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Re: [TCML] homemade pulse cap

Hi Scott,

I didn't see any replies to your question, so I'll take a stab at it...

The strategy of using two (or more) thinner layers of dielectric between plates has historically been used for decades by commercial and homemade capacitor builders for many of the reasons you mentioned.

Multiple dielectric layers are not inherently self-healing. A defect in one dielectric layer is protected by the remaining good layer(s), making the cap considerably more robust than one made using a single layer. However, if the remaining layer(s) are punctured, the entire capacitor catastrophically fails.

Problems of corona in voids between dielectric layers, or between metal and dielectric layers, were initially resolved by inserting a thin Kraft paper layer in film-foil capacitors. The Kraft paper layer was saturated with a dielectric fluid having a comparatively high dielectric constant (~3 - 7). Capacitors using this approach were sometimes called "soggy film" caps.

Most commercial capacitor manufacturers eventually migrated to a "hazy film" dielectric where one or both surfaces were mechanically roughened. The roughened surfaces were more easily wetted by dielectric fluid, and hazy film eventually eliminated the need for Kraft paper in all but the toughest energy-discharge applications.

Gaps between the foil plates and dielectric were eliminated by evaporating a thin layer of metal directly onto one or both surfaces of the dielectric. And, by carefully controlling metallization thickness (and sometimes adding thin-film fusible isolating links), dielectric failures could be isolated hundreds, or even thousands of times, resulting in graceful degradation rather than sudden death. "Self-healing" technology also eliminated the need for multiple dielectric layers per capacitor element, and permitted manufacturers to more highly stress the dielectric system. This revolutionized HV film capacitor technology, allowing manufacturers to build caps with much higher energy-density.

It turns out that your idea of stacking a series of "floating" conductive and dielectric layers is actually being used (albeit in a slightly different configuration) in the very snubber caps that most folks use for MMC's. CDC effectively stacks two (or more) identical single-layer self-healing capacitor elements in series (using a clever metallization pattern and proprietary construction techniques) to create reliable "dry" film HV capacitors.

Conceptually, your idea could work. However, almost nothing "sticks" very well to polyethylene (PE) or polypropylene (PP) film... unless the film has previously been corona-treated. Simply painting an adherent conductive layer onto untreated PP or PE film will not work. Your approach might work if you can locate some metalized PP capacitor film for a reasonable price. Metalized PP film does appear every now and then on the surplus market, but it's usually not cheap. You would also need to make an appropriate multilayer winder and may also need to construct a filtered clean air work environment. End termination could also be a problem unless you use foil for inner and outermost layers. I also suspect you'll find that MMC caps are really not all that expensive in comparison... :^)

Good luck,

Bert Hickman
Stoneridge Engineering
World's source for "Captured Lightning" Lichtenberg Figure sculptures,
magnetically "shrunken" coins, and scarce/out of print technical books

Scott Bogard wrote:
Hey Guys,
      Ok, I know this is a dead horse, but since I already have a vacuum
setup, and MMCs for large coils are so pricey I feel inclined to look into
it.  Here is my idea, corona is the number one killer of plastic caps yes?
In addition, multiple thin layers helps because it gives a self healing
effect if you puncture one layer, and it reduces corona, because the
voltage drop across each layer is small, theoretically can be made a few
hundred volts.  This is the idea I've been toying with the past 5 mins, why
not make a cap out of really thin (like one mil) plastic, in many layers
wiht just the outside layers with solid metal plates?  The obvious answer
why you can't do that, is at each interface for a homemade cap there will
be a tiny gap and your capacitance will likely be very low...  So to combat
this, why not paint each inside layer, with graphite based or other
conductive paint, this way even though it is slightly higher resistance, it
is better than a gap between layers, it only needs to touch in a few spots
per layer to make conductivity even, and should all but eliminate corona.
One can then do standard precautions like filling with oil and vacuuming.
Any thoughts?

Scott Bogard.
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