"First" Coilgun / Electromagnetic Projectile Accelerators / Forums

Forums

4hv.org :: Forums :: Electromagnetic Projectile Accelerators

« Previous topic | Next topic »

"First" Coilgun

1 2 3 next

Move Thread

LAN_403

Michael Witt

Sun Mar 05 2006, 07:16PM

Registered Member #282 Joined: Sat Mar 04 2006, 11:45PM
Location: North Chicago, US
Posts: 28

I am considering building a CG, but I have a few questions before I start to go build it and spend money on it. I built a small one with a disposable camera flash unit. I know, high ESR, but that wasn't the point. The point was that I could make a coil sufficient to move a nail, even if only a quarter of an inch, with no more than $6 (USD) of parts (I know it's not a CG, but I call it one anyway). I succeeded, so now I am moving up to one that will cost me some money, but I dont want to spend a lot of money then find out that I need a bigger capacitor, transformer, etc.... So I looked at oz.net

, and got a few general ideas, but had a few lingering questions, ones that I didn't find answers to, so, I looked here. These are the questions I have:

IGBT or SCR? (The price difference doesn't matter to me)
Capacitors; Higher voltage, lower capacitance; or Higher capacitance, lower voltage?
Magnet Wire: Thick or Thin?
How do I calculate the number of Joules?
and Finally: Where do I find HV transformers (or turn relatively low voltage into higher voltage)

I have other questions, but I'll post them later, when I get to that stage in construction.

Sebastian

Sun Mar 05 2006, 07:37PM

Registered Member #233 Joined: Wed Feb 22 2006, 09:06AM
Location: Dortmund, Germany
Posts: 7

Energy stored in a cap in Joules: 1/2 * C * VÂ²

If you choose too thin magnet wire you will see that it gets too hot even after one shot. So you have to see what size caps you are gonna use and calculate from there.

Michael Witt

Sun Mar 05 2006, 08:28PM

Registered Member #282 Joined: Sat Mar 04 2006, 11:45PM
Location: North Chicago, US
Posts: 28

Thanks, that helps with two questions.

I just found my schematic utility, and I'll post a draft of the circuit I was thinking about soon.

the_anomaly

Sun Mar 05 2006, 08:55PM

Registered Member #19 Joined: Thu Feb 02 2006, 03:19PM
Location: Jacksonville, FL
Posts: 168

Well I would go with the scr for this one. IGBTs' are really for performance and are usually driven with a single pulse. For the caps look for 300volts or above and a few hundred uf. Thick wire is a must to handle the high current and lower the resistance.

Michael Witt

Sun Mar 05 2006, 10:30PM

Registered Member #282 Joined: Sat Mar 04 2006, 11:45PM
Location: North Chicago, US
Posts: 28

Ok, I've made a rough draft of my schematic, I know, the image quality is poor, but ym schematic prog doesn't like it when I try to print to an image.

The second attachment is a ZIP file of the image as an MDI.

Please tell me if you see any problems with this CG, I'll fix them.

Notes on the Schematic:
S1 and S2 select capacitor state; Pole 1 is default pole, and is an open circuit, used when finished charging, before discharge into coil; Pole 2 is for charging capacitor; Pole 3 is for slowly discharging capacitor, and should be used when capacitor has been charged, but will not be fired, and must be slowly discharged, CG should be stored in position 3; Position 4 is for quick discharge, in case of emergency, should not be used if possible, as it may cause damage to the capacitor.

CS1 and CS2 are for selecting which coil the charged capacitors will be discharged into, they are also how the energy is recovered, through a half wave bridge rectifier, and being charged back into capacitor.

S3 is for selecting which capacitor to monitor with the voltmeter. Set to position 3 before discharging caps. NOTE: the number of poles on the switch is one more than the number of capacitor banks used, the final pole being open, and the previous poles being conneced to one side of each of the banks.

L1-L6 are all part of a single stage, and layered on top of each other, they are not multiple stages. Yet.

Please note that I most definitely made mistakes, as I often forget polarity stuff, so feel free to correct me.
]1141597726_282_FT3201_cgdraft1.zip[/file]

Simon

Mon Mar 06 2006, 01:34AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

Whoa! That's pretty complicated for a "first" single-stage cg circuit!

As to design, I'd recommend getting the capacitors, something like 300V and whatever energy rating you feel like, and then designing the rest of the cg around them.

Bjørn

Mon Mar 06 2006, 01:49AM

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

Do as Simon says, simpler is better in this case. Make a simple reliable design and make an effort to measure your efficiency and try different things to improve it.

You can improve a lot on your first attempt without spending any money, a properly designed coil gun with a single photo capacitor is capable of sending a projectile accross a room at significant speed.

Michael Witt

Mon Mar 06 2006, 02:56AM

Registered Member #282 Joined: Sat Mar 04 2006, 11:45PM
Location: North Chicago, US
Posts: 28

Oh, I just remembered why I didn't like that idea

. I dont have a switch that can handle that kind of voltage, so I have just been touching the lead of the wire to the other post of the cap, which is where I think I am loosing the energy. Is this assumption correct? Is thee a quick fix for it? I always get HUGE sparks this way.

[Edit: Fixed double post]

...

Mon Mar 06 2006, 04:43AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

First look arround and find a cap in the range of 300-500v at a like 1000uf. That would give you about 100j, which with a 1% efficiency (a resonable goal) give 1j out-40m/s for a 1g projectile if I did my math right.

From there, you need to calculate the size of the coil, for which I recommend barry's coil gun simulator. (use the calculator for transistor if you use a IGBT or SCR if you use a SCR) Aim for current in the 800a range.

As to the switch, I would recommend a SCR for your first coil. I could sell you SCR's that will handle 800a for $1/scr (sk065k's). If you want to put some more effort into the driver (you will need a pulse generator instead of a switch), then an IGBT will increase your efficiency a little.

I would recommend magnet wire around 20awg or so.

I wouldn't recommend using a transformer to charge it, but use either a voltage tripler/quadrupler or a boost converter. If you go the tripler/quadrupler path, please use a pair of 12v transformers back to back to limit the power and isolate teh cap bank from ground.

Good luck!

Bjørn

Mon Mar 06 2006, 01:44PM

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

A simple switching method like you describe can be very lossy. It is also not very consistent and ruins much of the science in it. As described above, a SCR is cheap and very suitable for this kind of work.

Because the coil is inductive the current will not start out very high and then ramp up to maximum current. That makes it possible to use mechanical switches quite successfully at these energy levels and voltages because they can get a reasonable contact before maximum current is reached.

If you use a SCR or a mechanical switch then you must think about how long it takes to discharge your capacitor, if not you will most likely have some problems with "suckback". This happens if there is still significant current in the coil when the projectile exits. It has happened that the projectile has been ejected in the wrong direction on particularly bad designs.

1 2 3 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob