Hi,

Could some one help me..

I am considering the various option to power the 8 white leds in series for the LCD back light.
When I use 42 VDC the power dissipation would be 42 VDC – 32 VDC = 10 VDC x 20 mA = 200 mW right?

Now suppose if I use Boost converter from 5 V in put to 32 V what would be the power disipation?

 

What does '32 VDC' represents?

What is the rated voltage of each diode?

 

WHY USE 42V for 8 leds IS THAT WHAT YOU HAVE? 8 leds are probably 2-3v forward drop @10-15ma so now 42-[8x[2.5]?]=20-42- 22v * .015a=330mw you need a resistor of 1.5k 1/2w. that is the most efficient way adding of 5 v PWM is lame. just tie all those LED in series.