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FIG. 9.119
Using PSpice to determine the Thévenin voltage for a network using a very
large resistance value to represent the open-circuit condition between the
terminals of interest.
result in 3 V across the 1-T resistor. The open-circuit Thévenin voltage is
therefore 3 V which agrees with the solution of Example 9.10.
Maximum Power Transfer The procedure for plotting a quantity
versus a parameter of the network will now be introduced. In this case
it will be the output power versus values of load resistance to verify the
fact that maximum power will be delivered to the load when its value
equals the series Thévenin resistance. A number of new steps will be
introduced, but keep in mind that the method has broad application
beyond Thévenin’s theorem and is therefore well worth the learning
process.
The circuit to be analyzed appears in Fig. 9.120. The circuit is constructed
in exactly the same manner as described earlier except for the
value of the load resistance. Begin the process by starting a New Project
called MaxPower, and build the circuit of Fig. 9.120. For the moment
hold off on setting the value of the load resistance.
The first step will be to establish the value of the load resistance as
a variable since it will not be assigned a fixed value. Double-click on
the value of RL to obtain the Display Properties dialog box. For
Value, type in {Rval} and click in place. The brackets (not parentheses)
are required, but the variable does not have to be called Rval—it
is the choice of the user. Next select the Place part key toobtain the
Place Part dialog box. If you are not already in the Libraries list,
choose Add Library and add SPECIAL to the list. Select the SPE-
CIAL library and scroll the Part List until PARAM appears. Select
it; then click OK to obtain a rectangular box next to the cursor on the
COMPUTER ANALYSIS ⏐⏐⏐ 363