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What kind of SCR do i need?

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LAN_403

badastronaut

Sat Dec 27 2008, 09:20PM

Registered Member #222 Joined: Mon Feb 20 2006, 05:49PM
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At 1.3kJ your rails would be just a few inches long, if that at all. I guess it would look something like a stubby pistol lulz. The biggest thing going against small railguns is mass and resistance.

rp181

Sun Dec 28 2008, 12:54AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Speaking of resistance....
I was reading some technical papers, And it turns out velocity skin affect is the main cause. (on yours bad, Your bus bars where unneccesarily thick, due to skin affect, layered thin plates would be better, just saying =p) You can think of it has water in a bottle. When you move the bottle forward, the water goes to the back. Its the same thing. Thats means projectiles with V or U tails will have high resistance as they go fast. To combat this, they put a resistive coating on the side of the projectile (10x resistance of the projectile) and it was eliminated. Further, by introducing a liquid lubricant (indium), the resistive coating could be reduced to 3x the resistance of the projectile.

This is not a concern for 1.3kJ though =p. It will be hard making a railgun with 1.3kJ in a handheld design that is actually reliable. You would be better of doing electrothermal gun. Ive been doing a electrothermal gun with 1.3kJ, and it works great. On youtube, one guy pierced a can with 100J. You would be able to get considarable power.

hotcrazyfruit

Sun Dec 28 2008, 04:16AM

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how long would the rails last with 1.3kj? can any one of you guys give me a good charging circuit? ill just figure out the rest of the gun.
thx.

badastronaut

Sun Dec 28 2008, 06:25AM

Registered Member #222 Joined: Mon Feb 20 2006, 05:49PM
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Velocity skin effect is just another form of AC resistance. On the cap bank bus bar, with multiple plates, you have to take into consideration the proximity effect.

Bigger bus bars allow for a larger perimeter for a lower resistance. They are not oversized although the dimensions were chosen arbitrarily. Bigger would still have been better. Using hollow tubing would be more material efficient but the effect is the same. One of the goals was to also minimize inductance. The total circuit inductance ended up being about 200nH.

No one really uses indium, and the velocity skin effect is beyond the scope of most small scale railguns. It's really the thermal problems that worry people rather than the losses.

The capacitors plus bus bars absorbed less 2kJ out of 16kJ delivered energy. The railgun itself absored 4.5kJ, and that energy is the KE plus all the energy lost in the copper rails. The majority of the energy was lost between the cap and the railgun, which contained the switch and the exploded part of the rails. My guess is that the majority of the energy got wasted in expanding the rails. I don't have the data to prove it either way though, which is why more testing is necessary.

Rail life depends on how good of a railgun you have. The more contact force you have to prevent melting, the longer it will last. Generally with such small energies, I would recommend the plasma armature railgun. You'll be able to fire it many times and you won't have to worry about having a good contact. The limit would be how well the projectile fits inside. In this way, it will also act as an electrothermal gun. You'll definitely need an SCR to do well, and I would recommend steel wool as the fuse.

rp181

Sun Dec 28 2008, 04:57PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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I never said it was practical to do that stuff =p I havn't heard about the proximity affect, ile look into that. At 550 volts, you could use a mains voltage multiplier, but only if its not portable. You could use a boost converter, or a Switch mode power supply. Take a look at uzzors big mofo core thread, its basically that scaled down.

hotcrazyfruit

Mon Dec 29 2008, 07:34AM

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Registered Member #1877 Joined: Mon Dec 22 2008, 02:03AM
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We need some kind of current limiter for the charging circuit. can you recommend any kinds of protection/charging circuits to charge the capacitor bank, other than just rectifying transformer output. will using 4 150watt resistors in series be enough to provide a good amount limiting, but keep the capacitor charge time in a reasonable amount?
any help is appreciated. xD

El_Roberto

Mon Dec 29 2008, 07:56AM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
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That would be fine, Just depends on what value the resistors are, To charge my coilgun I just rectified the mains and put a 140watt 1Kohm resistor bank on the output and it charges to 300volts in around a minute.

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