EXTENDING THE RANGE OF DC VOLTMETER ; VOLTMETER SENSITIVITY; LOADING EFFECT

As within the case of an ammeter, to get a multi range ammeter, variety of shunts are connected across the movement with a multi position switch.

Similarly, a dc voltmeter can be converted into a multi range voltmeter by connecting a number for resistor (multipliers) along with a range switch to provide a greater number of workable ranges.     

              

Figure 1 Multipliers Connected In Shunt String

Figure shows a multi range voltmeter using a three positions switch and three multipliers R₁, R₂, R₃ for voltage values V₁,V₂ and V₃.

R₁ = V₁/I_m – R_m for the voltage range V₁

R₂ = V₂/I_m – R_m for the voltage range V₂

R₃ = V₃/I_m – R_m for the voltage range V₃

Figure 1 can be future modified to figure 2 which is a more practical arrangement of the multipliers resistors of a multi range voltmeter.

                
                  Figure 2 Multipliers Connected In Series String    

In this arrangement, the multipliers are connected during a series, string, and therefore the range selector selects the acceptable amount of resistance required serial with the movement.

This arrangement is advantageous compared to the previous one, because all multiplier resistances except the first have the standard resistance value and are The first resistor (or) low range multiplier, R₄ is the only special resistor which has to be specially manufactured to meet the circuit requirements.

VOLTMETER SENSITIVITY:

The sensitivity (or) ohms per volt rating of a voltmeter is that the ratio of the entire circuit resistance R_t to the voltage range. Sensitivity is essentially the reciprocal of the full scale deflection current of the basic movement.

S = 1/I_fsd Ω/V

The sensitivity ‘S’ of the voltmeter has the advantage that it can be used to calculated the value of multiplier resistors in a dc voltmeter.

As, R_t = total circuit resistance (R_s +R_m)

      S = Sensitivity of voltmeter in ohms per volt

      V = Voltage ranges as set by range switch

      R_s = R_t – R_m and R_t = S* V

      R_s = (S * V) – R_m ,

LOADING EFFECT:

When selecting a meter for a particular voltage measurement, it's important to think about the sensitivity of a dc voltmeter. A low sensitivity meter may give a correct reading when measuring voltages in a low resistance circuit, but it is certain to produce unreliable readings in a high resistance circuit.

A voltmeter when connected across two points during a highly resistive circuits, acts as a shunt for that portion of the circuit, reducing the entire equivalent resistance of that portion as shown in fig. The meter then indicates a lower reading than what existed before the meter was connected. This is called the loading effect of an instrument and is caused mainly by low sensitivity instruments.