Re: rolled caps
From: Geoffrey Schecht[SMTP:geoffs-at-onr-dot-com]
Sent: Saturday, September 06, 1997 9:46 AM
To: Tesla List
Subject: Re: rolled caps
Answers to questions:
1.) Having a PE-al-PE-al cap will give you about about half of the
capacitance of a PE-al-PE-PE-al cap since capacitance is directly inversely
to plate separation (I think that I understood the question...maybe you
should repost with a layer scheme like the one I just used).
2.) LDPE can be contaminated by any fatty acid such as you find in soap.
One of the cleanest solvents that you can buy is, believe it or not,
Everclear 190 proof grain alcohol. We used to use it to clean nuclear
detector circuits which couldn't tolerate leakages even in the low nA
range. It was one of the only things that would properly decontaminate
teflon standoffs after normal flux removers were used (plus it was great
fun to have around after-hours). Having dust buried in the dielectric-al
interface probably doesn't hurt anything; I'd think that leakage over and
around the dielectric area is what could invite a flashover betwen plates,
assuming all other things are properly spaced.
3.) Polyethylene has a breakdown strength of 1200V/mil at 60 Hz and 25 deg.
C. Theoretically, you should be good for 48kV but I think that I'd derate
that by 50% since you probably don't have critically-controlled thickness
in your poly sheeting and it could have thin spots or possibly some voids.
Sounds like you're pretty safe at 15kV but I'd give it a once-over under
polarized light to check for thin areas (that might work, it'll at least
find stress areas).
I'd recommend wearing clean latex or nitrile gloves after you clean the PE
to avoid recontamination from skin oils and only work on clean,
decontaminated surfaces until the cap is rolled up and secured/sealed.
> From: Mad Coiler[SMTP:tesla_coiler-at-hotmail-dot-com]
> Sent: Friday, September 05, 1997 4:16 PM
> To: tesla-at-pupman-dot-com
> Subject: rolled caps
> To all that have made rolled caps,
> I have just recieved my order of 30 mill LDPE sheets. I plan on making
> the dielectric three sheets thick. This would be 90 mills, and I hopped
> this would be enough for 12000V.
> Question (1): Should you put aluminum flashing between each sheet of
> the dielectric or just set three sheets together without anything
> Question (2): I assume the plastic should be clean. Is LDPE sensitive
> to soap, or should a damp cloth work fine to get the dust off?
> Question (3): Will three layers of 30 mill also work for 15kV or do i
> need more?
> Mad Coiler
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