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1 large cap vs multiple smaller caps (1kj gun)

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ben5017

Thu Oct 14 2010, 07:40PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

new here help me out.

Dave Marshall

Thu Oct 14 2010, 09:27PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

First of all, you need to expand on your post considerably. One liner posts aren't permitted, particularly not in the technical forums.

You're missing a huge amount of pertinent data. Voltage? Current? Coil size? Capacitor specs? Type of project (inductance, reluctance, reconnection, ETG, or other type of capacitive projectile accelerator?)

Reread the forum rules. Then do some searching in the archives, then flesh out the specifics in this thread so we can help.

-Dave

ben5017

Thu Oct 14 2010, 09:42PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
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Posts: 156

sry i just joined here... Gun will be a single stage, voltage around 500, .01 F capacity. dont know about the coil size, this will be my first 1. the reason why i am askin is i was lookin around at bus capacitors and found a 450v 12000uf cap that would put me over my disired 1 kj range. i didnt know if i would be better off getting 2 or 3 smaller ones that still hold around 1kj.

Artikbot

Thu Oct 14 2010, 10:14PM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Is that a capacitor or a supercapacitor? If it's a super, it won't work. ESR is so high that it would take ages for it to discharge.

By the way, if it's your first coilgun, I suggest experimenting with disposable cameras until you get around with the basic concepts. It really helps because you don't need to buidl ALL the stuff concerning the whole circuitry, avoided by using a mechanical switch and a photoflash circuit.
You can also experiment by adding more caps to that charger.

But well, if you wanna start the house from the roof, hey, it's your decision

I recommend doing this because I've started recently too, and it takes A LOT of experimentation before you come up with the necessary knowledge and concern about the concept to build a functional coilgun.

ben5017

Thu Oct 14 2010, 10:18PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

it is a bus capacitor

yes i plan on building a few smaller prototypes before construction on it. It is still in the planning stages and i juss am trying to learn so i can make an effective coil gun. one of my friends is an electrical engineering student with acess to to alot of pieces, sothe circuitry is up to him ha

Artikbot

Thu Oct 14 2010, 10:20PM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Dude... you'd better start with something smaller, because if you're learning and wanna use THAT beast... You could kill yourself just because of a little mistake!

By the way, start by reading the project logs on barry's site, they will help you to get an idea of how the whole thing works and the dimensions of such a project ;)

ben5017

Thu Oct 14 2010, 11:24PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

i plan to make a few and work my way up. but for my actual gun i would use only 1 of those. from watchin videos onl;ine i want a 1kj gun or its not worth it. but as for my origional question whats better 1 big cap of 3-4 smaller caps with eq energy to the large 1

Artikbot

Fri Oct 15 2010, 08:02AM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

What do you mean by "it's not worth it" just from videos? I'm currently shooting at 200J with a "starting to be" optimised system (correct projectile placement, correct barrel, polished barrel+round, apropriate switch) and hey, it has A LOT of power. And it's just 200J.

It's not all about raw power, efficiency has a lot to do. If it's wrongly designed, even with 1kJ raw electrical energy it will barely go through a can. If it is efficient enough, with 400J it can pierce from side to side a soup can easily.

More than the stored energy, is the overall concept. I remember somewhere on YouTube a guy using 2kJ electrical energy and he still wasn't able to make it through a soup can and a potato inside it shooting with a huge nail. 2kJ! That's 10 times the energy I'm using and I still pierce the first wall and almost stick half round into the potato.

According to your question, depends on the ESR of each capacitor. If the 3-4 are very low ESR ones, might be better than the big one. If otherwise they have similar ESRs, I'd say go with the big one just for versatility.

GhostNull

Fri Oct 15 2010, 09:35AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

lol, if you don't get the design right, it might not work at all. From what you have said, you really need to do some research. =S

But back on what I think your original question is: Which is better? A single large capacitor or several smaller capacitors?

Which of the two options is better comes down to you and personal variables but I think several smaller capacitors would be better

In terms of pure functionality to store energy, if you are only putting them in parallel then there is not much difference. The capacitance will just add up. However, If you are putting them in series (to raise the voltage it can handle) it can be more trouble. When putting electrolytic capacitors in series, you must account for the leakage current of each capacitor using balancing resistors. Look here for more info:

and if one capacitor fails in a series capacitor bank, then it will take all of them with it. In terms of just energy storage, there is not much difference as far as I know but if you are going to put some capacitors in series, it would be better to use a single, higher voltage rated capacitor instead.

Several smaller capacitors allow for greater flexibility but a single large capacitor offers greater simplicity and safety. With many smaller capacitors give have greater flexibility: you can put them together in parallel to make a big high capacitance bank, in series to make a higher voltage bank, a combination of the two, use them separately to make a smaller coilgun or use them to power several coils and make a multistage (usually more powerful) coilgun. The problem with many small capacitors is that there is a lot more to go wrong; both in terms of safety as well as reliability. There is more work and chance for error in putting together many capacitors than there is putting in a single capacitor. The greater amount of connections required with multiple capacitors a greater chance of something going wrong. However, should one smaller capacitor of many fail, that is much less of a loss than a single large capacitor. A single capacitor means simplicity and easy of use and many capacitors means flexibility but more complexity and chance of failure.

Another factor is cost. You might find that many smaller capacitors are much cheaper than a single large capacitor, or you may not. If you spend too much on your capacitors you might find you don't have enough money for other parts such as the switching device and charger. Take into account the cost.

I suggest you get many smaller capacitors because you don't currently have a clear plan. It would be better to have the flexibility of multiple capacitor than getting a big capacitor and being stuck with it or having to get more capacitors.

Cheers
-Ken

ps. I think if you look around some more you'll get a better price. In my experience, eBay isn't always the cheapest place to get this kind of stuff. Look for surplus sites

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