Electrical – Does a dc generator need a flyback diode

Technical Explanation: The open circuit voltage produced by your generator is too low to drive any current through the LED.

Straightforward Explanation: Your problem is probably that there aren't enough volts produced by the generator, even when the LED isn't connected. You can measure this with a simple multimeter when spinning the generator. Set the multimeter to 20V AC or DC and see what voltage you measure. To get the LED working you should see at least 1.4 volts for a RED LED, 2.5 volts for blue, or 3 volts for a white LED.

Others have suggested a resistor is necessary to prevent damage to the LED. Normally, this would be true, but in this case as the LED starts to light up, it will slow down the wind turbine, protecting itself in all but the strongest winds. Additionally, most generators have the equivalent of a resistor inside them.

One suggestion is to use a microcontroller/capacitor. The problem with this solution is you will not be able to get a high enough voltage to power up the microcontroller.

My solution:

• Use a Red rather than white/blue LED. Use no resistor (or if you do, choose something below 300 ohms)

• Spin the generator faster - use gears to up the speed and lower the torque if necessary

• Get a different type of generator designed for slow speed use.

• If you have multiple LED's, connect them all in parallel (ie. every LED connects direct to the generator)

• Make sure you're spinning the generator the right way - test spinning it backwards, or connect the LED the other way round.

Note: Human eyes are very sensitive - if the output voltage of the generator is high enough, even a tiny current flow will cause the LED to visibly light up in the dark.

Additionally, there are many types of generator, AC/DC, brushed and brushless etc. In this situation, it doesn't really matter which you choose.

Bridge rectify the AC and smooth (smoothing almost optional) supply a resistive load that will provide somewhat more than the rated power
eg ~= R <= V^2/P <= 144/50 <= 2.88 ohm
So a say 2 ohm 100 W resistor.
This could be eg a coil of suitably thick Nichome wire which is vailable from industrial suppliers.

Then PWM this with a transistor (MOSFET liable to be easiest) and vary the PWM duty cycle. PWM duty cycle can be automatically adjusted to act as a regulator. Circuit can be provided but that may be enough.

I did essentially this some years back to provide 500 Watt+ loads for exercise machines. Works well. I used ~= 20 kHz PWM mainly to get PWM rate above audible so "singing" alternator was not objectionable. In your case a very low PWM frequency may be acceptable - essentially a subsonic buzz.