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422 ⏐⏐⏐ CAPACITORS
E
20 V
a
+
–
b
(a)
a
+
–
b
(b)
0
e
20 V
e
20 V
0 t
FIG. 10.75
Establishing a switching dc voltage level:
(a) series dc voltage-switch combination;
(b) PSpice pulse option.
V1
0
TD
TR
V2
PW
PER
TF
FIG. 10.76
The defining parameters of PSpice VPULSE.
t
t
applying a pulse waveform as shown in Fig. 10.75(b) with a pulse
width (PW) longer than the period (5t) of interest for the network.
A pulse source is obtained through the sequence Place part key-
Libraries-SOURCE-VPULSE-OK. Once in place, the label and all
the parameters can be set by simply double-clicking on each to obtain
the Display Properties dialog box. As you scroll down the list of attributes,
you will see the following parameters defined by Fig. 10.76:
V1 is the initial value.
V2 is the pulse level.
TD is the delay time.
TR is the rise time.
TF is the fall time.
PW is the pulse width at the V2 level.
PER is the period of the waveform.
All the parameters have been set as shown on the schematic of Fig.
10.77 for the network of Fig. 10.74. Since a rise and fall time of 0 s is
unrealistic from a practical standpoint, 0.1 ms was chosen for each in
this example. Further, since t � RC � (5 k�) � (8 mF) � 20 ms and
5t � 200 ms, a pulse width of 500 ms was selected. The period was
simply chosen as twice the pulse width.
Now for the simulation process. First the New Simulation Profile
key is selected to obtain the New Simulation dialog box in which
TransientRC is inserted for the Name and Create is chosen to leave
the dialog box. The Simulation Settings-Transient RC dialog box will
result, and under Analysis, the Time Domain (Transient) option is
chosen under Analysis type. The Run to time is set at 200 ms so that
only the first five time constants will be plotted. The Start saving data
after option will be 0 s to ensure that the data are collected immediately.
The Maximum step size is 1 ms to provide sufficient data points
for a good plot. Click OK, and we are ready to select the Run PSpice
key. The result will be a graph without a plot (since it has not been
defined yet) and an x-axis that extends from 0 s to 200 ms as defined
above. To obtain a plot of the voltage across the capacitor versus time,
the following sequence is applied: Add Trace key-Add Traces dialog
box-V1(C)-OK, and the plot of Fig. 10.78 will result. The color and
thickness of the plot and the axis can be changed by placing the cursor
on the plot line and performing a right click. A list will appear in which
Properties should be selected; then a Trace Properties dialog box will
appear in which the color and thickness of the line can be changed.
Since the plot is against a black background, a better printout occurred
when yellow was selected and the line was made thicker as shown in
Fig. 10.78. Next, the cursor can be put on the axis, and another right
click will allow you to make the axis yellow and thicker for a better
printout. For comparison it seemed appropriate to plot the applied pulse
signal also. This is accomplished by going back to Trace and selecting
Add Trace followed by V(Vpulse:�) and OK. Now both waveforms
appear on the same screen as shown in Fig. 10.78. In this case, the plot
was left a greenish tint so it could be distinguished from the axis and
the other plot. Note that it follows the left axis to the top and travels
across the screen at 20 V.
If you want the magnitude of either plot at any instant, simply select
the Toggle cursor key. Then click on V1(C) at the bottom left of the
screen. A box will appear around V1(C) that will reveal the spacing
between the dots of the cursor on the screen. This is important when