Thevenin Equivalent
Engr 240, Spring 2004, By Sin Ming Loo
In PSpice, you cannot calculate the Thevenin equivalent with respect to a pair of nodes directly. In
order to replace a circuit with its Thevenin equivalent at a pair of nodes, one needs the open circuit voltage Voc (which is the Thevenin voltage, Vth), and the short circuit current, Isc (Using
Ohm’s law and Vth calculated, Rth can be determined). One of the methods that we can use to find
the open circuit voltage and short circuit current is by adding a dummy resistor to the given terminals. Enter the following circuit (Figure 1) into PSpice.
a
b
Figure 1
To determine the Thevenin equivalent of the above circuit, we are going to need a dummy resistor
to assist us. Add the dummy resistor (with unknown resistance!) to terminals a-b as shown in Figure 2. A ground (reference) is added at terminal b.
dummy_resistor
Figure 2
In order to find the open circuit voltage, set the resistance of the dummy resistor to very a large
value (1e12 Ω). Using Bias Point analysis, you should obtain a open circuit voltage of 32V. This
is shown in Figure 3.
dummy_resistor
Figure 3
To determine the short circuit current, set the resistance of the dummy resistor to a very small
value (1e-12 Ω). Using Bias Point analysis, you should obtain a short circuit current of 4A. This
is shown in Figure 4.
dummy_resistor
Figure 4
Now, apply Ohm’s law to find Rth. In this case, Rth is 8Ω.
Figure 5
The Thevenin equivalent problem that was solved in the previous example consists of independent sources. In the following example, dependent source are involved.
Enter the circuit shown in Figure 6. The similar circuit enter into PSpice is shown in Figure 7.
Figure 6
_
Figure 7
Once again, we need to find the open circuit voltage, set the resistance of the dummy resistor to
very a large value (1e12 Ω). Using Bias Point analysis, you should obtain a open circuit voltage
of 8V.
To determine the short circuit current, set the resistance of the dummy resistor to a very small
value (1e-12 Ω). Using Bias Point analysis, you should obtain a short circuit current of 8A.
With Ohm’s law, Rth is 1Ω. The Thevenin equivalent is shown in Figure 8.
1
8
Figure 8