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By
Dr. G.N.Kodanda Ramaiah /
P. Siva Nagendra Reddy
Functional elements of Instruments
Primary sensing element
 The quantity under measurement makes its first contact with
primary sensing element of a measure...
Variable manipulation element
 The main function of variable manipulation element is
to manipulation element is to manipu...
Data presentation element:
 The information about the quantity under measurement
has to be conveyed to the personal handl...
PERFORMANCE CHARACTERISTICS
 The performance characteristics are divided in to :-
1.Static characteristics
2.Dyanamic cha...
STATIC CHARACTERISTICS
Instrument: A device or mechanism used to determine
the present value of the quantity under measure...
ACCURACY:
The degree of exactness or closeness of a measurement of a
measurement compared to the expected or desired value...
SENSITIVITY:
The ratio of change in output or response of the
instrument to a change of input or measured value.
DEFLECTION FACTOR:
The reciprocal of sensitivity of is called as deflection factor.
 deflection factor = 1/ sensitivity
RESOLUTION:
 Resolution is the smallest change in the measured value to which
a instrument can respond .
CALIBRATION
 Ca...
ERROR:
It is the difference between true value and a measured value.
e = Y n - X n
Where e=absolute errors; Yn=expected va...
The errors that may occur in an
instrument
 i. Gross errors or personal Errors
 ii. Systematic errors
 iii. Instrumenta...
Grass error:
 This class of errors mainly covers human mistakes in reading or using
instruments and in recording and calc...
Systematic error:
 These types of errors are divided into three categories such as
instrumental errors, Environmental err...
Random errors:
 This occurs are due to unknown causes and are observed when the
magnitude and polarity of a measurement f...
2)Fidelity:
 It is the degree to which an instrument indicates the
changes in the measured variable without dynamic
error...
inputs
Comparison between Electronic
meters and Conventional meters
BASIC PRINCIPLE OF ANALOG
METER
This permanent magnet moving coil meter movement
is the basic movement in most analog (me...
Basic Principle Operation Of Permanent-Magnetic Moving-Coil
Movement
 The permanent magnet moving coil instruments are most
accurate type for direct current measurements.
 The action of the...
 The pointer is carried by the spindle and it moves over a
graduated scale.
 The pointer has light weight so that it def...
 The deflecting torque produced is described below in
mathematical form:
Deflecting Torque, T = BINA
Where
B = flux densi...
DC VOLTMETER
 A basic d’Arsonval movement can be converted into dc
voltmeter by adding in series resistor multiplier as
s...
IM = full scale deflection current of the movement (Ifsd)
RM = internal resistance of the movement
RS = multiplier resista...
DC AMMETER
 The PMMC galvanometer constitutes the basic movement of a dc
ammeter. The coil winding of a basic movement is...
Multi Range Ammeter
R1 R2 R3 R4
+
_
+
_
Rm
D’Arsonval
Movement
Figure 2.3: Multirange Ammeter
S
Rc
Rb
Ra
Rm
D’Arsonval Meter
+
_
3
1
2+
_
Figure 2.4: Aryton Shunt
ARYTON SHUNT AMMETER
 Aryton shunt eliminates the possibility of having the
meter in the circuit without a shunt.
 Reduce cost
 Position of ...
Ayrton shunt or universal shunt
 The selector switch S, selects the appropriate shunt required to change
the range of the meter.
 When the position of t...
where I2 is the second range required.
In position 3, R1 + R2 + R3 is in parallel with Rm .
where I3 is the third range re...
REQUIREMENT OF A SHUNT
1) Minimum Thermo Dielectric Voltage Drop
Soldering of joint should not cause a voltage drop.
2) So...
DC VOLTMETER
 A basic D’Arsonval movement can be converted into a DC voltmeter by
adding a series resistor (multiplier) a...
 From the circuit of Figure 2.5:
Therefore,
m
m
s
m
mm
mm
s
msm
R
I
V
R
R
I
V
I
RIV
R
RRIV




 )(
2.5: MULTI-RANGE VOLTMETER
 A DC voltmeter can be converted into a
multirange voltmeter by connecting a number of
resisto...
Solid State mV Voltmeter using
OpAmp
AC Differential Voltmeter
EXAMPLE
A basic D’ Arsonval movement with a full-scale
deflection of 50 uA and internal resistance of
500Ω is used as a DC...
 Sensitivity and voltmeter range can be used to
calculate the multiplier resistance, Rs of a DC
voltmeter.
