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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

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Kiteman

8 years ago

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

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steveastroukKiteman

Reply 8 years ago

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

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Kitemansteveastrouk

Reply 8 years ago

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?)

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KryptoniteKiteman

Reply 8 years ago

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

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KitemanKryptonite

Reply 8 years ago

I acquired a carrier-bag full yesterday afternoon.

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KryptoniteKiteman

Reply 8 years ago

You are well on your way, grasshopper!

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steveastroukKiteman

Reply 8 years ago

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......

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caitlinsdad

8 years ago

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

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Kitemancaitlinsdad

Reply 8 years ago

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

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karlpinturrKiteman

Reply 8 years ago

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.

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CameronSS

8 years ago

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.

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KitemanCameronSS

Reply 8 years ago

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

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kelseymh

8 years ago

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.

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Kitemankelseymh

Reply 8 years ago

"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.