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ECE 3300 Lab 2
ECE 1250 Lab 2
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, 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.
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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
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.
2
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.
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.
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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
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 20
pts)
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.
Extra Credit (10 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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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
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. Fig. 2 shows the circuit schematic. The user will connect a resistor they want to test,
(called R1 in Fig. 2), and the LED will light up if the resistance is less than 1 kΩ. We will use
a standard red Light Emitting Diode D12 (LED) as the light. The other resistors in the circuit,
R2 and R3, have been calculated to accomplish two goals:
1) Limit the LED current to at most 10 mA to prevent the LED from burning out when the
user chooses a wire for R1, and
2) Start turning on the LED when the user selects 1 kΩ for R1.
The circuit uses the +15V source from the myDAQ for power.
Fig. 2: Resistance Indicator circuit to turn on an LED when R1 < 1 kΩ.
1. Model the LED: Determine VF (Forward Turn on voltage) and RLED (equivalent
resistance)
The LED is a nonlinear device that turns on when the voltage across it reaches a certain value
called the forward voltage. If the voltage is increased further, the current through the LED
rises very rapidly. Consequently, we have to control the CURRENT through the LED. To do
so, we need a model of the LED.
You have already seen the LED I-V curves in Fig. 3 from the application section of some of
our early lectures. LEDs are diodes that turn ON at currents of a few mA. They have
maximum current ratings of typically 20 mA or 30 mA. If we look at the I-V curve for a
"standard RED" LED in the graph on the right in Fig. 3, we can approximate it as two
intersecting straight lines. (The actual curve has a small elbow that we, as engineers, will
ignore.) One line is on the x-axis where the LED is off, and the other line rises steeply where
the current in the LED rises rapidly.
WRITEUP:Put a ruler on the "standard red" LED I-V line and record the value where it
intersects the bottom axis. This voltage is called the forward voltage of the LED and is the
voltage where the LED starts to turn on. Record this value of VF.
VF = ________ volts.
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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
Also, determine the slope (rise over run) of the "standard red" I-V line where it is going up,
and use Ohm's law to determine the equivalent resistance of the LED.
RLED = ________ ohms.
Using the forward voltage and the equivalent resistance, the LED may now be modeled by the
circuit shown in Fig. 4 when it is on. (The LED may be modeled as an open circuit when it is
off.) Fig. 4 also shows the values of R2 and R3.
Fig. 3. Light Emitting Diode (LED) I-V curves. The curve on the right is slightly more linearized
(idealized) and easier to read.
Fig. 4. Resistance Indicator circuit model when LED is turned on.
2. Evaluate the circuit.
WRITEUP: Verify that the Resistance Indicator circuit satisfies two conditions:
1) The voltage across the LED is close to its forward voltage, VF, when R1 = 1 kΩ. Since
this is the point where the LED is just about to turn on, you may treat the LED as an open
circuit and use the circuit model in Fig. 5(a).
2) The LED carries approximately ILED = 10 mA when R1 = 0 Ω (a wire). Since RLED
found earlier is much smaller than R2 and R3, we may treat it as a wire and use the circuit
model in Fig. 5(b).
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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
(a)
(b)
Fig. 5. Resistance Indicator circuit models: (a) with R1 = 1 kΩ, (b) with R1 = wire.
Experiment 4: Simulate the circuit with Multisim3 (10 points)
Simulate the circuit in Multixim, as shown in Fig. 6, (but use the values of R2 and R3 from
Fig. 4.) 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 R1. The LED will NOT actually
turn OFF, because Multisim allows ‘dim’ LEDs to continue to show as being ON in the
simulation.
OFF
ON
Fig. 6. Multisim circuits.
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.
b) Exceeding the power rating of the myDAQ.
c) Components not being exactly as designed. What is the expected range of R2 and R3?
Approximately how much will this affect your circuit?
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Multisim files are available for download from the lab assignment on Canvas, 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.
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7
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
WRITEUP:
a) Calculate the maximum power for the resistors, and verify that you will not exceed the
1/8W power rating in any configuration (on,off). If the power does exceed 1/8 W, use
parallel resistors to get more power dissipation capability.
b) 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) What is the expected range of R2 and R3 for 5% tolerance in the value?
Approximately how much will this affect your circuit?
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 Vs. 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 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. 6 for two cases: R1 = 1 kΩ, and R1 = 0 Ω (wire).
WRITEUP:
 Indicate your measured values corresponding to those shown in Fig. 5.
 Comment on how they compare to the expected values.
 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 = ___________
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