The sizes and types of electrodes for shielded metal arc welding define the arc voltage requirements (within the overall range of 16 to 40V) and the current requirements (within the overall range of 20 to 550A). The current may be either alternating or direct, depending upon the electrode being used, but the power source must be able to control the level of current within a reasonable range in order to respond to the complex variables of the welding process itself