Electric heater safety circuit - has diode in parallel to triac in series with heating conductor

摘要

The safety circuit for an electric heater employs a triac (Tr) in series with the heating conductor (RH) across the AC supply. A resistor (RN) of low ohmic value in comparison to the heating conductor (RH) is connected in series with a diode (D2) across the latter. A control oscillator (SG) synchronised with the AC supply is coupled to an ignition signal generator (ZG) for the triac, monitoring the half waves of the AC supply corresponding to the direction of conduction of the diode (D2). An ignition signal for the triac is blocked during these half waves. A controlled switching unit (TG) receives the output of several monitoring devices (U1 .. U4) and is coupled between the control oscillator (SG) and the ignition signal generator (ZG).