Re: Poly surprise

From: 	Adam[SMTP:absmith-at-tiac-dot-net]
Sent: 	Monday, December 08, 1997 11:14 PM
To: 	Tesla List
Subject: 	Re: Poly surprise

>Why? Because the poly apparenly absorbed some of the oil, and expanded.
>Less than 5%, but for 10", that amounts to 1/2" exapansion, more than
>enough to be a problem. I've reworked the tiedowns, but thought I would
>warn others, as I have not seen this mentioned in any of the
>documentaion for capacitor constrution.  I was using Diala-AX for the

This happened to my cap too, causing my first tight-fitting 0.25" 
plexiglas box to bow and leak Exxon Univolt N61 through the corners.  The 
dielectric in my cap started out as 19.25" x 13.75" sheets of 30mil poly. 
 The sheets are now about 19.625" x 14.125".  This is less expansion than 
you saw, but it was enough to ruin my first cap container.  I also 
noticed that the oil absorption caused considerable differential 
swelling, and the sheets are no longer perfectly flat.  They have a wavy 
kind of rippling appearance that is certainly non-ideal.  Electrically 
the properties do not seem to have changed, and I pressed the cap (very 
hard) between two boards overnight and strapped it together with three 
bands of 3" wide PVC electrical tape before putting it in its new 0.5" 
plexi housing.  It's fine now, no leaks :-)

On the down side, my cap is huge for a 0.02 uF unit- 25" High, 16" wide 
and 5.5" thick overall, and too awkward and heavy to safely move without 
a hand truck.  The finished product cost me about $210 in materials and I 
spent another $67 in materials for the first two housings that leaked.  
0.02uF for $280 is not really a great bargain.  On the plus side, it's 
clear and very pretty, fully repairable, and it works wonderfully!!!  I 
chose to hand build this one capacitor mostly to get the experience, and 
will definitely choose commercial caps in the future.


Adam Smith
Epoch, Inc. Digital Music Project

www.tiac-dot-net/users/absmith/                 MP3 Demo Tracks Now Available!