Harnessing the power of electromagnets


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), basically it made an electromagnet, and could pick up like a couple of paperclips or something similarly pathetic.

The turns of wire create a magnetic field, and the more current that is fed through these turns of wire, the greater the magnetic field. Simple.

When the resistance is too low, current will increase to a point where the power supply or the fuse will just blow up.

Herein lies the workings behind a coilgun, a device designed to propel metal 'bullets' using no gas, no gunpowder and no moving parts - only magnetism.

The problem here is that most power supplies you would have used for the paperclip device would only be capable of say, 2 or 3A maximum. 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 does one obtain 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, say in the vicinity of 10ms. The solution is a capacitor, of course. Preferably one with low ESR (equivalent series resistance), to allow higher peak current.

The one that I use in the coilgun is an inverter grade, relatively low ESR capacitor sourced from our good friend eBay.

Coilgun capacitor - there is a standard AA battery for scale.

For a bit of a reality check... a capacitor such as this one, which stores 3900uF at 400V, can store up to 312J of energy, which is theoretically able to kill a person 20 times over. It can also provide a discharge current of over 2000A, a factor of 80 times greater than what a standard kitchen oven would draw at any given time.

So, needless to say, capacitors can be quite dangerous in the wrong hands. Please be careful!

The idea of the coilgun then, if you haven't already realised, is to discharge this capacitor into a carefully tuned coil so that it produces an intense 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 is sucked into the coil at a very high speed. At this time, the magnetic field collapses, and as a result the moving 'bullet' continues to move out of the barrel at high speed.

My MS Paint skills at work

All wonderful for the theory. But what about reality? Well here is my coil + barrel. My barrel is just some random piece of aluminium tubing that fits snugly inside the coil former. There is nothing glamous-looking about this setup, that's for sure.

The wood blocks secure the coil, and the red tape covers a protective layer of iron laminations that are wrapped around the coil. In fact, these laminations assist in strengthening the magnetic field within the coil.

Here's a different view of the same thing. Pretty self explanatory I think. Again, there is nothing nice about the look of this thing. But hey, if you're a regular reading of this site then you'll know that ain't my style anyway.
And a view down the barrel. Very menacing. Below the barrel you can see the end of the big capacitor. I'll show you what's inside later. But first, the bullets.

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, though, that this might improve the aerodynamic stability. Just a theory...

Semikron stud SCR, 2100A surge

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 arse (so to speak) until you find a mechanical one, because there is no such thing as a small push button switch that can handle 2000A+.

A more practical way is a to use a semiconductor, such as a silicon controlled rectifier (SCR) which effectly acts as a high current switch. However these don't come cheap, but I scrounged around and found a supplier that sold used SCRs for $5.

This particular one was a Semikron brand stud SCR, which can handle a whopping 2100A surge current.

I also needed a hefty protection diode, because the voltage kickback from the coil would potentially destroy the capacitor, and the diode would be employed to absorb this kick. The diode chosen was slightly overkill but is also a stud variety.

Here's a picture of the internal setup with parts labeled. The rectifier diodes serve to convert the 300v AC supply into an equivalent DC voltage to charge the capacitor.

Entire coilgun setup

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 battery into at least 300VAC. This was achieved using an ordinary inverter circuit, such as the one used in my fluoro tube driver.

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 broke, and I was forced to dismantle the drill handle and ended up using it for the new improved night vision module.

This mechanism is something I came up with to ensure that the bullets are always loaded in the optimal position prior to shooting. It resembles, somewhat, the bolt of a gun, and 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 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. Unfortunately, this statue happened to be the first thing I could find. Thank you so much Nick... hahaha.

After a fun day of sharp shooting... a box of broken CD's, cases, and other random stuff. I've also broken a few tiles and put small dents in the wall (oops).


Stroboscopic photographs

Using the Penguin's Lab stroboscopic equipment detailed elsewhere, I attempted to capture images of the bullet coming out of the barrel at high speed.

Captured at ~20Hz flash rate. The power to the coilgun was turned down almost all the way because anything faster than that would not have been captured on time.

The bullets trajectory at extremely low firing power. Again captured at ~20Hz flash rate. I'm actually surprised at how the bullet doesn't swerve and turn and do all sorts of funny things in the air, because some of the impacts in the coke cans suggest that the bullets have hit the can side on.




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