Scr Operation - Lincoln Electric IDEALARC DC 1000 SVM123-A Service Manual

E-5

SCR OPERATION

A silicon controlled rectifier (SCR) is a three termi-
nal 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 rectifi-
er, 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 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.
THEORY OF OPERATION
FIGURE E.5 - SCR Operation
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
junction between gate and cathode, the voltage
between these terminals must be slightly greater
than 0.6 V. Once the SCR has fired it is not neces-
sary 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 hold-
ing 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.
IDEALARC DC-1000
E-5