Re: Plate Caps of PCB

Subject:      Re: Plate Caps of PCB
       Date:  Fri, 18 Apr 1997 22:15:41 -0700
       From:  Bert Hickman <bert.hickman-at-aquila-dot-com>
Organization: Stoneridge Engineering
         To:  Tesla List <tesla-at-pupman-dot-com>

> >I just made a stacked cap with 16oz. copper (measured thickness is 25> mils) and 6 plates
> >each with 206 sq. inches of area. 120 mil> ployethylene dielectric and light mineral oil as
> >the drowning agent.
> Hi all,
> 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
> single-sided board, you
> could double the voltage rating.
> I won't bore you here with the construction details, but I've got it
> sussed. So, I figure
> I'm not the first person to have thought of this - I wonder if anyone
> here has actually made
> one ?
> 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
> (2.75") dia. clear
> acrylic tube, 3mm (~ 1/8") wall.  A 1.83m (6ft) length was a mere $55.00
> !  This is for my
> _first_ coil. A 2m length 0f 200mm (8") dia. 5mm (1/4")wall, inc tax, is
> a tad under $400
> so I decided against it for the 'big coil'
> What I mean, though, is for stacked plate caps.  All the talk is about
> 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.
> 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.
> But mainly I'm wondering about PCB caps - any comments ?
> Thanx,
> Phil Chalk.


The best dielectric should have the right combination of high breakdown
voltage, low dissipation factor, good solvent resistance, and low cost.
The GOOD news is that, for coilers at least, the best choices are Low
Density PolyEthylene (LDPE), High Density PolyEthylene (HDPE), or
PolyPropylene (PP), all of which are comparatively inexpensive.
PolyStyrene (PS) doesn't have the solvent resistance, mylar (PET),
Plexiglas/acrylic (PMMA), PolyVinyl Chloride (PVC), Cellulose Acetate
Butyrate (CAB), polyamide (nylon), and PolyCarbonate (PC) all have a
significantly higher dissipation factor. Teflon has low losses, but it's
actually somewhat porous so that its dielectric standoff voltage is not
as good as some of the other plastics - plus it's _quite_ expensive!
Epoxy-glass printed wiring boards also has a relatively high dissipation
factor, and the dielectric breakdown strength for this material in Tesla
Coil use will be MUCH lower. You COULD make caps from all these other
materials... they just wouldn't work as well as caps made from LDPE/HDPE
or PP.

Silicone oil, depending on the type, may perform significantly better
than mineral oil. Refrigerator compressor oil, for example is made to
have high dielectric strength, and will work very well....at a
significantly higher price. I'm not sure how well brake fluid would
work, however.

Safe cappin' to you!

-- Bert H --