Small Motor Power Harvest

You've lost me here. You will need to drive the motor somehow, if you expect it to produce an output. The losses will make this approach a non-starter.

"The motor generates about 100mV when rotating.." What makes the motor rotate? Another motor? A propeller?

Use the power required to drive the motor that drives your computer motor to charge the capacitor and throw the computer motor on the shelf.

Or, have I missed something?

What makes the motor rotate?

My fingers. Right now.

However, I plan to attach it to a mechanical propeller and I want to get the power to light some LED or even charge a capacitor.

Thanks, the propeller was the part I was missing. Same problem here. The fan you use to make the prop turn the motor will consume more energy than you can produce.

But, have fun. You may not need any circuit, just hook 'em up and see what happens.

Have you tried an internet search for "stepper motor as generator"?

You are not going to be able to get much voltage or power out of one of these. Yes you can add a step-up DC-to-DC converter or circuit to increase the generated voltage to a more usable level, but for that expense you are better off just buying a larger second-hand DC motor which can produce a much higher voltage that can then be directly used and/or stored using a simple diode rectifier and capacitor(s).

What are you planning on using the harvested energy for?

The only use for a rectifier on a d.c. motor (generator) would be to block discharge of the capacitor (if the motor speed drops, the emf will reduce and the generator would want to run as a motor).

This, of course, presupposes that you have a very large capacitor...

You would get more from a cooling fan. Mine has LED's on it and when disconnected from the computer, I can blow on it and light up the LED's.

If you can drive an LED, the you must have much more than 100mV output.. at least 1.2V to drive an infrared led..

I measured my current over a 10 ohm resistor and I got 20uA, for a total power of about 2uW, which is too small to do anything,

however, if I can step up the 100mV efficiently, that would be different

Two years ago for $13.45 I bought a batteryles flashlight. It has a USB port and hand crank.Turn the crank nnd 3 white LEDon the front glow bright enough to be usable as a flash light.

Plug in a cell phone and it gets recharged by me turning the cranlk.

I m really bummed because last week I saw almost the same thing for$4.95 at a dollar store.

Anybody else want to join the "re-invent the wheel" club. Gee $4.95 wouldn't even pay for the electric power for my soldering station.

That's about the usual price drop over a two year period.....always wait two years and you will save money!!

Power = i²R = Well, not much and stepping up to a higher voltage won't give you more power.

http://www.maxim-ic.com/app-notes/index.mvp/id/2031 will get you started on the circuit.

As FFEJ states below, you're not going to get more power by increasing the voltage. If you get 2uW now, that's what you will get at a higher voltage (actually, less, since no DC-DC conversion is going to be 100% efficient). You need to study Ohms law a bit.

Find a small permanent magnet generator that is designed to be used on a bicycle, that will make a much better "generator" than your surplus motor.

Tom D.

There are several issues here.

First, you said you got this motor out of a computer disk drive. You didn't say whether that was a floppy or a hard drive. I've never seen a DC motor in a hard drive - only variations of stepper motors. even most floppy drives used mostly steppers, although I do have at least one spin motor from a 3.5" floppy that is indeed a DC motor. For those floppies that had eject motors (mostly from Macs) the eject motors were DC. Most DC motors have only two wires, although a few have one or two more for temperature sensors. A previous post gave info on steppers.

Second, you indicated you got about 100mV, but then say you got 20µA across 10Ω. Perhaps the 100mV was with no load other than the voltmeter. I=E/R, so 100mV/10Ω gives 10mA of current, not 20µA. If you measured the current by inserting a microammeter in series with the 10Ω, then the resistance of the meter must be taken into account. In fact I just measured the DC resistance of a 100µA DC meter at 1.5kΩ, and the 200µA range of a cheap DMM at about half that. In either of these two cases, the majority of your power would have been lost in the meter, not in the 10Ω resistor.

Third, you need to study power transfer. The 10Ω load is probably too low a resistance for this job. Red LEDs commonly have limiting resistors of one to several hundred Ohms when operated from 5V or so.

Fourth, the output voltage of DC motors used as generators is often directly proportional to the speed of rotation. For testing, try mounting the shaft of the motor in the chuck of a portable drill or drill press to get a consistent speed.

Do keep on experimenting!

