AC controls all have microprocessors that activate contactors to control the time and amount of power that is delivered to the part to be welded. These contactors could be considered electronic switches. They are made up of two SCR’s. There is one SCR for the positive and a second for the negative half of the AC sine wave.
These SCR’s not only turn the power on and off but they also control the magnitude by controlling the amount of the sine wave that is allowed to be on during each cycle. This phase shift changes the power or amperage that is transmitted.
Reference: RWMA - Resistance Welding Manual 4th Edition