ECE 3300 Lab 2
ECE 1250 Lab 2
Name ________________________ Lab Section ___ Student Folder # ____
Measuring Voltage, Current & Resistance
Building: Resistive Networks, V and I Dividers
Design and Build a Resistance Indicator
Overview: In Lab 2 you will:
 Measure voltage and current
 Calculate, Simulate, Build, Test (and compare these) for
o Serial/Parallel resistive network
o voltage divider
o current dividers.
 Design and build a resistance indicator (a light that will go on/off depending on
the resistance).
This lab will build on Lab 1 by using the series potentiometers as the variable resistance
for your resistance indicator. This lab will also help you think through some beginning
debug skills. (Check out the 'Sherlock Ohms' Extra Credit.)
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 Lab 1:
o Breadboard & wire kit
o Resistor (1 kΩ, 4.7 kΩ)
o Potentiometers (10kΩ and 100Ω)
 Additional parts:
o Red LED
o Resistors: 160 Ω, 470 Ω, 510 Ω, 1.5 kΩ, 2.2 kΩ, 3.3 kΩ, 10 kΩ
Safety Precautions:
1) Blown Fuse: If you use the myDAQ as an ammeter (as you are told to do in the lab
manual) and accidentally try to read the current across a short circuit (which is a very
easy mistake to make), you may blow out the fuse in your myDAQ. A better way to
measure current (to prevent this problem) is to measure voltage across a shunt resistor
and calculate the current using Ohm’s Law. Please do it this way in our labs, it will
save you and the TAs a lot of grief:
http://www.youtube.com/watch?v=V6Fv79uVrcw
2) Short Circuiting the Power Supply: When you are using the power supplies on the
myDAQ, you will have several wires screwed to the black holder on the side of the
myDAQ, all hanging loose together. If their ends touch, they will short-circuit. If
you short-circuit the power (+/- 15V or 5V) together or to ground, you may blow the
fuse in the myDAQ. Take care to prevent this. Keep your bench and wiring neat,
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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 2
always hook the power and ground to the same points on your circuit board, always
use the same colors so you recognize them (red, black, white are typically used), etc.
3) If you do blow a fuse: Instructions for changing it are on the class website (on the
Canvas Home page: Resource pages by topic: Labs then click on myDAQ Resources
then click on myDAQ Manual–3rd item on first line: then go to p. 14), and the ECE
stockroom has them in stock.
4) General Electrical Care: It is pretty hard to actually hurt yourself with this
equipment and circuits. Throughout our labs, it is possible you may miswire
something and create a short circuit, which can make parts get hot, or even pop. We
call this ‘letting the magic smoke out of the box’, after which these parts don’t work
any more, and you can get new ones in the stockroom. If you smell something hot,
ok, unplug your circuit and try to figure it out. Try to be aware and prevent short
circuits. For instance, it isn’t really a great idea to probe around in your circuit with a
metal screwdriver, which can easily create short circuits. Mistakes happen, and the
myDAQ has a fuse, which should protect it from any circuit mistakes you might
make in this class.
5) A few hints I’ve used for wiring circuits: Keep your circuits neat. Label the nodes
on your diagram, and keep track of where they are on your board (label them with
tape, if necessary). Don’t hook up the power until you are ready to use it. Measure
your voltage before you hook it up. Disconnect between circuit revisions. Build your
circuit in stages, testing as you go. Measure your resistors before you put them on the
board (colors can be easy to mistake).
Instructions & Reference Material:
 myDAQ Quick Start Guide
 myDAQ as voltmeter
https://utah.instructure.com/courses/266578/assignments/1347122
 myDAQ measuring current through shunt resistor
http://www.youtube.com/watch?v=V6Fv79uVrcw
 myDAQ measuring resistance
http://www.youtube.com/watch?v=nE6123mquhI
 Multisim demos : See DVD in back of your book.
 Data Sheets:
Light Emitting Diodes (LED) http://media.digikey.com/pdf/Data Sheets/Fairchild
PDFs/MV5x64x, HLMP-15x3,130x.pdf
Prelab: Run Multisim Simulations
1. Review the videos (on website) and written material.
2. (Optional) You will be faster if you do the circuit calculations and Multisim
simulations before you come to lab.
WRITEUP: Take notes during the videos and information from written
information so you don’t have to go back and watch or review them again.
