A power supply for arc processing which can be used with a commercial AC power supplying either a high or low voltage, with the low and high voltages having a ratio of 1:2. The power supply includes two or three rectifying circuits connected in parallel, each of which comprises three series diodes, with a first series connection point of each series connected diodes being connected to one of the input terminals (phases) of the commercial power supply. The power supply further includes two or three voltage change over circuits, comprising a capacitor and a voltae change-over switch, closed at a low voltage input, connected between a second connection point of its corresponding rectifying circuit and the first connection point of another rectifying circuit. An inverter circuit for inverse conversion is connected in parallel between the terminals of the three rectifying circuits.