An SCR can be triggered or switched from a non-conducting state to a conducting state in several ways as follows,

• If the voltage across the SCR exceeds the rated forward break-over voltage, the SCR will start conducting due to avalanche breakdown.
• If the dv/dt across the device is large, the device may turn on even though the voltage appearing across the device is small.
• With an increase in temperature, the leakage current through the reversed-biased p-n junction increases. At high temperatures, the SCR may turn ON.
• The SCR can also be turned ON by light which is achieved by irradiating the device. This may be used in conjunction with the gate triggering. Such triggering may be used in applications that require a response to light or electrical isolation between the trigger signal and the load.
• The gate triggering is the most commonly used method to turn ON an SCR. The gate triggering provides a large turn ON gain which is the ratio of the anode current to the gate current. The control so obtained consumes low power.

Unijunction Transistor (UJT) :

A unijunction transistor (UJT) does not belong to the thyristor’s family. It has a fast switching action. The conventional UJT is made up of an N-type silicon base with terminals B₁ and B₂, to which is alloyed a P-type emitter with terminal E, as shown in the below figure.

With positive voltage, V_b applied between B₂ and B₁, the potential of point C will be ηV_b. Where η is referred to as the intrinsic stand-off ratio which is determined by the relative magnitudes of the internal resistances R_B1 and R_B2 of the top (base 2) and bottom (base 1) regions of the device respectively.

The UJT will be in reversed biased condition until the emitter voltage V_E will become equal to the emitter peak point voltage V_P. Thus there will be no conduction except a small leakage current. Once the emitter voltage V_E reaches V_P, the emitter current, I_E will be greater than the peak point current, I_p thus turning on the UJT.

When UJT is turned ON, the resistance between the emitter and base-1 becomes low and the emitter current will be limited primarily by the series resistance of the emitter to base-1 external circuit.

SCR Triggering Using Unijunction Transistor (UJT) :

To fire the SCR, a short-time current pulse at its gate is to be applied. The delay of firing and controlling the instant at which the pulse occurs within each half cycle is to be provided by the firing circuit. A UJT can be used to perform this function as shown in the below figure.

The input voltage to UJT is applied through a capacitor C whose charging rate can be adjusted through resistance R₃. As the capacitor C charges, voltage V_E increases until its value reaches the firing potential, forcing the UJT to conduct discharging the capacitor through base resistance R₅.

The spike of current occurring with the capacitor discharge is observed as a voltage spike developed across base resistance R₅, part of this current is applied to the gate of the SCR. The duration of the firing pulse can be varied by adjusting the discharge rate of the capacitor through R₅.

The switching speed of UJT determines the rise time of the triggering pulse. Discharge of capacitor C is accompanied by a decrease in voltage V_E till UJT stops conduction for values of V_E below V_min. The capacitor will again start charging and the circuit is ready for the next operation.

The capacitor can be made to discharge through a pulse transformer which isolates the gate circuit of SCR from the power circuit. The secondary of the pulse transformer will be given to SCR, which supplies the firing pulse to the SCR.