Potentiometer
- 1. POTENTIOMETER A potentiometer measures the unknown voltage by comparing it with a known voltage source rather than by the actual deflection of the pointer. This ensures a high degree of accuracy. • As a potentiometer measures using null or balance condition, hence no power is required for the measurement. • Determination of voltage using potentiometer is quite independent of the source resistance. BASIC dc POTENTIOMETER First, the switch S is put in the ‘operate’ position and the galvanometer key K kept open, the battery supplies the working current through the rheostat and the slide wire. The working current through the slide wire may be varied by changing the rheostat setting. The method of measuring the unknown voltage, E1, depends upon the finding a position for the sliding contact such that the galvanometer shows zero deflection, i.e., indicates null condition, when the galvanometer key K is closed. Zero galvanometer deflection means that the unknown voltage E1 is equal to the voltage drop E2, across position a–c of the slide wire. Thus, determination of the values of unknown voltage now becomes a matter of evaluating the voltage drop E2 along the portion
- 2. a–c of the slide wire. When the switch S is placed at ‘ calibrate’ position, a standard or reference cell is connected to the circuit. This reference cell is used to standardize the potentiometer. The slide wire has a uniform cross- section and hence uniform resistance along its entire length. A calibrated scale in cm and fractions of cm, is placed along the slide wire. A basic potentiometer circuit contact can be placed accurately at any desired position along the slide wire. Since the resistance of the slide wire is known accurately, the voltage drop along the slide wire can be controlled by adjusting the values of working current. The process of adjusting the working current so as to match the voltage drop across a portion of sliding wire against a standard reference source is known as ‘ standardisation’. CROMPTON’S dc POTENTIOMETER It consists of a dial switch which has fifteen (or more) steps. Each steep has 10 Ω resistance. So the dial switch has total 150 Ω resistance. The working current of this potentiometer is 10 mA and therefore each step of dial switch corresponds to 0.1 volt. So the range of the dial switch is 1.5 volt. The dial switch is connected in series with a circular slide wire. The circular slide wire has 10 Ω resistance. So the range of that slide wire is 0.1 volt. The slide wire calibrated with 200 scale divisions and since the total resistance of slide wire corresponds to a voltage drop
- 3. of 0.1 volt, each division of the slide wire corresponds to volt. It is quite comfortable to interpolate readings up to of a scale division and therefore with this Crompton’s potentiometer it is possible to estimate the reading up to 0.0001 volt. Procedure for Measurement of Unknown emf • At first, the combination of the dial switch and the slide wire is set to the standard cell voltage. Let the standard sell voltage be 1.0175 volts, then the dial resistor is put in 1.0 volt and the slide wire at 0.0175 volts setting. • The switch ‘S’is thrown to the calibrate position and the galvanometer switch ‘K’ is pressed until the rheostat is adjusted for zero deflection on the galvanometer. The 10 kW protective resistance is kept in the circuit in the initial stages so as to protect the galvanometer from overload. • After the null deflection on the galvanometer is approached the protective resistance is shorted so as to increase the sensitivity of the galvanometer. Final adjustment is made for the zero deflection with the help of the rheostat. This completes the standardisation process of the potentiometer. • After completion of the standardisation, the switch ‘S’is thrown to the operate position thereby connecting the unknown emf into the potentiometer circuit. With the protective resistance in the circuit, the potentiometer is balanced by means of the main dial and the slide wire adjustment.
- 4. • As soon as the balanced is approached, the protective resistance is shorted and final adjustments are made to obtain true balance. • After the final true balance is obtained, the value of the unknown emf is read off directly from the setting of the dial switch and the slide wire. • The standardisation of the potentiometer is checked again by returning the switch ‘S’to the calibrate position. The dial setting is kept exactly the same as in the original standardisation process. If the new reading does not agree with the old one, a second measurement of unknown emf must be made. The standardisation again should be made after the measurement.
