How do I calculate the resistance between the Anode and the Cathode of an Optocoupler (while the optocoupler is on). I am messing aroung a little bit with optocouplers and found out using an multimeter that my optocoupler has an resistance of about 100 Ohms between the Anode and the Cathode. How do I calculate this resistance and why is it that high ?

I was using the digital pins of my Arduino Uno to turn the optocoupler on.

Thank you for every hint!

  • \$\begingroup\$ The very first thing you do is to look at the data sheet. The second thing you do is realize that an optocoupler probably doesn't have an output which can be easily modelled as a resistor (a few do). That means that you need to learn both how an optocoupler output works, and how a multimeter works. \$\endgroup\$ Jun 8, 2016 at 17:47
  • \$\begingroup\$ Does the LED part of the coupler have a current limiting resistor?. \$\endgroup\$
    – Spoon
    Jun 8, 2016 at 19:51

1 Answer 1


You could calculate the resistance with Ohm's law: \$R = U / I\$.

However, this does not really make sense, because neither the input nor the output of an optocoupler have a constant resistance:

  • The input is an infrared LED, i.e., a diode. This means that the voltage between the anode and cathode pins is more or less constant, while the current is whatever amount your circuit sends through it.

  • The output is a transistor (for most optocoupler models). This means that the voltage between the collector and emitter pins is mostly determined by your circuit (typically, you have a load resistor to drop most of the voltage when current flows, so the voltage is more or less constant), while the maximum current that can flow between them is somewhat proportional to the current through the input LED.

Calculating the resistance does not solve any problem. To help you understanding how it works, measure the current at the input, and how much current can then flow through the output.


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