resistance and a high reverse resistance.
 Controlled from an off state (high resistance) to
the on state (low resistance) by a signal applied to
the third terminal, the gate. Once it is turned on it
remains on even after removal of the gate signal,
as long as a minimum current, the holding current,
Ih, is maintained in the main or rectifier circuit. To
turn off an SCR the anode-cathode current must
be
reduced to less than the holding current, Ih.
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6. I-V Characteristics
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7.  (a) Peak forward and reverse
breakdown voltages
 (b) Maximum forward current
 (c) Gate trigger voltage and current
 (d) Minimum holding current,Ih
 (e) Power dissipation
 (f) Maximum dV/dt
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8.  Three terminals
 anode - P-layer
 cathode - N-layer (opposite end)
 gate - P-layer near the cathode
 Three junctions - four layers
 Connect power such that the anode is
positive with respect to the cathode -
no current will flow
 NOTE: Blocked by the reverse bias of
junction 2
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9. Positive potential applied to the gate
Current will flow - TURNED-ON
Once turned on, gate potential can be
removed and the SCR still conducts
CALLED LATCHING
 Holding current maintains latch
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10. Silicon Controlled Rectifier
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11.  Phase controller produces a trigger
pulse to permit on a portion of the
positive half cycle to get through
 Average power delivered is thereby
controlled
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12. Silicon Controlled Rectifier
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13. Silicon Controlled Rectifier
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14. SCR Application
• Gate Turn Off switch (GTO)
• Electronic (pointless) automotive ignition
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