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My Coilgun test and delema

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David.Lightman

Fri Mar 14 2008, 01:46AM

Registered Member #1327 Joined: Mon Feb 18 2008, 12:13AM
Location: North Ridgeville, Ohio.
Posts: 38

For now I have strung several 12VDC batteries together to make 76VDC. Made a simple, really basic coil gun setup. I do not have any resistors inline or diodes to prevent back flow, and use standard light switches for triggering the circuit. Not the best idea but what I have, and good enough for the simple testing I am doing.

I need to learn how to figure out what size diodes and resistors I should be using. I want to use resistors big enough to handle the 450VDC in case I ever get there, but not so big that they will hamper my lower voltage testing.

The capacitor bank I have put together has bleeder resistors, but nothing on the powering circuit...

D.L

Dave Marshall

Fri Mar 14 2008, 03:57AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Ok I'm confused. A bit more info is needed here. Why are you using batteries to charge a capacitor bank, and what's keeping the battery's current from just drawing the projectile into the center of the coil and locking it in place?

Rectified 220V mains will give you 311VDC. Even 110V would far surpass your battery bank, and you could get the necessary bits and pieces at radioshack for a few bucks.

Using batteries as the primary power source is viable, and actually presents a few advantages over capacitors, but they require more complex switching and triggering.

Dave

Bjørn

Fri Mar 14 2008, 05:58AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

There seems to be one switch for charging the capacitors from the batteries and one to discharge them through the coil. That should work unless you get confused and turn them both on at the same time and set fire to the whole thing.

I like to use lamps as current limiting resistors, that way I can see the current flowing and any short circuits are clearly visible. It is much more practical than waiting until you can smell the problem or you touch a smoking hot resistor.

Have a look in the wiki

and search the forum and archives. Almost anything you will ever ask about coil guns have been answered there.

David.Lightman

Fri Mar 14 2008, 11:35AM

Registered Member #1327 Joined: Mon Feb 18 2008, 12:13AM
Location: North Ridgeville, Ohio.
Posts: 38

That is correct BjÃ¸rn, I have two light switches one to charge, one for discharge. I am using the batteries because it is what I have on hand. I still have to check out the wiki. I need to learn a few more things before I try and rectify ac. I am generally not one for doing research. I started this just in having the idea. I have just recently started to do the research after reading excerpts from the book "The Radioactive Boy Scout". I was telling a friend about the experiments I was working on and he then shared the story with me. I figured I better do a little research before I loose a testicle.

I like the lamp idea. I will have to see what I can come up with.

My dilemma is I want to put together the cap bank and coilgun to run off the power I can make here for now, but for it to be able to handle the power I get when I next learn how to rectify AC and increase my power.

D.L

Dave Marshall

Fri Mar 14 2008, 02:48PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

What are you hoping to use the current limiting resistors for? To control charging rate?

You certainly don't want to use them for controlling the pulse shape of the coilgun. You'd be much better served by doing that with more turns on your coil. Using a resistor to control pulse width will just reduce your efficiency by creating heat, and likely causing suckback.

For controlling charge rate, any old power resistor ought to do, as long as its got 10W or so of power dissipation. You can figure out approximate charge time with the equation T=5RC where T is time in seconds, R is resistance in ohms, and C is capacitance in Farads.

Dave

David.Lightman

Fri Mar 14 2008, 07:36PM

Registered Member #1327 Joined: Mon Feb 18 2008, 12:13AM
Location: North Ridgeville, Ohio.
Posts: 38

I suppose I want to control the charging rate. I'm guessing if I through 450vdc directly at my cap bank that would be a bad thing. I like the idea of using a lamp. I don't know for sure but it would seem that a lamp would be less resistive over time and more resistive @ the start. This would lend it's self nicely to charging a bank. Maybe I have that backwards.

as for the trigger, I can easily enough get an On/On switch to replace the light switches. This will ensure that I don't get 'em wrong.. I put this demo setup together to do a little testing. to get an idea of how I want to build the design. I think I am going to make a 3 part modular setup. Capacitor Bank, Power Supply, Coil Gun. This way I can test the 3 different areas and see what parts improve design. I am interested to learn the difference in output power with different hardware. This same setup but with an SCR instead of toggle switches, etc.

D.L

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