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Re: Fw: Fair Radio Sales HV capacitor
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 --