homemade TIG from defunkt MIG box

[snowman]

I have an old MIG welder, it's a century I believe. It's a 110 volt box, still works, WeldPak100...possibly a Lincoln. I can't even remember.

I've examined the schematic, and it appears to be just a transformer with a motor controller for the wire feed adapter. It is NOT a CV machine, not that fancy. Basically it's what you'd get if you put a constant speed wire feeder onto a buzzbox and added a solenoid.

I also have an old TIG torch, can't remember the brand, but I know it's decent. Have a receptacle for it as well, I think I already modified it so that I can plug the argon line in to it and run gas.

I've come to the point where occasionally I need to do a little stainless TIG welding. Not often enough to warrant a $1000+ purchase, and anything less than that, short of the HarborFreight Inverter TIG is the size of a compact car, and I simply don't have the space. I know the options available as far as using a DC buzzbox and a HFreq box, please don't suggest them as it's a waste of bandwidth.

I'm wondering if the transformer and subsequent diodes would work to weld DC stainless with a scratch start. I can always make up a reactor to smooth the fire out of some scrap welding leads I have in the attic. The one thing I would lack is any type of incremental power control, as the MIG just has A/B/C/D settings. I'm not concerned about duty cycle, I don't do anything in TIG that would require a lot of welding.

Just curious if anyone has done anything similar. I'd hate to waste the time, and the hassle of switching one of my other tanks to argon if it's going to be a lesson in humility.

[torker]

Yabut...that's a CV power source....

[snowman]

I take it that is sarcasm?

[torker]

Nope...no sarcasm...any tig machine I've ever used has been a CC power supply...

[snowman]

So, please explain the difference in power sources (ie, the difference in design of the transformer and rectifier cricuits) between CC and CV machines.

[snowman]

I will add that I understand that on a CC machine, voltage is regulated by arc length with minimal variance in amperage...and on a CV machine, amperage is regulated by wire speed.

What I don't understand is the mechanism by which this happens. That is, what is the difference in the transformers?

[gnm109]

I will add that I understand that on a CC machine, voltage is regulated by arc length with minimal variance in amperage...and on a CV machine, amperage is regulated by wire speed.

What I don't understand is the mechanism by which this happens. That is, what is the difference in the transformers?

This is only a guess, but there is no difference in the transformers. A transformer is just a transformer. It converts one voltage to another voltage that is nore useful. The difference would be in how the output of the transformer is regulated. That is handled in the control circuitry of the machine, somewhere in the motherboard.

If you can devise a control board for your transformer that converts CV to CC, then you will be fine. It sounds like a daunting task to me.

:) No sarcasm intended.

[snowman]

There is no control circuitry downstream from the transformer, or upstream for that matter.

[Albireo]

I'm not a welding expert, but I have done some reading about welding supplies.
I also work in the electronics field.
There are such things as special use transformers that because of the construction and characteristics of the magnetic core will give more of a CC behavior. It is really more of a slope in the volt vs. current graph. That's what you would want for TIG or ARC welding.
Have you considered stainless wire in the MIG?

[snowman]

Well done...my question was answered!

Copied and pasted from another site:

Start with the use. What load would require a constant current? A good
example is arc welding or arc lights. An arc light produces it's light
from the electrical arc drawn between two carbon electrodes. What limits
the current flow? Nothing in the arc or the electrodes. It is limited by
the transformer.

Originally, this current control was provided by actually moving the
primary winding of the transformer away from the secondary winding to
reduce the current flow by increasing the inductive leakage, and of
course, moving it closer to increase the current. Today, it is easier to
build a transistorized control circuit that monitors the current, and
adjusts the incoming voltage to insure the current in the secondary
circuit remains the same.

A device called a "Ballast" is used in older fluorescent lamps to "choke"
excessive current flow through those tubes. Ballasts are transformers
that produce large changes in reactance when the current tries to increase
above their design value. Modern fluorescent lamps use a chip to do the
same job.

