Figure 1: DC Voltmeter.
The DC Voltmeter is an electrical measuring instrument which is used to measure line potential difference (P.D) between two points. The voltage to be measured be DC. Basic Connection Diagram of DC Voltmeter is shown in Fig. 1.
Rm – multiplier resistance connected in series with voltmeter coil having its own resistance
Range Extension of Voltmeter by Multiplier
Range extension of voltmeter means increasing the capacity of same voltmeter to measure higher values of voltages safely.
Need of range extension of Voltmeter
Voltmeter range extension voltmeter coil is thin and delicate and it can withstand for small voltages applied across it so voltmeter cannot be used directly if high voltage is to be measured.
How voltmeter range is extended?
Figure 2: DC Voltmeter with multiplier.
A high resistance called as multiplier is connected in series with voltmeter when it is used for high voltage measurement (see Figure 2). Multiplier creates major voltage drop across it and remaining voltage of safe value gets applied to the voltmeter. The requirements of multiplier are same as that of shunt.
Let
IV = current producing full scale deflection.
Rm = resistance of multiplier Ω.
v = voltage across voltmeter in volts
V = high voltage to be measured in volts
\[v={{I}_{v}}{{R}_{v}}\]
\[V={{I}_{v}}\left( {{R}_{m}}+{{R}_{v}} \right)\]
\[V-{{I}_{v}}{{R}_{v}}={{I}_{v}}{{R}_{m}}\]
\[{{R}_{m}}=\frac{V-{{I}_{v}}{{R}_{v}}}{{{I}_{v}}}\]
\[{{R}_{m}}=\frac{V}{{{I}_{v}}}-{{R}_{v}}\]
Multiplying power of multiplier is given by,
\[m=\frac{V}{v}=\frac{{{I}_{v}}\left( {{R}_{m}}+{{R}_{v}} \right)}{{{I}_{v}}{{R}_{v}}}=\frac{{{I}_{v}}{{R}_{m}}+{{I}_{v}}{{R}_{v}}}{{{I}_{v}}{{R}_{v}}}\]
\[=1+\frac{{{R}_{m}}}{{{R}_{v}}}\] \[{{R}_{m}}=\left( m-1 \right){{R}_{v}}\]
Multirange Voltmeter
- Using individual multiplier
Figure 3.
- Using universal multiplier or potential divider
Figure 4.
Separate multipliers can be used for different ranges of voltage. Fig. 2 shows 4 multipliers. The respective multiplier is selected by operating selective switch in Fig. 4 a single multiplier known as universal multiplier is shown tapping’s are provided on the multiplier.
Disadvantages of Multipliers
The multipliers can be used safely upto 1000 volts beyond this value it is not applicable because of following reasons :
1. Power consumption increases as voltage increases.
\[P=\frac{{{V}^{2}}}{{{R}_{m}}}\]
2. Construction of multipliers used for high voltage are costly and difficult.