Page 1000 - Basic Electrical Engineering

The SCR can be turned off if the current flowing through it, i.e., I AK is
reduced to a value lower than the holding current, I . The SCR can also be
H
commutated (switched off) by applying a reverse voltage across the SCR by

means of a commutating circuit. Inductors and capacitors are used in the
commutating circuits which facilitate the development of a reverse voltage
across the SCR for its commutation. The method of turning on an SCR is

called triggering and the method of switching off or turning off an SCR is
called its commutation.

14.10.2 Two-transistor Analogy of an SCR

An SCR can be considered made up of one p–n–p and one n–p–n transistor
sandwiched together as shown in Fig. 14.38 (a). Two-transistor-equivalent

circuit has been shown in Fig. 14.38 (b). T and T are the two equivalent
1
2
transistors of the SCR. The gate current, I acts as the base current for
G
transistor, T .
2
As shown in Fig. 14.38 (b) the anode is made positive with respect to the
cathode and a gate current, I is supplied. This gate current acts as the base
G
current of transistor, T . This transistor gets turned on. The emitter current of
2
T now becomes the base current of transistor, T . Transistor T now gets
2
1
2
turned on. The collector current of T adds to the base current of transistor
1
T . This way current multiplication takes place. Even if the gate current is
2
removed, the device will not get turned off as long as the current flow does
not come down to the level below the holding current.