Electrical Engineering ⇒ Topic : Extension of Range of PMMC Voltmeter

Extension of Range of PMMC Voltmeter

The range of a permanent-magnet moving coil voltmeter can be increased by connecting a high resistance R_s called multiplie in *series with it as shown in Fig. (a)

Let                   R_m = meter resistance

                         R_s = series resistance i.e., multiplier

                         I_m = full-scale deflection current

                           V = full-range voltage of the meter

Voltage across AB = Voltage across the meter and R_s

                       or V = I_m(R_s+ R_m)

                                   figure (a)

Clearly, greater the value of R, greater is the voltage amplification. For this reason, R, is called voltage multiplier or simply multiplier. The important requirement of a multiplier is that its resistance should remain constant, i.e. it should have low temperature coefficient of resistance