NU-MIG135 Blows fuse when trigger is pressed.

[a111r]

I would guess that that's a primary tap (usually a loop) that's broken, which is common with rotary voltage switches that you don't have.
Maybe not... this is a strange example of a strange machine!

That twist won't be good, anyway. High resistance.
Try using a proper screw terminal connection.

 

[DSR]

Normally I'd say don't disturb the wires or remove the transformer and my initial reaction was when I saw you had removed it but TBH, you've probably saved your life

General rules with the transformers in welders :-

Big fat wires - Secondary - low voltage high current, few turns.
Skinny Wires - Primary - high voltage, low current, lots of turns.

The primary will be tapped, and switched, the taps and switches is what varies the output. Different manufacturers have different tapping arrangements but the principle remains the same.

Importantly, the primary and secondary are separate from each other and held in place on the former. This ensures that should a fault occur, the primary mains voltage doesn't find its way onto the secondary coil.

Your transformer has a split primary wound on the former, the winding order looking at the picture is primary, SECONDARY (fat wire), primary.

That twisting together and soldering (I'm hoping somebody's tried soldering it !!!) could best be described as not good practice, best way to deal with it its to separate the join and either crimp it back together using a barrel crimp or using a heavy duty chock bar connector adding insulated sleeving in the process. Soldering and aluminium is rarely effective.

What's more worrying is that the primary wires are all bunched together and relying on the enamel insulation, there should be sleeving over each of the wires to keep them well separated especially where they come out of the former where they are likely to be stressed and the insulation damaged.

Couple this with the primary cable being very close to the choke (that's the coil stuck on the end of the transformer) there is a risk that should the enamel get damaged etc. there is a chance that mains could get into the welding side and the associated risks of electric shock.

Can you just confirm how many thin primary wires you have and also how many voltage settings you have on the front of the welder - a pic of the front etc. would be good.

Also take a good picture of the ends of the transformer, I think the former at the left has posts on it that are for anchoring wires to so may be useful for tidying up the mess.

 

[eddie49]

Is the wire that has been twisted copper or aluminium? If the former, has that twisted joint been soldered? If so, I think it will be conducting OK, but probably some insulating tape would be a good idea.

From the wire thicknesses, I would say that the very thick and relatively short coil on the left is the inductor, the second coil is primary, the third coil is the secondary, and the fourth coil is also primary. Is this theory confirmed by where the wires went?

( Also, refer to my post #16 about two spade tags on the primary winding soldered together at one of the front-panel power select switches.)

 

[bloobloo]

6 primary wires that go to the switches. I have used a brass straight screw connector, put heat shrink insulation on both ends and then over the brass block.

 

[bloobloo]

Also done more tests on the diodes. I think I need two new ones.

 

[DSR]

`kinell you're sorting it quicker than I can type


Happy new test meter

Good. It's only got four settings and the six wires is what is expected. It looks a lot better (and safer) now you have tidied it and insulated it. I think the primary circuit will look something like below:-

Get some more sleeving and cover the primary wires (screwfix sleeving is probably the cheapest - 100m for 7 quid).

Looking at how they were coiled near the switch, it looks as if somebody's had a go at removing a few turns from the primary for some reason, possibly to up the secondary voltage.

Also get some decent crimps on the ends that have been soldered. Crimps work better on aluminium than solder ever will.

 

[bloobloo]

Is the wire that has been twisted copper or aluminium? If the former, has that twisted joint been soldered? If so, I think it will be conducting OK, but probably some insulating tape would be a good idea.

It was ally, just twisted together.

There are 2 sets of primaries that are soldered together. just to confuse things even more!

 

[DSR]

Also done more tests on the diodes. I think I need two new ones.

View attachment 297149 View attachment 297151

Does look like two broken diodes. Do the diodes read 0 whichever way you connect the meter ?

 

[DSR]

Check that the studs are insulated from the plates and each other as well. They were saying 0 and that could be because of the diodes being short circuited.

 

[DSR]

It was ally, just twisted together.

My charity was misplaced

 

[bloobloo]

Does look like two broken diodes. Do the diodes read 0 whichever way you connect the meter ?

0.002 on both, both ways around. Unless I use the RS meter and then it says 24!

 

[bloobloo]

Check that the studs are insulated from the plates and each other as well. They were saying 0 and that could be because of the diodes being short circuited.

Studs are insulated. No continuity when tested. I’ve also put 25mm M8 washers on so that they now sit in the center. The other washers are M10 and make them look off kilter.

