Working principle of an electronic voltmeter, Electrical Engineering

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Q. Explain the working principle of an electronic voltmeter.

Sol. Electronic voltmeter

Basic Voltmeter: The addition of a series resistor, or multiplier, converts the basic D' Arsonval movement into a dc voltmeter, as shown in. the multiplier limits the current through the movements so as not to exceed the value of the full-scale deflection current (Ifsd). A dc voltmeter measures the potential differences between two points in a dc circuit and is therefore connected across a source of emf or a circuit component. The meter terminals are generally marked "Pos" and "Neg" since polarity must be observed.

The multiplier is usually mounted inside the case of the voltmeter for moderate ranges up t 500 V. for higher voltages; the multiplier may be mounted separately outside the case on a pair of binding posts to avoid excessive heating inside the case.

 Ayrton Shunt: The current range of the dc ammeter may be further extended by a number of shunts, selected by a range switch. Such a meter is called a multi-range ammeter. Fig. shows the schematic diagram of a multi-range ammeter.

The circuit has four shunts, Ra' Rb' Rc and   R d' which can be placed in parallel with the movement to give four different current ranges. Switch S is a multi-position make-before-break type switch, so that the movement will not be damaged, unprotected in the circuit, without a shunt as the range is changed.

The universal, or Ayrton, shunt of fig. eliminates the possibility of having the meter in the circuit without a shunt. This advantage is gained at the price of slightly higher overall meter resistance.

The Ayrton, shunt provides an excellent opportunity to apply basic network theory to a practical circuit.

Direct current ammeters are commercially available in a large number of ranges, from 20 µA to 50 A full-scale for a self-contained meter and to 500 A for a meter with external shunt. Laboratory-type precision ammeters are providing with a calibration chart, so that the user may correct his reading for any scale errors.  

Multirange Voltmeter: The addition of a number of multipliers, together with a range switch, provides the instrument with a workable number of voltage ranges. Fig. shows a multi-range voltmeter using a four-position switch and four multipliers Ra' Rb' Rc and R d'  for the voltage ranges V1'  V2'  V3 and V4' respectively.

The values of the multipliers can be calculated using the method shown earlier or alternatively, by the sensitivity method. A variation of the circuit of is as shown.

Where the multipliers are connected in a series string and the range selector switcher the appropriate amount of resistance in series with the movement.

This system has the advantage that all multipliers except the first have standard resistance values and can be obtained commercially in precision tolerances. The low-range multiplier R4 is the only special resistor that must be manufactured to meet the specific circuit requirements.

Electronic voltmeters use amplifiers, rectifiers and other circuits to generate a current proportional to the voltage being measured. The current so produced is measured by a permanent magnet moving coil instrument (PMMC) having taut band suspension instead of pivot and jewel mechanics. This type of voltmeter is called as analog voltmeters.


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