ELECTRONICS 151
Laboratory Exercise 22 – Thevenin’s Network Theorem
OBJECTIVE: To gain understanding of Thevenin’s network theorems, and ability to thevenize
a circuit.
MATERIALS: Protoboard, multimeter, DC-power supply, and 0.25W resistors:
120150.
DISCUSSION: Thevenin’s network theorem is used to simplify complex circuits. Based on this
theorem, any circuit with many voltage sources and components can be reduced and represented
by a simple equivalent circuit with respect to a pair of terminals.
Thevenizing a circuit is the procedure of replacing a complex circuit with an equivalent circuit
consisting of a single voltage source, VTh, and a single resistance, RTh.
PROCEDURE:
1) In a circuit connected as in the figure below, remove the RL and measure the voltage
across the open terminals A and B. This voltage is the VTh.
2) Calculate the VTh using methods of circuit analysis
3) Turn off the power supply and remove it from the circuit.
4) Complete the circuit using a jumper wire in place of the power supply.
5) Measure the resistance across the points A and B. This resistance is the RTh.
6) Calculate the RTh using methods of circuit analysis.
7) Connect a circuit using the VTh as power supply in series with a resistance, RTh.
8) Connect the RL, which was removed in the first step, across the terminals A and B of the
main complex circuit. Measure the voltage drop VL and current IL.
9) Now connect the RL across the terminals C and D of the simple Thevenin circuit.
Measure the voltage drop VL and current IL.
10) Redo the steps 8 and 9, using RL1=120and RL2=
11) Fill out the table. Compare the results and answer the following questions.
R2
R4
15 0ohm
V2
5V
22 0ohm
RL
A
B
33 0ohm
R1
47 0ohm
R3
39 0ohm
RTh
C
V Th
D
Calculated
Measured
Complex
Circuit
Thevenin
Circuit
VTh
RTh
VL
IL
VL1
IL1
VL2
IL2
REVIEW QUESTIONS:
1. Do the values of VTh and RTh depend on the value of RL?
2. Is the open circuit voltage across terminals A and B equal to voltage drop across RL?
3. Does a voltage source have a series resistance?
4. Does a current source have a series resistance?