ECE 240 – Electrical Engineering Fundamentals
PSPICE Tutorial 2
Determining Equivalent Resistance with PSPICE
This tutorial presents a method for using PSPICE to verify the equivalent resistance of a circuit.
It assumes that you have completed PSPICE Tutorial 1 and understand how to perform a basic
DC Bias Analysis of a resistive circuit.
In class, the equivalent resistance of the circuit shown in figure 1 was determined to be 10Ω.
51
101
71
41
31
121
71
Req
Figure 1. Resistive Circuit
By definition, if we apply a DC voltage source of value VDC to a resistive circuit as shown in
figure 2, and measure the current leaving this source, IDC, the equivalent resistance of the circuit
is:
Req =
VDC
I DC
IDC
VDC
+
-
Resistive
Circuit
Req
Figure 2. Equivalent Resistance
We can use the following procedure to determine the equivalent resistance of the circuit in figure
1 using PSPICE.
1. Enter the circuit shown in figure 3 into PSPICE. Note that a 1-volt DC source has been
added to the input of the resistive circuit.
Figure 3. PSPICE Version of Circuit
2. Run the DC bias analysis as described in PSPICE Tutorial #1. Display the current values
on the circuit. The results shown in figure 4 should be obtained.
Figure 4. Results of DC Bias Analysis
3. Note that the current leaving the 1V source is found to be 100mA. Thus, using the
formula given above:
Req =
VDC 1
= = 10!
I DC .1
4. The equivalent resistance can also be verified by constructing the equivalent circuit as
shown in figure 5 below. Note that both 1V sources have the same current. This verifies
that the 10Ω resistor in the bottom circuit is equivalent to the total resistance in the top
circuit.
Figure 5. Comparison of a Resistive Circuit with an Equivalent Resistance