EE105.7.ppt

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DEPARTMENT OF TECHNICAL EDUCATIONANDHRA PRADESH

Name : K.Chandra Sekhar Designation : Lecturer Branch : Electrical & Electronics Engg. Institute : Govt. Polytechnic, Hyderabad Year/Semester : I Year Subject : Elements of Electrical Engg. Subject Code : EE-105 Topic : Electric Current- Ohm’s Law,

Resistance Duration : 50 Mts Sub Topic : Give the concept of resistance to

the flow of electrons Teaching Aids : PPT, Animation, Diagrams

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Objectives

On completion of this period , you would be able to know

• Definition of resistance• Distinguish between resistance and resistor.• Types of resistors

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Resistance

• The property of a material due to which it opposes the flow of electrons (current) through it.

• It may be termed as electrical friction.• It affects the movement of electricity in a same manner

similar to the effect of friction on mechanical objects.

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Resistance

• It is denoted by ‘R’• It is measured in Ohms (Ω)• It is symbolically represented as shown in fig.1

R

Fig.1

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Resistance

• Good conductors exhibit less resistance

Why ?

• Due to presence of large number of free or loosely attached electrons in their atoms

Example:

Copper

Aluminum

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Resistance

• Properties of resistancePower dissipating propertyCurrent circulating property

• These properties will result in heating of the material

• Heat due to collision of electrons of one atom with another

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• Substance which offer relatively greater difficulty or hindrance to the passage of the electrons – Insulator

• Substance which offer relatively lesser hindrance to the passage of electrons – Conductor

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Distinguish Between Resistor and Resistance

Resistor Resistance

•

• It is noun It is an adjective

• It is a material It is a property

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Types Of Resistors

• Carbon Composition • Deposited Carbon• High Voltage Ink Film• Metal Film• Metal Glaze• Wire-wound• Cermets (ceramic-metal)

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• Most popularly used resistors in electrical and electronics engineering are

1. Carbon Composition Resistors

2. Wire-wound Resistors

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Carbon Composition Resistor

• This resistor is enclosed in a plastic case to prevent the entry of the moisture from outside

• Available in power rating 1/8,1/4,1/2,1 and 2 Watts, in voltage ratings of 250, 350 and 500 volts

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Carbon Composition Resistor

• Have tendency to produce electrical noise due to current passing from one carbon particle to another.

• This noise appears in the form of hiss in a loud speaker

• Extensively used in entertainment electronics

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How to find the value of resistance of a carbon composition resistor ?

• It’s value will be marked with color bands• The first band indicates the first number, which is close

to the edge• The second band indicates the second number• The third band indicates the decimal multiplier , which

gives the number of zeroes after the two digits• Fourth band indicates percentage tolerance

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How to read color strips on carbon resistors ?• Fig.2 shows color coding carbon resistor

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Fig.2

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Carbon Composition Resistor

Color value

• Black 0• Brown 1• Red 2• Orange 3• Yellow 4

Color value• Green 5• Blue 6• Violet 7• Gray 8• White 9

Table.1

Table 1. shows the color code values

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• How to remember Color Codes ?

B B R O Y of Great Britain has Very Good Wife

BlackBrown

RedOrange

Yellow

Green BlueViolet

Gray

White

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Carbon Composition Resistor

Fig.3

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Wire-wound Resistor

• Consists of a ceramic core wound with a drawn wire

• Have highest stability and highest power rating

• Is coated with an insulating material such as baked enamel

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Different configurations of wire-wound resistors

1. Fixed resistor

2. Variable resistor Fixed Resistor:• Is one that has a single value of resistance which

remains constant• It has only two terminals

• Fig.4 shows a fixed resistor

Fig.4

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Variable Resistor

• It is also called varistor• Are used to vary the amount of resistance , so as to

change the current or voltage in a circuit

Two typesRheostat

Potentiometer

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Rheostat

• It is a variable resistance with 2 terminals connected in series with a load along with a jockey which slides on the tube

• It is used to vary the amount of current• Fig.5 shows a simple rheostat

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Rheostat

Fig.5

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A

V

Fixed Resistance Variable resistance / Rheostat

1 2

Fig.6

Rheostat

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• Fig.6 shows a simple rheostat connected in series with the source

• When sliding contact is in position 1, the current flowing through the circuit is maximum

• When sliding contact is in position 2, the current flowing through the circuit is minimum

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Potentiometer

• It is a variable resistance with 3 terminals connected across voltage source along with sliding arm.

• It is used to vary the voltage division between the sliding arm and any one fixed end

• It is also called as pot• Fig.7 shows a simple potentiometer

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Potentiometer

Fig.7

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Potentiometer

Input Voltage

Output Voltage

3

2

1

Fig.8 shows potentiometer connected across voltage source to function as a voltage divider

(a) Wiring diagram (b) Schematic diagramFig.8

1

23

(b) Schematic diagram

1

23

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Potentiometer

From the fig.8 • When the variable arm is at middle then the output

voltage is half of the input voltage between 1 & 2 terminals

• When the variable arm is closer to terminal 3(maximum R) more input voltage is available between 2 & 1 as output.

• When the variable arm is closer to terminal 1(minimum R) then the output voltage between 2 & 1 is minimum.

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Comparison Between Rheostat & Potentiometer

Rheostat• Three terminals• In series with load &

voltage source• Varies the current

Potentiometer

• Three terminals• Ends are connected

across voltage source• Taps off part of voltage

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Summary

In this lesson you learnt about

• Resistance

• Types of resistors

• Rheostat and Potentiometer

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QUIZ

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1. What is the color for 7 ?

a) White

b) Red

c) Violet

d) Yellow

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2. Potentiometer has how many terminals ?

a) 1

b) 3

c) 2

d) none

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1. List out the various types of resistors .

2. Define Resistance and give its units .

3. Compare Rheostat and Potentiometer .

Frequently Asked Questions

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Thank You