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Re: Fair Radio Sales HV cap---cool with liquid N2?
Hello Doug, all cap owners,
I think (aside from the trouble of always having to have a pot of liquid
nitrogen around while firing up your coil) this wonīt work too well. Iīll try
to explain why I think so: The cap (probably having Mylar as its dielectric)
heating comes from the inside and spreads outward. The whole cap is encased in
plastic (read: a good insulator temp wise). So in my opinion no amount of
"outside" cooling will keep the innermost layers cool enough to stay on the
safe side.
Besides, as the material is RF-lossy you will keep adding heat AND you will be
wasting power (being turned into that heat you are feeling) running such a cap
in a Tesla tank circuit.
I could also image the dielectric heating might contribute to what I like to
call "sub-corona-voltage-creepage" along the surface of the dielectric,
eventually leading to the destruction of the cap. I call it sub corona voltage
creepage because you donīt notice anything (no corona) until itīs too late.
The charge on the surface of the dielectricum adds up very slowly and this may
take quite a while to reach dangerous levels (oil WILL NOT prevent this). The
interesting (and being so, it can be very misleading) thing about voltage
creepage along the surface of something is that the path along which flashover
can occur is much, much longer than than the same voltage could jump across in
pure air.
What I mean to say is the voltage needed to jump a certain distance along the
surface of something can be a lot lower when compared to the voltage it would
take to jump this same distance without the surface material being present
(e.g.: in an air gap).
Iīm not trying to say these Fair caps arenīt any good (a lot of coilers seem
to get good results using them), but I think you can make a rolled PE cap
which is just as durable as the Fair Radio Cap for less than $90. If I
remember correctly the Fair cap only has around 10 nF.
A series / parallel arrangement of PE caps will give you safety (keeps the
volts per mil down) and PE caps, even when run dry, do not exhibit heating,
even if cheaply build (i.e: non virgin material or not tightly rolled, etc). I
have done quite a few rolled PE cap experiments and have found (by adding
faults during construction on purpose) that homebrew PE caps are very, very
durable and can take quite a beating before they die and go to high voltage
heaven.
Coiler greets from germany,
Reinhard
Original Poster: Doug Brunner <dabrunner-at-earthlink-dot-net>
I'd like to be able to use one of these caps in my next coil, but I'm
concerned about the dielectric heating. I had the idea of immersing it
in liquid nitrogen. Do you think it would work?
--
--Mr. Postman (Doug Brunner)
<dabrunner-at-earthlink-dot-net>