Operating principle of Electrical Measuring Instruments:
Electrical measuring instruments are generally of two types. The first group is the AC- alternating 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 / VoltmeterConnection / 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.
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) Check condition of instrument and leads before use, never use a broken instrument
2) Select the appropriate Function or Parameter
3) At each measurement, confirm the range to be used.
4) 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)