This article is very old, from around 2006. It was originally published on my old website, flylab.ovh.org, now defunct. It’s unlikely to have much educational value. I’ve republished it mainly for sentimental reasons. Grammar and style were left “as is”.
I would like to describe one of my projects I’ve done during my secondary school (in 2005). It’s a didactic model of coilgun, alternatively called Gauss gun or simply electromagnetic rifle. Besides a large amount of fun, it can deliver some knowledge about electromagnetism an electronics. The kinetic energy of the projectile thrown out of the rifle is relatively small (ca. 3 J), but it’s enough to pierce an aluminium soda can. When you are playing with it, you have to be extremely cautious because of the risk of being injured. The author takes no responsibility for any actions and accidents caused by this device.
The principle of operation
A coilgun or Gauss gun is a type of weapon that uses one or more electromagnetic coils winded on the diamagnetic barrel to accelerate a magnetic projectile to high velocity. Before the shoot, the projectile stays just at the inlet of the first coil. When the coilgun fires, the current flows though the coil, causes the projectile to go inside the coil. The current should disappear when the projectile reaches the geometric center of the coil. In other case, the projectile would be strongly slowed down. The principle of operation of basic one-stage coilgun illustrates the animation below:
In multi-stage coilgun there are more than one winding. They are released one-by-one in appropriate moments with the help of optic sensors (slotted optocouplers). In the more advanced gun’s optocouplers are used to detect the moment when the projectile reaches the center of the coil as well.
In the simplest coilguns, like the one presented below, the sensors are not used at all. The initial position of the projectile and the capacitance of the capacitor set are experimentally adjusted to achieve optimal results. The capacitors have to lose their charge completely when the projectile is in the center of the coil, so it isn’t slowed down. Instead of transistors, the thyristor, alternatively called SCR (Silicon-controlled rectifier) is used. This kind of switching element, once triggered, keeps being open till the moment when the current drops to the specific low value.
Construction
Building a Gauss gun on your own is relatively simple, all you need is some basic electronic and mechanic knowledge as common tools. The custom parts which you have to made manually, are described in detail in separate paragraphs. In Poland, we have 230 V 50 Hz in the mains. If your power line voltage is noticeably lower (like 115 V) and you want to achieve good results, you should use the transformer to increase the voltage.
Required parts and tools
| Part | Description |
|---|---|
| Capacitor set min. 2000 μF 400 V | In my own set I’ve used capacitors from an old broken computer power supplies. There are usually two 330 μF 200 V caps in each. If you connect them together in series, you’ll obtain the 165 μF 400 V cap. My own set has 2420 μF total. It would better to use the capacitors designed for the full voltage, instead of connecting the smaller ones, especially if they are low ESR. Taking them from scrap is cheaper, though. |
| Thyristor (SCR) min. 40 A 500 V | Minimal continuous current 40 A. I’ve used one manufactured in Poland. |
| About 25 meters (27 yd) of 1 mm enameled copper wire (18 AWG) | |
| Glass pipe, Φ10 mm, 10 cm long | I’ve used a chemical dropper with each side cut of. Warning! The pipe breaks easily, so be careful during the process. |
| Transformer 230 V / 30–40 V | It’s needed to increase the voltage for capacitors. |
| Bridge rectifier 1000 V 2 A | |
| Light bulb 230 V 15 W | With a proper socket. |
| Switches and buttons | Toggle buttons: 1 on-off button and 1 on-off-on one, push-button (microswitches are not recommended). |
| Voltmeter 400 V | It’s not essential, but useful (in measuring the voltage stored in capacitors). You need it to set up the device. |
| two rectifier diodes 1 A 1000 V | |
| Resistors | 1x 1 kΩ 0.5 W and 1x 2 kΩ 5 W |
| Other stuff | Like wires, the chassis, insulating tape and so on. |
The barrel
Once I said, that I’ve made the barrel from glass chemical dropper Φ10 mm. After some tests I had made it happened to be an optimal diameter. If you don’t have a glass pipe, or you’re afraid the projectile may break it, use a plastic pen or something like this (however I didn’t test this idea). Glass is better, because it’s hard, smooth and resistant to attrition. You may grease the barrel inside, to decrease the resistance.
