I have a circuit into an optical isolator which is used to detect the status of a contact in the field.
Zener diodes are used in series to drop the voltage to the optical isolator as well as as series resistor to limit the current to about 3mA when the Zeners breakdown.
The puzzling phenomenon is that with field contract open in that there is no source voltage, one can read the sum of the Zener voltages at the input to the circuit.
Is it possible that the wire to the field contact provides enough "noise" so that it is possible to read a voltage at the input even though the contact is open and no source voltage is being applied?
Best Answer
We have seen long field wiring with water leakage and the circuit is no perfectly open circuit (4 V between poin + and X). Try putting a resistor, says 50k to 200k, Point X to point N/-.
Since 48V, you likely in telecommunication facility. If RF transmitter on site, it may explain the reading too.