After the first cycle. the relay will normally be energized. When normally closed pushbutton 51 is activated, the SCR turns off, the relay is de-energized, and power is applied to the relaxation oscillator and the load. Mter a time delay varying from less than a second to approximately 2.5 minutes, as determined by the setting of the 10-M!l potentiometer, the unijuction will fire and tum on the SCR.
The relay will energize until power is removed from the oscillator and the load, and will stay energized until button 51 is pushed again. The UJT trigger output from base 1 directly drives the gate of the SCR. However, where isolation between the UJT trigger or any other type of trigger and the thyristor power circuit is required, then a simple pulse transformer, interfacing the two elements, will suffice.