A standard AC weld control is made up of a modest number of components each with a specific purpose. The major components are:
Disconnect
Welding Contactor
Weld Control Logic
Valve Power Supply
The Disconnect is a safety in case of an electrical failure.
The contactor is an SCR control device which controls the amount of power and time of power conduction.
The Weld control logic is a microprocessor unit which receives all external inputs and initiation signals and activates all sequences including the force and power.
The Valve Power Supply provides the low voltage and signal to the cylinder or servo unit for the force component of the weld system.
Reference: RWMA - Resistance Welding Manual 4th Edition