Abstract: An arc welder has a transformer comprising a pair of flat aligned primary coils of aluminum strip wound on opposite parallel legs of a rectangular open frame core and a pair of aligned secondary coils of the same strip material wound on the same opposite parallel core legs in lateral spaced relationship with the primary coils, a flexible strip encircles each pair of aligned coils tightly banding them against lateral distortion, a flux shunting core encapsulated in molded plastic material is slidably movable through the core between the pairs of aligned coils, and mechanism for slidably moving and locking the shunting core in adjusted positions is operated by a single handle. One secondary coil of the pair comprises two windings, one wound over the other to provide two output ranges. In a modification, the addition of an outboard choke coil of round wire provides a third output range.