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Resistor Suggestion for Cap Bank

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ScotchTapeLord

Tue Nov 23 2010, 04:25AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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A large capacitance can create the same short and powerful pulse as long as you use thicker wire and fewer turns. It's like 120v into a 120 ohm heater vs 1 into a .008333 ohm resistor, except with magnetism... just different ways of achieving the same power.

Sir Fixalot

Tue Nov 23 2010, 06:17AM

Registered Member #2810 Joined: Sat Apr 17 2010, 07:17PM
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Hmm....In that case I'll Probly go with 8P and see how that performs. If It doesn't turn out too well, at least I Know what resistors to use for 2S4P.

ScotchTapeLord

Tue Nov 23 2010, 11:55AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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It will work if you plan and model and construct and tune it properly.

Sir Fixalot

Thu Nov 25 2010, 07:26AM

Registered Member #2810 Joined: Sat Apr 17 2010, 07:17PM
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Ok...I want to thank everyone for all the help that was provided! I have greatly appreciated and considered all of your comments and suggestions and am thankful for all! Special thanks to you, STL for the numerous responses to my questions! Ill try and post some pics when I get it done(If I ever actually get around to finishing it).

Thanks again,
Sir fixalot

ScotchTapeLord

Thu Nov 25 2010, 05:29PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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Don't thank me; my first post was wrong!

klugesmith

Thu Nov 25 2010, 07:37PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Sir Fixalot wrote ...
BTW my Projectile is a stainless steel rod I tore out of an old printer and cut into sections.

Many stainless steels are nonmagnetic, or much less magnetic than other steels. You can check yours with a permanent magnet -- the stronger the better. I just checked some old printer rods -- they seem plenty magnetic and also show signs of rust. Maybe their lack of rust in service is due to a superficial coating.

As STL and others keep saying, you can design your coil to fit either capacitor configuration (8P or 2S4P) with practically the same results. Barry's simulators will show you that -- note that the coil for 8P will have 1/2 as many turns, 1/4 of the inductance, and twice the peak current, with the same timing. If it has the same physical dimensions and uses wire 3 AWG's thicker.

Artikbot

Thu Nov 25 2010, 07:45PM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
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I think rods from printers are nickel/chrome coated steel rods ;)

Barry

Fri Nov 26 2010, 03:51PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

ScotchTapeLord wrote ...

If you're going to saturate your projectile, it's going to happen no matter how you configure your capacitors, since the energy output is the same. The solution is not to lower the voltage but to either split your banks and use multiple coils or to use a larger projectile.

Actually, the timing is also important. The same amount of energy spread over twice the time will have a lower peak field. You can reduce the magnetic saturation by spreading out the energy over time.

Although the total stored energy is the same for a given box of capacitors, some arrangements have much higher (or lower) capacitance than others.

Bottom line... You need to match the magnetic pulse to the acceleration of the projectile. Many factors are interrelated and can be used to tune the coilgun: voltage, capacitance, number of turns and wire size, projectile mass and length, external iron, and coil geometry. This is what makes coilguns so interesting.

ScotchTapeLord

Fri Nov 26 2010, 05:33PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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I meant to say that there is an ideal setup for whatever configuration you choose that will give you the same overall performance.

Sir Fixalot

Sat Nov 27 2010, 03:38AM

Registered Member #2810 Joined: Sat Apr 17 2010, 07:17PM
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Right...I understand what all of yall are saying now. I decided to go with 8P so I don't have to worry about balancing reistors.

Also, I've been playing around with the inductor and rlc simulators and Im gonna design my coil to match my cap bank. I will Probably start with about 2-3 layers of my 14 gauge wire, a smaller barrel, and an IRON Projectile. After that, I will play around with different wire sizes and number of turns etc...

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