Multirange Voltmeter

The range of the basic d.c. voltmeter can be extended by using number of multipliers and a selector switch. Such a meter is called multirange voltmeter and is shown in the Fig. 1.

Fig.1 multirange voltmeter

       The R₁, R₂, R₃ and R₄ are the four series multipliers. When connected in series with the meter, they can give four different voltage ranges as V₁, V₂, V₃ and V₄. The selector switch S is multiposition switch by which the required multiplier can be selected in the circuit.

       The mathematical analysis of basic d.c. voltmeter is equally applicable for such multirange voltmeter. Thus,

1.1 Practical Multirange Voltmeter

       More practical arrangement of multiplier resistances is shown in the Fig.2.

Fig. 2

       In this arrangement, the multipliers are connected in series string. The connections are brought out from the junctions of the resistances. The selector switch is used to select the required voltage range.

       When the switch S is at position V₁,  R₁ + R₂ + R₃ + R₄ acts as a multiplier resistance. While when the switch S is at position V₄ then the resistance R₄ only acts as multiplier resistance. The V₄ is the lowest voltage range while V₁ is the maximum voltage range.

       The multiplier resistances can be calculated as :

       In position V₄, the multiplier is R₄ only. The total resistance of the circuit is say R_T .

       In position V₃, the multiplier is R₃ + R₄.

       In position V₂, the multiplier is R₂ + R₃ + R₄.

       In position V₁, the multiplier is R₁ + R₂ + R₃ + R₄.

 

       Using equations (1), (2), (3) and (4) multipliers can be designed. The advantage of this arrangement is that the multiplier except R₄ have standard resistance values and can be obtained commercially on precision tolerances. The first resistance i.e. R₄ only is the resistance having special value and must be manufactured specially to meet the circuit requirements.