Cheap MIG w/ 2 Amp settings - hackable?

[hutchwilco]

I have been on the lookout for a welder mainly for automotive work (rust repairs) but also the odd project constructing toys (land yachts etc).

I've noticed in the local autoparts stores (Repco) in New Zealand, they have deals on 100 Amp gas/gasless MIGs for NZ$280 (~130 GBP).

The caveat seems to me to be only two power settings - 55amp or 90amps.
Having a degree in physics, and also first welding experience on an arc welder with a current adjustment that was a mechanical adjustment of the coils in the transformers - I feel like the limitation on these cheap MIGs is just a marketing ploy - remove features on the base models - it's really not hard to implement the feature, right?

Does anyone have any idea on the innards of these - I'm wondering if buying a cheap model, opening it up and having a tinker (ie adding in fine grained current control) would net me an equivalent of a higher priced model. I frequently dismantle laptops, cell phones, cars, household appliances and build my own guitar effects pedals so I have plenty of experience with 'lectronics, but I've never looked inside one of these welders so I don't know what to expect.

Am I correct about this reason for no fine grained control? Has anyone pondered this themselves? I'd love to hear people's thoughts....

Hi from New Zealand, by the way!

 

[rtbcomp]

Welcome to the forum.

Have a look here

http://www.clarkeservice.co.uk/mig_welders.html

Clarke manuals include circuit diagrams which should give you the basic principles.

 

[rikrobson]

the power settings for migs are by taps on the primary coil on the transfomer, thus changing the coil ratio on the transformer and hence the output voltage.

with a two stage setting there is probably only 3 taps for full,halt and common.

on my boc 130 there are 4 settings with two switches each going to taps on the coil.

they are constant voltage circuits and i doubt that you will be able to have a fine control.

cheap is cheap for a reason!!!

 

[eddie49]

Those two settings - 55 and 90A - are so coarse, that machine must be next to useless as a general-purpose MIG. If they've saved on a multi-position switch and a few tappings on the transformer, you'd have to question the design, materials, and build quality of all of it. Far better to buy a used real machine than waste money on this thing.

 

[SirDick Diodenob esquire]

I have a collection of old welders and the worst one is my Nu Tool Mig which easily takes bottom position , its even worse than my SIP Migmate . I paid £22 for the Nu Tool and managed to get it working but its a horrible machine and I will not spend any more time working on **** like this , for £60 I bought a Thermal Arc 180 which I felt was worth spending time on to make it work the way it should , its still not a top end machine but worth the effort . There is a very interesting thread on here on improving the wire feed on a Migmate and for the challenge of making something work better I can see where its coming from and I admire the ingenuity of the contributors but personally I would rather buy a duff Murex or similar for the same money and get that working as you then end up with a very good machine .

 

[GeorgeB]

If they've saved on a multi-position switch and a few tappings on the transformer, you'd have to question the design, materials, and build quality of all of it.

I think I'm right in saying you're being a bit unfair on the manufacturers. They haven't simply saved on a few tappings. To achieve lower output current/voltage down to, say 30A (many machines) or even 15A (a few) requires a lot more wire to be used in the primary windings, and perhaps even larger transformer laminations to hold it.

 

[Luke]

George, as eddie mentioned, they're 55 and 90 amp windings, not 30 or 15 amp.

As has been mentioned, you're looking at a cheap machine that, by the time you've bought or wound a new transformer (which is the only way to improve the number of tappings), you'll still not have a very good machine, and you'll have spent the time and money required just to source a decent mig in the first place.

Luke

 

[rtbcomp]

Could you use a variac on the input side of the transformer?

Mind you - I guess a variac capable of handling that power would cost more than the welder!

 

[GeorgeB]

George, as eddie mentioned, they're 55 and 90 amp windings, not 30 or 15 amp.

I know. I'm sorry if what I wrote was not crystal clear. I meant that for the manufacturer to move from (a) a machine running at 55 and 90 amps to (b) a machine running DOWN to 30 amps OR 15 amps, needs a fair bit more wire.

 

[eddie49]

Yes George, you are right. With a fixed number of turns on the secondary, to reduce the secondary voltage, they would have to have put more turns on the primary, which would mean more wire.
Yes rtbcomp, a Variac ( variable autotransformer ) could be used on the input. The welding power of my BOC Transmig 140 has stepless adjustment by means of a Variac, rated at 120V in, 0-140V out, 10A, 1.4KVA. In fact, this MIG has two of them - the wire feed speed control is also a small ( 0.28KVA ) Variac. Money was no object in those days !