Calculating the resulting resistance

Discussion in 'Homework Help' started by activee, Apr 20, 2014.

  1. activee

    Thread Starter Member

    Jan 16, 2014
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    Hey I'm not sure how to do this, I'm actually blocked at R4.
    [​IMG]

    I first thought I had to do like R4 and R5 weren't even there but I don't have the right answer.

    I did : R1 + (1/(1/(R2+R6) +(1/(R3+R7))) but that's not right. I got 3600.

    Could you help ? thanks
     
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Try redrawing the circuit from the prospect of kirchoffs current law
    t
     
  3. activee

    Thread Starter Member

    Jan 16, 2014
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    Do the current even go through R4 and R 5?

    Well there will be more current in R3 than in R2. Does that mean that there will be a ddp between the two ends of R4 ?
     
    Last edited: Apr 20, 2014
  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,386
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    You will have to do Delta-to-T transformation.

    R4 and R5 are the easy part. They are in parallel, so find equivalent resistance and replace R4 || R5 with their equivalent.
     
  5. activee

    Thread Starter Member

    Jan 16, 2014
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    like this ? R2, R3 and R4 in delta to Y. Same with R5,6,7.

    [​IMG]

    still not the good answer though
     
  6. amilton542

    Active Member

    Nov 13, 2010
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    This breed of finding equivalent resistance gets tricky here.

    Choose a voltage source of arbitrary amplitude and apply KCL and KVL to find the current through this source.

    Your equivalent network resistance will be V divided by the current through this source.
     
  7. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    I got 2769.4 Ohm.
     
  8. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    Replace R4 and R5 with a single 5000 ohm resistor; call it R45.

    Now do a delta-wye (same as delta-T) transformation on R45, R6 and R7. DO NOT transform R2, R3 and R4.

    I get 48700/17 which equals 2864.706
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    Your diagram in Post #5 is wrong. Look at the right hand transformation. Carefully label the three nodes of the delta circuit and then label the corresponding nodes of the wye circuit and ensure that they connect to the exact same nodes in the overall circuit that they did originally.
     
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