Coil Gun [Part 1] – Theory and Proof of Concept

I would venture a guess that 95% of all electrical engineers find their way into the field by trying to build a coil gun as a kid. In some way, it speaks to every boy’s innate desire to put their life in danger. The definite possibilities for electrocution and spontaneous fire have resulted in dozens of Instructables and home-brew web pages dedicated to building this infamous weapon.

A desktop unit can be put together on a Saturday afternoon using only some wire, a pen case, a nail, and a few 9V batteries connected in series. On the other hand, building a high voltage coil gun with an electronically controlled trigger requires some electronics knowledge and a healthy fear of high voltage. While not the most complex project, it does provide some valuable lessons in building high voltage switching circuits and isolation techniques.

Principle of Operation

The operating principle behind a coil gun is based on Ampere’s law. A result of this law is that electrons moving through a coil of wire, or solenoid, will generate a magnetic field along the axis of the solenoid. This is called an electromagnet.

solenoid
In this cross-sectional view of a solenoid, the current is moving in a counter-clockwise direction when viewing the solenoid from the right end. The magnetic field inside the solenoid is shown by the red lines.

If you were to hook up a battery (DC voltage source) to a coil of wire and put a ferromagnetic object near one of the ends of the coil, the object would be attracted towards the center of the coil. If the object was a tiny ball bearing small enough to fit inside the coil, it would be pulled inside the coil and stay at the center (try it yourself!). Therefore, a DC voltage source generates a magnetic field, but doesn’t behave how we would want for a coil gun.

What we want is a magnetic field that turns on, waits for the object to reach the center of the coil, then immediately turns off. With no magnetic field to pull the projectile back to the center of the coil, it continues on its merry way at high velocity. This can be accomplished by applying a very short duration, high voltage pulse to the coil.

Proof of Concept

To demonstrate the idea, I built a little desktop unit in about 10 minutes. I disassembled an old pen and cut off the ends to have a uniform tube I could use as the barrel. I then took some 28 gauge magnet wire (any wire will do, but thinner wire allows for more turns) and wrapped it around the case a few hundred times. Some electrical tape was used to keep the coil from unspooling.

Next, I connected six 9V batteries in series (that’s 54V for you math-challenged folks out there). I connected the negative end of the battery bank to one end of the coil. For a projectile I used a little screwdriver tip that fit the barrel perfectly. To fire, I quickly tapped the positive power lead to the other end of the coil to provide a short pulse of current. Getting the timing down is difficult: if the pulse is too long, the projectile gets pulled back to the center; if the pulse is too short, the projectile doesn’t gain enough speed to leave the barrel.

Below is a video of the desktop coil gun in action. I would guess the projectile comes out out of the barrel at a blazing speed of 1 meter per second.

Next Steps

Putting aside that little toy, it’s time to build something with a bit more power! I won’t try to build the most powerful coil gun in existence. Rather, I will be building a design with lots of nice safety and ease-of-use features that can be extended upon. The main features will be an electronic trigger (so I don’t have to worry about getting electrocuted when I fire the thing), an adjustable control for changing the voltage the capacitor is charged to and a cool retro voltage readout using Nixie tubes.

Charging Circuit

In order to get the projectile to a healthy velocity, you need a lot of energy. Since I will be using a capacitor as the power source, it would be helpful to know how to calculate the energy inside of a capacitor. This is calculated using this simple equation

U = C V^2

where C is the capacitance and V is the voltage. This equation tells us that we’ll get a lot more bang from our coil gun if we charge our capacitor up to a higher voltage, since the energy is proportional to the square of the voltage. Energy only scales linearly with capacitance, so we’d need to add a lot more capacitors to achieve the same effect as doubling the voltage.

Creating high voltages (> 100V) from a battery is fairly easy, but can be a little dangerous and requires careful selection of components to prevent any fires or explosions. The next post in this series will cover how to build such a charging circuit, as well as covering some safety precautions that most Instructables tend to gloss over.

Electronic Trigger Circuit

Once you have an energy source and a way to charge it, the next task is to build an electronic trigger that will safely and quickly discharge the capacitor through the coil. This is accomplished using a silicon controlled rectifier (SCR). Proper operation of an SCR is a little tricky (I have smoked at least three while building this circuit), so I will be writing a brief section on their theory of operation before diving into the trigger circuitry. This will be the third post in the series.

Making it Pretty

I will be taking this project one step further a la Fallout 4 style by using some Nixie tubes to display the current voltage stored in the capacitors. One of the weapons in Fallout 4 is named “The Gauss Rifle”, which displays the % charge of the gun using two Nixie tubes. I’ll be doing something similar to that.

The fourth and (possibly?) final post in this series will be showing you how to wire up Nixie tubes to make some badass voltage meters. I’ll also be showing you how I utilize a microcontroller to tie everything together. The microcontroller will serve several purposes, such as monitoring the capacitor voltage and display it on the Nixie tubes, setting the capacitor threshold voltage on the fly, one-click charging (rather than holding a button down until it reaches the threshold voltage) and of course the firing mechanism.

Stay tuned for more updates!

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