Re: Simple disposable camera coilgun
GimpyJoe, Fri Jun 23 2006, 06:46PM

Cool, Alfons! I like the way it's nicely put together instead of just being a jumble of camera junk the way most simple CG's are. Does it penetrate a can?

Re: Simple disposable camera coilgun
Alfons, Fri Jun 23 2006, 07:13PM


Thanks! It allmost penetrates a can, but not quite. I guess it's the coil, since I do have quite a lot of energy stored in these 5 caps... But, as mentioned, I didn't tune it or anything so I guess I shouldn't count on it penetrating a can. I don't really mind. Maybe I'll try to upgrade it when I'm into it

Re: Simple disposable camera coilgun
Alfons, Sat Jun 24 2006, 10:48AM

I just measured the projectile's speed using the Horizontal Ballistic Speed Trap Method and the speed is about 7,437 m/s (about 26,775 km/h, that is).

That isn't really fast, is it? Would winding a better coil help?

Re: Simple disposable camera coilgun
Bjørn, Sat Jun 24 2006, 12:08PM

Find the weight of your projectile and calculate the energy at 7,437 m/s. Then calculate your efficiency. 1% is not too bad for a small coil gun, 3% is good, 10% is brilliant.

Re: Simple disposable camera coilgun
Alfons, Sat Jun 24 2006, 12:45PM


Ok; my projectile weigs 1,2g; so if my calculations are correct, the Ek=(0,0012*7,437^2)/2=0,033J

There's one cap in my circuit on which no capacity nor voltage are printed; it only says 9(2) and 3e. Does anyone know about the capacitance / voltage of this cap?
Since it's about as big as the other caps, I'll assume it's a 80µF, 330V cap.

The other caps are all rated 330V and wired in parallel; so the total capacitance is 120µF+100µF+80µF+80µF+80µF=460µF.

To calculate the stored energy; should I just do ((1/2)*0,000460F)*(330V^2)=25,047J?

So my efficiency is... 0,033J/25,047J=0,13%
Too bad...

Re: Simple disposable camera coilgun
Alfons, Sat Jun 24 2006, 01:40PM

Oops... I just blew up a cap...

I'm going to fetch some used disposable camera's in the local photostore to make another cg.

I do have a question: if I test the working of the coilgun, most of the time I get a huge ZAP between one wire of the coil and the wire I touch it with (which was connected to the flash-tube before I removed it). I thought to put a resistor in between but of course the resistor went up in smoke...
Does anyone know how to prevent a zap, but still get a decent result from the coilgun?

Re: Simple disposable camera coilgun
GimpyJoe, Sat Jun 24 2006, 03:30PM

Find an SCR! You can read up on them on the wiki, using an SCR will reduce your switching losses enormously and maybe get you to .5% efficiency. From there, try winding a better coil and remember this is a magnet; you want the windings as close to the projectile as possible. Use the thinnest barrel material you can find.

Re: Simple disposable camera coilgun
Electroholic, Sat Jun 24 2006, 03:45PM

does the huge zap discharge the cap?
whats the problem?

and as stated, SCRs will also improve the performance A LOT.

Re: Simple disposable camera coilgun
Alfons, Sat Jun 24 2006, 04:18PM


Yes, it does discharge it into the coil.

I just got some *new* used disposable camera's, and I made an array using 10(!) photoflash caps; thus giving 1000µF at 330V.
I think that the standard flash circuit will not be good enough to charge these, would it? Any suggestions which kind of charging circuit I could use instead?

Re: Simple disposable camera coilgun
GimpyJoe, Sat Jun 24 2006, 06:36PM

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.

Re: Simple disposable camera coilgun
Bjørn, Sat Jun 24 2006, 06:49PM

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

Re: Simple disposable camera coilgun
Alfons, Sat Jun 24 2006, 07:04PM


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.

Re: Simple disposable camera coilgun
Madgyver, Sat Jun 24 2006, 07:41PM

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.

