Question about self-charging flashlight
lardom
Posts: 1,659
I have a novelty flashlight that recharges by shaking a short dowel shaped magnet back and forth through a coil. The pcb has an electrolytic cap and some diodes. It's very inefficient but the idea behind it is intriguing. I initially thought that the circuit would not add a load as it charged but then I thought the energized coil would have to exert an opposing force on the permanent magnet.
I guess in this sense any material that can conduct electricity has magnetic properties. Is it correct to say that ferrous metals are unique in their ability to be magnetic in the absence of an electric current?
I guess in this sense any material that can conduct electricity has magnetic properties. Is it correct to say that ferrous metals are unique in their ability to be magnetic in the absence of an electric current?
Comments
I believe all ferrous metals have magnetic characteristics - aka "permeability". However, that doesn't mean they are all magnetized.
Think about the difference between a "permanent" and "non-permanent" magnet.
There is probably some amount of back-EMF that will oppose the movement of the shaking magnet, but it'll be overcome by your manly strength.
Ferromagnetism is the general term given to a material that can exhibit magnetism without electric current running through it. However, check out this from wikipedia:
"Ferromagnetism is a property not just of the chemical make-up of a material, but of its crystalline structure and microscopic organization. There are ferromagnetic metal alloys whose constituents are not themselves ferromagnetic, called Heusler alloys, named after Fritz Heusler. Conversely there are non-magnetic alloys, such as types of stainless steel, composed almost exclusively of ferromagnetic metals."
http://en.wikipedia.org/wiki/Ferromagnetism
Also consider if you take a sheet of aluminium and run a strong magnet over it, you will induce magnetic fields in the aluminium and cause the sheet to move with the magnet, though somewhat out of phase.
Oh - forgot one thing
THAT WAS TOO COOL!!!!!!
Any way if you crack open an old fashioned "mechanical" speedometer you will see that that is how it works. A spinning magent turns an ally disk against spring. Faster the car, faster the magnet spins and the more the disk is turned.
And consider this. if the tube were superconduting the magnet could never fall. As there are no resitive losses in the eddy currents the reverse field they create would exactly ballance that of the magnet.
...but isn't movement required for the eddy currents to be formed?
BTW - are you 6V or 12V?
Well, assuming you have just moved the maget up close to the semiconductor, that motion created the eddy currents, that made the field that resist the falling of the magnet. When you are not pushing anymore the eddy currents only have to balance the force of gravity on the mass of the magnet. The thing reaches eqilibrium and can fall no more. There are no resistive losses to the eddy currents in the superconductor so the magnet is stuck there for good.
Or that is about how I convince myself a magnet levitating over a superconductor works. See various youtube videos, for example:
http://www.youtube.com/watch?v=CpH_TD_SVTc
http://www.youtube.com/watch?v=pO2eDJBr50E
Although the second video stumps me because the magnet is resting on a warm super conductor and it then jumps up when the superconductor is cooled!
How so, where did the energy come from to push it up?
Sometimes I'm 4V like this big bugger that I have and am I'm still hoping to light up one day:
http://www.tubecollector.org/vcr97.htm
More pactically (and often) I'm 6.3V
http://www.tubecollector.org/ecc83.htm
Superconductor do more than just conduct really well. The repel magnetic fields. Or so I "learned" (memorized) in my modern physics class.
It's not clear to me what you're trying to do, but part of what you said sounds a little bit like how BLDC controllers work:
From Wikipedia:
"...Some designs use Hall effect sensors or a rotary encoder to directly measure the rotor's position. Others measure the back EMF in the undriven coils to infer the rotor position, eliminating the need for separate Hall effect sensors, and therefore are often called sensorless controllers...."
http://en.wikipedia.org/wiki/Brushless_DC_electric_motor
Hope that helps???
That would work fine. Take a look at the way brushless DC motors work http://en.wikipedia.org/wiki/Brushless_DC_electric_motor
Not a great analogy. I'm trying to go from head knowledge to hands-on.
...ahhh - things of beauty, they are!
Very cool...literally!
And so I'm led to believe as well. So let's say in simple terms the super conductor is expelling the magnetic field and hence pushing the magnet up.
But my question still stands.
To lift the magnet requires energy, E = mgh, (mass of magnet * height lifted * gravitational constant)
But we have just sucked all the energy out of the plate my coolin it to make it a super conductor.
So where did the energy required to lift the magnet come from?