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ECE 3300 Lab 2
ECE 1250 Lab 4
Measuring: Voltage
Building: Thevenin Equivalent Circuits
Designing: Voltage Reference with Specified Output Resistance
Overview: In Lab 4 you will:
 Build simple linear circuits consisting of v-sources and resistors.
 Measure the output voltage of the linear circuit with various load resistances.
 Find an equivalent Thevenin equivalent circuit consisting of a single voltage
source and resistance that has the same output characteristics as the linear circuit.
 Design a voltage reference whose Thevenin equivalent matches given
specifications.
This lab introduces the idea of Thevenin equivalents, which consist of a single voltage
source and a single resistor. Any linear circuit with two output terminals behaves exactly
like its Thevenin equivalent circuit insofar as its output voltage and current are
concerned. This means that any linear circuit or part of a linear circuit with two wires
connecting it to other circuitry may be replaced by a single voltage source and resistor.
In this lab you will verify the equivalence of a circuit and its Thevenin equivalent.
Equipment List:
 MyDAQ board with cables. (You can hook them to the lab computers if you don’t
want to bring your laptop.)
 Multisim software.
 From earlier Labs:
o Protoboard & wire kit, red LED
 Additional parts:
o Resistors: 100 Ω, 680 Ω, 2 x 1 kΩ, 2 kΩ, 10 kΩ, 100 kΩ, and a few more
values to be determined by student
Safety Precautions:
1) myDAQ power supplies: If you follow the instructions in this lab, your myDAQ
should be perfectly safe, but just be sure you never place a wire directly from a
myDAQ power supply to gnd. Doing so could cause the myDAQ to output too much
current, although the myDAQ probably is designed to handle even this.
2) myDAQ current meter: We are avoiding the use of the myDAQ current meter in this
lab for two reasons: first, a current meter acts like a wire and must be inserted in
series with components of a circuit in order to function properly—if you accidentally
placed the leads across a component you might cause a large current to flow that
might damage the circuit; second, using the current meter requires moving the red
multimeter lead from the voltage plug to the current plug on the myDAQ—we would
rather avoid the hassle! Instead of using the current meter, we use the voltmeter to
measure the voltage drop across a resistor. Then we use Ohm's law to find the current
in the resistor. This is safe, convenient, and straightforward.
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
ECE 1250 LAB 4
Instructions & Reference Material:
 MyDAQ as voltmeter
https://utah.instructure.com/courses/347027/pages/labs-mydaq-resources
Prelab: Videos and Multisim
1. Review the videos and written material on Thevenin equivalents on the class website.
2. (Optional) You will be faster if you do the Multisim simulations before you come to
lab. If you do not have Multisim running on your PC yet, then just do the Multisim
simulations on the lab computers.
WRITE-UP: In your lab notebook, take notes during the videos and record key
information from reading assignments so you don’t have to go back and watch or
review them again. Make sure to label each section in your notebook with the
same headings as highlighted above and below in yellow as you write the
information mentioned for each write-up section.
Experiment 1: Equivalence of Circuit and Thevein Equivalent (65 points)
A. Thevenin Equivalent Calculation
Consider the circuit shown in Fig. 1. In your lab notebook, derive the circuit's Thevenin
equivalent relative to the a and b terminals. Remember that the Thevenin voltage is the
output voltage, vo, when there is no RL, (or we could say when RL ® ¥ ). Also,
remember that we find the Thevenin resistance by turning off the independent sources
and looking in from terminals a and b.
Draw the calculated Thevenin equivalent in your lab notebook with the numerical value
of vTh and of RTh shown.
Fig. 1. First circuit for which Thevenin equivalent will be found.
WRITEUP: Explain your procedure. Show and explain your calculations for the
Thevenin equivalent.
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
ECE 1250 LAB 4
B. Multisim Model of Circuit with Load Resistance
Using Multisim, create a simulation of the circuit in Fig. 1, and record the output voltage,
vo, at the a and b terminals for the values of RL listed in Table I below. That is, fill in the
missing values in Table I.
Table I
Simulated Output Voltage of Circuit
versus Load Resistance
RL (Ω)
vo (V)
∞ (open)
100 k
10 k
1k
100
0
WRITEUP: Explain your procedure. Show the completed Table I.
C. Measured Circuit Output with Load Resistance
On your breadboard, using your resistors and the myDAQ 15V and AGND outputs as the
voltage source, build the circuit in Fig. 1. Using the myDAQ voltmeter, measure the
output voltage, vo, at the a and b terminals for the values of RL listed in Table II below.
That is, fill in the missing values in Table II. You would expect the values in Table II to
be close to those in Table I.
