Re: Air Caps

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

Not if you stack the caps up...

Figure 40 kV for the breakdown voltage required (allowing for edge effects,
etc.) you need a spacing of around 1.5 cm.  A stack 1.5 meters high will
give you 100 plates, dramatically reducing the area requirement.
"Ballparking" the numbers.. C = 8.85 *A/d pF where A in square meters, d in
meters...

Say you need 0.01 uF, i.e. 1E4 pF, you need total plate area of 590 square
meters, spaced 1.5 cm apart. That's only 5.9 square meters (about 2.5
meters (8 feet) on a side) if stacked up as described above.
Sink the sucker in a tank of oil (one of those polyethylene fruit bins
springs to mind (Macrobin, $100 or so, roughly 40x48x30"))..you could move
the plates much closer together (say to 0.5 or 0.25 cm) and the increase in
dielectric constant (figure 2.5 for mineral oil, 4-6 for castor oil) will
also help.  For that matter, how lossy is water as a dielectric at 100kHz?

Oddly, your big losses will probably be: skin effect on the large plate
area, resistive losses in the plates (but, to a first order, this should be
the same as for any other cap using similar plate material), and mechanical
losses from the plates moving under electrostatic forces (I suspect this
last will dominate... There's a lot of force between two plates 1 cm apart
at 40 kV... you'll have made a very large electrostatic speaker...)

Tesla list wrote:
> 
> Original poster: "Allanh by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<allanh-at-starband-dot-net>
> 
> Not a bad idea but given the value of capicitance needed
> and the voltage rating required for TC's, I figure you will need a pair of
> plates about 4 acres.
> 
> allan
> 
> ----- Original Message -----
> From: "Tesla list" <tesla-at-pupman-dot-com>
> To: <tesla-at-pupman-dot-com>
> Sent: Thursday, February 14, 2002 8:24 PM
> Subject: Re: Air Caps
> 
> > Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
> >
> > Hi Cory,
> >
> > Usually the capacitance value of air caps (few hundred pF)is too small to
> > be useful.  Some people put them in oil to increase the value and add
> > dielectric stength.
> >
> > Cheers,
> >
> > Terry
> >
> > At 07:57 PM 2/14/2002 -0800, you wrote:
> > >Hello all,
> > >
> > >Anyone here tried using air caps for their TC?  a
> > >pretty big one would be needed for a large tesla coil
> > >but hey other than that it sounds like a good
> > >thing........ i dunno just wondering, i saw somone's
> > >site who had them on theirs and was jw about their
> > >effectiveness....
> > >
> > >Sincerely,
> > >
> > >Cory Roussel
> > >
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> >
> >