A power supply unit for a resistance welding apparatus, said power supply unit comprising a converter circuit adapted to convert an alternating power supply current, AC, applied via a residual current protection circuit to an input of said converter circuit into a pulsed electrical current supplied from an output of said converter circuit to a primary coil of a transformer of said resistance welding apparatus, and at least one switching element provided between the output of said converter circuit and protective earth.

A resistance spot welding method including: a pulsation process of clamping a sheet assembly of two or more overlapped steel sheets including at least one high tensile steel sheet using a pair of welding electrodes that are connected to a spot welding power source employing an inverter direct current method, and performing plural repetitions of current passing and current pass stopping, while pressing the steel sheets with the welding electrodes; and a continuous current pass process in which, after the pulsation process, current is passed continuously for a longer period of time than a maximum current pass time of the pulsation process, while pressing the steel sheets with the welding electrodes.

An economical capacitor-type welding device and capacitor-type welding method that have a small power loss, that can be made compact, and that reliably control a charging circuit without being affected by the inductance of a charging path. In an exemplary capacitor-type welding device and an exemplary capacitor-type welding method of the invention, a bypass switching element having a forward-blocking function is connected in parallel to output terminals of a charging circuit, and the bypass switching element is brought into a conduction state to allow a backflow current, which is caused to flow by a magnetic energy stored in the inductance of a primary winding or an inductor included in the charging path, to bypass the charging circuit. A discharge switching element is brought into a conduction state after the bypass switching element enters the non-conduction state and recovers the forward-blocking function.

A power supply unit for a resistance welding apparatus, said power supply unit comprising a converter circuit adapted to convert an alternating power supply current, AC, applied via a residual current protection circuit to an input of said converter circuit into a pulsed electrical current supplied from an output of said converter circuit to a primary coil of a transformer of said resistance welding apparatus, and at least one switching element provided between the output of said converter circuit and protective earth.