I would like to describe one of my projects, which I completed in 2005 during my secondary school years. It is a didactic model of a coilgun, also known as a Gauss gun or simply an electromagnetic rifle. Besides providing a large amount of fun, it can offer valuable knowledge about electromagnetism and electronics.
The kinetic energy of the projectile fired from the rifle is relatively small (approximately 3 J), but it is sufficient to pierce an aluminum soda can. When using it, you must be extremely cautious due to the risk of injury (especially to the eyes). Even more hazardous is the mains voltage present in the device. The author takes no responsibility for any actions or 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 wound around a diamagnetic barrel to accelerate a ferromagnetic projectile to high velocity. The projectile rests just at the entrance of the coil. When the coilgun fires, the current flows through the coil, causing the projectile to be pulled into the coil’s interior. The current flow should stop just as the projectile reaches the geometric center of the coil. Otherwise, the projectile would be significantly slowed down. The principle of operation of a basic one-stage coilgun is illustrated by the animation:
Multi-stage coilguns have several coils that are activated in sequence. Each stage is designed to accelerate the projectile even further. The activation of the next stage is triggered by the projectile passing the correct position with the help of optical sensors (slotted optocouplers). In more advanced gun designs, additional optocouplers are used to detect the moment when the projectile reaches the center of the coil. This can be used to switch off the coil current at the correct moment.
In the simplest coilguns, like the one presented here, the sensors are not present at all. The resting position of the projectile and the capacitance of the capacitor bank are determined by trial and error to achieve optimal results. The capacitors must lose their charge completely when the projectile is at the center of the coil to prevent it from being slowed down. Due to the large current involved and the need for greater robustness, thyristors, also called SCR (Silicon-controlled rectifier), are used instead of power transistors. A thyristor, once triggered by a pulse of small current through its gate, keeps conducting until the anode-cathode current drops below a specific value.
Construction
Building a Gauss gun on your own is relatively simple; all you need is some basic electronic and mechanical knowledge, along with some common workshop tools. There are some custom parts you have to make by hand, which 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 (such as 115 V) and you want to achieve better results, you should use a step-up transformer to increase the voltage.
Required parts and tools
| Part | Description |
|---|---|
| Capacitor bank minimum 2000 μF 400 V | In my own bank, I’ve used capacitors from old, broken computer power supplies. Each power supply usually contains two 220–330 µF 200 V capacitors. If you connect two of them in series, you’ll get a 165 µF 400 V capacitor. My own bank has a total of 2420 µF. It would be better to use capacitors rated for the full voltage, instead of connecting smaller ones, especially if they have low ESR and are designed for pulse applications. Taking them from scrap is cheaper, though. |
| Thyristor (SCR) minimum 40 A 500 V | Rated for a minimum of 40 A continuous current. I’ve used Unitra Lamina T R51-63-12-76 (datasheet), 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 both ends cut off. Warning! The pipe breaks easily, so be careful during the cutting process. |
| Transformer 230 V / 30–40 V | It’s needed to increase the charging voltage to achieve higher performance. |
| Full bridge rectifier 1000 V 2 A | |
| Light bulb 230 V 15 W | With a proper socket. Limits the charging current. |
| Switches and buttons | Toggle buttons: one on-off button and one on-off-on button, push-button (microswitches are not recommended). |
| Voltmeter 400 V | It’s not essential, but useful for measuring the current capacitor voltage. You need it to set up the device anyway. |
| 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 etc. |
The barrel
My barrel is made of a glass chemical dropper with a diameter of 10 mm. After some tests, I determined that this is the 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 similar, though I haven’t tested this idea. Glass seems to be better because it’s hard, smooth, and resistant to wear caused by the rapidly accelerating projectile. You may grease the inside of the barrel to decrease the resistance a bit.
So, you have the glass pipe. Now, you need to cut it to be about 10 cm long (if yours is between 7–13 cm, don’t cut it at all, it’s too risky). First, mark the places you’ll be cutting using a waterproof marker. To protect the pipe and yourself from the glass shards, wrap insulating tape several times around both ends of the barrel. Next, take a diamond glass cutter and roll it a few times around the markings. WARNING! Now be very CAREFUL. This is a critical moment. Carefully break the ends. You may also tap lightly on the end with a wooden stick. The ends are quite sharp, so you may want to grind them using sandpaper or cover them with insulating tape.
