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Re: Museum Coil Revisited
Chuck Writes:
<big Snip>
<< Tom Brown, V.P. at Plastics Caps called me this
morning and went over my request for a quote. Real nice of them, but
the quote came in for a .05mfd-at-15,000 VAC when I asked for 30,000 VAC.
Price was $554 which is about what I expected, but Condenser Products
had their .05mfd.-at-20,000VAC priced at $450.00 last summer. >>
After discussing design parameters at some length with Beau Meskan at Plastic
cap
(who it turns out happens to be the owner of the company) It turns out that
he believes that the cylindrical cap design that Condenser Products uses is a
flawed design approach. Heres why: cylinder caps are (because of the nature
of the case geometry) very rigid, they don't flex. Most high performance
caps use a Silicone oil bath (which is the best from a dielectric
standpoint).
Unfortunately, the silicone has a rather high coefficient of expansion. This
means that the oil expands quite a bit if any heating occurs. This in turn
requires an air bubble in cylindrical cap case designs (a la Condenser
Products). This in turn introduces additional potential corona and heating
problems, particularly if the cap is mounted and used horizontally, which can
cause the cap to fail prematurely.
The solution to this is to use a rectangular case design (a la Plastic Cap),
This design does not require an air bubble of any sort since the wide flat
walls of the rectangular case are designed to flex with any heating should
it occur. In fact the rectangular cap case style that Plastic Cap uses comes
with the walls slightly concave (due to the high vacuum they pull during
manufacture). This concavity also increases the amount of flexure that can
occurs and increases the operating temperature range of the cap even further.
The rectangular cap case design, however, does result in slightly higher
labor costs for the cap.
After discussing the merits of various cap case designs, I am convinced that
the rectangular cap design is clearly superior to the cylindrical style and
is well worth the extra money.
-DavidF-