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Help with capacitors

Hello everyone. I've seen this instrucable (https://www.instructables.com/id/ETJBV68MFMEV2ZAU7T/?ALLSTEPS) that uses a charged capacitor to light up a LED.
I found this very interesting, so I started investigating what a capacitor is.
So far so good and my questions are:
- Can capacitors be used as replacements for batteries? I'm thinking specifically to light 1, 2 or 3 LEDs.
- If so, how can I calculate the specs of a capacitors for a determined amount of LEDs.
- Is it possible to calculate how much time the capacitors will keep the LEDs lit?
- How can I determine if the capacitors have enough charge to light the LEDs?
- Would you be kind enough to suggest a circuit to achieve any of these?
Thank you.

Fat Bagel9 years ago
would you be able to use a bike dynamo to charge a capacitor.? Do you have to y have to use a low voltage capicitor if the dynamo only produces about 12v?
westfw10 years ago
Caps can be used as replacements for batteries under some circumstances. They're not very GOOD batteries; the capacity of even state-of-the-art "supercaps" is MUCH less than even "average" rechargable batteries, and the discharge curve is NOT flat at all. OTOH, they have simplified and fast charging and possibly other advantages.

Any cap charged to a voltage higher than Vf of the LED will light the LED. the question is: "for how long."

As a first-estimate guess, figure your LED will start on for R*C seconds, where R is the current-limiting resistor for your LED and C is the capacitance in Farads. So a 1F supercap lighting an LED through a 100 ohm resistor (which is likely to be "about" correct") will last "about" 100 seconds. It's more complicated than that (exponentials, dependency on start voltage, etc), but as a guess it'll work ok.

You can measure the voltage on a cap with a voltmeter.

    +++ ------------+------Switch----Resistor----+Charge               |                                               |Source              === Cap                                  v  LED     ----                  |                                               T        ---------------+-----------------------------------+
westfw westfw10 years ago
(oops)
cap.png