Re: The Ultimate Adjustable Tesla Coil Capacitor

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Mddeming@xxxxxxx 




Hi Jeff, Peter, Ed, All,

    Some time in the latter part of 2001, I posted my work to the 
list on a binary-switched PFC cap bank, using 7 switches to vary the 
value from 0 uF to ~492 uF in steps of ~3.87 uF. This was a large, 
heavy box on wheels  attached to the low voltage side using dozens of 
~40 uF caps in parallel and series, controlled by seven ordinary 20 A 
household switches .
( Mostly because I had dozens of 40 uF 600V caps and a box of 20 A 
switches lying around.) At that time, I didn't see a practical and 
safe way to switch values on the 15 kV side tank capacitors.
    If the dielectric stands up, then you'll have moved things 
another big step forward. Nice work!
Matt D.


In a message dated 11/2/06 11:14:10 P.M. Eastern Standard Time, 
tesla@xxxxxxxxxx writes:
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

----

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

---

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.