UJT trigger waveform for thyristor firing When a unijunction transistor (UJT) is used as a relaxation oscillator to generate thyristor gate signals, what is the principal waveform produced across the timing capacitor?

Introduction / Context:
A UJT relaxation oscillator is a classic method to produce periodic firing pulses for SCRs and TRIACs. The waveform across the timing capacitor determines pulse timing and linearity for phase control circuits used in AC voltage regulators and controlled rectifiers.

Given Data / Assumptions:

• UJT configured as a relaxation oscillator with an RC timing network.
• Device used to provide periodic gate triggers.

Concept / Approach:

In a UJT relaxation oscillator, the capacitor charges exponentially toward the supply through a resistor until reaching the UJT’s peak point voltage (η·VBB approximately). At this instant, the UJT conducts sharply, discharging the capacitor rapidly toward the valley point, after which the cycle repeats. The capacitor voltage therefore ramps up (approximately exponential but often treated as a ramp) and then falls quickly, producing a sawtooth-like waveform.

Step-by-Step Solution:

Verification / Alternative check:

Oscilloscope traces in standard UJT oscillator circuits show a rising ramp followed by a sharp fall, i.e., a sawtooth across the capacitor, and narrow trigger pulses at the UJT emitter resistor.

Why Other Options Are Wrong:

• Sine/square: not generated by the UJT RC relaxation process.
• “Either”: depends on circuit; the canonical UJT oscillator produces a sawtooth across C.

Common Pitfalls:

Confusing the sawtooth on the capacitor with the narrow gate trigger pulses delivered to the thyristor gate through a coupling network.

Final Answer:

Sawtooth wave