Great points. I got the motor from a cd drive. I dont know if it is a stepper motor or and 100mV was when no load was connected, ie. just hooked up to the voltmeter.

I understand ohms law very well, and ppl jumped on my comment without reading the entire post as usual. If you see my OP, i talked about charging a capacitor... and from there, trying to get enough power to light an led..

My apologies for questioning your knowledge of Ohm's Law, but when you said;

I measured my current over a 10 ohm resistor and I got 20uA, for a total power of about 2uW, which is too small to do anything,

however, if I can step up the 100mV efficiently, that would be different

I thought you were inferring that you would get more than 2uW of power by stepping up the voltage. I should know better than to "assume"

Tom D.

Economist, is this just for fun and education or is there a practical need associated with this project? At first glance it seems like a lot of effort to make an LED light up. Ther are easier ways.

Generally it takes several milliamps pf power to illuminate any LED yet you seem to be talking about microwatts or about 1000 times less energy.

Now that you mention it, the DC motor I was trying was indeed the spin motor from a CD drive. If it has only two leads, it is probably a DC motor (or a DC brushless motor, which is much more complicated).To be certain, turn the shaft the opposite direction; if it is a DC motor, rotating opposite directions will produce opposite polarities of output.

If the hub is still in place, it is difficult to spin the motor very fast. A knife under each side will let you pry the hub off, so you can mount the shaft in the drill chuck. With the hub in place, I could only produce a couple hundred milliVolts spinning with my fingers. Once I mounted it in the drill chuck, I could easily get a Volt and a half. This particular motor has a female connector with the same spacing as a standard LED, so I just plugged an LED in the socket; once the polarity is correct, I could make it light the LED without difficulty.

Note that it's not like an incandescent lamp that will glow dimly with low voltage. The LED won't glow at all 'till it reaches the threshold voltage, then it will suddenly come on. Without a limiting resistance, only a little additional voltage above the threshold would blow the LED, but the motor winding has some resistance (about 30Ω on the one I'm using) so there is little danger of that unless you spin it really fast.

That may not be the best picture but yes, that i what it looks like.. I was able to get opposite polarities, with opposite rotations and I was getting 100mv with my fingers..
I am interested in finding way to power a small chip without using any batteries or other power supply,, the EM reciever seems like a nice idea, but in this case, I have shaft that rotates at about 90 rpm so I was thinking of using this to save the power somehow and show it can work..
If I can prove the concept, maybe I can look into buying a motor suited for this purpose..

As a method of storage, look into "super capacitors", these have low series resistance, and very low leakage current. They are being used more and more as back-up power sources for memory, microcontrollers, etc. in place of the old lithium batteries. Larger versions are being used for regenerative power capture.

Tom D.

OK. That looks like an inject/eject motor. I just tried one of those, and it puts out way less voltage, just over a quarter of a volt, even with the drill.

The one I was using previously is the spin motor from an older (over an inch thick) CD unit. That motor is about an inch in diameter and half an inch thick. Especially if you only have 90 RPM, you should look for a larger diameter motor. Larger diameter means higher speed of the windings relative to the magnets for a given RPM value, and therefore more voltage.

The concept is valid, as long as you have a shaft already rotating for some other purpose, the power you extract from it is a fraction of the power available, and the source of energy rotating that shaft is free, like wind. If that shaft was being rotated by an electric or other motor of some kind, whatever power you extract (plus losses) will be reflected as increased power required by that motor.

Has anyone heard of the (new?) antenna based energy scavengers. Using a diode/capacitor bridge work (voltage rectifier) one can take advantage of parts of the electromagnetic waves proliferating in our atmosphere today - filtered energy suficient for many low powered electronic devices. NO moving parts & free for the scavenging. Google 'antennas scavenge energy' or 'antennas create energy'. A recent article from a reputable college describes some of the applications that could take advantage of this technology. Maybe even enough current to turn that fan? Carlos I apologize for not getting the exact website(s) - I read these at work & can't afford the time for it. Good luck.

Years ago (~1980?) I won a ham radio constructional contest with an 'off air' powered crystal set. The antenna collected enough power from the BBC World Service transmitter at Crowborough to operate a single transistor amplifier and a moving coil 'S' meter.

I was rather pleased with that...

What was your prize?

The only thing I won was the admiration of the judge(s).

Story of my life, really...

Just about better than nothing I suppose, sad....