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Re: Fw: Fair Radio Sales HV capacitor
to: Gary, Bert, et al
Could the case possibly be actually PCI instead of FCI ---- Plastic
Capacitors, Inc. in Chicago?
DR.RESONANCE-at-next-wave-dot-net
----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Fw: Fair Radio Sales HV capacitor
> Date: Monday, December 28, 1998 8:44 AM
>
> Original Poster: Bert Hickman <bert.hickman-at-aquila-dot-com>
>
> Tesla List wrote:
> >
> > Original Poster: Gary Lau 26-Dec-1998 0943 <lau-at-hdecad.ENET.dec-dot-com>
> >
> > > > I ran across a HV cap advertised in Fair Radio's latest catalog
> page 32,
> > > > 0.01 mfd at 100KV, wonder if anyone had had any experience with
> them in
> > > > Tesla service? They've lowered the price to $87.50. I imagine
it
> could
> > > > work in, say, a 15kV/30 or 60 mA neon transformer type coil.
Sure
> beats
> > > > the mess of "rolling yer own" and probably less money in the
bargain.
> > > >
> > > > What's the consensus?
> > > >
> > > > Stephen, KJ6LH
> >
> > >Stephen and all,
> > >
> > >There's good news and bad news with these caps. These are
> > >Mylar-dielecric DC-rated caps, and they WILL heat up under Tesla Coil
> > >use. As long as you keep run-times relatively short, and allow
> > >internally generated heat within the capacitor rolls to distribute
> > >before overheating occurs, they will work. Spark output will not be as
> > >"hot" as with polypropylene or polyethylene caps, and the caps cannot
be
> > >operated for extended periods without risking melting & punch-through
of
> > >the dielectric. But... they are relatively inexpensive alternatives to
> > >true pulse caps, and they will save you the time and effort of rolling
> > >your own.
> > >
> > >-- Bert --
> >
> > Hi Bert:
> >
> > I've tried my best to learn more on the construction of these caps,
> > scouring the 'net for a company named "FCI" that's stamped on these
caps,
> > but could find nothing. This is the first suggestion I've seen that
they
> > are mylar units. Have you found a source of info, or is this infered
> > from their self-heating behavior?
> >
> > I also suspected that their dielectric might be compromising
performance
> > so I built some extended-foil rolled poly/oil units, coincidently
measuring
> > in at precisely the same capacitance value. This made for an easy
> > performance comparison since no re-tuning would be necessary. Before
the
> > rolled cap's died, they delivered identical performance.
> >
> > Regards, Gary Lau
> > Waltham, MA USA
>
> Gary and all,
>
> It's really just an educated guess, Gary! High voltage film capacitors
> tend to use polypropylene or Mylar as the dielectric, and older caps may
> even use dielectric-fluid-soaked kraft paper. Based on the manufacturing
> date (per Fair Radio, November 1983), the plastic case, and the
> relatively small physical size, it's most unlikely that these are kraft
> paper caps. The relatively small (#12) stud size suggests that this cap
> is probably not rated for pulse duty, and is instead a DC cap. Fair
> Radio calls them HV DC capacitors in their ads, and Mylar caps are used
> quite extensively for high voltage DC/filter caps. But, by far the most
> convincing argument is the relatively rapid heating under TC use - this
> very convincingly suggests that they are Mylar dielectric capacitors.
> The other (but remote) possibility is that you are running with
> relatively high tank currents and are encountering ohmic heating of the
> interconnects.
>
> Now if you REALLY want to non-destructively identify your capacitor's
> dielectric, you can do this by measuring it's dielectric loss at RF
> frequencies. This will tell you whether the dielectric is Mylar (loss
> factor of 0.005 - 0.016) or Polypropylene (<0.0002). Determining this
> with any precision requires having access to calibrated laboratory
> equipment, using a specifically-configured RF impedance bridge (called a
> Schering bridge). The measurement technique is described in "Dielectic
> Materials and Applications" by Arthur Von Hippel. However, I also
> suspect that you can use the much simpler "finger test" - measuring case
> temperature rise after relatively brief running times - to determine
> virtually the same thing!
>
> Now it is interesting that you didn't see any performance difference
> between your rolled PP/oil and this cap! Is it possible that there were
> other significant system losses that masked the loss from the commercial
> cap? For example, could your home-brew cap have had substantial internal
> corona losses? The latter may be why the homebrew unit subsequently
> failed.
>
> Safe coilin' to you, Gary!
>
> -- Bert --
>