Shunts and Multipliers

Chapter 5.9 Shunts and Multipliers

Fundamental Electrical and Electronic Principles Third Edition Book
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Fundamental Electrical and Electronic Principles Third Edition Book

  • 162Fundamental Electrical and Electronic Principles 5.9 Shunts and Multipliers The basic instrument so far described is capable of measuring only small d.c. currents (up to 50 A typically). This obviously severely limits its usefulness. The fi gure of 50 A quoted is the value of coil full-scale defl ection current ( Ifsd ) for an AVO meter. As you will now be aware, this meter is in fact capable of measuring up to 10 A, by using the current range switch. This is achieved by means of what are known as shunts. Similarly, this type of meter is also capable of measuring a range of voltages. This is achieved by means of multipliers. Both shunts and multipliers are incorporated into the instrument when it is assembled. 5.10 Shunts A shunt is a device connected in parallel with the meter ’ s moving coil, in order to extend the current reading range of the instrument. It consists of a very low resistance element, often made from a small strip of copper or aluminium. Being connected in parallel with the coil, it forms an alternative path for current fl ow. It can therefore divert excessive current from the coil itself. In this instance, ‘ excessive current ’ means current in excess of that required to produce full-scale defl ection of the pointer. The latter is known as the full-scale defl ection current, which is normally abbreviated to Ifsd . The application for a shunt is best illustrated by a worked example. Fig. 5.23