How long can I practically power an IC with a capacitor?

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elin05
 
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Joined: Tue Jun 11, 2013 4:27 pm

How long can I practically power an IC with a capacitor?

Post by elin05 »

If I have, for example, an IC that draws 10mA @ 3.3V, how large of a capacitor would I need to power it for 2 minutes? 5 minutes?

I understand that using capacitors to power an IC is not the most practical, since they discharge very quickly, but am curious if the required capacitance is within the range of those commercially available.

I would like to understand the math behind calculating a problem like this as well.

Thank you!

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jcgoodman
 
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Joined: Thu Jan 23, 2014 6:03 pm

Re: How long can I practically power an IC with a capacitor?

Post by jcgoodman »

elin05 wrote:If I have, for example, an IC that draws 10mA @ 3.3V, how large of a capacitor would I need to power it for 2 minutes? 5 minutes?
Here's the equations you need.
Charge on a capacitor is proportional to its voltage:
Q = C V
So the change in the charge on a capacitor is proportional to change in voltage:
deltaQ = C deltaV
Change in charge equals current times time:
deltaQ = I T
so that means
I T= C deltaV
T = C deltaV / I

So the answer depends on how high you can charge the capacitor before it blows up, or it blows up your device. Example: say you charge a 1 microfarad capacitor up to 5 volts (so deltaV = 5 - 3.3 = 1.7). It'll power a 10 milliamp load for

T = 1e-6 * 1.7 / .010 = .00017 seconds

So that's no good.

There are some really impressive advances in capacitor technology lately, though. You can now easily buy capacitors as large as 5 farads (not microfarads). They'd power your IC for
T = 5 * 2.1 / .01 = 1050 seconds

which isn't so impractical! Of course, a LiPo battery the same size would last 50 hours...

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