# Optocouplers common resistor

I have seen optocoupler series with common cathode and resistor on every anode. My question is whether or not one resistor on the cathode side will be sufficient, with the right wattage, of course, and what is the reason behind these many resistors.

simulate this circuit – Schematic created using CircuitLab

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• If you're driving the LED with a current source, it doesn't matter what resistor you use. You could use no resistor at all and have no trouble. – The Photon Jul 2 at 19:49
• Well, I just didn't see the 24 volt voltage, so the input is 24 volts. – user6740407 Jul 2 at 19:52
• I've added the 24 V sources for you. If you don't have constant current sources then remove them. Double-click the resistors to set their values. – Transistor Jul 2 at 20:01
• If only one LED or optocoupler will be on at a time, you may use a single resistor. If more than one LED or optocoupler will be on at the same time, the current will be divided between the LEDs, which may make them too dim to operate the phototransistor in the optocoupler. – Peter Bennett Jul 2 at 20:08

In Figure 1a a typical LED wired as shown would receive a current of about $$\ I = \frac {24 - V_f}{480} = 45 \ \text {mA} \$$ if the forward voltage, Vf is about 2 V.