So you have the pipe. Now you have to cut it. It should be about 10 cm long (if yours is 7–13 cm, don’t cut it, it’s too risky). At first, mark by waterproof marker, where will you cut. To protect the pipe and yourself, wind an insulating tape several times round the both endings of the barrel. Now take a diamond glass cutter and roll it many times on the planned endings. WARNING! Now be very CAREFUL. It’s a critical moment. Break the endings carefully. Now they are very sharp, so protect them using an insulating tape.
The coil
After you make the barrel, it’s time to wind a coil. At first, you have to make a carcass. You need a piece of paper and cardboard, scissors, a insulating tape and a good glue. Cut a 5 x 10 cm rectangle from paper, wind it on the barrel (not tightly) and wind the tape on it. Make 2 cardboard rectangles with 11 mm holes inside. Stick them to both sides of the carcass. Make two 1 mm holes on each side for wire.
Now you can wind the coil. Mine has five layers. I’ve wound the whole carcass, so it has approximately 250 turns. Protect each layer using the insulating tape. Measure the resistance of the coil. Mine has about 0.5 Ω. Don’t forget to leave 20 cm endings to connect the coil to the rest of the coilgun.
The capacitor set
The energy that is transferred to the coil when you press the trigger is accumulated in the capacitor set. More capacitance your set will have, better effects will you achieve. It should be at least 2000 μF at the voltage of 400 V. There’s no risk of overkill, up to value of 8000 μF. The capacitors will discharge before the projectile reaches the center of the coil.
I made my set of electrolytic capacitors taken from old AT/ATX power supplies. Every supply has two of them, with capacity varying from 220 to 680 μF, 200 V. The pair of them (necessarily the same nominal capacitance, taken from the same power supply) be connected in series, then in a parallel configuration. Do it using a thick cooper wire. I’ve used 2 mm wire, main lines — 2x 2 mm.
For countries, where the power line voltage is 115 V AC. You can connect all of your 200 V capacitors in parallel. In this way, your set will have the capacity twice as big as my one. It won’t compensate the difference with my set, but it’s better than nothing (it’s better to increase the voltage than capacity, because voltage is squared in the energy equation, you can see it below).
Connecting everything together
All electrical connections are shown in the schematic below:
S1 is main power switch. The transformer connected in this way increases the voltage to ca. 260 V AC, so it charges the capacitors to ca. 365 V DC. Remember that the transformer’s windings should be in good phase. If you discover, that the voltages are subtracting rather than adding, simply swap the polarity of one’s winding, it doesn’t matter which one.
The light bulb limits the charging current up to 0.5 A. The S2 switch has three positions: CHARGING, DISCONNECTED, DISCHARGING. The wires which conduct the large amount of current (on the loop: SCR, capacitors, coil) should be thick e.g. Φ3 mm, to make as low power loss as possible.
Because of high voltages, the coilgun has to have an insulating cabinet. Mine, made of polyurethane, is not safe, but provides proper access to all essential parts of the device.
Gallery
The photos below show my version of the coilgun — the electromagnetic gun. Sorry for bad quality, they were taken many years ago.
Calculations
Below there are simple equations which can help you get some information about your coilgun.
Energy accumulated in capacitor set:
Where:
- E — stored energy [J]
- C — capacitor’s capacitance [F]
- U — voltage [V]
Projectile kinetic energy:
Where:
- Ek — kinetic energy [J]
- m — projectile’s weight [kg]
- Vo — speed of the projectile [m/s]
Approximate initial speed of the projectile:
Where:
- Vo — initial speed of the projectile [m/s]
- g — standard gravity — 9.81 m/s²
- l — the distance, where the projectile has fallen down [m]
- h — height above the ground [m]
Projectiles
As projectiles, I’ve used properly cut iron nails (look at the photos below). The biggest one was made of 7 mm steel rod and I achieved the best results with it. You can use every ferromagnetic you want, like nails, screws, bearing balls and so on.
Experiments
I’ve tested my coilgun on old coke tanks. It’s possible to perforate two cans standing in line. From the distance of one meter — just one can, from 2 meters it only perforates one side of the can and stays inside. Perhaps the aerodynamic shape of the projectile is bad.
I calculated the speed and initial kinetic energy of the projectile. I shot it from the balcony (5.5 m above the ground). It reached the ground from the balcony, so the initial speed was at least 28.7 m/s (103.4 km/h), energy 2.0 J (projectile weight 5 g).