(EN) An induction-hardening machine for the contour hardening of machine components such as gears includes a system processor which controls thyristor power switching circuits which supply high-power signals to an RF generator. Power switching circuits include silicon controlled rectifiers or SCR's. In order to overcome the variable "on time" characteristics of SCR devices, a zero crossing detector is implemented and time periods are calculated so that the system processor activates the SCR circuits to supply power to the RF generator at predetermined times. The system processor 12 will deactivate the SCR circuits at or just prior to a zero crossing referenced from the predetermined activation time thereby effectively controlling the on time of the SCR circuits with an accuracy of up to five ten thousandths of a second. The signal produced by the RF generator is supplied to an induction heater coil which is used to case harden the gear teeth of a machine component or gear. In another embodiment, a phase angle detector circuit produces a pulse for each corresponding detection of a predetermined phase angle of an AC signal. A start switch and the pulse produced by the phase detector provide inputs to a circuit which requires concurrence of the pulse and activation of the switch before a predetermined width signal pulse is produced. The predetermined width signal pulse activates power switching devices to supply a predetermined power signal to an RF generator coupled to an induction heating coil. Precise induction heating is accomplished via precise control of power input to the RF generator.
