Use of a UJT (uni-junction transistor) in SCR control Which of the following is a standard application of the UJT in thyristor circuits?

Introduction / Context:The UJT is widely used as a simple, reliable relaxation oscillator for generating gate pulses. In phase-control circuits, it triggers the SCR at a controlled delay each half-cycle.

Given Data / Assumptions:

• UJT exhibits negative resistance and can form timing networks (RC) to create sawtooth or pulses.
• SCR gate requires a properly timed pulse to initiate conduction.

Concept / Approach:The UJT relaxation oscillator charges a capacitor through a resistor until the UJT fires at its intrinsic standoff ratio, discharging the capacitor and producing a sharp pulse—ideal for SCR gate triggering.

Step-by-Step Solution:Design RC values for desired firing angle.Use UJT pulse to drive an isolating pulse transformer or gate circuit.SCR turns on at each intended point on the AC waveform.

Verification / Alternative check:Classic lamp dimmers and AC controllers employ UJT-based trigger modules feeding SCR/triac gates.

Why Other Options Are Wrong:Turning off an SCR (B/C): Direct turn-off requires current to fall below holding current or forced commutation; UJT alone cannot perform this.

Common Pitfalls:Assuming any transistor can both fire and commutate an SCR; commutation typically needs additional circuitry.

Final Answer:Firing an SCR (gate triggering)