Phase controlled thyristor triggering circuit - reduces bulk by eliminating isolating transformer whilst keeping galvanic isolation between control element and thyristor power circuit

摘要

The circuit is for the phase control of a thyristor (2) or triac in series with a load (1) connected across an a.c. single phase power supply. The circuit reduces the bulk of the triggering circuit by eliminating an isolating transformer whilst maintaining galvanic isolation between the control element and the thyristor power circuit. A d.c. supply is derived from the single phase mains by means of a rectifier (7). This supply is connected across a switching transistor (9) and an auxiliary thyristor (8). An optically controlled transistor (10) provides the link with the control element setting the required trigger delay for the main thyristor (2). With no light signal, the switching transistor (9) conducts and there is no signal to the auxiliary thyristor (8) gate. A light signal brings the opto-transistor (10) into conduction turning off the switching transistor (9). A trigger voltage is then available to the auxiliary thyristor (8) and is turned on, triggering the main thyristor (2).