A commercially available power source is rectified by a diode bridge and a capacitor and supplied to a transformer via a terminal. One end of a primary coil is connected to the terminal and the other end is connected to a switching device. One end of a feedback coil is connected to the switching device and the other end is connected to the terminal. A negative feedback circuit is provided between the switching device and the terminal. A rectifying circuit comprising a diode and a capacitor is provided on a secondary coil. A node of the coil and the capacitor is connected to the terminal via a resistor. In detecting circuits, a voltage across the terminal is detected and a current across the resistor is detected. When the detected voltage and current are equal to predetermined values, a control signal is supplied to a feedback circuit.