ECE 240 – Electrical Engineering Fundamentals
PSPICE Tutorial #7
Pulse and Exponential Waveforms in PSPICE
In this tutorial, we will describe the use of the pulse and exponential waveforms as voltage
sources in PSPICE. In previous tutorials we have described the DC voltage source, VDC, and
the sinusoidal voltage source, VSIN. In each case we will want to observe voltages and/or
currents as a function of time. Thus, the Transient Analysis will be used.
Pulse Waveform
A voltage pulse or pulse train can be applied as an independent source in PSPICE using the
VPULSE element. VPULSE has seven parameters that describe its shape as shown in figure 1.
TR
TR
TF
TF
V2
PW
PW
V1
PER
TD
PER
Figure 1. Parameters of VPULSE
As an example we will simulate the simple RL circuit shown in figure 2 below. Note that the
voltage input is a single 1-volt pulse that is 1 second in duration.
vs(t)
11
i(t)
1
1
t
vs(t)
+
-
Figure 2. Circuit with Pulse Input
1H
The PSPICE schematic for this circuit is shown in figure 3.
R1
V1 = 0
V2 = 1
TD = 0
TR = 1E-9
TF = 1E-9
PW = 1
PER = 10
V1
V
2
1
L1
1
I
1
0
Figure 3. PSPICE Schematic
Note that the pulse parameters are set to be:
TD = 0 (no delay, start the pulse train at t = 0)
TR = TF = 1n (10-9 makes the rise and fall times almost zero)
V1 = 0 (pulse starts at zero)
V2 = 1 (rises to one volt)
PW = 1 (pulse width is 1 second)
PER = 10 (the pulse will repeat every 10 seconds)
The transient analysis is set to run for 10 seconds and the initial inductor current is set to zero.
Note that even though the pulse repeats in 10 seconds, the analysis ends before this repeat
occurs. Thus, we are effectively simulating a single pulse input. The results of the analysis
showing the input pulse voltage and the inductor current are shown in figure 4.
1.0
0.5
0
0s
-I(L1)
1.0s
V(R1:1)
2.0s
3.0s
4.0s
Time
Figure 4. Input and Output of RL Circuit with Pulse Input
5.0s
Exponential Waveform
A voltage pulse or pulse train can be applied as an independent source in PSPICE using VEXP.
The VEXP voltage source generates a waveform as shown in figure 5, where TC1 and TC2 are
the time constants of the rising and falling exponentials, respectively.
V2
TC1
TC2
V1
TD1
TD2
Figure 5. VEXP Model
The RL circuit shown above is modified to have an exponential input as shown in figure 6.
R1
V1 = 0
V2 = 1
TD1 = 1E-9
TC1 = 1E-9
TD2 = 1E-9
TC2 = 1
V1
V
2
1
L1
1
I
1
0
Figure 6. PSPICE Schematic with VEXP source
In this example we want to model a decaying exponential with a time constant of 1 second and
an initial value of 1 volt. In order to eliminate the rising portion of the VEXP waveform we have
set TD1, TD2 and TC1 equal to 1E-9. The resulting input exponential waveform and inductor
current found in the simulation are shown in figure 7.
1.0
0.5
0
0s
-I(L1)
1.0s
V(R1:1)
2.0s
3.0s
4.0s
5.0s
Time
Figure 7. Input and Output of RL Circuit with Exponential Source
Note that the pulse and exponential are also available as independent current sources using
IPULSE and IEXP respectively.