Which capacitor is right for me?

[DougD720]

Hey guys!

I'm working on a little side-project and basically i need a compact source of a burst of electricity, so naturally the high-school physics in my head said "capacitor." Now i just don't know what kind to get! I need the maximum amount of energy to store (joules) and a quick discharge (low ESR right?). So I'm thinking Supercapacitor but there are so many capacitors and calculating .5(F)(V^2) for all of them to figure out which best suits me is daunting, so does anyone have any experience with these sorts of things and have any recommendations? Like i said, i just need it to give one high-powered burst of electricity. Also, how do you boost capacitors power? Series or Paralell? I think it's in series but I'm not 100%. Currently the two best ones i can find are the Illinois supercapacitor w/ Capacitance of 400F and Voltage of 2.7 (does this also mean discharge voltage?) w/ ESR of .003.

Size, mass and shape are of a great concern, otherwise this would be easy! I'm trying to keep size and mass on the small side and shape=cylindrical. The Illinois one is 35mm in diameter which is on the big side but it puts out energy! I also saw a Cooper Bussmann Aerogel Supercapacitor B-Series w/ C=50F, V=2.5volts, ESR=.025 and diameter of 18mm. Its not as strong as the other one but it's about half the size and almost a fourth of the mass.

Thanks for any advice you can give!

Edit:

I may have misspoken when i threw around terms like power and energy, i need a capacitor that gives off the Most Current in the shortest amount of time.

Thanks again!

 

[mplayer]

Also, how do you boost capacitors power? Series or Parallel? I think it's in series but I'm not 100%.

This is the only part I think I can help with.

Capacitors add in parallel: C_eq = C₁ + C₂ + C₃ + ...

In series : (1 / C_eq) = (1 / C₁) + (1 / C₂) + (1 / C₃) + ...

 

[skeptic2]

Bigger is not always better. First you need to define your minimum requirements for voltage and current. What size wires will you use to connect the capacitor?

 

[berkeman]

I could be wrong, but I don't think supercaps are meant for high current applications. The ones I've seen are for long-term energy storage, like to replace backup batteries for medium-term power outage type situations.

What voltage do you want to run this system at? What are you trying to power? How are you planning on charging up the capacitors?

 

[es1]

I always used the term supercap in the same way as you Berkeman; small, ~300mF and a couple of ohms ESR, low current.

But it seems those monsters do exist.
http://www.tawelectronics.com/wima/WIMA_SuperCap_MC.pdf
110F @ 400A!

DougD720,
Please keep in mind these things are dangerous, especially on both the charge and discharge, and probably just in general actually. For example, unless precautions are taken with your example part, ESR=25mOhm & V=2.5volts, the initial charge current into this cap will be 100A and because of the large C it's going to hold a large current for over a second.

 

[Phrak]

What's the application, Doug?

 

[Dadface]

Depending on what your application is you could also consider using inductance.

 

[DougD720]

Sorry for taking so long to reply. The application is just an idea i had and a design I've been working on. It's just been a while since I've worked with the equations for circuitry and capacitors. Essentially I need to maximize current and minimize discharge time. I need a high-power pulse that's portable, which is why a capacitor came to mind. I had originally hoped for a very small capacitor which met these needs but as I've crunched the numbers for a few it seems there aren't any small enough to provide the "power" i need so now I'm redesigning things to accommodate a larger capacitor. I realize things of this magnitude are dangerous and safety is of the utmost concern and has been thoroughly taken into account in my designs, it was my biggest obstacle with using small capacitors, which I'm still trying to use to get the desired result. Recommendations are still welcome, as I said, I need a "pulse," that's just about it. Handling high-temperatures would be a plus but if one pulse wrecks the capacitor (at least for the small-one-application) it wouldn't matter, it would for the larger ones though as they're more expensive and would likely be re-used. The application is a rail-gun design idea I had. That being said I'm sure that no one will take me seriously now :P

 

[triden]

I remember building rail guns back in electronics class. We would use coils of copper wire and hook them up to huge cap banks which we scavenged from old power supplies. I remember the guy sitting beside me marking up the walls pretty good launching steel marbles. I am not quite sure (it's been a while), but I think we discovered that using a higher voltage would get better results when it came to launch time.