Finding Vth from thevenin theorem

Discussion in 'Homework Help' started by gbox, May 24, 2016.

  1. gbox

    Thread Starter Member

    Dec 29, 2015
    42
    0
    צילום מסך 2016‏.05‏.24 ב‏.13.39.23.png

    I have manage to find the Rth=200/3

    Now to find Vth I just need to find the voltage on the branch where the 200Omh resistor and the 6V battery are.

    The answer is Vth=2/3Vin-2 and I can find out why
     
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    Visit the ebooks here on DC Network Analysis.

    What is the source of that answer?

    You can't find out why because it's NOT true or you typed it incorrectly.

    Show you work ... otherwise the members here can only speculate how you achieved such an answer.

    on edit: added the statement about typing the answer incorrectly.
     
    Last edited: May 24, 2016
  3. MrAl

    Well-Known Member

    Jun 17, 2014
    2,418
    488
    Hi,

    What is the question here?
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    I believe the title is the question ... and the TS has yet to account for the diode in the circuit. From the diagram, I suspect the real question is What is Vo?

    We shall see if the TS returns.
     
  5. MrAl

    Well-Known Member

    Jun 17, 2014
    2,418
    488
    Hi Joe,

    Yes i see that diode as bring another problem that has to be addressed if Vin can go high enough. I would like to see more information about what Vin really is.
     
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    I'll give the TS a few more days before I mention my thoughts on Vin.
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    I am wondering how long a linear analysis will be applied to this non-linear circuit.
     
  8. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    As near as I can tell, the diode is being treated as the load and the rest of the circuit is what the Thevenin equivalent is modeling. If so, that's perfectly valid.
     
  9. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    There are several ways to find Vth. In this case, you might use superposition.
     
  10. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    Vth=2/3Vin-2

    Assuming this is

    <br />
V_{th} \; = \; \frac{2}{3}V_{in} \, - \, 2 V<br />

    What's wrong with it?
     
  11. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    Nothing is wrong with it as I just re-examined the schematic.
     
  12. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    I was wondering if perhaps you might have been looking at a different port than I was -- the TS wasn't too clear on what he was doing.
     
  13. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    It looks like the TS is not coming back.

    I suspect the TS mean to imply he can't find out why the answer was correct. In my first view, I had the batter polarity reversed when I said the answer was incorrect, but WBahn's posting reminded me to pay closer attention.

    I used superposition theorem to identify the Vth voltage which confirmed the correct answer.

    It goes as far as a perfect diode and not one step further. But for a first "cut" as what to expect, who knows. What we don't know is if the TS had drawn the diode characteristic curve in their labs. We don't know a lot about the TS's studies and labs. All we have is the diagram and a member interpreted question.
     
  14. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    As long as the diode is not part of the circuit that the Thevenin equivalent is modeling, it is every bit as valid as not using a Thevenin equivalent at all. We do this all the time. For instance, reducing a biasing network to it's Thevenin equivalent and using that to determine the operating point of a transistor circuit.
     
  15. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    I agree.

    Here is an "excel" graph

    160524-Vth-inquiry.png

    Then the simulation:

    Vo-Vin-simulation.png

    And of course the Vth without the diode:

    Vth.png
     
Loading...