PURPOSE: To prevent an erroneous ignition by firing a photothyristor, which is connected to an ignition phase control capacitor in parallel, when the speed control signal disappears and by shorting the capacitor compulsorily.
CONSTITUTION: When output of the amplifier Amp is zero, the transistor Tr2 is turned off and the one Tr3 becomes "on", current flows in the LEDs D 7 ∼D 12 , the photothyristors TH1∼TH6 are fired to short the capacitors C 1 ∼C 3 . In this state, the trigger diodes TD1∼TD3 are not activated, the main triacs TRI 1 ∼TRI 3 are not fired. When output voltage is developed in the amplifier Amp, the Tr2 is turned on and Tr3 becomes OFF, no current flows in the LEDs D 7 ∼D 12 , and the thyristors TH1∼TH6 are not fired. Accordingly a reliable ignition circuit, wherein the main triac is not fired by the reverse voltage, can be obtained.