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Re: Doorknob caps
I dunno if anyone else has already answered this, but here goes.
Hello, John.
I've read that the ceramic dielectric used in most doorknob caps and
ceramic caps iin general will always drift acording to temperature. They
are also rather lossy for RF, so when you used them in your TC, they
heated up, changed capacitance, and the system went out of tune.
That's one reason why ceramic caps are not recommended as candidates for
TC tank service.
Grayson Dietrich
http://www.electrophile.8m-dot-com
On Sun, 02 Jan 2000 18:17:49 -0700 Tesla List <tesla-at-pupman-dot-com> writes:
> Original Poster: John Williams <jwilliams-at-edm-dot-net>
>
> Hi,
> One other thing you might want to do with doorknob
> caps is check to see if the capacitance doesn't drift in use. A
> friend and I had a pile of such caps and they seemed to work
> just fine for a couple of minutes and then the system they
> were in started to behave like a high value resistor had been
> introduced in the transmission line from the cap to the primary.
> The secondary behaved as though it were oscillating in several
> different modes and arcing to itself all the way up and down
> its' length.
> It turned out that several of the caps were changing
> capacitance under load, by as much as 40%. When they cooled
> down they recovered the original value.
> Perhaps they were leaking? Developing high resistance
> paths internally that faded when they cooled. I don't know.
> I haven't seen anything like it before or since.
> They are old and out of a pulse discharge apparatus. I
> doubt you'd want to use dc only rated caps in such a critter
> considering the wave form that such a beast produces.
>
> John
>
>
>
>
>
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