- 5. APPLICATIONS OF dc POTENTIOMETERS 1. Measurement of current 2. Measurement of high voltage 3. Measurement of resistance 4. Measurement of power 5. Calibration of voltmeter 6. Calibration of ammeter 7. Calibration of wattmeter Measurement of Current by Potentiometer The circuit arrangement for measurement of current by a potentiometer is shown in figure. The unknown current I, whose value is to be measured, is passed through a standard resistor R as shown. The standard resistor should be of such a value that voltage drop across it caused by flow of current to be measured, may not exceed the range of the potentiometer. Voltage drop across the standard resistor in volts divided by the value of R in ohms gives the value of unknown current in amperes. Measurement of High Voltage by Potentiometer Special arrangements must be made to measure very high voltage by the potentiometer (say a hundreds of volts) as this high voltage is
- 6. beyond the range of normal potentiometer. The voltage above the direct range of potentiometer (generally 1.8 volt) can be measured by using a volt–ratio box in conjunction with the potentiometer. The volt– ratio box consists of a simple resistance potential divider with various tapping on the input side. The arrangement is shown in figure. Each input terminal is marked with the maximum voltage which can be applied and with the corresponding multiplying factor for the potential scale. High emf to be measured is applied the suitable input terminal of volt–ratio box and leads to the potentiometer are taken from two tapping points intended for this purpose. The potential difference across these two points is measured by the potentiometer. If the voltage measured by the potentiometer is v and k be the multiplying factor of the volt– ratio box, then the high voltage to be measured is V = kv volt.
- 7. Measurement of Resistance by Potentiometer The connection diagram for measuring unknown resistance with the help of potentiometer is shown in Figure. The unknown resistance R, is connected in series with the known standard resistor S. The rheostat connected in the circuit controls the current flowing through the circuit. An ammeter is also connected in the circuit to indicate whether the value of the working current is within the limit of the potentiometer or not. Otherwise, the exact value of the working current need not be known. When the two–pole double throw switch is put in position 1, the unknown resistance is connected to the potentiometer. Let the reading of the potentiometer in that position is VR. Then IRVR Now the switch is thrown to position 2, this connects the standard resistor S to the potentiometer. If the reading of the potentiometer is that position is VS then, Measurement of resistance by potentiometer The value of R can be calculated accurately since the value of the standard resistor S is known. This method of measurement of resistance is used for low value of the resistor. Rheostat Stabilized D.C supply A
- 8. SX V V R IS IR V V hence ISV S R S R s Measurement of Power by Potentiometer In measurement of power by potentiometer the measurements are made one across the standard resistor S connected in series with the load and another across the volt–ratio box output terminals. The arrangement is shown in Figure The load current which is exactly equal to the current through the standard resistor S, as it is connected in series with the load, is calculated from the voltage drop across the standard resistor divided by the value of the standard resistor S. Load current where VS = voltage drop across standard resistor S as measured by the potentiometer. Voltage drop across the load is found by the output terminal of the volt–ratio box. If VR is the voltage drop across the output terminal of the volt–ratio box and VL is the voltage drop across load then, VL = k × VR where k is the multiplying factor of the volt–ratio box.
