WARNING: A coil gun can produces voltages that can cause serious electrical burns or easily kill you if not handled properly. Please do some research before beginning this project to ensure you are taking all proper safety precautions when working with high voltage. If you are at a beginner level, I strongly encourage you to start with low voltage projects before taking on a high voltage project. This resource is a great place to start educating yourself. Then search Google for “high voltage safety tips” and read through as many links as you can.
Review of Schematic
Here is the complete schematic of the coil gun. We have discussed each element of this circuit in detail, but it is worth reviewing.
- The 14.4V LiPo battery (V_BAT1) is the power source for the charging circuit.
- A regulated 9V is generated by the LM317 to supply power to the 555 timer.
- When SW_CHARGE is pressed, power is supplied to the LM317 and subsequently the 555 timer. The 555 timer is configured in astable mode, generating a 14.5kHz square pulse train which is applied to the gate of M1.
- As M1 is turned on and off, C_BANK is charged to a high voltage. Once the desired voltage is achieved (maximum of 300V), SW_CHARGE is released.
- Important point: V_BAT1 must be disconnected from the circuit before firing. If not, a short circuit is formed through L1, D1, L_COIL and U1, frying one of the components in the process within a matter of seconds.
- After disconnecting the battery, SW_FIRE is pressed to trigger the SCR. R6 limits the gate current to a maximum of 75mA. R5 and C6 are included to prevent spurious signals from falsely triggering the SCR during charging.
- As C_BANK discharges through L_COIL, the current through the SCR will eventually reach zero. At this point, the SCR acts like a diode and turns off. To prevent damage to the capacitor, D2 is included to allow the remaining energy stored in L_COIL to safely discharge via the voltage drop in D2 and the resistance of L_COIL.
- R_BLEED is included to bleed charge of C_BANK. This is necessary to allow C_BANK to safely discharge if left unattended, or if some part of the circuit should fail and C_BANK is left at a dangerous voltage.
Here is a slow motion video of the coil gun firing I took with my iPhone.
The coil is not optimized for the circuit I built, so the exit speed is quite slow. Regardless, this circuit has successfully demonstrated the design. Performance can be improved by electromagnetic modeling of different projectiles and coil designs in an FEM program, something far beyond what I will be getting into here!
Some More Pictures
Here are a few pictures I took of the prototype circuit. It definitely isn’t the prettiest thing I’ve ever built, but it works!