Fault · auxiliary power
Welder Has No Power or the 24 V Rail Is Missing
Use this page when an inverter welder has no fan, display or relay activity, when the 24 V control rail is absent, or when the auxiliary supply repeatedly starts and collapses. The goal is to identify the first missing stage before replacing a PWM controller.
First five minutes
Fast checks before replacing any controller
1. Define “dead”No fan, no display, no relay, or only one missing load are different faults.
2. Check for a main busIf the rectified bus is absent, stay on the input and rectifier side.
3. Look for VCC behaviorStable, never rising, or repeatedly rising and falling each point to a different branch.
4. Isolate loadsA shorted fan, relay, driver board or 5 V load can collapse an otherwise working supply.
Reference ground: primary-side controller measurements normally reference the controller ground pin and primary negative bus. Secondary 24 V / 15 V / 5 V measurements reference the matching secondary return. Never bridge primary and secondary grounds with a non-isolated instrument.
Confirm which “no power” symptom is present
| Observed symptom | What it proves | First direction |
|---|---|---|
| No fan, no display, no relay sound | Nothing downstream is visibly powered; it does not prove the main DC bus is absent. | Input protection, rectifier, bus, auxiliary startup. |
| Display flashes or fan twitches repeatedly | The supply may start, lose sustaining power or hit an overload. | VCC capacitor, auxiliary winding, secondary short, feedback. |
| 24 V missing but 5 V present | The auxiliary converter is operating at least partly. | 24 V secondary rectifier, filter, connector or load. |
| 24 V present but machine still appears dead | A single rail is not proof that logic and driver rails are healthy. | 5 V/15 V distribution, enable, display connector, protection. |
Power-off checks
- Discharge the DC bus and verify the remaining voltage with a rated meter.
- Check the fuse, inrush resistor or NTC, bridge rectifier and visible bus-capacitor damage.
- Measure resistance across the 24 V, 15 V and 5 V outputs to their own secondary return; investigate a near-short before applying power.
- Check the auxiliary switching MOSFET for drain-source and gate shorts.
- Check the startup resistor chain for an open section and inspect heat-damaged solder joints.
- Inspect the small VCC electrolytic for leakage, bulging, poor soldering or elevated ESR.
- Check secondary rectifier diodes and output capacitors for shorts.
- Inspect PC817/TL431 feedback parts and isolation slots without assuming they are the primary cause.
First live measurements under controlled power
| Order | Measurement | Reference | Interpretation |
|---|---|---|---|
| 1 | Main DC bus | Primary bus negative | Absent: stop and repair input/rectifier path. Present: continue to startup supply. |
| 2 | Controller VCC ramp | Controller ground pin | Never reaches start region: startup feed, VCC loading or controller short. Cycles: sustaining supply or overload. |
| 3 | Controller VREF | Controller ground pin | Nominal reference appears only after valid VCC. A low VREF may be an external load. |
| 4 | RT/CT activity | Controller ground pin | No oscillator with valid VCC/VREF: timing network or controller problem. |
| 5 | Output pulse | Controller ground pin | Pulse present: move downstream to gate resistor, MOSFET and transformer. |
| 6 | Secondary rails | Matching secondary return | Missing all rails: switching/transformer/rectifier. One missing rail: branch-specific fault. |
Decision branches
Bus absent
Stay on the mains input, fuse, inrush path and bridge rectifier. The KA3845 stage cannot start without its source.
Bus present, VCC absent
Check the startup resistor chain, VCC capacitor leakage, controller VCC short and any clamp device on the bias rail.
VCC cycles, no stable output
Separate a weak VCC capacitor, missing auxiliary-winding handoff, shorted secondary rail and overactive feedback/protection.
VCC and output pulse present
Move to the MOSFET gate, switching drain waveform, transformer continuity, secondary rectifiers and downstream loads.
One rail missing
Do not condemn the PWM controller. Diagnose that secondary winding, diode, filter capacitor, regulator and connector.
Rails recover after disconnecting a load
The auxiliary supply may be healthy. Find the short or excessive load on the fan, relay, driver, display or control board.
Component checks that commonly resolve the fault
| Part | Evidence before replacement | Common misleading symptom |
|---|---|---|
| Startup resistor chain | Open circuit, drifted high, cracked body or heat-damaged joint. | Controller VCC remains near zero even with a healthy DC bus. |
| VCC electrolytic | High ESR, leakage, low capacitance or VCC collapse under startup current. | Ticking, fan twitch or repeated display flash. |
| Switching MOSFET | Shorted junction, damaged gate resistor, weak drive or abnormal drain waveform. | Fuse failure, lamp limiter bright, or no energy transfer. |
| Secondary rectifier/filter | Diode short/open, capacitor short/high ESR, rail short to return. | VCC hiccup because the controller repeatedly hits overload. |
| PC817/TL431 network | Open LED path, wrong reference divider, cracked joint, unstable compensation. | Output too low, too high or cycling even when power parts are good. |
Repair verification
- Power through the approved current-limiting method with power devices and loads connected only as required.
- Confirm VCC rises once and remains above the controller turn-off region.
- Confirm VREF and oscillator activity are stable.
- Confirm the gate waveform reaches the switching device without excessive ringing or missing pulses.
- Measure every secondary rail at no load, then with the intended loads connected one at a time.
- Verify relay, fan, display and control-board behavior without bypassing protection.
- Run a staged output test and recheck temperature, ripple and restart behavior.
Stop conditions before another power-up
- The DC bus cannot be discharged or measured safely.
- The switching MOSFET, bridge rectifier or a secondary rectifier remains shorted.
- A secondary rail has a near-short that has not been isolated.
- Controller ground, bus negative and secondary return cannot be identified confidently.
- The gate waveform is missing or abnormal while the full power stage is connected.
- The current limiter remains bright, the fuse opens, or a component heats immediately.
Technical sources
- Texas Instruments UC3845 product page — pin functions, UVLO, VREF, current limit and maximum duty-cycle reference.
- UCx84x current-mode PWM controller datasheet — controller-level limits; it does not replace the actual board schematic.
- Texas Instruments TL431 product page — secondary feedback reference when the board uses TL431/KA431 regulation.
- WelderData internal technical records — used for measurement sequence and repair-pattern organization; exact readings remain board-specific.
Related repair map
Follow the evidence path from symptom to measurement, circuit and repair case.