Re: museum capacitors?

Tesla List wrote:
> Original Poster: corr-at-enid-dot-com
> With all this talk about capacitors I began to wonder :
> What do Tesla Coils used in museums use as capacitors.
> I have heard something about some of them have been there
> for thirty or fourty years.  Do the caps in them ever fail at all?
> Or are they just replaced every once in awhile, and if so, how
> long is that interval.

Anything I build for use by someone else (as opposed to tinkering in my
shop) is generously overdesigned. With commercial components, with known
characteristics, this is somewhat easier (you don't need to have such a
large design margin).

At home or in the lab, though, you tend to push the system ever further,
until something breaks. You've got to evaluate the satisfaction from
tweaking it just a little bit more against the cost of replacing the
broken parts. 

> Do they know something about the construction of caps that
> we don't?
Commercial mfrs of capacitors don't necessarily have more knowledge than
we do, they just are able to (and do) apply that knowledge. The
reliability of most electronic devices is directly influenced by the
care, cleanliness, and consistency in construction. When you are making
thousands of capacitors, you can afford to invest in specialized
equipment to carefully control the construction: winders that maintain
perfect tension, etc. Not only that, but a commercial manufacturer makes
more caps than they sell, so that they can test some to destruction,
etc. to get a better handle on reliability and rating. As a result, you
can sort of "trust" the ratings on a commercial cap. Very few hobby
constructors are going to go to this sort of trouble, instead building
their caps quickly and cheaply, with a bit of overdesign, and accepting
the inevitable failures.  It is a tradeoff between dollars and/or time
against reliability.
> Jeff Corr
> Oklahoma Tesla Coil Builders Assoc.
> corr-at-enid-dot-com

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