# lab University of Texas Pan American

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```					University of Texas - Pan American                                     Electrical Engineering
LAB 7: Introduction to PSpice Transient Simulation

A. OBJECTIVES

   Gain further practice in electric circuit analysis
   Become familiar with PSpice abilities to model transient and AC circuits

B. EQUIPMENT REQUIRED

   Computer with PSpice or similar product loaded. The instructions given here are
for OrCad PSpice, student version

C. PARTS REQUIRED

   None

D. PRIOR TO LAB

NOTE: this is an individual instruction lab. Each person will be assigned a slightly
different circuit to analyze and then simulate.

I1           VA
R1

VPULSE                          C1

Figure 1 - Circuit to be used in this lab

1. Consider the circuit shown in Figure 1 above. The pulse source, resistor and capacitor
are individually assigned based on your student ID number and Table 1 on the next page.
6th digit of   Pulse Height    7th digit of    R1               8th digit of   C1
ID Number      (Volts)         ID number                        ID number
0              3V              0               1K               0              10uF
1              4.5V            1               1.5K             1              6uF
2              6V              2               2K               2              5uF
3              7.5V            3               2.5K             3              4uF
4              3V              4               3K               4              3uF
5              4.5V            5               4K               5              2uF
6              6V              6               5K               6              2uF
7              7.5V            7               6K               7              1.5uF
8              3V              8               7K               8              1.5uF
9              4.5V            9               8K               9              1uF
Table 1:   Values to be used in circuit of Figure 1

2. Considering the bottom node to be the reference or zero node, find an expression for
the voltage VA and the current I1 using a pencil-and-paper calculation. You must
complete this step before proceeding to the simulation.

E. IN THE LAB

1. Modify the circuit from the previous part by adding a 1mH inductor L as shown in
Figure 6 below. This is classified as a second order circuit because it has two energy
storage elements, the L and the C. This type of circuit can produce an oscillating voltage
if the rate of power loss in the resistor is small compared to the peak energy stored in the
capacitor and inductor. The capacitor and inductor periodically exchange energy, losing
a small amount of energy into the resistor at each cycle.

The resulting waveform is a damped sinusoid of the general form

V (t ) Ae  at cos(t   )

R                             Scope
Function
Generator
L           C
Figure 6: Resistor-inductor-capacitor (RLC) circuit

2. The output waveform produced in response to a falling edge should resemble a
damped sinusoid. You will need to adjust the oscilloscope scales and triggering to get a
good display.

3. From the display, solve for:

a. the frequency  (from the period)
b. the exponential constant a (from the envelope, which is a smooth curve
drawn through the peaks of the sinusoid.)

4. Make a careful, accurate sketch of the oscilloscope display for your notebook.

Start PSpice

1. Log in to a workstation that has OrCad PSpice installed. If there are regular and
student versions, select the student version.

2. Under the PSpice group in the Windows Start menu, select and open the Capture
program. This is a schematic capture tool that can be used to enter circuit diagrams
either for simulation, record keeping, or printed circuit layout.

3. When the program opens, go to New and Project... A pop-up box will give you a
choice of several project types. Select the first option for Analog A/D. This tells PSpice
that you are planning to do simulations as opposed to a schematic only or a printed circuit
layout.

In this pop-up you will also need to choose a project name (your choice) and a location to
store the files.

When you finish it will present a second pop-up asking if you want to base the project on
an existing project, or open a blank project. Choose the Blank project.

4. After you finish with the above step at least two windows should open up. One is a
project map showing all the files involved; the other is a blank schematic.

Drawing the Schematic

5. The first thing you will need to do is add some parts to draw a schematic. There are
many ways to do this, the most basic is to go to Place and Parts...
A popup box will appear with a menu of parts; however, the menu will most likely be
empty. That is because you need to add some libraries of parts. Use the Add Libraries
option twice to load the following libraries:

Source
Analog

6. The libraries you have loaded will appear in the lower list box. When you click on
them, a list of parts available will appear in the upper list box.

7. To add a pulsed voltage source, find the VPULSE part in the Source library. It will
place a voltage source on the schematic, which you can move around using the mouse.
You can click on the voltage values beside the part, or click on the part itself, to change
the values. The important parameters are as follows

TR
V2

V1
TF
TD

PW
PER

In this lab we are trying to model a step function. Set V1=0 and V2 to the pulse height
from Table 1. We want the pulse to start immediately and rise instantly, so set TD=0,
TR =0 (rise time). The PW should be long enough that the transient response has had
time to get very close to steady state, which means PW >>the time constant.
Therefore, calculate, and then set PW to be more than 10 times the time constant. The
period PER should be longer than the pulse width PW.

8. Add the remainder of the components using the methods you learned in the previous
PSpice lab. Remember to rename the ground to "0".

Setting up the Simulation

9. Under the PSpice menu choose New Simulation Profile... Pick a name, after which a
pop-up with several tabs will appear. On the first tab, you can choose the simulation type
(Time Domain/Transient, AC Sweep, DC Sweep, Bias Point, and possibly others). For
this lab choose Transient.

You will need to tell PSpice the length of time to simulate. Click on the tabs and find the
tab that allows you to set the transient simulation time. Set the final time to five (5) times
the time constant .

Markers

10. Use the V button on the main toolbar to add a voltage \ markers. Put the marker on
the positive terminal of the capacitor.

Running the Simulation

11. Under the PSpice menu choose Run.

12. If all is going well, a new window with a graph will appear, with messages at the
bottom indicating a successful simulation. You should see a plot of the voltage versus
time. Print this page.

13. If you get an error, go to Window and view the Session Log. The error messages
tend to be cryptic so feel free to call an instructor for help if you reach this point.

Repeat for current

14. Change the voltage marker to a current marker and repeat the simulation.

Comparison

15. Your results should match your calculations almost exactly since PSpice uses the
same method, nodal analysis, that you used for your calculation. If they do not, you will
need to find and resolve the discrepancy.

16. Repeat the previous steps to start a New Project in order to do a New Simulation for
the circuit shown in the next figure (The results should be the same as the ones obtained
in the previous lab given that is the same circuit and component values.

17. For this part you will need to use and inductor value of 1mH, a capacitor value of
0.01μF, a resistor value of 10KΩ and VPULSE values of TR=0, TF=0. PER=2ms,
PW=1ms, V1=0V and V2=5V.

18. Remember you will need to tell PSpice the length of time to simulate. Click on the
tabs and find the tab that allows you to set the transient simulation time. For this part
choose your own time in order to show 3-4 periods.
VB
R

VPULSE                               C

19. Compare your results with the results obtained in the previous lab. Note: for this
schematic you don’t need to do a pencil-and-paper calculation.

F. AFTER THE LAB

Turn in a brief paper with (a) your hand drawn schematic including values, (b) your
detailed calculation using nodal analysis, and (c) a PSpice printout with voltages and
currents shown.

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