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DEPARTMENT OF TECHNICAL EDUCATION ANDHRA PRADESH

Name : P. VENKAT RAO.Designation : Lecturer.Branch : Electronics and Communication Engg.Institute : Govt. Model Residential Polytechnin Gajwel.Year / Semester : I Year.Subject : Electronic Components & Materials. Subject Code : EC-105Major Topic : Resistors. Duration : 100 Mts.Sub Topic : Rheostat and its applications.Teaching Aids : ANIMATIONS & DIAGRAMS

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On the completion of this topic, you would be able to learn the following:

• What is a Rheostat ?

• The diagram of a Rheostat.

• The specifications and applications of Rheostats.

Objectives

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In the previous topic, you have learnt about the following points:

• The comparison between Carbon and Wire – wound

Potentiometer.

• The need for tapering in Potentiometers.

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Rheostat

• A rheostat is a multi-(two)terminal variable resistor.

• Rheostats are designed to handle much higher voltage and current.

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Rheostat – Circuit symbol

Or1

2 1

3

2

Fig 1

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Cylindrical and Circular Rheostats

Fig 2

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A high power toroidal wire wound Rheostat

Fig 3

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Construction of a Rheostat

• Rheostats are constructed as a resistive wire

• wrapped to form a toroid coil with the wiper moving

• over the upper surface of the toroid, sliding from

• one turn of the wire to the next.

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Construction of a Rheostat

• Sometimes a rheostat is made from resistance wire

• wound on a heat resisting cylinder with the slider made

• from a number of metal fingers that grip lightly onto a

small portion of the turns of resistance wire.

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Construction of a Rheostat

• The 'fingers' can be moved along the coil of resistance

wire by a sliding knob thus changing the 'tapping' point.

• They are usually used as variable resistors rather than

variable potential dividers.

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Rheostat with various ratings and sizes

Fig 4

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Rheostat with various ratings and sizes

Fig 5

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Rheostat with various ratings and sizes

Fig 6

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Rheostat with various ratings and sizes

Fig 7

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Rheostat with various ratings and sizes

Fig 8

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Rheostat with various ratings and sizes

Fig 9

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Rheostat with various ratings and sizes

Fig 10

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Rheostat with various ratings and sizes

Fig 11

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Rheostat with various ratings and sizes

Fig 12

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Rheostat construction

• A rheostat is constructed by winding a resistance wire

around a former.

• The former can be round and flat or cylindrical.

• A wiper blade is then arranged to touch the surface of the

resistance wire, and is made to be moveable.

• Power is supplied to both ends of the resistance wire, and a

wire is attached to the slider.

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Rheostat and its construction

• The voltage at the slider wire is determined by the position of the slider on the resistance wire.

• By moving the sliding connection along the winding the resistive wire is made effectively longer ( or shorter) thus increasing ( or

reducing) the resistance in the circuit to which it is attached.

• If both ends of the winding are used and the central slider (tapping) is still connected then as one side gets shorter ( lower resistance ) the other gets longer ( higher resistance) and a voltage or potential divider is formed.

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Rheostat and its construction

• A rheostat consists of nichrome wire resistors connected in series or parallel and connected to wound rotor of motors by slide wire contacts.

• The resistors are encased in a metal housing where it is immersed in transformer oil coolant.

• During start up of motors, the rheostat is set at maximum resistance.

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Rheostat and its construction

• Gradually the setting is increased reducing the resistance connected to the rotor such that the motor speed also increases.

• Finally, the resistance is zero when the wiper contacts are short-circuited at the last stage and the motor rotates at rated rpm.

• Rheostats are used in wound rotor motors to reduce its starting torque and current.

• Rheostats can still be used as dimmers of lighting circuits.

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Specifications of Rheostats

• Resistance value : 1 Ω to 10 K Ω

• Power ratings : 100 watts

• Temperature : withstand up to 300 degrees centigrade

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Applications

• Used in Heater and Oven controls.

• Used in Light dimming controls.

• Used in speed control of DC motors.

• Used in arc welding process.

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Summary

In this topic , you have learnt about the following points:

1. Definition of Rheostat.

2. Construction of a Rheostat.

3. Specifications and Applications of Rheostats.

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QUIZ

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1). A Rheostat is a ________ terminal electrical

device

a. Two

b. Three

c. Four

d. Five

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2). A Rheostat is designed to handle

a. Low currents

b. High currents

c. Low and high voltages

d. High currents and high voltages.

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Frequently asked questions

• What is meant by a Rheostat ?

• Describe the construction of a Rheostat.

• Mention the applications of a Rheostat.

• List the specifications of a Rheostat.