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.
In many cases, the use of ammeters and electrical multitesters involves connecting in series with current circuits to read the current. In other cases, the ammeters' shunt resistors will be connected in series for the same current reading purpose. Either way, the current must pass through a shunt or meter. To measure small currents, ammeters with Westin type of movements are used.
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