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362 ⏐⏐⏐ NETWORK THEOREMS
FIG. 9.118
Using PSpice to determine the Thévenin resistance of a network through the
application of a 1-A current source.
can be used to determine the magnitude of the Thévenin resistance in
the following manner:
�RTh� � � �V
s
�
I � s
� � � Vs
�
1 A � � �Vs� In Eq. (9.16), since Is � 1 A, the magnitude of RTh in ohms is the same
as the magnitude of the voltage Vs (in volts) across the current source.
The result is that when the voltage across the current source is displayed,
it can be read as ohms rather than volts.
When PSpice is applied, the network will appear as shown in Fig.
9.118. The voltage source E1 and the current source are flipped using a
right click on the source and using the Mirror Vertically option. Both
voltage sources are set to zero through the Display Properties dialog
box obtained by double-clicking on the source symbol. The result of the
Bias Point simulation is 2 kV across the current source. The Thévenin
resistance is therefore 2 k� between the two terminals of the network
to the left of the current source (to match the results of Example 9.10).
In total, by setting the voltage sources to 0 V, we have dictated that the
voltage is the same at both ends of the voltage source, replicating the
effect of a short-circuit connection between the two points.
For the open-circuit Thévenin voltage between the terminals of interest,
the network must be constructed as shown in Fig. 9.119. The resistance
of 1 T (�1million M�)isconsidered large enough to represent an
open circuit to permit an analysis of the network using PSpice. PSpice
does not recognize floating nodes and would generate an error signal if a
connection were not made from the top right node to ground. Both voltage
sources are now set on their prescribed values, and a simulation will
Th