Using a 6 V DC motor for a 3.3 V LED

We are tasked to do a generator with LED for our project. We were given this video as a guide.

I am planning to buy a 6 V DC motor to power a 3.3 V LED. Is that okay? Although I have read somewhere that as long as I spin the motor slowly it would be okay, I just want to make sure.

• That video is slimy and noxious and, from the few seconds that I watched, not relevant to your question. However, if there is anything important in the video (that I was too nauseated to watch) you should capture it and, post as an image into your question. What is relevant are including links to data sheets for the LED and motor but, the problem is that it's a motor and not a generator and, it's difficult to make assumptions about how it operates as a generator so, you need to test it first. Mar 2 at 10:05
• I have found the Joule-Thief circuit to be very useful for doing this, giving much better and much more even LED illumination, but the 1K resistor needs to be replaced with a lower value, and you'll have to do your own experiments to determine the resistor value that is best for your situation. Mar 9 at 5:59

Sorry, I'm not going to watch a video for you, but there are a few general points:

• There is a relationship between rotational speed and the voltage when the running as a motor. Motor speed rises with applied voltage.
• When run as a generator the same relationship will apply (but the generated voltage will be a little less due to losses).

That means that if you run the motor at 6 V and measure the RPM that you can establish a relationship between the two. (You might find that you get 123 RPM/volt, for example.) From this you can then figure out what RPM you need to apply to generate a specific voltage.

• LEDs will draw little current until the voltage applied reaches the Vf, the forward voltage, 3.3 V in your case, and then the current rises very steeply with further rise in voltage. That means you need some means of limiting the current to a safe value. This is most easily done using a resistor.

What to do?

• Look at the datasheet for the LED and find out what the maximum current allowed is. Run at 50% to 75% of that for extended life.
• Calculate R for the maximum voltage output by the motor, 6 V, as follows.

$$R = \frac {6 - V_f} {i_{max}}$$

Choose the nearest standard value.

Assuming a brushed DC permanent magnet motor, the motor will output somewhat more than the rated voltage if you spin it at the no-load speed. For example, this motor will spin at about 10,800 RPM if you feed it with 6V. If you spin it at 10,800 RPM you'll get a bit more than 6V out of it (the difference being due to losses in the motor such as windage and friction when it is spinning itself). The open-circuit voltage will be proportional to the angular velocity, so at 1,100 RPM you'll get only 0.6 or 0.7V.

You can operate a 3.3V LED from 6V or 12V with choice of a suitable resistor but you cannot directly operate it from a 2V source. It is better to have at least 4 or 5V so that it's harder to pass excessive current through the LED.

You can measure the open-circuit voltage with the motor spinning at a reasonable rate and then calculate a suitable resistor.

For example if you want 15mA with 5V out, then you would use a resistor of R = (5-3.3)V/0.015A = 113Ω, so you might use 100Ω or 120Ω.

Given the relatively high RPM of typical toy motors you might prefer a higher voltage motor so you don't have to spin it as fast, but that depends on your mechanical drive arrangement and how well that works. I did not attempt to estimate how good the arrangement shown in the video will work.

What you're looking at are called nominal ratings. "Nominal" means "in name only", or more accurately in an electronics context, it means the numbers that are on the data sheet or name plate; the numbers the part is designed for.

You don't have to use the part at those numbers.

A 6 volt motor is designed to be run at 6 volts. However, no law of physics stops you from plugging in 3 volts. It will run slower. No law of physics stops you from plugging in 9 volts. It will run a faster, and wear out sooner (still not for a long time), and it might overheat a little. No law of physics stops you from plugging in 48 volts. It will probably spin very fast, make some sparks inside it, then catch fire and you'll have to blow the fire out and buy a new motor.

When you use it as a generator, the reverse applies. You can spin it faster than normal and get more volts. You can spin it slower than normal and get less volts. It even works in both directions. If you connect something that limits the voltage, once the voltage tries to go above that, it will increase the current instead, putting a drag on the generator and making it harder to turn.

If you have a plain LED without a resistor in it, the LED acts mostly like a voltage limiter. Once the voltage tries to go above the LED's forward voltage (about 3 volts for blue or white, 1.5-2 volts for other colours) the current will start flowing through the LED, holding the voltage down and putting a drag on the generator. If you try hard enough you can still make the voltage get up to 6 volts, but probably by that point, the LED's already burned out from too much current.

If the LED has a resistor, the same thing will happen but the resistor will 'absorb' some voltage making a 'softer' curve - less extra current flows when the voltage increases, making it easier to turn the generator faster, and the excess power will be spent in the resistor. Blue and white LEDs also have higher internal resistance than other colours and behave a bit like this.