Abstract: A constant power supply unit of an electric heating apparatus is disclosed. A voltage sensing circuit generates a DC reference signal corresponding to the nominal value of an AC supply voltage. A timing pulse generator generates a rectangular waveform signal during one of every four cycles. A trigger generator generates a trigger signal at zero-cross points determined by the combination of outputs of a voltage sensing circuit and the timing pulse generator. A power control circuit controls the input power to the electric heating apparatus by the trigger signal, thereby allowing a constant power supply to the electric heating apparatus regardless of the nominal value of the AC supply voltage which may be either 220 volts or 100 volts.