1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Thevenin Equivalent Resistance for a Black Box

  1. Mar 21, 2009 #1
    1. The problem statement, all variables and given/known data
    I am trying to find the theoretical Thevenin equivalent resistance for a black box that I used in a circuits experiment. The measured Thevenin equivalent resistance was roughly 4786[tex]\Omega[/tex], so the theoretical should be somewhere around that. However, I have tried several times to solve the problem, but keep arriving at a value of around 3511[tex]\Omega[/tex]. That just seems to be a little too low...

    I have attached the circuit schematic, with resistor values included.
    Could someone please look at the problem and see if I am on the right track. If so, any pointers on how I should finish it?

    2. Relevant equations
    R[tex]_{Th}[/tex]= V[tex]_{Th}[/tex]/I[tex]_{sc}[/tex]
    Resistors in Parallel: R[tex]_{eq}[/tex]= (1/R[tex]_{1}[/tex]+...+1/R[tex]_{n}[/tex])[tex]^{-1}[/tex]
    Resistors in Series: R[tex]_{eq}[/tex]=R[tex]_{1}[/tex]+...+R[tex]_{n}[/tex]
    Y to Delta Transformation:
    3. The attempt at a solution
    STEP ONE: R2, R3 in parallel
    R23 = [(1/R2)+(1/R3)]^-1 = [(1/9.97k[tex]\Omega[/tex])+(1/1.001k[tex]\Omega[/tex])]^-1 = .9096k[tex]\Omega[/tex]

    STEP TWO: R23 IN SERIES WITH R5
    R235 = R23 + R5 = .9096k[tex]\Omega[/tex] + 10.0k[tex]\Omega[/tex] = 10.9096k[tex]\Omega[/tex]

    STEP THREE: Y TO [tex]\Delta[/tex] TRANSFORM R4, R6, R7
    Rc = [(R4*R6)+(R4*R7)+(R6*R7)]/R4 = 24.95k[tex]\Omega[/tex]
    Ra = [(R4*R6)+(R4*R7)+(R6*R7)]/R6 = 24.85k[tex]\Omega[/tex]
    Rb = [(R4*R6)+(R4*R7)+(R6*R7)]/R7 = 12.39k[tex]\Omega[/tex]

    STEP FOUR: Rc PARALLEL to R235
    [(1/24.95k[tex]\Omega[/tex] )+(1/10.9096[tex]\Omega[/tex] )]^-1 = 7.59k[tex]\Omega[/tex]

    From here, I have tried different combinations of source transformations, converting the delta back to a Y, etc. to try and get an equivalent resistance.
     
  2. jcsd
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted



Similar Discussions: Thevenin Equivalent Resistance for a Black Box
Loading...