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Big home made pulse caps

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Pinky's Brain

Fri Jun 25 2010, 03:49PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837

Polystyrene is slightly soluble in oil ...

Any way, as I said you can buy BOPP fairly cheaply (it's used to pack flowers and fruit baskets). Packaging grade BOPP should have around 200 Volt/um dielectric strength, which is so much better than anything else (except Mylar and capacitor grade film) that it doesn't really make sense to use anything else if you are going to make something big.

pauleddy

Sat Jun 26 2010, 01:16AM

Registered Member #2909 Joined: Wed Jun 09 2010, 12:31AM
Location: fort belvoir, Va USA ( south of DC)
Posts: 145

so if i made a roll with like 200m X 50cm (X2 one for each plate) of Al foil does anyone think it will run a coil gun or something like that?

GhostNull

Sat Jun 26 2010, 08:55AM

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

Only electrolytics can really get the necessary capacitance for a coil gun without being too big.

pauleddy

Sat Jun 26 2010, 02:23PM

Registered Member #2909 Joined: Wed Jun 09 2010, 12:31AM
Location: fort belvoir, Va USA ( south of DC)
Posts: 145

so what if i modify an electrolytic to work at higher voltages, i recently found a pair of 100,000+ MF caps but are only rated for 15v. is their a way i could dismantal these and increase the max volts. ( something tells me this will end in some catastrophic failure)

big5824

Sat Jun 26 2010, 04:41PM

Registered Member #1687 Joined: Tue Sept 09 2008, 08:47PM
Location: UK, Darlington
Posts: 240

Im just going to put it simply, it isnt possible. You would get the same energy storage for 1/10th of the price just by buying some electrolytic caps, and even then a farad at lets say 500v is still larger than most universities have access to with their millions of pounds of funding.

klugesmith

Sat Jun 26 2010, 05:04PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

pauleddy wrote ...

so if i made a roll with like 200m X 50cm (X2 one for each plate) of Al foil does anyone think it will run a coil gun or something like that?

Yup. for example using dielectric film like Pinky described. Though I would stay below 1000 volts for 20 um film.

Suppose you had 2 foils 50 m x 0.5 m, and 2 similar areas of dielectric film -- that's 1/10 of Pinky's example, and you could get about 66 microfarads. 21 joules at 800 V - enough for a small coilgun.
I would start by making and testing a much smaller capacitor. Then if it works, make many more small ones & use them in parallel, so a single film failure doesn't destroy the whole thing.

It's a lot of work, compared to what you could do with a few free photoflash capacitors.

pauleddy

Sat Jun 26 2010, 05:37PM

Registered Member #2909 Joined: Wed Jun 09 2010, 12:31AM
Location: fort belvoir, Va USA ( south of DC)
Posts: 145

yes originally i set my goals way too high with out doing much looking around and realize that it was practily impossible. so im looking to do somthing a little more on the possibity side

i have two 196,000MFD 15VDC caps the tops are broken but the foil looks intact. ( i found them from an R&D dump site they wher part of a 12v 80A PSU that where crush, but some of the caps lived) is there a way to pump higher voltages with these caps?

Pinky's Brain

Sat Jun 26 2010, 07:03PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

I don't see how really.

It's not so much impossible, it's that you want low rep-rate pulse capacitors ... and for those commercial electrolytics are unbeatable, because they are consumerized devices.

Very high voltage (>>1 kV) high rep-rate pulse caps however are boutique items, so there it makes more sense to DIY. It's just that that's not the kind of capacitor you need in a coilgun.

IntraWinding

Sun Jun 27 2010, 01:20AM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

pauleddy wrote ...

yes originally i set my goals way too high with out doing much looking around and realize that it was practily impossible. so im looking to do somthing a little more on the possibity side

i have two 196,000MFD 15VDC caps the tops are broken but the foil looks intact. ( i found them from an R&D dump site they wher part of a 12v 80A PSU that where crush, but some of the caps lived) is there a way to pump higher voltages with these caps?

To increase the voltage of your electrolytics you'd need to grow a thicker oxide layer and that would mean the capacitance would drop.

pauleddy

Sun Jun 27 2010, 11:44AM

Registered Member #2909 Joined: Wed Jun 09 2010, 12:31AM
Location: fort belvoir, Va USA ( south of DC)
Posts: 145

thanks i thought as much.

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