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 (I_C / I_F). When reading the I_C value at I_F = 10 mA from the I_C - I_F curve, you can see that I_C = 20 mA. Here, if we assume that the CTR is almost the same as I_F = 5 mA, it can be calculated as follows:

In the case of GR rank (100% to 300%)

      I_C = 10 mA (I_F) x 100% to 300% (CTR) = 10 mA to 30 mA

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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