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Help choosing a capacitor for a toy

Jesss

Aug 29, 2014
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I have an small electronic toy bug that jumps around, powered by a LR44 1.5V button cell battery. I want to replace the battery with a capacitor so that I can use a hand crank to charge the capacitor and the bug can run off person power.

I've seen this toy set up like this before and am keen to replicate it. The only thing I am unsure about is what specific electrolytic capacitor to choose. Any advice? V? uF? I have no idea how to choose so any advice would be great! I only want it to work for a few minutes before needing to be charged again. (The hand crank I have says 'max volts 8V DC out' if that makes a difference.)

Cheers!
 

Arouse1973

Adam
Dec 18, 2013
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Thats easy to workout if you know how much energy the toy uses. Does it have lights and sound? You might be looking at a super capacitor. I think you will be supprised in the size of capacitor needed to run something like that for two minutes and still hold the voltage.

Are you able to measure the current the toy uses. Otherwise its trial and error. The other thing is your toy uses 1.5Volts. Your wind up thing could give out 8Volts. If the toy ran from this, at best it might seem that the toys been taking acid at worst no more toy.

So you might need to limit the voltage also. Not an issue you just need to think about it. See if you can find out how much energy it takes and mean while I will work out an example for you.
Adam
 

Arouse1973

Adam
Dec 18, 2013
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Ok so a starting voltage of 1.5 Volts and a finish voltage of 1.3 Volts for 125 seconds is roughly 6F thats 6 Farads of capacitance. You should be able to get a 2.5 Volt version no probs.
Adam
 

Jesss

Aug 29, 2014
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Cheers Adam! I don't know how much current it uses but will check today. It doesn't have lights or sound, just jumps around (www.jaycar.com.au/productView.asp?ID=YG2891). Once I know how much current it uses, how do I use that information to work out which capacitor is best? This is all totally new to me!

The shop where I plan to buy the capacitor is www.jaycar.com.au/productResults.asp?MID=1&SSUBID=641&SUBCATID=967&keyform=CAT2#1

They have lots of capacitors that are 10V but only two, which are super capacitors, that are 2.5V. They are a 22 Farad and 55 Farad option. Is a 10V capacitor possibly ok? If not, do you think of one of these super capacitors would work?

I'm guessing the 2.5V 22F super capacitor is probably the best option, but am happy to buy a few to experiment with.
 

BobK

Jan 5, 2010
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Adam,

At what current? I have not seen any post that specified the current it uses.

Bob
 

ketanrd01

Aug 11, 2014
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Ok so a starting voltage of 1.5 Volts and a finish voltage of 1.3 Volts for 125 seconds is roughly 6F thats 6 Farads of capacitance. You should be able to get a 2.5 Volt version no probs.
Adam


How did you come up with that Adam??
 

Jesss

Aug 29, 2014
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Aug 29, 2014
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Cheers for your interest guys. I visited an electronics shop today and decided to make it super simple instead, so wires from the motor come out of the bug and connect directly to the generator. Appreciate your input!
 

Arouse1973

Adam
Dec 18, 2013
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Ok so a starting voltage of 1.5 Volts and a finish voltage of 1.3 Volts for 125 seconds is roughly 6F thats 6 Farads of capacitance. You should be able to get a 2.5 Volt version no probs.
Adam


How did you come up with that Adam??

C=I*(dV/dt) = 0.001*(0.2V/0.33) = 6.6F

Edit: This is wrong it should be C=I*(dt/dV)=0.01*(1200s/0.2V)= 60F

So the voltage at the end is 1.3 which is a difference of 0.2. The current used is 10mA over a time period of 20 minutes which is 0.33 of an hour. I just used some figures to show you. When you find out the real current you use that.
Adam
 
Last edited:

Arouse1973

Adam
Dec 18, 2013
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But the unit...how can it be fahrad????.....
aint it too large Adam?....

That's the whole point, to try and match the energy inside even a little battery takes an enormous amount of capacitance.
Adam
 
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