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How to find Rth in Thevenin

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  1. May 22, 2017 #1
    • Member advised to use the homework template for posts in the homework sections of PF.
    This is the question
    negative-thevenin-resistance-jpg.jpg
    First,I use thevenin theorem,when I close the voltage source, i apply nodal analysis to find rth,is my answer correct?
    IMG20170523084138.jpg
     
    Last edited by a moderator: May 23, 2017
  2. jcsd
  3. May 22, 2017 #2
    Just a question but aren't those all resistors if the units are kila-ohms?
     
  4. May 22, 2017 #3
    I don't understand what you mean
     
  5. May 23, 2017 #4

    NascentOxygen

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    Staff: Mentor

    Dependent source 0.003vo should, with your nomenclature, be written 0.003V1
     
  6. May 23, 2017 #5

    NascentOxygen

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    Note: we are not able to see what's at your facebook link, if it's applicable
     
  7. May 23, 2017 #6

    gneill

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    Moderator's note: I've restructured the original post to make the relevant images visible, and removed the facebook link as it was not accessible. @qwertyuiop is advised to upload relevant content rather than link to a facebook page.
     
  8. May 23, 2017 #7

    The Electrician

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    Halfway down the page of your calculations you have:

    (V1-V2)/22k + 0.003 V2 +1 + V2/30k = 0

    This should be:

    (V1-V2)/22k + 0.003 V1 +1 - V2/30k = 0

    Which should then become:

    30 V1 - 30 V2 + 1980 V1 + 660000 - 22 V2 = 0

    Which becomes:

    2010 V1 - 52 V2 = -660000
     
  9. May 23, 2017 #8
    Thank you for you helping,now i know my mistake
     
  10. May 23, 2017 #9
    Thank you
     
  11. May 23, 2017 #10
    Thank you for your help.
    I don't understand why - V2/30k ?
     
  12. May 23, 2017 #11

    The Electrician

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    Because you have chosen to define currents into a node as positive. The other choice people make is to define current out of a node as positive. But whichever you choose, you must be consistent.

    The expression V2/30k gives the current out of the node, so its sign must be negative.
     
  13. May 23, 2017 #12
    ok, thank you for your help. In this circuit, I find the rth = -1363ohm. Is possible rth is negative? why?
     
    Last edited: May 23, 2017
  14. May 23, 2017 #13

    The Electrician

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    The dependent source in this circuit is a transconductance source. The value 0.003 is the transconductance. Substitute a variable x for the value 0.003 and solve the circuit for Rth. You will get this expression: Rth = 3.75/(.00025-x). You can see that if x is greater than .00025, Rth will be negative; since .003 is greater than .00025, we get a negative value for Rth.

    The circuit of this thread is an example of a circuit with an active element connected in such a way as to generate a negative resistance. There are other well known circuits that can do this, for example: https://en.wikipedia.org/wiki/Negative_impedance_converter
     
  15. May 24, 2017 #14
    Thank you
     
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