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New coilgun (coilgun casing help!)

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zero7525

Fri Feb 01 2008, 01:49AM

Registered Member #1281 Joined: Fri Feb 01 2008, 01:24AM
Location:
Posts: 4

Hey, I'm new to this site! I am in grade 8 and graduating in a couple of months (i have made several successful coilguns already don't question my age!). I reside in Canada.

I'm creating a new coilgun, the total power is 150.4joules input power
-4 capacitors (470uF @ 400v each) in parallel,1880uF, 400v config
-The coil is 22gauge wire with 12 layers, 30mm length, and 6mm inner diameter
-For switching i used a 40amp coil auto-relay (you can simply expect that I don't know a thing about SCRs)
-I don't really plan on using a protection diode(??!!) I can't find any with a rating that can handle it
-I plan on using 10 camera charger circuits wired in parallel to charge the caps
-The battery pack consists of 2 C batteries in series connected to the charger circuits in parallel

THE PROBLEM:
I don't know where to find a gun casing! I want this thing to be portable, not a desktop model. I figured that I want it to be shaped as a pistol or a rifle depending on the size of the final project.
Any suggestions?

UPDATE, i changed the capacitors from series to parallel, this is too complicated!!
it is now 150.4joules becuase i also broke some capacitors and now i have only 4 of them

Backyard Skunkworks

Fri Feb 01 2008, 03:50AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Many weed trimmers have pistol-style grips on them that would look pretty good for this application, you could use some plastic pipe to build a housing for the components and spray paint it all black. That would be the easiest way.

Also 338.5J is a ton of power at 400V, be UBER careful with this, it will not only stop your heart but might even cause burns or nerve damage if you get shocked. So BE CAREFUL! All of the disposable camera charging circuits I've tried need a seprate battery for each one, only one will start if you hook two or more up to the same battery. I'm not 100% sure on this but, if this thing is single stage you might wanna ditch the relay and just trigger it with a switch (I've triggered 56J flash cap gun using just the weed trimmer grip). Protection diodes are a really good idea to save the capacitors from reverse voltage building back up the things that really hate it are voltage sensitive electronic components (SCRs, IGBTs, etc),but I'd say to get some though, maybe 10 to 20 1N4007s in anti-parallel (the cathode of the diode hooked up to the side of coil thats hooked up to the + of the capacitor bank), nothing sucks worse then seeing your beautiful electrolytic capacitors smoke and bulge (and maybe cause a nasty explosion, ending your electrical hobbies carrer).

Shaun

Fri Feb 01 2008, 05:12AM

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

You do need a protection diode otherwise you will permanently damage capacitors, if not cause them to gas internally and rupture/explode.

40A relay I'd say has a good chance of working, but 338J is still a lot.

Something you may want to consider since this is to be portable and mounted is building a single dedicated charger, because 10 camera flash chargers could get a little bulky, and they're not the most reliable things. I have a 30W boost converter the size of an Ipod that could charge this bank in just over 10s (btw it was the first circuit I soldered together, so don't think its beyond you).

The good thing about your design is it uses many smaller capacitors instead of one huge one, so its easier to rearrange them to fit in odd shapes. As for something to mount it in, I would look for something that already has a pistol grip on it. Backyard's weedwhacker idea sounds good. You might want to also look at those big pistol-grip spotlights you get at Radioshack for like 10 bucks now. Those are good because they come with a nice lead-acid battery and charger which does very good powering a boost converter hint hint...

EDIT: Oh and you won't be able to charge your bank to 1200V no matter how many chargers are in parallel as this does not increase voltage just current. So they will charge to 320V but really fast. And I'm pretty sure you can't series the outputs of camera flash chargers with the inputs in parallel, someone correct me if im wrong.

