Fault
Auxiliary power supply no 24V
KA3843 / KA3845 startup, output feedback, PC817, zener, current-sense and load-pulling faults.
Why the 24V rail matters
Many Chinese/OEM inverter welders use a small auxiliary switching supply to power the fan, relay, control board, driver circuit and sometimes the display. If this supply is missing or weak, the machine may look completely dead or may power up without enabling the PWM section.
Auxiliary supply path
DC bus or auxiliary AC source
↓
Startup resistor / VCC capacitor
↓
KA3843 / KA3845 / UC3843 / UC3845
↓
Auxiliary transformer
↓
Secondary rectifier
↓
24V / 15V / 12V / 5V rails
↓
PC817 feedback and reference
Measurement process
| Check | Normal / Expected | Abnormal Meaning |
|---|---|---|
| Bulk DC bus | Present | Input rectifier or mains side fault if missing |
| PWM chip VCC | Rises to startup threshold | Startup resistor, VCC capacitor or chip load fault |
| Output pulse | Switching waveform exists | PWM chip disabled or damaged |
| 24V rail | Stable under load | Secondary short, feedback fault or transformer issue |
| PC817 feedback | Regulates output | Feedback open/short causes high, low or pulsing output |
Common causes
- Open startup resistor.
- Dried small electrolytic capacitor on VCC.
- Shorted secondary rectifier diode or overloaded 24V load.
- PC817 optocoupler or reference zener/CJ431 fault.
- Bad KA3843/KA3845 only after surrounding parts have been checked.
Repair method
Use current limiting and isolate downstream loads when possible. If the 24V rail comes back after disconnecting a load, the auxiliary supply may be good and the fault is on the fan, relay, driver board or control-board load.