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Good coilgun circuit?

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Jack A

Wed Jul 07 2010, 01:14AM

Registered Member #2975 Joined: Wed Jul 07 2010, 12:19AM
Location:
Posts: 28

Hey, I'm about to start getting stuff for a first coilgun that i designed, could you guys please check my schematic?

Thanks in advance.

The capacitor is 330v 300uf and in the future i may change it into a bank(parallel).
The coil is 12 layer and 2 cm, with this low power level(<40 Joules) is it OK to use 33 SWG(0.25mm) enameled wire?
The controller is my Lego NXT(no problem here as i just use two of the wires to connect the relay to the NXT's motor port).
Are the diodes necessary? apparently they protect the capacitor from reverse currents.
I'm assuming that I can just connect a 9v battery to "charge" and "ground" although I'm not sure. Do I need a resistor?

Cheers,
Jack Allison

P.S. I based this circuit on the second diagram on this page, http://www.anothercoilgunsite.com/nf-single.htm, and know that its bad karma not to acknowledge someone!

Z28Fistergod

Wed Jul 07 2010, 07:24AM

Registered Member #2040 Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180

The circuit is fine, but don't expect it to do anything. Charging the 300uF capacitor to 9V will store 12.15 milli joules. Lets say you have an efficiency of 5%, which is optimistic, then you will end up with 608 micro joules of kinetic energy. Assuming a 5 gram projectile, it will travel at 493 cm per second. So it might leave the coil if your lucky.

You need more energy, and when you get more energy the relay contacts are going to fuse on the first firing. So that's what you need to think about, how to store more energy and how to switch that energy.

GhostNull

Wed Jul 07 2010, 09:06AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

As always I recommend Barry's site:

1. Use the Inductor sim to find the aprox. values for your inductor

2.Use the RCL sim to find the pulse characteristics:

What you are aiming for is a pulse length between 1ms and 7ms

3. I'm guessing that the motor output of the Lego NXT will be able to handle inductive loads so that should be okay but reed relays have very bad voltage and current ratings but at 9v I might be okay but you will probably have to go high than 9v as you will find with the rlc sim.

4. The diodes do protect the capacitor and the switching device from reverse polarity and inductive kick back but may not be necessary. In the rlc sim if capacitor voltage does not go negative then it should be okay with out. and if they are needed you will probably be okay with just one diode in anti parallel. Multiple diodes are used when a single diode cannot handle the reverse current; if the (surge) current rating of the diode can handle the current than multiple diodes are unnecessary. Another thing to consider with diodes is that they will increase the pulse time.

5. Yes you could probaly just charge it directly from a 9v battery but the capacitor will only be charged to 9v and due the the characteristicts of electrolytic capacitors the capacitance will probaly be greater than the marked value. From the rlc simuations you have probaly found that 9v will be WAAAAAAYYY too low. To charge a capacitor of that size you could simply take out the circuit from a disposable flash to charge it. When charging capacitors you need to look at it's current rating, the current rating should preferable not be exceeded greatly during charging.

Hope that helps

-Ken

Jack A

Sat Jul 10 2010, 12:18AM

Registered Member #2975 Joined: Wed Jul 07 2010, 12:19AM
Location:
Posts: 28

Ok, Thanks guys!
Thanks in particular for telling me about the charger, i thought that the 9v battery would take the capacitor all the way to 330! Its ok because ill just use the charging circuit from the disposable camera i got the capacitor from.
With the diodes, ill get rid of three of them like you said.
By the way i plan to shoot a very small load, 2 grams of nail. This means that with the optimistic 5% efficiency itl fly at around 29 m/s after i fix it up.

So, if the relay will screw up should i use a 400V 4A SCR instead?

Thanks heaps!
Jack Allison

GhostNull

Sat Jul 10 2010, 07:47AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

An scr would be over kill for a tiny 300uF cap. A proper relay (not reed) rated for mains should do.

Keep us posted on your progress so we can catch any potential problems!

Jack A

Tue Jul 13 2010, 11:22PM

Registered Member #2975 Joined: Wed Jul 07 2010, 12:19AM
Location:
Posts: 28

Great advice guys,

As you said I played around with the all the tools on the website, and got all the timing stuff worked out, as well as the coil. I ended up with a 1.5 mH coil that had 10.45 Ohms of resistance, problem is, the right result in the simulator (76 A, 2.54 ms) came from my capacitor and the coil, with 2.5 Ohms. Is the differing resistance fixable?

Also, I'm wondering if my 33 SWG wire will be able to take 76 A for 2.54 ms.

Thanks again!

GhostNull

Tue Jul 13 2010, 11:49PM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Resistance will mean engery loss. You want to have the lowest possible resistance that will still work. To achieve lower resistance, thicker wire must be used. From 4hv wiki "coils are in general wound using enameled copper wire with sizes ranging from AWG28 and AWG14" 33 SWG wire is very thin and a resistance of 10.45 ohms is too high. I recommend you get some thicker wire for your coil. Also note the inductor sim is in AWG

klugesmith

Wed Jul 14 2010, 02:29AM

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

Jhackulon wrote ...
Also, I'm wondering if my 33 SWG wire will be able to take 76 A for 2.54 ms.

For a practical estimate of temperature rise, assume the entire stored energy from capacitor goes into heating the coil.
Look up specific heat of copper [edit]I did it for you: 0.387 joules per gram per degree C.
Then all you need to know is the total mass of copper wire.
Wire tables give the mass per unit length for each gauge.
Barry's inductor calculator gives the coil mass.

Where did you get 40 joules; I figure 300 uF at 330V holds just 16 joules.
So 0.5 grams of copper would be enough to absorb a single shot without overheating,
no matter what the wire gauge, length, or how it is wound.

[edit] In a sanity check using I-squared-T ratings, AWG33 wire is inadequate for your proposed pulse (I don't know about SWG).
If your RLC modeling is correct, then we can infer that 2.5 ohms of that wire is less than 1/2 gram.

As others have said, get some thicker wire. Unwind a motor or transformer!
Any length of AWG30 could handle 76 A for 2.5 ms.
And 1/2 gram of AWG30 could handle the output of your capacitor, no matter how the current and time are traded off.

Jack A

Tue Jul 27 2010, 12:00PM

Registered Member #2975 Joined: Wed Jul 07 2010, 12:19AM
Location:
Posts: 28

OK, sorry for taking so long to reply (holidays, homework, etc)
I played around with the simulator and i got to a satisfactory result. 191 Amps for 1.01 Ms with a 0.225 mH 26 AWG coil. This is pretty much perfect time and resistance wise (the coil has about 0.01 Ohms more resistance than it should, 1.03 instead of 1.02).
The reason i have made the times for exactly 1 Ms is that it could be more complex than its worth to get microseconds on the NXT.
This is the setup I'm going to go with. And i expect that the 26 AWG wire will be able to take the power i put into it.
Finally, and this is the part i need confirmation for. Do i just keep the capacitor in the camera circuit and short circuit the capacitor through the relay to the coil, etc, after it has charged?

Thanks again, Jack Allison

GhostNull

Thu Jul 29 2010, 09:19AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Yep, To fire just discharge your fully charged capacitor into the coil

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