430J Coil Gun

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Specs

Bank Voltage: 435V

Bank Capacitance: 4700 µF

Bank Energy: 435 Joules

Charger: Boost Converter, 15W

Coil Inductance: 80 µH

Coil resistance: 0.18 ohms

Estimated Peak Current: 1.4kA 

Projectile weight: 17 grams

Projectile speed: 27 m/s

Efficiency: 1.45 %

Intro

A coil gun is among the most easy to make electromagnetic projectile accelerators. How can a hollow coil and electricity accelerate things? The coil is energized with very high currents for a short time. With such high currents passing through the coil a strong magnetic field is created, and sucks anything magnetic into it. If a projectile is placed partially in one end the coil it will be sucked right through the coil and out the other end at high velocities. The coil gun itself consists only of an air cored coil, a high current source and a switch. For the current source a capacitor is often used for its high pulse current capability and a SCR is used for switching. Components can be seen below. The coil shown was upgraded later.

Some coil guns are multi-staged and use electronic control to switch several coils in succession as the projectile passes through. That way one can achieve higher efficiency.

 The SCR should have a large pulse current rating. The 50RIA120 can pass 1.5kA for 8ms, which is perfect for my coil gun. The peak current, discharge time, reverse current/ voltage can be calculated, and are characteristic for each coil gun. Here is a RLC calculator: 

http://www.coilgun.info/mark2/rlcsim.htm

Reverse recovery diodes are used to prevent the flyback spike from charging up the capacitors backwards, which would severely shorten their lifespan. 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. A transformer charger could also be used, something like the Mazzilli ZVS for example.

Coil 


This is what the coil looks like. External iron/steel is used to confine the magnetic field and concentrate it on the projectile, but mostly for holding the coil together. The wire should be pretty thick, around 14-17 AWG for 1.5kA peak currents. My coil uses 1.2mm wire and it gets warm after a few shots. The coil uses 10 meters of wire, wound on a 5.5 cm long, 1 cm diameter former. No idea how many turns or layers though. The inductance was measured to 80µH, which allows for a short high current pulse.

Projectiles

Here are the projectiles I use, weighing in at 9, 11 and 17 grams. The projectiles should fill as much of the barrel as possible, to minimize the air gap. 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 on hand. At least this coil gun is portable.

  

The loading mechanism and coil. A magnet is glued in place 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 hold the projectile from firing, and actually helps by holding it in place until the magnetic field is strong enough. The coil is held down and together with zip-ties.

 

An 0wned soup can, victim to several shots. I even got the blunt 17 gram slug through from 1 meters distance. Once a projectile was able to fly sideways straight through a shoebox, while leaving it standing upright, that's one shot I wish I had filmed.