Table II
Measured Output Voltage of Circuit
versus Load Resistance
RL (Ω)
vo (V)
∞ (open)
100 k
10 k
1k
100
0
WRITEUP: Explain your procedure. Show the completed Table II.
D. Measured Thevenin Resistance
Remove the myDAQ 15V and AGND wires from your circuit entirely. Place a wire
connecting the points where the 15V and AGND used to be connected. Remove the
RL. Using your myDAQ Ohmmeter, measure the resistance at the a and b terminals.
This should match the Thevenin equivalent resistance found in part A.
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
ECE 1250 LAB 4
WRITEUP: Explain your procedure. List the measured value of the Thevenin
resistance.
E. Thevenin Equivalent Output with Load Resistance
A close approximation to the Thevenin equivalent of the circuit in Fig. 1 is shown in
Fig. 2. (This circuit should be very similar to your calculated equivalent in part A.)
Build the circuit in Fig. 2 using your resistors and the +5V and DGND outputs of your
myDAQ as the power source. Using the myDAQ voltmeter, measure the output voltage,
vo, at the a and b terminals for the values of RL listed in Table III below. That is, fill in
the missing values in Table III. Once again, you would expect the values in Table III to
be close to those in Tables I and II.
Fig. 2. Thevenin equivalent of circuit in Fig 1.
Table III
Measured Output Voltage of Thevenin Equivalent Circuit
versus Load Resistance
RL (Ω)
vo (V)
∞ (open)
100 k
10 k
1k
100
0
WRITEUP: Explain your procedure. Show the completed Table III.
E. MATLAB plot of Circuit and Thevenin Equivalent Output Voltage versus Load
Resistance
Make a MATLAB plot superimposing the data in Tables I, II, and III. The horizontal
axis will be the log base 10 of the RL values, and the vertical axis will be the output
voltage values. Omit the data for RL equal to infinity. Plot the data in Table I as a green
line, the data in Table II as a black line, and the data in Table III as a red line.
Are the three lines almost the same? If the theory is right, they will be almost the same.
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
ECE 1250 LAB 4
WRITEUP: Explain your procedure. Show your MATLAB plot. Comment on the
similarity of the curves and why they might differ slightly.
Experiment 2: Design and Build a Voltage Reference Circuit (35 points)
A. Design Voltage Reference
Design a circuit using the myDAQ +15 V power supply and your choice of resistors with
a Thevenin equivalent of 5 V (like a USB adapter) that will deliver 9 mA when a red
LED is connected directly to its output. Treat the red LED as a 2V source. The needed
Thevenin resistance gives a 5V – 2V = 3V drop when 9 mA flows through it. Thus, by
Ohm's law, the needed Thevenin resistance is about 330 Ω.
WRITEUP: Explain your procedure. Show and explain the design of your
voltage reference circuit.
B. Build and Test Voltage Reference
Build the voltage reference circuit you have designed for part A. Connect a 1 kΩ resistor
to the output and measure the voltage across it. Do a calculation to determine whether
the current in the 1 kΩ resistor is what it should be for a circuit with vTh = 5V and RTh =
330 Ω, (see Fig. 3).
Fig. 3. Thevenin equivalent of voltage reference circuit with load resistance attached.
If your voltage reference circuit is working as expected, connected the red LED to its
output and verify that it lights up.
WRITEUP: Explain your procedure. Explain your calculation of the expected
current in the 1 kΩ load. Comment on whether the LED lights up.
Extra Credit: Find the voltage for a Thevenin on left driving Thevenin on
right (10 points)
When designing electrical circuits and systems, we can treat a linear circuit's output as a
Thevenin equivalent, and we can also treat a linear circuit's input as a Thevenin
equivalent. If we want to design a large circuit, it becomes essential to think of the
circuit as consisting of building blocks. The Thevenin equivalent allows us to model
circuit outputs and inputs as very simple circuits. This approach, in turn, allows us to
determine how our circuit building blocks behave when connected together. For
example, it allows us to determine how much one circuit loads down the voltage coming
out of a circuit that it is connected to.
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
ECE 1250 LAB 4
Fig. 4 shows one Thevenin equivalent connected to another. This models the connection
of one linear circuit to another. Using the symbolic voltage and resistance names, vTh1,
RTh1, vTh2, and RTh2, find a formula for vo, which is the input voltage to the system on
the right.
Using your myDAQ and breadboard, design and build an example of the circuit in Fig. 4,
and verify your formula for vo. The values of vTh1, RTh1, vTh2, and RTh2 are your choice.
Fig. 4 One Thevenin equivalent connected to another Thevenin equivalent.
WRITEUP: Explain your procedure and results. Write your summary in such a
way that a student next year would understand what problem you are solving and
how you solved it.
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu