How to Use a Photocoupler “Output-Side Resistor”

Step 3. Design output-side resistor R_L

Determine R_L from the I_C-V_CE characteristics of the output transistor. In order to use for signal transmission, it is necessary to fully satisfy the "L" level of the device connected to the load side.

Here, we set V_CE = 0.3 V as the target value.

When I_F = 10 mA, V_CE = 0.9 V at R_L = 1 kΩ, which is insufficient. At R_L = 2 kΩ, V_CE = about 0.2 V, which satisfies the target value. Therefore, choose R_L = 2 kΩ. In actual design, the impedance on the load side must also be considered.

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