How percent heat works. A single-phase resistance welding control uses a pair of SCRs as a fast electrical switch. Every half-cycle of the AC line, the SCRs wait, then fire. Once fired, they conduct until the current returns to zero at the end of that half-cycle.
Lower heat = later firing. At 100% heat the SCRs fire right at the start of each half-cycle and pass the full sine wave. At 50% they wait a quarter cycle (90°) and pass only the back half. At low heat they fire late and pass only a thin sliver.
Heat vs. weld time. Heat controls how much of each cycle gets through. Weld time (in cycles, 60 per second) controls how many cycles you get. Both together set the energy delivered to the weld.
Simplified drawing for illustration. On a real welder the transformer’s inductance shifts the current waveform, and the current does not scale one-for-one with percent heat. Questions? Call Spot Weld at (800) 582-5229.