SCR OPERATION
A silicon controlled rectifier (SCR) is a three terminal
device used to control rather large currents to a load.
An SCR acts very much like a switch. When a gate
signal is applied to the SCR, it is turned ON and there
is current flow from anode to cathode. In the ON
state, the SCR acts like a closed switch. When the
SCR is turned OFF, there is no current flow from anode
to cathode; thus the device acts like an open switch.
As the name suggests, the SCR is a rectifier; so it
passes current only during positive half cycles of the
AC supply. The positive half cycle is the portion of the
sine wave in which the anode of the SCR is more pos-
itive than the cathode.
When an AC supply voltage is applied to the SCR, the
device spends a certain portion of the AC cycle time in
the ON state and the remainder of the time in the OFF
state. The amount of time spent in the ON state is
controlled by the gate.
An SCR is fired by a short burst of current into the
gate. This gate pulse must be more positive than the
cathode voltage. Since there is a standard PN junc-
tion between gate and cathode, the voltage between
these terminals must be slightly greater than 0.6V.
Once the SCR has fired, it is not necessary to contin-
ue the flow of gate current. As long as current contin-
ues to flow from anode to cathode, the SCR will
remain on. When the anode to cathode current drops
below a minimum value, called holding current, the
SCR will shut off. This normally occurs as the AC sup-
ply voltage passes through zero into the negative por-
tion of the sine wave. If the SCR is turned on early in
the positive half cycle, the conduction time is longer,
resulting in greater SCR output. If the gate firing
occurs later in the cycle, the conduction time is less,
resulting in lower SCR output.
THEORY OF OPERATION
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E-6
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FIGURE E.5 – SCR Operation
GATE
