Re: Capacitor Help Needed

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Sean Taylor" <sstaylor@xxxxxxxx>

Hi Wilson,

IMO, the best cap for "typical" size TCs is an MMC.  You can get a single
string of 10 caps (CDE 942C series, 0.15 uF, 2kV) for a 15/60 NST for
about $30.  This will prove pretty reliable, especially for the run time
*most* of us put on our TCs.  You may wish to go with more per string and
more strings for more reliability, but this should be ok.

If you want something extremely cheap, a beer bottle cap is the way to
go.  I made a few in 5 gallon paint buckets out of champagne/martinelli's
apple cider bottles that worked pretty well, and the lossy-ness isn't so
bad.  I never had a bottle crack or any of those types of problems.  I'm
guess when you had a bottle crack, you were close to resonance with your
HV transformer and the voltage climbed rapidly.  FWIW, I never used any
sort of foil on the SW caps, just all-thread in each bottle and a couple
of pieces of all-thread in the bucket.

I've also made a few rolled caps, but the only one that's still working is
the one I've only used once!  It cost me more than an equivalent MMC would
have too!  It's 60 mil PP sheet, 24 inches wide.  I used about 30 feet of
this, and aluminum foil for the plates.  In order to avoid the inductance
of a rolled cap, I used foil about 23 inches wide offset from the PP about
1" (so there is a 2" gap from edge of the foil to the edge of the PP).
This way there are 4 layers: foil, PP, foil, PP.  Each of the two foil
pieces is offset the opposite direction so when it's rolled up they stick
out by 2" on opposite sides.  All this aluminum is bunched together with a
hose clamp and clamped to a hefty copper wire.  All of this was put in a
piece of 6" PVC, covered in oil, and vacuummed to remove any trapped air.
Sound like a PITA?  It was!  It turned out to be about 12 or 15 nF if
memory serves, and I'm confident it would hold up to a 15 kV NST just fine.

Hope that helps make a decision!  BTW, I'm guessing your homemade caps
were cracking/smoking/popping because of sharp edges.  There is a much
larger stress (and lots of corona in the air) that will help break down
any dielectric rather quickly from localized stress and heating.  The
metalized polyester caps are no good for TC use as polyester is fairly
lossy at RF, and the matallization lacks in peak current capacity.  The
end connections tend to fail!

Sean Taylor
Urbana, IL


On Tue, 19 Jul 2005 18:59:40 -0500, Tesla list <tesla@xxxxxxxxxx> wrote:

> Original poster: "Wilson Ng" <metalstorm2002@xxxxxxxxxxx>
>
>
>
>   Hello list members.
>
>      I have had bad experience with my capacitors. My first one that
> worked okay with my tesla coil was a jellyroll cap made from plastic
> baggies and metal foil tape. It works well up to about 3 kv. Beyond
> that, there will be smoke and popping sounds. Adding additional baggies
> for the dielectric decreased the capacitance dramatically. Later I
> tried MMCs with a string of 30 200v 2uf metalized polyester capacitors.
> It was just
> a test to see if the setup works. After several seconds, I smelled
> smoke even though the test voltage was only 3kv! I've also tried
> foil wrapped bottle caps but it was messy and one bottle cracked.
> UGGHH!
>
>      After sorting a while through the pupman archives, I was unable
> to find anything other than glass bottle caps, MMCs, and homemade
> plate caps. These caps have downsides:
>
>       MMCs                 - expensive
>       bottle caps          - messy and lossy
>       plastic/plate caps  - a pain in the ass to make and not very
>                                  reliable
>       rolled caps           -   not reliable
>       Microwave cap MMC   - possibility of explosion
>
> My question is, is there another way to make a reliable,
> cheap, and effective cap without breaking my bank?