How do I find the circuit impedance in Multisim?Primary Software: Electronics Workbench>>MultisimPrimary Software Version: 10.1.1 Primary Software Fixed Version: N/A Secondary Software: N/A
Problem:
Solution: Many of Multisim analysis have a powerful Add Expression feature that allows you to enter a mathematical expression. Since frequency is a variable to calculate impedance, you should run the AC Analysis. In the following example, we will analyze a simple RC circuit as shown in figure 1:  Figure 1: Simple RC Circuit Calculating the circuit impedance
Z= R+jXc
At 1Khz
Z = 1000 + j159
The polar form for Z is:
 From Ohm’s law we have
V=ZI
Working in polar form we have
 The impedance magnitude is give by
And the impedance phase is
 Multisim AC analysis setup:
1. Add a measurement probe on node 1, this allows the AC analysis to find current and voltage. On the menu select Simulate>>Instruments>>Measuring Probe, click on the node 1 wire to place a probe.
 Figure 2: Simple RC Circuit with Measurement Probes 2. On the menu select Simulate>>Analysis>>AC analysis>>Output>>Add Expression
3. Under Functions double click on mag()
4. In the Expression field, double between the brackets and under Variables double click on V(probe1)
5. Equation 1 should appear in the Expression field as shown below:
mag(V(Probe1))/mag(I(Probe1))
6. Click OK
7. Click Add Express again
8. The function ph() is phase, the expression for equation 2 is shown below:
ph(V(Probe1))-ph(I(Probe1))
9. Now select Simulate
10. Double click the upper plot Magnitude vs. Frequency on the Grapher View to make it the active plot.
11. Select View>>Show Cursors
12. Slide the cursor to 1Khz
 Figure 3: Grapher View
13. On the chart, Y1 shows magnitude = 1.012K and phase 157m which is approximately equal to the calculation.
Related Links: Developer Zone Tutorial: NI Multisim Fundamental Circuits Attachments:  Impedance.ms10
Report Date: 04/08/2009 Last Updated: 05/29/2009 Document ID: 4W7DH5MC |
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