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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 (CTR=IC / IF). From the IC - IF curve, you can see that IC=10 mA(@IF=5mA, VCE=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 IC = 20 mA (@IF=10 mA、VCE=5V、Ta=25℃).
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.