# Understanding UJT relaxation oscillator

I can't wrap my head around how the capacitor gets discharged in UJT relaxation oscillator. I understand when $$\ V_E \$$ reaches or exceeds a threshold (say $$\ V_{ON} \$$), current flows from emitter to $$\ B_1 \$$. But $$\ V_S \$$ supply the circuit with a constant DC voltage and eventually after an $$\ \tau = RC \$$, the capacitor locks in a charged up state with voltage of $$\ V_{ON} \$$, as $$\ V_E \$$ also holds onto a constant $$\ V_{ON} \$$ passing current from emitter to base.

The capacitor has no where to discharge the current unless $$\ V_{S} \$$ is removed and only then can the capacitor supply voltage to the UJT instead of $$\ V_{S} \$$.

I've read a few tutorials which all sum up, without much explanation, that once the capacitor charges up to a threshold voltage, it discharges through the UJT. And once the capacitor gets fully discharged it gets charged up again and the cycle goes on producing a pulsed waveform out of the base. I do not see how the capacitor can discharge unless $$\ V_{S} \$$ is taken out.

• Try reading these: UJT and PUJT. Then tell me what else is left over that wasn't covered and you still don't follow.
– jonk
Commented Jun 24, 2020 at 4:38