1. Analog And Digital METERS (14MARKS)
Unit-2
Analog And Digital METERS
(14MARKS)
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2. Mrs.V.S.KharoteChavan,E&Tc,PC poly
. classification of analog meters:4M
The main types of instrument used as ammeters and voltmeters are as follows:
Permanent magnet moving coil instrument (PMMC)
Electro dynamometer type instruments.
Moving Iron type instruments Attraction type moving iron instruments.
Repulsion type moving iron instruments
Thermocouple instruments.
Electrostatic instruments.
Induction instruments.
Hot wire Instruments.
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3. Mrs.V.S.KharoteChavan,E&Tc,PC poly
D Arsonval movement
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4. Basic d’Arsonval meter movement.
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PMMC instruments
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Construction:
A coil of thin wire is mounted on an aluminum frame (spindle) positioned between the poles of a U shaped permanent magnet which is made up of magnetic alloys like alnico.
The coil is pivoted on the jewelled bearing and thus the coil is free to rotate. The current is fed to the coil through spiral springs which are two in numbers.
The coil which carries a current, which is to be measured, moves in a strong magnetic field produced by a permanent magnet and a pointer is attached to the spindle which shows the measured value.
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7. Mrs.V.S.KharoteChavan,E&Tc,PC poly
Principle of Operation
When a current carrying conductor is placed in a magnetic field, it experiences a force and tends to move in the direction as per Fleming’s left hand rule.
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8. Mrs.V.S.KharoteChavan,E&Tc,PC poly
Advantages of PMMC meter:
It has uniform scale.
 Power consumption is low
 It can be obtained in wide ranges.
 High sensitivity & accuracy
It is unaffected by external magnetic field.
 Additional damping device not required.
 Hysteresis problem is not there.
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9. Mrs.V.S.KharoteChavan,E&Tc,PC poly
Torques which deflect the pointer from its zero position is known as deflecting torque. The deflecting of pointer is directly proportional to quantity to be measured.
The deflection torque produced due to current flowing through coil.
Let length of coil be L meter and width of coil be d meter. Assume, I is the current flowing through coil having N turn. B is consider as flux density produce in air gap.
Therefore the force exerted by coil is
Where, A = l x d = Area of coil former.
F= BiL
The deflecting torque is given by
Td = Force x distance
Td = F x S
= B x l x I x N x d (1)
Td = B x A x I x N (2)
Where, A = l x d = Area of coil former.
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10. Mrs.V.S.KharoteChavan,E&Tc,PC poly
Disadvantages of PMMC meter:
1.It is suitable for d.c. measurement only.
2.Comparatively highcost than moving iron type instrument.
3. Ageing of permanentmagnet & spring introduce
errors.
4. Friction due to jewel- pivot suspension.
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11. Mrs.V.S.KharoteChavan,E&Tc,PC poly
Applications:
Ammeter:
Voltmeter:
Ohm Meter:
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Moving Iron Instruments
Attraction type
Repulsion type
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LCD characteristics
They are light scattering
they do not actively generate light and depend for their operation on back lighting.
LCD operates at low voltages 1-15V
They operate on reflective or transmissive configuration
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Analog meters are characterized by the fact that they use a pointer and scale to indicate their value.
One of the common types of meters uses the d’Arsonval type of meter movement.
Analog meters use a moving coil placed between the poles of a magnet.
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31. Mrs.V.S.KharoteChavan,E&Tc,PC poly
Shunt
resistance ammeter.
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32. Mrs.V.S.KharoteChavan,E&Tc,PC poly
The basic movement of dc ammeter circuit consists of D’ Arsonval galvanometer.
When large current is to be measured then some extra modification is required.
For measurement of large current by using same movement a shunt resistor is connected as shown in circuit.
The value of shunt resistor is very small so that most of the current pass through it and
only small current allow to pass through the coil.
The coil winding of basic movement is small and light therefore it carries very small current.
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33. Mrs.V.S.KharoteChavan,E&Tc,PC poly
The voltage across the shunt and
movement must be same.
Vsh=Vm
IshRsh=ImRm
Rsh=ImRm/Ish
· Rsh=ImRm/(I-Im)
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34. Mrs.V.S.KharoteChavan,E&Tc,PC poly
A 2mA meter with an internal resistance of 100Ω is to be converted to 0-150mA ammeter. Calculate the value of shunt resistance required.
