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Re: plastic capacitors???

Subject: 
        Re: plastic capacitors???
  Date: 
        Sun, 30 Mar 1997 15:53:55 +0300 (EET DST)
  From: 
        Kristian Ukkonen <kukkonen-at-cc.hut.fi>
    To: 
        Tesla List <tesla-at-pupman-dot-com>


On Sat, 29 Mar 1997, Tesla List wrote:

> these are orange oil filled cylinders with screw terminals at each end. 
> bought one rated 14KV, .001 microfarads to play with. The vendor has a
> 
> 1. are these useful for tesla coil work

That's impossible to say with the information you provided. The EASY
method is to test the cap with a TC - it works fine or not. The next
easiest (IMHO) is trying to get information about the intended use
of the capasitor (data sheet, contact manufacturer etc.). Perhaps
measuring ESR, ESL etc. comes next but that requires equipment..
Here are some general guidelines I do use:

One good hint about the use are the terminals. If they are just wimpy
wires it _won't_ work but some LARGE screw terminals (or bolts) _might_
work.

Physical size of the capasitor may help as well. If the capasitor has
"good"  voltage rating, some capasitance and STILL is small in physical
size then it's propably not of use in TC. Why? Because the di-electric
material used is some lossy one with high di-electric constant - like
barium titanate. The good di-electrics (PP,PTFE etc.) have di-electric
constant near 2.. This is a bit of a simplification but usually
volymetrically inefficient objects rule in 'coiling.

The capasitance value of the capasitor is a hint too. If it's LARGE
(like
uF-value or larger instead of nF..) then it's usually a
filter-capasitor.

These "facts" are not universal. There are pulse-caps with large
capasitance and good voltage rating (good for rail-guns etc.) etc. 

Finding a DECENT impedance meter should help too.. Like the one I saw in
one place : it measures L,C,R,ESR,ESL etc. for a frequency band of mHz
to
GHz.. Cost? About 100k USD.. :( They use it for impedance spectrometry.
Perhaps I can some day talk the person operating it to measure my
coiling
caps.. >:) 

> metal cases - I figured on putting 6 in series to use with the small

Perhaps you want to be certain those don't get too hot - they might
EXPLODE because of internal pressure rising.. There have been some
horror-stories posted to this list..

> 4. I found a guy who said he had a .8 microfard 45,000 volt capacitor -
> I 
> didn't ask a price.

Filter-caps. Like those 1uF 80kV caps I salvaged from an old x-ray tube
power-supply recently - those weight about 40kg EACH..  Will serve fine
as
a filter-cap of a DC TC, perhaps.. :) 

'Coiling,

    Kristian Ukkonen.

---------------------------------------------------------------------------
| Kristian Ukkonen       |  Do as thou wilt shall be the whole of the
law |
| kukkonen-at-alpha.hut.fi  |_____  Chance favours the prepared mind 
|-------
| http://www.hut.fi/~kukkonen |  Fear is the mind-killer  |---------
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