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LED CONSTRUCTION & OPERATION
LED LED Schematics 



LED Circuit
LED Construction 

The formula to calculate the correct resistance to use is

Resistance (R) = (power supply voltage (Vs) - LED voltage drop (Vf)) /divided by LED current (I)

where power supply voltage (Vs) is the voltage of the power supply, e.g. a 9 volt battery, LED voltage drop (Vf) is the forward voltage drop across the LED, and LED current (I) is the desired current of the LED. The above formula requires the current in amperes, although this value is usually given by the manufacturer in milliamperes, such as 20 mA.

Typically, a LED forward voltage is about 1.8–3.3 volts; it varies by the color of the LED. A red LED typically drops 1.8 volts, but voltage drop normally rises as the light frequency increases, so a blue LED may drop around 3.3 volts.

The formula can be explained considering the LED as a  {V_f \over I} \;\Omega  resistance, and applying Kirchhoff's voltage law (KVL) (R is the unknown quantity):

V_s=V_r+V_f=R I+V_f

R I=V_s-V_f \;

R={V_s-V_f \over I}

 
Standard discretes LEDs are often tested at 20 milliamps of current - approximately 50 ohms with a Vs-Vf of 1 volt.  As one can see, the closer the supply voltage is to the LED voltage drop, the lower the value of the resistor to maintain a given current.  
   
   
The OpenLED Project

Updated to 08/07/13 v1