Secondary coil burn on some generic MIG-130 Flux Core welder

[drhousedk]

Hi,

Took a chance and bought a known-defective generic MIG-130 Flux Core welder today for toying around with. Looks a bit like a Rossi (but isn't, no branding other than MIG-130 visible). Relay clicks and the primary coil seems to "humm" when I pull the trigger but the wire doesn't feed.

Turns out that the secondary coil has been "a bit" hot. Pulled the coil out to see if I could see any specs on it, but unfortunately not. No visible damage to the PCB which seems to be in good condition. My thought was to buy some cheapo coil to see if it'll bring it back to life. From what I can see on the wiring it's a 230volt primary side, but the secondary side of the coil is unknown. Anyone in here knows if it should be 12v, 24v or maybe something else? And what should the specs (amperage) be?

Attached is a pic of the welder, and pics of the secondary coil and the control PCB, if that's of any help.

 

[Pete.]

Looks like that machine is dead unless you can find a replacement transformer.

 

[drhousedk]

Looks like that machine is dead unless you can find a replacement transformer.

Well that was kind of my idea. If it's a simple 230 to 24v transformer it should be only a few bucks.

 

[Pete.]

I'm on about the big transformer in the second pic. From here it looks fried.

 

[drhousedk]

I'm on about the big transformer in the second pic. From here it looks fried.

That's the secondary transformer, which I'm inquiring about

 

[Nix2]

This transformer can be replaced with a similar sized one, but it must come from a MIG/MAG machine (appropriate current-voltage characteristics and regulation taps)

 

[drhousedk]

This transformer can be replaced with a similar sized one, but it must come from a MIG/MAG machine (appropriate current-voltage characteristics and regulation taps)

They have to come from somewhere, so figuring they might be found brand-new somewhere. I don't know the specs which is what I was hoping someone would have an idea about. The current burned transformer is completely non-descript. I was thinking something like 220v to 24v (or maybe 12v) and perhaps 50VA.

 

[eddie49]

There seems to be some confusion here....

This type of generic gasless flux-core MIG welder should contain two transformers. One is very small, the other is large.
Both have 230v primaries. The small one is mounted on the control PCB - it is coloured blue in this case. The secondary will be 12 or 24v, and the rating is typically a couple of Watts ( VA ). It is powered as soon as the welder is switched on, and only runs the On/Off power control relay, when triggered by the torch switch.

[ The wirefeed motor is powered from the main welding output.]

The large transformer provides the main welding output. As @Nix2 said, it may have 2 or 4 input tappings on the primary, selected by the MIN/MAX and/or 1/2 power switches, and one secondary wound with very thick wire. The output will be from 15 to 35 volts, and rated at 2 to 3 kW for a 120A welder.

[ These welders often do not have a rectifier, nor an inductor ( choke ), so the welding output is AC.]

The number E123995 on your PCB leads to this website:

E123995 MIG24754 HB17716 M276S012A XY7317 30762-b YFT149388

E123995 MIG24754 HB17716 M276S012A XY7317 30762-b YFT149388

plytkadomigomatu.pl

and the alternative number MIG2475 for that replacement board leads here:

https://www.elektroda.pl/rtvforum/topic2626823.html

which shows the typical size of the main transformer, and that the PCB-mounted transformer is 24v and 2VA.

The burnt transformer, the item that you are calling the secondary coil, could be about 10 or 15 cm square, and as you said, rated at 50 or 100VA. The question is, what is it, and why is it there?

 

[drhousedk]

My train of thought is that it's for the wire feed motor. Two of the wires on the 6 pin plug on the PCB goes to the 'secondary coil' and another two goes to the wire feed motor. The rest I haven't traced yet. The welder seems to work still, relay clicking, big transformer humming when I pull the trigger. Only the wire feed doesn't run. I did check the small glass fuse and it's okay. (Relay etc actually still clicks when pulling the trigger even without the glass fuse which I found a bit confusing)

I don't belive 2VA would be enough for the wire feed motor given that a popular SIP welder mod is to tap in a laptop power supply to have a more stable wire feed.

 

[eddie49]

That is an unusual configuration for a ( sub- ? ) hobby-level machine, it is mostly larger professional welders that have a separate power source for the wirefeed motor.
The motors on plastic wirefeed assemblies are 10 to 15 Watts, and 20 to 40 Watts on the die-cast alloy units, so a 50VA 24volt transformer will be sufficient. It is going to cost 30 to 50 Euros - about what the welder is worth....!

If you do have a redundant 19.6v laptop power supply, you could trace the existing wiring to check if you could get the speed control circuit running on a pure DC supply. I think there is a small circular bridge rectifier on the board, and the TO-220-style power device is a BT151 thyristor, which probably relies on the supply dropping at every half-cycle.

 

[drhousedk]

I agree that the welder likely isn't worth too much. A new equally cheap unit is around 270 eur here, I paid 40 used for this knowing that "the wire feed doesn't work", leaving a bit of room. A known good used unit is approx 100 eur, so... If I can DIY my way out of this, worst case (if it's fixable, I know...) I'm break-even with the price of a known good unit and I've had some tinker-time too. :-)

If the wirefeed is 10-15 watts, a 2VA transformer on-PCB won't be enough for the wirefeed, I take it?

I did cut out the old transformer but did put tags on the transformer and the wires, where they each went to.

By no means am I an expert probably nor intermediate, but from this I gather that the wires tagged 3+4 is secondary going to the connector on the PCB and 1+2 is primary - 1 going to what I believe to potentially be a thermal fuse mounted on the big transformer and 2 going to a connector on the PCB (JP3 seen in the photo in the original post) which is shared with the middle connector on the 1/2 (not min/max) switch on the welder.

