PEC MACC, Manual, Page: 12 / 26

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3.1.3 Silicon Controlled Rectifier
The silicon controlled rectifier (SCR) is a
regenerative semiconductor switch. See
Figure 10 . It is a silicon diode with a third
element, a gate, which controls the flow of
current through the SCR. The gate
determines the point in each half cycle
when the SCR will "fire" or start to
conduct.
Without a gate signal, the SCR blocks the
flow of current in both directions. Not until
a signal is applied to the gate does the SCR
behave like a diode.
SCR firing is accomplished by introducing
a DC voltage between the gate and the
cathode. Conduction through the SCR
starts within microseconds after voltage is
applied between gate and cathode. When
sufficient current has started to flow
through the SCR, it "latches" into
conduction until the current falls below the
value of holding current at the end of that
half cycle. Holding current is the current
necessary to keep the SCR latched in
conduction. When the SCR stops
conducting, it returns to the blocking state
and will not turn again until the gate is
"pulsed" and the anode is positive with the
respect to the cathode.
Figure 11 shows two SCR’s connected in
a back-to-back arrangement. SCR1 will
conduct current when T1 is positive with
respect to T2. SCR2 will conduct current in
the opposite direction when T2 is positive
with respect to T1. Assuming that voltage
is applied between gate and cathode at
midpoint in each half cycle, 90 ° of
conduction angle can be obtained for each
SCR resulting in a current flow through the
load as shown in
Figure 12. Therefore, it is
possible by introducing gate-cathode
voltage at other times during the half cycle
to control the time the SCR’s are "on" in
each half cycle and vary the load current.
Figure 10 Silicon Controlled Rectifier (SCR)
Figure 11 Two SCR’s in Back-to-Back
Arrangement
Figure 12 SCR Current Flow (90 ° Phase Angle)