theskinnynewsdaily Getting the skinny on today's tech and gadgets

Coils can be constructed in a widevariety of diameters and with anythingfrom 32-gauge magnet wire to thick12-gauge copper. The bigger the wire,the lower the resistance and the greaterthe current that can be pumped into thecoil, because Ohm’s Law tells you thatI = V/R. Higher currents translatedirectly into stronger magnetic force, soNewton’s Law tells you that the pro-jectile’s acceleration, given by a = F/m,will increase directly with the magneticforce F.

To get high currents flowing, a 2-Abench DC power supply is not going tocut it. You have to pump out 20-A cur-rents and maybe even higher. The com-mon solution is to use capacitors to storeenergy from a DC supply, and thenquickly discharge that energy at a cur-rent level limited only by the resistanceof the coils, internal capacitor resistance,and interconnecting wire resistance. Thebigger the capacitors, the better, becauseyou can sustain high currents through-out the length of the coil firing time.Coils can be well modeled as induc-tances. The inductance depends onthe number of turns of wire in thecoil, the length of the coil, and thecross-sectional area of the coil. Thereare many parameters to trade-off andoptimize in designing a coil gun, but it essentially boils down to a simple circuitcomprising a capacitor (or a bank ofcapacitors in parallel) and an inductor(the coil) with some series resistance. It’sa perfect circuit for students to build inone semester and for a hands-on intro-duction to the important concepts ofresistance, inductance, and capacitance.

Photo 1a—The coil gun controller is mounted to a piece of shelf board and attached to a standard bench powersupply. A homemade coil is attached with alligator clips. b—The coil gun launcher circuitry attaches to the coil atthe bottom, to the external DC power supply on the left, and to the LM3S811 microcontroller board at the top.

HARDWARE

The main parts of the design are shownin Figure 3. Note that the project sup-ports either one or two launching coils.The ability to use two coils is a bit of“extra credit”—the optional second coilcan be used to accelerate the projectile asecond time for higher projectile velocity.The heart of the coil gun launcher isthe coil driver: a three-stage optoiso-lated current amplifier (see compo-nents IC8, T4, and Q4). The mainswitching transistor is a 100-V MOS-FET with a low on resistance of only14 mΩ. It adds as little resistance tothe coil as possible. Another 15 mΩ ofresistance is added by a current-senseresistor (e.g., R17), which can be usedto monitor coil current in real time.

Figure 1—A coil gun fundamentally consists of a projectilewithin a launching tube, a coil wrapped around the tube, anda current passing through the coil to effect projectile motion.

Components such as F4 and D14enabled me to “student-proof” the proj-ect. They protect against overvoltage,undervoltage, and overcurrent at theinputs the students would hook up.

The coil currents observed across thecurrent-sense resistors are buffered andamplified by rail-to-rail op-amps andthen digitized by a Linear TechnologyLTC1861 12-bit ADC. One of the keyfeatures of the project is the ability torecord and analyze real-time coil cur-rents to see how changes in designparameters, such as coil windings andapplied voltage, affect the coil gun per-formance. The ADC’s SPI pins are con-nected to the microcontroller by anoth-er isolated interface, this time usingan Analog Devices ADuM3401 isola-tor. Standard optical isolators are notfast enough to support the 40-µs sam-pling period necessary for getting a good look at the transient coil current.

By CIRCUIT CELLAR THE MAGAZINE FOR COMPUTER APPLICATION #216 July 2008