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[moved] Very High Voltage Coil Gun

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LAN_403

Quantum Singularity

Mon Apr 14 2008, 02:25PM

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

Are you talking about wire for winding the coil? If so, I'd guess you probably dont want excessively thick insulation. But wire is cheap, you can always experiment. If it were me I'd just go to Lowes or Home Depot if you have them nearby, they have any wire you would need (other than magnet wire) and you can buy per foot so just what you need. Just look for the regular single conductor interior wire, I'd guess perhaps around 8AWG would work and be a good starting point, at least that worked for me in the past with my HV pulse caps, though I never did get a chance to try other sizes.

Travlax1

Tue Apr 15 2008, 01:16AM

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

Ok sounds good. I'm going to go look for wire like that. Will having the threaded wire versus the single chunk of wire make a difference? I quickly got some 10 gauge single conductor wire with fairly think insulation. is it even worth trying it with that thick a wire?

Also another question about the design of the coil that i've seen in some pics. People all seem to make a recessed circle in their wood or lexan base in order to put the coil in like in this pic:

What does this do for the launcher? And why are there two layers of wood with the metal screws in this design? Most of the ones I've seen have just a single wood or lexan layer with the coil either on top or sandwiched between two thin pieces, whereas this design shows the coil on top of one of the two layers of wood. Any insight?

Andyman

Tue Apr 15 2008, 06:02AM

Registered Member #1083 Joined: Mon Oct 29 2007, 06:16PM
Location: Upland, California
Posts: 256

You don't want super thick insulation as that will decrease the amount of windings you can have in a small space. You want relatively thin insulation so you can pack as many windings as you can in a smaller area, thus concentrating the magnetic field.
Ah, that recess is just what the instructions on amazing1.com said to do. I wound my coil a bit too large and it doesn't fit. I have it on a flat piece of wood the same size for now, I'll probably never get around to enlarging the hole.

As for the two layers, the bottom one is up against a stop and the front one is separated from the back one by springs on the bolts. This would create a recoil absorber so the entire assembly doesn't self destruct from the force of the projectile launching. I have yet to actually use it though.
All of that stuff is mostly for looks, to tell you the truth. All you need is a coil of wire glued together and a piece of wood or something to set it on. Nothing more. The design is like the "magnetic cannon on amazing1.com

As you can see, any great force would rip the cannon right off the base. Thats what the springs are for.

Barry

Tue Apr 15 2008, 12:52PM

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

The recessed area in the wooden form may be designed to get the aluminum plate closer to the coil. This is fairly important -- the force you get depends on the magnetic coupling between the coil and plate. The closer you can have the plate to the coil, the more eddy currents you can get.

Ideally you would launch the disc from a starting position that is resting directly on the coil itself.

Cheers, Barry
Error. Keyboard not detected. Press F1 to continue.

Quantum Singularity

Tue Apr 15 2008, 02:24PM

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

I believe the recess is just for coil support. You would wind a coil to fill the recess; the 'walls' of the recessed area would help hold the coil in. It is not necessary and makes winding a coil a bit more challenging actually. It wont really provide a huge amount of support either, the forces in the coil are compressive, meaning when it fires the coils pull in on themselves and not so much outward.

You can see that my very first coil I ever made was just duct taped to a piece of plywood, i.e. 'ghetto rigged'. I don't recommend that of course, but just showing that a #8 AWG coil of standard insulated wire (THHN I think) from lowes, duct taped to a chunk of scrap wood, could launch platters out of site and held together fine. Might not have been that efficient but it worked and was easy and cheap to do, great for 'getting my feet wet' in the hobby when I was in college . I did try regular #12 but it wasn't rigid enough and the coil greatly deformed after several shots at a couple kJ.

BTW, when I said single wire conductor I didn't mean solid wire (as in solid vs stranded) I meant single as in single wire vs multi conductor like 10/2 or 10/3 like you would use to wire an oven with (two hots, one common, a ground, all in an additional insulated jacket). Just get the individual wire, and stranded is fine. Solid will work its just much more hard to bend, if your winding it with a nice jig though that might not even be a concern. But the audio cable usually uses very small strands and is highly flexible. That can actually cause problems since it will be easier for the coil to deform; you want a more rigid coil if possible.

Travlax1

Wed Apr 16 2008, 04:54AM

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

Thanks, thats very helpful, I got some 6 and 8 gauge wire today that should be good. Next question: Some pics that I have seen just show a single pancake coil thats about 8-12 wraps and thats it. Others I've seen have a PVC tube in the middle. I'm curious because my prototype I built (a crappy several hundred coil launcher connected to a wall outlet with induction in the center) had the pvc tubing. In the pancake model is it simply there to keep the launching of the disc straight or is there some sort of induction inside or something?

O yea and one last (i hope) item I need is a bleed resistor for my capacitors. I have four 375 uF 4000 V caps and i'm going to start with just one and work it up from there. Any Ideas on what kind/size/place to get it?

