Step 2. Calculate output current of the phototransistor from IF and CTR.
What is the value of the photocoupler’s output current (IC)?
Calculate the variation of the output current (IC) at the input current (IF) = 10 mA from the current transfer ratio (IC / IF). When reading the IC value at IF = 10 mA from the IC - IF curve, you can see that IC = 20 mA. Here, if we assume that the CTR is almost the same as IF = 5 mA, it can be calculated as follows:
In the case of GR rank (100% to 300%)
IC = 10 mA (IF) x 100% to 300% (CTR) = 10 mA to 30 mA
Next, we derive RL using the value of IC obtained here. In this calculation, design the value of RL so that VCE becomes saturation voltage even at the minimum value of IC.