How to Use a Photocoupler “Output Current”

Step 2. Calculate output current of the phototransistor from I_F and CTR.

What is the value of the photocoupler’s output current (I_C)?

Calculate the variation of the output current (I_C) at the input current (I_F) = 10 mA from the current transfer ratio (CTR=I_C / I_F). From the I_C - I_F curve, you can see that I_C=10 mA(@I_F=5mA, V_CE=5V, Ta=25℃). So the CTR of this sample is 200%, which is the same value of minimum BL rank (200% to 600%). Also you can see that I_C = 20 mA (@I_F=10 mA、V_CE=5V、Ta=25℃).
Next, we derive R_L using the value of I_C obtained here. In this calculation, design the value of R_L so that V_CE becomes saturation voltage even at the minimum value of I_C.

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