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POTENTIOMETER

POTENTIOMETER. SUBMITTED BY OMAR AZEEM EE-01083-133 ZAIGHAM MUNIR EE-01083-157 SAIM ASIM EE-01083-177 MUHAMMAD OMAR EE-01083-208 SECTION B SUBMITTED TO SIR SALMAN BILAL.

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POTENTIOMETER

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  1. POTENTIOMETER SUBMITTED BY OMAR AZEEM EE-01083-133 ZAIGHAM MUNIR EE-01083-157 SAIM ASIM EE-01083-177 MUHAMMAD OMAR EE-01083-208 SECTION B SUBMITTED TO SIR SALMAN BILAL

  2. POTENTIOMETER A potentiometer is a device used to make very accurate voltage measurement. Resistance changes manually by sliding contacts. Principle: NULL technique Resistance of the wire directly proportional to length of the wire Vo/Vs = Rac/Rab

  3. SIMPLE SLIDE WIRE POTENTIOMETER It consists of resistance wire sliding contact reference voltage galvanometer unknown voltage Concept: Achieve a NULL point.

  4. TYPES OF POTENTIOMETER Potentiometers are of following types Manual potentiometer (wire wound) (Book) Self balancing potentiometer (Book) Carbon film potentiometers (Additional) Plastic film potentiometers (Additional)

  5. MANUAL POTENTIOMETER It permits the measurement of higher voltages than the reference voltage. straight resistance is replaced by switch selectable resistor rotary slide wire Types of rotary control wire: Nalder resistance network Feussner resistance network Varley resistance network

  6. Nalder: • It substitutes adjusted resistances. • Resistances are arranged in circular pattern. • Feussner: • double dials with double contact arms • Varley: • pair of sliding contacts • two resistances of circularly arranged group. • parallel resistance = 1 unit of main dial • 1 unit is subdivided into 10 parts

  7. STANDARD MANUAL POTENTIOMETER • switch A to “calibrate” position. • current limiting resistor switched (off) • rheostat is set to null the galvanometer • current limiting is switched (on) • it establish correct voltage across divider • then switch A turned to the “measure” position. • commercial manual pots have accuracy of 0.05%

  8. SELF BALANCING POTENTIOMETER galvanometer is replaced with servo amplifier and motor. voltage difference and error voltage. motor slows down null point.

  9. CARBON FILM POTENTIOMETERPLASTIC POTENTIOMTER • carbon deposited insulating body • cheap • better resolution (compared to wire wound) • reasonable life. • plastic coated • rotary and translational slider movements • good life • better resolution • low noise

  10. EXAMPLE # 1 Data: V(pot)=1.5v, R(m)=150 ohm R(s)=350 ohm, R(ts)=600 ohm, V(emf)=? Figure:

  11. solution: Vthevenin = (1.5V)250/600 = 625 mV Thevenin resistance is Rthevenin = 350//250 = 146 ohm since, no current flows through the meter, the thermocouple EMF equals to the Thevenin voltage, or Vthermocouple = 625mV

  12. METER CALIBRATION AND RESISTANCE METHOD A potentiometer can be used to calibrate • DC-Voltmeter • DC-Ammeter • Measure Resistance DC-Voltmeter: • Potentiometer measure the voltage across R2 • V2 = R2/(R1 + R2) * Vdc • Vm = Vdc

  13. EXAMPLE # 2 Solution: When potentiometer reads 247.1mv, the dc voltmter under test should read Vm=247.1(85/10)=2.1v

  14. DC-Ammeter: • Current is determined by ohms law Measure Resistance: • Ru is the unknown resistor • Rk is the known precision resistor Ru= (Vu/Vk)*Rk

  15. Important parameters of potentiometer • Resolution • Non-Linearity • Accuracy (0.1% or better) Operational problems with potentiometer • Dirt accmulation • Friction problem

  16. Comparison DC-voltmeter and Potentiometer: • meter loading is not a problem. • permits comparison between unknown and reference voltage. • eliminates most errors due to the PMMC movement (such as bearing friction) <<<<<<<<<<<<<<THANK YOU>>>>>>>>>>>>>>

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