- 9. Calibration of Voltmeter by Potentiometer In case of calibration of voltmeter, the main requirement is that a suitable stable dc voltage supply is available, otherwise any change in the supply voltage will cause a change in the calibration process of the voltmeter. Calibration of voltmeter by potentiometer The arrangement for calibrating a voltmeter by potentiometer is shown in Figure. The potential divider network consists of two rheostats. One for coarse and the other for fine control of calibrating voltage. With the help of these controls, it is possible to adjust the supply voltage so that the pointer coincides exactly with a major division of the voltmeter. The voltage across the voltmeter is steeped down to a value suitable for the potentiometer with the help of the volt–ratio box. In order to get accurate measurements, it is necessary to measure voltages near the maximum range of the potentiometer, as far as possible. The potentiometer measures the true value of the voltage. If the reading of the potentiometer does not match with the voltmeter reading, a positive or Stabilize d D.C supply V Fine Adjust ment Course Adjust ment
- 10. negative error is indicated. A calibration curve may be drawn with the help of the potentiometer and the voltmeter reading. Then the power consumed, Calibration of Ammeter by Potentiometer Figure shows the circuit arrangement for calibration of an ammeter using potentiometer. Calibration of ammeter by potentiometer A standard resistor S of high current carrying capacity is placed in series with the ammeter under test. The voltage drop across S measured with the help of the potentiometer and then the current through S and hence the ammeter can be computed by dividing the voltage drop by the value of the standard resistor. Current, is the voltage drop across the standard resistor S. Now, compare the reading of the ammeter with the current found by calculation. If they do not match, a positive or negative error will be induced. A calibration curve may be drawn between the ammeter reading and the true value of the current as indicated by the potentiometer reading. As the resistance of the standard resistor S is exactly known, the current through S is exactly calculated. This method of calibration of ammeter is very accurate. Calibration of Wattmeter by Potentiometer In this calibration process, the current coil of the wattmeter is supplied from low voltage supply and potential coil from the normal supply through potential divider. The voltage V across the potential coil of the wattmeter under calibration is measured directly by the potentiometer. The current Fine Adjustme nt Course Adjustme nt Stabilized D.C supply A
- 11. through the current coil is measured by measuring the voltage drop across a standard resistor connected in series with the current coil divided by the value of the standard resistor. The true power is then VI, where V is the voltage across the potential coil and I is the current through the current coil of the wattmeter. The wattmeter reading may be compared with this value, and a calibration curve may be drawn. The arrangement for calibrating a wattmeter is shown in Figure. Calibration of wattmeter by dc potentiometer A . C POTENTIOMETERS An ac potentiometer is same as dc potentiometer by principle. Only the main difference between the ac and dc potentiometer is that, in case of dc potentiometer, only the magnitude of the unknown emf is compared with the standard cell emf, but in ac potentiometer, the magnitude as well as phase angle of the unknown voltage is compared to achieve balance. This condition of ac potentiometer needs modification of the potentiometer as constructed for dc operation. The following points need to be considered for the satisfactory operation of the ac potentiometer: 1. To avoid error in reading, the slide wire and the resistance coil of an ac potentiometer should be non-inductive.
- 12. 2. The reading is affected by stray or external magnetic field, so in the time of measurement they must be eliminated or measured and corresponding correction factor should be introduced. 3. The sources of ac supply should be free from harmonics, because in presence of harmonics the balance may not be achieved. 4. The ac source should be as sinusoidal as possible. 5. The potentiometer circuit should be supplied from the same source as the voltage or current being measured. CLASSIFICATION OF AC POTENTIOMETERS Polar Potentiometer The voltage is read in the form V – θ. Example: Drysdale polar potentiometer Coordinate Potentiometer Example: Gall–Tinsley type potentiometer Drysdale Polar Potentiometer Consists of 1. phase shifting transformer 2. Transfer instrument The slide wire S1–S2 is supplied from a phase shifting circuit for ac measurement. The phase shifting circuit is so arranged that the magnitude of the voltage supplied by it remains constant while its phase can be varied through 360°. Consequently, slide wire current can be maintained constant in magnitude but varied in phase. The phase shifting circuit consists of two stator coils connected in parallel supplied from the same source; their currents are made to differ by 90° by using very accurate phase shifting technique. The two windings produce rotating flux which induces a secondary emf in the rotor winding which is of constant magnitude but the phase of which can be varied by rotating the rotor in any position. The phase of the rotor emf is read from the
- 13. circular dial attached in the potentiometer. Before the ac measurement, the potentiometer is first calibrated by using dc supply for slide wire and standard cell for test terminals T1 and T2. The unknown alternating voltage to be measured is applied across test terminals and the balance is achieved by varying the slide wire contact and the position of the rotor. The ammeter connected in the slide wire circuit gives the magnitude of the unknown emf and the circular dial in the rotor circuit gives the phase angle of it.