Circuit
Control board shutdown and no-PWM path
Protection, VRD, current feedback, optocouplers and transistor stages that block PWM output.
What no-PWM actually means
No-PWM is different from no power. The control IC may be powered and still produce no output because a shutdown, VRD, thermal, overcurrent or feedback path is holding it off. This is common in SG3525 and UC3846 welder control boards.
Shutdown logic flow
Auxiliary supply OK
↓
PWM IC powered
↓
Reference voltage OK
↓
Protection / VRD / feedback signal checked
↓
Shutdown inactive
↓
PWM output appears
↓
Driver stage enabled
Where to measure
| Check | Normal / Expected | Abnormal Meaning |
|---|---|---|
| PWM IC VCC | Stable | Auxiliary supply issue if low |
| Reference pin | Stable reference | IC or supply fault |
| Soft-start node | Ramps normally | Capacitor or clamp fault |
| Shutdown/protect pin | Not active | Protection path is blocking PWM |
| Output pins | PWM waveform | Control IC disabled or damaged |
| Driver input | Waveform reaches driver | Broken trace or driver transistor fault |
Likely failed parts
- PC817 optocoupler in protection or VRD path.
- S8050/S8550 small transistor holding shutdown active.
- Current-sense resistor or peak-current feedback network.
- CA3140/LM324/LM358 feedback comparator stage.
- Soft-start capacitor, diode or clamp component.
Repair principle
Never bypass a shutdown path permanently on customer equipment. It can be useful for fault isolation on the bench, but the final repair should restore the intended protection behavior.