Voltage Divider Rule question

Discussion in 'General Electronics Chat' started by Xufyan, Sep 13, 2011.

  1. Xufyan

    Thread Starter Member

    Aug 3, 2010
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    Please look at the attached circuit ,

    According to Voltage divider rule ,

    V1 should be equals to,

    10volts x (33k + 68k) / ( 33k + 68k + 100k ) = 5.02volts .

    but the circuit says its 2.812 volts

    and V2 equals to,

    10volts x (68k) / ( 33k + 68k + 100k ) = 3.38volts .

    but according to the circuit its 1.641 volts,
    please explain where i am wrong :confused:
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Current will flow in the two 47k resistors which will upset the divider relationship.
     
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  3. Tealc

    Member

    Jun 30, 2011
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    I'm completely new to this but simulating the initial divider circuit in Multisim with just the 100k, 33k and the 68k give the results you have calculated but as soon as I add in the 47k and additional 100k the voltage on my meters changed and when adding the op-amp in they changed again to the figures you found in your simulation.

    As a guess I would say that for the bottom meter reading the 47k and 100k are working in parallel with the 68k to change the voltage divider values.
     
  4. Xufyan

    Thread Starter Member

    Aug 3, 2010
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    can you please drive the equation, how 47k resistors effect the voltage ?
     
  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    The attached analysis may help with understanding why the values aren't what you expect.

    The difference amplifier effectively loads the voltage divider.
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    The image didn't show that well - I'll try again
     
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  7. Xufyan

    Thread Starter Member

    Aug 3, 2010
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    thanks alot !!!! Great Explanation !!! Awesome !!!! :) :)
     
  8. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Why wouldn't the 100K in the lower leg play a roll as well, given it connects to ground??
     
  9. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    It does - included in my analysis.
     
  10. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Originally you didn't mention it, so I skipped the analysis, I'll read it now!

    Thanks
     
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