Rs=(S x Range)...
 In order to measure the alternating current with the d’Arsonval
meter movement, we must rectify the alternating current ...
 The peak value of 10 Vrms sine wave is,
or
 If the output voltage from the half-wave rectifier is 10V only, a dc
voltme...
Example 5.1: D’Arsonval Meter Half-Wave Rectifier.
Compute the value of the multiplier resistor for a 10 Vrms ac range on ...
 Commercially produced ac voltmeters that use half-wave
rectification have an additional diode and shunt as shown in
Figu...
 The full-wave rectifier provide higher sensitivity rating compare to
the half-wave rectifier.
 Bridge type rectifier is...
Operation;
 (a) During the positive half cycle (red arrow), currents flows through
diode D2, through the meter movement f...
 From the circuit in Figure 5.9, the peak value of the 10 Vrms signal with
the half-wave rectifier is,
 The average dc v...
Example 5.2: D’Arsonval Meter Full-Wave Rectifier.
Each diode in the full-wave rectifier circuit in Figure 5.10 has an ave...
 The equivalent DC voltage is,
(b) The ac sensitivity,
(c.) The dc sensitivity,
 .


K
mA
V
I
E
R
VVVE
T
dc
T
rms...
Multi Range AC Voltmeter
VOLTMETER LOADING EFFECTS
 When a voltmeter is used to measure the voltage across a
circuit component, the voltmeter circ...
OHMMETER
1. An ohmmeter is an instrument used to measure resistance and
check the continuity of electrical circuits and co...
 The purpose of an ohmmeter, of course, is to measure the resistance
placed between its leads. This resistance reading is...
OHMMETER (Series Type)
 Current flowing through meter movements depends on the magnitude
of the unknown resistance.(Fig 4...
OHMMETER (Series Type)
From equation (2-8) and (2-9):
(2-10)
V
RRI
RR
hmfsd
h 1
Figure 2.7: Measuring circuit resistance with an ohmmeter
2. When RX = 0 (short circuit), R2 is adjusted to get full-
scale current through the movement. Then, I = Ifsd. The
pointe...
Shunt type ohmmeter
1. Figure shows the basic circuit of the shunt-type ohmmeter where
movement mechanism is connected par...
3. Let the switch be closed. When RX = 0 (short circuit), the
pointer reads zero because full current flows through Rx and...
Digital ohmmeter
 Digital ohmmeter s are used to measure the
resistance accurately
 It shows the exact reading of the re...
Analog ohmmeter
 Analog ohmmeters are used to measure
the resistance of the circuit with respect to the
current flowing t...
Multimeter
DEPARTMENT OF ECE,
KUPPAM ENGINEERING COLLEGE,KUPPAM.
DEPARTMENT OF ECE,
KUPPAM ENGINEERING COLLEGE,KUPPAM.
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  1. 1. By Dr. G.N.Kodanda Ramaiah / P. Siva Nagendra Reddy
  2. 2. Functional elements of Instruments
  3. 3. Primary sensing element  The quantity under measurement makes its first contact with primary sensing element of a measurement system here, the primary sensing element transducer.  This transducer converts measured into an analogous electrical signal. Variable conversion element  The output of the primary sensing element is the electrical signal.  It may be a voltage a frequency or some other electrical parameter. But this output is not suitable for this system.  For the instrument to perform the desired function, it may be necessary to convert this output to some other suitable form while retaining the original signal.  Consider an example, suppose output is an analog signal form and the next of system accepts input signal only in digital form
  4. 4. Variable manipulation element  The main function of variable manipulation element is to manipulation element is to manipulate the signal presented to it preserving the original nature of the signal. Here, manipulation means a change in numerical value of the signal.
  5. 5. Data presentation element:  The information about the quantity under measurement has to be conveyed to the personal handling the instrument or system for control or analysis purposes.  The information conveyed must be in the form of intelligible to the personnel.  The above function is done by data presentation element.  The output or data of the system can be monitored by using visual display devices may be analog or digital device like ammeter, digital meter etc.  In case the data to be record, we can use analog or digital recording equipment. In industries , for control and analysis purpose we can use computers.
  6. 6. PERFORMANCE CHARACTERISTICS  The performance characteristics are divided in to :- 1.Static characteristics 2.Dyanamic characteristics  Static characteristics indicate the response of the instrument for slowly varying data or time invariant data.  Dynamic characteristics denote the behaviour of the instrument for the time varying quantities.  The instrument design ,testing and evaluation is performed based on the parameters.
