Coil Pistol v2, now with half decent relay switching. / Electromagnetic Projectile Accelerators / Forums

Forums

4hv.org :: Forums :: Electromagnetic Projectile Accelerators

« Previous topic | Next topic »

Coil Pistol v2, now with half decent relay switching.

Move Thread

LAN_403

Ragnarok

Sun Apr 22 2007, 08:18PM

Registered Member #659 Joined: Fri Apr 20 2007, 09:14AM
Location: SW Hertfordshire, UK
Posts: 75

Well, I only joined a couple of days ago, so it's probably worth showing you what stage I'm at currently.

This is my most successful coilgun so far (only finished it earlier). It's half made of tape and has wires all over the place at the moment, but it's working.

Sorry, I've removed these links. The hosting site was quite heavy on spyware I'm sure you don't want. I'll try to replace them soon.

The muzzle energy is somewhere between .5 and 1 Joules (estimated), which is fairly low I know, but it's nowhere near optimal. It runs on a 4 cap bank (Total 480 uf), hidden in the grip (There is space for more later on). This is usually charged to between 300 and 350V by a camera circuit hidden in the rear.(Running on 2AA batteries rather than the normal one, and with the normal switch replaced with a new one), giving a maximum stored electrical energy of about 29 J. Charge time is about 30 seconds.

The coil is 3 cm long, 2cm in diameter and has a resistance of about 6 ohms (A bit over 600 coils I think).

The whole pistol is about 9" long, 2.5" wide, 5" deep, and masses 360 grams. 6mm (plastic) barrel, firing 2cm long chunks of 6mm threaded rod (it's what I had available. I'll use smooth rod when I can get some).

The switching circuit is a relay with two microswitches attached in series (run by a 9V battery). One acts as a trigger, the other is installed above the breech to cut the relays power shortly before the round has moved entirely into the coil (after the round had moved out of the breech). This also neatly keeps the round in place.
The relay cuts the power after the P.D. has fallen to about 30 volts, which may not be ideal, but at least it's sort of working. The exact wiring both helps protect the camera flash circuit from accidental high currents, and the capacitors from back EMF.

So far, it'll punch through a few sheets of paper, but baulks at cardboard.

Now you know how much of a newb I am, we can begin.

uzzors2k

Sun Apr 22 2007, 09:02PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Not bad. There are a few things you can do to improve performance though. First, ditch the 6 ohm coil! You want high peak current, quick pulse, but with a 6 ohm coil it'll be way under par. Not only will it slow the pulse down and limit current, but it will burn power too. Power that should be going into the projectile! Aim for <0.5 ohms of coil resistance. Use thicker wire too. There is a great coil gun simulator at the magnetic gun club, however their site is down. You can put in coil inductance, resistance, bank voltage, etc, and get a simulation of the pulse and peak current. Very handy.

30 seconds for 30 joules is pretty slow, so you might want to build a better charger, this will be a life saver for those 500J banks later on.

Aahz made a nice boost converter design a while ago, check it out and build it. The self-stopping function will be required, as 30 joules will go in less than a second. Using a SCR to trigger would also save alot of power too. If you drop the coil resistance, the peak currents will be capable of welding the relay shut, so a SCR might be necessary.

Ragnarok

Mon Apr 23 2007, 07:35AM

Registered Member #659 Joined: Fri Apr 20 2007, 09:14AM
Location: SW Hertfordshire, UK
Posts: 75

The 6 ohm coil was made from what enamel coated wire I could find, finally taking the stuff out of the back of a smashed TV, so I'm not surprised it's rubbish.

Really this was a test project to see if I could work the system, so I wasn't going to blow much money on it. I added the relay and new switches yesterday to try and coax a little more out of it (before it was running on a simple button switch)

Total cost was about Â£7 ($14), and most of that went on the relay and microswitches. For under a tenner, I'm happy with the result.

I don't feel the need for a boost convertor yet - 30 seconds is survivable for this.
However, when it does come down to a 600J bank (which is what I hope for on my next project), I will certainly not be using a camera flash for that. (Judging by a 1W output, that'd be around ten minutes of waiting).

On other news, I tried adding a negative feedback loop onto the relay to help control pulse length better, and it seems to have improved power a bit (which still needs measuring properly though).

In the end, it's fun to use, which is probably the most important thing.

uzzors2k

Mon Apr 23 2007, 08:55PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Yeah, first coil guns tend to like that, just a quick and cheap lash-up. I was just thinking you might want to tweak it so you get a feel for things before you start the 600J gun. Still, you have made a pretty good coil gun given the components used. gj.

Ragnarok

Tue Apr 24 2007, 10:38AM

Registered Member #659 Joined: Fri Apr 20 2007, 09:14AM
Location: SW Hertfordshire, UK
Posts: 75

Thanks - I appreciate the postive comments.

For me, big jumps in what I've built aren't uncommon or impossible. You should see the huge difference between my early spudguns and my most recent ones - I'm not far off breaking the sound barrier now using some projectiles.

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob