Migmate 150 PCB transformer issue....

[Martin77]

Hi everyone, I've got a Migmate 150 which I've been using for the last 15 years or so, without too many problems and it's seen me through a lot of welding. However this morning it died.. The mains supply was there (fan goes ok), but nothing happened when I pulled the trigger. I checked the usual wires, and check the motor, all fine.

After a bit of research here I checked inside the pcb 12v relay, which was pretty blackened on the contacts, which I cleaned up with emery. However it didn't click or move when the trigger was pressed. I moved it (with great caution!) by hand and sure enough the feed motor whirred.

rightly or wrongly I put 12v DC from a car battery on the inputs of the relay and it clicked as it should, which suggests the relay is fine.

I then checked (carefully while it was switched on) the small transformer for 12v AC output, and couldn't find anything. This would suggest it's the transformer that's died...correct? and I don't suppose there's any chance of a replacement new or used?

Seems a shame to chuck the welder for a small component...

 

[piman]

Hello Martin,

you could try here:- http://uk.rs-online.com/web/c/power-supplies-transformers/transformers/pcb-transformers/

You need to find the input voltage, output voltage and capacity (Power VA) from your transformer. Hopefully it will be marked on it? If necessary you can fit a larger power as long as the input and output voltage match, but I wouldn't go smaller.

Alec

 

[Martin77]

Ah, there should be something in there. I know it'll surely be 240v input, 12v output, but can anyone help with the amps?

 

[Martin77]

Thinking about it, if the welder works ok when I manually move the relay to make the contact, does that mean the transformer is ok and it's just the relay?

 

[piman]

Hello Martin,

If you are not getting a feed to the relay coil, you need to find out why, either in the circuit or a lack of supply, e.g. a faulty transformer or fuse?

I'm not familiar with the insides of welding sets but is this transformer a small one just for the control circuit and there is also a larger one for the main welding current?

Alec

 

[Martin77]

Yeah, this transformer is just a small one on the circuit board to control the feed 12v motor. Someone will be along soon who will know what to test hopefully...

 

[piman]

Hello Martin,

you said there was no output from the transformer, did you check for an input to it?

Alec

 

[Martin77]

Alec, I've just been inspecting a little more. Yes, there is 240v across the input connections. I removed the transformer from the board and measured a resistance across the output connectors, but it's dead across the input side. This then must be the issue. All I need is a replacement transformer of the correct rating. Sadly there are no marking on the transformer.

 

[mike 109444]

Martin pretty much any 240 to 12v transformer that's physically bigger (which should have a higher va) than the one on the pcb should do. You don't have to fit it on the pcb if it's to big or the pin arrangement is incorrect as you can mount it near by and run the 4 connections via wire to the pcb ( remember to insulate any bare wires.)

 

[Martin77]

That was my thoughts, but it'd be nice to be sure which size to go for. I'm not so confidant about electrical theory so like to be safe. Do you think 2VA would be ok?

 

[mike 109444]

Think you would be looking for aprox 50va If current needed is the relay coil + drive to wire feed motor + ?. Call it 3 amps of load (there is usually a 2 amp fuse on some pcbs)

 

[mike 109444]

Or just find an old CB power supply and pull out the transformer! plenty big enough.

 

[Martin77]

That's what confuses me a little, the relationship between va and amps. And if this transformer doesn;t work for the relay, where does the 12v for the feed motor come from? (because that works when the relay is moved by hand)

 

[mike 109444]

That's what confuses me a little, the relationship between va and amps. And if this transformer doesn;t work for the relay, where does the 12v for the feed motor come from? (because that works when the relay is moved by hand)

I should have looked at the schematic. It gets it's power from the main transformer (the VERY big one! that also produces the welding voltage). I work in electronics but I don't in my job need to work out va etc.
Unfortunately most manufactures of migs don't make available schematics of there pcbs.

 

[Martin77]

Ok..yeah, that makes sense. So does that mean I can get a slightly smaller transformer than a 50va? (excuse slightly dim questions)

 

[mike 109444]

You should be able to go a bit smaller. But better to err on the over size. Note that some have a center tap so would be labeled 240-- 12-0-12 which is not what you need as some of the size of the transformer is taken up by the additional winding, so might look big enough but may not be. Busted vcr and such like are a good source for such parts.

 

[Martin77]

So, I found a pic on here of another identical SIP board that used a 1.5va transformer, so that's what I ordered. Fitted the board perfectly, and now I have 12v ac on that side. However..it still doesn't move the relay. I checked and there ain't 12v DC or AC getting to the relay when the trigger is pressed.

I'm now a bit lost.... I feel like giving up and buying a new welder....

 

[Martin77]

As a last resort I've ordered a replacement rectifier, 12v AC going in but nowt coming out.

 

[mike 109444]

Can you post a clear pic of both sides of the pcb (as I said before makers don't inc pcb schematics) it should be possible to trace the circuit and work out what could be wrong.

 

[dannyp]

going to say ive met a few of the small pcb transformers in friends littel sip's fail. believe it or not it's undersize over long extension leads that kill them.