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I'm thinking of building a simple steam generator (the design is irrelevant here). I'd design it so that it spins a propeller that would be connected to a generic DC motor like this one. The generator would spin the motor, making electricity.

Nothing too special, just a DC motor. This question indicate that so long as it has permanent magnets, I can just spin the metal rod, and power will come out of the two wires. However, I don't want just any amount of power. For what I'm trying to do (light a generic LED), I'd like a specific amount of power. If I'm going to connect the LED with a resistor, I'd like 5-ish volts out, and I don't need many amps. Or, I can just connect the LED without a resistor, so long as I only give it three-ish volts.

Chances are, this motor won't output exactly 3 volts. I don't know how many amps. So, given an unknown amount of DC voltage (say, 1 V to 24 V is my guess), and an unknown number of amps, how do I convert that to a specific output voltage, probably either 3 V or 5 V? I don't care about the output amperage, so long as it can light up my LED. I'm fine with sacrificing amperage if it means I get the voltage I want.

I've seen buck boost converters, but I find that most of them require at least 8 V to start with, often more, and I don't think I'll be able to produce that much constantly. I'm aware that using math I could simply use a voltage divider, but that doesn't work if the voltage from the motor fluctuates, which it will.

Phrased differently, given an unknown amount of input voltage and current (but within a reasonable range, say 1 V to 24 V), how can I make a circuit that will output a specific voltage, assuming I don't care about amps (so long as it's not, like, no amps). I'm open to using an existing component, like a buck boost converter, or a DIY circuit, more like a voltage divider. But it needs to work even when the input voltage fluctuates.

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  • \$\begingroup\$ Start with conservation of energy conversion then try to understand impedance of source vs load before any design ideas. What energy source to power a 0.1 W to 1W LED and how to convert efficiently \$\endgroup\$ Jan 24 at 4:38
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    \$\begingroup\$ First get the motor working as a generator, then measure the voltage and current it can produce. Then you can work out how to control the current to your LED. Please note that LEDs should be fed a controlled current - the voltage across an operating LED is determined primarily by the LED itself, but does vary somewhat with current. \$\endgroup\$ Jan 24 at 5:05
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    \$\begingroup\$ cocomac, The datasheet (if one can be found) of a specific motor will disclose a lot of important information. If you are buying, then you probably want a DC permanent magnet brush motor that is specified to operate at about 2-3 times the power you expect to get out of it. You don't need much. If you have a junk box of motors, you can experiment. Use your fingers and just spin the motor freely (use meter to check + and -.) Then jumper the motor leads, spin by hand again. Note difference? Add an LED, try again. I'd probably play around to match the motor+RPM to the LED. No need for a circuit. \$\endgroup\$
    – jonk
    Jan 24 at 6:26
  • \$\begingroup\$ I'll try just experimenting with a multimeter (I have no idea what the motor is, I just found it in a drawer, but it's a small DC motor). Ideally I'd like to be able to power something like an Arduino Micro, so having consistent voltage output would be nice. \$\endgroup\$
    – cocomac
    Jan 24 at 6:28
  • \$\begingroup\$ @cocomac You can also consider just re-writing your motor. I haven't reviewed the following link, but it seems promising: re-wire toy DC motor. Also, for at least seeing what a datasheet curve might look like, see this article. (I actually own a bunch of the Digilent gearbox motors mentioned in that article.) You won't see it shown directly on that chart, but enough information is there to find the output voltage for a given rpm. You just have to crunch numbers. \$\endgroup\$
    – jonk
    Jan 24 at 6:31

3 Answers 3

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For just one LED, you can use a current limiter circuit built from 2 transistors. Here's an example (simulate it here):

enter image description here

More here: Controlling High Current LEDs with an ATmega328

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  • \$\begingroup\$ That seems good for an LED, and I might try it. This might be a separate question... but what if I wanted to power something that wanted a specific voltage (say 5v), but didn't care about current, so long as it was above some threshold. An Arduino Micro might be one such device. Would a device like this one work? \$\endgroup\$
    – cocomac
    Jan 24 at 6:26
  • \$\begingroup\$ @cocomac There is a design process you can follow with DC brushed motors and their datasheet curves, if you are being serious about using one as a source of power and voltage. There are a number of details to consider then. But the process is boilerplate and just works well. Someone would need to write a walk-through for that, though. (Maybe one is already here... don't know.) \$\endgroup\$
    – jonk
    Jan 24 at 6:28
  • \$\begingroup\$ @jonk I'm more trying to learn and experiment than create something super specific. If I just took the DC motor, spun it, and measured the voltage output, would I be likely to cause major issues (like setting it on fire), or is it generally safe to experiment with low-voltage DC motors? \$\endgroup\$
    – cocomac
    Jan 24 at 6:33
  • \$\begingroup\$ @cocomac It is quite safe to play around with toy DC motors. Absolutely do it. You can run them with a variable DC supply or you can hook them up to a voltmeter and give them a spin. If you have a Dremel and can connect the two, you can just drive them at a fixed rate and see. I don't know what you expect to use in causing the motor to rotate, but if you know what you want to try... then try it! Strikes me a bit funny, you worrying about a fire. Not because it can't happen. But because I was experimenting, making rocket fuel in a double boiler of sulfuric acid at 16 yrs age. What? Me worry? \$\endgroup\$
    – jonk
    Jan 24 at 6:36
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Your biggest concern when using a small motor as a generator won't be getting too much voltage or current. Your problem will be getting enough voltage and current.

My son has a small steam engine.

enter image description here

Not that model, but one like it. It produces more power than your home made steam turbine is likely to.

We built a generator from a small motor and used it to power an LED. At full speed, the motor produced enough current to light a high efficiency white LED. It wasn't terribly bright. The steam engine (even with the best pulley ratio we could cobble together) couldn't turn the generator fast enough to reach the rated voltage of the motor.

I'd suggest you buy a motor from a good supplier rather than salvaging one. If you buy a motor you can get a datasheet that tells you what RPM to expect when the motor is driven at its rated voltage.

When operated as a generator, a motor will generate approximately its rated voltage when spun at the rated speed.

Take, for example, this motor.

The seller claims 12000 RPM when the motor is powered with 3V.

If you drive that motor at 12000 RPM with your steam turbine, then it will output about 3V. At lower speed, the voltage will be lower.

The current you get out of it as a generator will be lower than the rated current consumption as a motor - I don't know by how much.


The power plant (the thing driving the generator) matters a great deal. You need the correct speed to get the required voltage, and you need enough mechanical power to maintain the rotating speed when you connect an electrical load to the generator.

It would greatly surprise me if your steam turbine with a "propeller" could generate enough mechanical power to even turn the generator.

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I assume you want to build a proof-of-concept, rather than an optimised turbine that produces a significant amount of power.

From what I understand, a generator behaves more like a current source than a voltage source. Because of this, you can take advantage of the LED's nonlinear voltage-current relationship and dispense with a regulator entirely, as long as you generate less power than the LED is rated for. The voltage will stop increasing when the LED starts conducting, and it will not rise to levels that would damage it.

So, assuming that your turbine will be pretty weak, if you pick a high brightness LED rated for a few hundred mA, you should be able to supervise the process "by hand" with enough safety margin.

If you want a regulated output, this will actually be more complicated -- most buck or boost circuits expect a low-impedance power source and will not work properly with a generator. You would need a battery in-between, or a dc/dc converter which is specially designed to regulate its input as well as its output.

You may be interested in searching for DIY bicycle dynamo LED lights circuits which are essentially similar to what you want to do.

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