Download ECE1250 Lab 7 Capacitors

ECE 3300 Lab 2
ECE 1250 Lab 7 – Capacitive Sensors
Name ______________________ Lab Section ___ Homework Folder # ____
Build and test Capacitive Sensors
Background:
 In our previous labs, you have used resistors as sensors. In this lab, we will use
capacitors instead. You have measured DC voltage, and current, but in this lab, you
will measure voltages and currents that change with time using a function generator
and an oscilloscope.
Overview:
In this lab you will:
 Explore the signal generator and oscilloscope capabilities of your MyDAQ, and learn
more about sine waves and how they are measured.
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. (This is also running on the lab computers)
 Protoboard & wire kit
 Parts
From previous labs:
1kΩ resistor
Two alligator clip leads
New:
0.1µF capacitor
1 MΩ resistor
Aluminum foil (two pieces each about 1 ft2)
Wax paper (two pieces just slightly larger than the Al foil)
Absorbent material approx. ¼” thick and 2x6” in area (Sponge/stack of
napkins/washcloth, etc.)
Small Ziploc bag (put the absorbent material into it)
Water (to be poured into the Ziploc bag to change permittivity of the capacitor)
Instructions & Reference Material:
 MyDAQ and Multisim resources (see LAB page)
 How to use the NI ELVISmx Function Generator (FGEN):
http://decibel.ni.com/content/docs/DOC-12940
 How to use the NI ELVISmx Oscilloscope (Scope):
http://decibel.ni.com/content/docs/DOC-12942
I.
Learn to use the MyDAQ Function Generator & Oscilloscope
A. Connect the Function Generator to the Scope following this tutorial:
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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 3300 LAB 7
http://decibel.ni.com/content/docs/DOC-12940
On the MyDAQ:
 The Function Generator outputs a time varying waveform (a square wave, triangle
wave, or sine wave). The AO (Analog Output) has 0 (channel 1) and 1 (channel
2) outputs. Connect these wherever you want the voltage source(s). Also connect
AGND to the ground of your circuits. The frequency, magnitude, shape of the
waveform, etc. are controlled from the computer screen.
 The Oscilloscope measures a time varying waveform. The AI (Analog input ) has
0+ and 0- (for channel 1) and 1+ and 1- (for channel 2). The + side is like the
‘red’ lead on the DMM and represents the higher voltage measurement point. The
– side is like the ‘black’ lead on the DMM and represents the lower voltage point.
B. Generate a 1kHz sine wave. You choose the amplitude. Adjust the scope settings
until you can see it well on the scope. Be sure to use ‘Edge’ triggering so it holds
still. Sketch the sine wave as you see it on the scope or attach a screen shot.
a. Verify & label the magnitude of this signal from your graph. Should you read
the magnitude from the ‘peak’ value or the ‘peak-to-peak’ value?
b. The scope also provides an RMS value below the display. Verify that the
RMS voltage is Vrms = Vpeak / sqrt(2):
c. Verify & label the frequency of this signal from your graph. The time period
and frequency are related: T(seconds) = 1/f (Hz). Label T on your graph.
(See chapter 7-1 of your Ulaby text for a refresher on frequency and period.)
C. Experiment with each of the knobs & settings on your function generator and scope
so you know what they do. Explore the help buttons too.
II.
Build and Test a series RC circuit (20 points)
Do section 2-1 in the attached PDF file.1
III.
Measure Capacitance (20 points)
A. Measure capacitance in your series RC circuit above by adjusting the sine wave
frequency until the voltage measured across the resistor (channel 1 on your scope)
and capacitor (channel 2 on your scope) are equal. Follow section 2-3 in the
attached PDF file for details on how to do this. Remember  rad/secf(Hz)
C (measured) =
B. Measure capacitance using the time constant on a 1kHz square wave.
a. Adjust the settings on your scope until you see approximately the voltage
charging graph shown below2. You may not be able to show exactly one
period of the output in the scope window, so show a little more rather than
a little less. Sketch or print out a copy of your scope window.
