Scr operation, Scrs (see, Theory of operation – Lincoln Electric LN-8 SVM132-A User Manual
Page 49
E-5
THEORY OF OPERATION
E-5
SCR OPERATION
A silicon controlled rectifier (SCR) is a
three terminal semi-conductor device
used to control currents to a load. Refer
to Figure E.4. 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 cath-
ode. In the ON state the SCR acts like a
closed switch. When the SCR is turned
OFF there is no current flow, 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 posi-
tive 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 cur-
rent into the gate. This gate pulse must
be more positive than the cathode volt-
age. Since there is a standard PN junc-
tion between the gate and cathode, the
voltage between these terminals must
be slightly greater than 0.6 VDC. 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 time is later in the cycle the
conduction time is less, resulting in
lower SCR output.
FIGURE E.4 – SILICON-CONTROLLED RECTIFIER OPERATION
LN-8
INPUT
CATHODE
GATE
GATE
OUTPUT
ANODE
NOTE:
AS THE GATE PULSE IS
APPLIED LATER IN THE
CYCLE, THE SCR OUTPUT
IS DECREASED.