Reading current through a capacitor

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Discussion Overview

The discussion revolves around the setup of an experiment using Langmuir probes to measure current density in plasma. Participants explore the necessity and configuration of resistors in relation to ammeters and data acquisition systems (DAQs).

Discussion Character

  • Technical explanation, Debate/contested, Experimental/applied

Main Points Raised

  • One participant suggests that a resistor is needed between the probes and the ammeter to read current density accurately, particularly when interfacing with a DAQ system.
  • Another participant argues that a resistor is not necessary when using an ammeter, stating that resistors are only needed when using a voltmeter to derive current from voltage.
  • A later reply acknowledges the internal resistors of the ammeter but raises the question of whether a resistor should be placed in series or parallel with the probes for the DAQ setup.
  • Another participant, unfamiliar with the topic, provides a perspective on the function of the double probe, suggesting that the plasma acts as a non-linear current sense resistor and that varying the bias voltage helps in measuring current-voltage characteristics.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and placement of resistors in the experimental setup, indicating that multiple competing views remain unresolved.

Contextual Notes

There are assumptions regarding the configuration of the DAQ system and the nature of the plasma's behavior as a current sense element, which are not fully explored or defined.

limofunder
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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?
 
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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.
 
f95toli said:
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.

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.
 
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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