Tuesday 19 September 2017

resistance - how to calculate resistor value for optocoupler - 230V



I am trying to implement the following circuit. I am trying to understand how the resistance values are calculated. I followed this - "how do I calculate resistor value"


Mains detection with optocoupler


According to my optocoupler (SFH6202) datasheet, following are the calculations:


VF(typical) = 1.25V


If = 60mA


So



Vres = V - Vf = 230 - 1.25 = 228.75


R = Vres / If = 228.75/.06 = 3812.5


But the circuit shows two 100K resistors. Am I doing something wrong here?


Also the Power dissipation is:


P = Vres * Ifmax = 228.75 * 0.06 = 13.72 W


The power dissipation seems too high for 1/4, 1/8 or 1/10 W resistors. Am I doing something wrong in this calculation also? Or do I need some special kind of resistors?




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