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First attempt at a coil gun

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aonomus

Thu May 22 2008, 05:37AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Well after a long time of procrastination, I'm going to start tinkering with HV... I already salvaged a large pile of photo flash caps so I can put together any configuration (paralleled, or paralleled series), but the main problem I'm concerned about is charging and triggering as well as coil design for maximum efficiency... I'd rather try to get something as efficient as possible but I have a few questions. Sorry if this comes off as newbish...

1. Will a disposable camera board withstand the charging time, and if possible withstand being overdriven just a bit (say 3v?). Also, despite protection diodes, will any back-EMF damage the board through repeated firings?

2. Can I use several lower rating (likely TO-220 package) SCR's in parallel without worrying about delayed triggering, overheating, or, where should I look to find better SCR's (stud mounted, puck would be overkill) since I'm in Canada and eBay + customs = pain.

Would

, 50RIA60: 80A continual, 1200A surge be enough, or sheer overkill?

3. What would be the best way to design my coil, are there better ways than using FEMM since I can't predict how a mixed cap bank (for example 10 random photoflash caps charged to 300v) would discharge in terms of peak current and discharge time. More in particular is there a way to predict peak current and energy?

If I can get a CG to work, I plan to build a larger, more serious charger and cap bank to make a mini-railgun... but thats for another day.

Backyard Skunkworks

Thu May 22 2008, 06:54PM

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

This is pretty similar to my first coilgun. I used 1200ufd at 330V and a weed trimmer pistol grip to trigger.

1. If you've got say 10x 120ufd caps charging to 300v, it will work but take a while (30 sec maybe). If you do still want to use the camer boost regulator, use NiMH batteries. Alkalines drain really fast like this, you'll have maybe 20 shots max.

Not 100% sure on 2, someone correct me if I'm wrong, but I think paralleling scrs is fine for coilguns. The current shouldnt be rising fast enough to blow an scr before they have all had a chance to close. I can say that 1.2kA surge should be fine, I doubt you'll get that much current but... a good margin of overkill is always good, dead scrs are always bad. Speaking of dead scrs, you need some diodes to quench the back voltage spike, or your scrs are done for.

Not 100% sure on 3 either but, when your dealing with this small a coilgun I doubt the cap bank will be the main issue. You should get plenty of current at this voltage and capacitance. I'd reccomend using somewhat thick (18-20 gauge) wire for the coil so you get a higher peak current and less rise time.

You can calculate coil inductance on Barry's site:

And then figure out the discharge waveform:

aonomus

Fri May 23 2008, 06:40PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

The more I think about it the less I want to bodge the cap bank with photoflash caps, what about 400V 3900uF caps, say 3 in a bank? Where would I source copper buss bars, or could I perhaps just hammer some copper pipe flat?

Quantum Singularity

Sat May 24 2008, 02:55AM

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

I have used flattened copper pipe and also copper bars from mcmaster-carr. The pipe might be hard to get perfectly flat so might not be as well for terminals that you cant really clamp down on hard (i.e. aluminum studs on 'lytic caps). But it worked fine on heavier studs that I could really clamp down on to flatten out any small imperfections that might exist.

aonomus

Sat May 24 2008, 05:15AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

I don't know how many Canadians are around but shipping copper bars across the border would practically double it since shipping goes up considerably (ie: non-linear) relative to distance once it crosses the border. I think I'll take my chances with flattening pipe with a vise and wood block + hammer.

I've picked out a pair of SCRs which each have 150A (3500A surge) capacity at 800V, maybe overkill, but I'd rather order something once than several times.

At this rate I'm going to need a better charging circuit.... can I parallel several camera boards in a bodgy attempt with diodes to prevent reverse flow?

Edit: on a more important note I have two old(ish) RURG8060 diodes sitting around rated for 800A surge, seems ok right?

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