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The puzzling part to me is that the difference in potential energy between the 2 possible banks is only 17.5 Joules, but I am not sure if the longer pulse time of the 40v capacitors is going to negatively effect performance of my gun. If someone could help me out with this one it will be greatly appreciated!
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
You definitely want to go with the 250v caps (I would do a 2 in series, 3 in parallel arrangement), as you pulse will be way to slow with the other caps. Unless of course you are shooting 10lb blocks of iron
Registered Member #16
Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554
Yeah If you're using a system that isn't optimized for long pulses (ie a slow, massive projectile or a very long barrel) you're definitely going to want to take ...'s advice.
On the plus side, at that voltage, and 330 joules, you should be able to get away with a typical house light switch if you aren't looking for a fancy setup, or you haven't played with SCRs before. Only down side is eventually you'll spot weld the light switch closed, requiring you to buy another $0.75 switch.
Just be careful, as my mother would say... "You'll shoot your eye out!"
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
It doesn't matter. At least, it doesn't matter if you just look at the pulse time. However, there are lots of other things that do matter which I'll get to in a moment.
First a few words about timing ... It's about the same amount of stored energy with either choice of capacitors, and you can achieve the same pulse time by making the appropriate coil. With higher capacitance you would build a coil with lower inductance. The bottom line is that the time of the first half-sine-wave is about pi*sqrt(LC).
Now some other factors which can be very important ...
Your power supply voltage is important. What voltage level do you like to work with? This can be a personal choice; I prefer 100v and less because it's easy to produce and has less chance of arcing. A lot of people have no problem with kilovolt levels. This choice is often determined by parts availability; can you afford the power supply and switch that handles your voltage and current requirements.
The peak current is important. Note that one of your capacitor banks would result in a much higher current peak than the other. To observe this, dial your part values into the RLC simulator. This will help you choose a switch that has sufficient peak current capability.
Here's an example ... The RLC simulation of your capacitors with just enough inductance for a 1.0 msec pulse will tell us the peak current. I found that one arrangement resulted in almost twice the current peak than the other. The 4*10,000uF bank peaked at 4500A, while the 6*1,800uF bank peaked at 8000A. Same energy, same time, but twice the current. By the way, the magnetic field strength (1/2 L I^2) is approximately the same because the inductance is different.
Coil construction is important. For both capacitor banks, take a look at how difficult it might be to build the coils. These will both be low-inductance coils (you should figure out how much first) on the order of a few microhenries. What size wire and what coil dimensions will be needed? Can they be built with low enough resistance to remain a high-Q circuit? Be sure to keep total series resistance below the point of critical damping, i.e. R < 2*sqrt(L/C).
On the other hand, you could just build it and see. Don't suffer paralysis by analysis; just build it!
Cheers, Barry PS - If it were me I'd use the 40v cap bank, because it's easier to work at lower voltage. Plus, I'd make it portable by charging it from a couple ordinary batteries. It's your hobby, so just choose whatever turns you on (pun intended).
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Which Capacitors?
