NU-MIG135 Blows fuse when trigger is pressed.

[bloobloo]

Readings 1 and 2 show a dead short circuit between the terminals. When you measured it, were you measuring across the two connectors as circled below and the transformer wires were disconnected from them ?

Yes top and bottom with nothing else connected

 

[bloobloo]

wire going to rear of rectifier

 

[bloobloo]

2 "B" and 1 "A"

 

[bloobloo]

Under that blob of silicone sealant is the thermal cut out thingy

rear of rectifier

 

[DSR]

The rectifier looks seriously out of order. Those studs and washers circled in red should be concentric, the lower one looks as if the stud is off centre as well, possibly touching the aluminium plate. Coupled with the front plate insulators being cracked, I'd say that the rectifier has been taken to bits and re-assembled incorrectly. There are two options at this point, the simplest is to replace the rectifier, the hardest is to remove the rectifier and strip it down and try and work out the correct order of all the parts and re-assemble it. Once stripped down the diodes will have to be tested as well to make sure that they are all in good order otherwise reassembling will be a waste of time.

Before shelling out on a new rectifier, we need to find out why you are getting weird voltages from the transformer. (See next post)

 

[a111r]

Mental note made to avoid buying broken Nu-Migs.
Like Hutcho did with SIPs...

 

[DSR]

Is this the other 'A' wire (you said 2B and 1A in an earlier post).

I'm also concerned about the winding voltages measured. I've never come across this particular model of welder before but there is nothing in it that looks particularly special, including the transformer. Normally, if there is one thick winding, the winding will go to a bridge rectifier. If there are two windings the two windings will give out the same voltage and normally be wired in series and go to a bi-phase rectifier, the output from the rectifier will (typically) go to the torch and the centre of the windings will go to the welding earth clamp.

Yours is odd in that it has two windings and what looks to be a bridge rectifier which would suggest that the windings need to be wired in parallel.

Note that the metal chassis of the welder will be connected to your mains plug earth but the welding earth will not normally be connected to the chassis, the high current welding circuit is normally isolated from the metal body of the welder (it's why your rectifier is mounted on a plastic block). When measuring your welding output voltages, you should always measure them across the welding earth and torch, never with reference to the chassis.

We need to work out why you are only getting a few volts out of winding B. Have you measured the resistance of both windings ?

 

[bloobloo]

Is this the other 'A' wire (you said 2B and 1A in an earlier post).

Yes, that's the other A wire.

I'll take some more readings once i've finished my tea.
Thanks for all the help so far, I really appreciate it.

 

[Hutcho]

Mental note made to avoid buying broken Nu-Migs.
Like Hutcho did with SIPs...

Ha didn’t last long lol - this nu-mig needs a separate psu for the feed - it’s all over the place

 

[Hutcho]

View attachment 296979

Is this the other 'A' wire (you said 2B and 1A in an earlier post).

I'm also concerned about the winding voltages measured. I've never come across this particular model of welder before but there is nothing in it that looks particularly special, including the transformer. Normally, if there is one thick winding, the winding will go to a bridge rectifier. If there are two windings the two windings will give out the same voltage and normally be wired in series and go to a bi-phase rectifier, the output from the rectifier will (typically) go to the torch and the centre of the windings will go to the welding earth clamp.

Yours is odd in that it has two windings and what looks to be a bridge rectifier which would suggest that the windings need to be wired in parallel.

Note that the metal chassis of the welder will be connected to your mains plug earth but the welding earth will not normally be connected to the chassis, the high current welding circuit is normally isolated from the metal body of the welder (it's why your rectifier is mounted on a plastic block). When measuring your welding output voltages, you should always measure them across the welding earth and torch, never with reference to the chassis.

We need to work out why you are only getting a few volts out of winding B. Have you measured the resistance of both windings ?

That wire that’s close to the cover goes to the rectifier - same as mine

 

[DSR]

That wire that’s close to the cover goes to the rectifier - same as mine

@Hutcho - has yours got a double heavy duty winding as well ? - your pic only seems to show two wires.

 

[bloobloo]

We need to work out why you are only getting a few volts out of winding B. Have you measured the resistance of both windings ?

24.5 ohm on both "A" and "B"

I will look for my other meter. I'm not convinced it is reading accurately

 

[DSR]

24.5 ohm on both "A" and "B"

I will look for my other meter. I'm not convinced it is reading accurately

Definitely - with 24.5 ohms, you would be seriously limited with the output current.

Is the battery good in your meter ?

 

[Hutcho]

@Hutcho - has yours got a double heavy duty winding as well ? - your pic only seems to show two wires.

Mine seems to be the same setup - what Iam thinking is has someone unwound a turn of that coil

 

[Hutcho]

Found this on the net - nu-mig again , same rectifier layout but with only two wires again

 

[bloobloo]

plot thickens......

"A" front - "B" silicone
1-3=43.7v
1-4=49.2v
2-3=52.5v
2-4=61.2v

"A" twisted - "B" silicone
1-3=18.6v
1-4=21.8v
2-3=22.9v
2-4=26.4

"A" front - "B" twisted
1-3=19.5v
1-4=21.9v
2-3=23.2v
2-4=27v

"A" twisted - "B" twisted
1-3=4.5v
1-4=5.3v
2-3=6.2v
2-4=7.3v

 

[bloobloo]

could it be as simple as putting both "A" to one and both "B" to the other on the rectifier?
The way it was when we started we had an A+B on the same nut, that might explain popping of fuses?

 

[DSR]

plot thickens......View attachment 296985

"A" front - "B" silicone
1-3=43.7v
1-4=49.2v
2-3=52.5v
2-4=61.2v

"A" twisted - "B" silicone
1-3=18.6v
1-4=21.8v
2-3=22.9v
2-4=26.4

"A" front - "B" twisted
1-3=19.5v
1-4=21.9v
2-3=23.2v
2-4=27v

"A" twisted - "B" twisted
1-3=4.5v
1-4=5.3v
2-3=6.2v
2-4=7.3v

That doesn't make sense from what I can see of the windings. We can only see four fat wires coming from the transformer so we either have two separate windings or a single winding with two taps.

61.2V is way too high for MIG welding, 4.5V is way too low. Your centre two "A" twisted - "B" silicone and "A" front - "B" twisted are pretty much what would be expected.

The resistances you measured - 24 ohms are way too high as well, I'd expect well under an ohm.

The high voltages and low voltages could just be due to your meter being faulty and picking up stray currents, the resistance readings could be too.

Did you manage to find your other test meter. The one you are using is a good quality bit of kit (RS just don't do cheap - ever) but the numbers it is giving certainly aren't right. Are the test leads in good condition, does it show 0 when on the ohms ranges when you short the test leads out ? I'd be looking for another meter to check it against.

TBH, you'd be better off with a battery, a couple of lengths of wire and a torch bulb if you can't find another meter to check your current one against.

 

[DSR]

Mine seems to be the same setup - what Iam thinking is has someone unwound a turn of that coil

View attachment 296986

I was wondering that too - looking at how loose some of the wires are on the primary and the secondary, there is definitely something afoot

 

[bloobloo]

Took the tranny out. Should that twisted joint be there? Doesn’t look like something that would be done in the factory. Also wound that long one back around the coil. Soo….. what now?