Download Operating principle of Electrical Measuring

Operating principle of Electrical Measuring
Instruments:
Electrical measuring instruments are generally of two types. The first group is the ACalternating current instruments, these instruments are not polarized and in variable is of the
moving iron type. The second group is the DC direct current instruments, and these will be
discussed later.
Whenever a piece of iron is placed nearer to a magnet it would be attracted by the magnet. The
force of this attraction depends upon the strength said magnetic field. If the magnet is
electromagnet is electromagnet then the magnetic field strength can easily be increased or
decreased by increasing or decreasing current through its coil. Accordingly the attraction force
acting on the piece of iron would also be increased and decreased. Depending upon this simple
phenomenon attraction type moving iron instrument was developed.
Whenever two pieces of iron are kept side by side and a magnet is brought nearer to them the
iron pieces will repulse each other. This repulsion force is due to same magnetic poles induced in
same sides the iron pieces due external magnetic field. This repulsion force increases if field
strength of the magnet is increased. Like case if the magnet is electromagnet, then magnetic field
strength can easily be controlled by controlling input current to the magnet. Hence if the current
increases the repulsion force between the pieces of iron is increased and it the current decreases
the repulsion force between them is decreased. Depending upon this phenomenon repulsion type
moving iron instrument was constructed.
Construction of Moving Iron Instrument
The basic construction of attraction type moving iron instrument is illustrated bellow
A thin disc of soft iron is eccentrically pivoted in front of a coil. This iron tends to move inward
that is from weaker magnetic field to stronger magnetic field when current flowing through the
coil. In attraction moving instrument gravity control was used previously but now gravity control
method is replaced by spring control in relatively modern instrument. By adjusting balance
weight null deflection of the pointer is achieved. The required damping force is provided in this
instrument by air friction. The figure shows a typical type of damping system provided in the
instrument, where damping is achieved by a moving piston in an air syringe.
Theory of Attraction Type Moving Iron Instrument
Suppose when there is no current through the coil, the pointer is at zero, the angle made by the
axis of the iron disc with the line perpendicular to the field is φ. Now due current I and
corresponding magnetic field strength, the iron piece is deflected to an angle θ. Now component
of H in the direction of defected iron disc axis is Hcos{90 – (θ + φ) or Hsin(θ + φ). Now force F
acting on the disc inward to the coil is thus proportional to H2sin(θ + φ) hence the force is also
proportional to I2sin(θ + φ) for constant permeability. If this force is acting on the disc at a
distance l from the pivot, then deflection torque,
Since l is constant.
Where k is constant: Now, as the instrument is gravity controlled, controlling torque will be
Where k’ is constant.
At steady state condition,
Where K is constant:
DC - instruments
The principle of a moving coil within a magnetic field is what constitutes a moving coil
instrument used to measure DC values. Because of the problems associated with moving iron
instruments, and the fact that semiconductor technology allows to easily converting an AC
quantity to DC then modern instruments use the moving coil for both AC and DC instruments.
In your day today life, many times you are require to measure different electrical quantities like
current, voltage, resistance, etc. While doing experiment, there is necessity of multimeter. As we
have already discussed about multimeter, how it measures different electrical quantities like
electrical current, voltage, resistance, etc. But the basic instruments for the measurement of
electric current and voltage are ammeters and voltmeters respectively. Let us discuss these
instruments one by one, operating principle or working principle of ammeters and voltmeters,
finally major differences between ammeters and voltmeters.
Operating Principle:
Analog ammeters and voltmeters are classed together as there are no fundamental differences in
their operating principles. The action of all ammeters and voltmeters, with the exception of
electrostatic type of instruments, depends upon a deflecting torque produced by an electric
current. In an ammeter this torque is produced by a current to be measured or by a fraction of it.
In a voltmeter this torque is produced by a current which is proportional to the voltage to be
measured. Thus all analog voltmeters and ammeters are essentially current measuring devices.
