Two questions / Electromagnetic Projectile Accelerators / Forums

Forums

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

« Previous topic | Next topic »

Two questions

Move Thread

LAN_403

lhl_henrylui

Thu Jan 01 2009, 02:15PM

Registered Member #1498 Joined: Thu May 22 2008, 07:08AM
Location: Hong Kong
Posts: 72

1.I paralled three 1000uF (3000uF) capacitors and charged them to 450V in my coilgun . I triggered the gun by connecting wires by hand .But I want to use a more convenience way and I have decided to use semiconductors .I think SCR is a good choice .Which SCR should I use ?Or any other components are suitable? (Impedance of the coil : 1.2Î©)

2.What is the relationship between the length of a wired coil and the strength of the magnetic field?(The length of the wire in the coil unchange)

blackgrunge

Thu Jan 01 2009, 03:40PM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
Location:
Posts: 55

It all depends on current, inductance and resistance. Here's some links and resources that'll give you all the answers in a easy to understand package. Also try an 2N5207 SCR its what I use and is quite sufficient. If you go onto sites like electronics goldmine, alltronics, and all electronics you will be able to find stub SCRs with the part numbers. Then type those part numbers into google and it'll find some data sheets on them. Take a look at the data sheet and find the value called maximum continuous forward current. To give you an idea of the pulses it can handle here's some estimations: 30A FCC (forward continuous current) can handle around 800-900 amps, 40A FCC can handle 1000-1100 amps, 50A FCC can handle 1200-1350 amps. Beyond that you need not be worried. Be sure when you look up the data sheet that you look at the maximum forward voltage rating and be sure its rated for at least 500v. The reason you try and choose a number above your operating voltage is that when capacitors are pulsed the voltage jumps above the initial voltage for a split second (1-2 milliseconds) and that could possible damage the SCR if its an older one.

I've posted a data sheet with my SCR, 2N5207, just to give you a reference point for any others you look at. The 2N5207 is rated for 1200v RMS and has a high FCC rating so with 450v @ 3000uf you shouldn't need anything else because I've been pulsing 450v @ 4500uf as a testing platform for my optics and its been just fine. The data sheet says 2N5204 series but the 2N5207 is part of that.

Here's the links for the second question I was talking about:

The main idea behind what your asking is the RLC Circuit. It means resistor, inductor and capacitor in series. The resistor is the wire and the resistance of the coil, the inductor is the coil, and the capacitor is well the capacitor. The larger the coil the more inductance (takes longer to discharge) but the stronger magnetic field. The smaller the coil the shorter the discharge time but the weaker the magnetic field because magnetic field strength is a function of turn density.

All of the links my be confusing but if your really interested in the science behind what your asking I'd be more than will to help you out with reference material and to answer some more of you questions.
]2n681.pdf[/file]

rp181

Thu Jan 01 2009, 03:40PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

1) A SCR is the most cost effective choice, although IGBTS would also work. Use this RLC simulator to see the peak current of you coil, and look for a SCR with proper voltage and ITSM of the peak.

2)More windings = more resistance, so long pulse, with weaker field. less windings is high current peak for brief strong field. The windings you want depends on alot of things, you should just try it out yourself.

Marauder709

Thu Jan 01 2009, 07:46PM

Registered Member #1895 Joined: Thu Jan 01 2009, 03:12AM
Location:
Posts: 22

1) An SCR is the best all around way although spark gaps are cheaper (you can make them yourself if you want). I get most of my components from various online distributors like Mouser and Newark.

2) The Magnetic feild strength at any point is an inverse square law. In your case, the more turns you add, the less you benefit from adding those turns.

Barry

Fri Jan 02 2009, 07:35AM

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

Marauder709 wrote ...

2) The Magnetic feild strength at any point is an inverse square law. In your case, the more turns you add, the less you benefit from adding those turns.

Sorry, but I don't understand this -- can you explain these two items fully or provide web links so I can read more about it?

Marauder709

Fri Jan 02 2009, 07:44AM

Registered Member #1895 Joined: Thu Jan 01 2009, 03:12AM
Location:
Posts: 22

Its based on the Law of Biot and Savar. If you want more info on that then simply google it. Those websites (and I think yours as well Barry) explain it much better than I ever could. The magnetic feild is constant (in theory) throughout the inside of a solenoid. If we were to choose a point P at one end of the solenoid, and keep the current constant then the contribution of each successive length of wire will contribute less and less to the dB at point P. From this comes my statement "In your case, the more turns you add, the less you benefit from adding those turns." Another, more technical, and perhaps more accurate way to say it is that the marginal benefit of each additional turn and/or length of wire is constantly decreasing. The inverse square comment was simply to pay homage to the fact that the contribution of the current dI to dB at point P varies as the inverse square of the distance. In retrospect, the comment was quite vague; sorry.

Barry

Sat Jan 03 2009, 01:07AM

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

lhl_henrylui wrote ...

2.What is the relationship between the length of a wired coil and the strength of the magnetic field? (The length of the wire in the coil unchange)

Thanks for the explanation, Marauder! Now it makes sense.

I think LHL is asking about the preferred coil geometry for a given length of wire. If I restate his question as I understand it, then it might come out as: "If all you have is, say, exactly 5 meters of wire then is it better to wind a long coil of a single layer, or a very short fat coil with many layers of a few turns, or something in between?" (Or have I misunderstood his question?)

To calculate field strength, the magnetizing force is N*I where 'N' is the turn density (turns/cm) and 'I' is current. So for greatest strength on the axis at the coil's midpoint, you'd want the highest turn density.

However, this is not optimum in coilguns for a few reasons. First the inductance is proportional to N^2 and also in part to the diameter^2. A short fat coil will quickly have too much inductance for high speed pulses.

Secondly the additional layers don't couple as well to the iron projectile since they're farther away. So a fat coil can be ineffective. My experience is that the best design tends to be a few layers (2-3) of thick magnet wire in a coil about the same length as the projectile.

Hope this helps answer more questions than it raises.
Barry
I'm confused. What exactly IS an electron???

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