Question

How do I set up a basic DC generator circuit?

I have a tiny generator that I need to use to harvest energy and power an LED. The first way I set up the circuit didn't work. I'm thinking it should be done like this:

red cable from generator -> resistor -> long end of LED -> short end of LED -> black wire to generator

Note that the prof calls it a motor. I'm not sure if this would affect how the circuit should be built?

Answer 1

Hi,

While your connections look ok I can think of the following reasons why it did not work out:

1. Low speed of rotation. This is a very likely cause of the problem. The output voltage from any DC generator is proportional to its speed of rotation. LED would not glow unless the voltage across it is at-least 1V (assuming that it is a red LED, other colours will in general requires even higher voltage) How are you rotating it?

2. You should check if you DC generator is generating any voltage at all. To do this disconnect the LED and connect a multimeter (oscilloscope would be better though). Give the shaft a quick rotation and see if multimeter reading changes a bit? It is not a very accurate method but should give you an idea if there is any voltage output at all.

3. Incorrect direction of rotation. Long leg of LED is its anode and it needs to be more positive with respect to the shorter leg (cathode). In a DC generator the polarity depends on the direction of rotation.

4. Check if LED is okay. You can check it with a multimeter. Put the multimeter in diode testing mode. Connect the read probe of multimeter to the long lead of LED and black probe to the shorter lead. See if LED glows faintly.

And yes a DC motor can work as DC generator in general. You should do test #2 above to check if your motor is working properly as generator.

Hope it help.