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Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158
I'm currently driving my coilgun with a 1100uF 350v capacitor charged up to 300v. coilgun.info's RLCSim program suggests that at 550uF capacitor charged to 600v ( a voltage my power circuit is capable of delivering ) would give a really nice coilgun performance. I figured that if I bought another 1100uF 350v capacitor and hooked it up to my existing one in series, I could make a 550uF/700v capacitor. Is this feasible?
I read coilgun.info's page on capacitors, and apparently I need a pair of high power (like 1 watt) 100kOhm resistors for "balancing".
Yes I have done that, it works just fine. Seriesing capacitors increase the equivalent resistance but i think that is not too bad for coilgun applications.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
You can get the same results with the 2 caps in parallel, as in series. Then you would not need to think about balancing resistors. More generally, series vs parallel decisions are about matching the capacitor bank to your power supply and switch. And matching the capacitor bank to the coil, if that's already made.
Let me explain.
Compared to the 2-series case, the 2-parallel would store the same energy at half the voltage. It would want a coil with half as many turns, using wire with twice the area (3 AWG's bigger). That coil would have same size and weight, but 1/4 as much inductance and resistance, as the coil for 2-series. Then with 4x the capacitance and 1/2 the voltage, the current pulse in 2-parallel system would have the same time and shape as that in the 2-series system. Peak current would be double, but there are half as many turns. So you have the same number of ampere-turns and same magnetic results.
Capacitor ESR and ESL effects also work out to be the same for parallel and series, given the different coils.
The switch in 2-parallel case must handle twice the current and half the voltage. The R and L of interconnecting wires will be more significant in the parallel case.
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Using series capacitors for coilgun cap bank?
