SCR Firing and Triggering Explained

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

Figure 1: SCR device symbol and construction

As shown in Figure 1, the SCR is a 3-terminal device viz. gate, anode, and cathode. It has four semiconductor layers.

Let’s understand SCR operation with its characteristics as shown in Figure 2.

Figure 2: SCR characteristics

About SCR Firing or SCR Triggering

The voltage vs. ampere characteristic of SCR for I_G = 0 is shown in Figure 2.

• At low currents in region OA, junction J2 acts as a reverse-biased PN junction.
• As the applied voltage is increased, the current slowly increases until V_BO (breakover voltage) is reached.

The current then increases enormously and can only be limited by resistor R. Junction J2 becomes forward biased, and hence the SCR junction J2 acts as a forward-biased PN junction. This is indicated in Figure 2 by CD. In this CD region, the current is high and the voltage drop is low.

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

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

This four layer device is also known as a 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.