RESISTORS
DEFINITIONS AND USE
TYPES; MATERIALS, SIZE
LABELING AND COLOR CODES
VARIABLE RESISTORS
LIMITATIONS AND PROBLEMS IN USE
Definition
• An electrical component that opposes the
flow of either direct or alternating current
• Employed to protect, operate, or control
the circuit
• To limit the current
• Used in electrical circuits to maintain a
constant relation between current flow and
voltage: V = IxR
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Example in Current Limiting
• In the following circuit enough current must flow to
make the LED light up, but not so much that the LED
is damaged. The LED turns on at 1.7 V and the
maximum voltage it can have is 2 V. Calculate the
resistance necessary to have 10 mA through the LED.
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Types of Resistors
• Fixed valued
–
–
–
–
–
–
Wire wound
Carbon composition
Carbon film
Metal film
Metal oxide
Semiconductor
• Variable
– Potentiometer and rheostat
– Sensors
• Strain gage
• Thermistor
• Light dependant resistor (LDR)
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Examples of Resistors
Potentiometer
Rheostat
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Construction of Resistors
Carbon resistor
Carbon film resistor
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IC Diffused Resistors
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Cutaway View of a Resistor
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Types of Resistors
Constant Value resistors
• Wound-wire Resistors – Higher Power; low
values
– Made of Nickel-Chromium alloy wire.
– Few turns of the wire is wound around the
resistor’s body
• The turns have some space between themselves.
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Types of Resistors
Constant Value resistors
• Carbon Resistors – Higher Values; Low Power
– Made of conductive Carbon powder spread
on a non-conductive rod (Ceramic).
– Two conductive wires are attached to the
ceramic rod; one wire at each side.
• To facilitate the soldering of the Resistor to
another components such as Capacitors,
Transistors and etc…
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Carbon Composition and Metal
Film Resistors
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The Resistor
Color Code
Color
Number
Black
0
Brown
1
Red
2
Orange
3
Yellow
4
Green
5
Blue
6
Violet
7
Grey
8
White
9
Silver
10%
Gold
5%
Red
2%
Brown
1%
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Labeling of Resistors
using Color Codes
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Preferred Values in E 12 Series
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Tolerance of resistors (fourth
band of color code)
• The tolerance of a resistor is shown by the
fourth band of the color code.
• Tolerance is the precision of the resistor and
it is given as a percentage.
• For example a 390  resistor with a tolerance
of ±10% will have a value within 10% of 390
, between 390 - 39 = 351  and 390 + 39 =
429  (39  is 10% of 390 ).
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Precision Resistors
• Precision resistors will have a tolerance of 2%,
1%, 0.5%, 0.25% or 0.1%
• Some precision resistor values are identified using
the color code with 5 color bands,
– the first 3 bands indicate the rated resistance value
– the 4th band is the multiplier
– the 5th band indicates the tolerance
• Other precision resistors use an alphanumeric
coding
• The cost of the device is directly related to the
precision – higher precision = higher cost
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Color Coding of Precision
Resistors
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Alphanumeric Labeling
• Two or three digits, and one of the letters
R, K, or M are used to identify a resistance
value
• The letter is used to indicate the multiplier,
and its position is used to indicate decimal
point position
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Fixed IC Resistors
• Often it is convenient to use multiple
resistors mounted in IC packages in many
of today's modern circuits
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Types of Resistors
Variable Resistors
• The value of the resistor is varied by means of
either one of the following methods:
– Mechanically --- a sliding metallic contact
– Optically sensitive --- LDR
• The resistance decreases when the Light intensity increases
– Thermally sensitive --- The resistance value varies
with the temperature changes
• PTC – Positive Temperature Coefficient
– Resistance increases with temperature increase
• NTC – Negative Temperature Coefficient
– Resistance decreases with temperature increase
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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
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Examples of Variable Resistors
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Linear vs Logarithmic Taper
Potentiometers
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Wattage vs. Size
• The wattage increases with the
size of a Carbon resistors
•
•
•
•
•
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1/8 watt
1/4 watt
1/2 watt
1.0 watt
2.0 watt
0.145 inch long
0.250 inch long
0.375 inch long
0.562 inch long
0.688 inch long
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Power Resistors
• Used when the device must accommodate high
currents
• Physically large to surface can dissipate heat into
the environment
• Heats the air around the device – may be very hot
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Resistors in Different Sizes
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Resistors on Board
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Surface Mount
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Zero Ohm Resistors
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Resistors at High Frequencies
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Noise Produced by Resistors
Pure signal used for
evaluation; input voltage (y
axis) versus time (x axis)
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Effect of commercial
components on signal; output
voltage (y axis) versus time (x
axis)
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Noise Produced by Resistors
Pure signal used for
evaluation; input voltage (y
axis) versus time (x axis)
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Signal when foil-based resistors
are used; output voltage (y axis)
versus time (x axis)
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Criteria for Selection of Resistors
•
•
•
•
•
•
•
•
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Value of resistance
Power rating
Frequency of operation
Temperature dependence
Size
Type: axial lead or surface mount
Tolerance
Price
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For next lecture (September 26,
2011)
• Study the behavior of passive components
(resistor, capacitor and inductor) in DC
and AC circuits
• Define the impedance
• Study the characteristics of series R-C and
R-L in DC and AC circuits
• Pretest (from the above EE 250 topics),
you may use EE 250 textbook for
preparation to the test.
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