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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 2
Experiment 1: Measure Voltage (10 points)
The myDAQ puts out two voltages (+15V and – 15V relative to the ground, which is
labeled AGND, and 5V relative to digital ground, labeled DGND). It also provides a
variable DC voltage using the function generator.
Find the +15V, -15V, and AGND pins on the long side of the myDAQ, and screw wires
in to them. Be careful their ends do not touch each other and short out. Use the myDAQ
as a Voltmeter to measure the voltages to see how close they are to what you are
expecting. You may need to use alligator clips to connect the wires on the voltage pins to
the Voltmeter probes on the bottom side of the myDAQ.
meas V from +15V to AGND = _________________ This is the power used for
the rest of this lab.
meas V from -15V to AGND = _________________
meas V from +15V to -15V = _________________
Repeat for
meas V from +5V to DGND = _________________
Repeat for variable voltage source. See MyDAQ quick start guide section F.7 for
information on how to use the function generator as a variable DC voltage source.
What is the largest and smallest voltage you can measure on the MyDAQ?
WRITEUP: Explain your procedure, including a diagram of your connections.
What voltages are available on the MyDAQ? How accurate is the expected
voltage compared to your measured voltage? Note any abnormalities or
unexpected information that happens. Explain why, if you can.
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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 2
Experiment 2: Resistive Networks, Voltage & Current Dividers (30
points)
Calculate, simulate, build and test the circuit for problem m2.3 on page 93 of your text
(Fig. 1, below).
1. Extra Credit: Calculate the total resistance using the methods in section 2-3.1 of your
text. See Additional file for this extra credit, with hints, etc.
2. Calculate the voltages using voltage dividers, described on page 55 of your textbook.
3. Calculate the currents using current dividers, described on page 57 of your textbook.
Fig. 1. Circuit for problem m2.3, page 93 of the Ulaby textbook.
TABLE I
RESISTIVE NETWORK VALUES
Value
Total Resistance
connected to V1
voltage across R1
voltage across R2
voltage across R4
voltage across R6
Current through R1
Current through R2
Current through R4
Current through R6
Calculated
(Extra Credit)
Simulated (Multisim)1
Measured
U1=
U2=
U3=
U4=
WRITEUP: Explain your procedure in your own words. Sketch how the
voltmeter and current meter are connected to the circuit (for at least one
measurement). Provide the solution for the circuit above. Explain any anomalous
results.
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Multisim files are available for download from the lab site, or you can create your own.
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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 2
Extra Credit (5 points): Sherlock Ohms Debugs a Circuit
Have another student or the TA change any one of your resistors for another resistor with
the WRONG value. Using your myDAQ as a voltmeter, find which resistor it is, and
determine if the resistance value is too large or too small. WRITEUP: Record
information in your notes and turn in: (a) Indicate which resistor was changed on Fig. 1,
(b) describe how you tested, and the (c) reasoning behind your testing method, and (d)
anything you found that complicated your testing.
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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 2
Experiment 3: Resistance Indicator (30 points)
Now let’s build a resistance indicator to turn on a light when the resistance is below a
certain value, namely R1< 1 kΩ. Fig. 2 shows the circuit schematic. We will use a
standard red Light Emitting Diode D12 (LED) as the light. R1 will be the resistance we
want to sense. R2 will be the current limiting resistor (to keep the current below 20 mA
and prevent the LED from burning out when R1 = 0). R3 controls the turn on/off of the
LED. Vs is the +15V source from the myDAQ.
Fig. 2: Resistance Indicator circuit to turn on an LED when R1 < 1 kΩ. Equivalent circuits for diode
on and off are also shown.
1. Determine VF (Forward Turn on voltage)
The LED turns on when the CURRENT through it is large enough. You have already see
the LED I-V curves in Figure 3 from the application section of some of our early lectures.
LEDs are diodes that turn ON at approximately Ion = 20mA. They turn OFF at
approximately Ioff= 5 mA. They are used in the approximately-linear region shown (a
straight line that intersects the x-axis at the turn on voltageVF).
From Figure 3 read the value of VF (on the x-axis). VF = ________ volts
Figure 3 Light Emitting Diode (LED) I-V curves. The curve on the right is slightly
more linearized (idealized) and may be easier to read.
