ECE3413:
MAXIMUM POWER TRANSFER
Exercise shw2
vers 1.1
Context of exercise: Thevenin theorem and power transfer
Electronic circuits is of the generic form of a source and a load as shown by figure 2.1. The source is
usually an extended network which may contain several electrical energy sources. The load is passive,
usually designated by a dissipative component as represented by resistance RL.
Figure 2.1(a) Source and load context
Thevenin’s theorm reduces the source to an equivalent single voltage source Vth with equivalent source
resistance Rth.
Figure 2.1(b) Thevenin Theorem context
Requirement:
1. (10 pts) Set up figure 2-3 as your figure 2-1. It is a generic test platform for assessment of The
Thevenin theorem and maximum power transfer. Note that the resistance R2 has a default that essentially
eliminates it (GΩ is equivalent to an open circuit = no resistor). Screen snapshots of these circuits are
accomplished by the Edit > copy-to-clipboard feature of pSpice.
Figure 2-3. Test platform for Thevenin theorem.
Set up a DC sweep with Rx as the sweep variable. Note that you will have to select the ‘Sweep Var.
Type’ as ‘Global Parameter’. The sweep should be from 0.1k to 50k in increments of 0.05k. One caveat
of using resistance as a parameter is that zero resistance is NOT allowed since it is also an infinite
conductance. Circuit simulators use a technique called modified nodal analysis for which an infinite
conductance will not compute.
2. (10pts) Execute the simulation. Display the power dissipated in resistances R1 and R3 and the power
contributed by V1. For the power dissipated in the resistances it is recommended that you use the
formABS(I(R3)*V(VL) as shown by figure 2-4 below.
Show two figures (as your figures 2.2(a) and (b)), one with linear x-axis and one with logarithmic x-axis.
Mark the peaks for the power to the load in each case with cursors. Your figures should look something
like figure 2-4 (below).
Figure 2-4. Power transfer vs variation of load RL. Figures (a) and (b) are the same except for axis type
choice.
3. (10pts) Using the menu ANALYSIS>SETUP>PARAMETRIC and Rs as a Global Parameter step
R1 through the values 4k,6k,12k, which you will see is one of the options under ‘Sweep Type’. Display
your results (your figure 2.3) in the same way as you did for part 2 (two figures, one with linear x-axis,
one with logarithmic x-axis) but only for power to the load. Determine the maximum PL and value of RL
for each of the peaks and include in an Excel table (your table Table 2-1) along with an analytic
calculation for comparison.
4. (10pts) Change the value of R2 from Rz = 99G to Rz = 12k and repeat part 3. Show as your figure
2.4. Enter results in your table 2-1.