Firing and triggering of SCR | SCR Firing or SCR triggering

This page covers SCR firing or SCR triggering. It describes firing and triggering of SCR with its VI characteristics.

SCR device symbol and construction

As shown in the figure-1, SCR is a 3-terminal device viz. gate, anode and cathode. It has four semiconductor layers. Let us understand SCR operation with its characteristics as shown in the figure-2.

SCR characteristics

➨About SCR firing or SCR triggering: The voltage vs ampere characteristic of SCR for IG = 0 is shown in the figure-2.
➨At low currents in region OA, junction J2 acts as reverse biased PN junction.
➨As applied voltage is increased, current slowly increases till VBO. Here VBO is breakover voltage. The current is increasing enormously and it can be limited only by resistor R, the junction J2 becomes forward bias and hence SCR junction J2 acts as forward biased PN junction. This is indicated in the figure-2 by CD. In this CD region current is high and voltage drop is low.

The SCR firing or SCR triggering is done to bring SCR in ON state. This state can be stopped only by reducing current to below IH (i.e. holding current). As soon as current falls below IH value, conduction gets ceased and SCR operating point goes from C to O point as indicated in the SCR characteristics.

• Once SCR has been fired, gate terminal losses its controlling action.
• In this state, SCR can not be turned OFF by its gate signal anymore.
• The only way to bring SCR back to its normal OFF state is to reduce anode voltage sufficiently enough such that anode current drops below the holding current (IH).

This four layer device is also known as thyristor. SCR has many applications which include regulated power supplies, motor controls, time delay circuits, battery chargers, inverters, heater controls, phase controls, overvoltage protection etc. Refer Difference between SCR, DIAC, TRIAC, UJT>>.

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