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Name: ______________________________
ECET 231 - Circuit Analysis I
Lab 8
Thevenin’s Theorem and Maximum Power Transfer
Objective:
Students successfully completing this preparatory exercise will accomplish the
following objectives:
1. Gain increased familiarity with the Superposition Theorem.
2. Determine the Thevenin equivalent of a complex circuit by both calculation
and measurement.
3. Determine the value of a load resistor which will allow maximum power
transfer to the load.
Equipment:
Power supply, Digital Multimeter (DMM), breadboard, resistors, jumper wires.
Procedure:
1.
Select four resistors: R1 = R2 = R3 = R4 = 2.2 kΩ. Measure the values of each of these and
record your results in Table 1 below.
Table 1: Measured resistor values
Resistor
Measured Value
R1
R2
R3
R4
2.
Using your measured resistor values, calculate the Thevenin equivalent external to RL for
the circuit of Figure 1. Use superposition to determine ETh. Enter your calculations into
Table 2.
Figure 1: Parallel circuit with two sources
Table 2: Thevenin equivalence calculations by superposition
Quantity
Vab1
(Due to E1)
Vab2
(Due to E2)
VTh
(Vab1 + Vab2)
RTh
Calculation
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3.
Construct the circuit of Figure 1, leaving out RL. Measure the terminal voltage Vab. This is
the Thevenin voltage. Does this compare favorably with your calculated value?
____________
Also measure the short circuit current Iab. Record your results in Table 4 below.
4.
Install a 130 Ω resistor in the circuit as RL. Measure Vab and IL. Record your results in
Table 4.
5.
Replace RL with each of the following resistors one at a time: 470 Ω, 1 kΩ, 2.2 kΩ and 4.7
kΩ. Repeat the measurements of step 4 for each resistor. Record these results in Table 4.
6.
Replace RL with a potentiometer. Adjust RL until the voltage Vab is equal to half the
Thevenin voltage (Eth / 2). By the Maximum Power Transfer Theorem, this will be voltage
at which maximum power is transferred to the load resistor. Carefully remove the
potentiometer so as not to accidentally readjust it. Measure its resistance and record the
result below. Calculate the percent difference between this measurement and the
calculated value of Rth using the following formula:
⎛ Quantity A − Quantity B ⎞
⎟⎟ × 100%
% Difference = ⎜⎜
Quantity B
⎝
⎠
Table 3: Comparison of RL to RTh at VRL = Eth / 2
RL (Potentiometer)
7.
RTh
(Previously calculated)
% Difference
Set a voltage source to your calculated value for the Thevenin voltage, Eth. Construct the
Thevenin equivalent circuit using a potentiometer to achieve RTh. Measure the terminal
voltage Vab and the short circuit current, Iab. Also measure the load resistor voltages and
currents for each of the resistors of steps 4 and 5. Record your results in Table 4.
Table 4: Measured load voltages and currents
Load
Resistance
0Ω
130 Ω
470 Ω
1 kΩ
2.2 kΩ
4.7 kΩ
Open circuit
Figure 1
Vab
Thevenin Equivalent
Iab
Vab
Iab
% Difference
Vab
Iab