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Re: The Ultimate Adjustable Tesla Coil Capacitor



Original poster: "Jeff Behary" <jeff_behary@xxxxxxxxxxx>

Peter & Ed,

I know such a simple concept can't be that original, but its the first time I've seen it applied to tuning for Tesla Coils. I've seen adjustable caps, but never made by tapping the number of plates based on the place digits of binary numbers. It really does work extremely well...even if horribly simple! in the end its only a capacitor with a several wires instead of 2 and a possibility of 100s or even a 1000 different values possible using only those wires. Even if a single capacitor section doesn't equal .001 mfd, you still have the maximum number of values based on the smallest capacity as a whole (2 plates 1 dielectric)...so if a single section is ..0001 mfd or .001 mfd or even .012 mfd, it makes little difference to the flexibility as a whole when compared to just a normal plate capacitor.
There's really no extra work involved in making it compared to a normal cap.

If MMCs can be found with the values I mentioned (.001, .002, .004, .008, ..016, .032, .064, etc) they would be a great solution for many applications as opposed to building-your-own. The best bet would be to have commercial pulse capacitors made with these specific values. Or one large capacitor that is tapped and has a series of leads from the case:

On the MMC concept, if this concept was expanded to those values you can control the frequency of a coil (or the tuning) remotely using only a few relays/etc.

The concept can be used for rolled caps having one continous metal sheet- a dielectric - and then a series of individual metal sheets that are separated and tapped to form the other leads. I've tried this with microwave oven caps. Its messy but works. A microwave cap around 1 mfd can create an adjustable cap 2000V using the same concept. The biggest problem is trying to work with them and separate the less-than-paper thin plastic and metal...

Your microwave oven challange is great. I built an interesting twin Pancake Tesla Coil that gives a mean 8-10" hot discharge (melts wire)using a single MOT, and 4 of those caps in series. Doesn't fit the challange, but maybe will spark some ideas:

http://www.electrotherapymuseum.com/2005/KilowattKinraide/index.htm

and a movie,
http://www.electrotherapymuseum.com/Films/PragueII_Low.wmv

Jeff Behary, c/o
The Turn Of The Century Electrotherapy Museum
http://www.electrotherapymuseum.com

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Interesting concept and I imagine, a necessary device for your smaller fixed inductance coils. I doubt that plastic sheets/foil will last long for a larger coil compared to an MMC though. I am also not sure how your setup will go under oil as you would need a vertical setup to get bubbles out.


Last week, for my entry in the "Tesla coil using parts from a single microwave oven prize" I was using a rolled cap with variable pressure on the roll giving a cap range of about 3 times for tuning. This is a low powered coil however with the limitation from the parts available. I haven't posted the cap pics yet but here is the prize thread.
http://4hv.org/e107_plugins/forum/forum_viewtopic.php?15477.0

Peter

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The same general "binary" design, sometimes using special switches and sometimes how, has been in use for a very long time. I have a bunch of power factor correction capacitors fof WW2 airborne radios which operated from 400~ power. They go from 1 ufd to 21 ufd in 1 ufd steps.
I just connect to them with toggle switches and find them very useful.

I'm pretty sure a lot of precision resistor boxes do the same thing, adding in series rather than in parallel.

Ed

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