this is my first post

Discussion in 'Homework Help' started by Favour, Jul 15, 2008.

  1. Favour

    Thread Starter Member

    Jul 15, 2008
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    This is my first post.
    What is the likely solution to the attached problem. Appreciate your assistance.
     
  2. Ratch

    New Member

    Mar 20, 2007
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    Favour,

    One of the resistors connected to the dependent voltage source is missing a value. Ratch
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
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  4. Favour

    Thread Starter Member

    Jul 15, 2008
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    Ratch
    Thanks for your observation.
    All.
    Please find attached the updated attachment.
    Will appreciate your assistance.
     
  5. Favour

    Thread Starter Member

    Jul 15, 2008
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    thingmaker3,
    I opened two thread for two different circuit questions. Are you now saying that I should have merged the two attachment in one thread?

    Sincerely, I do not know how to go about the two problems. Can someone help?
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
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    My apologies, the second file was similar enough to the first I thought it was the same one. Comparing them side by side, they are obviously different. I have re-opened the second thread.
     
  7. Ratch

    New Member

    Mar 20, 2007
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    Favour,

    Looks to me like they want you to disconnect the east loop and Thevininize at the point of disconnection. Do you have a problem doing that? When you have a Thevinin impedance and voltage, connect up the east loop again and figure the voltage across Ro. Then check by doing a loop analysis to find the current of the east loop and the voltage across Ro. Can you do loop analysis? Ask for help if you have difficulty. Ratch
     
  8. Favour

    Thread Starter Member

    Jul 15, 2008
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    Ratch
    Thanks for all your effort.
    If I apply loop analysis (say mesh), how do I handle the dependent source (2vx), 2A current between two loops. It seems I will end up having four loops with four unknown. I am confused and do not know how to obtain a solution. Appreciate if you will assist.
     
  9. studiot

    AAC Fanatic!

    Nov 9, 2007
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    When you are doing mesh analysis involving dependant sources, you do not know the voltage drop in the loop containing the dependant (voltage) source.

    So the trick is to establish node current equations to include the current through the dependant source.

    This will not yield enough equations to solve the mesh by itself.

    So you then form voltage loop equations, from loops not involving the dependant voltage source.

    You will then have enough equations to solve the mesh.

    Have a go yourself at this before we do it for you.
     
  10. Favour

    Thread Starter Member

    Jul 15, 2008
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    I have tried solving it. Got Rth as 3 ohms (though not sure). Having problem with the appropriate and correct loop equation. Please help me!
     
  11. Ratch

    New Member

    Mar 20, 2007
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    Sorry I did not catch it before, but you have to specify the polarity of VX. In other words, is the dependent voltage source sensing VX as if the negative lead of a voltmeter were on the north side or south side of resistor VX? Ratch
     
  12. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Sorry Ratch, partly my fault, there are no voltage sources in this circuit, only current sources.

    Attached set of equations for the loop analysis. Best layout I could get in Word.
     
  13. Ratch

    New Member

    Mar 20, 2007
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    studiot,

    Isn't that a dependent voltage source in the southwest loop dependent on the voltage across the 2 ohm resistor in the northeast loop? If so, we have to know how to connect the voltage sense leads across the 2 ohm resistor.

    The problem states to first Thevinize the circuit, and then check it be analyzing the whole circuit using loop analysis.

    Ratch
     
  14. studiot

    AAC Fanatic!

    Nov 9, 2007
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    No Ratch it was me not reading the circuit thoroughly. The symbol with the arrow inside is a current source. The voltage dependency is a constant times some voltage elsewhere in the circuit. In this case twice the voltage across resistor CF.

    The other two sources, shown by arrows in circles, are independent current sources.

    You were correct in noting that a dependent voltage source has polarity markings, but it has no arrow.

    Feel free to do the Thevenin analysis, I did the (bulk of) more onerous loop analysis.

    I think one of the points of the exercise was to show how judicious use of Thevenin can shorten an analysis. The loop analysis involves solving 9 simultaneous equations.
     
    Last edited: Jul 20, 2008
  15. Ratch

    New Member

    Mar 20, 2007
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    studiot,

    OK, you convinced me that it is indeed a dependent current source. But we still need polarity signs on on the 2 ohm resistor in order to know which way the voltage is being measured by the dependent current source. Ratch
     
  16. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I apologise I have to admit to a bobo the last equation in my loop analysis should read

    Loop EFCHGE 2I7 -2I4 + I3 + 2I3 -4I6 = 0

    The loop I too hastily chose contains one of the independent curent sources so, of course, the voltage across it can't be included.
    I have uploaded a replacemnt page in this post.
     
    Last edited: Jul 20, 2008
  17. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Ratch, please check your textbook on circuit analysis.

    I said in post 12 and I will say again.

    There are no voltages sources, dependant or independent in this circuit.

    All the voltages are entirely determined by the displayed circuit parameters; the polarity of the voltage VX across the 2 ohm resistor can be found in the analysis.
     
  18. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I superimposed my stuff on the original word document. I have just noticed that resistor DE was not labelled in the original so I have updated my loop analysis to include the value of 2 ohms from the subsequent pdf.

    I have also added another equation to account for the unknown voltage VX.

    So here is the latest version, please ignore (or mods remove) versions 1 and 2.
     
    Last edited: Jul 20, 2008
  19. Ratch

    New Member

    Mar 20, 2007
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    studiot,

    I am still looking for some polarity marks by the resistor for VX. For instance, would the negative voltage sense lead from the dependent current source connect to the north or south side of the resistor? In other words, where are the polarity markings for the sense resistor? Ratch
     
  20. Ratch

    New Member

    Mar 20, 2007
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    studiot,

    Since this particular problem in not in any of the books on circuit analysis I have, they do not help me.

    True, but irrelevant. There is a voltage across resistor VX, and I have to know how it is measured before analysis can begin. Look at this voltage controlled current source and notice how the voltage leads are labeled. http://www.freeda.org/doc/elements/vccs.pdf That is the way the resistor VX should be marked. Ratch
     
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