Basic D.C. Voltmeter

The basic d.c. voltmeter is nothing but a PMMC D'Arsonoval galvanometer. The resistance is required to be connected in series with the basic meter to use it as a voltmeter.

This series resistance is called a multiplier. The main function of the multiplier is to limit the current through the basic meter so that the meter current does not exceed the full scale deflection value. The voltmeter measures the voltage across the two points of a circuit or a voltage across circuit component. The basic d.c. voltmeter is shown in the Fig. 1.

Fig. 1 Basic of d.c. voltmeter

       The voltmeter must be connected across the two points or a component, to measure the potential difference, with the proper polarity.

       The multiplier resistance can be calculated as:

        Let         R_m = Internal resistance of coil i.e. meter

                      R_s  = series multiplier resistance 

                      I_m  = full scale deflection current

                      V   = full range voltage to be measured

       From Fig 1,  .^..    V = I_m  (R_m + R_s )

.^..                                V  =  I_mR_m + I_m   R_s 

 .^..                               I_mR_s = V -  I_m  R_m

       The multiplying factor for multiplier is the ratio of full range voltage to be measured and the drop across the basic meter.

       Let    v = drop across the basic meter =  I_mR_m 

                m = multiplying factor = V/v

       Hence multiplier resistance can also be expressed as,

                               R_s = (m-1) 

       Thus to increase the range of voltmeter 'm' times, the series resistance is (m-1) times the basic meter resistance. This is nothing but extension of ranges of a voltmeter.