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THEORY OF OPERATION
E-7
E-7
SQUARE WAVE TIG 355
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 sup-
ply. The positive half cycle is the portion of the sine
wave in which the anode of the SCR is more positive
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 continue
the flow of gate current. As long as current continues
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 supply volt-
age passes through zero into the negative portion of
the sine wave. If the SCR is turned ON early in the
positive half cycle, the conduction time is longer,
resulting in greather SCR output. If the gate firing
occurs later in the cycle, the conduction time is less,
resulting in lower SCR output.
FIGURE E.7 – SCR OPERATION
NOTE: Unshaded areas of Block Logic Diagram are the subject of discussion.
