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Find Resistance one and two

  1. Sep 30, 2015 #1
    1. The problem statement, all variables and given/known data
    Consider the voltage divider circuit below. The output voltage is with V0 =20v with RL removed ( RL= ∞Ω) from the circuit. With RL in the circuit and RL=380Ω , the output voltage is V 0=19.192 v . Determine values for R1 and R2. I attached a picture of the circuit.

    2. Relevant equations


    3. The attempt at a solution
    For RL=∞Ω
    20=100((R1)/(R1+R2))

    For RL=380Ω
    19.192=(100)((380//R2)/(380//R2)+R1))

    Am I on the right track with this? I tried to make a substitution by solving for R1, but it came out really messed up.
    Is there a better way to work this problem?

    Here is the answer: R1=80Ω R2=20Ω
     

    Attached Files:

  2. jcsd
  3. Sep 30, 2015 #2

    gneill

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    Staff: Mentor

    Your approach looks fine, but check your voltage divider equation for the unloaded circuit. The output is across R2, right?
     
  4. Sep 30, 2015 #3
    Ohh ok! I fixed my unloaded circuit equation and came up with R1=4R2

    I think something might be wrong with my RL=380Ω equation though. I substituted R1=4R2 and wound up with -497.56. I've redone my algebra three times and keep getting the wrong answer.
     
  5. Sep 30, 2015 #4

    gneill

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    Staff: Mentor

    Sometimes when you hit a barrier due to a slip you can't see, it can help to change the approach slightly to take a different path to the solution. Why not try first reducing the voltage and R1, R2 to a Thevenin equivalent? You'll have two expressions for Vth and Rth (involving R1 and R2) but the result will be a purely series connected circuit.
     
  6. Sep 30, 2015 #5
    Thanks!!
     
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