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Registered Member #690
Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616
uF, or the Farad in general is a measure of capacitance and cannot be applied to batteries. If you want to find the total energy stored in your batteries, see if you can find an Amp-Hour rating on the battery. Amp-hours*Voltage*3600 will give joules stored. For batteries your size, it will be huge.
Far example, I have a medium-size 7Ah lead-acid battery (12V). So 7*12*3600=302,400 or about 300kJ.
Also, one thing you left out is the SCR is a polar device, and its gate must be connected to a proper low-voltage power source with limiting resistor. If you were to use an SCR, it would look something like this:
EDIT: oops, remember to put a fire switch in series with the SCR gate! I didn't!
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
How big of a diode, and whether you need one at all, depends on how your circuit is configured. If you want, you can actually eliminate the reverse spike completely. Of course the math behind this is pretty nasty, but you can play with Barry's calculator to see what is going on with the circuit (make sure to turn on the voltage trace at the bottom).
High speed (or super, ultra, hyper, for that matter) diodes don't really help for this application, since all silicon diodes turn on at the same speed, it is just how long they take to turn back off that changes, and you could care less about that for this application.
You could, if you wanted to, calculate the uF stored in the batteries by finding the energy available (E=V*A*T, all SI units) for a 12v 18a.hr battery that is 0.8MJ, and then use the E=.5cv^2 formula, which give 5kF (ie, 5x10^9uF).
You could replace the switch between the battery and teh caps with a resistor 10-100ohms, if you want to simplify the design a bit. I would also recommend putting the batteries in series, and if you caps are rated for <25v then also split it into 2 banks of 4 parallel caps that are in series.
Registered Member #1106
Joined: Thu Nov 08 2007, 01:15AM
Location: Seattle, Washington
Posts: 25
shaun: yeah i forgot that part, but im going to do a toggle for now.then move to scr after a lil but thanks =)
...) yeah the caps are rated for 15v. so by making 2 banks of parellel that are in series, you essiciently double the voltage right? and if i do that, what happens to my uf.? does it split into 400,000? and by making a series with my caps, i would also have to make my batteries series in order to charge it?
Registered Member #690
Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616
Actually 4 parallel strings of 2 series caps each would be 30V rated, 200,000uF. When you put two caps in series, its half the original capacitance (of just one of the caps), and double voltage.
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New Coilgun plans..need feedback
