Finding Req for thevnin theorem

Discussion in 'Homework Help' started by Muqaddas Jamil, Dec 25, 2008.

  1. Muqaddas Jamil

    Thread Starter New Member

    Dec 23, 2008
    3
    0
    Hi !
    we are currently doing thevnin and norton theorems in class and therez some problem with me in finding req.I am posting a problem from sergio Franco book....Plz see attached file.

    Plz i need step by step explanation 4 every step.
     
  2. silvrstring

    Active Member

    Mar 27, 2008
    159
    0
    It's hard to tell without drawings of how you manipulated the circuit before you decided to combine R5 and R3. R5 and R3 are the first ones I found the equivalent to, but it was 4 ohms--not 10; they are in parallel.

    If this is a class assignment, don't count on simulation--it's result will be off by at least 30 mΩ.

    Tryto redraw the circuit in a way that A and B are at one end before you start making calculations. Sorry, but I don't have scanner to send help.
     
  3. vvkannan

    Active Member

    Aug 9, 2008
    138
    11
    hello jamil,
    I think you can convert the star connected R9,R10 and R7 into delta and then it would become easier.
    Or you can consider it like this. The current through the parallel combination of R5 and R3 can reach B through R1 or R4 and hence they are in parallel.
    these two parallel combinations are in series and this combination is again parallel with R2 .

    http://www.allaboutcircuits.com/vol_1/chpt_7/3.html

    Read this and try applying that concept for your complex circuits.
     
  4. Muqaddas Jamil

    Thread Starter New Member

    Dec 23, 2008
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    0
    i really cant get it....can u plz give me sum demonstration through diagrams?
     
  5. silvrstring

    Active Member

    Mar 27, 2008
    159
    0
    Jamil,

    look at this walkthrough. Hope it helps you understand.

    Take care
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You have to use a delta to star resistor transformation for resistors R8, R9 and R10. After that, the rest will be parallel and series resistor combinations.
     
  7. Ratch

    New Member

    Mar 20, 2007
    1,068
    3
    Muqaddas Jamil,

    You almost solved it in your third diagram. Put R9 and R7 in parallel for a resistance of 5 ohms. Then you have 10 ohms in parallel with 9 ohms for a Req of 4.74 ohms. For Voc, a three loop mesh gives a voltage of 35.2 volts. For Isc a four loop mesh gives 7.2 amps. Using the resistance formula, 35.2/7.2 gives 4.89 ohms, which is with the limits of error for 3 significant digits.

    Ratch
     
  8. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    2,285
    329
    I get 4.73684 ohms for Rth, obviously the same as you get.

    But, I get 23.6842 volts for Vth, and 5 amps for Isc.
     
  9. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    Did you take the voltage of the floating battery to be 100.3 volts? It looks to me like it's 100/3 volts.
     
  10. Ratch

    New Member

    Mar 20, 2007
    1,068
    3
    The Electrician,

    That voltage figure is rather murky, but I think you are right. It should be 100/3. I calculated it for 100, so lets go with your answer. Of course no matter what the voltage, Req will remain the same. At least we appear to have that right.

    Ratch
     
  11. Muqaddas Jamil

    Thread Starter New Member

    Dec 23, 2008
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    0
    oh yes!
    if i consider 10ΩII 10 =5Ω,which is in series with 4Ω=9Ω,and the 9 Ωis again in parallel with 10 Ω,i get req=90/19 or 4.736Ω which is the same...;)Thnx
     
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