Reading current through a capacitor

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SUMMARY

The discussion focuses on the use of Langmuir probes in plasma experiments to measure current density. It establishes that when using an ammeter, no additional resistor is necessary, as the ammeter contains internal resistors for current measurement. However, when interfacing with a Data Acquisition (DAQ) system, a resistor is required to measure the current sent to the computer, and the placement of this resistor—whether in series or parallel with the probes—needs clarification. The conversation highlights the importance of bias voltage in extracting current versus voltage anomalies in plasma measurements.

PREREQUISITES
  • Understanding of Langmuir probes and their function in plasma physics
  • Knowledge of ammeter operation and internal resistance
  • Familiarity with Data Acquisition (DAQ) systems and their configuration
  • Basic principles of current and voltage measurement in electrical circuits
NEXT STEPS
  • Research the configuration of resistors in series and parallel circuits
  • Learn about the operation and calibration of Data Acquisition (DAQ) systems
  • Study the principles of plasma physics and the role of Langmuir probes
  • Explore current versus voltage measurement techniques in experimental setups
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Researchers and engineers working in plasma physics, electrical engineering students, and professionals involved in experimental setups using Langmuir probes and DAQ systems.

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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