Circuito
fuente auxiliar
24V, 15V, 12V y 5V rails generated by KA3843/KA3845 style auxiliary supplies.
Why the 24V rail matters
Many chinas/OEM soldadoras inverter use a small auxiliary switching supply to potencia the fan, relé, placa de control, driver circuit y sometimes the display. If this supply is missing o weak, the máquina may look completely dead o may potencia up without enabling the PWM section.
Auxiliary supply path
bus DC o auxiliary AC fuente
↓
Arranque resistencia / VCC capacitor
↓
KA3843 / KA3845 / UC3843 / UC3845
↓
Auxiliary transformador
↓
Rectificador secundario
↓
24V / 15V / 12V / 5V rails
↓
PC817 realimentación y reference
Measurement process
| Comprobación | Normal / valor esperado | Significado si es anormal |
|---|---|---|
| Bulk bus DC | Present | entrada rectificador o mains side falla if missing |
| PWM chip VCC | Rises to arranque threshold | Arranque resistencia, VCC capacitor o chip load falla |
| salida pulse | Switching forma de onda exists | PWM chip disabled o damaged |
| 24V rail | Stable under load | Secondary short, realimentación falla o transformador issue |
| PC817 realimentación | Regulates salida | realimentación open/short causes high, low o pulsing salida |
Common causes
- Open arranque resistencia.
- Dried small electrolytic capacitor on VCC.
- Shorted rectificador secundario diodo o overloaded 24V load.
- PC817 optocoupler o reference zener/CJ431 falla.
- Bad KA3843/KA3845 only after surrounding parts have been comprobacióned.
Repair method
Use current limiting y isolate downstream loads when possible. If the 24V rail comes back after disconnecting a load, the auxiliary supply may be good y the falla is on the fan, relé, placa driver o placa de control load.