  7. 7. STATIC CHARACTERISTICS Instrument: A device or mechanism used to determine the present value of the quantity under measurement. Measurement: The process of determining the amount, degree, or capacity by comparison (direct or indirect) with the accepted standards of the system units being used.
  8. 8. ACCURACY: The degree of exactness or closeness of a measurement of a measurement compared to the expected or desired value . PRECISION: A measure of the consistency or repeatability of measurements i.e, succesive readings do not differ. or precision is the consistency of the instrument output for a given value of input. precision where, xn=value of the nth measurement xnbar=average value of the set of measured values
  9. 9. SENSITIVITY: The ratio of change in output or response of the instrument to a change of input or measured value.
  10. 10. DEFLECTION FACTOR: The reciprocal of sensitivity of is called as deflection factor.  deflection factor = 1/ sensitivity
  11. 11. RESOLUTION:  Resolution is the smallest change in the measured value to which a instrument can respond . CALIBRATION  Calibration is the process of making an adjustment or marking a scale so that the readings of an instrument agree with the accepted value and the certified standard.
  12. 12. ERROR: It is the difference between true value and a measured value. e = Y n - X n Where e=absolute errors; Yn=expected value; Xn=measured value; Therefore %error = (absolute value/expected value)*100=(e/Yn)*100  Therefore %error =  It is more frequently expressed as an accuracy rather than error. Therefore A=1 - Where A is the relative accuracy Accuracy is expressed as % accuracy a=100% - %error a=A*100% (where a=%accuracy)
  13. 13. The errors that may occur in an instrument  i. Gross errors or personal Errors  ii. Systematic errors  iii. Instrumental errors  iv. Environmental errors  v. Observational errors  vi. Random Errors
  14. 14. Grass error:  This class of errors mainly covers human mistakes in reading or using instruments and in recording and calculating measured values As long a human beings are involved.  Some gross errors will definitely be committed. Although complete elimination of gross errors is probably impossible, are should try to anticipate and correct them.  Some gross errors are easily detected while others may be very difficult to detect. The experiment may grossly misread the scale.  Great care should be taken in reading and recording the data.  Two , three or even more readings should be taken for the quantity under measurement.  These readings should be taken preferably by different experimenters and the readings should be taken at a different reading point to avoid re-reading with the same error.  Never place complete dependence on one reading but take at least three separate readings. Preferably under conditions in which instruments are switched off- on.
  15. 15. Systematic error:  These types of errors are divided into three categories such as instrumental errors, Environmental errors and observational errors. Instrumental errors:  These errors arise due to inherent short comings in the instruments misuse of the instruments and loading effects. Environmental errors:  These errors are due to conditions external to the measuring device including conditions in the area surrounding the instrument.  These may be effects of temperature, pressure, humidity, dust, vibrations or of external magnetic or electrostatic fields.  The connective measures employed to eliminate to reduce these undesirable effects.
  16. 16. Random errors:  This occurs are due to unknown causes and are observed when the magnitude and polarity of a measurement future in an unpredictable manner.  Some of the more common random errors are: (i) Rounding error: This occurs when readings are between scale graduations and the reading is rounded up or down to the nearest graduation. (ii) Periodic error:  This occurs when an analog meter reading swings or fluctuates about the correct reading.  In addition, the meter reading quickly changes in the immediate vicinity of the corrected value, but changes slowly at the extremes of the swing.  Since it could be easier to read the meter when it is slowly changing, the correct value would be less likely read than an incorrect value.  The other random errors are due to noise backlash and ambient influence.  Random errors cannot normally be predicted or corrected but they can be minimized by skilled observes using a well maintained quality instrument.
  17. 17. 2)Fidelity:  It is the degree to which an instrument indicates the changes in the measured variable without dynamic error (faithful reproduction).
  18. 18. inputs
  19. 19. Comparison between Electronic meters and Conventional meters
  20. 20. BASIC PRINCIPLE OF ANALOG METER This permanent magnet moving coil meter movement is the basic movement in most analog (meter with a pointer indicator hand) measuring instruments. It is commonly called d'Arsonval movement because it was first employed by the Frenchman d'Arsonval in making electrical measurements.
  21. 21. Basic Principle Operation Of Permanent-Magnetic Moving-Coil Movement
  22. 22.  The permanent magnet moving coil instruments are most accurate type for direct current measurements.  The action of these instruments is based on the motoring principle. When a current carrying coil is placed in the magnetic field produced by permanent magnet, the coil experiences a force and moves.  As the coil is moving and the magnet is permanent, the instrument is called permanent magnet moving coil instrument. This basic principle is called D’Arsonval principle.  The amount of force experienced by the coil is proportional to the current passing through the coil.
