Re: The Ultimate Adjustable Tesla Coil Capacitor

["Tesla list" <tesla@xxxxxxxxxx>]

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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