The LED circuit above is an example of using 25 white LEDs in series connected to the 120VAC line. It can be modified for more or less LEDs by changing the R value. The exact resistance value will depend on the particular LEDs used.

In operation, a DC voltage of around 170 is produced from the bridge rectifier and 50uF capacitor. The capacitor value is not critical and can be anything from 20uF or more. The capacitor voltage must be 200 volts or more. You can find 200 volt 470uF capacitors in old PC computer power supplies.

To find the resistor value and wattage, multiply the number of LEDs by the individual LED voltage. Then subtract this number from 170 and divide the result by the desired current (usually 20 miliamps).

So, for example, using 25 LEDs with a forward voltage of 3 volts, the total will be 75 volts. Subtracting this from 170 leaves 95. Dividing 95 by 0.02 (20 milliamps) yields 4750 or 4.7K. The resistor power rating will be the current squared times the resistance or (.02)^2 *4700 = 1.88 watts.

So, the circuit below, using 25 LEDs, at 3 volts each, will require a 4.7K resistor at 2 watts or more.

Copyright 2006 Bill Bowden