Re: Bubbles

Subject:      Re: Bubbles
       Date:  Fri, 13 Jun 1997 07:57:38 +1200
       From:  Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Organization: Wellington Polytechnic, NZ
         To:  tesla-at-pupman-dot-com

Hi Viv,

> I have read many references in this list about failure of capacitors due
> to
> air bubbles.  I am currently making a flat plate capacitor of 0.015uF. 
> I'm
> using Polystyrene Sheet (80 mil thick), and sticking aluminium foil to
> it
> with insulating varnish.
> Because I know I have air bubbles what can I
> expect the failure mode to be?  I expect the breakdown voltage to be
> about
> 40Kv so should be ok with by 10KV 50ma Neon transformer.

Speaking from experience with run-in-air LDPE rolled caps, main thing 
is heating at any favourable corona spots (foil edges) wnich seems
to actually "crack" or decompose the plastic. Shortly before the caps 
that I described in that article fail, the sheeting inside them is 
noticeably oily and has a strange pong to it. Appears the plastic 
degrades to the point where a spark eventually punches through.

> Also are their any other coilers in the UK,  I don't see much UK
> correspondance? 

I have a good friend in the UK (Richard Craven) who has just got 
himself set up with proper Web access and says he will rejoin the 
list soon.