Circuit
ZX7-250 Overcurrent Feedback and Thermal Protection Checks
A ZX7-250 protection reference covering CT feedback, thermal switch input and the difference between protection signals and real current measurement.
Database summary
This WelderData protection record covers the ZX7-250 current-transformer feedback path and the thermal switch input. In this board family, the protection decision is normally built from overcurrent feedback and temperature state, not from the front-panel current display alone.
The service rule is simple: when the welder blocks PWM or enters protection, verify the feedback inputs before replacing the PWM controller, driver transformer or power devices.
Current transformer feedback
The current transformer is placed around the output-current path so that a heavy current event or output short can be converted into a small feedback signal for the control board. If this path is open, shorted, contaminated or incorrectly connected, protection behavior can be missing, unstable or permanently active.
CT feedback is a protection signal. It is not the same as a calibrated output-current measurement. Use the CT path to understand shutdown behavior; use an external shunt to verify true output current.
Thermal switch input
A thermal switch around the 85°C class may be used as a temperature-protection input. When it closes or pulls the protection input into the active state, the board can stop output even if the low-voltage rails are present and the panel display is alive.
Check the thermal switch, its connector and the small-signal input path before assuming that a no-output condition is caused by the power stage.
Protection interpretation table
| Input path | Normal repair meaning | Abnormal pattern | Next check |
|---|---|---|---|
| Current transformer | Feeds overcurrent information into protection logic | False trip, no trip or unstable trip | Check CT winding, connector and input components |
| Thermal switch | Blocks output when heat is excessive | Permanent protection or no thermal protection | Check switch state around operating temperature and wiring |
| Panel current display | Shows control-side set or display value | May not match real current | Verify actual current with shunt measurement |
| PWM output | Should not be judged until protection inputs are known | No PWM while protection is active | Clear protection cause before replacing controller |