Electrical Engineering ⇒ Topic : Applications of D.C. Potentiometers

Applications of D.C. Potentiometers

A potentiometer basically measures the potential difference between two points in an electrical circuit. On this basis, it can make several other measurements. A few applications of a potentiometer are discussed below by way of illustration.

(i) Measurement of current. A potentiometer can be used to measure the current accurately.For this purpose, the unknown current I is passed through a standard resistance S of known value as shown in Fig. (a). The potential difference (P.D.) across the standard resistance Sis measured with a potentiometer. The value of I is given by ;

                                                     figure (a)

(ii) Measurement of resistance. A potentiometer can also be employed to measure the unknown resistance. For this purpose, the unknown resistance R is connected in series with a known standard resistance S and a suitable current is passed through this series combination as shown in Fig. (b).The potential differences across R and S are measured with a potentiometer. Then,

                                                                    figure (b)

Since the p.d.s across R and S are measured and the value of Sis known, the value of unknown resistance R can be determined

(iii)Calibration of ammeter. A potentiometer can be usefully employed to calibrate an ammeter.By calibration of ammeter means to determine the extent of error in the reading of the ammeter throughout its range. The ammeter A to be calibrated is connected in series with a standard known resistance S and variable resistance R across a d.c. supply as shown in Fig.(c). The magnitude of circuit current (i.e., current through ammeter and S) can be changed by the variable resistance R.

                                                    figure (c)

     The circuit current is adjusted with the help of variable resistance R to give a particular value of current on the ammeter. This current is now measured by the potentiometer and is given by p.d. across S indicated by the potentiometer divided by S. The cun-ent measured by the potentiometer is accurate and is compared with that indicated by the ammeter to find the error in the ammeter reading.The test is repeated for various values of current over the entire range of the ammeter.

Note. The value of standard resistance S should be such that when full scale current of ammeter is flowing in the circuit, the potential difference across S does not exceed the range of potentiometer.

(iv) Calibration of voltmeter. A voltmeter can be calibrated with a potentiometer in conjunction with a volt-box as shown in Fig (d). A high variable voltage to the volt-box is provided by the potential divider arrangement. The voltmeter V 'to be calibrated is connected across the output of the potential divider and indicates the voltage fed to the volt-box. We can vary the voltage across the voltmeter through the adjustment of the potential divider.                                                    

                                                                  figure (d)

The potential divider is adjusted to give a particular value of voltage on the voltmeter. This voltage is now measured with a *potentiometer. The voltage measured by the potentiometer is accurate and is compared with that indicated by the voltmeter to find the error in the voltmeter reading. The test is repeated for various values of voltages over the entire range of the voltmeter