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Transcript
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 infer current from voltage measurements.
 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 help you on future labs, because we will later
use the resistance indicator circuit in a different way, as a voltage indicator for our
variable voltage source. 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 Protoboard & wire kit
o Potentiometers (10k and 100 Ω)
o Resistor (1k ,4.7kΩ)
 Additional parts:
o 1N4728A Zener Diode1
o Red LED
o Resistors: 510, 1.5k, 2k ,3.3k, 10kΩ
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 will 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
1
1N4728A Zener Diode Data Sheet https://www.fairchildsemi.com/ds/1N/1N4749A.pdf
1
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 3300 LAB 2
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 will 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.
3) If you do blow a fuse: Instructions for changing it are on the class website under
Labs, 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 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://www.electronics-tutorials.ws/diode/diode_8
Zener Diodes http://en.wikipedia.org/wiki/Zener_diode
LED Specs (abbreviated) https://www.sparkfun.com/products/9590
I.
PreLab:
A. If you haven’t already, get the MyDAQ and Multisim running on your computer.
B. Review the videos and written material on the prelab site.
C. (Optional) You will be faster if you do the circuit calculations and Multisim
simulations before you come to lab. IF you do not have Multisim running on your PC
yet, then just do the calculations, and do the Multisim on the lab computers.
II.
Measure Voltage (10 points)
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 3300 LAB 2
A. The MyDAQ puts out two voltages (+15V and – 15V relative to the ground, which is
labelled AGND. We often need voltages other than these, so are going to build a
circuit that provides them. 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.
B. 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.
+15V to AGND = _________________ ** This is the configuration we will use
the rest of this lab.
-15V to AGND = _________________
+15V to -15V = _________________
III.
Resistive Networks, Voltage & Current Dividers (30 points)
Calculate, Simulate, Build and Test the circuit for problem m2.3 on page 93 of your text.
a) Calculate the total resistance using the methods in section 2-3.1 of your text.
b) Calculate the voltages using voltage dividers, described on page 55 of your
textbook.
c) Calculate the currents using current dividers, described on page 57 of your
textbook.
Figure 1 Circuit for problem m2.3, page 93 of the Ulaby textbook.
Value
Total Resistance
across V1
voltage across R1
voltage across R2
voltage across R4
voltage across R6
2
Calculated
Simulated
(Multisim)2
Measured
U1=
U2=
U3=
U4=
Multisim files are available for download from the lab site, or you can create your own.
3
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 3300 LAB 2
Extra Credit (20 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. Turn in a separate sheet with:
(a) Indicate which resistor was changed on Figure 1, (b) describe how you tested, and the
(c) reasoning behind your testing method, and (d) anything you found that complicated
your testing.
IV.
Resistance Indicator (50 points)
Now let’s build a resistance indicator to turn on a light when the resistance is below a
certain value. We will use a Light Emitting Diode LED13 as the light, a Zener Diode
D14 to control the turn on voltage, and a resistor R1, that we want to sense. V1 is the
+15V source from the MyDAQ.
Figure 2: Voltage Indicator with a Zener Diode
A. Design
1. Determine VF (Voltage across the LED)
Figure 3 shows the LED I-V curves. LEDs are fully ON at approximately Ion = 20mA
(this is also the maximum recommended current)5, and OFF at approximately Ioff= 5
mA, as shown. At what voltage is a red LED ON? VF = _______________6
3
Light Emitting Diodes (LED) http://www.electronics-tutorials.ws/diode/diode_8
Zener Diodes http://en.wikipedia.org/wiki/Zener_diode
5
LED Specs (abbreviated) https://www.sparkfun.com/products/9590
6
Answer: VL = Approximately 1.8V
4
4
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 3300 LAB 2
Figure 3 Light Emitting Diode (LED) I-V curves
2. Determine VZ (Voltage across the Zener Diode)
A Zener diode can act like a voltage-controlled switch, which is how we will use it in this
application. If the voltage across the Zener is above VD, current can flow (switch is
ON). If not, no current will flow (switch is OFF). Each Zener has a different Vz, as
shown in Figure 4. For the 1N4728A, circle the typical value of
Vz = ________________7
Figure 4 Electrical characteristics of Zener Diodes. From [4].
3. Find VR (voltage across the resistor)
From Figure 2, find VR = __________________. Fill in the voltages in Figure 2, and
label the current I = 20 mA (ON) and 5 mA (OFF) in the circuit.
7
Answer: Vz = Approximately 3.3V
5
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 3300 LAB 2
4. Find Ron and Roff
What resistance is needed in this circuit to turn the LED on and off?
Ron = _________________ Standard Value8 = __________________
Roff = _________________ Standard Value = __________________
OR you could use a potentiometer in series with Ron. Redraw Figure 2 so that Ron and a
series potentiometer can create Roff, indicating the potentiometer connection. What size
pot do you need in order to meet (or exceed) Ron?
Potentiometer > _______________________
Could you use the two potentiometers in series from Lab 1? _____________
5. 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.
What would happen if you used ONLY a potentiometer for R?9
Calculate the power through the resistor, and be sure you will not exceed the
1/4W power rating in any configuration (on,off):
8
StandardResistor Values http://ecee.colorado.edu/~mcclurel/resistorsandcaps.pdf
Consider your calculations in section 4. What would happen in the circuit if the pot was adjusted to its
lowest (R=0) value?
9
6
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 3300 LAB 2
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 VZ
and R? Approximately how much will this affect your circuit?
B. Simulate the circuit with Multisim10
Simulate your circuit with your values of Ron and Roff. I have given you an
approximate circuit below. Use Multisim to evaluate the current (U1) and voltages (U2U4) as shown.11 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 Figure 5:
Calculated
Values
Simulated
Values
Measured
Values
(a) ON
10
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. D1 is a Zener (diode), just type Zener. 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.
11
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 3300 LAB 2
Figure 5: Voltage Indicator (TOP) ON (BOTTOM) OFF (or dim)
C. Build and test the circuit.
Black Line
on Zener
At top
Flat side / short leg of LED
Figure 6 Build the Resistance indicator. This one is shown with a simple Ron resistor, although you
may prefer to use your potentiometers in series with Ron instead. Write in the measured resistances
and voltages.12
Check your Ron resistance and be sure the light comes on, and an Roff resistance, and
show that it is off. Then, you may want to put your pots in series with Ron, to see what
range of resistances actually turns your light on and off. WARNING: Do NOT use just
the pots, because if you turn them down to R=0, you have effectively short circuited your
power supply, and then you will need a new fuse ….
12
A close-up of this figure is available on the lab page.
8
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 3300 LAB 2
Record your results:
 Indicate your measured values in Figure 6.
 Compare them to the expected values in Figure 5.
 At what range of resistances is your light ON?
Ron min =
Ron max =
V. Discussion and Conclusions: How to Debug a Circuit (10 points)
You have now calculated, simulated, built a few simple circuits. You have measured
resistance, voltage, and (using voltage and ohms 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.
9
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