In this topic, you study the conversion of galvanometer into ammeter.
The galvanometer can be converted into an ammeter (to read currents) by connecting a small resistance in parallel with the instrument. This resistance is called “shunt”. The value of shunt is to be designed according to the “range” of the currents under measurement.
EXTENSION OF RANGE OF AMMETER BY SHUNT
A “shunt” is employed for extension of an ammeter’s range. A shunt is a resistance of small value connected in parallel with the ammeter. The combination is then connected in series with the circuit, whose current is to be measured. The major part of the current is bypassed through the shunt. The resistance of the shunt can be calculated (Fig. 3.5.) as explained below :
Let Rm the internal resistance (of the coil) of the galvanometer.
1m Full scale deflection current of the meter, which can be safely carried to the instrument.
Fig. 3.5. Extension of Ammeter Range.
I the current to be measured 1m)
Ish, R sh — current and resistance of the shunt.
As the shunt is connected parallel with the meter, the voltage drop across shunt and the ammeter should be the same.
The ratio of total current to the current taken by the meter is called “multiplying factor” of the shunt.
(From eq (ii)l
or
Multiplying factor m —
Rsh
Resistance of shunt
The resistance of shunt now can be calculated if Rm and m are known.
Required Properties of Shunt Material
The material for the shunts should have low temperature coefficient of resistance and their resistance should not have “ageing” effect. Also, they should carry the excess current without temperature rise. The materials used for making shunt are manganin and constantan. Shunts for low currents are enclosed in the meter case, but for currents beyond 200 A, the shunts are mounted externally. See Fig. 3.6 (a & b).
Fig. 3.6. Mounting of Shunt.
Swamp Resistance. (Fig. 3.7)
For eliminating temperature errors, a wire is connected in series with the meter as shown. The resistance of this wire is about 30 times to that of (the coil of) the meter itself like shunt. It is also made of manganin. Such a resistance is called “swamp” resistance. When used for ac measurement, a capacitor is connected across the swamp resistance to make the measurements independent of frequency as shown.
Fig. 3.7. Swamping Resistance