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Re: Capacitor check (fwd)



---------- Forwarded message ----------
Date: Fri, 24 Apr 1998 08:48:40 +1200
From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Capacitor check (fwd)

Hi Sulaiman,
             I think you might be headed for grief if you try and 
pack your voltage ratings into a single unit.

 > ---------- Forwarded message ----------
> Date: Thu, 23 Apr 1998 14:48:27 MYT
> From: Sulaiman Abdullah <sulabd-at-hotmail-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Capacitor check
> 
> Hello all,
> I'm about to assemble my flat-plate capacitor, almost all materials
> ready and waiting for your feedback!
> My main question is regarding tracking-length;
> I designed the capacitor (PE and Aluminium) to have 1" space to
> the 'edges' and 2" to the 'ends'.

If it's good enough to have 2" to the ends, why not the sides?  I 
don't think 1" is anywhere near good enough but it isn't going to be 
your #1 problem....

> I certainly expect 30 kV peak, and I'd value all your opinions
> as to whether this is enough allowance, and would you care to
> 'guestimate' what clearance would produce a RELIABLE capacitor
> for 50 kV peak. Assume I'll clean all parts fairly well and
> vacuum out most air-bubbles.

ALL air bubbles. If you are talking about anything other than DC, 
forget it. The real problem with designing caps for TC work is the
reversals they suffer (typically 90% for coupling k around 0.1). 
Things move and rub inside caps and additionally, the oil gets 
chemically altered even if the plastic doesn't. The pros build their 
caps with many large capacitance sections in series. This not only 
keeps voltage across each unit to a reasonable value but also takes 
advantage of the phenomenon that a thin film has a higher Volts/mil 
rating than a thick one.

 > I asked this question a while ago in a different way, I'm asking
> again because I REALLY don't want to waste a significant ammount
> of my hard-earned cash!

My advice: build four of 4x the required value, match them well and 
run them in series. A shunt resistance with appropriate voltage 
rating and a value which permits a least as much current as you get 
leakage in a single cap to flow should ensure voltages are roughly 
equal across each cap. Best of luck.

Malcolm