PURPOSE: To control the current in charging operation and to efficiently utilize discharge energy by winding around one core a first winding having the discharge current passing therethrough, a second winding wherein welding current passes through and a third winding having charging current passing therethrough to be insulated from each another.

CONSTITUTION: In a charging period, the current IC is applied to charge a capacitor 3 from a charge control circuit 2 through a main capacitor 3 and coil 10. The current I'C is not applied through a SCR7 and coil 8 because the voltage induced in the primary coil at the beginning of the charging operation is blocked by a diode 11 and the SCR is turned off. In the discharging operation, the current ID is applied to the coil 8 from the capacitor 3 through the SCR7. At this time, the direction of the magnetic flux generated by the current ID may be opposite to that of the current IC generated, therefore a core is magnetically neutralized. The diameter of the primary coil 3 is increased to lower impedance and the diameter of the charging coil is reduced to obtain a high impedance. According to such a construction, a size of the equipment can be reduced to permit a control with a high reliability by utilizing a simplified circuit.