1. Not finding help here? Sign up for a free 30min 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!

[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):

    h_1390930293_9756129_f0b4069220.png

    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

    mfb

    User Avatar
    2016 Award

    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

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    That is even better.

    Right.

    Smaller load resistances lower the voltage and increase the current, so the smallest current corresponds to the largest voltage.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted