Impedance calculations. Multisim VS Hand-calculated

Discussion in 'Homework Help' started by JamesCA, Jul 13, 2015.

  1. JamesCA

    Thread Starter New Member

    Sep 4, 2013
    2
    0
    Hello all,

    i'm doing some impedance calculations, and what was going to be prove of concept has failed :) . The diagrams are pretty self-explanatory:

    [​IMG]

    The circuit, simulated in multisim. What i'm interested in is the output RMS voltage at probe 1 (with no load).

    My calculations:

    Xc = 1/(2*pi*f*c) = 1/(2*pi*100hz*1e-6) = 1591.5ohm
    R = 1k ohm

    for completeness:
    [​IMG]

    Z1 is just the real part of Z1 which is R = 1k
    Z2 is just the imaginary part of Z2 which is Xc= 1591.5 ohm

    which gives me, using voltage divider formula ==> vout = 20*(Z2/(Z1+Z2)) = 20*(1591.5/(2591.5)) = 12.28v RMS

    Multisim gives however 16,9 volts. I must be doing some fundimental error here. This is not homework, but seemed the best section to fit this in.

    I appreciate all comments in advance! Thanks
     
  2. Lool

    Member

    May 8, 2013
    117
    12
    I got the same answer you did.
     
  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Your calculation are wrong. The correct one looks like this

    Vout=Vin*\frac{Xc}{\sqrt{R^2+Xc^2}}=20*\frac{1.59}{\sqrt{1^2+1.59^2}}=16.9V
     
  4. Lool

    Member

    May 8, 2013
    117
    12
    Yep, we both forgot the j=sqrt(-1) for the capacitive reactance. Xc=-j/(2piC) which will give a phase shift and magnitude 16.9V
     
  5. WBahn

    Moderator

    Mar 31, 2012
    17,757
    4,800
    The sqrt(-1) is not part of the reactance.

    Z = R + j X

    where R is the resistance and X is the reactance.

    In this case, X = -1/(2piC)
     
  6. Lool

    Member

    May 8, 2013
    117
    12
    Yes, sorry for my miswording. I'm rusty on this topic.
     
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