Re: Hollow tube PP capacitors

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Original poster: "R.E.Burnett by way of Terry Fritz <twftesla-at-uswest-dot-net>" <R.E.Burnett-at-newcastle.ac.uk>


Hi Terry,

I recently did a "post-mortem" on a Polypropylene PFC cap.  The foil &
dielectric were rolled on a 4mm hollow metal tube.  It was made by
Cambridge Capacitors (UK) in 1979,  so it seems some capacitors are
already made this way.  I never thought much about it until now.

BTW,  it was one of the foil-end connections that had failed ;-))

							Cheers,

							-Richie,

> Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>
> 
> Hi David,
> 
> ..... The problem area is
> were the leads attach to the end plates (easy to fix) and where the plates
> attached to the end plates (not so easy).  Cheaper film caps use evaporated
> film for the electrode plates which have a very poor attach that will fail
> under very high current....
> 
> .....  So the thermal design of the things could stand a ton of
> improvement to even out the temperature.  The metal foil plates help draw
> heat out a little but it could be far far better.  "i" would make them
> hollow so the cap is more of a tube.  Then the heat would not build up in
> the center and I think you could get far higher currents through them
> without overheating.  Most of the capacitance is in the outer layers anyway
> so making them hollow would not reduce the capacitance/volume that much.  I
> would not be surprised if you could get 4X the current through one.  Maybe
> Chris could mention this to his pals at CD ;-))  You need fancy stuff to
> make poly film/foil caps so it is not a home project...
> 
> Cheers,
> 
> 	Terry