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Re: Caps: Stacked vs Rolled

Subject: 
            Re: Caps: Stacked vs Rolled
       Date: 
            Sun, 13 Apr 1997 19:07:42 -0700
       From: 
            Bert Hickman <bert.hickman-at-aquila-dot-com>
Organization: 
            Stoneridge Engineering
         To: 
            Tesla List <tesla-at-pupman-dot-com>

> > Chris,
> >
> > Flat plate caps work great and if made properly have very low
> > inductance. However, you can save some money by skipping the copper
> > plates and using either aluminum flashing or heavy-duty aluminum foil.
> > There will be NO measurable performance difference but you'll see a
> > sizable cost savings.
> >
> > Your total dielectric thickness should be at least 90 mils to 120 mils
> > (for a 15 KV source). By using LDPE, your cap will stay cool even if you
> > ran it continuously. Don't clamp it too hard - the oil needs to
> > completely displace any trapped air - mounting the cap so that the
> > plates are aligned vertically will help this process. I'd also skip
> > using paraffin - make a top out of plexiglas and screw it down using a
> > buna-n or neoprene gasket.
> 
> Is it REALLY a must to submerse all this in oil? I have made some
> experimental caps without oil, and they seem to work well, I think
> soaking them in oil will not change the opperation, but prevent presures
> from being created between stacks, and maybe to remove the heat better.
> 
> Also, what exactly is this mil measurement?
> 
> And does anyone here notice that trash bags give a µm measurement? What
> is this for? Is it not what I'm thinking?
> 
> --


Jeremy,

Immersion in oil is essential to prevent corona from destroying your
dielectric. Corona is the electrical breakdown of air under electrical
stress. If you operate your capacitor in a darkenned room, you will see
the glow and little fingerlets of corona coming off the edges of your
capacitor's plates. Running for any length of time under these
conditions WILL blow your cap if you're using a thermoplastic (Low
Density PolyEthylene or Polypropylene for example) dielectric. The oil
displaces the air, stopping the corona. Corona is highly energetic and
hot - it will easily track and melt your dielectric. It's also contains
high concentrations of ozone which will chemically attack the dielectric
with time. 

A "mils" is 0.001" - so 120 mils is slightly less than 1/8" thick.
Trashbags give thickness as a relative measure of physical strength.
Some vendors quote this thickness using the metric system. However,
colored trashbags contain materials which degrade their dielectric
performance. Stick with clear LDPE film (like dropcloth material) or
heavier LDPE sheet material from a plastics supplier. Another potential
alternative (for a flat plate cap) is polypropylene folders (used for
securing 8 1/2" x 11" paper sheets in a binder) that you can pick up at
an office supply store. This material is typically 3 mils thick. 

Safe cappin' to ya!

-- Bert H --