Passive Electronic Components and Circuits
Laboratory 7
Thevenin’s and Norton’s theorems
Objective:
o Verify the Thévenin’s theorem by obtaining the Thévenin equivalent voltage (VTH) and
Thévenin equivalent resistance (R ) for the given circuit.
TH
Equipment:
o Digilent Electronics Explorer Board,
o Digital Multimeter,
o Resistors.
Theoretical support:
o Lecture 7 (Microsoft Power Point Support).
Prelab:
1. Describe the transient behavior of a RC circuit: charging and discharging a capacitor.
2. Describe the meaning of the time constant.
Problems:
For the circuit from Fig.1, are given: R1=1kΩ, R2=2.2kΩ, R4=3.3kΩ, R5=2.2kΩ, R6=1kΩ,
RL=1kΩ, V1=5V and 𝑣2 (𝑡) = 5 sin(1570𝑡) , [𝑉].
Fig.1.
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Passive Electronic Components and Circuits
1. Find VTh: Remove the load resistance RL, and calculate the open circuit voltage across the
terminals AB. This is equal to VTh. (See Fig.2). Use the superposition principle to find
this voltage.
Fig.2.
2. Calculate the equivalent resistance at the terminals AB (See Fig.3.). This is equal with the
equivalent Thevenin resistance.
Fig.3.
3. Determine the equivalent Thevenin circuit at the gate AB.
4. Calculate the voltage drop across the load resistance, RL.
5. Plot the input signal and the signal across the load resistance.
Procedure:
Verifying the Thevenin’s theorem:
1. Set up the circuit shown in the Fig.1. using the components chosen before.
2. Open the application “Power Voltage” from the Waveform Generator Menu and apply a
5V as input voltage V1 to the circuit.
3. Open the application “Arbitrary Waveform Generator” from the Waveform Generator
Menu and apply sinusoidal signal v2(t) as input voltage to the circuit.
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Passive Electronic Components and Circuits
4. Open the application “Scope” from the Waveform Generator menu. Make sure the signal
v2(t) is read on channel 1 of the Scope module, the signal on the load resistance on
channel 2, and the “Base” Time and “Range” are properly set.
5. Accuretly measure the signal vRL(t) using “Oscilloscope”. This signal will later be
compared to the one find using Thevenin equivalent circuit.
6. Find VTh: Remove the load resistance RL, and measure the open circuit voltage across the
terminals AB. This is equal to VTh. (See Fig.2).
7. Find RTh: Remove the source voltages and construct the circuit as in Fig.4. Measure the
resistance looking into the opening where RL was with an ohmmeter. This gives RTH.
Make sure there is no power in the circuit before measuring with an ohmmeter.
Fig.4.
8. Obtaining VTH and RTH, construct the circuit of Fig.5.
Fig.5.
Questions for Lab Report:
1. Compare the results with the calculus.
2. Using voltage and currnet dividers, calculate VRL and IRL. Explain the differences
between the results obtain with this method and with Thevenin.
3. Using Kirchhoff theorems, calculate VRL and IRL. Explain the differences between the
results obtain with this method and with Thevenin.
4. Using superposition theorem, calculate VRL and IRL. Explain the differences between the
results obtain with this method and with Thevenin.
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Passive Electronic Components and Circuits
5. Instead of RL, connect RL=10kΩ. Repeat the calculus from 1.a-1.e. Explain what happens
if you connect a larger resistances at the terminals.
Problems:
6. For the circuit from figure 3: R1  5k , R 2  2 k , R 3  R 4  R 5  R 6  8k .
Calculate the equivalent Thevenin circuit at the gates AB.
7. For
the
circuit
from
figure
Figure 3.
4, the current
source
generates
the
signal


i(t )  3  2sin  200 t   [mA], and V  5V , R1  1k , R 2  1.5k , R 3  2 k ,
2

R 4  2.5k . Determine:
a) The equivalent Thevenin circuit at the gates AB.
b) The equivalent Thevenin circuit at the gates AB.
c) Represent on a graphic the voltage at the gates AB.
8. For
the
circuit
from
figure
Figura 4.
5, the voltage
source
generates
the
signal


v(t )  3  sin 100 t   [V] and I  2mA , R1  1k , R 2  1.5k , R 3  2 k ,
4

R 4  2.5k .
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Passive Electronic Components and Circuits
Figure 5.
a) The equivalent Thevenin circuit at the gates AB.
b) The equivalent Thevenin circuit at the gates AB.
c) Represent on a graphic the voltage at the gates AB.
a) The equivalent Norton circuit at the gates AB.
b) The equivalent Norton circuit at the gates AB.
c) Represent on a graphic the voltage at the gates AB.
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