P.S. I'm writing a fairly large paper on this and was wondering if anywhere on this site or somewhere else theres an in depth writeup on the theory of operation

ramses

Thu Apr 17 2008, 01:14AM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

for the resistor, I'm thinking a salt water resistor. basically salt water and 2 pieces of aluminum foil in a cup. add salt or make a connection to bleed. oh and if and when you present this beast, MAKE SURE THE PIECES OF FOIL DON'T TOUCH.

big boom, bright flash, soiled pants, etc.

as to the theory of operation, copy pasted from wikipedia:

Lenz's law states that the induced current in a loop is in the direction that creates a magnetic field that is parallel to the change in magnetic flux through the area enclosed by the loop. That is, the induced current tends to keep the original magnetic flux through the field from changing. This law is similar to Le Chatelier's principle.

ramses

Travlax1

Thu Apr 17 2008, 04:03PM

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

Ok, I like the simplicity of building your own resistor however, I'm looking for one that I could buy that could handle a few of the 4000V 375uF caps. Also, Heres a pic of my switch with two copper posts that I'll join with another piece of thick copper tubing from about 20 feet away with a PVC arm. Any thoughts? Think that should work? Also, Any advice for wrapping the coil? I've got 8 gauge wire that I'm going to sandwich between thin lexan sheets and I've got strong glue that everyone recommends.

ramses

Thu Apr 17 2008, 09:02PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

several things:

1: you might be better off using a spring to force the pipe between the contacts because manually closing it can take a long time, resulting in arcing and welding.

2:I'm not sure you will be able to use a 20 ft pipe with a heavy copper pipe on the end with any amount of dexterity.

3: you haven't mentioned a charger yet. this is a rather important component. I would suggest a 9000V 60mA Neon sign transformer. you may need to rectify this, i forget

4: for coil winding, watch this video:

It's not mine, but it was made by another member of the forum here.

5: the resistor will probably be very expensive to buy, even from ebay.

assuming that you put the caps in series/parallel:
a 25W resistor would have to have 2.5MOhms resistance. that gives you a time constant of over 97 hours to discharge! and you need like 5 time constants to drain it to a safe-ish level.

now that I think about it, a salt water would suck too much power from the supply to charge the bank. maybe you could use a tap water resistor that is connected to the bank with a switch. 100ml of water in the resistor should safely discharge the bank without boiling.

don't think you can get away without a bleed resistor, either. there is a video on youtube of someone dead shorting around a MEGA joule of caps. you don't want that person to be you!

you may want to give some thought as to the projectile as well. a thick aluminum disk would be good, as would a copper disk.

to add body to your paper, you could compare the efficiencies of copper vs. aluminum, as well as velocity.

When is the deadline?

Quantum Singularity

Sat Apr 19 2008, 05:51AM

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

Travlax1 wrote ...

Ok, I like the simplicity of building your own resistor however, I'm looking for one that I could buy that could handle a few of the 4000V 375uF caps. Also, Heres a pic of my switch with two copper posts that I'll join with another piece of thick copper tubing from about 20 feet away with a PVC arm. Any thoughts? Think that should work? Also, Any advice for wrapping the coil? I've got 8 gauge wire that I'm going to sandwich between thin lexan sheets and I've got strong glue that everyone recommends.

Bleed resistors. Well the easiest thing I can think is also to buy them... I've picked up some mighty big resistors at hamfest for just a few bucks before... foot long resistors. You might be able to find some that would work on ebay or somewhere. I've never designed a bleeder network for such a cap bank, but just calculating out some rough estimates... too keep the power dissipation relatively manageable, lets say <50W, thats going to put you into a few minute long bleed time. others can advise what would be acceptable here but I'll just calculate some numbers. A 500K ohm power resistor would make a time constant of 187 seconds (~3 minutes) but its going to take several time constants to fully bleed down, but the bulk of the charge will bleed in the first tc. BTW, the TC = R*C (500,000*.000375). And the max power dissipation by the resistor at max voltage is V^2/R so 4000^2/500,000 = 32W. These numbers are all based on 1 resistor per cap. And a quick search on ebay turned up a 500k 50W power resistor here:

for just $1.99 but it looks like there is only 1. And if you doubled the number (2 per cap in parallel) then youd hald the discharge time as well, but unless these are only engaged when you need them it will draw too much power from your supply I'd think. If it were me, I'd have built/purchased a contactor/relay unit that would engage & disengage the bleeders so they arent drawing power while your charging up. if you cant do that then youre going to be limited by how much you supply can handle also given the load of the resistors, and you might have to use higher value resistors and longer discharge times. I'd also have a voltmeter monitoring your cap voltage, either built in or they also make some hand held HV probes that adapt DMM's for HV.

As for the crude switch, well for a first attempt it would probably work. Of course its not great, but similar to what I did back when I first started in the hobby. 20' is a bit unreasonable though for PVC, I'd think 5'-8' should be plenty. Better if you can design a spring powered one like earlier mentioned... you could simulate your with a much shorter piece of PVC like 1' on a hinge, with a spring pulling them together.

Coils, make sure to keep the lexan on the top of the coil very thin. For the best coupling you want only enough thickness to insulate the wires and provide a little rigidity. For insulation you probably dont need anything more than the wire, back when I used #8awg insulated wire I used it up to about 5kV with no arc through. So unless you need it for support, I might not even use it. I have no idea how well glue sticks to the insulated wire your using though. if your just needing something to stop the coil from gluing to its rig you can also just use paper on top.

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