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EE 1106: Introduction to EE
Freshman Practicum
Lecture 3: Measurements, Precision
Credit: Dr. Bill Dillon, EE Dept., UTA
Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015
Measurements
• Essential to Engineering
• Multitude of Sensors
– Electromagnetic
– Hall Effect
– Photonic
– Simple
• Yardstick
• Spring Scales
• Measuring cups
Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015
Measurement
Accuracy and Precision
Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015
Measurement Errors
•
•
•
•
Inherent Instrument Error
Poor Calibration
Improper Use of Instrument
Application of Instrument Changes
What was to be Measured
– Ideal Voltmeters have Infinite Resistance
– Ideal Ammeters have Zero Resistance
Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015
Measuring Resistance
• Indirect
– Measure Voltage across Resistor
– Measure Current through Resistor
– Calculate Resistance (Inaccurate)
• d’Arsonval Ohmmeter
– Very Simple
– Inaccurate
• Wheatstone Bridge (Most Accurate)
Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015
D’Arsonval Meter Movement
•
•
•
•
Permanent Magnet Frame
Torque on rotor proportional to coil current
Restraint spring opposes electric torque
Angular deflection of indicator proportional to rotor
coil current
S
Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015
N
D’Arsonval Ohmmeter
Rb
Vb
Rx
Radj
Need to adjust Radj and zero setting each scale
change.
Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015
Ohmmeter Example
45
0
10 mA Full Scale (Outer Numbers)
Rb+Radj+Rd’A=150 W
Inner (Nonlinear) Scale in Ohms
Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015
10
0
0
50
8
150
5
7.
2.
5
5
Vb=1.5 V
Measuring Voltage and Current
Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015
A Digital Voltmeter
•
•
•
•
Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015
Integrating Converter
Dependent on Vref
Dependent on Time
Independent of RC
How the DVM Works
Vout up
Vin

tu
RC
Vout down  0  
Vref
RC
td  Vout up
Vref
Vin
td  
tu
RC
RC
Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015
td
Vin  Vref
tu
DVM Example
Vref  5V
C  10 F
R  10k W
tu  100ms
td  50ms
50ms
Vin    5V 
 2.5V
100ms
Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015
Calculating Resistance
R
 l
When conductor has uniform
cross-section
A
cu  1.67 10 W  cm
6
 al  2.70 10 W  cm
6
Area,
A
l
Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015
Temperature Coefficient of
Resistance
Metallic conductors have a linear increase of resistance with
increased temperature.
R(T )  Ro 1  a T  To 
To is the reference temperature (usually 20oC) and Ro is the
resistance at the reference temperature. a is the temperature
coefficient of resistance for the material. At 20oC, some
values for a are:
Material
Aluminum
Copper
Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015
Alpha @ 20oC
0.004308
0.004041
Color Coding of Resistors
http://www.csgnetwork.com/resistcolcalc.html
Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015
Resistor Measurements on
Breadboard

Error  %  
Measured value  Color band value
100%
Color band value
Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015
Prelab 4 due next class!!
Available online at course website
Acknowledgements: Dr. Bill Dillon, and NI
Labview 3 hour courseware
Questions?
Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015
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