DC Welding
DC welding was first used in an attempt to avoid Johnston's patent; however, it is obvious that a DC power supply with a constant power output would create a superior weld seam when compared to AC. Unfortunately, this increased weld quality came at a price.
A simple DC welder schematic is shown in Figure 3. The main characteristic of a DC welder is that AC line current is immediately transformed into low voltage/high current. This low voltage/high current then goes through a full-wave rectifier which converts the AC to DC. The DC power unit must be located as close as possible to the weld point to minimize power loss.
Figure 3:DC Welder Schematic
In addition, the low voltage/high current DC is then transmitted through large slip rings to the rotating unit on the tube mill and the welding current is transmitted to the tube through rotating copper electrodes.