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# What is the current output of a capacitor? Answered

I want to make a circuit where a light is powered by a capacitor until it is discharged, and then powered by a battery until the capacitor is full again.

How long is it possible to power a light on a capacitor?

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## 12 Replies

kaboom! (author)2011-05-27

Thats the highest there is at my local electronics store.

iceng (author)2011-05-27

Sorry that is way too small.
For a long LED light time Use something like 2.2 Farad.

A

steveastrouk (author)2011-05-26

Look at the energy balance

E = CV^2/2

Energy used by an LED = IVt

Where I= LED current, V = LED volts/ capacitor volts and t=time

IVt=CV^2/2

t=CV/I

Its not accurate, because the real answer needs calculus, but its a start

ARJOON (author)2011-05-26

calculus my worst nightmare

steveastrouk (author)2011-05-26

Why ? I loved it when I learned it. If you're struggling, find Silvanus Thompson's book

ARJOON (author)2011-05-26

thns. it is just that i did not have a good teacher.

iceng (author)2011-05-26

You should to stay away from girl horses at night :-Þ

A

orksecurity (author)2011-05-26

Sanity check: Why not power the light directly from the battery?

iceng (author)2011-05-26

Batteries do not recharge forever :(
Ultra Caps will still be recharging when our descendents
have long forgotten us :-Þ

rickharris (author)2011-05-25

A super capacitor (10 farads ) will keep an LED lighted for several hours. (at a price!)

http://www.mindsetsonline.co.uk/product_info.php?cPath=16_443_194&products_id=311

lemonie (author)2011-05-25

What capacitor?
Short answer: not long enough for you - use a rechargeable battery instead.

L

iceng (author)2011-05-25

A friend I have makes a police flash light that charges an ultra capacitor
array to 63 VDC in less then 90 sec and provides full light output
for over 45 min.

A