Experiment # 4
Thevenin’s and Maximum Power Transfer Theorem
Objective
The objective of this experiment is to study the principles of Thevenin’s Theorem and
Maximum Power Transfer Theorem. This will be accomplished by first analyzing the
particular circuits to be used in this experiment numerically in the preliminary lab
exercise. These same circuits will then be experimentally examined in the lab. The
theoretical and experimental values will then be compared.
Preliminary Lab Exercise
Thevenin’s Theorem
The circuit shown in the following Figure 1 will be used to investigate Thevenin’s
Theorem.
RL
1k - 10 k
R1
A
R3
4.7 k
B
R5
10 k
5.1 k
VS2
VS1
15 V
R2
R4
20 k
10 V
36 k
0
Figure 1
1. Compute the open-circuit (Thevenin’s) voltage, VOC = Vth, with respect to the
terminals A and B with the load resistor RL removed.
2. Compute the Thevenin resistance, Rth, with respect to the terminals A and B by
replacing the voltage sources with their internal resistances. Given Vth and Rth,
compute the short-circuit current ISC.
3. Compute the short circuit current directly by placing a short-circuit between the
terminals A and B. Compare this result with that obtained in the previous step.
4. Use PSPICE to construct the above circuit with RL removed. Run the dc analysis and
compare the voltage you obtained between terminals A and B with the open circuit
voltage you obtained in step 1. Now using the PSPICE again, construct the above
circuit without RL and placing a short circuit between the terminals A and B. Run the
dc analysis and compare the current flowing through the short circuit between A and
B with the ISC you obtained in steps 2 and 3.
Maximum Power Transfer Theorem
The circuit shown in the following Figure 2 represents the Thevenin equivalent circuit of
the circuit shown in Figure 1 with a load resistor RL connected between the terminals A
and B.
A
Rth
xk
Vth
y V
RL
1k - 10k
B
0
Figure 2
1. Determine what value of RL will achieve maximum power transfer to the resistor RL.
2. Compute the maximum power transferred to the resistor RL.
3. Calculate the value of the voltage VAB when a variable resistor is placed between the
terminals A and B. Let the variable resistor vary from 1 k to 10 k in 1 k increments.
Use the following table to put down your calculated values:
RL(k Ω)
1
2
3
4
5
VAB(Volts)
PRL(µWatts)
RL(k Ω)
6
7
8
9
10
VAB(Volts)
PRL(µWatts)
Procedure
The circuits of Figures 1 and 2 will now be used to verify the principles of Thevenin’s
theorem and maximum power transfer.
Test Circuit
1. Construct the circuit shown in Figure 1.
2. Measure the voltage VAB using a digital multi-meter in voltmeter mode. This is the
open-circuit voltage or Thevenin’s voltage Vth.
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3. Measure the short-circuit current ISC by placing the digital multi-meter in ammeter
mode between terminals A and B. Measure the Thevenin resistance, Rth, by
disconnecting the voltage sources from the circuit and reconnecting the resistor
terminals to the proper nodes. Remember to turn the power off before making any
changes to the circuit or the digital multi-meter.
4. Place a decade resistance box across the terminals A and B. Measure the voltage VAB
when the variable resistor is set at discrete values from 1 kΩ to 10 kΩ in 1 kΩ
increments.
Thevenin Equivalent Circuit
1. Construct the circuit shown in Figure 2 using the value of Vth and Rth measured in
steps 2 and 3 of Test Circuit.
2. Place a decade resistance box across the terminals A and B. Measure the voltage VAB
when the variable resistor is set at discrete values from 1 kΩ to 10 kΩ in 1 kΩ
increments.
Analysis
Thevenin Equivalent Circuit
1. Compare the measured values of Vth, Rth, and ISC with the theoretical values.
2. Compute Rth from the measured values Vth and ISC and compare with the measured
value of Rth.
3. Compare the measured values of VAB for the results of step 4 of Test Circuit and step
2 of Thevenin Equivalent Circuit (Procedure) with the theoretical results.
Maximum Power Transfer
1. Plot the power delivered to the decade resistance box as a function of the decade
resistance for the data collected in step 4 of Test Circuit and step 2 of Thevenin
Equivalent Circuit (Procedure).
2. From the plot determine the value of resistance which corresponds to maximum
power transfer and compare this with the theoretical value.
3. Compare the value of maximum power transferred to the load resistor with the
theoretical value.
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