Question on first "complete" coil gun / Electromagnetic Projectile Accelerators / Forums

Forums

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

« Previous topic | Next topic »

Question on first "complete" coil gun

1 2 next

Move Thread

LAN_403

equilibrium

Mon Jul 30 2007, 10:04PM

Registered Member #932 Joined: Mon Jul 30 2007, 09:30PM
Location:
Posts: 9

I am currently working on a coil gun thats going to be running off a rechargeable AA battery. The barrel is a little less than 1mm in diameter and it is going to be using a sewing needle as a projectile. The coil is about 1 inch and about 3 or 4 coils thick. My plans are to in case the gun in an eye glass case or perhaps an altoids can. The test firing was great however it didn't leave me satisfied.

I have a couple capacitors to choose from:

A 650uf 200V cap.
A 1600uf 16v cap.
An unmarked cap that came from disposable camera.

My questions is, which capacitor would give me the power I'm looking for?
Please correct me if I'm wrong but the more micro farads the longer it will take to disperse the energy? I am quite confused by the capacitor ratings so hopefully you guys can help me out

.

Also I took a look at the thread about plasma guns. Would it be a bad idea to try an propel the plasma with an additional smaller coil?

Dave Marshall

Mon Jul 30 2007, 11:02PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Most likely, the photo flash cap is 330v. Capacitance could range from 180uF-600uF, but most often its on the low end of that scale, 180uF-220uF is a safe bet.

Discharge time is a function of capacitance and system impedance. Time=5*R*C where R is impedance in ohms (including cap ESR) and C is capacitance in Farads.

I suspect you aren't going to see a dramatic increase in velocity, even with a better cap. Your projectile is probably very nearly, if not already saturated. Your best option would probably be 2-5 (depending on how much room you have) caps from disposable cameras, and a projectile made of some small diameter cold rolled steel. You can get 3 feet of .125" cold rolled steel for a couple dollars at practically any hardware store.

You can probably even get some .0625" at some of the better stocked stores. That would improve your efficiency a bit and get you a better velocity without increasing the projectile mass too much.

Dave

equilibrium

Tue Jul 31 2007, 12:00AM

Registered Member #932 Joined: Mon Jul 30 2007, 09:30PM
Location:
Posts: 9

Thank you Dave for your help.

One more question:

Would it be ideal to link my capacitors like this?

Capsgp1

Barry

Tue Jul 31 2007, 12:51AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

equilibrium wrote ...

A 650uf 200V cap.
A 1600uf 16v cap.
An unmarked cap that came from disposable camera.

The potential energy depends on both capacitance and voltage:
PE = 0.5 C V^2

so your three capacitors work out to be:

(0.5)(0.000 650)(200^2) = 13 joules
(0.5)(0.001 600)(16^2) = 0.2048 joules
(0.5)(0.000 180)(300^2) = 8.1 joules (minimum)

So the best capacitor is most likely the first one: 650uF at 200v.

Regardless of which capacitor is used, you need to match the discharge time to the projectile. This usually ends up in the range of 1 to 5 milliseconds. The discharge time is roughly sqrt(LC) times pi, for the first half of a sinusoid.

So plug in the numbers and see ... When I assume t=5msec and C=650uF, then I get L=3.89mH, but you better check my math to confirm.

Now "all you have to do" is build a 3.89mH coil, and measure the speed while gradually removing turns to find the best performance.

Keep us posted! ~~Barry~~
PS - Yes, your diagram for connecting capacitors in parallel is correct.

GimpyJoe

Tue Jul 31 2007, 11:19PM

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

How are you charging your capacitors? You said one of your capacitor choices was 16v which makes me think you're using a very low voltage.

equilibrium

Wed Aug 01 2007, 01:10AM

Registered Member #932 Joined: Mon Jul 30 2007, 09:30PM
Location:
Posts: 9

My plans, which are looking very promising at the moment, were to charge the capacitors with a rechargeable AA battery. I am most likely to use the PC board from a disposable camera because its pretty small. So to answer your question, no I don't plan on working with very high voltages because I want this CG to be portable as well as safe (to and extent).

And thank you Barry, you helped me a lot there. The CG is 80% finished all I need to do is fit it inside the case.

Simon

Thu Aug 02 2007, 12:29AM

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

Remember that rechargeable batteries will have a lower voltage than most non-rechargeables (1.2V compared to 1.5V). The disposable camera circuits are built on the cheap and work to the limit so this will almost certainly affect performance. (Or are you replacing the board parts, too?)

The overwhelming majority of caps from disposable cameras I've seen have been between 80 - 120uF. 180uF sounds more like a small proper camera's flash. The small external flash units I've gutted have had caps on the order of 300 to 800 uF.

I really hope you're not putting all these caps in parallel. The 16V will pop.

equilibrium

Thu Aug 02 2007, 01:03AM

Registered Member #932 Joined: Mon Jul 30 2007, 09:30PM
Location:
Posts: 9

No (but thanks I wasn't thinking of that), I am trying to use all the same caps. Preferably the flash caps because I can find space for them easily. I'm really not concerned with voltage because this is my first real project with a CG and I'm just trying to get a working piece together. I'm working on a design that will allow me to switch coils when I have a projectile thats bigger in diameter. Along with that, a stand for the charger.

Ragnarok

Thu Aug 02 2007, 07:58PM

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

Dave Marshall wrote ...

Most likely, the photo flash cap is 330v. Capacitance could range from 180uF-600uF, but most often its on the low end of that scale, 180uF-220uF is a safe bet.

I've never got that lucky! All my photo flash caps are between 80uf and 120 uf, damn it! (Curiously, 100 uf less for both than you quoted)

I've found that because many rechargables have very low internal resistance, they have an incredible ability to burn out photo flash circuits with high currents.

Danielle

Thu Aug 02 2007, 08:17PM

Registered Member #632 Joined: Mon Apr 09 2007, 01:09AM
Location:
Posts: 267

you need to go buy a cap bank like mine 4 450V 2400uf for a 900V 2400uf capacitor bank and use huge SCRs to trigger it.

1 2 next

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