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Lab 2 from Tinker, Learn by Barry Paton http://www.ntspress.com/publications/tinker-learn-and-doengineering-with-ni-mydaq-4/
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From http://lwn.net/Articles/250967/
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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 3300 LAB 7
b. Find RC. During charging, 1RC is the time from when the voltage is 0 to
(1 – 1/e) or about 66%. During discharging 1RC is the time from when
the voltage is maximum until it drops to 1/e or about 33% of the original
value.
RC (measured) =
R (measured) =
C=
Compare to the value in (a) above. Method (b) is the method you will
need to use to measure your capacitance for the rest of the lab.
IV.
Build and measure a Capacitor (20 points)
C 
A
d
A. Build a parallel plate capacitor from two pieces of aluminum foil ‘plates’ (each
approx. A=1 ft2) with wax paper between them. The wax paper should completely
separate them to prevent short circuiting the two plates together. Fold the edges of the
foil if needed to make it smaller than the wax paper. Estimate your capacitance:
Permittivity: ɛ (F/m) = ɛo ɛr =
ɛo = (8.854x10-12 F/m) and ɛr paraffin wax = 2.1 to 2.5 3
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http://www.engineeringtoolbox.com/relative-permittivity-d_1660.html . Which value should you use?
This is a matter of engineering judgment and depends on your application. If you want to know the largest
your capacitor could be, choose ɛr= 2.5 . If you want to know the smallest, choose ɛr= 2.1 . If you want to
know the average or ‘nominal’ value, choose the average of these two. I’m not going to tell you what to
do. You choose, and make a note on what value you chose and why.
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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 3300 LAB 7
Area: A (m2) = (measure it)
Distance between plates: d (m) ̴ (measure it approx.)
C (F) =
B. Connect the RC circuit
C. AI (0+)
AO (0)
AI (1+)
=1MΩ
AI (0-)
C
AI (1-)
AGND)
Connect the series RC circuit shown above. Use alligator clips to connect the
aluminum foil capacitor to the circuit. Each clip should connect to a separate
plate and should not touch the other plate.
a. Line up the two plates directly on top of each other with the wax paper
between them, and put your book on top to hold it as flat as you can. Use the
method shown in III.B.b to measure capacitance throughout the rest of this
lab.
C=
How well does this agree with your estimate in part A above?
b. Experiment with reducing the overlap areaA of the plates either by sliding
them apart or folding them. Does your capacitance increase or decrease as
you reduce the overlapping area of the plates?
c. Put another sheet of wax paper on top of the top plate, and experiment with
rolling the capacitor like a jelly roll. Does your capacitance increase or
decrease? Why?
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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 3300 LAB 7
d. Experiment with the permittivity ɛ. Fold or cut your top plate into a section
about 2”x6”. This will be the new area of your capacitor. Fold paper towel or
napkin until you have a pile about the same size and around 5-6mm thick. Put
the folded paper towel in a Ziploc bag, but don’t seal it yet. The bag will
insulate the paper towel and be sure you don’t have short circuits. Place the
bagged paper towel between your two plates, and connect the plates in series
with the 1MΩ resistor. Observe the RC time constant on the scope. Without
moving your capacitor overly much, add water to the Ziploc bag until the
paper towel is wet through. Did your capacitance increase or decrease?
e. Experiment with the spacing d. Manually compress the wet paper towels with
your hands or book. As you decrease d, did your capacitance increase or
decrease?
f. What was the approximate range of capacitances (smallest to largest) you
were able to build with the two pieces of aluminum foil and wax paper? (Feel
free to try any other configurations as well)
V.
Discussion and Conclusions (20 points):
The purpose of this writing assignment is for you to get practice describing an electrical
engineering procedure precisely yet succinctly. Describe how to measure a capacitor
using the method in III.B.b Include equations and figures if necessary to help you
describe this procedure.
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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