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Author Topic: capacitor discharge  (Read 680 times)

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Offline vipulan12Topic starter

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capacitor discharge
« on: July 27, 2013, 07:57:56 AM »
how can i determine the current flowing out of a capacitor?
i have a 330 v 80 uf cap
if you multiply them you get 26400 columns

Offline jkerns

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Re: capacitor discharge
« Reply #1 on: July 27, 2013, 09:34:59 AM »
I = C* dV/dt

You would also want to account for any resistance in the circuit and, potentially, the internal resistance of the capacitor.
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Offline jwatte

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Re: capacitor discharge
« Reply #2 on: July 27, 2013, 01:29:02 PM »
With "ideal" components, "dt" will go towards zero, so "current" will go towards infinity.

Note that the 330V rating has little to do with the actual current -- what matters is the actual voltage you put across the capacitor, and the delta in that voltage.

Your calculation is off by a factor of 1,000,000, btw, because you multiply micro-Farads with Volts. This yields micro-Columbs, not Columbs.

If your circuit is fully resistive, then add your resistive load to the ESR of the capacitor for an estimate of R, and calculate the time constant RC. The time constant RC tells you how long it takes to drop voltage by about 63% across that resistive load. From that you can calculate the current.

Also, the ESR of the capacitor becomes very important here -- multiply current squared times ESR to get the power that the capacitor will see as heat. If this is too high, then the capacitor will overheat and blow up. This is why electrolytic capacitors often have a "ripple current" rating, which is roughly speaking the max changing current they can see during normal operation without overheating.

Offline vipulan12Topic starter

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Re: capacitor discharge
« Reply #3 on: July 27, 2013, 04:36:16 PM »
then formula can i use exactly?

Offline jkerns

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Re: capacitor discharge
« Reply #4 on: July 27, 2013, 04:43:58 PM »
If your circuit is resistive, and if my memory is correct,

I = (V0 -e^(-t/(RC)))/R

Where R is the total resistance including the equivalent resistance of the capacitor, V0 is the initial voltage, and t is time, and e is ... well .... e.
« Last Edit: July 27, 2013, 07:01:05 PM by jkerns »
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Offline waltr

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Re: capacitor discharge
« Reply #5 on: July 27, 2013, 05:22:22 PM »
The formulas can easily be obtained by using Google and Wikipedia.

Also, If discharging a cap with a constant resistance the current is NOT Constant but decreases exponentially. That's what the equation jkerns posted tells you.

Offline sherby

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Re: capacitor discharge
« Reply #6 on: July 28, 2013, 06:42:15 AM »
I would like to get some information about selecting capacitor too!
Is there a post or some information, to get help in selecting proper capacitors with proper values?

Thanks in advance!

Offline jwatte

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Re: capacitor discharge
« Reply #7 on: July 28, 2013, 10:23:05 AM »
That kind of depends what your intended use is!

If you need a particular time constant at a particular resistive load, then you can calculate the capacitance needed from that.
If you need a particular ripple current tolerance, then that's typically listed for the capacitors, and is searchable in most parametric search tools. You can also calculate it from the power and ESR, given the above formula.
If you have a particular voltage you need to use, make sure the rating is higher than the voltage by a safe margin (50% is a good margin.)

I like the digikey.com parametric search tool:
http://www.digikey.com/product-search/en/capacitors/aluminum-capacitors/131081

Note that if you need very low ESR, or very small form factor, or some other extreme parameters, you may want to look at chemistries other than aluminum. (Tantalum, ceramic, PTFE or mylar film, etc.) Those are also searchable on digikey, if you navigate through their parts index.

 


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