 

[Nix2]

This "small" transformer on the board does not power the feeder, it only powers the relay coil on the board, so it has a power of about 2-3 VA.
Of course, you can use a much larger one (e.g. 50-100VA) and it will work the same, but it is not economically justified.

 

[drhousedk]

This "small" transformer on the board does not power the feeder, it only powers the relay coil on the board, so it has a power of about 2-3 VA.
Of course, you can use a much larger one (e.g. 50-100VA) and it will work the same, but it is not economically justified.

But I'm not talking about the PCB transformer but on the one on the table. From schematics that's the one feeding the wire feed.

 

[Nix2]

ok, if we're talking about the "large" transformer, that's what powers the wire feeder (via the board - usually PWM type regulation)

 

[eddie49]

The excellent Polish electronics website "elektroda" that I linked to in Post #8 has a hand-drawn circuit diagram for the PCB, and an explanation of the pins 1 to 6 of the white connector. This states that, as usual for this class of machine, the wirefeed motor is powered by the output of the main welding transformer. That 15 to 30 volts AC input is connected across Pins 1 and 2, the motor positive is Pin 5, and the motor negative is Pin 3.
Are you sure that this additional "mid-size" transformer is an original item, and not perhaps a modification by a previous owner?

I think that before buying a replacement transformer, you should confirm that the welder output, measured from the tip of the torch to the workpiece clamp, is correct. That is, 15 to 30 or 35 volts, possibly AC, varied in steps by the switches, and triggered by the torch switch.

You may also want to consider making a temporary test of the full functionality of this welder by just wiring it in the typical way, with the AC welding output from the main transformer linked to Pins 1 and 2 of the PCB. This would avoid the need to purchase that transformer at all.

What work do you intend to do with this gasless fluxcore welder? Basically, what metal thickness?
Could you please supply a photo of the dataplate?
In the worst case, the electrical symbol will just show two overlapping circles, meaning only a transformer and confirming that there is no rectifier on the welding output.
It is also typical that the welding current range will show as starting at 50 or more Amps. This would mean a rather large minimum metal thickness, at least 2mm.

 

[drhousedk]

First of all thank you for your very valuable input. Without it I would never have been able to gain as much insight into this machine as I do have now.

I haven't found clear inside photos of an identical welder yet, but yeah it is possible that this was a modification done by a previous owner as I don't know it's full history for obvious reasons. I've read plenty stories about a poor wire feed stability on similar-ish welders and people mounting a separate DC power supply in order to have a stable wire feed, so this might be why there's a separate transformer with a from-the-walls power input.

That said the mounting holes and hardware (nuts and bolts) for the "extra" transformer is identical to the other mounting hardware in the enclosure, so it might be original equipment - who knows. I found a second hand transformer, 24v / 2.35A (should be approx 56 VA), for 20 euros including shipping. This is more a tinkering project more than anything else. If it doesn't work out in the end, at least I had a bit of fun tinkering with it, getting to know the absolute basics of the innings of a cheap MIG welder.

If I figure out the wiring and how to connect the PCB ("pin 1 and 2" as per the polish website) to the main welder transformer I might give that a shot.

Only plans I've got for it is to spot weld a few corners on some thick sheet metal based flower bed edges just to keep it together, so nothing that needs to be pretty at all (or pass an MOT :-))

It is indeed a pure AC welder so it's not going to work wonders at all.

 

[drhousedk]

Ok, so curiosity got me. I checked with a voltmeter and there is indeed 22 VAC when at it's minimum settings and 44 VAC when the switches are "maxed out" - measured from clamp to torch.

So I connected pins 1 and 2 on the control PCB to the clamp and torch respectively, and wouldn't you know it, the wire feed whirrs away happily. Yay!

Unfortunately I don't have any obvious points to tap into the welding transformer, but I do see why people like to create a separate power source for the wire feed - given that you range from 22 VAC to 44 VAC on the welding transformer depending on settings, the wire speed gets a lot higher on the max settings compared to when on the min settings, meaning that wire speed isn't consistent if you change the voltage settings on the switches.

Given the success I've seen so far (and that I don't have any obvious tap-into-points on the welding transformer) I'm inclined to wait 'till the 230VAC to 24VAC transformer arrives in two days and mount that, and that way have a consistent wire speed setting which doesn't change depending on the settings on the switches, but only on the potentiometer setting.

 

[Nix2]

You can use a separate transformer to power the feeder, but this will not help much in terms of welding quality.

 

[drhousedk]

You can use a separate transformer to power the feeder, but this will not help much in terms of welding quality.

But from what I can gather, it would help have the wire speed consistent at least

 

[eddie49]

Thank you for the updates and photos, and congratulations on the progress to a working wirefeed!

The 44v AC out of the welding transformer at the max setting is too high for the motor, and that may be why they added the ( third ) 24v transformer, so I now agree that you should replace it with a new one.

Increasing the wirespeed automatically at higher settings of the voltage switches is actually not considered to be a problem. Most low-end machines that borrow the welding output to drive the wirefeed motor will inevitably do this. The Danish Migatronic welders have a separate transformer/rectifier/capacitor as the source of power, but the speed control circuit is biased by a signal from the rotary voltage step switch to increase the wire speed with every step up. This is known as "semi-synergic" control, and is regarded as an advantage.

In fully-synergic machines, there is only a single control, often calibrated in millimetres of metal thickness, and this manages both voltage and wirespeed in a fixed pre-programmed ratio. Well-designed machines also allow the voltage to be manually "trimmed" by a few percent up or down.

The "SIP welder mod" of providing a separate stabilised DC power source for the wirefeed motor instead of the welding output, via the original or a new PWM speed controller, is done to overcome short-term variations - stuttering or stumbling of the wire - caused by variations in the welding voltage when the arc is struck or the arc length changes.