capacitors

Frequently asked questions on capacitor types, ratings, brands, use and abuse.

capacitors

Postby TragicTravisty » 03 Jan 2010, 20:06

wasnt sure where to post this...

can a capacitor absorb a constant DC current? if a pedal had a constant dc offset of 1V, could the appropriate capacitor absorb this constant current, even with ac (guitar) signal on top of it, so that the net dc offset would be zero? or would it be necessary to have an equal and opposite dc current at the output?

thanks for answering my noob question...
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Re: capacitors

Postby lolbou » 03 Jan 2010, 20:11

TragicTravisty wrote:can a capacitor absorb a constant DC current?


Well, a cap just blocks any DC current (its impedance rises to infinity).

TragicTravisty wrote:if a pedal had a constant dc offset of 1V, could the appropriate capacitor absorb this constant current, even with ac (guitar) signal on top of it, so that the net dc offset would be zero? or would it be necessary to have an equal and opposite dc current at the output?


Well, don't mix voltage and current... There's a voltage drop across the cap (DC and AC), but only AC current can flow. Then only the AC signal actually travels the cap...

Hope this helps! :D
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Re: capacitors

Postby TragicTravisty » 03 Jan 2010, 20:36

thanks, very helpful
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Re: capacitors

Postby TragicTravisty » 04 Jan 2010, 00:53

sorry, for my ignorance, but wouldn't the capacitor eventually reach full capacity and let dc current leak?
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Re: capacitors

Postby lolbou » 04 Jan 2010, 10:39

The more it is charged, the less the current flows... In fact some current flow at the charging start, and it decreases until zero when the cap is fully charged, blocking any DC. But charging the cap is a matter of milliseconds...
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Re: capacitors

Postby TragicTravisty » 04 Jan 2010, 17:41

and how long would the cap take to discharge (I'm working on a design with a constant DC current running through it - from my obvious lack of knowledge i'm sure i'll fail miserably)
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Re: capacitors

Postby lolbou » 04 Jan 2010, 17:47

TragicTravisty wrote:I'm working on a design with a constant DC current running through it - from my obvious lack of knowledge i'm sure i'll fail miserably)


Sure you will, 'cause NO DC CURRENT will flow through the charged cap. The only way to discharge it is to remove the power supply, making the cap act like a power supply itself and freeing its electrons through a load.

Show the design, I'll tell you... Maybe you are working on a design with DC voltage across the cap, which is more than common... :)
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