Hi chinnaphat,
As H. Chang mentioned, there are three types of capacitator discharge stud welding. The initial contact method, the initial gap method, and the drawn arc method. The three differ primarily in the manner of arc initiation. Initial contact and initial gap capacitator discharge studs have a small, specially designed projection tip on the weld end of the stud. Drawn arc stud welding creates a pilot arc as the stud is lifted off the work piece by a gun, which is similar to arc stud welding. In the drawn arc method, arc initiation is accomplished in a manner similar to that of arc stud welding. The stud does not require a tip on the weld face. An electronic control is used to sequence the operation. Weld time is controlled by an electrical circuit in the unit. The welding gun is similar to that used for arc stud welding. The stud is positioned against the work;the trigger switch on the stud welding gun is actuated, energizing the welding circuit and a solenoid coil in the gun body. The coil motion lifts the stud from the work, drawing a low amperage pilot between them. When the lifted coil is de-energized, the stud starts to return to the work. The welding capacitators are then discharged across the arc. The high amperage from the capacitators melts the end of the stud and adjacent work surface. The spring action of the welding gun plunges the stud into the molten metal to complete the weld.