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new caps & basic launcher testing rig

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Quantum Singularity

Wed Mar 29 2006, 12:43AM

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

I just recieved my caps today I got off ebay. They were $5 each for 250V 9,100uF caps that had the large screw mounts (a hair smaller than 3/4"). I am sure there are better deals out there somewhere but I wasnt finding them, and the ebay auction was an easy solution. Plus they had a lot of 13 of them so I got a bunch of matched caps which should be better for making banks of them. I am hoping they will work out for some coil guns and maybe a rail gun.

I want to test them, but I just found my cap meter is shot. So I was thinking maybe just make a quick simple disk launcher and try each cap. I will rectifiy the 120V mains, charge the cap, and discharge it with a simple spark gap mechanical switch into a flat spiral coil with a hard drive disk on it. I havent run any calcs yet but one of these caps should get a disk to move shouldnt it? Assuming a get get it to go a certain hieght, I was gonna try it with each cap to make sure the results were reasonable similar and none of the caps had any major problems. DOes this sound like a good way to test em and have some fun at the same time?

Here are the caps I got:

Caps1

FastMHz

Wed Mar 29 2006, 01:01AM

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

That's 3.7kj worth of capacitors if you use all of them on one bank - you'll have a blast (pun intended)! I'm sure you've seen my post about my 3.5kj bank...And yes, you will get a launch with just one. Rings hit the ceiling with my bank charged to just 200v.

Wilson

Wed Mar 29 2006, 01:33AM

Registered Member #78 Joined: Thu Feb 09 2006, 11:27AM
Location: Sydney, Australia
Posts: 133

wow, i really envy you guys in the US. Each one of those caps store 280J of energy at full charge, so assuming even 3% eff. you should be able to get a 20g platter moving at 20+m/s hopefully. It might be worth investing in some large SCRs in the long run, as you could probably explode most mechanical switches with that kind of energy ;)

Quantum Singularity

Wed Mar 29 2006, 01:41AM

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

FastMHz wrote ...

That's 3.7kj worth of capacitors if you use all of them on one bank - you'll have a blast (pun intended)! I'm sure you've seen my post about my 3.5kj bank...And yes, you will get a launch with just one. Rings hit the ceiling with my bank charged to just 200v.

Well of course I saw you post

I think I probably posted on your post too! But have you seen my 6kJ launcher? Or my plan this summer to try 12kJ? That will really be a blast (or a large explosion). But these new caps are for an easier / more portable / safer design. My big caps weigh like 100# each and I can barely lift them enough to get em in my truck. Plus the extra danger with 10kV. And I hope I can try solid state switching with these caps - like a scr - something I cant do with the big cap. I really hope to build a battery supply for them so I can make it really portable, atleast for a couple caps - maybe not a bank of all 13 of them.

I forgot to ask - since this is the first time I am trying electrolytics on a coilgun, will the caps survive the V reversal from the coil? Do I need to use clamping diodes to protect them? My big maxwells had V on them after I fired, I dont know if it was reversed or not cause as soon as I would fire I would quickly throw the rod across my discharge gap to make sure they were fully discharged. Then again that was probably mostly due to the spark gap & char buildup not conducting under a few kilovolts probably.

FastMHz

Wed Mar 29 2006, 01:51AM

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

I saw your 6kj...nice stuff and very dangerous

I would definitely have a clamp diode...and if you use SCRs, you must, or you'll lose them for sure. I have a huge diode rated for 6000a pulses hooked in parallel to the coil, so backwards induced voltages are shorted. This protects caps and SCRs.

12kj...if you use any coil sized for HDD platters I think it might detonate...Now with these smaller designs and SCRs you have a lot more efficiency than with a spark gap...you might find similar performance with 3.7kj SCR switched rig to your 6kj spark gap one. I'm getting close to 10% efficiencies with mine and 3 SCRs in parallel.

I charge mine on 5ah 12v gel cell with a cheap inverter which powers a transformer from a 1950s era TV, which then charges my 1400v bank via a bridge reg and a center tap like a voltage doubler...

Quantum Singularity

Wed Mar 29 2006, 02:17AM

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

Ok, wasnt sure about the clamping. I'll hold off until I can get some big diodes. There were a few from digi-key mnetioned in another post here from Rupidust I might look into, but 6,000A pulse rated diode I havent ever seen... is this something commonly available (if so where?)?

BTW, I probably wont be firing a 12kJ coilgun... on my 6kJ one I dont think I ever went more than 3kJ. But I do want to parallel both my big caps and try to push more weight with a better more rugged launcher design. And the coil I used was #8 awg and with 3kJ wasnt having any problems (the 12 awg was destroyed though). And the disks I am using are probably bigger than yours too. Everthing I have used so far has been the 5" disks from 'older' hdd's. I even have a super large 12" disk I want to try from some kinda mainframe /server machine that looked like a small washing machine.

Wilson, just to let you know, when I said 'simple spark gap mechanical switch' I meant like a couple of copper pipe peices with another copper pipe piece that slams together. Simple, cheap, and rebuildable. If I have the time I will make it selenoid & spring powered so it can be triggered by remote instead of by hand. Until I get an scr bank built it will suffice.

