For some weldments, a resistance welding transformer with the same capacity but with long arms connected to length or with large openings between arms is required. The loop through which the welding current of the spot welder flows is called the welding loop, because this loop is directly connected to the outlet end of the secondary winding of the resistance welding transformer, so it can also be called the secondary loop.

　　When welding the (secondary) circuit, the welding current is also reduced, so when planning and selecting a resistance welding machine, pay attention to the welding circuit. Arrangement of graded joints, mobile fixtures and weldments between fixtures. Density and other requirements are reviewed. These parts can generally be made of copper alloys with poor conductivity but higher strength. In the case of a constant secondary no-load voltage, when the arms extend or the opening between the arms increases, the loop impedance increases and the welding current decreases.

　　In addition, for welding magnetic materials (such as low carbon steel), the welding circuit of spot welders and projection welders is mainly composed of the secondary winding of the resistance welding transformer, the secondary soft joint, the electric arm, the electric grip (spot welding machine) or the electric The weldment (projection welder) of the board is composed, and there are movable conductive connection points. The welding circuit of the butt welding machine is mainly composed of the secondary winding of the resistance welding transformer and the components in the secondary welding circuit. It not only transmits 1,000 amperes or more and 1,000 amperes of welding current, but also transmits appropriate mechanical force.

　　Therefore, the mechanical strength or stiffness of these components and their connection points (contact points) should be calculated. In general, the cross-section is usually larger than the cross-section required for the current density. Therefore, the planning of the welding circuit is generally based on the requirements of mechanical strength or rigidity.