Add-on Square Wave for Vintage Lincoln?

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The post by Mr. astuch regarding an add-on HF for his Interter reminded me of a question that I've had floating around in my mind for a few years. I have a Lincoln 250/250 TIG/Stick Idealarc, ca. 1985. It has AC-DC and HF start or continouus. It's a nice older machine and working with it has taught me a lot about welding! It does, of course, have its limitations and is not as sophisticated as the newer welders.I was wondering if any company had ever built a square-wave "black box" that could be put in line with the output leads when the machine is operating on AC for aluminum? I know a bit about electronics and I can sort of visualize how something like that might work. It would be fun to massage the AC current for better cleaning, penetration, etc. I've looked at the Lincoln site but didn't see anything. I can understand that they probably want to sell complete newer machines in any case. That said, I was wondering if anyone here had ever seen or heard of such a unit by any manufacturer. I'm not opposed to buying another machine with SW technology just for TIG but I'd still keep my old friend, Mr. Lincoln. I was just wondering if I could spruce him up a bit.Pleasant comments, scorn and disrespect equally appreciated. I'd rather be reviled than ignored.    .Last edited by gnm109; 06-11-2006 at 09:26 AM.
Reply:Interesting concept.  I don't pretend to be more than a hobbiest interested in welding and electronics, but here are some random thoughts.  I have a WWII vintage saturable reactor-controlled, AC output only, transformer machine with HF, so your question is intrigueing to me.I interpret your interest in deriving a "square wave" output from your welder as desiring (1) the capability to control the degree of balance of standard and reverse polarity when welding aluminum and (2) achieving the benefit of arc restrike without continuous HF due to the rapid transition from one polarity to the opposite.  Correct?First, I assume from your description that your Idealarc is a basic transformer machine.  The data sheet I pulled up doesn't describe the control technology, but the large dial on the front implies that it is a variable-magnetic coupling transformer with a moveable magnetic shunt or moveable primary coil.  This would mean that the AC output is a sine wave directly out of the transformer.  Unless there is a large bank of DC blocking capacitors or an auxiliary circuit providing DC offset voltage to the AC output, any deviation from balanced output (by, for example, modification of the duration or magnitude of the positive vs. the negative excursions of the output waveform) will be seen as DC current through the transformer secondary, leading to core saturation or at least reduced output capability.  Does your Idealarc provide any AC output balance offset capability for TIG?  I assume not, or you probably would not have raised the issue.Second, with a pure sine wave to deal with, the only fast switching you could achieve would be to turn on and off the sine wave at times when the output from the welder is appreciable, which implies relatively large "off" times as the sine wave passes through zero volts.  Such delay would (I think) cancel out the instant arc restrike benefit of the rapid polarity transitions of "square wave" (actually, a "rectangular wave" with variable duty cycle) outputs from inverter machines, since the arc plasma would have time to de-ionize during the off time. Based upon the above (and without benefit of actually knowing anything about the topic), I think a possibly more feasible (albeit expensive) option would be to provide an inverter fed by the DC output of your machine.  You would be rapidly switching the polarity of the DC output while the transformer saw only an AC load.  This approach would require that the switching frequency NOT be synchronized with the line frequency, which would be easy to achieve, since you probably want a higher frequency output anyway.  It would also probably be necessary to provide some filtering of the DC out of the machine to avoid nulls in the output voltage that would result in the arc not being sustained even though you were switching the polarity.  (+zero volts is the same as -zero volts.)  In this topology, whatever arbitrary duty cycle of the rectangular wave you selected would not result in DC current through the transformer.The obvious shortcoming of this approach is that you would have to build most of the guts of an inverter welder, less only the input rectifier and, I suppose, the current control, and you lose the advantage of having a lightweight inverter welder because you are still joined at the hip to your transformer monster.Ah, well.  I just talked myself out of the project.  I'd welcome input from a welder designer that proved me wrong.Note that add-on HF is a totally different question and is quite feasible and practical.awright
Reply:Thanks for the response. I think you talked both of us out of it.   I can see why it's likely that no one has done it. Your post only serves to confirm that. The machine does extremely well as is on nice new aluminum and even old castings if cleaned properly. I may also have succumbed, partially at least, to the raves that I hear from people who have the SW technology in their machines. The interesting thing about this machine has been that it really keeps you honest. With good metal joint preparation, scrupulous cleaning procedures and solid technique, one can get excellent results. That sounds like something we should be doing anyway. Thanks for the response.
Reply:I would think threough the use of caps and coils you could shape the wave to approximate a square wave.  The components would have to be pretty heavy to handle the current.  I doubt you could come close enough to a true square wave to eliminate the need for HF.  I think you guys are on the right track.  Pass this one up and save for an inverter!DennisThermal Arc 185-TSWMillermatic Challenger 172VictorO/AAtlas Craftsman 12 by 24 LatheEsab PCM-875Wholesalem Tool Mill-Drill