Basic Electronics and Electrical Engineering: Chapter 6: Measurement and Instrumentation
Multimeter
Construction, Working Principle, Equivalent Circuit
A single instrument is used to measure three quantities, viz current, voltage and resistance.
MULTIMETER
A single instrument is
used to measure three quantities, viz current, voltage and resistance.
Multimeters are also called AVO meters (Ampere Volts and Ohm Meter).
Multimeters are available as analog type, electronic type or digital type.
An analog multimeter is
basically a permanent magnet moving coil galvanometer. For measurement of current,
a selector switch is operated in the current measurement mode. As shown in Fig 6.21,
a number of low value resistors called shunts, are connected in parallel with
the instrument through a range selection selector switch.
The use of the
multimeter as a multirange voltmeter has been shown in Fig.6.22. The rectifier
diode D1, conducts during positive half cycle of the input waveform
and causes the meter to give deflection according to the average value of the
half cycle. Diode D2 prevents a reverse voltage appearing across the
diode D1 during the negative half cycle of the input voltage.
The calibration of the
scale is such that the some scale is used for both dc and ac measurements. The
rms value of the sine wave is Im/√2, In order to have the same
deflection of dc and corresponding ac voltage range, the value of the
multiplier (resistance to be connected in series with the voltmeter) for the ac
range must be reduced accordingly.
The multi‒range
resistance measurement, the basic circuit is similar to an ohm meter and is
shown in Fig.6.22. When Rx is zero, i.e., the terminals A and B are
shorted, meter current is zero. When Rx is open i.e. when Rx
is infinity, current finds path through the meter only. By adjusting R we can
set full deflection. Thus the scale of the meter will have a zero mark on the
left hand and on infinity mark on the right hand side.
In digital multimeters, the result of measurement is displayed at discrete intervals in the form of numerals in the decimal system. Digital meters provide reading in the form of numbers, and hence there is less chance of meter reading error and error due to parallax.
Basic Electronics and Electrical Engineering: Chapter 6: Measurement and Instrumentation : Tag: Basic Engineering : Construction, Working Principle, Equivalent Circuit - Multimeter
Basic Electronics and Electrical Engineering: Chapter 6: Measurement and Instrumentation
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