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THEORY OF OPERATION
E-6 E-6
COMPACT WIRE WELDERS
SCR OPERATION
A silicon controlled rectifier (SCR) is a three terminal
device used to control large currents to a load. An
SCR acts very much like a switch. When it is turned
on, 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 a remainder of the time in the OFF
state. The gate controls the amount of time spent in
each state.
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.
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 the 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 min-
imum value, called holding current, the SCR will shut
off. This normally occurs as the AC 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.
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