Wednesday, April 13, 2011

How it's done volume 4: Getting a decent cap

Any decent sized accelerator relies on it's capacitors. The most important part of a good cap is it's voltage. The because the coils are highly inductive. The change in current through an inductor rises linearly with the voltage across that inductor. So we need high voltage to raise the current going through them as quickly as possible. If it was cheap/easy to power those coils at 10,000V I absolutely would. It would save a lot of hassle of firing coils in advance and getting timing just right. Of course, 10kV is hard to manage from an arcing standpoint and generally capacitors rated to that voltage have terrible power density.

As it happens, the type of cap with the highest power density and lowest cost per Joule is the electrolytic. It is not the fastest firing capacitor, but we're using it on an inductive load anyways so there's not a lot of point in going for hyper-fast firing. If you do any research at all you'll find that electrolytics go up to about 400 or 500V. That's about where I draw the line at vaguely save voltages anyways! How convenient.

If you hit a place like digikey you'll be able to buy these things in bulk for a few hundred dollars per dozen. I got mine for 9.88 $/Joule brand new. You can also usually find them on ebay but they'll be of more questionable construction and history.

As long as we're here, let's get serious for a second. Here's something I did once: Take a couple hundred kJ cap with a few hundred volts on it. Get a wire, bend it into a U shape, and the jam it across the leads of that cap. I wasn't wearing a shirt at the time so I could feel the pressure wave across my chest as part of the wire vaporized. If you're a litigious person, don't do this because it really is dangerous and you'll find a way to hurt yourself. If you've got balls of steel, a serious expert understanding of how not to die doing it, and are willing to risk your life if you're wrong, do try it. It will give your animal instincts a better understanding of the risks: Take a misstep and it'll nerf your heart in a millisecond. That shit is lighting in a can. But you need it to run your machine.

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