I just watched a video on Facebook by somebody who built a project to run an LED using a 9 volt DC motor scrounged from a battery operated toy. They simply wired the LED to the motor and then used a pulley system to spin the motor.

Here's a link to the video: Creating a generator from a DC motor.

Doesn't a typical DC motor like those used in toys act like an AC generator if you spin it? When you move a magnet past a coil of wire you get an AC pulse out of the coil. I would be surprised if a cheap DC toy motor contained a rectifier diode since it's designed to be a DC motor, not a DC generator.

So my expectation is that a typical DC motor would act like an AC generator.

Further, a 9 volt DC motor spun at a fairly high speed would probably emit around 9 volts AC with a fair amount of current behind it, so I would think you'd risk exceeding the reverse breakdown voltage on a small LED and burning it out.

I think the project in the video in question would need a rectifier diode (ideally a full-wave rectifier) and a current limiting resistor or it would risk blowing out the LED.

  • 3
    \$\begingroup\$ For a small motor, the winding resistance is probably enough to make an extra current limiting resistor unnecessary. As others have already said, the commutator is the rectifier. \$\endgroup\$
    – brhans
    Mar 23 '16 at 14:26
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    \$\begingroup\$ It's the same thing in either direction. You feed it DC and the shaft turns. So swap cause-and-effect and you should know that if you turn the shaft you get DC generated. \$\endgroup\$
    – JDługosz
    Mar 23 '16 at 18:14
  • \$\begingroup\$ Depends what you mean by "DC" and "AC". If you spin the motor in one direction, the current will only flow in one direction, so in that sense, it's "DC". But, it won't be a constant current: It will be a periodic waveform to which, if you want to properly understand it, you must analyze with AC circuit analysis techniques. \$\endgroup\$ Mar 24 '16 at 14:01
  • \$\begingroup\$ Yeah, the old-style DC motors contained a commutator and would generate something approximating DC if turned. In fact, you could take a toy with a DC motor driving the wheels, and the same battery powering a light, and force the toy forward faster than it wanted to go. The light would shine brighter, indicating that the motor was generating more "juice" than the battery. \$\endgroup\$
    – Hot Licks
    Mar 24 '16 at 15:31

I would be surprised if a cheap DC toy motor contained a rectifier diode since it's designed to be a DC motor, not a DC generator

A cheap DC motor of the type that has a permanent magnet stator uses brushes and a rotor commutator to continually reverse the current into the rotor coil thus the effect is like feeding AC into the coil: -

enter image description here

If you didn't do this the rotor would spin maybe up to half a full turn and stop. Then it would take too much current and maybe burn out.

When driven as a generator the commutator does indeed work as a rectifier to produce a DC output: -

enter image description here

Maybe you are thinking that a cheap motor uses slip rings. This type of motor requires AC and will produce AC: -

enter image description here

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    \$\begingroup\$ There is yet another "kind" called "brushless DC motor" which is pretty much your AC motor with some electronics to convert DC into suitable polyphase AC. Of course, this electronics works one way only so the entire combo cannot be used as a generator at all. Unless one disassembles it into a regular AC motor. \$\endgroup\$
    – Agent_L
    Mar 23 '16 at 16:03
  • \$\begingroup\$ True, but it's unlikely that a "cheap DC toy motor" would be of the brushless type. \$\endgroup\$
    – gbarry
    Mar 24 '16 at 5:09
  • \$\begingroup\$ @gbarry I think that commutator motors are used mostly because of their wonderful load-speed-torque characteristics. I'm sure the electronics in low-power "brushless" is actually cheaper than commutator and it's assembly. \$\endgroup\$
    – Agent_L
    Mar 24 '16 at 13:33
  • \$\begingroup\$ Hey guys -- does the simple commutator in these things, simply swap the +/- every 180 degrees, or? Cheers \$\endgroup\$
    – Fattie
    Mar 24 '16 at 16:14
  • \$\begingroup\$ @JoeBlow effectively yes. \$\endgroup\$
    – cortices
    Mar 28 '16 at 7:13

A DC motor will generate a DC voltage if you spin it.

That is to say, mostly a DC voltage. It will have skips and jumps in it due to the brushes and the gaps in the commutator.

The commutator is the thing that makes the output DC, by the way.

When you apply DC to the motor, it moves and rotates the commutator. The commutator changes which coils are connected which way to the DC in. This keeps the motor moving.

When you rotate the motor with an external force, the commutator is also rotated. The connection changes needed to keep the motor running on DC are also the same changes needed to make DC out of the AC you expected.

The voltage that the motor puts out has very little to do with the voltage it is designed to run on. With no load, the voltage can be very high. If you put any kind of load on it, the voltage will drop drastically.

Small DC motors don't really generate a lot of power when you spin them.

An LED will operate on AC. An LED is after all a diode (Light Emitting Diode) and so will rectify as needed.

A current limiting resistor probably isn't needed for most small toy motors used as generators.

I've always found the problem to be getting enough current out of the things rather than having too much.

My son has a small steam engine, and we've built generators for it from various motors to power LEDs. We've never managed to kill an LED with any of them.

  • \$\begingroup\$ How high a reverse voltage can a typical low power LED tolerate? \$\endgroup\$
    – Duncan C
    Mar 23 '16 at 15:02
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    \$\begingroup\$ It depends on the LED. I find various ratings from 5V to 6V minimum with typical values that can be up to 15V. \$\endgroup\$
    – JRE
    Mar 23 '16 at 15:11
  • \$\begingroup\$ Ok, so if an LED is designed for 2.2v forward voltage then it can tolerate reverse voltages nearly double that? \$\endgroup\$
    – Duncan C
    Mar 23 '16 at 15:22
  • \$\begingroup\$ Probably. Check the datasheet. If it doesn't say (or you don't have a datasheet because you bought a bunch of them surplus somewhere) then connect one backwards and see. \$\endgroup\$
    – JRE
    Mar 23 '16 at 15:25
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    \$\begingroup\$ A thing to notice is that while the output will be spiky, the way commutators work should never allow the polarity to change (unless the shaft changes direction). A capacitor on the generator and a resistor to the LED would effectively smooth the output. I would also consider putting a diode in series before the cap (assuming unipolar electrolytic), just in case somebody spun the motor in the wrong direction. Schottky diode could be used to minimise voltage drop. \$\endgroup\$
    – Sebi
    Mar 24 '16 at 1:13

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