Lab 3: Digital Multimeter and Voltage Generator

Lab 3: Digital Multimeter and Voltage Generator
Lab Goals:
•
•
•

Learn how to use your myDAQ as a Digital Multimeter (DMM)
Learn how to output a signal to a specified output port on the myDAQ
and verify its functionality.
Design a Digital Multimeter and Voltage Generator to be used on
later labs.
Use the DMM to measure several resistor, voltage and current
values.
Credit: Part 1: Dr. Ed Yu, UT Austin, Part 2: Dr. Bill Dillon, UT Arlington
Required Lab Material:
•
NI myDAQ kit
Due at the end of Lab:
•
Lab report and copies of Vis you designed
Part1: DMM and Voltage Generator using NI myDAQ (est time: 60 mins)
Assignment: Build a Digital Multimeter
Create a voltmeter
1. On block diagram, Use Quick Drop (Ctrl-Space) to find Multimeter Select NI
ELVISmx Digital Multimeter. It’s called an Express VI because it has its own
configuration screen which comes up automatically.
2. Configure for DC Voltage measurement, Auto scale
3. Expand the Express VI down to see all the input/output terminals
4. Right click on Device Name input and Create Control (this places control on front
panel)
5. On front panel, use Quick Drop to find Waveform Chart and Numeric Indicator
6. On block diagram, wire together as shown.
7. On block diagram, use Quick Drop to find While Loop. Drag loop around your code
8. Right click on Conditional terminal and Create Control (this places stop button on
front panel )
Figure 3.1: Multimeter, Front Panel
Figure 3.2: Multimeter, Block Diagram
9. Test by measuring a battery, or 5V / DGND pins on myDAQ using DMM probes.
10. Save your VI as your_name_myDMM.vi
Add Current and Resistance measurements to your DMM
11. On block diagram, drag a Case Structure around the DAQ Assistant.
12. On front panel, use Quick Drop to create an Enum ( an enumerated list ). Label as
“Measurement Type” (Figure 3.3, 3.4)
13. On front panel, right click on Enum and select Edit Items (Figure 3.5).
14. Add Voltage, Current ,and Resistance as the three items on the list
15. On block diagram, wire the Enum to the Case Selector ( denoted by )
16. Right click on border of Case and select “Add Case for Every Value” ( This will create all
three cases you need, neatly labeled to match the “Measurement Type” list. )
Figure 3.3: Front Panel
Figure 3.4: Block Diagram
Figure 3.5: Enum Properties Menu
17. Add NI ELVISmx Digital Multimeter.vi to the “Current” case (Figure 3.6), and setup the
measurement as shown. The NI myDAQ has an internal shunt resistor to measure current
through the analog-to-digital converter.
Figure 3.6: Multimeter VI Properties Menu
18. Add NI ELVISmx Digital Multimeter.vi to the “Resistance” case (Figure 3.7), and setup the
measurement as shown. Note that in order to measure Resistance using an Analog-to-
Digital converter, you supply an excitation current ( Iex ) through a Shunt Resistor. myDAQ
has an Internal current source, allowing the measurement of Resistance.
Figure 3.7: Multimeter VI Properties Menu
19. Be sure to wire the “Data” output of each DAQ Assistant to the white tunnel on the Case
Structure. When all Cases are properly wired, the tunnel will turn solid and your VI should
run.
20. Now you are ready to test your DMM for all three measurements: Voltage, Current, and
Resistance.
21. Congratulations! You have just created your own Digital Multimeter using LabVIEW and
myDAQ!
22. Remember to switch the DMM probes to the correct when you want to measure current.
23. Test Resistance by touchi8ng probes together. Resistance should go to zero. Or get a
resistor and touch the probes to either side of the resistor.
24. Save your VI.
Acknowledgement: A Special thanks to Eric Dean (Academic Field Engineer of NI), and
Dr. Ed Yu from UT Austin for the example DMM.
Part2: Resistance Measurements with your NI myDAQ (est 60 mins)
Resistors are the most common of electronic components found in many circuits and systems. This lab
experiment is designed to:
• sharpen your skill at reading specified values and tolerances from resistor color bands, and
• introduce you into taking resistor measurements using the DMM.
Experiment Procedure
The lab GTA will give you a numbered plastic bag containing 5 quarter-watt axial-lead (through-hole)
metal film resistors of various values and tolerances. Your task is to record the number of your bag at
the top of Table 1 shown in Section IV on the following page, and complete the entries in this Table
using the resistor color guide and a DMM. The procedure for this job is as follows:
(a) Refer to the wiring diagram of your breadboard below. Install resistors on your toolbox
breadboard so that the resistor leads are not shorted.
(b) using only the resistor color guide, fill out columns 2 through 9 for each resistor’s specified value
and tolerance; show your results to the lab GTA before turning on the DMM,
(c) power up the DMM and measure the actual resistance of each corresponding resistor; record these
values in column 10 of the Table. Make sure you carry out several measurements (discuss in your
report how many measurements, how you carried out your probing, etc) then perform simple
statistics on your date. Record the average measured value and the standard deviation in your
notebook and then then include your statistics in the lab report.
(d) compute the error in percent (%) between the color band value and the average measured value
for each corresponding resistor; record these errors in column 11; use the color band or specified
value as the basis for the percent, that is
Error  %  
Measured value  Color band value
100%
Color band value
(1)
(e) compare this value with the standard deviation of the resistance measurement in part (c) and
discuss whether your measurement technique may have an influence on the overall error. If so,
propose a way to incorporate the standard deviation of your measurements in the final error
calculation.
The first two rows in Table 1 illustrate an example of the procedure on two resistors Ra and Rb.
Resistor Ra in the first row has 4 color bands with colors green (5), brown (1), orange (3), and gold
(±5%). The specified value of this resistor is determined from
Ra  5
1
(2)
000   51K 
green brown orange
with a tolerance of ±5%. However, its value measured with the DMM is 50.5KΩ as is recorded in
column 10. The error between its measured and specified values is computed from equation (1)
where
Error  %  
50.5K   51K 
100%  0.98%
51K 
(3)
This error is recorded in last column as indicated. The procedure is repeated on resistor Rb which has
5 color bands. The second row of Table 1 contains values for this resistor. Clearly, the differences
between the specified and measured values for both resistors are well within specified tolerances.
Table 1
Axial-lead resistor values
Bag No.________
Color band
4
5
4
green
brown
orange
gold
N/A
51K
±5
50.5K
-0.98
5
red
orange
violet
red
brown
23.7K
±1
23.8K
+0.42
Rb*
R4
R5
Error
(%)
3
Ra*
R3
Average
measured
value
(Ω)
2
Bands
R2
Color
band
tolerance
(%)
1
R
R1
Color
band
value
(Ω)
Lab Note
Important concepts / Key ideas
Procedure / DATA
Try and error / Thoughts
Date
.
. 2014

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