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"Mass driver" launcher project

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Jasonr

Sun Jan 20 2008, 06:20AM

Registered Member #167 Joined: Mon Feb 13 2006, 06:41PM
Location: Waterloo, WI
Posts: 54

you can easily see the caps reversing voltage with a meter.
Diodes are cheap insurance, I would use them weather you think you need them or not.

I would use one across the load. and one across the cap bank. I would use the highest current ones you can find.

Jason

Shaun

Sun Jan 20 2008, 08:58AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

As for many smaller diodes vs. a single high-current diode, if you have a huge amount of small diodes in parallel, each with a very slightly different turn-on time, the first one to turn on will get all the current and die.

Not to say that this will happen every shot, or that it even WILL happen. Plenty of people do this on a much larger scale than you and with no problems whatsoever, but if taken to an extreme level that is what it will come to.

Just my 2 cents.

Andyman

Tue Jan 22 2008, 07:58PM

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

I just bought 4 high current stud diodes. 85HFR-120, datasheet here: http://pdf1.alldatasheet.com/datasheet-pdf/view/209591/TEL/85HFR-120.html
They are rated at 1200V 85 amp continuous duty and 1800A max surge current for 8.3mS. I plan on putting one across the whole cap bank, one across the load, and one on each pair of paralleled capacitors. Is this too overhaul? Can I eliminate one or two diodes, or is there a better setup? Also, how much current should I expect from the discharge at 800v and 1250J?

ramses

Tue Jan 22 2008, 10:22PM

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

depending on your coil specs, i would estimate A LOT. at least for the first half wave. on the second wave, a lot of the energy(thus current) would go to the projectile, (through induction, the whole concept of the gun). i would think that they would be fine that way, but i would parallel 2 at the coil and 2 across the whole cap bank. putting them across the 2 parallel banks would do nothing(i think) a thin section of copper pipe would probably make one heck of a projectile.

Andyman

Thu Jan 24 2008, 05:21AM

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

I just finished the charging module for the capacitor bank. I put in a little extra effort to make the thing look nice, and not be hazardous for demonstrations.
First of the schematic. Voltage septupler based on the one from anothercoilgunsite.com. Output is 850V:

Mmmm... Diodes. The capacitors are 400v 22uF.

Heres the thing in its case with the lovely lamp turned on.

The guts of the thing. I found that big power resistor out in the desert among some other shot up electronics. I had no other use for it, and I needed to fill up some space.

Would it be alright if I put a 2.2k bleeder resistor across the outputs? As long as there's a load (the capacitor bank) I think it'll work, but I could be wrong.
Will I get more current with higher capacitance capacitors?

Shaun

Thu Jan 24 2008, 06:30AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Looks good overall, but I would watch out for shorts on you multiplier stack; those leads look a little precarious.

And about the bleeder resistor, a 2.2K resistor with 850V across it will dissipate 330W, so unless its the size of a TV remote you may want to consider a higher value.

Myke

Thu Jan 24 2008, 07:35AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Also don't depend on the bleeder resistor because if I should fail open for whatever reason then the charge will remain. Always discharge your caps before working on them.

teslacoolguy

Thu Jan 24 2008, 06:27PM

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

for the capacitors you can do what i do and go into walgreens with a big smile on your face and ask for some used disposible cameras for a school project

i use that line and it always works but anyway get like 8 120uf caps and make 1000uf at 300v cap bank assuming that the caps are rated at 330v and they are most likely to be found in the kodak powerflash and sometimes in the kodak funsaver

Andyman

Sun Feb 03 2008, 12:01AM

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

I've finished winding and potting the coil, and got my protection diodes in place. The project is now in it's final stage of completion which is basically just making the thing look nice and mount the flyway on to something.

This is how I attached the protection diodes. I don't see the reason why I need two at the coil and two at the capacitors, this way is more simple. As long as the diodes are preventing reverse voltage, they're doing their duty and I figure the closer they are to the capacitors, the more protected the capacitors are.

This is the finished coil. I used probond polyurethane glue, because according to fastMHz, it won't shatter, unlike epoxy. It is 15 layers of 2 turns each. I got tired towards the end, and it turned out a little sloppy.

This is a picture of the wear on the switch after about a dozen or so 3/4 charged shots.

The performance of the launcher is outstanding. With around 700J at 600v, the big platter goes as high as our two story house which is about 30-40 feet. The coil is taking the shots very well, and there are no outwardly visible signs of stress. I tried to split a piece of wood, but the result weren't like I was hoping. The wood launched probably 10-20 feet up and the platter was warped around it. I had to use a hammer to flatten it back out.

I had made a little mini coil that shot the spacer between the hard drive platters, and its almost too much for it.

At about 600v, the ring launches VERY high, almost out of sight. I would estimate about 60 to 100 feet.

You can see that the stresses are too much for the coil, as the bottom turns are getting smashed into the inside of the coil

Now the only thing left to do is make a pivot for the cannon, and assemble the recoiling base.
P.S. I've learned the hard way to discharge the capacitors in the charger before messing with them. I got a nasty shock of 650 volts through my hand, and on the finger that touched the positive end, there was a nice tiny hole where it touched. Lesson learned!

FastMHz

Sun Feb 03 2008, 08:04PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

Congrats!!! Induction launchers are great fun....the center post is not needed BTW, and neither is the hole in the center of the projectile. A round plate of aluminum will actually work better because it deforms less, especially when you start putting things on top of it to launch

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