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Reading current through a capacitor

  1. May 16, 2009 #1
    I am working on an experiment involving Langmuir probes. I believe the probes act as capacitors when placed in a plasma to get a reading of the current density in the plasma. However, since we are biasing a voltage, I know that in order to read the current we must insert a resistor between the probes and the ammeter. How should this resistor be placed, in series or in parallel?
  2. jcsd
  3. May 16, 2009 #2


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    You don't need a resistor if you are using an ammeter. The only time is you need a resistor in series is if you want to use a VOLTMETER to read a current (in which case you get the current from I=U/R).
    But maybe I missunderstood the question? There could be other reasons for putting a resistor in series (e.g. current limiting) but that has nothing to do with the ammeter.
  4. May 16, 2009 #3
    Yes, I apologize, I understand the ammeter already has internal resistors to measure the current. I plan to run this through a DAQ which requires a resistor to measure the current sent to the computer (the program I'm using asks for a resistance value since the DAQs are configured to measure potential), and I wasn't sure if I should have the resistor in series with or parallel to the probes.
  5. Jun 25, 2009 #4
    Totally out of my field but I think at least with the double probe it is biased between the 2 electrodes and current across or through the sensor is measured. The bias voltage is varied to extract specific current v.s. voltage anomalies. Can be thought of as a source/measure device. The plasma is the current sense "resistor" albeit very non-linear but that's what your looking for right ?
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