[Dawai]

Yes, I made a bunch of stainless tattoo machines using a lincoln Mig, take leads loose, hook tig torch to it, invert polarity. Something 130 is my machine.. I forget..

Mine, it has a transformer with each "Tap" coming out to the heat a-g switch.. ok? nothing complicated other than some diodes in there somewhere. It was purchased in the 80s/early 90s.

I was in a have-to case to make money when I was jury rigging mine. I also was polishing them machines with some Dangerous stuff. A flying tattoo machine frame will put a knot between your eyes like a billy club.

If you can get a decent dc welder that cranks all the way down to near nothing.. well.. You'd be a mile ahead.. if you can get a box that "lights" the arc, two miles.. scratch starting a tig is a annoying habit to try to UNLEARN.. I got a 200 miller tig rig now.

[snowman]

I'm just annoyed because when I got my new MIG machine I gave away my big bulky arc welder.

No big loss I guess. I'll just watch craigslist for a buzzbox. Or go to harbor freight for one of their cheapies.

[Dawai]

Lil brother had a 3inONE machine here.. A ramsond. Something 50?? I posted it's ebay ad here somewheres..

It was about four hundred or so?? plasma cut half inch steel like butter. Tig rig came with it, stick welded and would sink a 3/32nd rod through a piece of 1/2 steel..

He took it with him. I miss the plasma cutter. I got a small 35amp miller plasma cutter, it will cut 1/4" ragged like.

[Don Young]

An arc welding transformer often has a magnetic shunt with a small air gap as part of the core. The shunt provides an alternate path for the primary magnetic flux that does not pass through the secondary windings. The magnetic flux diverts through the shunt when the path through the secondary encounters the counter magnetic force caused by the secondary current. That will cause the output voltage to drop rather sharply as the secondary current increases, thus becoming more of a CC source. Some of them use an adjustable shunt to control the amperage of the arc.

Resistances (or reactors in AC machines) in the leads perform somewhat the same function but not as well.

[macona]

Yep, like other have said it is constant voltage. Nothing in that transformer to limit current. If it did have a current limited transformer then there would have to be voltage sensing circuitry to control the feed motor speed. The circuit looks at the arc voltage and either raises or lowers the wires speed to maintain the arc voltage at the set point. This is a much more expensive method than either making a tapped transformer or control circuit on the primary side. The wire feed can then be controlled with something as simple as a rheostat.

[John Stevenson]

Snowman,

Many years ago I took a small oil cooled welder and took to AC outlet off it thru some very heavy stud rectifiers to make a bridge up and onto a TiG torch.
You had to scratch start this but it worked OK on thinnish steel steel.
Never tried stainless.

I built it to weld patches into road going NSU motorcycle frame which were pressed steel to turn them into the Rensport versions for classic racing.

Gas was no good as I couldn't control the distorsion but the TiG worked fine and when one was welded, fettled and primed there was no difference between these and the original factory models.

When I got a proper AC/DC TiG I let a friend have it and he still uses it.

As I say never tried it with stainless.

.

[macona]

There is usually not any problems TIG welding with CV. You voltage setting will control the heat. Higher voltage, more current through the arc, more heat. Some MIG machines have a connection for TIG welding.

[dr pepper]

I have some links if they are any good to a couple of simple hf start modules, I looked into it for mine using either a neon sign tranny or a car ignition coil, as I wanted to build a mobile vehicle rig.

[rbjscott]

The differance between CC and CV is the transformer design. This is the basic differance.

[macona]

The differance between CC and CV is the transformer design. This is the basic differance.


Yes and no. Some machines like the Miller CP series use moving brushes on the secondary as a variable transformer for CV. And some machines use saturable reactors after the main transformer to limit current in CC machines.

But almost all new machines use just a basic transformer and either voltage regulators or current regulators, or both for multi-process machines. Cheaper to build this way.