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Compact Coilgun

(If you know how coilguns work, skip this paragraph)

The idea of a coilgun is that you make an electromagnet by coiling wire around a hollow tube, and then you get an iron projectile and put it just outside the tube. To fire the gun, you send a large pulse of current through the electromagnet coil. It pulls the iron projectile into it quickly. Then the pulse ends and the electromagnet turns off while the projectile is still moving, so it continues moving and flies out the other side. Below is an animation from the Wikipedia coilgun page. It shows a coilgun version with three electromagnets in a row.

Animation of 3-stage coilgun

For the record, there is another type of coilgun which works on a completely different principle. That type uses an aluminum or copper (nonmagnetic) ring for the projectile. It is placed in front of the coil. To fire, the coil is turned on quickly, which induces a current in the ring, which makes the ring act like an electromagnet pointed in the opposite direction. The two "North" poles repel, and the ring is shot away from the coil. My coilgun doesn't work that way at all, but I included the description here in case you've seen the ring type and were confused. That effect is undesirable here because we're trying to attract the iron into the coil, not repel it. That effect does take a little bit of efficiency away from the iron-projectile coilgun type.

My Circuit

The circuit that I use to produce the current pulse is a Marx Generator modified to use SCRs (explained below). The original Marx generator design uses spark gaps and is named for Erwin Marx, its inventor. His last name offends me though, so I will call this modification the Adam Smith Generator, because it is more practical to build and works efficiently.

(I'm sure this modification has been made by others too, but I haven't seen a catchy name attached to it. I'm also being a little facetious about the benefits of SCRs over spark gaps. SCRs are better for my 300V design, but spark gaps are better for the very high voltages that the Marx Generator was designed for.)

The Adam Smith Generator uses Silicon-controlled rectifiers (SCRs) to switch its current. SCRs are basically
transistors that, once turned on, get stuck on until the current through them stops, even if you remove the gate voltage. That is obviously inconvenient, but their advantage is that they can handle larger currents and voltages than regular transistors of the same size and cost could. And they can handle even more current if it's a short pulse. So they're a very good switching device for coilguns, which need a short pulse of high current. (The SCRs will naturally turn off once the capacitor discharges and current stops.)

The basic idea is that capacitors are charged in parallel but discharged in series, so the voltage can be made higher than the original voltage source. The Adam Smith Generator can be easily scaled to supply current and voltage at any level. It charges its capacitors with a 300V camera flash power supply from a disposable camera. The capacitors are also from disposable cameras (more on that later, on the Construction page.) The version of the circuit in my compact coilgun gives the coil 600V when it fires. The circuit is below, followed by an explanation of how it works.


In the Adam Smith Generator, the SCRs are off while the camera flash circuit charges the capacitors. So the capacitors, connected by the charging resistors, charge in parallel until each one has 300V. The equivalent circuit while charging is below.

Then, to fire, the SCRs get turned on (details of that later). The resistance of the SCRs and of the coilgun's coil is very low compared to the resistors, so very little current flows through the resistors. (Also, the camera flash charging circuit turns off automatically after the capacitors have been charged, so it's out of the picture too.) So basically the capacitors are connected in series, so if two 300V capacitors are in series, the coil is getting 600V.  The equivalent circuit while firing is below.

Triggering the SCRs
To turn on SCRs, you send a small current into their "gate" input. Below is the complete circuit, including the trigger part.

The datasheet for my SCRs says you should send about 40mA into their gates to turn them on. I tried to design the circuit to do roughly that. The trigger circuit just takes the 300V from one of the capacitors and sends it through a switch and some current-limiting resistors.

Other things to consider

SCR peculiarities
I worried a little about the fact that, once the SCRs turn on, their voltages relative to the trigger circuit and each other change a lot. However, my thinking is that, at the instant I push the switch, their cathodes (the side where current comes out) will all be at about the same voltage: the voltage of the downstream side of the coil. That will only change when they start to turn on. They should turn on at about the same speed, and once they're on, they'll stay on, so I decided I didn't need to worry about it.

Another feature of SCRs might come into play if the Adam Smith Generator were expanded to a lot of capacitor stages: SCRs can also be turned on by a sharp voltage rise across them, even if there's no input to the gate (called a dV/dt turn on). Also, they turn on if the voltage across them goes above their intended maximum operating voltage. So for a Smith Generator of many stages, you would probably only have to control the first few SCRs, and the later ones would be turned on by one of those two effects. (I tried to test that by making a 3-stage Smith Generator and only controlling the first two SCRs, but I couldn't get the third to turn itself on. It might work with 4 stages though; I haven't tried more than 3.)

Since building this, I've discovered one problem in the circuit: if you trigger the circuit, and then trigger it again when it's just starting to recharge, the recharging current will go through the SCRs, keeping them turned on, so the capacitors will never charge, and the charging circuit will keep going until the battery runs out or you take it out. This could be fixed in several ways, but I chose to just avoid doing that.

Optimizing and expanding the circuit
The Adam Smith Generator can be expanded to provide more voltage, a longer current pulse, or both.

Voltage is increased by adding more capacitor "stages," so more capacitors will be in series during discharge. A higher voltage means a higher current, a stronger magnetic field from the coil, and more force on the projectile. It also means the capacitors discharge sooner, so the current pulse will be shorter, which may or may not be a good thing. Ideally, the current pulse should stop when the projectile is in the center of the coil. If current is still flowing after that point, it will be pulling the projectile backwards and slowing it down. 900V gave me about 24 m/s, and 600V gave me about 20 m/s. I decided the extra 4 m/s wasn't worth the extra capacitor stage, so my compact design uses 2 capacitors for 600V.

Besides going to higher voltage, you can also increase the total energy by putting two or more capacitors in parallel at each stage. Using two parallel capacitors will double the capacitance (just like using a capacitor that's twice as big) and double the length of the current pulse, all else being equal. The circuit below is an example of an Adam Smith Generator with three stages (to make 900V) and two capacitors per stage. Note that both adding stages and adding parallel capacitors will increase the time needed to charge the circuit.

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