Induction heating apparatus with protective circuit arrangement

Claim 1

An induction heating apparatus comprising, in combination: a power supply circuit including a rectifier coupled to an alternating current power source for generating a full-wave rectified unfiltered unidirectional excitation potential; an inverter circuit coupled to said rectifier and including an induction heating coil, a commutating capacitor receptive of the excitation potential and connected to said induction heating coil, a silicon controlled rectifier having a control electrode, and a diode connected in inverse parallel relationship therewith to drive said induction heating coil with periodic energization current; a charge absorbing circuit including a charge absorbing capacitor and a semiconductive switching device for providing a conductive path, when energized, between said commutating capacitor and said charge absorbing capacitor so that a charge stored in said commutating capacitor can be discharged through said conductive path to said charge absorbing capacitor; a circuit for timing energization of said charge absorbing circuit at a zero voltage point of the alternating current voltage of said power source by counting number of zero voltage points from that instant said power circuit is energized; delay means for providing a delay interval from an instant a predetermined number of zero voltage points has been counted to allow said commutating capacitor to recharge to a predetermined level; and a gating circuit means energized upon expiration of the delay interval, said gating circuit means being coupled to said control gate of said silicon controlled rectifier for rendering the same repetitiously conductive at a predetermined frequency.