Power supply device for electric discharge machining apparatus
A power supply device for electric discharge machining apparatus, for supplying power to a gap formed between a workpiece and a tool electrode comprises a d.c. power source, a bridge circuit supplied with d.c. voltage from the d.c. power source and having switching transistors provided on each side, and a control circuit for controlling the on/off switching operation of switching transistors of the bridge circuit so that a.c. voltage pulses are applied from the bridge circuit to the gap. A first resistor limits current flowing when the workpiece is negatively poled and the tool electrode is positively poled. A second resistor having a larger value than the first resistor limits current flowing when the workpiece is positively poled and the tool electrode is negatively poled.