Electrical power converter thyristor firing circuit having noise immunity

Claim 1

In a power control circuit having a pair of buses connectable to an a.c. power source and having at least one controllable current conduction means having a power circuit and a firing signal responsive control circuit including a controlled gatable rectifier having an anode circuit and a cathode circuit for conducting controlled amounts of energy to a load, said power circuit being connected to said a.c. buses for receiving the alternating current, an improved firing circuit comprising: a light activated silicon controlled rectifier having an anode and a cathode coupled to said control circuit for providing firing signals to said control circuit; light means for activating said silicon controlled rectifier; resistive-capacitive means comprising at least one series connected resistor and capacitor pair connected across said a.c. buses and having a tap intermediate said resistor and capacitor connected to said anode of said controlled rectifier for providing a voltage that is reduced with respect to the voltage in said buses to said anode for providing firing signals when said controlled rectifier is activated while rendering said firing circuit immune to transient voltage spikes in said a.c. buses, and diode means electrically connecting said capacitor to said cathode circuit of said control circuit to maintain the charge at said capacitor.