  23. 23.  The pointer is carried by the spindle and it moves over a graduated scale.  The pointer has light weight so that it deflects rapidly.  The mirror is placed below the pointer to get the accurate reading by removing the parallax.  The weight of the instrument is normally counter balanced by the weights situated diametrically opposite and rapidly connected to it.  The scale markings of the basic d.c PMMC instruments are usually linearly spaced as the deflecting torque and hence the pointer deflections are directly proportional to the current passing through the coil.
  24. 24.  The deflecting torque produced is described below in mathematical form: Deflecting Torque, T = BINA Where B = flux density in Wb/m2 (Tesla) I = current (A). N = number of turns of the coils. A = area ( length X wide), (m2).
  25. 25. DC VOLTMETER  A basic d’Arsonval movement can be converted into dc voltmeter by adding in series resistor multiplier as shown in fig.
  26. 26. 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
  27. 27. DC AMMETER  The PMMC galvanometer constitutes the basic movement of a dc ammeter. The coil winding of a basic movement is small and light, so it can carry only very small currents.  The PMMC can use to build an ammeter with connected the shunt resistor and meter in parallel.  A low value resistor (shunt resistor) is used in DC ammeter to measure large current.  Rm = internal resistance of the movement  Rsh = shunt resistance  Ish =shunt current  Im = full scale deflection current of the movement  I = full scale current of the ammeter + shunt (i.e. total current)
  28. 28. Multi Range Ammeter
  29. 29. R1 R2 R3 R4 + _ + _ Rm D’Arsonval Movement Figure 2.3: Multirange Ammeter S
  30. 30. Rc Rb Ra Rm D’Arsonval Meter + _ 3 1 2+ _ Figure 2.4: Aryton Shunt ARYTON SHUNT AMMETER
  31. 31.  Aryton shunt eliminates the possibility of having the meter in the circuit without a shunt.  Reduce cost  Position of the switch:  ‘1’: Ra parallel with series combination of Rb, Rc and the meter movement. Current through the shunt is more than the current through the meter movement, thereby protecting the meter movement and reducing its sensitivity.  ‘2’: Ra and Rb in parallel with the series combination of Rc and the meter movement. The current through the meter is more than the current through the shunt resistance.  ‘3’: Ra, Rb and Rc in parallel with the meter. Maximum current flows through the meter movement and very little through the shunt. This will increase the sensitivity.
  32. 32. Ayrton shunt or universal shunt
  33. 33.  The selector switch S, selects the appropriate shunt required to change the range of the meter.  When the position of the switch is '1' then the resistance R1 is in parallel with the series combination of R2 , R3 and Rm.  Hence current through the shunt is more than the current through the meter, thus protecting the basic meter. When the switch is in the position '2', then the series resistance of R1 and R2 is in parallel with the series combination of R3 and Rm. The current through the meter is more than through the shunt in this position. In the position '3', the resistances R1 , R2 and R3 are in series and acts as the shunt. In this position, the maximum current flows through the meter.  This increases the sensitivity of the meter. The voltage drop across the two parallel branches is always equal. Thus, Ish Rsh = Im Rm But in position 1, R1 is in parallel with R2 + R3 + Rm where I1 is the first range required. In position 2, R1 + R2 is in parallel with R3 + Rm .
  34. 34. where I2 is the second range required. In position 3, R1 + R2 + R3 is in parallel with Rm . where I3 is the third range required.  The current range I3 is the minimum while I1 is maximum range possible. Solving the equations (1), (2) and (3) the required Ayrton shunt can be designed.
  35. 35. REQUIREMENT OF A SHUNT 1) Minimum Thermo Dielectric Voltage Drop Soldering of joint should not cause a voltage drop. 2) Solderability - never connect an ammeter across a source of e.m.f - observe the correct polarity - when using the multirange meter, first use the highest current range.
