Let's Make Robots!

Super Capacitor


Vendor's Description: 

Description: Yes you read that correctly - 10Farad capacitor. This small cap can be charged up and then slowly dissapated running an entire system for hours. Combine two in series for 5F/5V. Do not over voltage or reverse polarize these capacitors.

Description: Yes you read that correctly - 10Farad capacitor. This small cap can be charged up and then slowly dissapated running an entire system for hours. Combine two in series for 5F/5V. Do not over voltage or reverse polarize these capacitors. (from Sparkfun)

 

Just as a reminder:

Capacitors in series get a total capacitance(Ctot) of:  

(1/Ctot)=(1/C1)+(1/C2)+...+(1/Cn)   which is like saying   Ctot=[(1/C1)+(1/C2)+...+(1/Cn)]^-1

Capacitors in parallel get:

Ctot=C1+C2+...+Cn

The voltage rating is the max voltage it can take. (http://letsmakerobots.com/node/1988)

 

some formulae:

i=(f/R)*e^(-t/τ)

ΔV=f*e^(-t/τ)

 

where τ=R*C

and f is the potential difference of the capacitor at the beginning 


 

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i tought i browsed everything on sparkfun, but i don't remember this super cap :P
Btw, looking at the linked discussion, i'm a bit baffled at the 2.5 V specification. What does it mean? That i cannot use more than 2.5 V for charging it ?
Like batteries you can put them in series to increase their voltage.  2 in series will bring the voltate up to 5v and the capacitence down to 5F.  Putting 3 in series will give you 7.5v and 3.3F.

That rating of 2.5V does not seem like much. It only takes a second cell in your power supply to ruin this cap. So why is anyone interested in buying these?

The answer is in "solar"botics (.com).  And in BEAM robotics. The tiny solar cells that drive Beam bots do not produce a very high tension (voltage). And the circuits on these bots are optimized for that. They even collect energy until there's enough to do something with it. They play dead in the dark and jump around when the sun is beaming on their backs.

This supercap holds on to the energy until the bot decides to use it.

even if you are putting 2 of them in series, and that's 5V, you are still getting 5F of capacitance and that's a lot.

mutr.co.uk sell 20F caps I think. not sure what voltage rating 

Back in my car stereo competition days, we used caps the size of oatmeal containers!! -For the big SPL (sound pressure level (all sub-woofers)) they would team up 20 or 30 of them!!

I have a really stupid question...

Lets say you hook up batteries to this thing ti charge the capacitor. In theory can you then remove the batteries and let something run off of the charge in the capacitor until it is drained? Seems like a cheap option to recharable batteries... I need to dig into capacitors more to learn how to figure out which type/specs go with each use, but this seems feasable to me anyway.

As far as I understand, this is correct.  You can use this site to figure out how long it'll take to discharge:

http://www.cvs1.uklinux.net/cgi-bin/calculators/time_const.cgi

The resistance will be the resistance of the circuit plus any additional resister you add to slow the discharge.  Be sure to use owms law to calculate the Amps that it'll output as well.  You don't want to burn out your circuit.

uhm but what voltage will you have from it while it discharge?

let's say that i want to run an arduino board with those caps, how should i set it up ?