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Electronic Instruments

Electronic Instruments

Rectification for an Average Responding Voltmeter

Peak Reading Voltmeter

Form Factor

True RMS Meters

Electronic Multimeters

Working of individual Components

Digital Voltmeters

Ramp-type DVM

Staircase-Ramp DVM

Dual-Slope Converter

Successive-approximation DVM


Electronic Instruments

In this chapter the instruments to measure voltage, current, resistance, inductance, power, capacitance, etc are presented. The instruments which are basically Electro-mechanical Instruments use D’Arsonval meter which is shown in Fig. 1

Working of D’Arsonval meter
A commonly used sensing mechanism used in DC ammeters, voltmeters, and ohm meters is a current-sensing device called a D'Arsonval meter movement. The D'Arsonval movement is a DC moving coil type movement in which an electromagnetic core is suspended between the poles of a permanent magnet. The current measured is directed through the coils of the electromagnet so that the magnetic field produced by the current opposes the field of the permanent magnet and causes rotation of the core. The core is restrained by springs so that the needle will deflect or move in proportion to the current intensity. The D'Arsonval movement is a DC device and can only measure DC current or AC current rectified to DC.

Limitations of D’Arsonval meter
• An amplifier is required for increasing the
– Current sensitivity below 50μA
– Voltage below 10mV
• Power required greater than ½ μW
– Drawn from the circuit under measurement
– Varies with the voltage range
The next few sections illustrate the modifications used for measuring AC current with
a D’Arsonval meter.