Operations introduction 
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⚫  I: output current of LED constant current source; 
⚫  Vd: break-over voltage of diode (string); 
⚫  R: constant resistance. 
V-I curve of LED is as shown below. 
 
 
According  to  four  parameters  above  and  the  V-I  curve  of  LED,  you  can 
calculate the value of R and Vd . 
 
NOTE 
The value of V2, V1, I2 and I1should be close to the static working point of LED as shown 
in the red circle above.   
Or you can calculate the value of R and Vd by the following method. 
Vd=V*0.8 
R=0.2V/I