1. ## Capacitor question

I have a Supernova E3 dynamo powered light system that turns off when the wheels stop turning.I was told that I can use a capacitor to hold a 5 minuet charge which would be very useful to me,can anyone give me advice on this subject and tell me what capacitor to use and where to buy it?It can not be more than 6 volts or greater than 1 amp.

2. Radioshack? Capacitors should be wired in parallel to the power source. You should be able to find one that will fit your application from the "Shack".

Capacitors are spec'ed by the amount of charge they can hold. Check out Wiki's page on capacitors if you want to get into the electronics or math end of things... http://en.wikipedia.org/wiki/Capacitor

3. Perfect!Thanks Dnlwthrn.

4. Unlikely, 300s x 1 amp x 6 v =1800 joules
How many farads is that ?
1800 = 1/2 * C *V^2
C = 3600/(6^2) = 100 Farad

That is a whopping great capacitor.
A simple search :
The unit of capacitance is a farad. A 1-farad capacitor can store one coulomb (coo-lomb) of charge at 1 volt. A coulomb is 6.25e18 (6.25 * 10^18, or 6.25 billion billion) electrons. One amp represents a rate of electron flow of 1 coulomb of electrons per second, so a 1-farad capacitor can hold 1 amp-second of electrons at 1 volt.

A 1-farad capacitor would typically be pretty big. It might be as big as a can of tuna or a 1-liter soda bottle, depending on the voltage it can handle. So you typically see capacitors measured in microfarads (millionths of a farad).

* A typical alkaline AA battery holds about 2.8 amp-hours.
* That means that a AA battery can produce 2.8 amps for an hour at 1.5 volts (about 4.2 watt-hours -- a AA battery can light a 4-watt bulb for a little more than an hour).
* Let's call it 1 volt to make the math easier. To store one AA battery's energy in a capacitor, you would need 3,600 * 2.8 = 10,080 farads to hold it, because an amp-hour is 3,600 amp-seconds.

If it takes something the size of a can of tuna to hold a farad, then 10,080 farads is going to take up a LOT more space than a single AA battery! Obviously, it is impractical to use capacitors to store any significant amount of power unless you do it at a high voltage.

5. would this capacitor's be different than the ones i use in my car stereo? I have a 1 farad cap for my amps and its about the size of a 1-liter soda bottle. But your saying that a AA battery holds more energy than it?

6. The best cap I can think of would be a supercap:
http://www.nuintek.co.kr/data/ElectricDoubleLayer.pdf

You'll need a couple of them in series/parallel. A battery with a simple charger might be a better solution.

7. There are several reasons that I think this won't work:

• Dynamos often put out AC which will destroy most (all?) large capacitors.
• The big caps are often rated at less than five volts so, again, the dynamo is likely to destroy them and the cap wouldn't have the voltage to drive the lights anyway.
• When capacitors discharge the voltage goes down quite quickly (exponentially) so your lights would dim very quickly.
• As already mentioned the storage requirement is massive

In order to use a capacitor you'd end up with a load of electronics to overcome the problems listed. Not the sort of thing that you could just knock up.

You'd probably do better with a small relay powered from the dynamo to switch in a battery pack when the dynamo stops.

8. WOW!!!Thank you all,this is all very good information!
So I see that the general consensus is that the simplest and best solution is a small battery + recharger.Please forgive my ignorance on these matters but can anyone one tell me the best brand,specs (preferably a single unit that is weather resistant) and where to buy it (I was hoping for a store that is more bicycle specific than radio shack)?

9. To be honest I think that is a lot of hassle for nothing. If it was me I'd pick up some cheap LED lights that have a long life in flashing mode and run them in addition to the dynamo. That way you always have lights, even when stopped, plus you get the benefit of highly visible extra lights too. I have a couple of lights that I used through the whole winter, commuting about 40 minutes a day, and I never had to replace the batteries.

Just my two cents, hope it's some help.

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