Circuit reference
AC Transformer Welder Tap, Shunt and Current-Control Checks
A repair reference for traditional AC arc welder current-control paths, separating tap contacts, movable magnetic shunts, reactors, transformer evidence and secondary output losses.
Database summary
In many older AC arc welders, current control is not a PWM function. The machine may change welding current through primary taps, secondary taps, a movable magnetic shunt, a movable core, an adjustable reactor, or a combination of magnetic and contact arrangements.
For repair work, the technician should record whether the current-control mechanism actually changes the magnetic circuit and output evidence. A frozen shunt, burned tap switch or overheated contact can produce the same user complaint as a failed transformer.
Current-control methods
| Control method | What to inspect | Failure clue |
|---|---|---|
| Tap switch | Contact tightness, burned contact surface, loose terminal and correct position. | Output changes in steps, is intermittent, or disappears at one range. |
| Movable core / shunt | Mechanical travel, screw/linkage condition, scale pointer and jammed iron assembly. | Current knob moves but arc strength barely changes. |
| Magnetic reactor | Coil heating, loose core, insulation, terminal condition and vibration. | Output is unstable, overheats quickly, or hums abnormally. |
| Secondary output path | Holder, clamp, cable, secondary terminal and work return. | Machine setting changes, but arc remains weak at the workpiece. |
Field check sequence
- Confirm input voltage and that the primary switch/fuse path is not intermittent.
- Inspect the current-control contact or movement without load.
- Record transformer hum and heat behavior briefly under a controlled test condition.
- Measure or compare secondary output evidence at the machine terminals, then at the holder/work clamp.
- Stop if insulation smell, visible arcing, leakage or severe overheating appears.