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agrip
08-28-2005, 08:55 PM
My TIG welder quit. - - - This beast is a Lincoln AC-DC 250 amp single phase TIG rig, with various remote controls.

The “smoke cam out” of the coil for the main contactor. The power for same is a small transformer built into the machine. The transformer is OK. I suspect that the root problem was “stray” HF voltage impressed in to the control circuit.

The high frequency arc start/ stabilizing component is impressed on the welder output circuit, which then of course, gets coupled right back through murphy's law and the physics of the matter, into the 60 Hz control circuit.

The HF is generated by a spark gap rig. I do not know the frequency. But from some old experience around mobile radio transceivers, I suspect that it is around 28 mHz.

The welder may have 2- 300 hrs total welding time now. Principal use is in prototype shop duty. It sits a lot.

When the unit was built they didn't put any visible RF choke or bypass at this solenoid coil and the insulation of the coil finally said pbbbbbttt. They did put the solenoid and coil in a separate sheet iron box which itself is shielded by the main transformer and cooling baffles, but that did not prevent the problem.

Lincoln makes you buy a whole new solenoid rig for 450+ USD PLUS Freight.
EEK that is 25% of a new machine.
A workable replacement coil retails below 50 bux at supply houses. I don’t know the game they are playing but the tickets seem kind of unreasonable.

How do I figure/ guess an appropriate bypass capacitor value.
And should I bother with a choke or not since the coil itself is one.

MY idea is to bypass to ground before the circuit enters the solenoid box And bypass across the coil itself in the box.
Any guidance truly appreciated.

ag

J Tiers
08-29-2005, 09:33 AM
I would think that any such coil is plenty good enouigh to prevent RF getting through it, although that may have nothing to do with it giving up.

If there was an accidental resonance that allowed a very much higher than normal voltage to occur, that might account for it.

But it might just have been defective, or gotten too hot.

KDRedd
08-29-2005, 10:43 AM
Spark gaps generate RF across the range of DC to Light. This is why the FCC ban the from transmitter service. This is also why we use resistor in plug wires or in the sparkplug to reduce RF interferance.

Kent

agrip
08-29-2005, 05:01 PM
Thanks for reply fellas.
JT Yes the solenoid coil does resist the entry of RF. Lots of Henries.
The RF amplitude on the tungsten is intended to start the welding arc at close to a half inch gap which it easily jumps and more.

I have reason to believe that a coupled RF voltage across the solenoid coil,is what did the coil in. It wasn't hot,there was a localized arcing with "only 120vac" from a small auxillary control transformer.

The question is what is an appropriate capacitor size and rating to ground the RF in the control circuit, to prevent the RF from doing in the solenoid again.

KDR
Thanks for the frequency spread.


Both
I guess that I could come up with a capacitor value sporting reactance of an ohm or so at maybe 15MegHz and use that to bypass any stray RF to the chassis fo the welder.
That doesn't answer the rating question but at least its a start.