Re: Oil Filled Capacitors

Miroslav Pokorni

I guess, it does not help, but then everything has a resonant frequency. The
only way to cope with it, if you can not avoid operating at that frequency,
is to make cap mechanically lossy so that Q factor is down in mud.

Possibly, this acoustical phenomena, together with high current through cap,
made resonant power supplies pretty much disappear. The last one that I have
seen was a Hewlett Packard unit that used a cap branded with their name,
what I guess was a custom cap.

There seems to be no such a thing as a simple component, there is only the
simple demand.


Miroslav Pokorni

----- Original Message -----
From: "Craig Sawyers" <c.sawyers@...>
To: "TekScopes" <TekScopes@...>
Sent: Friday, May 30, 2003 12:27 AM
Subject: RE: [TekScopes] Re: Oil Filled Capacitors

Corona discharge would be pretty hard to induce at 200 Vac. You would
to hit frequency around cap's resonance
That reminds me of another failure mechanism - acoustic. Try the
experiment. Make a simple power amp for square waves with a couple of
transistors or FETS, driving +/- 30V (say). Connect capacitor from output
to low impedance load to get a reasonable current flowing. Now sweep an
audio generator.

Many capacitors howl like a banshee at particular frequencies where the
internal mechanics resonate. These are usually ones where the windings
inside are relatively "loose". Very tightly wound capacitors are very
quiet - but there are some real shocks out there when you try the
(like some respected 10kV polypropylene which were more like a loudspeaker
than a capacitor).

Needless to say, such mechanical vibration does nothing to help the
reliability of the capacitor.


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