Re: Re. Home-brew Capacitors

Tesla List wrote:
> Original Poster: Gary Lau  15-Dec-1998 0827 <lau-at-hdecad.ENET.dec-dot-com>
> >Original Poster: "Steve Young" <youngs-at-konnections-dot-com>
> >
> >Hi all,
> >
> >1) It's been a while since posts have been made about proposed home made
> >dry (no oil) caps made by connecting a bunch of them in series to minimize
> >corona and breakdown in the individual sections.  Has anyone achieved good,
> >long life results yet?  If so, many of us would appreciate the particulars.
> >
> >2) It's also bee a while since the list had posts of plans to build caps
> >using stacks of poly bags (e.g. from Associated Bag Company).  Has anyone
> >done it, and if so, what were the results?
> >
> >Bottom line - building caps is a major undertaking.  If people discover
> >better ways to do it, please share.
> >
> >--Steve
> I was one who proposed a dry construction capacitor by building a 20
> series element flat plate unit.  I have purchased 1000 13" x 13" x 2.0
> mil bags from Associated Bag Co for $29.20 plus shipping.  This was
> actually the price for the 1.5 mil thickness, but since they were out of
> that, substituted 2.0 mil (actually 1.875) for the same price.  I plan on
> having three 2-mil thicknesses between plates.   Using 13" wide bags was
> essential so that 12" wide Al foil could be used.  Unfortunately due to
> space constraints in my rig, I have to cut the length down to 10", which
> for the necessary 760 bags, is a LOT of cutting, even on one of those
> gillotine-type paper cutters.  I'm somewhat disappointed in the bags I
> received.  They were packed in a box smaller than 13" square and
> consequently were folded and creased.  Also, the bottom of the bag is
> heat-crimped and is a bit ruffled, making the stackup uneven.  I'm
> presently trying to find a good way to build each of the 20 units as a
> modular assembly that won't shift around as it's moved and bolted into
> the 20-unit stack.  I also plan to incorporate 6Meg balancing resistors
> in parallel with each element, and these are complicating the assembly.
> One thing is for sure, it will be a lot more work than a rolled poly cap.
> I promise to post results, good or bad, as they occur.
> Regards, Gary Lau
> Waltham, MA USA


As long as you construct the capacitor modules in an identical fashion,
the balancing resistors should not be necessary. They would be necessary
if you were operating the chain at DC for an extended period of time. In
this case, the long RC time constants from the various individual
leakage resistances and module capacitances will cause unequal
distribution of module voltages, overstressing of the "better" (less
leaky) sections. This long-term "transient response" of the various RC's
in the chain typically equires minutes to develop. However, with PP
dielectric, reasonably sized module capacitance, you should not begin to
see this effect in a TC tank capacitor. However, it COULD be a problem
for the DC energy storage cap in a DC resonant charging system...

-- Bert --