Electrical Engineering ⇒ Topic : DC and AC Potentiometers

DC AND AC POTENTIOMETERS

Potentiometers are used in high precision measurements. The potentiometer may also be considered as a bridge since it works on the principle of balance of two opposing voltages.The two voltage sources, known and unknown, are connected in series opposition See Figure (a). The variable contact of the known resistance R is varied until there is no deflection of the galvanometer.

                                                  figure (a)  DC potentiometer.

At balance, the unknown voltage is:

                                                                                   ............ (1)

I₂ is the whole length of potentiometer.

We may compare the e.m.f.s of two cells in the same way of obtaining the balance with each cell connected one after other.

                                                                .............. (2)

Because, at balance no current flows in the circuit as the voltage source is balanced, the potentiometer can be considered as a high impedance circuit.

If we consider the upper half of the variable resistor R as R₁ and the lower half as R₂ then at balance, it can be written by voltage division as follows:

Instead of varying the resistance, the pointer may be kept at a fixed reading of the scale and the supply voltage is varied by a rheostat. In all these measurements, the supply voltage V should be greater than the cell voltage E.

For measurement of current, it is passed through a standard resistance and the voltage drop is measured as before.

Potentiometric methods are used to calibrate ammeters, voltmeters, and wattmeters.

The principle of the ac potentiometer is same as that of DC potentiometer. In AC balance,the magnitude as well as phase has to be obtained. There are two types of AC potentiometers:

(i) Polar type and (ii) Coordinate type.

Polar type reads the unknown AC voltage in polar form A ∠ θ and in coordinate type the AC voltage is measured in the two coordinates, horizontal and vertical, i.e., in phase and quadrature. AC potentiometers are used in measurement of self inductance, calibration of AC ammeters, voltmeters, and wattmeters.