PURPOSE: To enable formation of uniform beads continuously in a wide range of current by connecting a resistance in parallel to one of a pair of control rectifier cells for double wave rectification of transformer output having an intermediate tap.

CONSTITUTION: Both ends of the secondary winding of a transformer 1 provided with an intermediate tap 0 are connected to control rectifier cells (SCR) 2, 3 and double wave rectifies the output of the transformer 1 and supplies a reverse polarity arc source to a wire 4 and a base metal 5. Here, a resistance 6 is connected to one of SCR2. In the output waveform, a period (c) is supplied to the load through the resistance 6. Then, SCR2 is ignited and gives a period (a). When the polarity is further reversed, the period (d) is reversed by the resistance 6 and the polarity is reversed by ignition of SCR3 and becomes a period (b). In the case where there is no ignition of SCR3, the period (d) increases to fit to small current arc which requires much positive polarity. Accordingly, waveform to suit to each welding can be obtained by conducting angle control of SCR3.