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What kind of SCR do i need?

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hotcrazyfruit

Mon Dec 22 2008, 02:09AM

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I am making a railgun. i have the capacitors. they total about 1.3 kj and a possibility of 3 , depending if we get more capacitors. at 550 volts.
but what kind of SCR do i need?what rating? any help is greatly appreciated.

Backyard Skunkworks

Mon Dec 22 2008, 04:53AM

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I think you are MUCH better off using compressed air injection of the projectile, this not only eliminates the need for SCRs; it lessens the chance of the railgun functioning more like a welder.

Also, get a large protection diode to place in anti-parallel with the capacitor bank (I'm out of my depth for railgun protection diode ratings, anyone?).

And of course you'll need decent precision on all the components, and very low resistance.

rp181

Mon Dec 22 2008, 04:49PM

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There is no problem of welding, especially at 1.3kJ, that is unless the projectile falls through. The projectile fit should be tight, it should be hard to push the projectile in. I do agree on the fact of compressed air/co2 injection, as SCR's are expensive.

hotcrazyfruit

Tue Dec 23 2008, 12:53AM

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Thank to both of your answers. and one last thing, what kind of diodes will i need? how much will they cost? and where can i get them (in canada)?

hotcrazyfruit

Tue Dec 23 2008, 12:56AM

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Also what is anti parallel? lol. i guess i thought i knew my stuff. xD

hotcrazyfruit

Tue Dec 23 2008, 01:06AM

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FURTHERMORE...will i need garolite to keep the rails together? how much force are we talking about with 1.3 kj?

rp181

Tue Dec 23 2008, 02:23AM

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For future reference, look at the top of your post and click edit rather then post 3 times.

Antiparrallel is when the anode of the capacitor bank is connected to the cathode of the diode. So when the bank charges, it should NOT conduct. If reverse polarity was applied to the capacitors, then the diode should short it.

Garolite is unnecessarily expensive. It is strong, but over kill. For 1.3kJ, 2 pieces of 1/4" polycarbonate sandwiching the rails will work fine.

By mentioning garolite, im assuming you got that from powerlabs. Some of his information on railgun's is misleading such as the welding and how you need to lengthen the pulse.

If you do plan on doing the compressed injection, use air rather then co2. co2 has a higher density, so the expansion of gas is slower, which means lower speed on the projectile.

Backyard Skunkworks

Tue Dec 23 2008, 02:28AM

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EDIT: Whoops! Typed my post at the same time as rp181!

I don't know if a reversing diode is totally necessary for a railgun; however it was mentioned in one of the IEEE papers I saw... It can't be a bad idea.

You would want a diode rated for a very high current spike (few hundred amps).

Anti-parallel means the diode is connected across the capacitors, in the oppisite direction of normal current flow.

I'm a bit out of my depth on this, rp181 is the railgun guy.

hotcrazyfruit

Tue Dec 23 2008, 03:40AM

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Thanks to both answerers. xD
i appreciate it.

rp181

Tue Dec 23 2008, 04:03AM

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Ive looked alot into theory of railguns, but all the people that have built one have disappeared...
A diode is not totaly necessary, if the rails are long enough. It would be a great thing to have though, as a precaution. A ISOTOP diode will work, surge rating goes up dramatically with short pulse times. Ive used ISOTOP's succesfully in inductive loads, which has way more back EMF. When i am actually worried about back EMF i use a 200A stud diode, thats rated for ~6kA surge for 10ms.
If you have the means to measure total circuit resistance (including ESR of the capacitor bank) and inductance accuratly, you will be able to calculate the current peak, pulse width, and any inductive kickback.

EDIT: STTH6010W from STmicroelectronics will work fine. You can just put in a sample request, and explain your project, and they will most likley give too you.

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