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I have a circuit into an optical isolator which is used to detect the status of a contact in the field.

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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?

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A schematic would be useful to be clear on what you are doing here. Also, how exactly are you reading the Zener voltages? –  Oli Glaser Dec 22 '12 at 21:54
    
Load meter with 1k then 100 k and observe voltages. Unloaded circuit will rectify mains pickup and will be limited by zeners. Can even get RF pikup in some cases. –  Russell McMahon Dec 23 '12 at 10:13
    
I set up a Google Drive. I hope this works. Here is the link:docs.google.com/open?id=0B_O3P__R_R1DZi1vTE9RMnpEcHc –  Harry Snowden Dec 23 '12 at 15:46
    
@RussellMcMahon I think your response is reasonable. Can you take a quick look at the circuit that I posted to a share on Google Drive just to confirm your answer? Thanks. –  Harry Snowden Dec 23 '12 at 16:05
    
@HarrySnowden - something along the lines of what Russell mentions is almost happening - if you are reading voltage it's coming from somehwere, but more info is required to know whether it's conducted (e.g. a common impedance), near field capacitive/inductive coupling, or RF (far field eletromagnetic) The schematic is not very clear - for example it does not explain what the voltages shown are (measured? Sources) it needs the values of components added - between which two points are you measuring Von/Voff?. Are there any capacitors in your circuit? –  Oli Glaser Dec 23 '12 at 18:23
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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.

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