  36. 36. DC VOLTMETER  A basic D’Arsonval movement can be converted into a DC voltmeter by adding a series resistor (multiplier) as shown in Figure 2.3.  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 Rs Im Rm Multiplier V + _ Figure 2.5: Basic DC Voltmeter
  37. 37.  From the circuit of Figure 2.5: Therefore, m m s m mm mm s msm R I V R R I V I RIV R RRIV      )(
  38. 38. 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. Figure 2.6: Multirange voltmeter R1 R2 R3 R4 + _ V1 V2 V3 V4 Rm Im
  39. 39. Solid State mV Voltmeter using OpAmp
  40. 40. AC Differential Voltmeter
  41. 41. EXAMPLE 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:  k uA V R I V R m m s 5.199500 50 10
  42. 42.  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 = 199.5 kΩ Vk uAI S m /20 50 11 
  43. 43.  In order to measure the alternating current with the d’Arsonval meter movement, we must rectify the alternating current by use of diode rectifier .  Figure 5.6 is the DC voltmeter circuit modified to measure AC voltage.  The diode, assume to be ideal diode, has no effect on the operation of the circuit .  For example if the 10 V sine-wave input is fed as the source of the circuit, the voltage across the meter movement is just the positive half-cycle of the sine wave due to the rectifying effect of the diode. D’Arsonval Meter Movement with Half-Wave Rectification. Figure 5.6: DC Voltmeter Circuit Modified to Measure AC Voltage.
  44. 44.  The peak value of 10 Vrms sine wave is, or  If the output voltage from the half-wave rectifier is 10V only, a dc voltmeter will provide an indication of approximately 4.5 V.  From the above equation, rms rms dc pave E E E EE *45.0 *2 * 2     m dc rms m dc dc s R I E R I E R  45.0 peakrms rmsp VV EE 14.14414.1*10 2   dcac SS 45.0 Cont’d…
  45. 45. Example 5.1: D’Arsonval Meter Half-Wave Rectifier. Compute the value of the multiplier resistor for a 10 Vrms ac range on the voltmeter shown in Figure 5.7. Solution: Find the sensitivity for a half wave rectifier. .       K V V RRangeSR VI SS macacs fs dcac 2.4300 1 10 * 450 * 4501 *45.045.0 Figure 5.7: AC Voltmeter Using Half- Wave Rectification.
  46. 46.  Commercially produced ac voltmeters that use half-wave rectification have an additional diode and shunt as shown in Figure 5.8, which is called instrument rectifier.  .Figure 5.8: Half-Wave Rectification Using an Instrument Rectifier and a Shunt Resistor. Cont’d…
  47. 47.  The full-wave rectifier provide higher sensitivity rating compare to the half-wave rectifier.  Bridge type rectifier is the most commonly used, Figure 5.9. 5.3 D’Arsonval Meter Movement with Full-Wave Rectification. Figure 5.9: Full Wave Bridge Rectifier Used in AC Voltmeter Circuit.
  48. 48. Operation;  (a) During the positive half cycle (red arrow), currents flows through diode D2, through the meter movement from positive to negative, and through diode D3. - The polarities in circles on the transformer secondary are for the positive half cycle. - Since current flows through the meter movement on both half cycles, we can expect the deflection of the pointer to be greater than with the half wave cycle. - If the deflection remains the same, the instrument using full wave rectification will have a greater sensitivity. (b) Vise-versa for the negative half cycle (blue arrow). Cont’d…
  49. 49.  From the circuit in Figure 5.9, the peak value of the 10 Vrms signal with the half-wave rectifier is,  The average dc value of the pulsating sine wave is,  Or can be compute as,  The AC voltmeter using full-wave rectification has a sensitivity equal to 90% of the dc sensitivity or twice the sensitivity using half-wave rectification. peakrmsp VEE 14.14*414.1  VEE pave 9636.0  VVEE rmsave 910*9.0*9.0  dcac SS *9.0 Cont’d…
  50. 50. Example 5.2: D’Arsonval Meter Full-Wave Rectifier. Each diode in the full-wave rectifier circuit in Figure 5.10 has an average forward bias resistance of 50 Ohm and is assumed to have an infinite resistance in the reverse direction. Calculate, (a) The multiplier Rs. (b) The AC sensitivity. © The equivalent DC sensitivity. Solution: (a) Calculate the current shunt and total current, . mAmAmAIII and mA mA R E I mshT sh m sh 211 1 500 500*1      Figure 5.10: AC Voltmeter Using Full- Wave Rectification and Shunt.
