Electrical Engineering

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ELECTRICAL INSTRUMENTS 31
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MULTIMETER
Multimeter is one of the most important and common (instrument) aids. It is nothing
but a multipurpose meter,in which all electrical parameters combined in a single
unit with a common movement and a common scale known as volt-ohm-
milliammeter (VOM).
D’ Arsonval Movement (PMMC) As A Galvanometer
i) The d‟ Arsonval movement (PMMC)
ii) Iron vane type
iii) Electrodynamometer type, but most of the Multimeter use the first type, i.e. d‟
Arsonval movement.
shows the construction of the movement. d‟ Arsonval is the name of scientist, it
is also known as PMMC “ Permanent Magnet Moving coil” mechanism
(Galvanometer).It consists of a soft iron core mounted in bearings, between two
poles of a permanent magnet, which are in horseshoe type. The coil is suspended
so that it can rotate freely in the magnetic field. An aluminium pointer is
attached to the coil assembly, one at the top and the other at the bottom. These
springs serve two purposes. First, they proved a path for the current to reach the
coil, secondly, when current is zero they keepthe pointer at the lower end of the
scale and they provide a restoring torque ( refer fig. ) when current flows through
the coil. The iron core helps in making the field radial, which uniform magnetic
field throughout the coil.
When current passes through the coil, the magnetic flux is produced. This flux
interacts with the flux produced by the permanent magnets. Torque is produced
on the coil known as a deflection torque, which results in deflection of the coil
with pointer, so the pointer moves on the scale. As the coil rotates,the restoring
force caused by spring goes on increasing.The coil stops its deflection at a point,
where the deflection torque is equal to restoring torque. This deflection produced
is proportional to the amount of current passing through the coil.
ELECTRICAL INSTRUMENTS

Basic Milliammeter
Now let us see how we can use the same movement for current measurement having
0.1FSDI mA and 500MR  . Let us convert this movement into a milliammeter. It is
obvious that maximum current through the movement is 0.1 mA for full-scale
deflection, for measurement of higher current it is necessary to provide other path
for excess current.
( )
Movement Shunt
FSD M SH SH
FSD M
SH
SH
SH Range FSD
FSD M
SH
Range FSD
V V
I R I R
I R
R
I
and I I I
I R
R
I I
 
  

 
 

 
In this equation for any required current range, substituting RangeI the value of SHR
can be easilycalculated. Selectingdifferent valuesof shunt resistance, a multirange
millammeter facility can be produced. Fig (3.4) shows the construction of a
multirange milliammeter.

 Multirange Milliammeter
The circuit consists of a PMMC movement. 1, 2, 3R R R are the necessary shunt
resistance and „S‟ is the function switch for selecting the range. The main
drawback of this method is that when switch changesits position from one range
to another, during this intermediate time, no shunt resistance is present in the
circuit and there is a possibility of passing an excess amount of current through
the meter. It may damage the meter refer fig. (3.4)
Ayrton Shunt Method
To overcome the possibility of damage to the meter this method is employed. Another
speciality of this method is the advantage for higher current range; shunt resistance
need not be low because the value of ,MR becomes high due to the circuit
arrangement. Fig. (3.5) shows Ayrton shunt or universal shunt method of a
multirange milliammeter.

When the switch is at 1 mA range position, 1 2 3( )R R R  are in parallel with meter.
Since FSDI is 100 A (0.1 mA) the shunt resistor required to pass the current will be
1 mA-0.1 mA=0.9mA. Thus we get the equation as
1 2 3 ........( )FSD m
Range FSD
I R
R R R I
I I

  

For 10 mA range shunt is 2 3( )R R and now meter resistance is in series with 1R
 1
2 3 .......( )FSD M
Range FSD
I R R
R R II
I I
 
  

 1 2
3 .......( )FSD M
Range FSD
I R R R
R III
I I
  
 

By solving these three equations 1R , 2R 3R (fig. 3.5) can be calculated.
Basic Dc Voltmeter
Which is greater than FSDI and this should not happen, so current flowing through
the meter must be reduced or excess voltage should be dropped across series
resistance.i.e. acrossmultiplier ( SR )to convert the basic movement into a voltmeter.
The basic construction of such a voltmeter is shown in fig. (3.6) Figure shows that
SR and MR are in series.

