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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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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 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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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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