1Press torch switch
Hand-switch input.
TIG start diagnosis
TIG Welder Gas Valve Clicks But No Output and No HF
A practical TIG start-fault route for machines where the gas valve operates but PWM output and high-frequency arc start are missing.
Quick Answer
Use this workflow when the TIG torch trigger makes the gas valve click, but the machine has no open-circuit voltage and no high-frequency arc start.
- First confirm 24 V control power and torch-switch response.
- If SG3525/CW3525 has clock but no output pulses, inspect the shutdown and feedback path before replacing the PWM IC.
- Check PC817, CA3140 and SG3525 pin 9 evidence before replacing the whole control board.
Use this page when
- Gas valve clicks after trigger.
- No OCV appears at output.
- No HF arc start appears.
- The machine is not completely dead.
Do not use this page when
- The breaker trips immediately at power-on.
- There is no control power at all.
- The torch switch creates no response.
- The main problem is only gas flow with normal OCV/HF.
Why this symptom is valuable
When the gas valve clicks but the machine has no open-circuit voltage and no high-frequency start, the solenoid is only one piece of evidence. The shutdown path may still block PWM even when the control power and main MOS/power devices look normal.
TIG start sequence
TIG gas / hand-switch / HF sequence
Redrawn TIG control sequence for gas, PWM, high-frequency start and post-flow diagnosis.
2Pre-flow gas
Solenoid relay ON.
3PWM enabled
SG3525 / CW3525 unblocked.
4HF start
High-voltage path ON.
5Arc transfer
Main current flows.
6Post-flow gas
Gas remains after output stops.
Gas valve clicks only
Check PWM shutdown, feedback optocoupler, CA3140 path and SG3525 pin 9.
Output without HF
Check HF relay, HV transformer and discharge gap.
HF but no output
Check power stage, output diodes and feedback/protection path.
TIG control goal
Gas first, current delayed, HF only during arc start, gas delayed after stop.
Repair case route
WS-250 gas-click / no-OCV / no-HF case
Redrawn repair case route from gas click to SG3525 pin 9 feedback shutdown.
Symptom
Gas valve clicks, but no OCV / no HF.
SG3525 / CW3525
Pins 11/14 no PWM; pin 4 clock present.
PC817 + CA3140
Feedback/shutdown path pulls pin 9 low.
R12 5.1 k open
Bias path broken, false shutdown.
Do not replace ICs first
Control supply and MOS/output parts may be normal.
Measure feedback chain
CA3140 output, PC817 input and SG3525 pin 9.
Repair result
Replace open 5.1 k resistor after confirming the path.
Case lesson
No PWM can be a feedback-shutdown fault, not a bad PWM controller.
Diagnostic table
| Evidence | Meaning | Next check |
|---|---|---|
| Gas valve clicks | hand-switch and gas relay path may be alive | continue to PWM permission and HF command. |
| SG3525/CW3525 has clock but no output pulses | PWM controller may be shut down, not dead | check shutdown/feedback pins before replacing IC. |
| Pin 9 pulled low by optocoupler path | feedback/shutdown path is active | inspect PC817, CA3140 output and resistor network. |
| 5.1 k path open | bias path can create false shutdown | verify related pull-up and feedback path. |
| HF absent too | HF command may depend on PWM/start state | check HF relay and start sequence after PWM permission. |