Combining series and parallel resistors

Discussion in 'Homework Help' started by WayneDegu, Feb 7, 2014.

  1. WayneDegu

    Thread Starter New Member

    Feb 6, 2014
    7
    1
    Dear Forum...

    I am working on electronic circuits and really struggling to determine when resistors are in series or parallel in a mixed circuit.

    Here is a question from an "All About Circuits" worksheet.

    Which resistors do I combine together as parallel or series? I'm o.k. with all the calculations i.e. Ohms law, but it is just deciding how to pair off resistors that is causing problems.

    It is question 40 on this sheet:

    http://www.allaboutcircuits.com/worksheets/dc_sp.html

    Thanks in advance.

    Wayne
     
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  2. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    Here is the original drawing:

    [​IMG]


    Would it help if we "transform" as shown below:

    [​IMG]

    Bertus
     
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  3. WayneDegu

    Thread Starter New Member

    Feb 6, 2014
    7
    1
    Hello Bertus...

    That's what I thought of doing. So, if R1 and R2 are in parallel, their combined resistance is 333.33 ohms. Then, combining in series with R3 and R4 we get:

    333.33 + 150 + 450 = 933.33 ohms....which is not the 875 ohms quoted in the answer.

    Am I missing something?

    Thanks again,

    Wayne
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    Have a close look at the schematic.

    R3 and R4 are in series.
    This combination is parallel with R1.
    This combination of R1, R3 and R4 is in series with R2.

    Bertus
     
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  5. WayneDegu

    Thread Starter New Member

    Feb 6, 2014
    7
    1
    Dear Bertus...

    Many thanks...that seems to work out o.k.

    How did you get the schematic from the original? Is it anything to do with the position of the battery?

    Thanks again,

    Wayne
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    Nothing will change as long as the nodes stay the same.
    I only moved the battery and a resistor outside the loop to make it more clear.

    [​IMG]

    Bertus
     
    WayneDegu likes this.
  7. WayneDegu

    Thread Starter New Member

    Feb 6, 2014
    7
    1
    Thanks Bertus, much clearer now.

    Wayne
     
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