Working principle of Ammeters:
Figure 1 – a micro Ammeter
The essential requirement of measuring instruments are (i) that its introduction into the circuit,
where measurements are to be made, does not alter the circuit conditions ;(ii)the power
consumed by them for their operation is small.
Ammeter
Ammeters are connected in series with the circuit whose current is to be measured. The power
loss in an ammeter is (I2.Ra) where I is the current to be measured Ra is the resistance of the
ammeter therefore ammeter should have low electrical resistance so that they cause a small
voltage drop and consequently absorb small power.
Working principle Voltmeters:
Figure 2 – 0-10 DC voltmeter
Voltmeter
Voltmeters are connected in parallel with the circuit whose voltage is to be measured .the power
loss in voltmeter is (V2/Rv), where V is the voltage to be measured and Rv is the resistance of the
voltmeter. Therefore voltmeters should have a high electrical resistance, in order that the current
drawn by them is small and consequently the power consumed is small.
Difference between Ammeters and voltmeters:
Parameters
Ammeter
Voltmeter
Connection
It is to be connected in series mode
It is to be connected in parallel mode
Resistance
It has comparatively low resistance
It has high resistance
Uses
It is used to find the amount of current
flowing in the circuit
It is used to find the potential
difference in the circuit
Circuit
Circuit must be disconnected in order to
attach the ammeter
Circuit does not need to be
disconnected
Accuracy
Considered as less accurate
Considered as more accurate
compared to ammeter
STUDY AND USE OF DIGITAL MULTIMETER:
With the development and the development in liquid crystal display LCD more modern
instruments are of the digital type. The simple block diagram for this instrument is shown below.
Analogue
System
ADC-
LCD
Analogue to
Digital
converter
Figure 3 – Block diagram and Complete Digital Multi Meter
Apparatus Required:
Digital Multimeter can measure, Resistors, A.C/D.C Voltage, A.C/D.C Current measurement
with the one movement. Instead of a pointer deflection as in analog meter, in digital meter an
additional circuit (ADC) is used to convert the analog signal to an equivalent decimal electrical
signal and a LCD display is used. A LCD display gives an accurate reading of the quantity
measured in the display. There is no parallax error in measurements.
Theory and Procedure:
Movement Protection Switch:
We have provided ON-OFF switch on the right side of meter case to short the movement which
prevents the damage during transit.
Function of measurement:
when using the multimeter you must first select, the function (Parameter) to be measured, that is
Voltage, current or resistance and in some case capacitance
Range of measurement:
For each parameter (Function) you will select the most suitable range for the measured quantity.
DC Voltage : 5V, 25V, 250V and 1000V
AC Voltage : 5V, 25V, 250V, 500V and 1000V
DC Current : 10mA 250mA
Resistance : Range R x 1, R x 10, R x 100 and R x 1000
Battery : Internal 1.5V Pencil Cell. (2Pcs.)
General Instructions:
1)
2)
3)
4)
Check condition of instrument and leads before use, never use a broken instrument
Select the appropriate Function or Parameter
At each measurement, confirm the range to be used.
When measuring an unknown voltage or current, always start from the highest range to
know the approximate value. Then switch it down to the appropriate range to check
accurate value.
5) By no means voltage should be measured on current or ohm range.
6) When the meter is not in use, set the selector switch on DC mA range or OFF
7) Do not expose the Meter in high temperature or humidity.
Features:
1. Wide Range of Measurement: Through the internal the batteries (1.5V x 2, 9V x 1). The meter
measures resistance up to 20 MW at maximum, while DC voltage ranges measure from 0V to
500V at full scale.
2. The meter is available for checking all kinds of electronic instruments and radio sets as well as
semi-conductors, in industrial and domestic purpose.
Measurement of DC Voltage
(a) The range selector switch is set at an appropriate DC Voltage range connecting the test prods
to their respective jacks.
(b) When checking T.V., radio and communication apparatuses the black test lead is connected
to the chassis which is -ve potential and the red test lead is applied to +ve potential is for
measurement.
