unit-1basis electronic component
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Classification of Resistor
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Resistors
Fixed Variable
Carbon Wire wound metal film
Carbon
Composition
Carbon film
Power type Precision type
potentio
meters
Carbon Wire wound
1. Rheostats
2. LDRs
3. Thermistors4. Varistors
5. Trimmers
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Fixed Resistors
Resistors whose ohmic values remain fixed at a
constant value
Carbon Resistors:
(i) Carbon composition resistors
(ii) Carbon film resistors
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Carbon composition resistors
made of finely divided carbon or graphite mixed
with a resin binder in suitable proportion needed
for the desired resistance valueavailable in values of 1 to 20 M
power rating 1/10, 1/8, , , 1, 2 watts
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Advantages
smaller in size compared to wire wound
resistors
very wide resistance range is available
cheapest resistors
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Disadvantages
No precision and have very high toleranceGet easily heated and crack down on soldering
Resistance values vary with aging
Not useful for applications involving power above5 W
Application
Used for all general purpose circuits (Radio, T.V.etc)
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Carbon film resistorsManufactured by depositing very thin film of
carbon on to a substrate of ceramic or glass tube
Thin film (5m) resistors
Advantages
Available in all resistor values, from
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Disadvantages:
Can not withstand high temperatures
Can not withstand to mechanical shocks
Can not withstand to atmospheric moisture and
humidity
Chemically reactive
Applications:
Used in high frequency performance circuits(computers, telephonic circuits and amplifiers)
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Wire wound resistors
The higher the resistivity of a conductor, the
higher its resistance.
The longer the length of a conductor, the higherits resistance.
The lower the cross-sectional area of a
conductor, the higher its resistance.
The higher the temperature of a conductor, the
higher its resistance.
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Constructed from a long fine wire (nickel-chromium)
wound on a ceramic core
Two types: (i) power wire wound resistors(ii) precision wire wound resistors
Power wire wound resistors types:
low power-----3, 5, 10 watts
medium power----- 10 to 60 watts
high power-----60 to 1000 watts
Precision wire wound resistors :
available up to 5 W with 0.5% and 1%
tolerances and useful upto 5 to 10 MHz
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Advantages:
accurate resistance values, very low tolerance(0.01%)
withstand large power dissipation
used in high temperature situationsCapable of carrying large currents
Can withstand mechanical shock and vibration
Used in high voltage circuits
Do not change much with aging i.e. stable
resistance values.
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Disadvantages:
very large in size and weight
Very costlyPower type wire wound resistors are not suitable
beyond 200 KHz due to inter-winding inductance and
capacitanceWires can break, leads to breakdown of the circuit
in which these resistors are used
Applications:Power type---loads in TV receivers
Precision type----bridges, voltmeters and other
instruments
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Variable resistors can have two or three
terminals. Most have three.
Variable resistors are classified as a rheostat or a
potentiometer, depending upon the application.
Rheostat: Two- or three-terminal device used as a
variable resistor
Potentiometer: Three-terminal device used for
controlling potential levels
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Rheostats
used to vary relatively large values of currents
either ac or dc
nichrome wire is wound on a groovedcylindrical tube
Wire is coated with various enamel
Silicone cement is used for secure the wire
permanently to the ceramic body
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Some have a sliding contact, some have fixed
tapping
Ends of resistance wire and movable contact
are connected to three lug terminals
circular type rheostat provided with an
enclosure for dust and moisture, called dimmer
stats or toroids
Available from 1to 10 K with a power rating
up to 100 watts
Can withstand max temp. 300 degree C.
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Circuit symbols
Applications:heater, oven controls, light dimming controls, dc
motor speed control, arc welding etc
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Potentiometer
Most potentiometers have threeterminals as shown.
Two of them connected to the ends
of the carbon track the other terminal
is connected to the arm that contactsthe resistor element by a metal spring
wiper.
As the shaft of the control is rotated
the variable arm moves the wiper tomake contact at different points on
the resistive element.`
Figure 3.24
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Smaller variable resistors----pots
Three terminals
Arrow indicates the movable contact
Position of arrow determines the resistance
value
Types: Carbon potentiometers and wire
wound potentiometers
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Parameter
1. Element
2. Resistance
values
3. Power
rating
4. Taper
5. operation
6. Applications
Carbon pots
Carbon
composition
High
Low
Easy
Silent
Volume and tone
controls in radio,focus & constract
control in TV
Wire wound pots
Wire
Low
High
Difficult
Noisy
Small motor control,
servo control cktsetc.
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Need for tapering
In pots, the manner in which the resistance
varies with the rotation of shaft, called tapering
Used to maintain various values of resistance
Types: Linear tapering and Non-linear tapering
Linear---resistance changes in direct proportion
to shaft rotation, found in general purpose control
and TV picture adjustment
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R changes more gradually at one end withbigger changes at the opposite end
Non-linear tapering----resistance change is
not direct proportion to shaft rotationLogarithmic taper used for audio volume
control and tone control
Pots are either left hand or right handtapered
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Types of Resistors
The resistance between theoutside terminals a and c isalways fixed at the full ratedvalue of the potentiometer,regardless of the position ofthe wiper arm (b).
The resistance between thewiper arm and either outsideterminal can be varied from aminimum of 0 to amaximum value equal to thefull rated value of the
potentiometer.The sum of the resistances
between the wiper arm andeach outside terminal willequal the full rated resistance
of the potentiometer.
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Temperature Effects
Temperature has a significant effect on the
resistance of conductors, semiconductors and
insulators.For good conductors, an increase in temperature will result in an
increase in the resistance level. Consequently, conductors have
positive temperature coefficients.
For semiconductor materials, an increase in temperature will result
in a decrease in the resistance level. Consequently, semiconductorshave negative temperature coefficients.
The resistance of alloys increases with the rise in temperature but
this increase isvery small and irregular.
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Temperature Effects
Inferred absolute temperature
Resistance increases almost linearly with an increase in
temperature to the inferred absolute temperature of 234.5C
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Temperature coefficient of resistance
The change in resistance of a material with rise in temperature
can be expressed by means of the temperature coefficient of
resistance
Consider a conductor having resistance Ro at 0oC and Rtat t
oC.
Then in the normal range of temperatures, the increase in
resistance i.e. (Rt - Ro) is
Directly proportional to the initial resistance
Directly proportional to the rise in temperature
Depended on the nature of the material
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Temperature EffectsTemperature coefficient of resistance
The higher the temperature coefficient of resistancefor a material, the more sensitive the resistance levelto changes in temperature.
When we use the temperature coefficient equationwe see that copper is more sensitive to temperaturevariations than is silver, gold, or aluminum.
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Thermistors
A thermistor is a two-terminal semiconductor device whose resistance istemperature sensitive.
Increase in current through the device will raise its temperature,
causing a drop in its terminal resistance, negative temp. coefficientMaterials employed in the manufacture of thermistors include oxides
of cobalt, nickel, strontium and manganese.
Resistance values: from 0.5to 10M
Temp. range: -100oC to +350oCShapes: beads, discs, rods
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Thermistor characteristics
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Applications:
Used for measurement and control of temp. as inovens
Used for power measurement
Used for detection of thermal conductivity
Used as a thermal relay
Used for fluid flow sensing and flow rate
measuring device
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Sensistors
Resistance value increases with increasing
temperature
Positive temperature coefficient of resistance
made by using very heavily doped
semiconductors so that their operation is similar
to PTC type thermistors.Temp. range: 100oC to 400oC
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Sensistor characteristics
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Applications:Used for temp. sensor
Used for thermal relay
Used for temp. sensitive controlled devices
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