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Ragnarok

Sun Jan 13 2008, 01:03AM

Registered Member #659 Joined: Fri Apr 20 2007, 09:14AM
Location: SW Hertfordshire, UK
Posts: 75

Well, I haven't been around for a while, I've been working on my air cannons mostly... the best of which can be seen in this YouTube video:

Anyway, I realised I'd been neglecting my coilgun plans, and I drew up a new brief. My old plans were quite a bit bigger than my new ones, but I decided to make something small because I'm a university student now, so my budgets and projects need to be small.

I want to get over low 1 ft-lb (~1.36 J) from a single stage reluctance coilgun, which in itself is quite easy. Some of my other requirements complicate that a little.
- Not much larger than a pistol. Should be no longer than about 30 cm in length, or 5cm wide. Weight should be under 2 kilos.
- An interchangeable magazine of at least 6 rounds. Rounds are to be 6mm in diameter, 30 cm long (just over 6g in weight). Manually cycled feed acceptable, semi-auto would be better.
- Needs to get at least 10 rpm.

The space requirements will make the project quite interesting, and the batteries and capacitors are going to be making severe requirements on the space. A small cap bank is going to be quite important for this project, and having fiddled with the gausspistol.com simulator, even with my rather lacking skills, I believe I stand a quite good chance of getting the required KE out of a 54 joule bank (600V 300uf), which should be quite modest in size.
Switching is also going to take some space, but I think that SCRs with quenching should be fine for the smallish power.

Obviously my first question is, does this sound a list of criteria that will be achievable for someone like me who is frankly pretty poor at this stuff?

Also I'd like to ask what people think about a charger, and I know I have asked before, but the circumstances are slightly different this time. Needs to be small, and capable of going from 4 NiMH or NiCd batteries (about 5V between them I find) up to 600V, with at least 10W output. Some sort of low voltage flyback, or should I use a boost converter?

Other ideas and suggestions are welcomed.

Electroholic

Sun Jan 13 2008, 02:05PM

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

these questions have been asked so many times...
try this site,

his site has everything you will need to build your coil gun, it is the portable cg bible.

btw, 10W from 4 x AAs? they are not going to last long.

Bjørn

Sun Jan 13 2008, 02:31PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

The person that made that site has been banned several times from 4HV so you might want to double check any information you find there.

The electronic part of that coil gun matches exactly that of modern electronic flash units, except they are smaller and more powerful. They extract about 30 kJ from four high capacity AA batteries of best quality and can go a lot higher than 10W. So maybe you want to look into finding one of those. Now that the old units no longer works well on modern cameras I expect you can get them very cheap.

Ragnarok

Sun Jan 13 2008, 09:06PM

Registered Member #659 Joined: Fri Apr 20 2007, 09:14AM
Location: SW Hertfordshire, UK
Posts: 75

BjÃ¸rn BÃ¦verfjord wrote ...
The person that made that site has been banned several times from 4HV so you might want to double check any information you find there.

I guessed there were tensions as the link redirected me to a 404 because I'd been linked from 4hv, as well as the fact that the front page contains the message "4hv sucks".

@Electroholic: I know they're common questions, but there are so many variables I wanted to be sure. If you looked at the Youtube video you'll see my area of expertise is very much more mechanical than electronic. I've read all the main sites in as much detail as I can, but I still find talking to people and getting their thoughts helps a lot more.

10W from 4 AAs won't be too bad- if I'm not being an idiot. Assuming I'm not there are about 35kJ in four 1.2V 2000mAH NiMH batteries, and I've got a large collection of rechargeables around up to 2400mAH, (as I understand, higher capacity batteries tend to supply higher currents - is that right?) and those energy levels should last a couple of hundred charges before they need changing.
I'll probably also build an external CW multiplier to charge the thing quicker if I'm near a power socket - I know I'll need to make an auto stopping function for that of course, a bank that size on a mains charger would go faster than blinking, but it should theoretically provide the option for short bursts of full auto fire.

Bjørn

Sun Jan 13 2008, 09:33PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

higher capacity batteries tend to supply higher currents - is that right?

Current and capacity tends to follow the same increasing trend because of developments in the design of the batteries. But very high capacity comes at a cost of higher internal resistance and higher self discharge rate. So usually you will find that the batteries capable of the highest current have lower capacity but they are not common so are difficult to find and expensive.

Electroholic

Mon Jan 14 2008, 12:32AM

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

10W of output is needed, including waste heat, say you need 12W from the batts.
At 6v, 12W means 2Amps, which is pretty close to 1C discharge for most high capacity NiMH in the AA size.

Bjørn

Mon Jan 14 2008, 01:20AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

My Nikon SB-600 does about 3.5A when the capacitor is empty. The datasheet for the batteries indicate that 3C is fine (7.2A) with some voltage drop

Ragnarok

Mon Jan 14 2008, 03:13AM

Registered Member #659 Joined: Fri Apr 20 2007, 09:14AM
Location: SW Hertfordshire, UK
Posts: 75

I've just surprised myself by looking back at the coilgun simulator, and with a few incredibly simple changes, I've found a way of getting the same KE for a much smaller bank of capacitors. Or even a way of getting almost three times the KE with a bank only fractionally larger if I'm up for making a three stage CG, and an offer like that is really hard to resist.
I don't think it will should complicate stuff beyond my ability either, so I think that changes my plans. I'm more than happy to build a bit bigger if I can get 4 joules from it instead of one and a bit. The efficiency would approach ~7%, and I'm lead to believe that's pretty good for a coilgun.

The problem was, I hadn't realised how much increasing the voltage harmed the efficiency - I'm used to the world of pneumatic cannons, so to me, the most logical way to get more power from something the same size is to crank up the pressure - for example at the moment, I'm using about 2 to 3 times the pressure most other people will, and I'm looking at adding almost 50% more again - and that works fine so far, it gives me the power of cannons over twice the size, but applying the same logic to coilguns is a really stupid assumption I shouldn't have made.

Anyway, thanks for the help so far, I may come back and ask more irritating questions in the near future after I've tried to find sources for all the parts.

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