A customer brought in an inverter-type welding machine TAYOR WSM-250I with a complete “no power” condition. The unit showed no LED indication, no fan operation, and no output response when connected to mains supply. This document describes the systematic diagnostic process and final repair solution.
External inspection showed no visible mechanical damage. However, upon opening the unit, the circuit boards were covered by dust layer. I had to clean the unit before testing for any fault. Later i observed that the main fuse for logic board power supply was open.
Shows internal circuit of the welding machine
Diagnostic procedure,
There are precautions to take before power up or insert power to these types of machines. the conventional lamp series method will not work for this type circuits. Bulk capacitors hold enough energy to blow up the mosfets when you accidentally trigger high side and low side mosfets.
Disconnect main DC bus from bulk capacitors. some manufacturers provide jumpers on the PCB for that purpose. otherwise you will have to cut the pcb gently. this will protect your mosfets.
STEP-2
Now you can use lamp series method to power up the primary power supply. primary power supply delivers power to display panel, MOSFET or IGBT gate drivers, inverter driver board and relays.
**The primary power supply of this WSM-250I machine was the fault. SMPS controller ic and Power mosfet was burned. I was able to bring it back to life replacing those parts.**
STEP-3
After working on primary power supply, inspect inverter MOSFET gate waveforms from PWM generation ic (SG3525,TL494,etc....) using a oscilloscope.
STEP-4
Now you can connect capacitors into the DC bus and power up the machine.
This is the very basic explanation on how to diagnose inverter type welding machines. please check following components before you move into the deep of the circuit.
- Power cable, fuse inside the plug
- VDR
- NTC thermister
- Bridge rectifiers
- Power Relays
- Schotkey diodes
- Capacitors in primary PSU
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