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a few questions about coilguns.

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LAN_403

Forty

Mon Jun 13 2011, 04:05PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

tapping the wires together will lose a bit of energy, and unless they maintain perfect contact throughout the firing, will likely send a weird shaped signal through the coil. you'll also have a greater chance of shocking yourself if you've got live wires around and youre holding them. capacitor bank shocks are not pleasant. If you dont yet have a solid state switching device, atleast use a relay instead. i've also had success in the past by using camera flash bulbs. wire the bulb in series with the coil and use a high voltage pulse around the bulb to fire it (the same way they are used in a camera flash circuit)

alf

Mon Jun 13 2011, 06:47PM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121

hi,

thanks the idea with the camera tube is nice, ima try it.

i have the measurements:

barrel internal diameter: 6mm, external diameter: 7.5mm.
coil diameter: 15mm.
wire diameter ~1mm.
barrel length: 120mm
projectile length: 17mm.
distance from the back end of the barrel to the coil: 12mm.
distance from the front end of the barrel to the coil: 78mm.

too many measurements probably, but i thought id take them anyway

and heres a drawing of what it looks like.
sorry for labeling obvious stuff, its just that i like to do that :D

and another problem is that, the big scr, it is acting as a diode, so i cannot use it as it conducts when its not triggered,
is it that the bolt isnt the anode like i think? or is it faulty?

thanks. Alf.

Forty

Tue Jun 14 2011, 12:00AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

the scr won't conduct unless the gate is signaled. and then it will stop conducting only after the current into the anode drops to ~0. it won't conduct the other way around unless it's broken. if you can't figure out the anode from cathode for your scr then just use an indicator led and a battery across it while you signal the gate.

Turkey9

Tue Jun 14 2011, 12:15AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Inductance has EVERYTHING to do with the peak current, it's not as simple as ohms law. Barry has a great simulator that can help you visualize it. Google "barry rlc simulator" and it's the first result.

According to your dimensions, your coil is 20uH and 64mOhm with 84 turns. Putting those numbers into the rlc simulator shows that you'll get around 1.8kA peak current pulse. I'd say that that is a good value for a coilgun of your size.

As for your SCR, try connecting the gate wire to the large wire coming out the back. If it still acts like a diode, your SCR is most likely dead.

Andyman

Tue Jun 14 2011, 01:56AM

Registered Member #1083 Joined: Mon Oct 29 2007, 06:16PM
Location: Upland, California
Posts: 256

You have a resistor, inductor AND capacitor which means you have an RLC circuit.
You can't just use the R of your coil, you have to use the total impedance Z, which takes into account inductive reactance X_L and capacitive reactance X_C. THEN you can use V=I*Z

Forty

Tue Jun 14 2011, 03:24AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

stud SCRs can come in both polarities, (with the bolt as the anode and the wire as the cathode or vice versa) If it still doesn't work in either direction, and you're sure you have a proper gate signal (i've found that the whole circuit needs to share common ground sometimes,) then it is most likely dead

alf

Tue Jun 14 2011, 06:52PM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121

hi,

thanks for all your replies.

and i dont know about the scr, im still testing it,

today i put the projectile further into the coil, and it worked considerably better,
it made a big dent in a cardboard box at 340v, i guess it would fire right through it at 400v,
at 340v the energy would only be 31J, the projectile is a 1.7cm long piece of metal and the end is completely flat,
dyou think this is working OK at such a low energy?

i intend to buy 4 capacitors of total capacitance 9000uf, that should work better ;)

thanks. Alf.

Forty

Tue Jun 14 2011, 10:20PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

get a projectile that is about as long as the barrel and make sure it is a good material by sticking a magnet to it (and then to other things for comparison.) I'd also make sure the projectile has a smooth surface and no sharp edges so it doesn't rip up your barrel/coil. 31J (stored) denting a box sounds reasonable.

alf

Fri Jun 17 2011, 07:08PM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121

hey,

ahh cool, i hoped youd say that :P
what would you guess the maximum capacitance to use at 400v would be with my coilgun, before the increase in capacitance would pull back the projectile?
i mean just a rough guess considering the measurements i uploaded :)

thanks. Alf.

Artikbot

Fri Jun 17 2011, 09:01PM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

It depends mostly on your coil.

Barry's sim will plot a very nice graph to show you when will the maximum power spike occur, so you can have an idea of the mass you'll need to achieve the sweetspot where your projectile uses all the available energy and the coil doesn't pull it back again (I recall seeing a guy in YT where his projectile went away, came inside again and smacked him in the chest lol)

31J denting a box sounds very good! My tests with 24J were barely able to leave a small mark on it :/

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