Barry's Coilguns


This coilgun successfully launches a variety of nails at impressive speeds. The nails fly at 50 – 55 fps using a 150 joule capacitor.


The efficiency was much lower than initially hoped for. We anticipated 1 – 2% efficiency based on earlier coilguns at lower energy. The measured efficiency was around 0.25%. The best efficiency occurred with projectiles of 3 to 4 grams. The external iron assembly proved to be successful, and improved efficiency by a factor of four.

The speed of the first coil tested could launch projectiles at 9 m/s. Tuning the coil such that the current pulse reduced from 8 ms down to 4ms resulted in speeds of 18 m/s. Further improvements are certainly possible, but we stopped here to damaging the SCR. The current peak was measured at 1300 A, and could be about as much as this particular device can handle.

The retention bolt using a small permanent magnet was extremely successful. It was easy to adjust, provided just enough holding force, and gave easily repeatable placement.

The critically-damped LCR circuit had mixed results. It was very successful in arriving at a simple schematic by eliminating a large and possibly expensive protection diode. It also served as a very convenient current-sensing resistor. We noticed the damping resistor must be adjusted whenever the coil’s inductance is changed. However, this design was much less efficient and had slower speeds than desired. Further mathematical analysis found that critically damped designs have a peak current only 36% (the reciprocal of e) of the maximum possible current.

Note the damping resistor should be a smaller value when the inductance becomes smaller. This suggests that “proper” tuning will be when the inductance is reduced to the point where the added damping resistor is zero ohms.

We look forward to building another coilgun of similar design and power levels. It will use a larger SCR for higher peak current, and be tuned for a pulse of one to two milliseconds. This is expected to double the velocity to above 30 m/s (or 100 fps). Also the next coilgun will replace the damping resistor with a V-switch design to quench the firing current, for even greater improvements.

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