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Subject:  Re: Plate Caps of PCB
  Date:   Fri, 18 Apr 1997 21:10:43 +0300 (EET DST)
  From:   Kristian Ukkonen <kukkonen-at-cc.hut.fi>
    To:   Tesla List <tesla-at-pupman-dot-com>


On Thu, 17 Apr 1997, Tesla List wrote:

>    From:  Phil Chalk <philoc-at-ozemail-dot-com.au>
> 
> I've been thinking of making a stacked cap from fibreglass PCB.  I read
> it has a Dielectric
> Constant (K) of about 5 (5.2), and a puncture voltage of 700V/mil.  The
> stuff I use is about
> 1.6mm thick, or about 63 mils, for 44kV.  This may be optimistic, but it
> could probably be
> conservatively rated around 30kV or so. Using back-to-back pairs of

"conservatively" :)

Seriously, glass and epoxy&fiberglass are lossy at the frequencies
of interest so the cap will heat (read: losses) and after getting
hot enough, get destroyed.

It should WORK though if you are careful and DO oil-immerse the
stacked cap. Corona is the killer of caps.

AND those 63mil->44kV are DC RATINGS most certainly, NOT pulsed/RF..
SO I'd _guess_ something like 10kV for 1.6mm to be carefull..

Just to compare : LDPE is usually rated for 20kV/mm in those same
tables that you read for that rating above. However, something
like 7kV/mm is more realistic for Tesla-work. Oil-immersed.

> Also I'd like someone to tell me, if cost were no object (which it is !)
> which would be
> THE BEST dielectric material for TC work ?  I have just bought some 69mm

No di-electric material at all would propably be the best.

(read: vacuum caps)

> polyethylene for its
> low dielectric loss.  Would any of: mylar, polycarbonate, polypropylene,
> polystyrene, PTFE
> (Teflon), acrylic, etc be any improvement - albeit costly ?  The list I
> saw quoted the
> puncture voltage of mylar at 7500V/mil.  If this is not a typo, that
> makes it look pretty
> attractive for TC caps.

Mylar is LOSSY so it heats..  Same applies for polycarbonate, -styrene,
acrylic etc.. Look at the dissipation factor for more info on the
details.

Polypropylene works fine. I use that as LDPE is not available as
thick foil here in Finland.

Teflon should work fine as well. It does, however, have a certain
amount of holes per cubic meter (always) so it has to be stacked
with a number of thinner sheets. The specs sheets for teflon foils
actually state this (max holes/m*m) parameter.

> Has anyone ever tried silicone oil in caps ?  Or know of its HV
> properties ?  It's used in
> e.g. high-quality brake-fluids, as I believe it has very good
> anti-hygroscopic properties.

They do use "silicone oils" in caps - but that term applies for
lots of grades, I'd guess.. I'm certain they don't use brake-
fluids for cap insulation commercially :)

> But mainly I'm wondering about PCB caps - any comments ?

It will work - but a LDPE/PP cap will work better.

I hope this answers some questions,

  Kristian Ukkonen.

---------------------------------------------------------------------------
| Kristian Ukkonen       |  Do as thou wilt shall be the whole of the
law |
| kukkonen-at-alpha.hut.fi  |_____  Chance favours the prepared mind 
|-------
| http://www.hut.fi/~kukkonen |  Fear is the mind-killer  |---------
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