I am Beginner and Enthusiast in Electronics, I want to calculate suitable resistance for white LED with different Supply voltages for example 3.6, 6 , 9 ,12.
I don't know operating Voltage of a White LED and How much Current It consumes.
Please some one guide me !
Thanks.

 

I've written an article on LEDs you might like.

http://forum.allaboutcircuits.com/showthread.php?p=117636#post117636

Figure you want 20ma. Take the battery voltage, subtract the Vf, and divide the result by the current (.02A). The resultant resistance is the answer.

3.6V probably won't power one very well since a typical white LED drops 3.5-3.8 V. Many cases you can use a 3V cell straight, since they can not provide much current. Many LED flashlights use them with no limiting resistor.

OK, for 6V plug the values into the equation...

(6V-3.5V)/.02A=125Ω, 120Ω is a standard value, and will work.

You can figure the rest, give the answers and we'll tell you if you got it or not.

 

You simply look at the range of its forward voltage in its manufacturer's datasheet.
A white LED is 3.2V to 3.6V.
The datasheet says what is its max allowed continuous current. It might be 30mA to 1A.
Then you use Ohm's Law to calculate a current-limiting resistor.

1) A 3.6V supply:
For a 3.2V LED at 25mA then the resistor has 0.4V across it and 25mA in it so it is 0.4V/25mA= 16 ohms.
If the LED is 3.6V then you cannot calculate a current-limiting resistor for it.
2) A 6V supply;
For a 3.2V LED at 25mA then the resistor is 112 ohms. Nobody makes a 112 ohm resistor so use 120 ohms for a current of 23.3mA.
If the LED is 3.6V then the resistor is 96. Nobody makes a 96 ohm resistor so use 100 ohms for a current of 24mA.

You can calculate the resistor value for the other supply voltages.