Circuit path · PWM and burn-back
NBC wire feeder PWM and burn-back control
This circuit reference explains how an NBC-style CO2/MIG wire feeder turns a torch command into gas-valve action, motor PWM, wire-speed control and burn-back timing. It is intended for board-level diagnosis, not blind component replacement.
Functional signal path
Burn-back and post-trigger behavior
Burn-back is the short timing interval after torch release that prevents the wire from sticking into the weld pool or burning back too far into the contact tip. In many NBC feeder boards it is controlled by a potentiometer plus capacitor/resistor timing path, then applied to the motor and output-stop sequence.
| Symptom | Timing clue | Check |
|---|---|---|
| Wire sticks to workpiece | Burn-back too short or output stop too early | Check burn-back pot, timing capacitor and stop command path. |
| Wire burns back into tip | Burn-back too long or motor stops too soon relative to arc | Check adjustment range and trigger-release sequence. |
| Wire keeps feeding | Not normal burn-back; likely command or power path stuck | Measure motor output after command is removed. |
Diagnostic measurements
Set voltage
Should change smoothly as the wire-speed control is turned.
PWM duty
Should respond to set voltage and enable/disable with trigger logic.
Motor output
Should appear only during feed command and follow speed setting.
Do not confuse relay click with motor command
A gas or relay click only proves part of the trigger sequence. It does not prove that the PWM IC is enabled, that the motor output stage is healthy, or that the feeder motor has a usable voltage under load.