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Simple disposable camera coilgun

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LAN_403

GimpyJoe

Sat Jun 24 2006, 06:36PM

Registered Member #316 Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212

You can charge it with the standard charger, it'll just take a while. If you have ten photoflash caps, I assume you have ten photoflash boards laying around. If you have a beefy 3v supply (yes, you can run a pfc charger at 3v) you can parallel a few or even all then chargers for more current.

A better but more complex way to charge your caps would be with a boost converter. You can find schematics for these all over the forum.

Bjørn

Sat Jun 24 2006, 06:49PM

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

Your math seems to be correct. Adding more capacitor does not automatically give higher power or efficiency, it all depends on what is causing the inefficiency. If the pulse is too long then adding more capacitors may lengthen the pulse and slow down the projectile and in worst case the projectile will exit in the wrong direction.

You will find many posts in the archive and there is also a useful article in the HvWiki http://wiki.4hv.org/index.php/Coil_gun

Alfons

Sat Jun 24 2006, 07:04PM

Registered Member #134 Joined: Fri Feb 10 2006, 10:44PM
Location: Belgium
Posts: 86

Funky wrote ...

You can charge it with the standard charger, it'll just take a while. If you have ten photoflash caps, I assume you have ten photoflash boards laying around. If you have a beefy 3v supply (yes, you can run a pfc charger at 3v) you can parallel a few or even all then chargers for more current.

A better but more complex way to charge your caps would be with a boost converter. You can find schematics for these all over the forum.

Ok; so the way I do it is: I replace the original cap with my string of parallel caps; I solder the load- and trigger-switches so they're always switched on ; I replace the xenon tube by one end of the coil and a switch, connected to the other end of the coil. So with parallel, you mean that I should connect all the wires where the xenon-tube was before in parallel, and put the coil there?
See my very crappy schematics for my own circuit, and what I think is the parallel circuit you're talking about.

Madgyver

Sat Jun 24 2006, 07:41PM

Registered Member #177 Joined: Wed Feb 15 2006, 02:16PM
Location: Munich, Germany
Posts: 214

He means, that you should parallel the part were the cap is being charged. So you connect one baord to your "string of parallel caps" and then another one and then another one.

What you drew makes no sense.

Alfons

Sat Jun 24 2006, 07:51PM

Registered Member #134 Joined: Fri Feb 10 2006, 10:44PM
Location: Belgium
Posts: 86

Madgyver wrote ...

He means, that you should parallel the part were the cap is being charged. So you connect one baord to your "string of parallel caps" and then another one and then another one.

What you drew makes no sense.

Ah lol

indeed it doesn't make sense I didn't really think clearly when I was drawing that...

Do I have to remove the xenon tubes from the other boards? Or should I just leave'm there?

Madgyver

Sat Jun 24 2006, 08:32PM

Registered Member #177 Joined: Wed Feb 15 2006, 02:16PM
Location: Munich, Germany
Posts: 214

It doesn't matter. leave them in if you like.

Alfons

Sat Jun 24 2006, 09:42PM

Registered Member #134 Joined: Fri Feb 10 2006, 10:44PM
Location: Belgium
Posts: 86

Ok; I now have the following setup: 3 flash boards connected in parallel; 10 caps in parallel; the caps load in about 45 seconds with the boards running at 4V.
Using the same coil as before, I hardly seem to have any difference in performance. I have not had the time to do the proper measurements, though.

I will probably wind a new coil during the following week; so then I hope to get a decent result.

Bjørn

Sat Jun 24 2006, 11:00PM

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

Remember to experiment with different start positions of the projectile, if the pulse is too long you may find better performance when you start the projectile further away from the coil.

Alfons

Sat Jun 24 2006, 11:51PM

Registered Member #134 Joined: Fri Feb 10 2006, 10:44PM
Location: Belgium
Posts: 86

I tried to use an SCR (2N6509) but I can't get it to switch decently!
I wired two SCR's in parallel not to blow them (I wasn't sure of the specs when I tried them at first); and I used a circuit similar to the one Barry Hansen used for his Mark III.

Problem is, that the SCR's don't seem to do their job well... They don't switch when I want them to, and sometimes they switch when I turn on or off one of the two power supplies (the one used for the cap charger / to switch the SCR).

Pictures: the complete setup and the circuit (Yes, I noticed I wrote SRC instead of SCR:-p)

Any ideas what can be the cause of this? Also; what value-range should I use for R1 (gate resistor)?

IamSmooth

Sun Jun 25 2006, 02:37AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Why don't you move the coil between the + end of the capacitor and the SCR?

So, + end of capacitor ---> coil ---> anode of SCR ---> cathode SCR to gnd. I can't tell from your diagram if the negative end of the capacitor is floating. I would make sure it is connected to a solid ground. See if this helps.

You can also use a switching transitor. Connect +V to a resistor/base on an NPN. Connect +Vc to the collector through a resistor. When you have a small base current it will give you the switching current you need to open the SCR. If you want to a sharper spike, you can have Vc charge a small capacitor while the transistor is off. This way, when it opens you will get the current from Vc and discharge from the capacitor.

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