# 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:

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:

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.

2. ### Lool Member

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

3. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,990
1,115
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
18,087
4,917
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.