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Railgun Switch Idea

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steve516

Sat May 14 2011, 09:28PM

Registered Member #3832 Joined: Thu Apr 14 2011, 11:57PM
Location: Downtown Chicago, IL
Posts: 37

I'm currently in a group building a 2.5kJ railgun for our AP Physics final. (I found awesome caps on eBay- 450V 8,000uF for only $7 a piece) Well, one of the issues we realized we have is the lack of an adequate triggering system. It seems fairly obvious that with the 10kA+ surge any normal switch would explode, and we even considered modifying or trying a circuit breaker, but I'm not even sure how those work or if it is plausible.

So today I was looking around my basement and noticed one of those wooden spring loaded Victor mouse traps. I played with it for a while and then started wondering- if I pulled out all the triggering crap and just had the spring w/ arm, and then welded/ mounted two copper plates, each about 2x2, one on the swinging arm and the other where it hits, I may have a damn good switch!

Google turns up results that show these things close in under 1/60th of a second, which means the time from no to full contact should be perfect with almost no bounce because of the spring being there.

Does this sound plausible? a 2"x2" copper plate could take a lot of current I believe!

Steve

Turkey9

Sat May 14 2011, 11:17PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

I would go with a railgun design that uses an injector. This way, the projectile is the switch.

Have your two rails connected directly to the caps and then use a spring or compressed air to shoot the projectile into the rails closing the circuit and firing the gun.

Inducktion

Sat May 14 2011, 11:44PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Why not try a two phase system? Make one branch charged up to 400 volts relative to ground, then the other branch charge up to -400 volts to ground. Put the two together over a coil, using two separate switches and basically halving the power requirements.

Just my two cents. :D

rp181

Sun May 15 2011, 12:04AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

I would go with an injector. Take a look at Quick Exhaust Valves (QEV's, common in the spudding world) a 1/4" QEV with a air tank will work well. With my 1/2" bore, I used a 3/4" qev:

Electroholic

Wed May 25 2011, 03:29AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

How about the sacrificial SCR idea?

Before triggering, SCR will block off the voltage, at trigger, junctions fuse together to form a conduction path. This way you only have to rate your scr to the standoff voltage. and to be structurally strong enough to hold together long enough for the pulse.

Dr. ISOTOP

Wed May 25 2011, 09:52AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

Puck SCRs have surge ratings of 30kA+; I'm sure one would work for your application.
They're also not terribly expensive new, and even less so on Ebay.

klugesmith

Thu May 26 2011, 01:35PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Let's say you want to start with a stationary armature between the rails, as opposed to shooting it in with an injector mechanism.

Your idea of a mechanical closing switch, with terminals slammed together by a rat or mouse trap bail, should be fine. Contacts can be much smaller (and lighter) than 2" square. Barry presents many details here:

You may also want extra series inductance to shape the current pulse, but I'm no railgun expert.

For voltages of at least 1000 V, I believe most can-crusher / coin-shrinker hobbyists have abandoned electronically triggered switches in favor of mechanically triggered spark gaps (in which the electrodes don't actually touch, and may be closed slowly). This can-crusher switch has just one moving part, not counting lanyard and return spring, and can handle 25 kA peaks (as can AWG10 wire, for short enough pulses).

The electrodes are from a drawer-knob and a water faucet handle.

steve516

Thu May 26 2011, 05:39PM

Registered Member #3832 Joined: Thu Apr 14 2011, 11:57PM
Location: Downtown Chicago, IL
Posts: 37

thanks for the advice guys! let you know how it turns out!

big5824

Thu May 26 2011, 07:15PM

Registered Member #1687 Joined: Tue Sept 09 2008, 08:47PM
Location: UK, Darlington
Posts: 240

Going from what iv learnt from the past 3 years of browsing, id say that amount of energy with a stationary projectile is going to cause too much heating at the area where it meets the rails, I can see it just turning into a glorified arc welder. An injector would spread the heat out more evenly, and it could probably get the projectile into tighter fitting rails than by hand (just a guess)

Inducktion

Thu May 26 2011, 08:36PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Is my idea not any good then? No one's said anything about it...

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