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[moved] Very High Voltage Coil Gun

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LAN_403

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Sat Apr 19 2008, 06:33AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

One thing to keep in mind when using excessively low resistance bleeder resistors is that you are going to load down your power supply considerably. If you are using a 10kv/20ma NST and a 500k bleeder resistor your bank is never going to fully charge... (assuming you are using the 10kv cap in the first post)

It really is better to use a long time constant bleeder resistor (something burning 50w at full power is the 'standard' I believe) and then a low resistance 'dump' resistor that is sized to handle the energy stored in the cap (6kJ), for this a water resistor would probably be good. Then you just need a switch (something pneumatic set up so that you need pressure on a hose to be able to charge the back would be sweet) that will actually connect the dump resistor. Then if that doesn't work or you forget the slow resistor takes over and when you come in the morning the bank is discharged.

Travlax1

Mon Apr 21 2008, 04:53PM

Registered Member #1388 Joined: Wed Mar 12 2008, 04:42PM
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Posts: 23

Ok sounds good. How bout the inductance? In my previous model I had the thin metal survey sticks in the center of the PVC but with all the pancake coil models, I see that most people dont have any sort of inductance. Will it work with just the projectile resting on top of the coil? Also, is a thin 1/16" piece of lexan on top of the coil too thick or will it still shoot pretty high?

Quantum Singularity

Thu Apr 24 2008, 12:20PM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

Travlax1 wrote ...

Ok sounds good. How bout the inductance? In my previous model I had the thin metal survey sticks in the center of the PVC but with all the pancake coil models, I see that most people dont have any sort of inductance. Will it work with just the projectile resting on top of the coil? Also, is a thin 1/16" piece of lexan on top of the coil too thick or will it still shoot pretty high?

As far as the ferromagnetic pieces in pvc it is not needed for a high power induction launcher. These can be used in much lower powered launchers setup similar to a 'Thompsons Ring Launcher'. I think with the very rapid discharge time of pulse caps the pieces might actually hinder performance.

About the lexan, you will just have to experiment. It will work i am sure, but the closer you get the disk to the coil the better. So if the disk is further away than it needs to be you just might loose a small amount of efficiency. For you first try I'd just do it as simple as you can. Once you get it working, then you can try to make different size coils and experiment with different aspects. When I used HV pulse caps and a simple coil of regular #8 insulated wire, i did a couple experiments like placing block of wood between the projectile and coil. I noted that after a space of a couple inches the effect is almost gone. So sub 1" should work fine, just a small amount of loss. But YMMV. Thats what experimenting is about

teslacoolguy

Thu Apr 24 2008, 12:47PM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
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Posts: 792

4g wire is 4g wire if it is a big fat piece of copper or a zillion hair thin strands it will still be able to withstand the current and the only reason they do it this way is so it is very easy to bend.

Travlax1

Fri Apr 25 2008, 07:22AM

Registered Member #1388 Joined: Wed Mar 12 2008, 04:42PM
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Posts: 23

Thanks, thats very helpful. I'm getting ready to launch it in the next couple days and Im pounding out my paper at the same time. I went to a machine shop today to look for a good projectile and ordered a few aluminum rings that are 3 and 4 inches in diameter with a 1 inch hole in the center and they're a little more than 1/8" thick. Will the aluminum be ok to use? I've heard Hard drive platters are a decent choice too but I assume those will warp pretty quickly. Thanks

Quantum Singularity

Fri Apr 25 2008, 12:50PM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

Those should work fine but depending on how high power you go you might even warp those. You might want to see if you can get some thicker ones to experiment with too, like 1/4-3/8" thick. Hard drive platters will work, but they are very thin. On my original launcher using maxwells that was very inefficient would still blast a hard drive platter into a shallow bowl in a single 2-4kJ pulse. They will warp fairly easily, and I think are too thin from an electromagnetic perspective too (higher resistance plus has to do with the skin depth, I dont have any formulas handy I just know I had better efficiency with a thicker disk). but of coarse they will work, just depends on how much you care about efficiency. Pretty much anything conductive will work, without much regard to size or shape either, thats the beautiful thing about induction launchers. From the same coil I could launch hard drive platters, pop cans, large copper washers, etc.

Travlax1

Sat Apr 26 2008, 06:44AM

Registered Member #1388 Joined: Wed Mar 12 2008, 04:42PM
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Posts: 23

Ok, so I tested it today with a smaller cap bank. I used one that my teacher already had built for a launcher of his own. I cant remember exactly what size but they were pretty standard like 5 or 6" tall and 2 or 3" diameter and we charged them up to around 350V. With 4 of those, the 4" aluminum disk went a measly 3 or 4 feet up. Will using one of a few of the big caps (4000V 375uF) make it into a real launcher (i.e. 30-40 feet?)

Also, I used the switch design I showed and just connected it with another piece of copper tubing and that at least seemed to work just fine.

Barry

Sat Apr 26 2008, 12:24PM

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

Aluminum is just about the best material for an induction launcher. You want something with high conductivity and low mass. The conductivity determines how much induced current will appear. The low mass will result in faster speeds.

I suggest making a table of common metals and for each list the unit conductivity and unit mass. Compute the ratio of each of these, and sort the table by the ratios. I think you will find aluminum at the top of the list, even better than copper and steel. Check it out ... comparing these ratios will tell you (in theory!) how much better one metal will be than another.

You're also wondering how thick the disk should be for best results. The concept is that induced currents are strongest on the surface, and diminish with depth into the disk. At some point there is no useful current flowing and additional thickness is simply extra mass along for the ride.

The eddy current profile is characterized by "skin effect" and it depends on the frequency being used (higher frequency has less depth) and the material's conductivity. Study it on the web at places like

and many others. You'll want to estimate your frequency from f = 1 / (2 pi sqrt(LC)) or better yet measure it with an oscilloscope.

Cheers, Barry

Quantum Singularity

Sat Apr 26 2008, 01:16PM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

Its impossible to even take a guess on how to compare your HV caps with the other (electrolytic?) caps without knowing their specs. We need to know the cap specs and how they are wired (series/parallel). The easiest way to get an estimation on how powerful it will be is simply the stored energy in the caps (.5cv^2). Remember the voltage is squared so the energy increases exponentially as you reach nearer to full charge on the caps. I do believe efficiency with your setup will favor the higher voltages of your caps compared to the 350V ones you tried. And I dont believe you realize how powerful those caps might be... at full charge those 4kV caps are 3kJ each, just one of those near full charge should launch a disk pretty much out of sight 100-200' up, and a well designed setup that got between 10-20% efficiency with 3 of those caps (just for example) would be capable of launching the disks I am using (about 60g) at 500-1,000 fps...

Something else that concerns me though, were you using a spark gap switch when you did the 350V cap test? Any protection diodes on the coil? If they are polarized caps you could be damaging them if you have no protection diode, and they could explode. Just wanted to make sure this wasnt the case.

Travlax1

Sat Apr 26 2008, 03:29PM

Registered Member #1388 Joined: Wed Mar 12 2008, 04:42PM
Location:
Posts: 23

I'm not positive, Although I do believe he has protection diodes and bleed resistors in the circuit. However, its an older setup and it wouldnt be a big deal if they broke. And as for exploding, they're cased in a half inch lexan box which I think should hold the caps in if they were to explode. For my HV caps,is there the same risk of explosion if theres no protection diode?

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