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Harvesting Capacitors for Backyard Munitions
Diabolus writes "This is the tale of a man, a bunch of disposable cameras, and his techniques for harvesting lots of capacitors to build a gauss gun. Insane..." A basic capacitor tutorial is probably in order.
And I bet it still won't work....
"The best argument against democracy is a five minute chat with the average voter."
--Winston Churchill
>Am I missing something, or is his time worth nothing?
.1 F.
Yes, you are missing something.
Car audio capacitor: 16-20 volts @
Camera flash capacitor: 330 volts @ 120 uF.
Now lets see, using the formula E = 1/2 * U^2 * C, how many joules are in each capacitor.
Car audio capacitor: E = 20 Joules
Flash capacitor: E = 6.534 Joules
Car Cap: $40 or $2 per joule.
Disposable camera: $5 or $0.76 per joule.
Camera caps are far cheaper, and this guy got them for free.
If you could be told what you can see or read, then it follows that you could be told what to say or think - BoC
Yes, you are missing the fact that most 'car audio' capacitors are 'rubbish' (to be polite) and are simply a method or removing money from peoples pockets.
For his purposes he requires an actual high capacity, high discharge rate capacitor, not an easy thing to create.
The challenge is having a VERY low ESR (effective series resistance), as well as a low inductance, and using a massive number of parallel capacitors is certainly one of the only economical ways of doing this. a LOT of care is also needed in how they are connected up to keep the inductance down.
You can, for example, by multi-farad 'supercaps', but these have charge/discharge rates in the milliamps and are used for memory backup and other purposes, you can also get kilovolt rated caps with very low capacitance, but it is very hard to get medium voltage very low ESR high energy caps, primarily because they are lethal. They are used in radar installations and a few other high energy 'toys'.
I don't know anything about car capacitors, so perhaps this is a useless answer, but.... capacitor banks, rather than single large capacitors, are used in high-current circuits because they can crank out a whole lot more current a lot quicker when you have lots of small ones in parallel. This is frequently done even on small bypass capacitors in circuits -- rather than putting a few thousand microfarads on a motor, you might put several 470uF caps, for instance. The response is much quicker. So not surprisingly, this is the way you always do experiments that require frightening amounts of juice.
The streets shall flow with the blood of the Guberminky.
First of all, there are no pictures of a gauss gun, because it doesn't exist yet. I have the parts for an inverter to charge it, but have not yet assembled it. Not having had much experience with switch mode power supplies, I'm just hoping it doesn't burn down.
For someone else's (working) effort, check out powerlabs.org
As to those who think car capacitors, etc might be better, they have a very low voltage rating (eg 15V). E=0.5*C*V^2, so even with a farad the energy isn't that great. Secondly, they are made from *thin* foil and have crappy current ratings, not much good for generating 10kA for 1ms. These are photoflash-rated caps, intended for 1ms discharge times - thick foil, good dielectric and some actual quality control.
Lastly, the capacitors will have to discharge through an inductor, even if it is only a couple of turns. Lack of voltage means the current rise is too slow (dI/dt = V/L) and so a low voltage, high capacitance bank will not discharge fast enough. Slow discharge means the ring has moved away before it receives much energy.
I'm also missing large silicon devices to actually discharge the thing. SCRs that can handle 10 or 20kA are not common and seem to cost many hundred of dollars. If anyone's got a spare one, please tell me! Otherwise I will have to make do with lots of smaller devices from surplus shops and build it multi-stage with messy triggering.
As for energy, consider 0.5*m*V^2. Given about 3 to 5kJ and about 2g of mass, you figure it out. It will be lucky to get 1% efficiency, but still. If it works well, I might have to look into firearms licensing. Big deal.
To those who say "you're a dickhead, that's lame", well, fair enough. Its not for everyone, and this page wasn't put up for the express of having it critiqued by /. bottom-feeders. I guess you could say "that's just sad", but if you're not an eleceng then you're not going to get what's interesting about this stuff.
Grandparent correct, As the owner of a few car audio shops, nothing made my day more than selling stuff to kids who thought they needed it. The sales pitch was always based on some fragment of truth then a wild jump to a $200 solution. In the case of the caps, the fragment of truth was that the voltage at the back of the car dropped when the amp sucked some current. The REAL answer is both of the following: 1: thats how electricity works 2: the effect can be minimized by making sure your connections are solid (the drop is caused by resistance, NOT the fact that your battery cant keep up to the demand.. the battery can run your starter at a couple hundred amps)
The real answers arent all that profitable, nor are they very cool, so we sell you big caps, and your friends drool with envy at your new equipment while you pretend to hear a big difference in the sound. The same principle drives the music/clothing/other industries... 'the dumbest buy the mostest'
Have you ever played with a car amp that needs 1000 watts?
No, and neither have you. 1K watts is about 1.5 HORSEPOWER. In recent years, many manufacturers of car audio equipment have been competing for your money by putting really large numbers on the cases of thier devices just because it makes kids buy them. Take the average radio.. a good one claims to do 40 watts per channel, on a 12 volt powersupply and a 4 ohm load, the maximum possible output wattage is 36.. and thats assuming a 100% efficient amplifier (which doesnt exist)
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