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POWER MIG 255
THEORY OF OPERATION
E-7 E-7
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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. See Figure E.6
for a graphical representation of SCR operation. 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 off state and the remainder of the time in the on
state. The amount of time spent in the ON state is con-
trolled 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 cath-
ode voltage. Since there is a standard PN junction
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 voltage 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 greater SCR
output. If the gate firing occurs later in the cycle the
conduction time is less resulting in lower SCR output.
FIGURE E.6 — SCR OPERATION.
INPUT
OUTPUT
CATHODE
ANODE
GATE
NOTE: AS THE GATE
PULSE IS APPLIED
LATER IN THE CYCLE
THE SCR OUTPUT
IS DECREASED.