Electrical Engineering ⇒ Topic : Ohmmeter

Ohmmeter

A device that measures the resistance directly is called an ohmmeter. The simplest direct reading ohmmeter is the basic series ohmmeter circuit shown in Fig. a (i). It consists of a permanent magnet moving coil (PMMC) instrument in series with a battery and a rheostat R. Note that A and B are the terminals of ohmmeter

                                                         figure (a)

(1) With terminalsA and B shorted together, R is adjusted for full-scale deflection (f.s.d.). Since terminals A and B are shorted, the ohmmeter should read zero resistance. Thus as shown in Fig. a (ii), the full-scale deflection should read zero resistance.

(2) When terminals A and B are open-cricuited, the pointer should indicate infinity Therefore, zero deflection point on the scale is marked as infinite resistance. [See Fig. a (ii)].

(3) When an unknown resistance R_x is connected to terminals A and B, the meter current I_m is

Since the value of I_m is more than zero and less than I_g (f.s.d.), the meter will give a reading between zero and infinity Thus the value of unknown resistance R_x can be determined.

Note. The circuit shown in Fig. a (i) relies upon battery voltage remaining absolutely constant. When the battery terminal voltage falls (as they all do with use), the instrument scale is no longer accurate. Thus some means of adjusting for battery voltage variations must be built in the circuit