The coil
When you’re done with the barrel, it’s time to make the coil. First, you need to make a bobbin. On the bobbin, you’ll wind the copper wire. You’ll need a piece of hard paper, cardboard, scissors, insulating tape, and strong glue. Cut a 5 x 10 cm rectangle from paper, wind it around the barrel, and secure it with tape. The cylinder should fit tightly on the barrel, but it must still be possible to slide it left and right. Cut two cardboard rectangles with 11 mm holes in the middle. Glue them to both ends of the bobbin. Punch two 1 mm holes for the wire on each side.
Now you can wind the coil. Mine has five layers. I filled the entire bobbin with the wire, the coil has approximately 250 turns. Cover each layer with insulating tape. Measure the resistance of the coil to ensure it has no breaks or shorts. Mine has about 0.5 ohms. Don’t forget to leave 20 cm of wire at both ends to connect the coil to the rest of the coilgun.
The capacitor bank
The energy transferred to the coil when you press the trigger is stored in the capacitor bank. The more capacitance your bank has, the higher the performance you will achieve. It should be at least 2000 µF assuming the voltage is 400 V. There’s no risk of overkill, up to a value of 8000 µF. The current during launch is so high that the capacitors will discharge long before the projectile reaches the center of the coil.
I disassembled a couple of broken AT and ATX computer power supplies to salvage the capacitors. Each power supply has two capacitors, with capacities varying from 220 to 680 µF, 200 V. Two of them, necessarily identical, are to be connected in series, which halves the capacitance but doubles the maximum voltage. Then, a number of pairs should be connected in parallel. Do this with thick copper wire to minimize the resistance and inductance. I used 4 mm² wire as bridge in each pair, for the main lines: 2x 4 mm².
For countries where the power line voltage is 115 V AC: You can connect all of your 200 V capacitors in parallel. This way, your bank will have twice the capacity of mine. It won’t compensate for the difference in voltage, but it’s better than nothing. To maximize the stored energy, it’s better to increase the voltage rather than the capacity, because voltage is squared in the energy equation.
Connecting everything together
All electrical connections are shown in the schematic below:
S1 is the main power switch.
The transformer, connected in this way, increases the voltage to about 260 V AC, so the charging voltage is about 365 V DC after rectification.
Remember that the transformer’s primary and secondary windings should be in-phase.
If you notice that the voltages are subtracting rather than adding, simply swap the polarity of one of the windings—it doesn’t matter which one.
The light bulb L1 limits the charging current to up to 0.5 A.
The S2 switch has three positions: CHARGING, DISCONNECTED, and DISCHARGING.
In the DISCHARGING position, the capacitor bank is discharged through the resistor R2.
V1 are the terminals for the voltmeter.
Diode D2 protects the system against back EMF generated by the coil.
The wires between the capacitor bank, the SCR, and the coil should be as short as possible and have a large cross-section to minimize inductance, which limits the current.
Because of the high voltages in the system, the coilgun must have a cabinet made of insulating material. Mine, made of Novotext, is not safe but provides proper access to all essential parts of the device. Don’t copy this; use a fully enclosed box.
After you’ve assembled the device, don’t fire it yet. Check every connection first. Then connect the device to the mains and start the process of reforming the capacitors. Leave the device in CHARGING mode for several hours, preferably overnight. Reforming is recommended for old capacitors, especially since we’ll be using them in a high-voltage circuit with high currents.
Gallery of the electromagnetic gun
The photos below show my version of the coilgun. Sorry for the bad quality; they were taken in 2005.
Calculations
Below are the basic equations that may help you gather some information about your coilgun.
Energy stored in capacitor bank:
Where:
- E — stored energy [J]
- C — capacitor’s capacitance [F]
- U — voltage [V]
Projectile’s 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 between the end of the barrel and the place where the projectile has fallen [m]
- h — height above the ground [m]
Projectiles
I made my projectiles from various pieces of steel rods and nails. The largest one was made from a 7 mm steel rod, and I achieved the best results with it. You can use any ferromagnetic material you want, such as nails, screws, bearing balls, and so on.
Experiments
I’ve tested my coilgun on empty soda cans. When the can touches the exit of the barrel, the projectile can perforate two cans standing in line. If the can is placed one meter from the barrel, the projectile goes through only one can. If placed two meters away, the projectile perforates only one side of the can and stays inside. This means that the aerodynamic characteristics of the projectiles are poor.
I calculated the approximate speed and initial kinetic energy of the projectile. I shot it from the balcony (5.5 m above the ground). It touched the ground 30.4 m from the balcony, so the initial speed was at least 28.7 m/s (103.4 km/h), with energy of 2.0 J (projectile weight 5 g).