See AUTOMATIC ARC WELDING CURRENT and RESISTANCE WELDING CURRENT.
Current for welding is controlled by a welding power source. The typical output of a welding power source may be alternating current, direct current, or both. It may be either constant current, constant voltage or both. It may also provide a pulsing output mode. Selecting the correct power source depends on the current output required for each of the arc welding processes. See VOLT-AMPERE CURVE.