The supply voltage of the secondary winding of transformer is rectified by the half wave rectifier diodes and given to the zener diode. The zener diode is used to clip the rectified to a fixed voltage This voltage is applied to the charging circuit R. The capacitor charges through the resistor R.
When the voltage across the capacitor reaches the UJT triggering voltage, the UJT turns ON. At this instant, the capacitor discharges through emitter (E), base (B1) and primary of pulse transformer. A pulse is produced at the primary, as well as at the secondaries of the transformer.
When the capacitor discharges to a voltage called vally voltage the UJT turns-off and capacitor charges again and the whole process repeats itself. The charging of the capacitor can be varied by varying the value of resistance R and therefore the firing angle can be controlled form 0 to 180 Degrees.