PURPOSE: To obtain a specified output without being affected by the fluctuation in an AC power source voltage and to stabilize welding by detecting the fluctuation in said source voltage and enabling the control of the firing phase of a phase control rectifier element.

CONSTITUTION: A power source for a welding machine consists of a detector which detects an AC power source voltage, a comparator which compares the output from said device with a reference voltage, a function generator and a control circuit 11a. The above-mentioned function generator consists of a variable resistor VRI, resistors R6, R5, capacitors 25, 26 and an operational amplifier 27 and generates a prescribed function in accordance with the output change of the comparator. The circuit 11a controls the firing phase of a phase control rectifier circuit corresponding to the output of said function.