Firing circuitry for semiconductive controlled rectifiers

Claim 1

In combination, a semiconductive controlled rectifier having an anode, a cathode and a gate electrode, a source of alternating current potential and a load impedance connected in series with said anode and cathode, isolation transformer means having primary and secondary winding means, means connecting said secondary winding means between the gate and cathode of said controlled rectifier, a normally-conducting transistor having its emitter and collector connected in series with said primary winding means, a source of potential of one polarity connected to the base of said transistor to render it conducting, means including a Zener diode for applying a logic pulse of the opposite polarity to said transistor, said pulse being of sufficient amplitude to override said source of potential and cut off the transistor, a common mode isolation amplifier having a pair of inputs connected to the anode and cathode of said controlled rectifier and adapted to produce an output of said one polarity when the anode of said controlled rectifier is negative with respect to said cathode, and means including an isolation diode and a Zener diode for applying said output of one polarity to said base of the transistor to maintain it conducting even though a logic pulse is applied to the base.