Transcript Chapter 5: Resistors

Chapter 5: Resistors
What is a resistor?

Definition: A device used in
electrical circuits to maintain a
constant relation between current
flow and voltage. Resistors are
used to lower the voltage at
different points in a circuit
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Resistors work as they turn
voltage energy into heat
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2 Functions:


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To limit current
To produce desired voltage
Analogy
Resistors
Resistor’s Schematic Symbol
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Composed of a zigzag line
What is the value of
the resistor on the
right?
Reference
Designators
300 Ω
30kΩ
Surface Mount vs. Through Hole
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Surface Mount
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Through Hole
Surface Mount Technology
Surface Mount Resistors (SMT)
Types of Fixed Resistors
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Carbon Composition
Old-skool
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Metal Film
High accuracy
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Metal-Oxide Film
Low noise and good thermal properties
High accuracy at high resistance
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Resistor Networks
Many packaged resistors
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Wirewound Resistors
High power applications
Resistor Properties
Value in Ohms
 Accuracy
 Power rating (We’ll look at this one first)
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www.digikey.com
What is Resistor Power Rating?
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Examples:
¼
watt, ½ Watt, 20 watts
P = V·I = V2/R = I2R
 Heat Sinks
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Determining Value and Accuracy
Tolerance
Color Code:
Brown = 1%
Red = 2%
Gold = 5%
Silver = 10%
Video
See lecture notes first
 http://www.youtube.com/watch?v=kvQBhX
o_tF0
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More in class practice problems
Blue Blue Blue Gold
66 MΩ +/- 3.3 MΩ = 62.7M to 69.3M
Brown Black Red Silver
1000 Ω +/- 100 Ω = 900 to 1100
Brown Black Orange Silver
10000 Ω +/- 1000 Ω = 9000 to 11000
Orange Violet Black Gold
37 Ω +/- 1.85 Ω = 35.15 to 38.85
Find tolerance range as well
… or you can just measure it with an Ohmmeter!
Reading Resistor Value on a
Surface Mount Resistor
324
320,000 Ω
670
67 Ω
103
10k Ω
100
10 Ω
Potentiometers are written on the same way
What color is the following?
Green
Black
Violet
Blue
What color is the following?
Violet
Brown
Orange
Red
Homework
Do problems 1 and 2 on page 85
 Worksheet – Resistance and Voltage
Worksheet
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 Include
the range (if applicable)!
Other Calculator Functions
Talk about change button
Put Calculator in Write View (If doesn’t say w-view at
top, go to Setup menu, press 2, press 0)
Talk about a/b button.
Moving left and right to correct mistakes.
Back space button
Using previous answer
Storing and recalling numbers
Changing from Degree to Radian in SET UP
x-1 button
Pi button
Sqrt button
Negative button
Using parenthesis a lot for parallel resistors
PEN vs DEC mode
Variable Resistors
• Variable resistors are designed so that
their resistance values can be
changed
• A potentiometer is a three terminal
variable resistor used to divide voltage
• A rheostat is a variable resistor used
to control current
Pictures of Variable Resistors
Linear vs. Logarithmic “Pots”
Linear vs. Logarithmic “Pots”
Knob rotation (Usually 20 turns)
See page 69 for schematic of application
Resistance of a Wire
•The longer a wire is, the more resistive it is.
•The thicker a wire is, the less resistive it is. (Think parallel)
•(Also think of a fire hose. There is always pressure drops across a
pipe, but less pressure drop for bigger hoses.)
•Resistance is given by the following equation
•Therefore as a wire gets longer, the resistance change gets
linearly larger.
“The larger the cross-sectional area of the conductor,
the more electrons are available to carry the current,
so the lower the resistance. The longer the conductor,
the more scattering events occur in each electron's
path through the material, so the higher the resistance.
Different materials also affect the resistance.”
Resistors that change
resistance with temperature
(Page 70 in book)
•RTD’s (Need to be powered)
•Bolometers - Thermistors (NTC) /Barreters (PTC)
•Thermocouples (produces a voltage)
•Negative vs. Positive Temperature coefficient.
Bad Resistors
Resistors can go bad when too much current is passed
through them.
Signs of a resistor that has gone bad are:
Burned or charred looking
Smoky smell
Melted
Random Video of the Day
Pinkey.wmv