Synchronous switching circuit

Claim 1

In a circuit for selectively triggering a control thyristor having anode, cathode and gate terminals, at the zero crossing of the voltage of an a.c. source, comprising a full-wave rectifier circuit having input terminals for operable connection thereof to the a.c. source, a zero-voltage crossing detector circuit having load terminals for connection across the anode and cathode of the control thyristor and across the output of said rectifier circuit and including a transistor having a control terminal and a pair of load terminals for controlling the application of current from one of said load terminals to the gate terminal of the control thyristor, said transistor conducting current through said load terminals to thereby shunt current away from said gate terminal when the transistor is in its conductive state, and permitting the flow of current at said gate terminal of the control thyristor for triggering the same when said transistor is in its non-conductive state, and switching means connected between the control terminal and the other of said load terminals of said transistor for continuously maintaining said transistor in conduction when said switching means is non-conductive, the improvement wherein said zero-voltage crossing detector circuit includes a first resistor connected between the anode of the control thyristor and said one load terminal of said transistor, a second resistor connected between the anode of the control thyristor and the control terminal of said transistor, a third resistor series connected with said switching means between said control terminal and said other load terminal of said transistor, said second and third resistors defining a voltage divider for applying a voltage to the control terminal of said transistor such that it is biased to its non-conductive state when said a.c. source voltage passes through zero and when said switching means is conductive, said transistor thereby providing an output pulse to the gate terminal of the control thyristor corresponding to the zero-voltage crossing of the a.c. source and the control thyristor being latched in a conductive state by current flow through said first resistor coupled between the gate and anode of the thyristor so long as said transistor remains nonconductive, said transistor being the sole three terminal transistor device in said zero-voltage crossing detector circuit.