Calculating Resistors in a Voltage Divider

Discussion in 'Homework Help' started by pelorus32, May 2, 2010.

  1. pelorus32

    Thread Starter New Member

    May 2, 2010
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    This is probably very simple but my brain won't get around it.

    How do I calculate resistor values in a voltage divider circuit as follows:

    Vin = 255V
    R1 = unknown
    Vout after R1 = 195V
    R2 = unknown
    Vout after R2 = 150V
    R3 = 10K ohms
    Vout after R3 = 0V

    What are the steps for calculating R1 and R2?

    Many thanks
    Mike
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    It is all a matter of proportion, involving fractions.

    Show a schematic to work with would help, otherwise too many assumptions creep in.
     
  3. pelorus32

    Thread Starter New Member

    May 2, 2010
    4
    0
    Hi Bill,

    thanks for the reply. I don't have the tools to draw a schematic. However it is a series circuit and the order of the items as set out in my original post is the order that they appear in the circuit.

    Regards
    Mike
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Here is a drawing made in paint:

    [​IMG]

    The current can be calculated using the voltage drop of R3 and the given value.
    With this current you can calculate the other resistors.

    Bertus
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    Take a look at my blog. I offer a set of templates under the name of PaintCAD that will work for any graphics program (I use MS Paint), along with a fairly extensive cookbook of drawings made with this package.

    Bill's Index

    Introduction and PaintCAD

    I am one of this sites graphic artists.
     
  6. pelorus32

    Thread Starter New Member

    May 2, 2010
    4
    0
    Thanks to both of you for your assistance. My answers so far from that are:

    Circuit Current: 15mA
    R2: 3 K ohms
    R1: 4 K ohms


    Does that sound right?

    Many thanks
    Mike
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You are correct.
    By making use of Ohms law the answers where calculated correctly.

    Bertus
     
  8. hgmjr

    Moderator

    Jan 28, 2005
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    214
    Your answers are the same as those that I calculated.

    hgmjr
     
  9. pelorus32

    Thread Starter New Member

    May 2, 2010
    4
    0
    Many thanks to all of the people who assisted.

    No doubt I'll have other questions -;)

    Mike
     
  10. bertus

    Administrator

    Apr 5, 2008
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