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My students are making a shake charger using a coil.....? Answered

and a neodymium magnet.  Can I overlap the coil??  Meaning if I start at the left of the tube and coil to the right (clockwise), when I get to the end can I go right to left?  A few of my students are done already and their chargers aren't even lighting an LED.  Can anyone guess why?  Thank you so much!



The more turns the merrier. Its hard to wind too many turns for this by hand.
Yes, you can overwind the coil ! In fact, you'll pretty much HAVE to do.
Are the coils a GOOD sliding fit on the magnet ?


You need to get as much magnetic flux to link your circuit as possible, so you need to have a "magnetic" circuit with as little magnetic "resistance" (the correct word is relucatance actually) as possible

Big hole - small magnet - low output.

If it's not lighting an led, then it either has a short circuit, or simply not enough turns to generate enough voltage to overcome the voltage drop in the bridge rectifier.

And yes, you can and must layer the turns. You want them as smooth and flat as possible to fit as many coils as possible in the available area. Each turn will generate its own voltage from the magnetic field (left hand rule), so when you put them all end to end they will generate a current. Bigger wires, and bigger magnet will mean more current, more turns, and faster magnet will mean more voltage.

Do I need a diode in series with it? Is the flux canceling itself out by producing AC?

No ! The diode will drop ~0.6 V - you can't afford to lose volts. Just settle for it developing an AC output. You can get clever, and put TWO leds in the project - in "anti- parallel" - ie, put the cathode of 1 to the anode of the other, and vice versa - one will light on the up stroke, one on the down......


true - but he still needs to make a phone charger, not just flashlight.

If she has to then, I'd go for schottky or germanium diodes, and since I have a choice, I'd wind it as a centre tapped coil, so I can use only one diode drop in each leg.

Can you explain what that means? This is the first time I'm doing this and I'm not really technically advanced as you can see. Thanks!

"Normal"diodes - ie the kind you'd get from Radio Shack are made of silicon.
When you use silicon diodes, you ALWAYS get a voltage drop THROUGH the diode which we say is "0.6V" you can argue a bit, and say 0.65, 0.7, but, fundamentally agree with me at 0.6v.

So whatever current you are generating passes through the diode, and if you manage to generate 2 v in your winding, then you will ONLY see 1.3 after the rectifier. There is NOTHING you can do about that, if you have to use Silicon.

HOWEVER, there ARE other diodes. A "Schottky" diode will drop around 0.4V, a germanium diode, around 0.3V.

Doing that halves the energy wasted in the diode.


This is a centre tapped coil, attached to only TWO diodes, but giving full wave rectification.