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Re: More newbie questions





---------- Forwarded message ----------
Date: Thu, 16 Oct 1997 20:01:00 -0400
From: Thomas McGahee <tom_mcgahee-at-sigmais-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Cc: ceg4760m-at-cs.wright.edu
Subject: Re: More newbie questions  



----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: More newbie questions  
> Date: Thursday, October 16, 1997 5:24 PM
> 
> 
> 
> ---------- Forwarded message ----------
> Date: Thu, 16 Oct 1997 16:34:52 -0400 (EDT)
> From: student_of_agoshtas <ceg4760m-at-cs.wright.edu>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: More newbie questions 
> 
> What is a good source for thick polyethylene sheet for caps?  The thickest
> stuff I can find at the hardware store is 6 mil, and with a 30,000V cap
> calling for around 75-90 mil of dielectric, that is a lot of layers!  The
> stuff I've found is intended for sealing windows during cold weather and
> as a light-duty tarp.  Is there a common application for stuff in the
> 20-40 mil range?  A local plastic supplier wants to charge $0.25/sq. foot
> for 15 mil PE.  This strikes me as being way to pricey.
> 
> Thanks for all the advice so far,
> Aric

Aric,
I pay between $25 and $35 for a 4'x8' sheet of 60 mil poly. That is four times
the thickness of 15 mil poly, and comes to about $1 per square foot on AVERAGE,
so the $0.25 per square foot for 15 mil poly is not really all that bad.
Actually, 15 mil poly is a great size to use!!! Instead of trying to build a
cap with 90 mils of insulation, build 4 15 mil caps and connect them in series.
Even though the total will only be 60 mil, it will have a breakdown equivalent
BETTER than 75 mils. It so happens that thinner dielectric sheets have a LARGER
breakdown voltage PER MIL that single thick sheets.

Use heavy duty aluminum foil and build flat plate or rolled caps. My personal
preference is flat caps, as they make better pulse capacitors due to their
lower inductance. But the bottom line is that with the same amount of poly you
can usually get about 20% more total capacitance with a rolled cap using the
same quantity of poly, since you don't have the "end loss" except at the
extreme ends. With a flat plate cap you have "end loss" at each and every poly
plate. And don't forget to use transformer oil and be vigilant concerning
bubble removal.

Hope this helps.
Fr. Tom McGahee