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Registered Member #963
Joined: Tue Aug 21 2007, 08:50PM
Location: USAF Academy
Posts: 3
I just need something cleared up about capacitors. How do you decide what the right capacitor to use is going to be. Ohms law basically says that a higher voltage causes more current and a higher current gives a higher magnetic field. Why not use an incredibly high voltage capacitor with a very low capacitance like 80,000V at 0.005uF this would create a huge amount of current for an incredibly small amount of time, or even a small ceramic disk cap rated for 2000V at .01uF.
I just don't understand what you gain with a larger capacitance other than moving a larger projectile.
I plan on moving a small steel bullet (1in by 1/4in)
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
I assume you want to make a coil gun. You have a coil and a capacitor, it sounds simple but is quite complicated when you factor in all the details.
When you connect your capacitor to the coil you get this circuit:
And a capacitor behaves like this circuit:
You also have to include your connections, wires, switch and your projectile in your calculations. You will get higher currents with higher voltages if you simply apply Ohm's law, but Ohm's law does not model these circuits correctly when applied in a basic way. You need to take into account and correctly model all the different parts of your circuit. Don't forget that your projectile will saturate at a certain point and greatly reduce the efficiency.
Registered Member #963
Joined: Tue Aug 21 2007, 08:50PM
Location: USAF Academy
Posts: 3
sorry, I forgot to clarify that I am in fact building a coil gun. If the problem is saturation, Is there a way to calculate a projectiles maximum saturation, or would it be easier to just try different projectile materials until I find what works best?
Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
You need total energy stored, not just a current peak. You cant beat physics and if it takes X joules to move your projectile to Y velocity, then your going to need to put X joules into it, regardless of how you store it.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
The simplest way is to just look it up in a page like this (rightmost column): Remember that 10 000 gauss = 1 tesla.
In general coil guns are very well suited for experimentation. If you try to calculate everything in detail most people find the math so complex that it takes all the fun out of the project.
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
Ohm's Law works for circuits at d.c. or steady-state. The refinement needed for energy-storage elements (capacitors, inductors, moving mass) is the timing.
Capacitors take time to discharge according to the current and capacitance. i=C dv/dt
Inductors take time to begin conducting, according to the applied voltage and inductance. V=L di/dt
Projectiles take time to begin moving, according to force and mass. F=ma
The original question was "how do you decide what the right capacitor to use is going to be." The general approach is to estimate the timing needed, then choose the capacitor and coil together to get that timing. One tool that can help is this RLC timing calculator.
The RLC discharge time depends only on capacitance, not voltage. So it is desirable choose the capacitance first, then find a real capacitor with the highest voltage rating you can afford, to get the most stored energy. However, you should revisit the estimated timing after you've tentatively chosen a capacitor. If you start with more stored energy then it usually means less time in the barrel.
If this sounds like a circular process, that's because it is. The projectile, coil and capacitor are interdependent.
Another question was "Is there a way to calculate a projectiles maximum saturation?" Yes, but it depends on the specifics of the projectile's shape and material, and the coil geometry. A computer program for finite element magnetics modeling is required. FEMM 4.0 by Dr. Meeker is free and does this job admirably. It does have a learning curve. Some examples can help you get started.
Cheers, Barry All those that believe in telekinesis, raise my right hand.
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Capacitors? (What type for what use)
The projectile, coil and capacitor are interdependent.