Control unit for an a-c control element with full-wave control

Claim 1

A control unit for use with an a-c supply voltage and for use with an a-c control element having full-wave control and including thyristors having firing electrodes, said control unit comprising: a push-pull oscillator for generating firing pulses for said thyristors; and control means for switching said oscillator on at the beginning of a first half-wave of said a-c supply voltage and for switching said oscillator off at the end of a second half-wave of said supply voltage, said second half-wave being opposite in sign to said first half-wave, said control means including: a bistable multivibrator; a delay line for delaying the signal generated by said multivibrator; a sawtooth generator operating at twice the frequency of said a-c supply voltage; an R-C circuit responsive to the output signal of said multivibrator; a comparator circuit responsive to the output signals of said sawtooth generator and said R-C circuit; and a digital logic means including an AND logic circuit responsive to the output signals of said comparator and said delay line for generating an output signal for switching said oscillator on and off.