Hello,

I want to use a TLP521-4 (

Datasheet) in my setup. I tried to read about opto-coupler and the datasheet, but I am not completely sure how to calculate.

I think the resistor for the LED is correct. I use Arduino Pro Mini with 3.3V. So:

U = 3.3

U_F = 1.15

I_F = 0.05

R = (U - U_F) / I_F = 43.00 Ω

But I am not sure about Work Resistor. Formula should be:

R_A = (V_CC * SF) / (I_F * CTR)

V_CC = 24

SF = 5 (Switching speed is no matter)

I_F = 0.05 (I read that this comes from LED?)

CTR = 1 (I am not sure about CTR from datasheet)

So R_A would be 2.4 kΩ from this calculation. But as you can see, I am not sure about CTR. In the data sheet I only see one graph which is only for 25 °C and two completely different curves Sample A and Sample B. To this I am not sure what I shoulg choose on x-axis (Forward current I_F (mA)). I talk about this graph:

Thank you and best regards!

[EDIT 20181118] The final and reviewed drawing