tesla coil varnish is dumb

[Fumeaux]

Dear High Voltage Forum!

Some time ago I started to wind a new coil. I chose a PU-dispersion paint, which turned out to be a mistake. My guess is that normal PU-paint would have worked, but the watersolvable-dispersion-stuff didn't. Now my coil has countless fingernail wide cracks. I don't think that is that big of an issue, as I have a layer of electricaltape underneath to make it structurally sound. But I'm still worried about the insulation aspect. That's why I'm asking for your thoughts on this.



Does anybody think is should fix it? And what do you suggest?

I can't reccomend this lacker.

[Mads Barnkob]

I tried something similar once, where I tried to fix it with a different type of varnish and that was just another mistake on a mistake. I used a oil based furniture varnish first and when that failed I tried to add spray on polyurethane and that just cracked up everywhere!

Since it cracks its either reacting with the electrical tape or you had huge temperature differences?

Fixing it.... hmm, try to fill out the cracks with another varnish type and just live with the fact that it will never be pretty

You have a sad doggo in the varnish.

[T3sl4co1l]

Huh.  What's the point of varnishing the outside, if there's air between the wires, under the tape?

I have one experience with water-based urethane; I was thoroughly underwhelmed by its performance.  It cost much more, went on much thinner (it's mostly water, so the dried film is quite thin and many coats are needed), is softer (though still strong enough not to be peeled off by fingernails alone), and seems to turn a bit gooey/sticky with prolonged contact with oils.

There probably are better-performing mixes, and I happened to get the worst one out there.  Ironically, I feel like the stuff I had, wouldn't have given you trouble -- it doesn't seem brittle enough to crack.  But then again, vinyl tape is rich with plasticizers (basically, oils that turn the plastic gooey -- pure PVC is a brittle white solid -- see where this is going?), and maybe that's strained or weakened your coating, causing it to crack.

Varnish is kind of dumb anyway, since at the voltages we're talking here, you need whole cm's of thickness to effectively insulate something.  Modest layers can still provide some protection, such as corona suppression or protecting against smaller streamers; I don't have enough experience in this direction to know how significant this is, so I'll let others fill in.

Tim

[Hydron]

Huh.  What's the point of varnishing the outside, if there's air between the wires, under the tape?

Kizmo kinda got around this by making a pipe lathe to cut a groove for a spaced secondary winding, which both cut down the secondary impedance and also gave better insulation properties (as there were spaces between turns for varnish to fill).

I tend to agree that varnish is probably fairly futile from an electrical insulation perspective, but in terms of holding the coil together mechanically it makes sense. Especially if you have the coil on a motorised winding jig it's pretty easy to slap on some polyurethane or epoxy to hold the windings together and give some protection from minor scrapes etc.

[Teravolt]

I just use minwax urithane in a spray can just after winding to hold the windings in place not to insulate.  the wire will strech after winding and I have had coils unravle

[MRMILSTAR]

Varnish is not for insulation. It is used to hold the coil windings in place during temperature extremes. If you don't varnish the coil, a big temperature swing can actually cause your secondary windings to collapse and fall off of the form. This has happened to a number of people.