Page 454 - Instrumentation Reference Book 3E

Input I d.c.
attenuator amplifier

Voltage measurement
r-----7

Resistance
measurement

Figure 20.14 Electronic multimeter. (a) Electronic input; (b) schematic of electronic multimeter.

was described. The same techniques can be operation of the ct depends upon the balance of the
applied in ax. measurements. However, in power ampere turns (the product of current and turns)
measurements with large currents the power dis- produced by the primary and secondary windings.
sipated in the shunt becomes significant (see If the transformer is ideal with no magnetizing
Table 20.3). For high voltage measurements the current or iron loss then
resistive voltage multiplier provides no isolation
for the voltmeter. FQ~ these reasons, range exten- IP
sion is generally provided by the use of current Is =
and voltage transformers. These enable single where nCt is the current transformer turns ratio
range ammeters and voltmeters, typically with given by
FSDs of 5 A and 110 V, respectively, to be used. ns
The principle of the current transformer (ct) is nCt = -
shown in Figure 20.17(a) and its equivalent circuit np
is shown in Figure 20.17(b). The load current being The ct is generally constructed with a toroidal
measured flows through the primary winding core of a high-permeability, low-loss material
whiist the ammeter acts as a secondary load. The such as mumetal or strip-wound silicon steel. This