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Transparent Electrothermal gun

Built a miniature cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Transparent Electrothermal gun

Unread postAuthor: Insomniac » Thu Oct 30, 2008 3:57 am

I was messing around with some disposable camera capacitors and ended up making a very very small ETG. It is basically two wires which go through a bunch of heatshrink tubing, which has a short length of drinking straw slipped over the top. If a small thin strip of aluminium foil is wrapped around the two wires, a spitball placed in the barrel (these seem to be the perfect ammo for it: Not too heavy, seals the barrel well, and doesn't get stuck), and the ends of the wires attached to a small capacitor bank, the spitball flys out with a bang. It is pretty weak, but it was fun to play around with.

The capacitor bank I'm using is pretty small, it is only 3 camera flash caps in parrallel. The are 130uf, 80uf and 160uf (I'm pretty sure mixing capacitences won't hurt them when in parrallel, correct me if I'm wrong) They are all rated for 330V.

Pros:
Bright muzzle flash
Satisfying noise
Dangerous amounts of electricity
See through
A small sheet of alfoil lasts hundreds of shots

Cons:
Longish charge time (I could use a battery that can supply more current, but don't want to fry anything)
Safe muzzle velocity (where's the fun in that?)
A bit fiddly to fire
Vapourized aluminium floating around my bedroom can't be good for me


The gun:
ImageImage

Electrodes with foil:
ImageImage


[youtube]http://www.youtube.com/watch?v=PsKhYB-VHqM[/youtube]
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Unread postAuthor: Hotwired » Thu Oct 30, 2008 5:45 am

Cute :P

Not much bigger than a LED without the barrel, scaling up from such a simple design would be easy too.
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Unread postAuthor: Insomniac » Thu Oct 30, 2008 6:30 am

I got sick of waiting for it to charge, and because I dismantled the capacitor bank so I had access to the individual caps, I did a few more shots with just the single 160uf cap. While it isn't quite as loud now, it is much more convenient. Faster charges, and I can simply hold the cap so that the bit between the leads is in contact with the black heatshrink on the gun, and by moving the cap backwards it comes in contact with the V shaped legs of the gun. This makes it easy to fire without fiddling around trying to make both leads touch. Because of that, I did a bit of shooting in the dark

Damn I love long exposure photos:

This first one was a blank shot with no ammo, with a super super short barrel to direct the sparks a bit:

ImageImage

These next two are real shots. The first one had a loose spitball that didn't fire very fast, and just bounced off the target. The second shot was much better and fired with such insane velocity it actually managed to stick to the plastic container I was firing at :lol: Still nowhere near as powerful as a mini combustion, but oh well. Makes for nice photos:

ImageImage

ImageImage

Another blank shot, this time with the 'real' barrel. Note the small cloud of plasma coming out the front (or is it just glare in the camera lens?):

ImageImage

These two were fired with no barrel, so I could see the pretty sparks :D It's strange, in both photos (epsecially the first), there is at least one spark that completely changes it's direction in midair and comes back towards the gun. It's amazing how perfect the one on the first pic looks (it's the spark furthest to the right I'm talking about) I wonder why this happens??? Other sparks, like from where the capacitor touches the leads, the sparks either fade or break apart. Aluminium sparks seem to be different.

ImageImage

ImageImage


Now for the important question:

What looks better? The green barrel in these pics, or the original orange one?
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I wonder how much deeper the ocean would be without sponges.
Right now I'm having amnesia and deja vu at the same time. I think I've forgotten this before.

Add me on msn!!! insomniac-55@hotmail.com
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Unread postAuthor: Gippeto » Thu Oct 30, 2008 9:21 am

I'm not really into mini's, but this has a certain "cool factor" to it. :D

Well done, and nice pics!
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Unread postAuthor: psycix » Thu Oct 30, 2008 9:53 am

Beautiful pictures!
And surely a fun little toy.

I think the green barrel looks better.
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Unread postAuthor: i-will » Thu Oct 30, 2008 2:25 pm

i had the same problem with my electromagnetic gun. i had about 9 caps with only one circuit charging it and took about 2 minutes. the solution was to wire a lot more charging circuits to the cap bank. if u put a stronger battery to one charger it will fry it. stick to one AA to a circuit. in the future leave the caps attached to the circuit. line them up and solder the caps in parallel. with every cap connected to its own circuit it will only take about 3-4 seconds to charge.
if u ever want to fire it with the speed of a semi-auto then u'll want to add 4-5 circuits to each cap. have fun with those flash caps. i now use them for grenade fuses. they're perfect for lighting a cannon fuse.
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Unread postAuthor: i-will » Thu Oct 30, 2008 2:32 pm

also if u solder a switch/trigger to complete the circuit u will prevent wasting energy on arcing the wires and the only sound u will hear will be in the barrel.
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Unread postAuthor: ramses » Thu Oct 30, 2008 4:41 pm

hurray for the edit button!!! (or not)

I also noticed the weird spark patterns. I used slightly more energy(243J), and possibly copper as well. I have since upgraded to nearly 3000j. I suppose I should make an ETG.
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Unread postAuthor: DYI » Thu Oct 30, 2008 6:39 pm

You lucky bastard, it took me 250J to propel just a little airsoft round to 600 fps, and it was so weak that it couldn't even fire when I used anything that weighed over 0.5g. I wasn't able to move a spitball in the barrel with TEN of those camera caps and an SCR for switching.

Great job on this extremely mini mini, but I was half expecting to see a topic about something Larda had made with some sort of transparent stainless steel bar :lol:
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Unread postAuthor: rp181 » Thu Oct 30, 2008 7:08 pm

:shock:

I just tried one. The barrel was a tent pole support sleaved with .5in PVC. the endcap was hotglue, and i used 800j through some screws, into the side of the barrel. I just tilted it so alu. ball could fall, and hit screws. it was LOUD, sounded like a bomb. The barrel is about 3ft long.

When i did it, there was no sparks, just a big flame (3ft) in a teardrop shape in front of the barrel.

At first i tried it with a epoxy putty tube, with a 8j cap dumped. It exploded, and set of the fire alarm =p. I did the above outside, neighbor hood playing children went quite :wink:
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Unread postAuthor: Insomniac » Thu Oct 30, 2008 11:35 pm

DYI wrote:I was half expecting to see a topic about something Larda had made with some sort of transparent stainless steel bar :lol:


Sorry if I made it sound more awesome that it really is :lol:

Actually, I'm not sorry at all. The slightly misleading title was semi-intentional :lol:


EDIT: Also, could someone explain the best types of switch for smallish capacitor banks? Is there a cheap mechanical switch that won't have huge losses, or will I have to use somthing like an SCR. What do I need to know to use an SCR with this circuit? Ie what numbers to pay attention to, how much would a suitable one cost, would most electronics shops have them, and how would I wire it up? (I'll probably google and figure out how to wire it, but I'd still like to hear from somone here)
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Unread postAuthor: Lentamentalisk » Fri Oct 31, 2008 1:49 am

I remember reading from back when I was designing a coil gun, that you need a SCR if you are going to go with much current, otherwise your switch will weld itself shut, by the arc melting the contacts.
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Unread postAuthor: starman » Fri Oct 31, 2008 2:24 am

DYI wrote:I was half expecting to see a topic about something Larda had made with some sort of transparent stainless steel bar :lol:


...or Scotty's favorite, transparent aluminum....
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Unread postAuthor: mark.f » Fri Oct 31, 2008 11:19 am

No no... you're thinking of Transparisteel :wink:
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