Explanation of Extending the Range of Ammeters and Voltmeters

Extending the Range of Ammeters and Voltmeters

The ranges of electrical measuring instruments (whether ammeter, voltmeter or any other type of meters) are limited by the currents, which be carried by the coils of the instruments safely. For example, the moving coils of the instruments can carry maximum current of about 50 mA safely and the potential drop across the moving coil is about 50 mV. Hence, it becomes necessary that the current and voltage being measured be reduced and brought within the range of instrument.

There are four common devices used for extending the range of the instruments, namely \

1.       Shunts

2.       Multipliers

3.       Current transformers

4.       Potential transformers

The let us discuss the first two topics i.e. shunts and multipliers

1.       Shunts: The range of an ammeters can be extended by connecting a low resistance, called shunts, connected in parallel with ammeter. The shunt by passes the extra current and allows only safe current

to flow through the ammeter

Properties of Shunts:

            1.       Temperature coefficient should be low.

            2.       Its resistance should not be vary with time.

            3.       Should carry the current without excessive temperature rise.

            4.       Should have low thermal e.m.f with copper.

2.       Multipliers: The range of voltmeter can be extended by connecting a high resistance, called multiplier in series with the voltmeter coil. The multiplier limits the current through the meter so that it does not exceed the value of full scale deflection and thus prevents the movement from being damaged.

Note: Materials used for multipliers are manganin and constantan.

Properties of Multipliers:

          1.       Their resistance should not change with time.

          2.       The change in their resistance with temperature should be small i.e. TVR should be small.

          3.       They should be non-inductively wound for A.C meters.