The current limiting resistors of the optocoupler input and output are both 1k, and the input voltages are the same, so by adjusting the voltage value of the voltage regulator source, the relationship between the optocoupler input current IF and the output current IC can be obtained.
The left side is the input current IF, the right side is the output current IC
The obtained experimental data show that the trend of the output current IC and the input current IF curve is basically the same, and the CE impedance changes linearly when the impedance is less than 1k. And the minimum impedance is greater than 100Ω.
Therefore, when using a linear optocoupler to transmit switching signals, it is necessary to reasonably match the size of the input resistance, the input resistance is 360Ω, the forward voltage drop of the optocoupler input is about 1V, and the input current IC is (5-1)/360≈11mA. The output CE impedance is more than 200Ω, and the coil impedance of the relay HFD2 is 2880Ω. At this time, the relay can be driven normally. If the IC current becomes smaller, the CE impedance will become larger and the relay cannot be driven normally.