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megajoule capacitor

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jnbrex

Sun Nov 27 2011, 04:01AM

Registered Member #3950 Joined: Wed Jun 15 2011, 12:45AM
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Posts: 51

With higher dielectric breakdown voltage comes lower permittivity, which is e. Thus with a higher dielectric breakdown voltage, the capacitance of your capacitor decreases.

Pinky's Brain

Sun Nov 27 2011, 07:23PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Nope, might be true in semi-conductor range ... but not for insulators. Look up the numbers, PVC and Mylar have similar dielectric constants.

Antonio

Sun Nov 27 2011, 08:31PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

There is no much sense in trying to use plate capacitors for this large energy storage. An assembly of commercial high-voltage electrolythic capacitors would be far simpler and smaller. 1000-uF, 400-V capacitors are quite easy to find, and can store 80 J. You need "just" 12500 for 1 MJ, in an array of 50x250 for 20000 V. I leave for you the estimation of the cost...:-p

Conundrum

Sun Nov 27 2011, 08:51PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Seem to recall that praesodymium oxide is supposed to have breakdown voltages in the MV per cm.
Exotic metal oxides vacuum deposited onto perfectly flat copper plates and then layered would probably work...

-A

Pinky's Brain

Sun Nov 27 2011, 09:38PM

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

wrote ...

There is no much sense in trying to use plate capacitors for this large energy storage. An assembly of commercial high-voltage electrolythic capacitors would be far simpler and smaller.

For the submicrosecond range electrolytics aren't well suited (lets say you're trying to make some huge nitrogen laser). Although at that point size becomes a problem as well ... I think you'd need something like a PFN Marx to deliver all the energy in a submicrosecond range with Megajoule range energy storage.

Tetris

Sun Nov 27 2011, 10:13PM

Registered Member #4016 Joined: Thu Jul 21 2011, 01:52AM
Location: Gainesville, FL
Posts: 660

Antonio wrote ...

Quite dangerous, since 10 J are enough to kill, but you eat more than this energy every day.

To Antonio: There was this one site... I forgot what it was, but you entered your BMI and it said how many AAs would you equal. If you spontaneously combusted, you'd destroy the earth. Fortunately, physics doesn't allow that, hee hee.

To Harry: Now cap sound- byoeeeeeeeeeee.... *two years later* ...eeeeeeeee... increasingly getting higher and higher pitched....

To thread: Not yet the flux capacitor

but that's still big. :O

Marko

Sun Nov 27 2011, 10:34PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I think one of most likely ways for a homemade capacitor to get into if not megajoule, but at least kilojoule range would be a stacked party plate capacitor.

Buy several hundred to several thousand of them and have a bunch of people cut up the foil into right shape before you stack it in with some mineral oil.

Then you could use the result to crush cans or just generally blow stuff up.

Marko

Herr Zapp

Sun Nov 27 2011, 10:55PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

All -

It may be entertaining to joke about building a megajoule capacitor, but it's quite another to see the results of an uncontrolled energy release from an energy storage capacitor only a tiny fraction of this energy level.

One of the only documented incidents of this type is an accidental discharge of a 278uF, 22kV capacitor that Brian Basura was using to power his beautifully designed and constructed pulse-discharge quarter-shrinker. At full charge, this capacitor can deliver ~67 kJ, and the accident occurred when the capacitor was charged to at least 45 kJ.

Read about "the incident" here:

Brian's "backyard science" home page with info on his Tesla coils and most of the data on his quarter shrinker is here:

Herr Zapp

2Spoons

Mon Nov 28 2011, 12:22AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

I looked into building energy storage caps many years ago, and noticed that each type of dielectric has a 'specific energy density' which is a result of the combination of dielectric strength and dielectric constant.
Just an observation.

StevenCaton

Mon Nov 28 2011, 05:57AM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

Can you guys imagine what it would be like to own and build a high voltage capacitor that can store 1 megajoule and up of electric energy?

I would defibrillate a blue whale with a capacitor of that size.

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