Given: Im=2mA, Rm= 100 Ω, I= 150mA
To find: Rsh=?
Solution: m= I/Im= 150mA/ 2mA= 150/2 =75
Rsh= 1/ m-1 x Rm
1/ 75-1 x 100
100/74
Rsh= 1.35Ω
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35. Mrs.V.S.KharoteChavan,E&Tc,PC poly
Calculate the value of multiplier resistance on the 50V range of a dc voltmeter that uses
a 200μA meter movement with an internal resistance of 100Ώ.
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36. Mrs.V.S.KharoteChavan,E&Tc,PC poly
Given: Ifsd = Im = 200 μA
= 200X 10-6 A
Rm = 100Ώ
V = 50V
solution:
RS1 = V/ Ifsd – Rm
(50/ 200 X 10-6 ) – 100
– 100
249900
249X 103 Ώ
RS1 = 249 kΏ
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37. Mrs.V.S.KharoteChavan,E&Tc,PC poly
Why ammeter never connected across source of emf ? Justify.
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38. Mrs.V.S.KharoteChavan,E&Tc,PC poly
While connecting an ammeter across the emf source always a series resistance should be used.
This is necessary to limit the current passing through the meter movement maybe damaged. This is because the meter is having a small internal resistance. So it maydraw very high current from the emf source.
3. The polarity of the meter should be first observed and then it should be connected
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39. Mrs.V.S.KharoteChavan,E&Tc,PC poly
MULTIRANGE AMMETER
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40. Mrs.V.S.KharoteChavan,E&Tc,PC poly
Aytron shunt type ammeter
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41. Mrs.V.S.KharoteChavan,E&Tc,PC poly
When switch is at 100mA range position ,
the series combination of R1,R2,R3 is in parallel with meter resistance Rm.
Hence I1(R1+R2+R3) = Im Rm (1)
When switch is at 500mA range position,
The resistor R2 and R3 are in parallel with R1& Rm
Hence I2(R2+R3) = Im( R1+ Rm ) (2)
when switch is at 1A position
The resistor R3 is in parallel combination with R1+R2+R3
Hence I3(R3) = Im (R1+ R2 + R3 ) (3)
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42. Mrs.V.S.KharoteChavan,E&Tc,PC poly
Design a multi range DC ammeter using a basic movement with an internal resistance Rm =50Ω and full scale deflection current Im = LmA. The range required are 0-10 mA, 0-50 mA, mA, 0-500mA.
Ans
Given= Rm =50Ω
Im =10mA-…… Consider
I1= 10mA, I2= 50mA, I3= 100mA, I4= 500mA
Rsh1=? , Rsh2=? Rsh3=? Rsh4=?
m1= I1/ Im= 10 mA/ 1mA = 10
Rsh1= Rm/ m1-1= 50/ 10-1= 50/9= 5.55Ω
m2= I2/ Im= 50 mA/ 1mA = 50
Rsh2= Rm/ m2-1= 50/ 50-1= 50/49= 1.02Ω
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m3= I3/ Im= 100 mA/ 1mA = 100
Rsh3= Rm/ m3-1
= 50/ 100-1
= 50/39= 0.505Ω
m4= I4/ Im
= 500 mA/ 1mA
= 500
Rsh4= Rm/ m4-1
= 50/ 500-1
= 50/9= 0.100Ω
Rsh1 = 5.55Ω
Rsh2= 1.02Ω
Rsh3= 0.505Ω
Rsh4= 0.100Ω
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44. Mrs.V.S.KharoteChavan,E&Tc,PC poly
A basic D’Arsonval movement withan internal resistance of 50Ω and a full scale deflection current of 2mA is to be used as multirangevoltmeter. Design a series of string of multipliers to obtain the voltage ranges of 0-10V, 0-50V.
Given:
Rm= 50Ω
Ifsd= Im= 2mA
To find: a) Rs1 b) Rs2
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45. Mrs.V.S.KharoteChavan,E&Tc,PC poly
For range (0-10V), V1=10V
Therefore,
Rs1= V1/ Ifsd – Rm
= 10/ 2x
4950Ω
Rs1= 4.95kΩ
For range (0-50V), V2=50V
 Therefore,
Rs2= V2/ Ifsd – Rm = 50/ 2x  
 24950Ω
Rs2=24.95kΩ
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46. Mrs.V.S.KharoteChavan,E&Tc,PC poly
Sensitivity is high as compared to normal shunt type ammeter
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47. Mrs.V.S.KharoteChavan,E&Tc,PC poly
Basic DC voltmeter
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48. 2.4: A DC VOLTMETER
A basic D’Arsonval movement can be converted into a DC voltmeter by adding a series resistor (multiplier) as shown in Figure .
