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An ambitious atempt

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Turkey9

Tue Oct 25 2011, 08:28PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

For the multiplier, which is a very simple and powerful choice, you need to rate the diodes at above your final output. Same with the capacitors. The higher the capacitance, the more current you can pull. I'd go with ~100 uF for both the doubler caps. For diodes I'd go with something above 5 amps at 600V (600V is a common rating). You can sample diode that will work from st microelectronics. Mouser and digikey are great places to buy components. Mouser can be a little cheaper and lower shipping.

One issue I can see with using this inverter is that it might not be able to handle the inrush current when charging. I'd put a limiting resistor in series with the output before the doubler and the caps in the doubler will also cause a large inrush.

The thinner the wire you use, the more resistance it will have. This means that it will generate more heat when the coil is fired. The physical size of the coil doesn't really matter to a certain extent because coilguns are so inefficient to begin with. You could probably go down to 20awg without much problem. I built a coilgun with 20awg wire and only 30J per stage and the coils would get warm after a couple shots. I recommended 14awg because it is cheap and readily available and will work fine in your application.

kdoh

Wed Oct 26 2011, 03:08AM

Registered Member #4165 Joined: Wed Oct 26 2011, 02:39AM
Location:
Posts: 2

i'm rob11095's IRL friend, also working on this project with him, pretty much the only one with any previous (albeit basic) knowledge of what we're doing.

the cost efficiency here is going to be kind of a big factor, being that we're a couple high school students with rather limited budgets as far as materials, but i'll let rob worry more about the economics of the project.

i've seen 12V niMH batteries used around the interwebs for similar projects and presently i see no reason why it wouldn't work. if anyone has any objections to this, please, let us know! haha

as far as getting our voltage up to 400V, it's typically more cost effective to just use a transformer to get the voltage up, but how much current can we get away with sacrificing at the expense of voltage? i.e. would it be more effective to simply pay more to increase voltage through diodes and caps as in a voltage multiplier, or could we get away with saving the money and going with a transformer. one idea to obtain the desired voltage was to use a step up inverter, as rob said, and a 4 stage voltage multiplier. it is to my understanding that when you invert and rectify you end up with sloppy waveforms. how will this affect the rest of the circuit, if at all? and if it does, will it significantly alter our output?

also turkey brought up the point of limiting resistors and the large inrushes of current. should we have a limiting resistor wired in series with the output before the multiplier AND after the doubler to handle both areas where it seems we will have large inrushes of current?

and just to be sure, as far as the scr goes, could we wire positive terminal of battery to 1K ohm resistor to a rocking switch to the SCR followed by inverter, multipliers, caps, coils, and finally to the negative terminal of the battery?
The rocking switch would just function as the main trigger mechanism for the gun. interesting point about the diode as well. also any technical reasoning behind the points you all bring up would be immensely beneficial, we're always eager to learn about this stuff.

finally, this has been something i've been curious about for quite a while, how much of a difference does external iron really make as far as the final output kinetic energy goes?

Turkey9

Wed Oct 26 2011, 06:00AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Well if you're looking for a charger that is easy and cost effective, I'd go with a ZVS driven rewound flyback. The whole idea of a commercial inverter followed by a doubler is a simple solution but not the cheapest. Follow Uzzors steps here for excellent construction examples.

When any ac waveform is rectified, it will have extreme ripple. Usually these supplies have a filter capacitor to smooth out the ripple. In the case of a capacitor charger, the bank is the filter. So in short, there is no issue in this waveform hurting your circuit.

I suggested current limiting resistors because often commercial inverters with shutdown if too much current is drawn. You won't have this problem with the mazilli circuit as the transformer itself will limit your current. There is no need to use these resistors if you are going to use the mazilli charger.

I'm not sure what you mean when you asked about the scr... are you just asking if you can use your battery to trigger it with the toggle switch? If so, you sure can!

External iron works surprisingly well for amateur coilguns. I am not completely sure on the physics, but I believe that it makes the field inside the coil (where the projectile passes) more symmetrical. You get the most force from the field when the lines are parallel to the path of the projectile(think vectors). Anyway on the subject of external iron, I used some on my first coilgun. The projectile went from sticking in a can to passing through. I wish I had better numbers for you, but I didn't measure anything at all on my first couple builds.

kdoh

Wed Oct 26 2011, 07:08PM

Registered Member #4165 Joined: Wed Oct 26 2011, 02:39AM
Location:
Posts: 2

Wow, thanks so much! That's so helpful!
I can see what you mean about the zvs flyback being the cheaper solution for the coilgun. We'll definitely try that, the link you posted looks extremely helpful! And we'll certainly implement the external iron in that case

earth3

Thu Oct 27 2011, 01:45PM

Registered Member #3721 Joined: Sun Feb 27 2011, 04:43AM
Location:
Posts: 40

guys can anyone tell me how to make an external iron?

Saz43

Fri Oct 28 2011, 03:41PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Coilgun systems did some great research:

It seems to work best with lower voltages and current densities.

klugesmith

Fri Oct 28 2011, 05:22PM

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

Saz43 wrote ...
Coilgun systems did some great research:

Yes, and I'd also recommend Barry's site

-- be sure to try the inductance calculator and the RLC waveform simulator.

Saz43 wrote ...
It seems to work best with lower voltages and current densities.

Thumbs up for doing (and reporting) experiments to back the theory. I didn't look to see if they analyzed the point where their external iron saturates.

People here talk a lot about series-vs-parallel banking, and other ways to trade off C against V^2 for the same stored energy. Often the idea is to find the optimum, for a given coil.

Less frequently mentioned: When you change to a different C and V for the same energy,
you can get back to the original pulse timing, current density, and field strengths
by rewinding the coil with a different wire gauge in the exact same volume.

More to the point: it's reasonable to start by picking a bank capacitance, voltage, and coil dimensions as suggested in the OP. Then do your homework (tools on Barry's site are handy for this) to choose the wire gauge you want, then shop for wire.

Rob11095

Tue Nov 01 2011, 01:30PM

Registered Member #4140 Joined: Sat Oct 15 2011, 05:37PM
Location:
Posts: 3

Thanks guys great tips on what to do. I'm still stuck on what wire to get. How is he strengh of the magnetic field related to the wire, like diameter and resistance. Oh and the caps I ordered are 20 450v 1000uf, but I doubt we will use all of emm, only 12, 16 if we are feeling ambitious. And with the charger you recommended, how much does that cost and how would we adapt it for our volt caps?

Forty

Tue Nov 01 2011, 04:44PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

you could start with this for wire

The magnetic field depends on the number of turns per length and the current. thicker wire means higher current but less turns, and thinner wire means less current and more turns. Since for a coilgun you want a fast discharge, you'll want thick wire as then your coil will have less resistance, be able to handle higher current, and have a lower inductance than a coil of the same out dimensions made with thinner wire.
the zvs charger is real cheap. free if you just sample or salvage all the parts.

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