B

#### Bill Bowden

- Jan 1, 1970

- 0

I have a series chain of 25 white LEDs at 3 volts each. I want to

connect them through a resistor to a bridge rectifier connected to the

120VAC line. There is a 50uF capacitor across the chain of LEDs. What

is the resistor value for a RMS current of 20mA?

The total LED voltage will be 25*3 or 75 volts so the conduction angle

will begin at about (asin) 75/170 =.44 = 26 degrees. So, out of a 180

degree half cycle, the resistor will conduct for 180-52 = 128 degrees

or about 71 percent. The RMS voltage across the resistor is the peak

input minus the LED voltage times 0.707 or (170-75)*.707 = 67 volts

RMS. But since the duty cycle is 71 percent, the RMS voltage should be

adjusted to 67*.71 = 48 volts RMS. Therefore, the resistor value for

20mA of current should be 48/.02 =2400 ohms.

Does this make sense, or did I miss something?

-Bill