Im =full scale deflection current of the movement (Ifsd)
Rm=internal resistance of the movement
Rs =multiplier resistance
V =full range voltage of the instrument + Im
Multiplier V Rm _
: Basic DC Voltmeter
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49. Mrs.V.S.KharoteChavan,E&Tc,PC poly
From the circuit of Figure
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50. Mrs.V.S.KharoteChavan,E&Tc,PC poly
A basic D’ Arsonval movement with a full-scale deflection of 50 uA and internal resistance of 500Ω is used as a DC voltmeter. Determine the value of the multiplier resistance needed to measure a voltage range of 0-10V.
Solution:
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51. Mrs.V.S.KharoteChavan,E&Tc,PC poly
Sensitivity and voltmeter range can be used to calculate the multiplier resistance, Rs of a DC voltmeter.
Rs=(S x Range) - Rm
From example 2.4:
Im= 50uA, Rm=500Ω, Range=10V
Sensitivity,
So, Rs = (20kΩ/V x 10V) – 500 Ω
= kΩ
Mrs.V.S.KharoteChavan,E&Tc,PC poly 51

52. 2.5: MULTI-RANGE VOLTMETER
A DC voltmeter can be converted into a multirange voltmeter by connecting a number of resistors (multipliers) in series with the meter movement.
A practical multi-range DC voltmeter is shown in Figure 2.6. Im V2 V1 V3 Rm + V4 _
Figure 2.6: Multirange voltmeter
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53. Mrs.V.S.KharoteChavan,E&Tc,PC poly
Basic d’Arsonval meter movement with rectifier to change AC voltage to DC voltage.
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54. Mrs.V.S.KharoteChavan,E&Tc,PC poly
Sensitivity AND Loading effect of voltmeter.
The sensitivity of voltmeter means the response given by a voltmeter to input signal.
 It is the ratio of total resistance (RT) to the voltage range
S = RT / V
Where, RT – Total resistance…… RT = RS + Rm
 V= Voltage range. OR
It is also defined as the reciprocal of full scale deflection current of the basic movement.
S = 1 / Ifsd
Ifsd = full scale deflection current.
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56. Mrs.V.S.KharoteChavan,E&Tc,PC poly
Rectifier type AC voltmeter
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Basic rectifier type AC voltmeter is a general rectifier type of voltmeter. In this case for the
rectification action two diodes namely D1 and D2 are used.
An a.c input signal to be measured is applied.
If a current passing through the diode is small then there is a non- linearity problem. But for higher current the diode shows linearity. So to increase the current passing through diode; a
resistance R2 is connected in parallel with the meter.
Now during positive half cycle of input signal, diode D1 is forward biased with the polarities.
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58. Mrs.V.S.KharoteChavan,E&Tc,PC poly
While the diode D2 is reversed biased. So during this cycle the current passes the deflection.rough diode D1 and the meter. Thus the meter shows
During the negative half cycle diode D1 is reversed biased and diode D2 is forward biased.
So the current flows in opposite direction.
In this case the meter is bypassed.
Because of the diode action an a.c input signal is converted into pulsating dc.
Thus the meter shows average value of an input signal.
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59. Mrs.V.S.KharoteChavan,E&Tc,PC poly
full wave rectifier type AC voltmeter.
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Here high stabilized amplifier is used to provide the amplification.
The one terminal of stabilized amplifier is connected to the attenuator network which
consists of five resistors. The other terminal is connected with feedback path.
Capacitor is used to block d.c entering into stabilized non –linearity problem created by
diodes. Also the variation
in impedance meter is compensated by negative feedback.
The D.C. milliammeter is calibrated in terms.
The average reading is
obtained by filtering the signal coming gives from the rectifier.
Thus for meter movement
gives response to average value.
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63. Mrs.V.S.KharoteChavan,E&Tc,PC poly
Analog Multimeter.
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