 

[bloobloo]

So, the next question.... ow the piggin hell does this go back together? where do the secondary wires go to and in which order?
I know theres not much point in doing it until I get the new diodes but I can mak a start on it

 

[DSR]

Probably not worth replacing the diodes - buy a whole new rectifier. What normally happens is that the part number is either not on the diode or when it is the diode is obsolete and hard to come by. However armed with this information you go and look around for an equivalent diode and find one that is suitable but to buy a couple of them will cost half as much as a new rectifer and as they aren't identical diodes, they will probably take a higher or lower share of the load and work themselves to death quickly or cook the remaining originals. They're also a sod to fit.

I've bodged a rectifier up in the past using high power diodes bolted to the old plates from an original rectifier - it makes a good heatsink but the time and effort involved is disproportionate to the cost of a new rectifier.

 

[Hutcho]

Is the wire that has been twisted copper or aluminium? If the former, has that twisted joint been soldered? If so, I think it will be conducting OK, but probably some insulating tape would be a good idea.

From the wire thicknesses, I would say that the very thick and relatively short coil on the left is the inductor, the second coil is primary, the third coil is the secondary, and the fourth coil is also primary. Is this theory confirmed by where the wires went?

( Also, refer to my post #16 about two spade tags on the primary winding soldered together at one of the front-panel power select switches.)

Those wires together Eddie are right compared to mine - two yellows together

 

[bloobloo]

I've just seen these. Would they work if i took out the old ones?

5 Pair 25HFR120/25HF120 Rectifier Diode, Iron Base Forward/Reverse Polarity Rectifier for Battery Charging and Converter, 25A 1200V : Amazon.co.uk: Automotive

5 Pair 25HFR120/25HF120 Rectifier Diode, Iron Base Forward/Reverse Polarity Rectifier for Battery Charging and Converter, 25A 1200V : Amazon.co.uk: Automotive

www.amazon.co.uk

 

[eddie49]

Those wires together Eddie are right compared to mine - two yellows together

It sounds like the ends of each section of the primary winding were fitted with crimped-on spade tags, then they were turned into tappings on one continuous coil by joining pairs of spade tags. Maybe the plan was to use 2-male 1-female spade tag splices, but instead they just soldered one tag to the other.

 

[a111r]

You could parallel three of these up for under a fiver, as an initial test (as I'd be wary of spending on that MIG).
I've done five before, yes it is a soldering challenge!

KBPC5010 Bridge Rectifier 1000V 50A Metal Case UK STOCK+SELLER Buy 2 get 1 free | eBay

BEN KBPC5010 1000V 50A Bridge Rectifier.

www.ebay.co.uk

 

[DSR]

I've just seen these. Would they work if i took out the old ones?

5 Pair 25HFR120/25HF120 Rectifier Diode, Iron Base Forward/Reverse Polarity Rectifier for Battery Charging and Converter, 25A 1200V : Amazon.co.uk: Automotive

5 Pair 25HFR120/25HF120 Rectifier Diode, Iron Base Forward/Reverse Polarity Rectifier for Battery Charging and Converter, 25A 1200V : Amazon.co.uk: Automotive

www.amazon.co.uk

They look good and are well priced but 5 pair is 5 * 25 = 125A so you will be pushing them to the limit, I'd order two packs, knock all the old diodes out of the rectifier and drill ten holes in the plates and mount all 20 of them then solder wire ends to them to followed by 8mm eyelets and fit them to the bolts.

You could make it look pretty and easier to solder by mounting then on A PCD round the bolts - 5 round each bolt on both plates.

It'll be a good rectifier for under 20 quid

 

[eddie49]

I've just seen these. Would they work if i took out the old ones?

5 Pair 25HFR120/25HF120 Rectifier Diode, Iron Base Forward/Reverse Polarity Rectifier for Battery Charging and Converter, 25A 1200V : Amazon.co.uk: Automotive

5 Pair 25HFR120/25HF120 Rectifier Diode, Iron Base Forward/Reverse Polarity Rectifier for Battery Charging and Converter, 25A 1200V : Amazon.co.uk: Automotive

www.amazon.co.uk

Five pairs would mean 10 diodes. A bridge rectifier needs four effective diodes, so if you used two of those diodes in parallel to share the current at each of the four places, you would use 8 of them. However, they are only 25 Amps each, so two in parallel would not be enough. Additionally, half the diodes are "reverse polarity", which means on some the stud is the anode not the cathode, so mounting them on the existing two aluminium plates would be complex.