Thanks for all the words of wisdom everyone. Its been a few years since I have done any HV or EMP projects, and because I found this site a month ago I have a rejuivinated interest!

FastMHz

Wed Mar 29 2006, 06:50PM

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

The diode is a Semikron and I got it surplus from someone on this forum a while back. It measures about 4" long by 1.5" wide and actually had two diodes in it.

Interesting that your 12awg was destroyed - I'm firing 3.5kj through 14awg. I have two 3.5" diameter coils of 14awg layered on top of each other and in parallel. It's completely epoxy coated which is probably what keeps it alive.

Your discs are bigger - mine are 3.5" in diameter.

EDIT: I noticed on your web site that your rings were cupped too...my 5/16" thick rings were also quite cupped and I need to hammer them flat repeatedly. This would indicate to me that the actual forces on the ring are around the outer edge...or may the magnetism in the ring itself be doing this?

Quantum Singularity

Thu Mar 30 2006, 07:33PM

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

I am sure if the #12 awg coil I used was epoxy coated and securly mounted it would have probably survived just fine. Back then I had just used the duct tape method onto a piece of plywood. The #12 wire developed all sorts of kinks around the coil. Almost looked as though it was trying to form the coil into a square? My pulse length was probably a lot shorter and peak currents much higher, so I assume my coil was seing alot more di/dt which probably helped in its demise. When I tried the #8 coil it held up fine and didnt distort, so we used it. It appeared no different in performance but how do you really tell when things are going up high in the air? I am sure a chrono would have detected which one was better.

The were only 2 rings that were severely cupped with a single shot, one is the one in my avatar and I dont know how much power went into it. Your setup is probably more efficient as my 2-3kJ shots only slightly cupped the ring (like for example the darker ring pictured probably underwent 2-4 shots to become cupped - I dont remember for sure. The other one is the strangely warped one, it went from slightly cupped (even) to that odd oppositely bent form in one shot (~7kV), and thats the shot that the coil severely distorted. 5/16" is also alot thicker and probably takes more force to bend! Mine are all like 1/8" I am guessing.

About the cupping, I cant remember for sure which way it cupped (away from the launcher on the outside or inside). I think it was the outside - so placed in its original launching position would look like a bowl. I am no expert but this is what I have surmised what happens to bend the disk. The magnetic force is purely repulsive, correct? So the disk should just fly away rather then bending. The force holding it back is obviously inertia and air pressure. Inertia alone wouldnt cause it to bend unless there was a substantial amount more magnetic repulsion around the outer parts of the disk compared to the inner parts - this is possible. But the air pressure created must be huge. I have found in my launcher that over 3/4 of the force is excerted in the first say 3 inches. Accelerating a flat disk several square inches to a few hundred feet per second in a matter of a few inches is an enormous force of air pressure against a flat object. I dont know that much of physics to calculate the air pressure seen. Now then, there is one key thing that can help determine what is truly going on and I think you might have this. When you make a somewhat aerodynamic body above the disk, like a rocket, you have eliminated air pressure as a cause. But to determine better, the housing should place equal force on the entire surface of the disk (not just a tube that touches the outer edges). If it meets this criteria, then any resulting warpage must be the result of inertia or other magnetic forces at play. So does your rocket evenly distribute the weight accross the disk? And does your disk bend as much as with nothing on it, or not at all? Sorry for the long post but I think this is very interesting.

FastMHz

Thu Mar 30 2006, 08:42PM

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

Excellent concepts in your post! My current rocket has the disc mounted to a piece of plastic which is then mounted to the base of the rocket. The plastic bends a little I'm sure, and the disc is severely cupped after just a 1.8kj launch. But the base of the rocket is solid great stuff, so it's not just the edge with pressure. These are 5/16" thick discs, much thicker than a HD platter but still cupping.

I'm in the process of building a new rocket and will be testing it with both a 5/16" disc and a new 1/4" I just cut out. I'll see what I can do to distribute the load as evenly as possible.

I also just built a new dual layer coil which is wrapped even better than my previous, and using a better fabrication method both sides of each layer are epoxied. My current dual layer coil suffered a bit of damage when the 1/4" disc went up 5 feet and landed edgewise...cracked the surface

Quantum Singularity

Fri Mar 31 2006, 03:33AM

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

What I was thinking of making that would definately disperse the weight evenly on the disk is a solid wood rocket out of large round stock (like a dowel) and turn it on a lath and cut a roundnose shape to one end and leave the other end perfectly flat. I figure for good ballistics the length should probably be double or so the width. Do you happen to have a lathe? I probably wont get around to high power testing for a few months yet. I am not 100% sure the best way to attach the disk to the base of the wooden slug though, but even if its not attached at all it should probably be ok, it will just seperate in the air at some point.

To protect your coil maybe put a piece of 1/8" polycarbonate sheet on top it? You could even epoxy it to the top for extra strength, but then it couldnt be removed or replaced.

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