To calculate the resistance when powering a LED use this:

R=(Vpow-Vled)/I

• R: is the needed resistance
• Vpow: is the voltage of our source or power supply (e.g. battery)
• Vled: voltage of the LED
• I: the maximum of current that goes through our LED.

for a 2V (diode forward voltage) LED we need 20mA =0.02A (diode forward current)

So, for instance, if we have a 12V battery then: R=(12-2)/0,02= 10/0.02=500 Ohm

Another example is the use of multiple LEDs in an array. So, for an array of 4 LEDs with diode forward voltage 2V, diode forward current (0.02mA) and a Vpow of 12V we need:

4 x 1 array uses 4 LEDs exactly

+12V       R = 220000 ohms
• each 220000 ohm resistor dissipates 0.088 mW
• the wizard says the color code for 220000 is red red yellow
• the wizard thinks 1/4W resistors are fine for your application
• together, all resistors dissipate 0.088 mW
• together, the diodes dissipate 0.16 mW
• total power dissipated by the array is 0.248 mW
• the array draws current of 0.02 mA from the source.

Here is a calculator to automatically find your needed resistance [>>].