LED diodes are non linear elements whom I-V characteristic depend from many external parameters such as temperature and manufacture. Mathematically I(U) dependence is described by Shockley diode equation. Unfortunately this exponential dependence, after adding resistor in series, becomes implicit function of current and supply voltage witch can be only solve numerically. That is why we have to simplify things for practical purposes. Here is useful table witch I use for fast choice of resistor for LED diode. It contains values of resistance calculated for four most common supply voltages at 20 mA current.
|Color||Voltage drop at 20 mA||Resistor for 3.3 V||Resistor for 5 V||Resistor for 9 V||Resistor for 12 V|
|Infrared||1.6 V||85 Ω||170 Ω||370Ω||520Ω|
|Red||2.2 V||55 Ω||140 Ω||340 Ω||490 Ω|
|Green||2.0 V||65 Ω||150 Ω||350 Ω||500 Ω|
|Blue||3.2 V||5 Ω||90 Ω||290 Ω||440 Ω|
|White||3.4 V||0 Ω||80 Ω||280 Ω||430 Ω|
|Ultraviolet||3.8 V||0 Ω||60 Ω||260 Ω||410 Ω|