Subject:
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Re: Electrolytic Capacitor Ratings [was Re: Choosing Parts?]
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Thu, 22 Apr 1999 20:21:14 GMT
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Original-From:
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Chuck McManis <cmcmanis@mcmanis.com!nospam!>
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Viewed:
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833 times
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Kind of. Electrolytics are indeed made by plates in a conductive solution,
however when the voltage increases, it pulls the plates together (standard
columbs law stuff). When you exceed the rated voltage the force on the
plates is enough to pull the plate _through_ the dielectric and to make
contact. The turns the capacitor into a wire and, if current continues to
flow, into a small explosive device. If it is oil filled (not as common
these days) you get a smokey orange fire ball. (very impressive but scary
as hell the first time!)
This is why electrolytics get bigger for a higher rated voltage but the
same capacity. The plates are made larger and kept farther apart.
As someone else stated this is also what electrolytics have such a wide
value range (+/- 20% or more). Voltage through the device changes its
geometry and its capacitance. (and its why they fail, the dielectric fluid
leaks out and they become short circuits)
--Chuck (exploder of many a cap) McManis
At 08:52 AM 4/22/99 -0400, you wrote:
> Electrolytic capacitors are formed by metal plates placed in a
> conductive solution. A thin dielectric coating formed on one of the plates.
> The rummer states that the thickness of this dielectric coating varies
> slightly with applied voltage. So (according to the rumor), if you want the
> capacitor to operate at its rated capacitance, you must operated it at its
> rated voltage.
> That is the rumor as I've heard it. I have not really heard anyone
> speak against it other than to say that it is common practice to use
> capacitors larger than is needed. In most power supply applications such
> attention to detail is usually not called for. And if your using capacitors
> in tuning applications, you probably aren't using electrolytics.
> Is there anyone on the list that is involved in the manufacture of
> electrolytics that can speak about the truth of this rumor?
>
> Pherd
>
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