Did you ever, as a young child, do the "wind lots of turns of wire around an iron rod and apply power from a battery" experiment? If you missed out (oh dear, you poor thing), then basically what it did was it made an electromagnet, and you could pick up like a couple of paperclips or something pathetic like that.
However, most people would just say, 'wow', and leave it at that. Excuse my badly drawn diagram to the right. Especially the paperclips.
When the resistance is too low, current will increase to a point where the power supply or the fuse will just blow up.
The turns of wire create a magnetic field, and the more turns there are, the greater the magnetic field. The amount of current that travels through the wire also affects the magnetic field proportionally.
The problem here is that most power supplies you would have used for this are only capable of say, a little bit, maybe 2 or 3A at the max. So even if you had less turns (less resistance = more current), your current would peak at around 3A, and simply stop.
So the next logical question would be, where do I get a power supply that can provide close to unlimited current?
The answer is, you can't. But you can get one that supplies almost unlimited current for a very short duration, perhaps 10ms.
Its called a capacitor. Preferably one with low ESR (equivalent series resistance), as they can provide more peak current.
The one that I use in the coilgun is on the right.
There is a standard AA battery for scale.
Just so you know what you're up against... it holds a max of 400V at 3900uF, which is 312J (able to kill a person 20 times over). It can provide a discharge of over 2000A (which is 80 times as much as your oven uses).
So, needless to say, quite dangerous in the wrong hands.
The idea of the coilgun, if you haven't already realised, is to discharge this capacitor into a carefully tuned coil so that it produces a HUGE magnetic field. This magnetic field will then attract a length of steel rod (the bullet), and accelerate it to a very high velocity.
As the coil will only be energized for a few milliseconds, the bullet be sucked at a very high speed into the coil and, as it approaches the middle of the coil (where it would stop if the coil was on all time), the coil will turn off, which means the bullet keeps going (due to its inertia) out of the barrel.
All wonderful for the theory. But what about reality? Well here is the coil + barrel. My barrel is just some random piece of aluminium tubing that fitted snugly inside the coil former.
The wood is to secure the coil, and the red tape crap covers a protective layer of iron laminations that are wrapped around the coil. Actually these laminations also help strengthen the magnetic field within the coil.
The bullets, I admit, are not made out of the ideal material. First I cut 2cm bits off a long steel thread, then one by one I sharpened them with an angle grinder.
The thread actually reduces the amount of iron, so that doesn't really help. I hoped that it might improve the aerodynamic stability though.
So how do we go about dumping the capacitor's energy into this coil? Quite clearly we need some kind of switch, but you can look up a donkeys butt until you find a mechanical one, because there is no such thing as a small push button switch that can handle 2000A+.
So it was onto find a semiconductor solution. However these don't come cheap, but I scrounged around and found a supplier that sold used SCR's (a type of semiconductor switch) for like US$5.
This particular one was a semikron brand stud SCR, which can handle a whopping 2100A surge.
I also needed a hefty protection diode, because the voltage kickback from the coil would potentially be able to screw up the capacitor, so the diode would be employed to absorb this kick.
Here's a picture of the internal setup with parts labeled. The rectifier diodes I haven't mentioned, but all they do is turn the 300v AC supply into an equivalent DC voltage to charge the capacitor.
The only other thing I haven't mentioned is the DC-AC inverter. Because I wanted to use a battery (since when did guns carry around a power cable?), I had to have something to boost the 12VDC into at least 300VAC. Oatley electronics sold me a kit for this, so no drama there.
Originally I had the bottom end of a drill as the handle, with the drill battery powering the gun. But after a while the battery screwed up, and so I dismantled the drill handle and used it for the new improved night vision module.
Here is the whole setup, with lid on.
Oh oh, almost forgot. This is the bit that is equivalent to the bolt of a gun. You put the bullet in that gap there, and slide the bolt forward to cock it.
It is configured so that the bullets are in exactly the right position behind the coil after the bolt is armed.
So... results?
Mmm, nice clean holes through coke cans.
Using a microphone and a computer, I recorded a shot over 2 meters or so, and used the delay to calculate the velocity. Turns out the bullet's velocity is around 30m/s, or just over 100km/h.
And a shot through a ceramic statue's head from a few meters away.
Actually this was given to me as a present by Nick. Thank you so much Nick... hahaha.
