A device for resistance welding includes a welding clamp housing, a welding transformer, a welding clamp, a welding clamp drive for operating the welding clamp and a power supply using alternating supply voltage. The device may be built as small as possible and may be simple to expand. The power supply is implemented by a main converter and by drive and welding converters. The output of the main converter is connected to the inputs of the drive and welding converters. The inputs of the drive and welding converters are connected to one another. These connections are implemented by an a.c. voltage intermediate circuit.