The voltage range of a current meter can be extended by adding:

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ALP CBT 2 Electronic Mechanic Previous Paper: Held on 21 Jan 2019 Shift 1

Answer 1

We can extend the range of ammeter by keeping a shunt resistance.

RRB JE EE 156 15Q RQBank 1(Hindi) images madhu

Here Rm = internal resistance of the coil

Rsh = Shunt resistance

I = Required full-scale range

Im = Full scale deflection of current

As the two resistances, Rm and Rsh are in parallel, the voltage drop across the resistance is equal.

\({I_m}{R_m} = \left( {I - {I_m}} \right){R_{sh}}\)

\({R_m} = \left( {\frac{I}{{{I_m}}} - 1} \right){R_{sh}}\)

\(\Rightarrow {R_{sh}} = \frac{{{R_m}}}{{\left( {\frac{I}{{{I_m}}} - 1} \right)}}\)

\(\Rightarrow {R_{sh}} = \frac{{{R_m}}}{{\left( {m - 1} \right)}}\)

Where \(m = \frac{I}{{{I_m}}}\)

‘m’ is called multiplying power

To increase the range of a voltmeter, we need to the series resistance and it is given by

\({R_{se}} = {R_m}\left( {\frac{V}{{{V_m}}} - 1} \right)\)

Where V is the required voltmeter range

Vm is the voltmeter range

Rm is the meter internal resistance

Note:

To increase the ranges of ammeter, we need to connect a small shunt resistance in parallel with ammeters.
To increase the ranges of a voltmeter, we need to connect a high series of multiplier resistance in series with voltmeters.