The firing circuit for a triac connects the turn-on capacitor to the triac gate and consists of a four-diode bridge with its second diagonal between the capacitor and the triac gate. The bridge first diagonal contains two transistors, acting as a thyristor, each with its collector coupled to the other one base, and its emitter coupled to one end of the diagonal. The turn-on voltage for the transistors is determined by a zener diode in series with a resistor in the first diagonal. The second transistor base is coupled to the zener-diode and resistor junction. The firing circuit can be easily integrated and has a low firing voltage.
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