E55 Ferrite transformer and Litz wire

...and don't forget concentrated phosphoric acid which is an excellent flux, relatively non-toxic and is easy to wash away. I works nicely for stainless steel as well as copper.

John

I often use large-diameter litz with hundreds of tiny wires. I follow my technician's approach, which is to use a solder pot. I dip the litz bundle into a gross- looking paste flux and then into the pot. Voila, an instant ready-to-go solder-encrusted termination.

"Large"? That's cute. ;-)

In the induction heating business, inch-wide rope is regularly used. The ends are crimped into several-aught size lugs, some rosin dripped in, then dipped whole into a solder pot. When the bubbling subsides, it's ready.

On a less bragging matter:

I've made litz myself, using whatever stuff I have laying around. On several occasions, I've made the mistake of using this old brown stuff that's classic "enamel". It just carbonizes harder and harder, on heating!

So I get out an abused teablepoon, and melt a lump of potassium chlorate(!) in it. Heat the wire briefly in the flame, then dip the wire in the salt: FOOF, no more enamel! Water quenches and dissolves the salt stuck in the wire, leaving perfectly clean, pink copper, ready to be tinned!

(!) Potassium chlorate is the schizophrenic little brother of potassium perchlorate, which itself is basically used in pyrotechnics and not much else. Perchlorate is remarkably stable, as long as you don't drip concentrated sulfuric acid on it, or heat it over 600C. Chlorate is a much weaker salt, and becomes a friction-sensitive explosive if paired with mildly acidic fuels like sulfur or phosphorus. It's still stable enough to melt, but it's bubbling with oxygen all the while...

Tim

You can also do it by heating the wire to red heat and then rapidly plunging it into alcohol (ethanol, methylated spirits etc.) whilst still red hot. If the wire has insufficient heat capacity to stay red hot long enough to do the above, then you can wrap it with some other copper wire just to increase the thermal mass. The added wire may get welded to the Litz wire if you heat it too much. That may or may not be a problem.

Obviously, messing with blowtorches around a pot of alcohol is likely to result in a fire, so do it in a place where a fire won't be a problem, and limit the quantity of alcohol to the amount that would not cause a problem if it catches fire.

The Aspirin trick is also described in Rev. Sci. Instr. vol.75 no.5 p.1169 (2004), a paper by I. R. Walker. Works well.

Regards, Mikko

Hmmm. That might work for modern enamels -- don't think it'll work for my particular example, which looks like it turns to glassy carbon on heating. (Which means it'll take a lot more heating to get it to oxidize to bare crusty copper.)

As for the active ingredient -- copper-catalyzed oxidation of light spirits (methanol, ethanol, acetone, etc.) is really cool to watch, and it does indeed produce a solderable surface that was (briefly) at red heat!

I'm surprised the chlorate doesn't burn the copper too, but hey, go figure. I'll take what I can get. :)

Additional hazard warning: definitely don't combine the two methods, and, say, accidentally plunge roiling, molten chlorate into a vat of alcohol. ;-D

Tim

I did it without blowtorches by putting a wad of cotton in a metal can lid, pouring some denaturated alcohol into it and firing it up. The Litz was put into the flame and then pressed to the wet wad and pulled out sideways. This worked fine for the wire in IF transformers in the tube era.

The alcohol does not need to denaturated, but due to the alcohol policies in the Nordic countries it was the only resonable. The connoisseurs could flamber it with Courvoiser.