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Electromagnetic induction questions. Answered


OK, this is another one of those questions I should know, but don't. 

Mainly because I have never done it before, I have a whim to build an electromagnetic launcher, something table-mounted with a range of around 3-5m (10-15 feet).  I'm not after launching nails through Coke cans.

I have been browsing the web, and the designs all seem to be much more complicated than is really necessary for toy.

The image below is a scan of my badly-drawn thoughts of how simple it should be.

  • Have I got that right?
  • I plan to use a bunch of capacitors harvested from dead electronics and connected in parallel.  Is there a recommended total capacitance for such a device?
  • If launching a ring, must the ring slide over the ferrous core, or would there be enough umph to launch a ring balanced just past the end of the core?






Discussions

(Oh, and where it says "Transformer + Rectifier", the final product will probably be "wall wart".)

Recommended capacitance is lots.

You really HAVE to couple the core to the ring - the reluctance of the magnetic circuit will be too high to work otherwise.

Steve

OK.

I'm currently harvesting 1000 and 1800uF electrolytic caps from a heap of dead PC boards. Do you think I'll have to pay actual money for some bigger ones?

(Go on, pick a number - how many Farads should I be aiming for?)

The most common method here is to harvest some caps from disposable flash camera circuit boards. 'tis what I did.

I acquired a carrier-bag full yesterday afternoon.

You are well on your way, grasshopper!

I suppose you could take a stab at it from conservation of energy considerations. Assign a speed to the final ring, which has to aquire the energy from the capacitors......

Coil guns are not toys. You'll put your eye out!

I want to fire rings or discs from point A to point B, not pierce things.

Given enough oomph (to overcome/work with the many factors involved), the bluntest of objects can pierce anything...

Not that I'm suggesting a desktop coil gun is likely to do anything drastic, if properly built and supervised - and I'm sure you know your class(es) well enough that having such a thing won't give cause for various authorities to become involved.

Is this going on your desk at school next to the catapults for disciplining unruly students?

No, I don't actually know anything about coilguns, but I will say that I've seen smallish ones with ten flash capacitors in parallel. The only dead flash unit I have handy has an unlabeled cap, but I think they're usually around 2200uF.

It will probably spend some time at school, but it's ultimate fate (if it works) will be different.

Check out the latest issue (24) of MAKE. You've just re-designed the aluminum levitator (page 100), which the author warns you (multiple times :-) not to crank up the voltage or you put an eye out.

"For example, do not try this project with 240V AC. Even if the magnet wire’s insulation holds up, you can start a fire or cause serious injury." (p107)

Bit of a bugger that - UK mains is 240VAC.

I've seen a lot of videos of that kind of launcher over the last day or so, and the vertical height they reach is rarely over a metre unless they cool the launched ring with liquid nitrogen.

So, I think I'm back to using a capacitor bank, with the added interest of varying charging-times to alter the potential range.