Before going through the intricacies of the luminance of power LEDs we must first know the characteristics of LEDs. A white LED is generally biased at a voltage range of 2.5 to 3.5V DC.
Depending on the wattage of LED, the input current is determined. White LEDs are used for illumination purposes and are available right from 0.1Watt to 3Watts in general, with the same voltage characteristic but different current rating.
Though very simplistic, it is interesting to use the following serially connected white LEDs in a chain.
Here in this circuit 25 LEDs in a series have been considered a safe zone operation. We decide the bias voltage to be 3V DC in each of this rung. The total potential required to bias the LEDs is 25x3= 75V DC.
A bridge rectifier is employed to rectify 120V AC line without any filter circuit being employed. The peak of the rectifier voltage would be 120x 1.414= 169.68 V DC.
To play with the value of R in the first circuit is very simple as R has to create a potential drop of 95V DC with a justified calculation of the current going through it of wattage is very simple to decide.
Now let us consider the second circuit with half watt rectification with single diode only. This circuit is cheaper and at the same time the heat dissipated in the current limiting resistance is much less. This happens as the circuit employs a half wave rectifier and using an anti parallel diode across a LED chain.
This circuit obviously pushes a larger peak current to the LEDs but this will not be harmful to the circuit operation because it lasts only half the cycle.
In this particular case each LED is biased in such a way that it delivers 0.06 watt of power, in total the entire chain is a 1.5 watt power source which probably is sufficient for reading purpose and worth its cost and life.
As the circuit is designed around live mains, user must be cautious of electrical seepage or short circuits and for this an insulated cover properly designed is a strong recommendation.
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