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Disclaimer: Hi, I was in the neighborhood (StackOverFlow) and just noticed this cool site. I have had this general wondering for a while now and I am not sure if my question is constructive enough for this site (not from EE background). But I am genuinely interested to learn/know if there's any practical use of the concept. So, please do keep an open mind and any explanation would be greatly appreciated.

Concept: Let's say I have a DC motor (12V, 0.5A) and I managed to keep it spinning by "strategically" attaching some neodymium magnets around it (my intention is not to start a war regarding whether or not it is possible, but for the sake of practical usage, let's assume it is possible). The output I am getting is 7-9V, unknown amperage.

Question: Is it possible to convert this variable output to a stable 5.0 volt (standard USB output) with 0.5-1.0 amp to charge cellphone and if so, could anyone please provide some resources / explanation / diagram?

JYelton
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J A
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    You can either use a buck convertor or linear voltage regulator to step the voltage down to a stable 5v. Linear voltage regulators are generally quite inefficient but are arguably easier to implement. – tangrs Oct 12 '14 at 01:05
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    The question is very unclear. Are you asking, "if I attached some magnets to a motor, could those magnets generate AC power?" Yes, they could, that's one way to convert from one voltage to another, or a way t make a back-up generator. It is essentially an electric motor driving a generator. If you are asking, "can a 7-9V AC voltage be converted to a 5V DC?", then yes. There are lots of ways, e.g. diode rectifier, capacitors and voltage regulator. If you are asking, "will it provide 0.5-1A?", 1A unlikely, 0.5A possible. Input: 12V*0.5A = 6W – gbulmer Oct 12 '14 at 01:06
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    Also, you should be aware that the motor voltage output will likely to change depending on how many amps you draw. – tangrs Oct 12 '14 at 01:07
  • possible duplicate of [Reducing 12V to 5V](http://electronics.stackexchange.com/questions/109039/reducing-12v-to-5v) – Ignacio Vazquez-Abrams Oct 12 '14 at 01:11
  • Do you perhaps mean "permanent magnet" instead of "perpetual magnet" in the title of the question? – JYelton Oct 13 '14 at 17:15
  • @JYelton: No, I meant perpetual {magnet-motor}. It is too-good-to-be-true concept of spinning a motor without feeding any electricity! – J A Oct 13 '14 at 17:18
  • Ok, it seemed likely with the body of the question but I just wanted to check. :) – JYelton Oct 13 '14 at 17:28

2 Answers2

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Assuming we can completely ignore the laws of physics, especially as regards conservation of energy, then yes it is possible*.

The device you are looking for is called a "Voltage Regulator", and it takes in a variable voltage and outputs a stable, lower, voltage.

They come in two basic flavours:

  1. Linear Regulator. These are like automatic variable resistors. They constantly adjust how much voltage is dropped across themselves in order to keep the output voltage stable. They are quite inefficient, and tend to get rather hot with higher currents.
  2. Switching Regulator. These chop the incoming voltage up into small chunks by rapidly switching it on and off. The ratio of on to off gives the output voltage. A feedback loop monitors that output voltage and adjusts the ratio to keep it stable. A low-pass filter then smooths that on-off waveform into a single constant voltage again. These are much more efficient because they don't waste the excess voltage as heat, they just turn it off instead.

Now the electronics are sorted out, why not head over to Physics.SE and discuss perpetual motion and why it doesn't work in the real world. :)

* for certain values of "possible"

Majenko
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  • Many thanks for the detailed explanation. The concept is too tempting (https://www.youtube.com/watch?v=jiAhiu6UqXQ) to dismiss without trying. Will check with Physics gurus. – J A Oct 12 '14 at 01:17
  • I knew someone that had one of those lightbulbs. Touch the side and bottom contacts and they light up. – Majenko Oct 12 '14 at 01:23
  • Lightbulb is not really my concern. The fan seemed to spin legitimately. – J A Oct 12 '14 at 01:24
  • Also I notice we never get to see the underside of the fan with the coin cells and reed switch. – Majenko Oct 12 '14 at 01:25
  • Yeah, I read some of the comments. Quite a few managed to do it (according to them). Anyway, it'll be a waste of time to argue over the legitimacy of the video. I'll update the comment if I can make it spinning efficiently for 24 hours. Thanks again. – J A Oct 12 '14 at 01:28
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    @JA Hi. I design electric motors for a living. Permanent magnet motors don't work. Every motor manufacturer in the world would be making them if they did. See here for one example of how to trick people into thinking they work: http://youtu.be/_I3Ly5mBhT4 – Eric Oct 12 '14 at 02:57
  • @JA, how did your experiment turn out? – Eric Oct 16 '14 at 18:34
  • @Brad, Well, could use some sarcasm at this point ;) Among many other limitations, precision magnet placement is most demanding task. In the hunt for some 3d printing services nearby. – J A Oct 16 '14 at 18:41
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Given that you want at least 0.5A, from "unknown current" the question can't be answered as is. Connect a resistor of about 15 ohms across the motor and spin it.

If you can still get 7-9V you will be putting about 0.5A through the resistor, so the answer is yes. Note that if so, the resistor will need to be rated at several watts. A 2W resistor would be OK for a short test (it'll get hot!), but a 5W one would be preferred for continuous operation.