Page 831 - Basic Electrical Engineering

The value of shunt resistance, R has to be calculated for each range of
s
current measurement. For example let us consider a 0–1 mA instrument

having a coil resistance of 80 Ω to be used for measurement of larger current
in two ranges, i.e., 0–100 mA and 0–1 A. The values of two shunt resistances

are calculated as follows.
For the range of current measurement of 0–100 mA, we have to take, I =
100 mA, i.e., full-scale deflection value of current.

Shunt current, I = I – I = 100 – 1 = 99 mA.
s
m
Shunt resistance,

For the range of current measurement of 0–1 A using the same instrument we
need to have a separate shunt whose value is calculated as

11.9.2 Use of Shunts and Multipliers

Shunts are small resistances connected in parallel to increase the range of an

ammeter. For multi-range ammeters, the moving coil remaining same,
separate shunts are used to increase the range of measurement of a single
instrument.

Multipliers are high resistances connected in series with the moving coil to
extend the range of measurement of PMMC-type voltmeters.

A dc ammeter can be used as a multi-range ammeter by using a number of
shunt resistances in parallel with the instrument. Their values are to be

calculated according to the range of measurement scale as has already been
explained with an example.