Abstract: A safety circuit for a silicon controlled rectifier controller of the type wherein the current delivered to a load is determined by controlling the firing angle of the silicon controlled rectifiers. The safety circuit is capable of sensing a shorted condition in the silicon controlled rectifiers and automatically disconnecting the load from the AC mains in the event of such a short circuit condition. Load current is sampled at a time in the cycle where it is expected to be zero. If at the sampling time a non-zero condition is detected, a relay is made to operate to actuate a circuit breaker or to perform other desired functions.

Abstract: A controller for controlling SCR's used for supplying power to loads, which provides digital control of the phase angle of the firing signal for SCR's during rapidly changing frequency of applied power. The digital control is based upon clocking signals which are derived directly from the rotation of a generator to always provide a set number of pulses for each electrical cycle independent of time (frequency).