How it works
Resistive (R) triggering: gate current Ig = (VAK − VD)/Rg flows only in the positive half-cycle; firing angle limited to 0°–90°. RC triggering extends the range to 0°–180° because the capacitor phase-shifts the gate voltage, allowing current to peak later in the cycle. The UJT (2N2646) relaxation oscillator is the most reliable triggering circuit: capacitor C charges through R until Vp = ηVBB (η ≈ 0.63 for 2N2646), then UJT fires, discharging C through R1 and producing a sharp pulse at B1. Firing frequency f = 1/(RC ln(1/(1−η))). Pulse transformers provide galvanic isolation between the power circuit (say 400 V) and the control circuit (5 V logic).
Key points to remember
Gate characteristics: the gate is a PN junction — VG typically 1–2 V, IG 10–200 mA. Gate sensitivity varies with temperature: higher temperature means device triggers at lower gate power. Minimum and maximum gate power limits define a safe triggering region on the VG–IG curve. Cosine firing control produces α proportional to a control voltage Vc: α = cos⁻¹(Vc/Vm), giving a linear relationship between Vc and Vdc. Synchronized triggering using a zero-crossing detector ensures the firing angle reference resets correctly at each supply cycle.
Exam tip
Every Anna University power electronics paper asks you to draw the UJT relaxation oscillator triggering circuit and derive the expression for firing frequency — use f = 1/(RC ln(1/(1−η))) and substitute η = 0.63 for the 2N2646.