Opto Isolation

Well, I have some PC817 opto's on order but what do I need to do to put into a circuit.


The VF is quoted as 1.2V to a max of 1.4 V at an IF of typically 20mA.

If I am driving them from  5V  then the series resistor needs to  R=V/I   = 5 - 1.4   =3.6V/20e-3A = 180R.

Perfect.

But the best CTR  (600%) occurs at 2.5mA     approx  With a current transfer ratio (CTR)

3.6/2.5e-3 = 1440 R or 1K4

Now I want to input 120Vdc


Using a potential divider

Vout =  Vin R2 /(R1+R2)

5V = 120 (R