Diagnostic workflow
Capacitor Discharge Stud Welder Common Fault Routing
Use this workflow to separate charging faults, capacitor-bank faults, trigger faults, discharge SCR faults and poor-weld-quality problems before replacing major parts in capacitor-discharge stud welders.
Symptom routing table
| Observed symptom | Likely section | First checks |
|---|---|---|
| Power indicator on but capacitor voltmeter reads zero | Charging path | Check charging transformer secondary, rectifier/charge SCR, control supply and meter wiring. |
| Charging voltage cannot be adjusted | Voltage reference / cutoff control | Check voltage-setting potentiometer, divider network, comparator/zener reference and charge-control transistor. |
| Charging voltage unstable or never reaches ready state | Capacitor bank or charge-control loop | Check capacitor leakage, sensing divider, charge-ready relay and charging SCR trigger behavior. |
| Gun button pressed but no discharge | Gun trigger / discharge trigger path | Check gun switch, gun cable, contact condition, relay contacts, UJT/pulse transformer and SCR gate pulse. |
| Capacitor bank charges but discharge SCR does not fire | SCR trigger or SCR gate/cathode circuit | Check isolated trigger pulse, gate resistor, pulse transformer secondary and SCR gate-cathode path. |
| Stud touches work and arcs immediately | Discharge SCR leakage/short or trigger interlock fault | Stop power. Check SCR for short, trigger relay contacts and gun cable insulation. |
| Arc does not stop or output terminal voltage remains high | Discharge path / charge cutoff abnormality | Check discharge SCR recovery, capacitor discharge resistor, charge-control shutdown and trigger relay sticking. |
| Poor weld quality, small fused area or excessive spatter | Energy setting / capacitor / contact mechanics | Check charge voltage, capacitance, stud tip, gun pressure, work return and surface condition. |
| Fuse blown normally versus violently vaporized | Overload vs hard short | Check whether the fault is a normal overload event or a severe rectifier/SCR/capacitor-bank short. |
Stop conditions
Stop the test sequence and discharge the capacitor bank before continuing if any of these conditions appear:
- Capacitor-bank voltage does not decay through the discharge path after power-off.
- The discharge SCR tests shorted or the output terminals remain energized unexpectedly.
- The gun trigger cable insulation is suspect or the gun fires without the button command.
- Charging voltage rises above the expected setting and charge cutoff does not act.
- A replacement fuse vaporizes immediately, suggesting a hard short rather than normal overload.
Measurement record template
| Record item | Why it matters |
|---|---|
| Machine type and capacitor rating | Stored-energy behavior depends on capacitance and charge voltage. |
| Selected charge voltage and measured bank voltage | Separates setting/cutoff faults from output discharge faults. |
| Charge-ready indicator or relay state | Shows whether the voltage-control loop recognizes the setpoint. |
| Gun trigger continuity and interlock state | Separates gun/cable faults from SCR faults. |
| Discharge SCR gate pulse evidence | Shows whether the trigger circuit reaches the SCR. |
| Weld result after controlled test | Links electrical evidence to stud fusion, spatter and work-return quality. |