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Re: Polyethylene Capacitors
Tesla List wrote:
>
> >From gowin-at-epic-1.nwscc.sea06.navy.milTue Sep 17 22:37:24 1996
> Date: Tue, 17 Sep 1996 16:50:54 -0500
> From: Dan Gowin <gowin-at-epic-1.nwscc.sea06.navy.mil>
> To: tesla-at-pupman-dot-com
> Subject: Polyethylene Capacitors
>
> All,
> I`m about to build my first set of Polyethylene/Oil
> Capacitors. I've already downloaded the construction tips by
> Richard Quick and I've found them to be very complete. I've
> also purchased some 14"x50' aluminum flashing and 3-(6"x19") PVC
> pipe with caps for containers. The one question I have is,
> why should I special order some 30 mil polyethylene ( stacked
> 3 sheets for 90 mil thickness)? When 15 sheets of 6 mil polyethylene
> will do just the same. And is readily available at a department
> store.
>
> Big Red, HV Capacitors. Burn Baby Burn.
> ITS Member
> D. Gowin
Dan,
You may run into a couple of problems. First, inexpensive LDPE sheeting
may have a fair number of defects and small flecks of debris due to some
of it being made from recycled/scrap material. The second problem is
that trying to stack and roll multiple sheets of this stuff is extremely
difficult! Its slippery and tends to easily go out of alignment. Winding
2 sheets is a more difficult than one. Three is more so... and manually
winding/aligning 6 or 8 sheets is virtually impossible.
Since the outermost sheets will always have a larger average radius/turn
than the inner sheets, all sheets need to slide (lengthwise) relative to
each other while you are winding to prevent any kinks or folds from
forming. However, you don't want them to slide any width-wise, since
they'll go out of co-alignment. Trying to manually keep all the sheets
and the foil equally taut and aligned while rolling becomes a major
excersize in futility!
The concept could work if you constructed a "web" machine which had
seperate feed rolls for the LDPE and foil, each one having a tensioner,
to keep the material aligned and tight while winding. The alternative
would be to use fewer dielectric layers per capacitor module, then
string these in series to get the desired total dielectric strength.
I used 4 mil material on my homemade caps, but went to a flat plate
design after admitting defeat on any multiple-sheet rolled capacitor
construction with this material. Using smaller stacked sheets allowed me
to inspect and reject sheets with any significant defects, and avoided
the problems of trying to roll up many sheets simultaneously. Four
identical low-inductance capacitor "modules", each using 8 thicknesses
of LDPE (32 mils/module) are series connected to provide 128 mils of
total dielectric thickness. These are aligned vertically to permit oil
to wick up between the sheets and displace any air during the break-in
period, and are housed in a Rubbermaid Roughneck trashcan. About 5
gallons of Shell Dial-X fills the unit. Each cap (I made 2) weighs 45
pounds, is 0.011 uF, and should withstand 20 KV RMS (supply) in Tesla
service. They've withstood (knock on wood) any/all abuse for the last 3
months with no problems.
Designing these was fun and educational. HOWEVER(!), after totalling up
the total material cost and time involved in building these beasts, I
will definately go the commercial route the next time around!
Safe coilin (and cappin') to ya! :^)
-- Bert --