Derivation
By Kirchhoff‟s voltage law,
Range S M
S M
V Voltageacross R Voltageacross R
V V V
 
 
FSDI Will be the current through the circuit when voltage connected across the
terminal is 10V. SR Should be calculated for FSDI and 10V, which is the range.
( )............. .......(3.2)
FSD S FSD M
FSD S FSD M
M Range
FSD
V I R I R
I R V I R
V
Rs R whereV V formula
I
  
 
  
For another voltage range we connect another multiplier Rs in the circuit.
Multirange DC Voltmeter
In a Multimeter different multipliers are connected in series with the movement for
different voltage measurement. Fig. illustrates the idea of multirange voltmeter.
When the switch is,
At 10V range
 
 
1 1
1 2
2 1
3
2
10
500 99.5
0.1
50
( )
50
500 99.5 500 100
0.1 10
400
S M
FSD
S M
FSD
M
FSD
V
R R R R K
I mA
At V range
V
R R R R
I
V
R R R
I
K K K
R K

     
 
    
 
 
   
 
 
        
 
 

 At 100 V range
 
 
 
1 2 3
3 1 2
3 3
100
0.1
1000 ( )
100
500 400 99.5
0.1 10
1000 500 500
S M
M
V
R R R R R
mA
R K R R R
R K K
K K K

 
     
 
    
     

    
Observe the calculations carefully, higher the voltage range the higher is the
multiplier value. i.e. more voltage is dropped across the multiplier.
MULTIRANGE AC VOLTMETER
To convert a basic d‟ Arsonval movement into a multirange AC voltmeter a rectifier
circuit is required in order to convert an AC into DC. The rest part of the circuit is
same as in multirange DC voltmeter. Generally, a full wave bridge rectifier circuit is
preferred.
RESISTANCE MEASUREMENT (OHMMETER)
The PMMC movement can be used to measure the resistance. Three common
methods are used for the measurement of resistance known as an Ohmmeter.
A) We can connect unknown „R‟ in series with meter to test the ability of unknown
R to prevent the current through it. This type of ohmmeter is known as „Series
type ohmmeter‟. It is suitable for medium „R‟.

B) By connecting unknown „R‟ in shunt with meter and determining the ability of
unknown R to bypass the current through the shunt path. This method is
known as „Shunt type ohmmeter‟. This method is suitable only for low – value
Resistance.
C) Third method is to apply a known voltage across unknown „R‟ and then to
determine the current through it. The ratio of voltage to current gives the value
of „R‟. This method is called as „Megger Type‟. This ohmmeter is used to measure
very high value of R.
SERIES TYPE OHMMETER
shows the construction of series type of Ohmmeter. IN this type an unknown
resistor (Rx) is kept in series with the meter. For Ohmmeter;a battery is necessary
which supplies a fixed current. Actually it measures current,but on the Ohmmeter
scale,it is calibrated in Ohm. The scale is marked in reverse dir3ection to the scale
of current and it is non-linear.
Now let us observe how this circuit measures unknown resistance. Supposes if the
probes are shorted (x-x) i.e. 0 .XR   The series resistor SR is adjusted suitably so
that FSDI flows through the meter and it shows 0  on the scale. This adjustment is
known as “Zero adjustment”. If the probes are open it shows infinite resistance, no
current flows through the movement, which is marked as  on the scale. SR is for
zero adjustment because the internal battery discharges with time. So it is necessary
to adjust FSDI . Multirange ohmmeter is same as current meter, a number of SR are
required to get multirange ohmmeter.

SHUNT TYPE OHMMETER
In shunt type ohmmeter current scale and resistance scale are in the same direction
i.e. zero deflection shows 0.
But it requiresan ON/OFF switch to disconnect the battery when meter is not in
use.
shows a typical shunt type ohmmeter.The meter, battery and unknown resistance
are in parallel. When Rx = 0 or when the probes are shorted, the meter current
will be zero because whole current flows through the probes. When Rx =  or
when the probes aren‟t shorted current flows through the meter only it is FSDI .
Usually this type of ohmmeter is rarely used. But it is useful for determining low
value resistance.IN shunt type Ohmmeter; the zero deflection corresponds to zero
Ohms and full deflection to infinite Ohms. The scale is not inve rted like series type.
COMPARISON
Series Type Ohmmeter Shunt Type Ohmmeter
1. Unknown Resistance, battery and
galvanometer are in series.
1.They are in parallel.
2. Current and Ohm scale are in
opposite direction.
2.Ohm scale is in the same direction
with Current scale.
3. It has right hand zero scale. 3.It has left hand zero scale.
4. Switch is not required. 4.Switch is required to disconnect the
battery when meter is not in use.
5. Circuit diagram 5.Circuit diagram

Electrical Engineering

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