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Neither parallel nor series

Discussion in 'Electronic Basics' started by Wong, Oct 20, 2004.

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  1. Wong

    Wong Guest

    What theorem or method can be applied to calculate the total
    resistance at point A to B?

    ----- A -----
    | |
    | |
    R1 R2
    | |
    | |
    ------R3------
    | |
    | |
    R4 R5
    | |
    | |
    ----- B -----

    Thanks.
     
  2. burbeck

    burbeck Guest

  3. Jack// ani

    Jack// ani Guest

    Star to delta conversion will work here.

    Your above star connection can be converted into equivalent delta
    connection, like this



    RB
    ___
    |--------------|---|___|----------------|
    | | | |
    | | | |
    | | | |
    .-. .-.
    B RA | | | | RC A
    | | | |
    | '-' '-' |
    | | | |
    | ___ | | ___ |
    |----|___|-----|-----------|-----|___|--|

    R5 R2

    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


    RA = ( R4*R1 + R1*R3 + R3*R4 ) / R1

    RB = ( R4*R1 + R1*R3 + R3*R4 ) / R3

    RC = ( R4*R1 + R1*R3 + R3*R4 ) / R4


    Now it is easy enough, for you to further simply it.
     
  4. KM

    KM Guest

    Use Delta to STAR conversion,

    the result became, and it is series with parallel series.

    A > | | |

    Ra = (R1+R2)/(R1+R2+R3)
    Rb = (R1+R3)/(R1+R2+R3)
    Ra = (R2+R3)/(R1+R2+R3)


    it is in Electrical technology by Edward Hughes

    KM
     
  5. Wong

    Wong Guest

    I think there is a correction here, Ra should equal to
    (R1*R2)/(R1+R2+R3) and Rb, Rc as well.
    So the total resistance is (Ra + ((R4+Rb) || (R5+Rc)))
    Let say, R1=1k, R2=2k, R3=3k, R4=4k and R5=5k. I will get the answer
    of 2.9k and it is same with my simulation. Thanks !!
     
  6. Jack// ani

    Jack// ani Guest

    whoops, what an ugly mistake i did; feeling very shameful.
    should be delta to star. really frustrating isn't it……should go and die somewhere.
     
  7. If that is the biggest mistake you make this year, you are doing
    better than most of us.
     
  8. Jack// ani

    Jack// ani Guest

    well i hope so.
    thanks
     

  9. You forgot to say "I don't like doing my own homework, please
    do it for me."


    And the correlary: if this ISN'T a homework problem, you
    should say so. It also would help if you mentioned what
    you've tried so far, and whether you're an absolute beginner
    or not.
     
  10. Bill Bowden

    Bill Bowden Guest

    I'm not following these formulas.
    If all the resistors R1-R5 are 1 ohm, then the
    total resistance from a to b will be 1 ohm.
    But, according to the formulas, Ra, Rb ,Rc will
    be 2/3 or 0.67 ohms. So, we have 1.67 in parallel
    with 1.67 which is 0.83 which is in series with
    Ra (0.67). That all adds up to 1.5 ohms, but it
    should be 1 ohm.

    What am I missing?


    -Bill
     
  11. Don Kelly

    Don Kelly Guest

    ---------
    Nothing:

    The formula is wrong for delta to star conversion- try Ra=(R1*R2)/(R1
    +R2+R3) etc
    This will give you 1 ohm.

    In the case where R1/R4 =R2/R5, you can simply remove R3 as it won't have
    any effect. (figure out why).
     
  12. Rich Grise

    Rich Grise Guest

    Because the current's flowing both ways simultaneously, and so, from the
    POV of our perceptive systems, cancel each other out and so don't exist.

    Cheers!
    Rich
     
  13. John Fields

    John Fields Guest

    ---
    Puh-leeze...

    If R1/R4 = R2/R5, then the voltages at the junctions of R1R4 and R2R5
    will be equal, and with no potential difference across R3 current
    can't flow through it, whether it's an open, a short, or anything in
    between.

    As a matter of fact that's why, at null, the detector across a
    Wheatstone bridge vanishes.
     
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