Abstract: A giant transistor-inverter power source controlled by a micro-computer TP801A which can be used as the power source for resistance seam welding of CAN, is described in this paper. A bridge type inverter circuit has been applied as the main circuit of the power source. The welding current of medium frequency (120~260Hz) is output from the secondary circuits of the power source to offer the high speed seam welding of CAN.