  51. 51.  The equivalent DC voltage is, (b) The ac sensitivity, (c.) The dc sensitivity,  .   K mA V I E R VVVE T dc T rmsdc 5.4 2 0.9 0.910*9.010*9.0       K RR RR RRR shm shm dTs 15.4 500500 500*500 50*24500 2 V VRange R S T ac /450 10 4500    V VS S or V mAI S ac dc T dc /500 9.0 /450 9.0 /500 2 11     Cont’d…
  52. 52. Multi Range AC Voltmeter
  53. 53. VOLTMETER LOADING EFFECTS  When a voltmeter is used to measure the voltage across a circuit component, the voltmeter circuit itself is in parallel with the circuit component.  Total resistance will decrease, so the voltage across component will also decrease. This is called voltmeter loading.  The resulting error is called a loading error.  The voltmeter loading can be reduced by using a high sensitivity voltmeter.  How about ammeter??
  54. 54. OHMMETER 1. An ohmmeter is an instrument used to measure resistance and check the continuity of electrical circuits and component. This resistance reading is indicated through a meter movement. 2. The ohmmeter must then have an internal source of voltage to create the necessary current to operate the movement, and also have appropriate ranging resistors to allow desired current to flow through the movement at any given resistance. 3. Two types of schemes are used to design an ohmmeter – series type and shunt type. 4. The series type of ohmmeter is used for measuring relatively high values of resistance, while the shunt type is used for measuring low values of the resistance.
  55. 55.  The purpose of an ohmmeter, of course, is to measure the resistance placed between its leads. This resistance reading is indicated through a mechanical meter movement which operates on electric current.  The ohmmeter must have an internal source of voltage to create the necessary current to operate the movement, and also have appropriate ranging resistors to allow just the right amount of current through the movement at any given resistance.  A more accurate type of ohmmeter has an electronic circuit that passes a constant current (I) through the resistance, and another circuit that measures the voltage (V) across the resistance. According to the following equation, derived from Ohm's Law, the value of the resistance (R) is given by R =V/I. Operation of an Ohmmeter
  56. 56. OHMMETER (Series Type)  Current flowing through meter movements depends on the magnitude of the unknown resistance.(Fig 4.28 in text book)  The meter deflection is non-linearly related to the value of the unknown Resistance, Rx.  A major drawback – as the internal voltage decreases, reduces the current and meter will not get zero Ohm.  R2 counteracts the voltage drop to achieve zero ohm. How do you get zero Ohm?  R1 and R2 are determined by the value of Rx = Rh where Rh = half of full scale deflection resistance. (2-8)  The total current of the circuit, It=V/Rh  The shunt current through R2 is I2=It-Ifsd m m mh RR RR RRRRR   2 2 121 )//(
  57. 57. OHMMETER (Series Type) From equation (2-8) and (2-9): (2-10) V RRI RR hmfsd h 1
  58. 58. Figure 2.7: Measuring circuit resistance with an ohmmeter
  59. 59. 2. When RX = 0 (short circuit), R2 is adjusted to get full- scale current through the movement. Then, I = Ifsd. The pointer will be deflected to its maximum position on the scale. Therefore, this full-scale current reading is marked 0 ohms. 3. When RX = ∞ (open circuit), I = 0. The pointer will read zero. Therefore, the zero current reading is marked ∞ ohms.
  60. 60. Shunt type ohmmeter 1. Figure shows the basic circuit of the shunt-type ohmmeter where movement mechanism is connected parallel to the unknown resistance. In this circuit it is necessary to use a switch, otherwise current will always flow in the movement mechanism. 2. Resistor ‘Rsh’ is used to bypass excess current.
  61. 61. 3. Let the switch be closed. When RX = 0 (short circuit), the pointer reads zero because full current flows through Rx and no current flows through the meter and Rsh. Therefore, zero current reading is marked 0 ohms. 4. When RX = ∞ (open circuit), no current flows through RX. Resistor R1 is adjusted so that full-scale current flows through the meter. Therefore, maximum current reading is marked ∞ ohms.
  62. 62. Digital ohmmeter  Digital ohmmeter s are used to measure the resistance accurately  It shows the exact reading of the resistance  It is mostly used to measure the earth resistance
  63. 63. Analog ohmmeter  Analog ohmmeters are used to measure the resistance of the circuit with respect to the current flowing the circuit  It shows the reading in the analog form so if there is mistake by the reader in taking reader there it will cause great difference in the calculations.  Many analog ohmmeters will, when switched to the ohm function, reverse the polarity of the test leads.
  64. 64. Multimeter
  65. 65. DEPARTMENT OF ECE, KUPPAM ENGINEERING COLLEGE,KUPPAM.
  66. 66. DEPARTMENT OF ECE, KUPPAM ENGINEERING COLLEGE,KUPPAM.

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