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Re: My 8" secondary
There was a trick to building that cap which effectively made
creepage distance between the plates head towards infinity:
> Original Poster: RWB355-at-aol-dot-com=20
<snip>
> Malcom told me he had designed a 100nF cap for his 9.9" coil:=20
> Malcom wrote:
> "Details of construction are online on Jim Fosse's homepage. There are=20
> (from memory) 12 plates, each 1 metre long. Dielectric between the=20
> plates consists of 2 layers of 0.25mm polyethylene. A 12 metre long=20
> roll of cooking foil just does the job."
>
> I would be interested in having a look at Jim=B4s site. Anyone know his w=
> eb URL?
> Jim: Are you out there?=20
>
> The only thing which "bothers" me is the very small (0.5mm=3D19.68 mils)
> dielectric distance. I was thinking of somewhere around 48-55 mils for my
> 7.5kV input to play it safe. However, I don=B4t know what voltage Malcom=
> was
> using to power his coil.
About 8kV peak. Here is the basic plate/insulation schema viewed end-
on before rolling the cap:
- = plastic
* = foil
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| ************************************
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| ************************************
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^
connect all foils ditto
It is a pig to roll. You really need three people - two to hold
the plastic and plates and one on top to roll. The plastic tries to
skew sideways due to differences in ROC as rolling proceeds. For this
reason, it is far easier to make a cap using 12 short plates than two
long ones. You end up with the odd fold in the roll but it doesn't
matter as far as voltage rating is concerned. Remembering that it is
dry-fired, there is going to be air trapped in it anyway so runtime
is limited. For the voltages I use at 100BPS, I get around 1/2 hour
runtime when used intermittently (two runs of about 30s to a minute
each) with complete cooling between lots of two runs before the
plastic finally gives up in disgust. Failure is soft - no explosions,
just a bit of a flare up in the weak spot with greatly diminished
coil output. I have recovered what is left of the plastic and made
smaller caps for use in Marx banks.
Malcolm
<snip>