Tesla Coil: Internal Arcing

I've just finished a 1/4 wave 1kw tesla coil off a 7 stage marx generator with polypropylene capacitors off of a microwave oven transformer. It was far too much work. The secondary coil is ~700 turns on a 3 in diameter acrylic tube, with 3 spray coats and 1 brush coat of polyurethane. The primary fits inside the secondary, with the wires to the primary running through holes in the secondary. The problem is, when I run it, it arcs along the inside of the winding, between the tube and the wire. I can see white flashes in the space wound areas of the tube, and the secondary has this cool looking branching lightning that's red because the light is coming through the enamel. Obviously, this shorts part of the coil, changing the inductance, and thus de-tuning the system. The topload is ~25 pf (calculated) and the secondary is 20.1 mh (measured) with resonance at like 230kHz, I can't remember the exact numbers right now. I can draw off about 6 inches to ground, but I should be getting 8 times that. Any ideas on how to fix this?

Reply to
bhauth
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If I wasn't clear enough, it's the secondary that's arcing. If it matters, the capacitors are storing about 3j.

Reply to
bhauth

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- my guess would be overcoupling - primary inside scondary is probably not helping

Reply to
Mike Harrison

Coilers usually have the primary outside the secondary. If the Q of both windings is high, you get good energy transfer with loose coupling.

Your replacement primary should have the same inductance as your old primary, which means fewer turns. The turns should be spread out somewhat, although with none getting too close to the top end of the secondary. Make sure the primary wire has adequate circumference since skin effect will play a major role - perhaps use nominally 1/8 inch (O.D. about 1/4 inch) copper tubing.

I think you will be doing very well if you get 2 foot sparks.

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

The primary wire is 10 awg; the secondary is 22 awg. The arcing happens in about a 10 inch region of the 19 inch winding, in an apparently consistent position. I don't see how skin effect affects this kind of arcing, to be honest. My guess was that there was some oil on the inside somehow that was getting arced through.

Thanks though.

Reply to
bhauth

Skin effect does not affect arcing, but affects losses in the primary. Since I suggested an external one, I think the wire length may be greater than with your internal one.

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

Presumably there is air but no varnish between the turns and the tube... If so, maybe vacuum encapsulating would be an impractical way of solving it... Chris

Reply to
Chris Jones

I fixed it by removing the internal primary and adding an external one. It was arcing through the primary through 3/8 in of acrylic. I'm now getting 3 ft peak sparks with Fres tuning from theory, without good spark gap adjustment, and with big unpatched corona leaks.

Chris, unintuitively, vacuum conducts better than air.

Reply to
bhauth

A really good vacuum does not. But air at any pressure less than atmospheric and more than usually some fraction of 1 Torr (depending on how short the longest spark path is) breaks down more easily than air at atmospheric pressure does.

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

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