unit-1basis electronic component

8/12/2019 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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