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Homework Help: [Circuits] Designing a simple voltage-divider

  1. Jan 28, 2014 #1
    1. The problem statement, all variables and given/known data

    Design a voltage divider to provide a voltage of about 5V (±10%) across the variable load. The load-current demand varies in the range of 0 to 5 mA and the available dc-supply voltage is 15V.

    2. Relevant equations

    3. The attempt at a solution

    Here is the circuit given (I've added ground, 15V, and 5V):


    First, I assumed that there was no load. This gave me the following relationship: ##R_1=2R_2## but I'm not sure how to proceed.
  2. jcsd
  3. Jan 28, 2014 #2


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    Staff: Mentor

    The maximal current for the load will correspond to the lowest acceptable voltage (can you calculate that?). This allows to determine R1 and R2 together with the equation you found.
  4. Jan 28, 2014 #3
    The lowest acceptable voltage would be 4.5V, right?

    How do you know that maximum current would correspond to lowest voltage? Does that mean lowest current would correspond to highest voltage? Should I have used 5.5V instead of 5V in my first calculation then?
  5. Jan 28, 2014 #4
    For no load set Vout at 5.5V, Now with 5mA load set Vout at 4.5V.
  6. Jan 28, 2014 #5


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    Staff: Mentor

    That is even better.


    Smaller load resistances lower the voltage and increase the current, so the smallest current corresponds to the largest voltage.
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