High-frequency heating device for variably controlling a high frequency output in a continuous or stepwise manner

A high-frequency microwave type heating device comprising a semiconductor controlled rectifying element, a transformer including a primary winding coupled through the semiconductor controlled rectifying element to an AC power source, a magnetron high-frequency oscillator coupled to a secondary winding of the transformer, a signal generating circuit operated in accordance with the cycle of a voltage of the AC power source to generate a signal at a predetermined phase angle of each cycle of the power source voltage, a pulse oscillator whose output pulse width is variable, and a control circuit supplied at least with an output of the pulse oscillator and an output of the signal generating circuit to generate a control signal, said control signal of the control circuit being used to control the semiconductor controlled rectifying element;

• and in which said semiconductor controlled rectifying element is a triac thyristor and said high-frequency heating device further includes an amplifying circuit for amplifying an output signal of the pulse oscillator for application to the control circuit, a delay circuit for supplying upon energization a signal to said amplifying circuit for a predetermined time period to cause the operation of the amplifying circuit to be stopped, thus preventing a pulse from the pulse oscillator from being applied to the control circuit, and a voltage detection circuit for detecting a voltage across the triac thyristor to prevent an output signal from the signal generating circuit from being applied to the control circuit when no voltage is applied across the triac thyristor.