Re: Simple disposable camera coilgun
Alfons, Sat Jun 24 2006, 07:51PM


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?

Re: Simple disposable camera coilgun
Madgyver, Sat Jun 24 2006, 08:32PM

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

Re: Simple disposable camera coilgun
Alfons, Sat Jun 24 2006, 09:42PM

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.

Re: Simple disposable camera coilgun
Bjørn, Sat Jun 24 2006, 11:00PM

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.

Re: Simple disposable camera coilgun
Alfons, Sat Jun 24 2006, 11:51PM

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)?

Re: Simple disposable camera coilgun
IamSmooth, Sun Jun 25 2006, 02:37AM

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.

Re: Simple disposable camera coilgun
Electroholic, Sun Jun 25 2006, 07:32AM

is it 26 or 24 awg?
too thin for 10 PF caps

it doesn't matter where you put the scr in the loop,
floating or not, performance would be the same.

try clean up the charger board a bit, i assume you still have the trigger cap and transformer on it? they might be the cause

Re: Simple disposable camera coilgun
Alfons, Sun Jun 25 2006, 08:46AM


It's 0,5mm, so that's about 24 awg.


I have already removed the small caps which were soldered to the boards, so that couldn't be the problem.

Re: Simple disposable camera coilgun
Alfons, Sun Jun 25 2006, 03:42PM

Ok; I just wound a new coil, still using 0,5mm wire (I don't have any other wire left and I don't have money at the moment so I can't buy thicker wire).

The coil draws 5,66A from a 18VAC source; so the impedance is Z=(18/5,66)=3,18 ohm, right? So if my calculations are correct, the inductance should be L=Z/(2*pi*f)=3,18ohm/(2*pi*50Hz)=10,1mH...

As I expected, there isn't much difference in performance.

So, what wire diameter would you guys use; and what inductance should my coil have if optimally tuned? (Or how do I calculate that?)



EDIT:
The SCR-issue still isn't sorted out. The problem is the following: I connect the SCR (see circuit some posts above), connect a 9V-source, switch opened. Then I load the caps, when they're loaded I push the SCR switch, but nothing happens. When I disconnect the 9V-source though (switch NOT closed!), the coilgun shoots! I don't understand how this can occur, and I'd like it if someone'd explain that to me.

Re: Simple disposable camera coilgun
rupidust, Sun Jun 25 2006, 05:34PM

Do you truely want the CG to first work at all and secondly work with performance >= 2%?

* Practice using the SCR at a 12v level using LEDs, lamps, or small motors as a load.
* Practice using the SCR as a LOW side switch, SCR cathode at ground level and load at +12v level.
* The SCR should be known and exprienced prior to high energy usage.
* Use 22 or 20 awg mag wire. Use 6 to 10 layers, make the coil length equal to projectile length.
* Bound the coil with more than helping hands. Any coil movement subtracts from projectile forward velocity.
* Make a graph or table of Velocity/Efficiency Vs. Projectile start position using a fixed voltage, coil, and projectile. The crest of this bell curve graph will be your tuned position. If you are not satisfied, change either projectile, voltage, or coil and remake a new graph or table.
* L inductance = Z/(2*pi*F) and Z = (2*pi*F*L). Better to use a Inductance calculator or a meter because this is not a 50HZ AC application.

Re: Simple disposable camera coilgun
Alfons, Sun Jun 25 2006, 05:46PM

Yes; it does even work, only not like I want it to.
Not necessarily. I'd be pleased with 1% too, but of course 2+% would be great
That's a good idea indeed. I don't have any experience with SCR's, but I thought maybe it'd work right away (which it didn't), in which case I'd already be pleased (I know, I know, wrong mentality)

EDIT: I practiced using the SCR at low levels, and I do get it to work. If I connect it to my CG circuit, it still doesn't work, though...

Ok; I'll try that out and see what results I get.
Indeed I know I should do that, but I haven't got access to an LCR meter and I don't have a signal generator or something, so I guess it'll be quite hard to determine the inductance using another method...