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high voltage capacitors

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high voltage capacitors

Unread postAuthor: singularity » Wed Aug 29, 2007 7:40 pm

i happen to need roughly (20) 250-400v 500uf capacitors and i was kinda hopping on spending less than $50 on them. i have already searched digikey but they would have ended up costing about $70, over the limit. so does anyone know of some place that would sell discount hv capacitors? or by some off chance happen to have some lying around which no longer have a purpose?
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Unread postAuthor: thespeedycicada » Wed Aug 29, 2007 9:18 pm

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Unread postAuthor: Velocity » Wed Aug 29, 2007 9:31 pm

Here are some ebay things I found

<a href="http://cgi.ebay.com/Lot-of-10-NEW-High-Voltage-Capacitors-200VDC-2900UF_W0QQitemZ170142957846QQihZ007QQcategoryZ36335QQssPageNameZWDVWQQrdZ1QQcmdZViewItem">Lot of 10, 200V, 2900uF</a>
<a href="http://cgi.ebay.com/Lot-of-10-NEW-High-Voltage-Capacitors-200VDC-2900UF_W0QQitemZ170142958867QQihZ007QQcategoryZ48700QQssPageNameZWDVWQQrdZ1QQcmdZViewItem">Another Lot of 10, 200V, 2900uF</a>
<a href="http://cgi.ebay.com/Huge-High-Voltage-Capacitor-Lot-of-300-Liquidation_W0QQitemZ300145432141QQihZ020QQcategoryZ4662QQssPageNameZWDVWQQrdZ1QQcmdZViewItem">Lots of random capacitors, good price</a>
<a href="http://cgi.ebay.com/Lot-of-200-High-Voltage-Capacitors_W0QQitemZ300145432194QQihZ020QQcategoryZ1502QQssPageNameZWDVWQQrdZ1QQcmdZViewItem">Lot of capacitors, voltage is higher than you requested, but might work</a>


Hope this helps.
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Unread postAuthor: Fnord » Wed Aug 29, 2007 9:44 pm

It'll probably be easier to track some really cheap ones down in person than online.
There is a kind of capacitor that fits what you are looking for pretty well, and they are used in conjuntion with large sodium lights(the yellow kind of lights you see in steel mills).

If you have any friends in this area(electricians), you may be able to get some cheap/free. The place that my dad works at had boxes full of 400v capacitors(BIG ones), but someone snagged them/hid them before I got to them.
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Unread postAuthor: noname » Wed Aug 29, 2007 10:09 pm

Go to a local drug store that developes disposable camera film and ask for all the used cameras. Say it's for a school project.
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Unread postAuthor: Hotwired » Thu Aug 30, 2007 6:12 am

I've got a 300v 1500uf capacitor bank made from 12 x 300v capacitors from free disposable flash cameras. If its on a tight budget, camera shops are the place to go.
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Unread postAuthor: singularity » Thu Aug 30, 2007 10:46 am

i dont think disposable camera caps are going to work the plan for them was an omega 8 stage coil gun, you know like a sniper type thing. anyways each bank was to contain 2 300v 500uf capacitors wired in series (giving me 600v at 250uf). now if i were to use camera caps (average 330v at 100uf) i would need a significant amount more (10 caps in each bank) which would mean there would mean the gun would be massive. the caps rmich732 found on ebay do look pretty promising might have to settle for 400v 1500uf.
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Unread postAuthor: jimmy101 » Thu Aug 30, 2007 12:47 pm

You might want to look at http://www.surplussales.com/Capacitors/ ... 999uF.html

Perhaps a bigger cap value like;
(CFE) 002200R00AFJ 2,200µF 450v 3"x6" $24.00
which would replace 4.4 of the 500uF caps.
or
(CFE) 002000R00AFA 2,000µF 400/450v 2"x5-3/4" $19.00
which would replace 4 of the 500uF's

What are these going to be used for? If it is for a coil gun or other device that is supposed to dump it's energy quickly then you want caps with low ESR (equivalent series resistance). The lower the ESR the faster a cap will discharge. The nice thing about photocaps is they have very low ESRs.
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Unread postAuthor: iPaintball » Thu Aug 30, 2007 12:52 pm

An eight stage coil gun would just be a waste of electricity. Adding on stages reduces efficiency and, eventually don't affect the projectile any more. The most stages you would want to build would be 3. Any more and the coils won't effect the projectile.
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Unread postAuthor: singularity » Thu Aug 30, 2007 4:11 pm

the most im going to get out of a 3 stage coil gun is about 150fps, my goal is 300fps (mid level airsoft gun speed). adding more than 3 stages will be very inefficient but i am not going for efficiently... im going for fps and the more stages the better the fps (assuming the cap banks can be dumped fast enough) also it appears as though they only sell low ESR caps up to about 100v (well the "ultra low ESR" series only goes to 100v)
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Unread postAuthor: TurboSuper » Thu Aug 30, 2007 5:54 pm

Just out of curiosity, what are you using as your voltage source?
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Unread postAuthor: iPaintball » Thu Aug 30, 2007 6:10 pm

If you want the extra coils to work, your caps are gonna have to have EXTREMELY low ESR. And, like you said, these don't come in very high voltages.
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Unread postAuthor: singularity » Thu Aug 30, 2007 8:36 pm

ill take my chances with the normal ones and optically triggered IGBTs, i still believe there will be a significant performance boost.

the voltage is coming from a dc/dc converter similar to this...

<img src="http://www.anothercoilgunsite.com/img-schematic-boost-b.gif">

but replace the ne555 and supporting components with a PIC micro controller
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Unread postAuthor: TurboSuper » Thu Aug 30, 2007 9:27 pm

Nifty
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Unread postAuthor: jimmy101 » Fri Aug 31, 2007 1:14 pm

Have you modeled your cap+coil setups? With multiple stages the problem is that you can't get the later cap+coil pairs to discharge fast enough.

Timing is actually the least of the problems with multistages since there are a lot of ways to do the timing. There are not a lot of ways to make an RCL work faster and have sufficient magnetic field strength.

Every seen the really handly "Barry's RLC simutator"? (http://www.coilgun.info/mark2/rlcsim.htm)

And my favorite lab intrument (a PC with a soundcard) is a great way to measure the muzzle velocity and the discharge characteristics of a coil gun.
Image
The first peak is the voltage profile of the cap discharging, the next two peaks are the projectile (BB) passing through a pair of speed traps.

The first peak gives a direct measure of the RLC of the gun as it actually fires a projectile (from here.)
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