Find Non-linear Current Across a Device

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Discussion Overview

The discussion revolves around finding the non-linear current across a device connected to a 'blackbox' circuit, where the Thevenin voltage and resistance are known. Participants explore the implications of non-linearity in the context of circuit analysis, particularly using Thevenin's theorem.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that for a non-linear device with a static V/I characteristic, the current can be found by solving the equation V_thv - R_thv I - f(I) = 0.
  • Another participant emphasizes the application of Kirchhoff's Voltage Law (KVL) to explain the relationship intuitively.
  • There is a question regarding whether nonlinear source circuits are included in the analysis of the 'blackbox' circuit.
  • Some participants clarify that the 'blackbox' must be linear to have a simple Thevenin equivalent, but the external circuit can be nonlinear.
  • Concerns are raised about the validity of deriving a Thevenin equivalent for certain 'blackboxes' that may not be approximately linear.
  • It is noted that Thevenin's theorem is strictly valid only for linear or nearly linear circuits, and deviations may require additional specifications like Total Harmonic Distortion (THD) and Intermodulation Distortion (IMD).

Areas of Agreement / Disagreement

Participants express differing views on the applicability of Thevenin's theorem to non-linear circuits, with some asserting that certain 'blackboxes' may not yield valid Thevenin equivalents. The discussion remains unresolved regarding the extent to which non-linear characteristics can be incorporated into the analysis.

Contextual Notes

Participants highlight limitations regarding the assumptions of linearity in 'blackbox' circuits and the conditions under which Thevenin's theorem can be applied. There is also mention of the need for additional specifications when dealing with non-linear devices.

ian2012
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I was just thinking about this the other day...

Given any 'blackbox' circuit, for which the values of the thevenin voltage and thevenin resistance are known. And given that a non-linear device is placed across the terminals of the circuit. What relation would one use to find the current across the device. Since it is non-linear, i am guessing V/R does not apply.
 
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If the nonlinear device had a simple static V/I characteristic in the form V=f(I) then you would solve the equation,

V_thv - R_thv I - f(I) = 0
 
Thank you for that. Is there an intuitive explanation?
 
ian2012 said:
Thank you for that. Is there an intuitive explanation?

Yes it's just a simple application of KVL (Kirchoff Voltage Law) around the loop.
 
ian2012 said:
Given any 'blackbox' circuit, ... (boldface added for emphasis)
However, are you also including nonlinear source circuits in your consideration?
 
turin said:
However, are you also including nonlinear source circuits in your consideration?

There's actually no problem with that turin. The "black box" itself must be linear in order to have a simple Thevenins equivalent, but there's no restriction on the nature (linear/nonlinear) of the external circuit when using a Thev equiv.
 
uart said:
The "black box" itself must be linear in order to have a simple Thevenins equivalent, ...
That's precisely the problem. I just didn't want the OP to think that they could make a few measurements to derive a Thev. eq. circuit for any blackbox. There are some blackboxes that are not even approximately linear.

EDIT: I reread the OP carefully, and now realize that the OP probably meant what you said. Sorry.
 
ian2012 said:
Thank you for that. Is there an intuitive explanation?

The part you had the Thevenin of didn't stop being linear so it still looks the same electrically.

BTW Thevenins are only strictly valid if the circuit they are representing is either linear or nearly linear (small signal assumption). Otherwise the Thevenin itself would be a problematic representation - though often enough it's still done and then you throw in new specs like THD and IMD to account for the variance.
 

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