anon5005 t1_j64bavl wrote on January 27, 2023 at 4:50 PM

First comment is to be very careful, the current those capacitor could put through a human body at 400 volts is very very far above lethal.

Second comment is, a resistor is going to drag down the cicruit and dissipate a lot of your energy. You know that the rate of change of current at the start, at time zero, is voltage divided by inductance, V/L. So at the beginning the graph of current over time looks like an increasing line of slope V/L. The total charge that has flowed is the integral of this which is 1/2 (V/L) t^2 if t is time. But then the voltage starts to decrease, the voltage is Q/C where Q is charge, and at the outset Q=Q_0 - 1/2 (V/L)t^2 where Q_0 is starting charge. The best way to continue is by differential equations because V is not really constant, but for small values of time, if we approximate the charge that has flowed as 1/2(V_0/L)t^2 where V_0 is initial voltage, then remaining charge is Q_0 - (1/2)(V_0/L) and voltage is V= (1/c) (Q_0 - (1/2)(V_0/L)t^2) so in this formula Q_0 is starting currentm V_0 is starting voltage and you see the voltage dropping off. It is clear that you need to increase L. As I say, this is just an approximation for small time.

By the way, you would also see if you write down the differential equation, an oscillation after the bullet has left. If you had an amazingly durable diode you could prevent reverse flow, but I do not know how expensive it would be to get a diode that can handle the very extreme currents you'd want to put through it.

You can see that you have a small coil --- that is the problem.

thebrews802 t1_j651qk6 wrote on January 27, 2023 at 7:35 PM

It's not reverse flow in this case, polarity shouldn't make a difference since it's just a magnetic pull. The problem OP is facing is that the coil is just a magnet pulling the projectile towards the center. At t=0, the projectile gains a ton of momentum rushing towards the coil, by the time it gets there the magnetic force has dropped a ton since the voltage of the caps has dropped. But there's still a little bit. Once the projectile gets past the coil, the magnetic field is now pulling back on the projectile, slowing it down. Since the voltage is much lower from when it started, it won't be an equal force, but it'll slow it down none the less.

There will certainly be a ringing after the coil dies down, but I don't think that's the source of OP's problem. Good point though, I recommended IGBT'S for switching the coil off and I didn't think of the flyback current on turn off. OP, look at guides to put a clamping/flyback diode across the IGBT. A 1N7007 should be fine. You can buy like 100 of them on Amazon for like $10. A great thing to keep in the electronics drawer too.

anon5005 t1_j67lc0s wrote on January 28, 2023 at 7:51 AM

>polarity shouldn't make a difference since it's just a magnetic pull

Good point, the math for that isn't as simple as saying when the current reverses so does the force....it would if the projectile were permanently magnetized (and that were the significant effect) but it is just iron and its own magentization is caused by the coil and reverses too. If that did happen, one could try to tune the circuit to oscillate just once and pull the projectile partway then push. But as you say, by the time the projectile is at the midpoint one wants to have the current shut off.