Download Pspice Lecture5_v1 194KB Sep 20 2011 08:28:26 AM

PSpice
Lecture 5 – Comparing DC Circuits
Date: 19 September 2011
Lecture 5A
1. PSPICE can be used to compare two circuits.
Suppose we make change to a circuit and would like to compare the modified
circuit to the original circuit.
2. First draw the original circuit as below:
3. Select F from the ANALOG library and use CTRL-F and CTRL-R to flip and
rotate CCCS.
4. Connect the left circuit as shown above. Note that the two left input of F are not
joined together.
5. Continue to finish the circuit as shown below. Remember to add ANALOG
GROUND from the PORT library.
6.
Select “BIAS POINT DETAIL” from the Analysis>>Setup Menu. Run
Simulation from the Analysis Menu.
7. Write down the voltages for node 1 and 2 are -5.882V and -12.24V.
Lecture 5B (Equivalent Resistor)
8. From the circuit above, note that resistor 25Ω is in series with the 15 Ω and
parallel to the 10Ω resistor. These three resistors form an equivalent resistor of
8Ω.
9. Create another Schematic file, Cut and Paste the previous circuit and replace the
three resistors to only an 8Ω equivalent resistor.
10. Run Simulation from the Analysis Menu.
11. Examine the output. Verify the simulation results are correct.
Note that the voltages for node 1 and 2 are -5.882V and -12.24V also. Similar to
the previous circuit with three resistors.
12. Now report the result.
The results for the equivalent resistor are similar to the previous circuit.
Lecture 5C (Thevenin Equivalent Circuits)
13. Continue for the previous circuit, Thevenin Equivalent is determined by finding a
value for Voc (Open Circuit Voltage) and Rth (Thevenin Resistance).
14. To calculate Voc, use an open circuit represented by a resistor having infinite
resistance, is connected to terminal a and b.
15. To calculate Rth, we need to calculate the Isc (Short circuit current) first. A short
circuit, represented by a 0V voltage source, is connected to terminals a and b.
Rth is then equal to
16. Now use OrCAD to describe the circuit having infinite resistance.
(Saved the previous OrCAD circuit as Lecture5C)
17. Change the resistor R4 (8Ω) with very large resistance.
Use 5G (Giga ohm)
18. Make sure Analysis>>Setup is selecting “BIAS POINT DETAIL”.
19. Run Simulation and take down the Voc which is -8.667V.
20. Now saved the circuit as Lecture5D.
21. Copy and paste the circuit and place it below the current circuit.
22. Change the 5GΩ resistor of the second circuit to short circuit (0VDC from the
SOURCE library)
23. Run Simulation.
24. Take down the Isc value, which is 3.714A
25. So the Thevenin resistance is
26. Verify the simulation results are correct.
27. If R for the Voc is similar to Rth as shown below, the Voc would be half of the
result, which is -8.667/2 = -4.334V.
28. Try change the R of the top circuit to -2.334Ω and run the simulation again.
29. The new Voc is -8.667/2 =- 4.334V which prove the simulation results for Voc is
correct.
30. For the circuit at the bottom, if Vab=Voc, Isc would become 0A.
31. Now change the 0V of the bottom circuit to -8.667V and run Simulation.
32. Note that Isc is now become 142.86uA which is almost 0A. This also prove that
the simulation result for Isc is correct.
33. Final step is to report the Thevenin equivalent circuit as
Isc = 3.714A