Barry's Coilguns

Coil #2 of 56 Turns

This was the fourth and final coil tested. The coilgun is in its finished form, with a fixed-voltage power source.

Finished coilgun photo, with coil of 56 turns

Coil #2 of 56 Turns

Because the power supply is now fixed at 160v, the testing consists of measuring speeds of all projectiles. See the Projectiles page for details of these projectiles. The projectiles are listed in order of mass.

Projectile Description Mass Velocity Velocity Efficiency
    (grams) (m/s) (feet/sec) (%)

3.5" finishing nail

4.301 13.36 43.8 0.25%
B 1.25" galv casing nail 2.800 17.03 55.9 0.26%
C 2.5" finishing nail 2.163 16.15 53.0 0.18%
D 2" finishing nail 1.464 17.23 56.5 0.14%
E 1.75" T-head nail 0.872 17.26 56.6 0.08%
F 1.5" finishing nail 0.735 17.41 57.1 0.07%
G 1.25" T-head nail 0.442 18.06 59.3 0.05%

Graphical Results

This graph compares the velocity and energy efficiency as a function of mass.

Graph showing velocity and efficiency as a function of mass


Although most projectiles fly at about the same speed, the heavier ones have better energy conversion efficiency.

The best efficiency of 0.26% is disappointing. We hoped for 1 – 2% efficiency with the use of external iron.

Additional testing using the same coil without the external iron, found much lower efficiencies in the range of 0.1% or less. Although it’s nice to know the iron makes four-times improvement in energy conversion, this is still very low.

We hope to find better coil geometries in the future that will improve energy conversion efficiencies.

Coil Timing

The 56-turn coil measured a 4.0 ms half-cycle discharge, as shown in the oscilloscope image that follows.

This waveform is somewhat over-damped, suggesting that a lower damping resistor would improve velocities by reducing the dreaded suck-back effect.

Oscillograph of 56-turn coil at 160v

The oscilloscope image above indicates peak currents with capacitor charge of 160v and peak current of:

Equation to calculate peak current

We don’t expect this SCR to survive much more than 1300 A in this application.

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