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From: Uwe Zimmermann
Newsgroups: sci.electronics.design
Subject: Re: Wrestlemania Royale: Tantalum vs. Electrolytic Vs. Polys, etc.
Date: Sun, 27 Oct 2002 23:15:57 +0100
Organization: Royal Institute of Technology, Stockholm, Sweden
Message-ID: <3DBC659D.9802318D@imit.kth.se>
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john jardine wrote:
>
> Basically, there are no clear rules for selecting caps, as at the end
> of the day they all do the same thing. A 10n capacitor of whatever
> type or maker will electrically always work as a 10n cap should work.
> Some types are good at holding their manufactured value over time.
> Some hold their value well against temp' changes. Some change in a
> fixed manner with temp'. Some types work well at high voltages. Some
> have exceedingly low losses. Some have 'self healing' aspects. Many of
> these products can (in varying degree) have a range of these
> properties.
That's quite an optimistic view of an ideal capacitor. However all
capacitors you buy, wherever you buy them, are far from ideal.
A capacitor as you buy it is never fully represented by its
capacitance and the deviation of capacitance over
time/temperature/voltage alone. These little things are rather
complicated circuit elements consisting of inductances and resistances
in the leads and possibly in the metal foil, leakage resistance
through the dielectric,....
____ ____ C
---|____|--|____|--o----||----o---
Ls Rs | ____ |
o--|____|--o
Rp
represents a simple model of a real capacitor. Whether or not the
parasitics can be neglected depends mainly on the type and size of the
capacitor and the frequency you intend to use. In the manufacturers
datasheets you should normally find an impedance vs frequency plot,
where you can see the frequency above which the parasitic inductance
starts to dominate the behaviour. This can be several to several tens
of kHz for electrolytics, some 100kHz to several MHz for plastic foil
capacitors and up to several 100MHz for single layer ceramic
capacitors.
http://www.vishay.com/products/capacitors/main.html
http://www.evox-rifa.com/europe/film_capacitor_catalog.htm
http://www.cornell-dubilier.com/mmultiframe.htm
(just to name a few)
Uwe.
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