30Views3Replies

# Dc generator and power led Answered

I have a project. My teacher wanted me to produce energy by using a dc motor and light a power led with it and calculate the power that generated. I choose 12 volt 15600 rpm 1.2a dc motor. I assume i can spin the motor 15600 rpm. Now which power led i have to use and how am i going to calculate the power. The teacher said you can add resistance. Can you help me please.

The forums are retiring in 2021 and are now closed for new topics and comments.

Electrical power is the product of two numbers. It is voltage V multiplied by current I. So you need to measure both of those numbers somehow.

One way to do it, is to use two multimeters: One configured as a voltmeter, to measure voltage. The other configured as an ammeter, to measure current.

Also it helps if you have an experimental setup, with DC voltage and current that changes slowly enough, for the experimenter to glance at both meters, and write the numbers down.

Actually, I think I already have a setup like this. Two cheap multimeters in a wood frame, set up for measuring DC voltage and DC current simultaneously.

Another way to do this, using just one multimeter, is to include a small current sensing resistor, e.g. a 1 ohm (or 10 ohm or 100 ohm) resistor, in series with the load, the thing dissipating electrical power. Again this relies on a setup with DC voltage and current changing so slowly, that there is time for the experimenter to measure: (1) the voltage across the 1 ohm resistor (2) the voltage across the load.

You know what I mean? There is sort of the assumption that things are changing so slowly, that nothing significant changed in the amount of time it took to measure one voltage, then move the meter's probes to a different place, and measure the other voltage.

The reason I suggested powers of 10 for the current sense resistor, is because that makes the math for calculating the current easier. Since I=Vsense/Rsense, with Rsense = {1, 10, 100, or whatever it is} ohm. In the case of an LED as the load, a larger current sense resistor might actually be a good idea, to protect the LED from drawing too much current.

You know, because it would be kind of dumb, or unfortunate, if you burned out the LED, in the process of trying to supply power to it.

By the way, may I ask how you are supplying mechanical power to your DC-motor-as-generator?

You write, "I assume i can spin the motor 15600 rpm."

But I am wondering how you plan on doing that. I mean, human hands do not move that fast. So you must be contemplating something else. I guess.

Power = Voltage * Current
Lets assume, you have 0% loss in the generator/motor. Thats false, but unimportant as you will see later on.
12V * 1.2A = 14.4W
So your LED needs to be able to drain 1.2A at 12V which is 14.4W.
Now, as your Motor will not be 100% efficient, your current or voltage will be less and therefore the power to the LED will be less. But thats only save and does not endanger the LED.
The nice thing is, that 12V is a quite common voltage to use in lighting of LEDs. That you can use as load is a simple 12V 15W LED spot bulp you can buy at your local hardwarestore.
12V & 15W are the key-features you are looking for.
Something like https://www.aliexpress.com/item/32808402682.html

That would not need any resistor to protect the LED. You can straight up measure the current flowing and the voltage applied to get your power generated:
P = U * I

How to measure:
Two options.
A: Your multimeter can measure current and voltage separately.
- Measure the voltage first while the LED is lit up by your generator.
- Measure the current in one of the leads going to or coming from the LED
Power = Voltage * Current
B: Your multimeter can only measure voltage.
- Add a very small value resistor to your LED. Like 0.1Ohm @ 0.2Watt or 1Ohm @ 2Watt
- Measure the voltage at the motor
- Measure the voltage accross the resistor
Power = VoltageMotor * (VoltageResistor / ValueResistor)
This works because the Reeistor will have a voltage accross him proportional to resistance and current flowing thru: Current = Voltage / Resistance
This makes
Power = VoltageMotor * (UltageResistor / ValueResistor)
to become
Power = VoltageMotor * Current and we have the same calculation as if you would measure the current directly

You can spin a motor at 15600 rpm... with your hand? Well, I do not know much about this but I can tell you a few things: you will be better off experimenting with it and measuring the result. To calculate this without doing that, though, we have to know if it is a brushed or brushless DC motor, also have to know the torque of the motor, and if you are actually going to put the calculations I am sure you will figure out eventually to the test, the motor will not be the advertised rpm and torque, rather it will be in a certain range, for example 15600 rpm +- 5%. However, I do not have enough knowledge in magnetism and inductance to tell you. the only real thing I can tell you is that it will vary, especially at startup and shutdown (something called "Back EMF)