PURPOSE: To keep output voltage constant regardless of the fluctuation of supply voltage by connecting a capacitor in parallel with the DC end of a diode bridge for converting a transistor as a DC element into the AC load of a feedback winding for a saturable transformer.

CONSTITUTION: When a commercial power supply 11 is closed, a DC output is generated at DC terminals (a), (b). FETs 21, 22 are turned ON alternately by relaxation oscillation, and AC voltage is induced in a secondary winding 42 for an inverter transformer 4. A control circuit 6 is mounted between an output winding 43 for the inverter transformer 4 and a feedback winding 34 for a saturable transformer 3. The control circuit 6 contains a diode bridge 69 and a capacitor 82 connected on the DC terminal side of the diode bridge 69, and voltage induced in the feedback winding 34 is rectified and smoothed and applied between a collector and an emitter in a transistor 68 as a variable impedance element. Accordingly, sufficient voltage between the collector and the emitter is applied to the transistor 68 at all times.