|Bank Energy||434.5 J|
|Bank Voltage||450 V|
|Bank Capacitance||4.7 mF|
|Charger||15 W Boost Converter|
|Coil Inductance||80 µH|
|Coil Resistance||180 mOhms|
|Estimated Peak Current||1.4 kA|
|Projectile Weight||17 grams|
|Average Projectile Speed||27 m/s|
|Average Efficiency||1.45 %|
|Peak Efficiency (at 200V bank voltage, 11g projectile)||2.65%|
It doesn't take many components to put together a single stage coil gun, but due to the high energy involved a single error could result in a violent failure. The peak current, discharge time, reverse current/ voltage can be calculated, and are characteristic for each coil gun. A coil gun makes up an RLC circuit, which makes determining the characteristics easy. Barry has made an excellent simulator here -> RLC Simulator By simply plugging in the component values one can predict how the circuit will operate. This is what my coil gun looks like:
The SCR should have a pulse current rating at least 10% over the estimated maximum current. The 50RIA120 can pass 1.5kA for 8ms, which is perfect for my coil gun. The reverse diodes are there to protect the capacitor from being reverse charged by the energy in the coil when the SCR turns off. They give a path for the negative current pulse, and should be rated for the estimated current. The 68 ohm resistor and 12V battery simply give some initial current to turn the SCR on. Once on it latches until the current falls below a certain level. The coil is wound on a non-conductive and non-magnetic form, which allows the projectile to pass freely through the coil. The coil former should be as thin as possible, but keep in mind that the coil shrinks with great force when firing, so the barrel must be strong. For charging the capacitor bank I use a boost converter, which is able to charge the bank to 430V in 30 seconds from a 12V source.
This is what the coil looks like. External iron/steel is used to confine the magnetic field and help concentrate it on the projectile, while holding the coil together. The wire needs to be pretty thick to sustain the high currents without shattering. 15 AWG should hold up to 1.5kA for short durations. My coil uses 17AWG and it gets warm after just a few shots. The coil is 5.5 cm long, and I used 10 meters of wire. No idea how many turns or layers though. The inductance with no projectile was measured to 80µH, which allows for a short high current pulse. The projectiles I used weighed 9, 11 and 17 grams. The projectiles should fill as much of the barrel as possible in order to experience the most force. I used thick projectiles in hopes that they would not saturate as fast, and thus absorb as much energy as possible.
The completed coil gun in all its ugliness, and no, I couldn’t have made it look better or I would have. A locomotive look was not what I was aiming for. The switch hanging out the side is the firing switch, which needs replacement. I didn’t have the proper type at hand. At least this coil gun is portable. The loading mechanism will hold the projectile in place, it's not to complicated yet. A magnet is glued to the bottom of the runway to hold the projectile. Once in place the projectile stays there, even when the gun is shaken upside down. The magnetic is too weak to restrict the projectile while firing, and actually helps by holding it in place until the magnetic field has built up enough. The coil is held together and fastened with zip-ties. Ghetto. I tried sending the blunt 17 gram slug through, but that only resulted in a huge bulge in the bottom of the can. However the 17 gram projectile was able to fly sideways straight through a shoe-box without the box so much as flinching, which was one of the most awesome shots I've ever seen.
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.
Disclaimer: I do not take responsibility for any injury, death, hurt ego, or other forms of personal damage which may result from recreating these experiments. Projects are merely presented as a source of inspiration, and should only be conducted by responsible individuals, or under the supervision of responsible individuals. It is your own life, so proceed at your own risk! All projects are for noncommercial use only.
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