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Phantom resistors

  1. Jul 19, 2014 #1
    hi guys

    I am attempting to solve a problem that has me scratching my head. I have a small connector PCB with one 8k resistor present. The connector is four way with the resistor terminals being the two inner pads. The outer two pads are short circuit together.

    I solder two wires on the two inner pads(to the resistor) on the PCB. Then i apply a current of 100uA with a constant current source using a multimeter and measure 8k. Then I place the pcb in a container filled with tap water. After a few minutes I remove the pcb from the container. I measure a resistance of 8k between two inner terminals, and a resistance in the megaohm region between the outer terminal to the inner terminal( resistance of water present between pads) with 10uA.

    Then i drive the circuit 100uA but now i measure a resistance of 2-3k, when measuring across the inner terminals.

    so then i decided to let the pcb set in a oven for 30 minutes and took another measurement using 100uA and the value obtained was once again 8k.

    so does this mean that with water present I was measuring 8k in parrallel with a phantom resistor, causing a lower resistance value read by the DMM?

    The pads are really close to each other that i was expecting the water to short out the circuit and give me a value near zero not 2-3k and interestingly with 10uA it still measred 8k but with 100uA it measured 2-3k. I dont fully understand what happening here?
     
  2. jcsd
  3. Jul 20, 2014 #2

    CWatters

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    Homework Helper

    The conductivity of water depends on impurities in it and can vary quite a bit.

    It's also possible you were seeing some electrolysis (powered by the meter) or some other electrolytic action going on (dissimilar metals forming a "battery"?).
     
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