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Light Emitting Diode (LED) http://www.digikey.com/product-detail/en/MV50640/MV50640-ND/403109
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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 2
2. Evaluate the circuit.
Verify that the circuit in Fig. 5 satisfies two conditions:
1) The LED is exactly at its turn-on voltage and has ILED= 0 when R1 = 1 kΩ.
2) The LED carries ILED= 20 mA when R1 = 0 (so the LED doesn't burn out if the user
puts the lowest possible resistance in the circuit.)
Hints: The LED voltage = v3 and is the voltage at the node below R2. Use your value
from above for VF.
Fig. 4. Resistance indicator circuit with LED modeled as V-source.
WRITEUP: Fill in Table II below with your calculated values. Do the calculations
and explain them.
TABLE II
LED INDICATOR CIRCUIT CURRENTS
R1
1 kΩ
0Ω
iLED
WRITEUP: Answer all above questions for this section. Add extra pages if
necessary.
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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 2
Experiment 4: Simulate the circuit with Multisim3 (10 points)
Simulate the circuit in Fig. 4 with indicated values of R2 and R3. Fig. 5 shows the circuit
in Multisim. Use Multisim to evaluate the current (U1) and voltages (U2-U4) as shown.4
Compare the ON and OFF cases, and experiment with the value of the resistor. The LED
will NOT actually turn OFF, because Multisim allows ‘dim’ LEDs to continue to show as
being ON in the simulation. Simulated values are given. Write your calculated /
expected values next to the currents, resistances, and voltages in
OFF
ON
Fig. 5
OFF
ON
Fig. 5: Resistance Indicator
What could go wrong in this circuit (and
check to make sure it won’t)?
Several potential problems occur when you build a theoretical circuit in real life. These
gremlins include (but unfortunately are not limited to):
a) Exceeding the current or voltage limits of the components.
Calculate the maximum power for the resistors, and verify that you will not
exceed the 1/8W power rating in any configuration (on,off):
b) Exceeding the power rating of the myDAQ. Look up the maximum current that
can be sourced by the +/-15V and 5V power supplies on the myDAQ. Will you
be exceeding that rating?
c) Components not being exactly as designed. What is the expected range of R2 and
R3? Approximately how much will this affect your circuit?
WRITEUP: Describe what you did for this section. Record all information as
mentioned above.
Experiment 5: Build and Test the Circuit (10 points)
Build the resistance indicator circuit on your breadboard, using your myDAQ's +15 V
power supply for V1. The long lead on the LED is the plus side. The short lead is
connected to reference (AGND on myDAQ). Insert different R1's or a potentiometer into
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Multisim files are available for download from the lab site, or you can create your own.
Find parts in multisim from’Select All Groups’ and typing the various component names. U1 is an
ammeter. U2,U3,U4 are voltmeters. Note their connection for measuring voltage differences across
components. LED1 is an LED, choose a red one.V1 is DC_POWER, and you can change its voltage once
you have it set down. Don’t forget the GROUND.
4
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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 2
your breadboard to test your indicator. It should start to light up when R1 is about 1 kΩ,
and it will get brighter as R1 gets smaller.
Using your myDAQ voltage and current meters, measure the currents and voltages shown
in the Multisim simulation in Fig. 5 for two cases: R1 = 1 kΩ, and R1 = 0 Ω (wire).
Record your results:
 Indicate your measured values corresponding to those shown in Fig. 5.
Comment on how they compare to the expected values shown in
ON


OFF
Fig. 5.
At approximately what value of resistance does your LED turn on ON? (You may
use your resistors from Fig. 1 in various combinations, or you may use the 10 kΩ pot
from Lab1. If you put the 10 kΩ and 100 Ω pots in series as you did in lab 1, it is
easier to ‘tune’ your resistance. )
Ron min = ___________
WRITEUP: Describe what you did for this section. Record all information as
mentioned above.
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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 2
Discussion and Conclusions: How to Debug a Circuit (10 points)
WRITEUP:
You have now calculated, simulated, and built a few simple circuits. You have
measured resistance, voltage, and (using voltage and Ohm's law) current.
1) List what you have found to be ‘best practices’ for building circuits.
2) Whether or not you actually made wiring mistakes as you built these circuits,
list at least three different ways you could figure out what is wrong in a circuit.
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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