The instrument which gives output that varies continuously as quantity to be measured is known as analog instrument. The instrument which gives output that varies in discrete steps and only has finite number of values is known as digital instrument.

1. Analog & Digital
Measuring Instruments

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CHARACTERISTICS OF IDEAL VOLTMETER AND IDEALAMMETER
1. Accuracy: The degree of exactness (closeness) of a measurement compared to the expected
(desired) value.
2. Resolution: The smallest change in a measured variable to which an instrument will respond
3. Precision: A measure of the consistency or repeatability of measurements i.e. successive reading
do not differ.
4. Error: The deviation of the true value from the desired value.
5. Sensitivity: The ratio of the change in output (response) of the instrument to a change of input or
measured variable.

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PMMC ( PERMANENT MAGNET MOVING COIL ) INSTRUMENT
• 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.
WHAT IS APMMC?

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WORKING PRINCIPLE OF PMMC INSTRUMENT
• D’Arsonval Movement Principle :
• An action caused by electromagnetic deflection, using a coil of wire and a magnetized field.
When current passes through the coil, a needle is deflected.
• Whenever electrons flow through a conductor, a magnetic field proportional to the current is
created. This effect is useful for measuring current and is employed in many practicalmeters.
• This type of meter movement is a current measuring device which is used in the ammeter,
voltmeter, and ohmmeter. Basically, both the ammeter and the voltmeter are current measuring
instruments, the principal difference being the method in which they are connected in a circuit..

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CONSTRUCTION OF PMMC INSTRUMENT
 The PMMC instrument consists of a permanent
powerful magnet of a horse-shoe form with soft-iron pole
pieces attached to it. Between the magnetic pole pieces a
soft iron cylinder is fixed, which serves to provide a
uniform magnetic field in the air-gap between the pole
pieces and the cylinder.
The coil is wound on a light metal frame and is mounted
so that it can rotate freely in the air-gap. The pointer
attached to the coil, moves over a graduated scale and
indicates the angular deflection of the coil and therefore
the current through the coil.

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Two Phosphor-Bronze conductive springs normally
equal in strength, provide the calibrated force opposing
the moving coil torque.
The entire moving system is statistically balanced all
deflection positions by three balance weights. The
pointer, springs and pivots are assembled to the coil
structure by means of pivot base, and the entire movable
coil element is supported by jewel bearings.
Parts:
Permanent Magnet, Controlled Spring, Former, Coil, Pivot,
Pointer,Scale,

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WORKING
When current is passed through the coil, force is
setup on its both sides which produces a deflection
torque and the direction can be determined by
applying Fleming’s Left Hand Rule.
Torque Equation
 The deflection torque Td is given by,
Td = Force * Perpendicular distance
Td = BNAI N - mt

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Td = BNAI N - mt
Where,
Td = deflecting torque in N-m
B = flux density in air gap, Wb/m2
N = Number of turns of the coils
A = effective area of coil m2
I = current in the moving coil, amperes
Therefore,
Td = GI
Where, G = NBA = constant
The controlling torque is provided by the springs and is proportional to the angular deflection of
the pointer.

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Tc = KØ
Where,
Tc = Controlling Torque
K = Spring Constant Nm/rad or Nm/deg
Ø = angular deflection
For the final steady state position,
Td = Tc
Therefore GI = KØ
So,
Ø = (G/K)I or I = (K/G) Ø
Thus the deflection is directly proportional to the current passing through the coil. The pointer
deflection can therefore be used to measure current.

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ADVANTAGES
 It has a very high torque to weight ratio.
 The PMMC has consumes low power.
 The PMMC has a very high accuracy.
 The PMMC is free from hysteresis error.
 Extension of instrument range is possible with the help of shunt and series resistances.
 The PMMC has efficient damping characteristics and is not affected by stray magnetic field.

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DISADVANTAGES
 It has comparatively high cost.
 The PMMC has only suitable for d.c. measurements.
 Aging of permanent magnets and control springs introduces errors.