Electroholic

Fri Feb 01 2008, 05:51AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

hey man welcome to the forum, if you live in ontario, pm me for some 1N5408, for free of course. As shaun said, diode is a must.

and your math ain't right, 6 470uf 400V caps holds 225.6J, a 2s3p configuration will give you 705uf at 800V, still 225.6J.

do not expect much efficiency, 22ga is a lil thin, and with 12 layers it will heat up real quick.
PF chargers will never output 1200V either.
so, what i would do is, just put all the caps in parallel, all the chargers in parallel.

as for the gun handle, have you seen the wiimote gunstyle mounts?

zero7525

Fri Feb 01 2008, 09:56PM

Registered Member #1281 Joined: Fri Feb 01 2008, 01:24AM
Location:
Posts: 4

edit: as for the capacitors, i wasted 2 of them for a prototype (now i have 4 in total). I lined them up in series parallel in a 2x2 config, i believe that i will get out 800v @ 470uF (225.6joules?). Here are my results...

-tested the 40A relay, works better than a regular push button switch i suppose, and i don't think that im ready for an SCR yet, because it's hard to find a mass production SCR store, never heard of any

-i might use a boost converter, i just don't know how to use one yet, im not an electronics expert, yet

-I got a bunch of 1N5393 protection (rectifier) diodes, i don't think that they will suffice though (I hardly understand the ratings) Probably 200v max reverse voltage and 50A peak current (??!)

-I'm still studying on the semiconductor thing (I am all self-taught, nobody in my class understands me, *sob* :P)

-Yes, i do live in Ontario, Canada

-correct my on the charger circuits or the capacitor config if im wrong, but here's the schematic (courtesy of rwilsford07)

Backyard Skunkworks

Sat Feb 02 2008, 05:20AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Your math is off on the energy of the capacitors, the energy is E= V^2*C/2 where E is energy in joules, V is voltage in volts and C is capacitance in farads.

So if you've got 800 volts and 470 uf then its...
800^2*0.00047/2=150.4

For higher spike currents then the diodes can handle, use more of them in paralell. Two of those could handle 100A spikes, twenty could handle a whopping 1kA.

zero7525

Sat Feb 02 2008, 09:46PM

Registered Member #1281 Joined: Fri Feb 01 2008, 01:24AM
Location:
Posts: 4

Backyard Skunkworks wrote ...

Your math is off on the energy of the capacitors, the energy is E= V^2*C/2 where E is energy in joules, V is voltage in volts and C is capacitance in farads.

So if you've got 800 volts and 470 uf then its...
800^2*0.00047/2=150.4

For higher spike currents then the diodes can handle, use more of them in paralell. Two of those could handle 100A spikes, twenty could handle a whopping 1kA.

a your right! I accidentally used the calculation for joules for parallel capacitors.

i think that my diodes will not handle the maximum reverse voltage and they will breakdown, does that matter?

UPDATE, i changed the capacitors from series to parallel, this is too complicated!!
it is now 150.4joules becuase i also broke some capacitors and now i have only 4 of them

Backyard Skunkworks

Sun Feb 03 2008, 12:22AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Mkay, first of all I could say "you're lucky to be alive" but whats more accurate is "you could have gotten very unlucky and been injured in a capacitor explosion". The fact that you've killed 2 caps means you've been getting a back voltage spike on the coil during testing, this is very bad and CAN cause the capacitors to explode (violently). Exceeding the diodes max reverse voltage is also very bad at this amount of current and quite likely WILL cause an explosion (although a smaller, less serious one), putting diodes in series raises the total breakdown voltage, so do this.

And please be careful, 150 joules in a capaciotr bank is a ton of power and will cause electric compents to violently explode and at 400 volts can deliver a lethal shock.

zero7525

Sun Feb 03 2008, 10:22PM

Registered Member #1281 Joined: Fri Feb 01 2008, 01:24AM
Location:
Posts: 4

although the capacitors did not actually "explode", but I failed to desolder them in a prototype experiment for a coilgun (it worked), now the leads are all messed up and i can't get them back, they're sitting in the corner here...I'm sure that they are damaged though, since i didn't put a diode with them in the last 20 shots of the coilgun

Backyard Skunkworks

Mon Feb 04 2008, 12:29AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Capacitor explosions are not that common but they do happen, while its quite likely that the capacitors have been damaged from firing without a protection diode its also quite possible they have held up fine. A somewhat flawed but still valid anagoly is that the protection diode(s) are kind of like locking your house up in an uber high crime neighborhood, any one day your out the odds are your house still will be fine. But sooner or later with an unlocked door your gonna lose all your stuff. Just use some diodes!

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