(c) In case of special circuits such as oscillating circuit where negative potential is generated, the
connection of the probes are reversed, the red one to the chassis and the voltage is checked by
applying the black lead.
(d) To measure transistor circuit P-N-P type, the probes are connected as indicated in (c) above.
with N-P-N type transistor circuit, the connection is as indicate in (b).
Measurement of DC current
(a) For current measurement the tester is always connected in series to circuit checked and the
circuit measured is disconnected.
(b) The center selector switch is set at an appropriate DCmA-mA range.
(c) To measure, the red probe is applied to (+) potential and black point, to (-) potential of the
circuit. It the pointer deflects to the left across the Zero point, just reverse the connections.
Measurement of Resistance
(a) The selector switch is set to an appropriate ohm range.
(b) The resistance value is known by applying the red and the black probes each to both
ends of the resistance of measure.
(c) The ohm value read on the scale indicates the resistance measured on R x 1 range. On
the other ranges, the value indicated is multiplied by 200, 20K, 200K, 20M
respectively as the case may be.
Measurement of AC Voltage
AC Voltage ranges are mostly used to check the output voltage of power transformers or mains
supply. Using a high quality midget copper oxide rectification.
(a) The selector switch is set at an appropriate range. Test probes used irrespective of polarity.
(b) Black AC Volt scale is used, The voltage value is obtained just in the same way as with
measurement of DC Voltage.
(c) Frequency ranges used are approximately from 40 Hz to 50 Hz
Analog multimeter:
A multimeter may be implemented with a galvanometer meter movement, or less often with a
bargraph or simulated pointer such as an LCD or vacuum fluorescent display. Analog
multimeters are common; a quality analog instrument will cost about the same as a DMM.
Analog multimeters have the precision and reading accuracy limitations described above, and so
are not built to provide the same accuracy as digital instruments.
Analog meters are able to display a changing reading in real time, whereas digital meters present
such data in a manner that's either hard to follow or more often incomprehensible. Also an
intelligible digital display can follow changes far more slowly than an analogue movement, so
often fails to show what's going on clearly. Some digital multimeters include a fast-responding
bargraph display for this purpose, though the resolution of these is usually low.
Analog meters are also useful in situations where its necessary to pay attention to something
other than the meter, and the swing of the pointer can be seen without looking at it. This can
happen when accessing awkward locations, or when working on cramped live circuitry.
Analog displays are also used to very roughly read currents well above the maximum rated
current of the meter. For this, the probes are just touched to the circuit momentarily, and how fast
the pointer speeds towards fsd is noted. This is often done when testing state of charge of dry
batteries.
Digital multimeter:
Modern multimeters are often digital due to their accuracy, durability and extra features. In a
digital multimeter the signal under test is converted to a voltage and an amplifier with
electronically controlled gain preconditions the signal. A digital multimeter displays the quantity
measured as a number, which eliminates parallax errors.
Modern digital multimeters may have an embedded computer, which provides a wealth of
convenience features. Measurement enhancements available include:
Auto-ranging, which selects the correct range for the quantity under test so that the most
significant digits are shown. For example, a four-digit multimeter would automatically select an
appropriate range to display 1.234 instead of 0.012, or overloading. Auto-ranging meters usually
include a facility to 'freeze' the meter to a particular range , because a measurement that causes
frequent range changes is distracting to the user. Other factors being equal, an auto-ranging
meter will have more circuitry than an equivalent, non-auto-ranging meter, and so will be more
costly, but will be more convenient to use. An other reason to 'freeze' the range is that this
somewhat avoids 'hunting' which is a situation where the meter continuously switches between
two neighbouring ranges as when the instrument is in the low range, the value is too large but too
small in the larger range, although this may seem counter-intuitive, for non-ohmic
systems/devices this is possible.
To convert Dc multimeter to measure Ac:
Converter the Ac to Dc using Rectifier and then feed it to the Dc Multimeter.
It gives you the Vdc. So calculate Vrms using the following formulas
Vdc = (2* Vpeak)/pi
Vrms = Vpeak/root of (2)