3.1 Thevenin’s Theorem
Objectives
1. To show the use of Thevenin’s theorem in circuit analysis.
2. To prove Thevenin’s theorem by experiment
Materials Required



DMM
Power supply: low-voltage, variable, regulated
Resistor, one each, 1.2kΩ, 3.3KΩ, and 10kΩ
Procedure
1. Construct the circuit shown in Figure 1. Let consider R3 represents a load.
R1
1.2k
+
12V
R2
_
10k
R3
3.3k
Figure 1
2. Measure the current through R3 and the voltage across R3. Record them.
IR3 = ____________ VR3 = ____________
3. Now, by using Thevenin theorem, we will calculate the current through R3. The
procedure is given step by step. Show all work in the space provided. First, we will
calculate the VTH. To find VTH, we remove R3 and find the voltage at open terminal ab as shown in Figure 2. Based on this figure, determine VTH. Show all work.
VTH = _______________(Calculated)
R1
a
1.2k
12V
+
_
+
R2
10k
VTH
_
b
Figure 2
4. Verify the actual VTH by measurement. Construct the circuit in Figure 2 and record
VTH.
VTH =
V (measured)
5. Next, we will calculate the RTH. To find RTH, we remove R3 and turn off the voltage
source (i.e. replaced by a short circuit) as in Figure 3. Based on this figure, determine
RTH, the equivalent resistance looking from terminal a-b. Show all work.
R1
a
1.2k
R2
RTH
10k
b
Figure 3
RTH =
kΩ (calculated)
6. Verify the actual RTH by measurement. Construct the circuit in Figure 3 and record
RTH.
RTH =
kΩ (measured)
7. Draw the Thevenin equivalent circuit using the calculated VTH and RTH in the box
below. This circuit is Figure 4.
Figure 4
8. Based on Figure 4, calculate I R3 .
IR3 = __________(calculated)
9. Construct circuit in Figure 4 . Obtain a resistor for RTH as close as possible to its
calculated value (or use a potentiometer).
Measure the current through R3 and the voltage across R3. Record them.
IR3 = __________(measured)
VR3 = __________(measured)
Question & Answer
1. Compare the current measured in Steps 2 and 8. Are they close? If not, find the
reason for the discrepancy.
2.
Compare those measured results in Step 2 with the measured results in Step 9. Are
they close? If not, find the reason for the discrepancy.
Conclusion
(In your own words describe the purpose of the experiment and what you have learned from the
experiment)
3.2 Power And Its Measurement
Objectives
1. To calculate power absorbed by individual components
2. To calculate power absorbed by the whole circuit.
3. To prove the power calculations through experiment.
Materials Required



DMM
Power supply: low-voltage, variable
Resistors, one each, 680 and 470 Ω
Procedure
1. Connect the circuit as shown in Fig.3.1.1.
2. Measure the loop current: IT =
3.
mA. Indicate the loop current in the circuit.
Measure the voltage drop across each resistor: VR1 =
V; VR2 =
V.
4. With the measurements obtained in step 2 and 3, calculate the power dissipated by each
resistor: PR1 =
W; PR2 =
W.
5. With the values from step 5 calculate the total circuit power dissipation: PT =
Show your calculations.
W.
6. Using the power supply voltage and resistances of R1 and R2, calculate the circuit current and
voltage drops. From these figures; calculate the power dissipated by R1 and R2: PR1 =
W;
PR2 =
W.
Question & Answer
1. Did the calculations in step 7 agree with the measurement or calculations in step 6?
Explain any differences observed between the measured and calculated values.
2. If the circuit had been a parallel circuit, the total power dissipation would have been (a)
the same, (b) greater, (c) less.
3. Write the formula for finding total circuit power when the power dissipation of individual
components is known.
Conclusion
(In your own words describe the purpose of the experiment and what you have learned from the
experiment)
3.3 Power Transfer And Its Measurement
Objectives
1. To show that maximum power is transferred from one circuit to another when the output
resistance of the source circuit is equal to the input resistance of the load circuit
Materials Required



DMM
Power supply: low-voltage, variable
Resistors, one each, 1k Ω and 2200 Ω, 3900 Ω, and 4700 Ω; two each, 3300Ω
Procedure
1. Connect the circuit of Fig.3.1.2.
2. Measure the circuit current in Fig 3.1.2 and record your measurement in Table 3.1.2.
3. Measure the voltage across RL and record your measurement in Table 3.1.2.
4. Calculate the power dissipated by RL and Record this value in Table 3.2.2.
5.
6.
7.
8.
Change RL to 2200 Ω, and repeat step 2, 3, and 4.
Change RL to 3300 Ω, and repeat step 2, 3, and 4.
Change RL to 3900 Ω, and repeat step 2, 3, and 4.
Change RL to 4700 Ω, and repeat step 2, 3, and 4.
Table 3.1.2. Power Transfer Measurements.
Value of R1
1k Ω
2200 Ω
3300 Ω
3900 Ω
4700 Ω
I
VRL
Power RL= I * V
Question & Answer
1. What value of RL caused the most power to be “transferred” to RL?
2. Did your answer to question 1 verify the theory found in the introductory information?
Explain.
3. What was the effect on power transfer when RL was less than R1?
4. What was the effect on power transfer when RL was greater than R1?
5. State a general law for maximum power transfer.
Conclusion
(In your own words describe the purpose of the experiment and what you have learned from the
experiment)