Scr Operation - Lincoln Electric IDEALARC SP-255 Service Manual

E-6
INPUT
CATHODE
ANODE
A silicon controlled rectifier (SCR) is a three terminal de-
vice 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 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.
THEORY OF OPERATION

SCR OPERATION

OUTPUT
GATE
Figure E.6 - SCR Operation
IDEALARC
This gate pulse must be more positive than the cathode
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 cur-
rent. 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 oc-
curs 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.
® SP-255
E-6
NOTE: AS THE GATE
PULSE IS APPLIED
LATER IN THE CYCLE
THE SCR OUTPUT
IS DECREASED.