What capacitor should I use to smooth 2 to 5 volts oscillating dc power?

Question

A homemade generator is producing 2 to 5 volts AC which have been converted to DC with a bridge rectifier, still the leds I want to use flash. What capacitor should I use? And can the capacitor regulate the output voltage to 3 volts? Thank you.

Answer 1

The cause and the rate of the variation are important.  Also is the current the LEDs are using.

If it is in the once per second area, you will need a very large cap.

If the variation is once per second, and the current is 100mA,

Then a C determined by:

CV = it

C = (0.1sec)(1sec)  / 0.1 volt = 1 farad, an impossibly large value.

If the variation is faster, try a 100,000uF cap, you can get these rated at 6 volts.

.

Answer 2

The capacitor will reduce the variation in the output, but will not regulate it.  You can get a 3-volt series regulator, but it won't work with an input under about 5 volts.  The capacitor by itself may be enough for your application, depending on how much the input varies.  The value of the capacitor depends mainly on how much current you need for the LED's, and the frequency of the AC.  Probably about 1000 uf would work.  If you use too much capacitance, the LED's will stay on a while after the generator stops.

Answer 3

If your generator has a field winding then an automotive regulator can be adapted to regulate the voltage. If the generator is excited with permanent magnets then simply regulate the voltage with the speed of the prime mover.