I have an input of pulse width 5 ms and which is fed to a Buffer IC and then the output of the buffer is FED to an opto isolator whose collector is pulled with a pull up of 7.5k resistor with an 24 V source. When no load is connected the output is fine but when connected to a load (PLC input pin) The output drops to 10V.

We increased the pull up to 15K the output is the same.

The opto used here is a 4N35

Please suggest some points to TAP on and a solution for the same.

Thank you

  • \$\begingroup\$ Increasing the pullup resistance will make it worse, not better. \$\endgroup\$ Jan 16, 2016 at 6:18
  • \$\begingroup\$ Okey then what should be done to avoid this? \$\endgroup\$ Jan 16, 2016 at 6:22
  • \$\begingroup\$ Name names. Doesn't the PLC have a datasheet? \$\endgroup\$
    – CL.
    Jan 16, 2016 at 10:11
  • \$\begingroup\$ What does FED and TAP stand for? \$\endgroup\$
    – Transistor
    Jan 16, 2016 at 10:42

2 Answers 2


Your PLC may have an opto-coupler on its input which is connected to 'ground' (common terminal) and needs to be pulled up by your opto-coupler. If so then you should wire the Collecter to +24V, and the Emitter to the PLC input. A pull-down resistor may not be necessary.

enter image description here


Schematic for @Bruce Abbott's answer.


simulate this circuit – Schematic created using CircuitLab

Figure 1. Opto isolator interface to PLC 'sinking' input.


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