How to drive an LED using a DIY steam generator?

Question

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.

Answer 1

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

More here: Controlling High Current LEDs with an ATmega328

Answer 2

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.

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.

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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.

Answer 3

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.