Science Buddies: "Ask an Expert"

Dear Science Buddies;

We are stumped.

My daughter and I are making a mini-hand cranked generator to light a 2.5mA LED.

I have a rectangular/cube shaped Neodymium N42 magnet attached to a dowel with the N/S poles of the magnet perpendicular to the axis of the dowel (so it looks like a ‘ T’ sort of). The magnet measures 1” x ¾” x ½”, and has a pull strength of ~70lbs…it is one strong little magnet.

Then we have ¼# spool (~800ft) of 30AWG magnet wire still wound from the factory. We lightly sanded about 2 cm of the tips to remove the enamel….and have good continuity reading with a multi-meter. The internal diameter of teh spool is a little over an inch so teh magent fits nicely inside the spool.

We bought a red LED from radio shack with a max current rating on 2.5mA, and we connect this to the end of each wire, at time with and with out the multimeter as well.

We mount the dowel in a cordless drill. And rotate at various speeds inside the coil. The magnet fits very snuggly in side the hollow part of the spool. We tried the hand crank and got nothing….so we went for the sure thing …..the cordless…

The problem is that the LED does not light at all…and we tried another LED as well….and also as we spin the magnet, we have the multimeter trying to read some current. The readings on the multimeter (set to mico-amps) is very jumpy…and sometimes negative.

I’m stumped on what could be the issue….

We’ve tried:
Flipping the magnet to multiple orientations..
Rotating the magnet CW and CCW
SLOW, medium, and fast rotations.
Testing the coil for continuity and resistance…the coil is solid….

One thing I am not sure about…is I think my multimeter reads only DC current….could that be why the reading is all jumpy. It jumps around between -1.2mA and +1.6mA….but it is all over the place with out any connection to rotation speed or position of the magnets (my observation)
I thought this was a no brainer straight forward demonstration of how to create a current…..I’m stumped….
Any suggestions…. THANK YOU so much in advance for your time.
Tom and Chrissi Sedlack.

Hi,

It sounds like your minigenerator is producing AC. LEDs work on DC. If your LED works from a battery, then it is functional. You could try adding a full wave rectifier to your circuit to drive the LED off DC, or perhaps convert your generator to DC by adding commutator strips and pickups. See this link:

http://forum.allaboutcircuits.com/showt ... hp?t=16227

http://en.wikipedia.org/wiki/Brushed_DC_Electric_Motor

Good luck!

Barrett Tomlinson

Tom and Chrissi - If I understand what you have done, the bar magnet is spinning inside the spool of wire, basically in the same axis as the windings. I don't think this will produce any flux change that would induce power into the coil. If the magnet were at right angles, it would react differently. Another suggestion, use the powerful magnet to couple to an iron armature that has the coil around it, that would probably produce more power as well. As the other response stated, this type arrangement wil product AC power. I don't think that it has to be rectified in this experiment as the LED should light when the polarity is correct and be dark during the reverse cycle. Here is another project that might by adapted to your experiment.

http://www.creative-science.org.uk/gen1.html

Rick Marz

Is the spool plastic or metalic? If it is NOT plastic, you have a shielding problem as well.

If you visualize magnetic lines of flux coming out of the north poll of the magnet and going back to the south poll as a bunch of ovals with one of the long sides through the bar magnet, they make up a something that looks like an apple with the bar magnet as its core.

In order for current to be induced in a wire, the lines of flux have to pass through the wire at 90 degrees. Your setup was rotating the apple in the same direction as the coil turns. Try turning your drill so that your rod is along the side of the spool and the magnet rotates into part of the center of the coil you should get much better results.

Barrett is correct that your generator will produce alternating current and Rick is correct that an LED (light emitting diode) will produce light during half of the cycle.

Your DC MA meter isn't going to be any use without the diode in the circuit. Even with the diode in series, it won't measure the instantaneous current.