Using PSpice to find the Transient Response
Objective
After going through this exercise, each student will learn how to use PSpice to
find the transient response for various circuits. Specifically, this handout shows how to
find the transient response of first order RL and RC circuits.
Procedure
Open PSpice Schematics and click on ‘Draw => Get New Part’. Scroll down
through the parts list and choose an inductor (represented by the symbol ‘L’). Place this
part on your worksheet. Similarly, place a resistor on your worksheet (symbol ‘R’) and a
ground (symbol ‘GND_EARTH’).
Finally, place a squarewave source (symbol
‘VPULSE’). Click on ‘Draw => Wire’ to bring up the wiring tool, and connect the
circuit elements together to obtain a circuit that looks like the one shown in Figure 1.
Note that the voltage marker can be added by selecting ‘Markers => Mark
Voltage/Level’. Tip: You can rotate a part by pressing ‘Control-R’.
Figure 1: RL Circuit
Now, if you have not already done so, set the resistance to 1 k? and the
inductance to 0.2 H by double-clicking on the respective value. Next, double click on the
voltage source. For the first voltage, ‘V1’, type in 0 and then click on ‘Save Attribute’.
Similarly, type in 8 for the second voltage, ‘V2’, and save the attribute. Next set both the
rise time, ‘TR’, and fall time, ‘TF’, to 0.000001. Set the pulse width, ‘PW’ to 0.001 and
the period, ‘PER’ to 0.002. Click on ‘Ok’ to exit this menu—the voltage source has been
properly set up to generate a 500 Hz waveform.
Now, click on ‘Settings => Setup’. Check the box next to ‘Transient’. Click on
the ‘Transient’ button. The ‘End Time’ denotes how far out the simulation will go in
time, and the print step determines the spacing between each point used in the graph. For
this problem, use an end time of 0.1 seconds and a print step of 0.00001 seconds. Click
‘Ok’ to exit all of the pop-up menus.
Click on ‘Analysis => Simulate’. At this point the probe window should pop up
and show you the voltage drop across the inductor. If you want to zoom in, one effective
method is to use the ‘zoom area’ feature—click on ‘View => Area’. Now, simply draw a
box on the plot around the area you would like to see in greater detail. Verify the time
constant by inspecting the transient response.
Now, draw the RC circuit shown in Fig. 2. Once the circuit is constructed
properly, click on ‘Analysis => Simulate’. Note: The capacitor component is simply
represented by ‘C’. Verify the time constant by inspecting the transient response.
Figure 2: RC Circuit