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I (short circuit) Question

  1. Sep 20, 2015 #1
    1. The problem statement, all variables and given/known data
    I'm in a lab and we have this circuit & question as part of the pre-lab
    SS 2015-09-20 at 3.53.36 PM.png
    SS 2015-09-20 at 3.54.07 PM.png

    3. The attempt at a solution
    SS 2015-09-20 at 3.54.13 PM.png
    I began trying to use nodal analysis to get Va & Vb & to then use Vab to get the voltage and then the current but after doing what you can see above, I stopped because I think there is another way that I'm supposed to do it.
    Someone please guide me in the correct direction?
    Thank you
    H
     
  2. jcsd
  3. Sep 20, 2015 #2

    Hesch

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    Gold Member

    Make a Thevenin equivalent for ( R2 and R3 ) and for ( R1 and R4 ).

    Vth = Vb = 5V*R3/(R2+R3). ( R2 and R3 is simply a voltage divider ).
    Rth = R2 || R3. ( || means in parallel ).

    Short circuit the two Thevenin equivalents to find Isc.

    REQ = Rth + Rth.
     
  4. Sep 20, 2015 #3

    gneill

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    Staff: Mentor

    You're looking for the short circuit current, so essentially the current flowing through a wire (replacing RL with a wire). My first instinct would be to use mesh analysis to find that current, but a niftier choice would be to work with what you've already got and proceed to find the Thevenin equivalent (or Norton equivalent), since really all you need to finish is the equivalent resistance of the circuit looking into the open load terminals; The network is not difficult to simplify. Then finding the short circuit current would be trivial.
     
  5. Sep 21, 2015 #4
    okay, using what both of you said, I tried to finish the problem. Tell me what you think,
    SS 2015-09-21 at 11.52.40 AM.png
     
  6. Sep 21, 2015 #5

    gneill

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    Staff: Mentor

    You were doing fine until you went to calculate ISC. It's not clear from your work what values you are using for Vth and Rth. What comprises your final equivalent circuit?
     
  7. Sep 21, 2015 #6
    I determined Vab by subtracting Vb from Va and then I divided that by my determined REQ
     
  8. Sep 21, 2015 #7

    gneill

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    Staff: Mentor

    Okay, can you check (and post) the values?
     
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