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Homework Help: Intro EE - Voltage Divider Design circuit

  1. Sep 21, 2012 #1
    The link is an image with the question and circuit representation:

    Here are the equations I used:

    V_out/V_in = R_2/(R_1 + R_2) {using the node method}

    R_thevenin (R_th) = R_2*R_1/(R_1+R_2)

    And given the conditions =>

    10 < R_th < 30

    Manipulating this inequality I found that 50 < R_1 < 150, which left me with three possible values for R_1 (56, 68, 82).

    Using the first equation, I put in the numbers, which gave me 0.2=R_2/(R_2+56) and solved for R_2 with every value I put in for R_1 (56, 68, 82).

    R_2 values were 14, 17, and 20, respectively, which came to equate to 15, 18, and 22 when compared to the original set of resistors and the 10% condition.

    I tried putting in the values of these resistors I used, because all gave me the right ratio of voltages, but they're incorrect.

    Pardon how nuanced I may seem, it's been too long since I've done any physics..
  2. jcsd
  3. Sep 21, 2012 #2


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    Staff: Mentor

    I think you are on the right track.
    This should be, 10,000 ≤ R_th ≤ 30,000
    Aren't you missing some candidates here?
    R₁ ∈ {56k,68k,82k,100k,120k,150k}
  4. Sep 21, 2012 #3
    Wouldn't it just be the three since your given all the resistances available in the set E12?
  5. Sep 21, 2012 #4


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    Staff: Mentor

    120kΩ = 12 x 104
  6. Sep 21, 2012 #5
    Right! Thanks.
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