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DC AMMETER &
VOLTMETER

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EXAMPLE

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EXAMPLE

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EXAMPLE

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EXAMPLE

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FET INPUT VOLTMETER

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CIRCUIT DIAGRAM OF FET INPUT VOLTMETER

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RAMP TYPE DVM

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PRINCIPLE
Input voltage is converted into digital equivalent by counting the time taken for the ramp
wave to decrease from the magnitude of input voltage to 0V.
CONSTRUCTION
The block diagram of the Ramp-type ADC can be divided into two sections as follows:
1. Voltage to time conversion section
2. Time measurement section

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VOLTAGE TO TIME CONVERSION SECTION
 In the voltage to time conversion section, the analog input voltage is fed to the attenuation
circuit. The attenuated signal is compared with the ramp signal generated by the ramp
generator given in the block diagram by the input comparator 'C1'. Similarly, The ramp
signal generated is compared with 0V via a zero-crossing detector 'C2'.
 A sample rate multivibrator is connected to the ramp generator whose purpose is to provide
an initiating pulse for the ramp generator to start the next ramp voltage for the next
measurement. It is also used to reset the counter before generating the next ramp voltage.

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 The counter is reset after each successful completion of time measurement by a control signal
from the sample rate multivibrator. The count produced is displayed by connecting suitable
display device.
TIME MEASUREMENT SECTION
 In the time measurement section, there is counter which is triggered by a gating pulse. The
inputs of the gating pulse are (i) Output of 'C1' (ii) Output of 'C2' (iii) Clock pulse from the
oscillator.

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OPERATION
 Initially, the attenuated signal is compared with a negative going ramp signal generated by
the ramp generator. When the ramp voltage coincides with the input signal, the output of 'C1'
becomes low. This point is called coincidence point. This initiates the counting process ( start
of count ). The counter continues to count until the ramp voltage reduces and crosses zero
(0V). This is detected by zero crossing detector 'C2'. The output of 'C2' becomes high which
ends the counting process (end of count).

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 The count displayed is the
count of number of clock
pulses produced by the
oscillator during the time in
which the ramp signal is less
than the input signal and
greater than 0V (ie) |input
signal| > ramp > 0V. This
count gives the digital
equivalent of input analog
voltage.

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RAMP TYPE DVM

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The basic digital frequency meter needs the following components:-
1. Amplifier
2. Schmitt trigger
3. AND gate
4. A counter
5. Crystal oscillator
6. Time base selector
7. A flip-flop

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OPERATION
 When we apply an input signal to the meter, the amplifier present in it starts amplifying the
signal. The amplification is done in order to strengthen the weak magnitude of input signal.
 The amplified input is now given to Schmitt trigger which then converts the input signal into
square wave.
 The square waves are then separated and clipped to obtain a train of pulses.
 Similarly, the oscillator produces a sine wave which is then converted into square wave which
produces a square wave and is applied to the flip-flop.
 Now, the first pulse activates the gate control FF connected after the Schmitt trigger which
enables the AND gate.

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 Now, the train of pulses is then applied to AND gate due to which AND gate produces an
Output.
 This Output is then given to the decimal counter which counts the first decimal number.
 Now when the second pulse is given to the Gate Flip-flop which removes the AND gate to “turn
ON”.
 Thus, when the third pulse arrives at the flip-flop the AND gate is then enabled until then the
counter shows the previous counted decimal.
 The decimal counter and display unit shows the output corresponding to the train of pulses
received for a precise time interval. Thus, the counter output shows only the frequency of signal.

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APPLICATIONS OF DIGITAL FREQUENCY METER
 It is used for testing radio equipment.
 It is used for measuring the temperature, pressure, and other physical values.
 It is used for measuring vibration, strain

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DIFFERENCE BETWEEN ANALOG INSTRUMENTS AND DIGITAL
INSTRUMENTS
ANALOG INSTRUMENT DIGITAL INSTRUMENT
The instrument which gives output that varies
continuously as quantity to be measured is
known as analog instrument.
The instrument which gives output that varies
in discrete steps and only has finite number of
values is known as digital instrument.
The accuracy of analog instrument is less. The accuracy of digital instrument is more.
The analog instruments required more power. The digital instruments required less power.

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Sensitivity of analog instrument is more. Sensitivity of digital instrument is less.
The analog instruments are cheap. The digital instruments are expensive.
The analog instruments are extremely portable. The digital instruments are not easily portable.
The resolution of analog instruments is less. The resolution of digital instruments is more.

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THANK YOU