Electronic Circuit Board Problem MIG Welder

[Lovel]

A dear friend of mine has asked me to look at his mig welder which has been playing up.

The symptoms are that when the power switch is turned on the wire in the gun keeps spooling out unless the power is switched off. I checked the torch switch but it seem to work fine and is switching off and on. I had a suspicion that one of the relays on the circuit board was jammed/burnt on. The clear relay housings showed some blackening from the outside.

When I checked the single point relay with 12v across the coil I could not get continuity across the terminals.

When I placed 12v across the coil to test the second DPDT relay instead of functioning there was a little puff of smoke appeared, but I could not see where it came from, it looked like it came from some other component.

So suspecting the relay or both relays I replaced them both with new only to find that although the torch is not spooling now with power on, the wire does not spool at all now. I also replaced the fuse with new.

Just wondering if anyone reading this would care to share their knowledge, as I am struggling to check the other components on the board. Although I did check the diodes, by disconnecting each one from the board to test.

Unfortunately I don't have an oscilloscope and tbh wouldn't know how to use it so I tried a ohmmeter across the large capacitor and got a kick on the meter but it is a digital meter and not so easy to see, I cant recall if you should get a kick then a drop down the meter scale with a capacitor?

Here's a photo of the circuit board in question for the Oxford 160 mig welder.

[fega72]

Check the transistors as well (or simple replace them?). You probably burnt one when you applied 12V to the relay.

[murphyv310]

Hi.
Looking at the component side of the board I'd say the Diode across the relays coil is most likely short circuit, It's there to suppress the back EMF of the coil when the drive is switched off, if it goes short then its driver will pass too much current and cook, this is the one next to the diode I'd think, no doubt an NPN and I'd say shorted.
Virtually any Si NPN that is suitably rated would do. Snag is any transistor down stream of the driver could also be duff.

[marinabrian]

Originally Posted by murphyv310

Hi.
Looking at the component side of the board I'd say the Diode across the relays coil is most likely short circuit, It's there to suppress the back EMF of the coil when the drive is switched off, if it goes short then its driver will pass too much current and cook, this is the one next to the diode I'd think, no doubt an NPN and I'd say shorted.
Virtually any Si NPN that is suitably rated would do. Snag is any transistor down stream of the driver could also be duff.

Well there only appears to be three smallish transistors, and perhaps one in the TO220 package attached to the heatsink.......

Given the crude simplicity of the circuit, it should be possible to repair that easily enough

Gary, if you check out the transistors with your meter on diode test, you should be able to identify which have failed, possible base emitter open, I should imagine they are being used in conjunction with the big boy on the heatsink??????

Brian

[Lovel]

Originally Posted by marinabrian

Well there only appears to be three smallish transistors, and perhaps one in the TO220 package attached to the heatsink.......

Given the crude simplicity of the circuit, it should be possible to repair that easily enough

Gary, if you check out the transistors with your meter on diode test, you should be able to identify which have failed, possible base emitter open, I should imagine they are being used in conjunction with the big boy on the heatsink??????

Brian

Had a good natter with Trevor this afternoon and drawing from his electoral knowledge. The upshot is I have ordered up some of the smaller d-shaped transistors and will swap them out to see where we are after that.

[Lovel]

Originally Posted by Lovel

Had a good natter with Trevor this afternoon and drawing from his electoral knowledge. The upshot is I have ordered up some of the smaller d-shaped transistors and will swap them out to see where we are after that.

Changed out the d-shaped transistors and all is working well now with the welder.

Thank you guys and especially Trevor for the time on the phone.

[tantallon]

Absolutely brilliant .What a team of men available .Very refreshing in this throw away society we live in .Well done to all involved.

[murphyv310]

Well done Gary.
Excellent news