INTRODUCTION TO DIGITAL INSTRUMENTS
An analog device in which the output or display is a continuous function of time and bears a constant relation to the input. Analog instruments find extensive use in the present day which are common to both analog and digital instruments and are fairly limited at present. Hence, it can safely be predicted that analog instruments will remain in extensive use for a number of years and are not likely to be completely replaced by digital instruments for certain applications.
Analog instruments may be classified according to the quantity they measure (ie.) An instrument meant for the measurement of current is an Ammeter, while an instrument for the measurement of voltage is a voltmeter.
Types of Analog Instruments:
1. PMMC instrument
2. DC ammeter
3. DC voltmeter
4. Rectifier type voltmeter and ammeter.
5. Series and Shunt type ohmmeters
6 FET input voltmeter.
7. Differential voltmeter.
INTRODUCTION TO DIGITAL INSTRUMENTS
The digital instrument basic circuits involve the use of logic gates and flip-flops. A logic gate is a circuit that produces a desired high or low output voltage when its several input terminals are all at the prescribed high or low input voltage level.
Digital instruments are rapidly replacing analog instruments, because of their easy readability, better resolution, and higher accuracy. Digital instruments display the output in numerical form i.e.; in digital form. Digital instruments are used to measure the voltage, current, power, frequency, etc. To enable digital instruments to recognize information inputs that are analog in nature must be converted to digital form.
Hence any digital instrument consists of an analog-to-digital converter in its input stages. The basic building of Digital instrument digital instrument may consist of the following components- Resistors, Capacitors, Transistors, Linear ICs, Digital ICs, Display devices, Analog Digital Converters, and Digital to Analog Converters. The display block is analog or digital. If analog readout is required, a stage involving digital to analog conversion can be included before display.
Characteristics of Ideal Voltmeter :
1. Internal resistance is infinite.
2. No loading effect.
3. No drift problem.
4. Input impedance is infinite.
5. Fidelity is 100%.
6. High accuracy.
7. High precision.
8. High resolution.
9. No temperature variation in output.
Characteristics of Ideal Ammeter:
1. Internal resistance is Zero.
2. No loading effect.
3. Fidelity is 100%
4. High accuracy
5. High precision.
6. High resolution
7. No temperature variation in output
8. Eddy current error, hysteresis error, stray magnetic field error, reactance error, and temperature error are zero.
CONSTRUCTION AND PRINCIPLE OF OPERATION OF PMMC INSTRUMENT
The permanent magnet-moving coil instrument is the most accurate type for de measurements. The working principle of these instruments is the same as that of the D'Arsonval type of galvanometers, the difference being that a direct reading is provided with a pointer and a scale.
The basic PMMC movement offers the largest magnet in a given space and is used when a large flux is required in the air gap. It has an instrument with very low power consumption. The low current is required for full-scale deflection.
Construction: The constructional features of the PMMC instrument. As the name indicates, the instrument consists of a permanent magnet of horseshoe form with soft iron pole pieces attached to it. A permanent magnet is made up of Alnico material. Between the pole pieces, there is a cylindrical core of soft iron, which serves to provide a uniform magnetic field in the air gap between the pole pieces and the cylindrical core. Surrounding the core is a rectangular coil of many turns wound on a light metal (aluminum) frame and is mounted so that it can rotate freely in the air gap. The coil attached to the pointer moves over the graduated scale and indicates the angular deflection of the coil.
The aluminum metal frame not only provides support for the coil but also provides damping torque by eddy currents induced in it. The sides of the coil are free to move in the two air gaps between the holes and core. Control of the coil movement is provided by two phosphor bronze air springs, one above and one below. These control springs also serve the purpose of leading the current in and out of a coil.
Control of the coil movement is provided by two phosphor bronze air springs, one above and one below as shown in Fig. 1.4. These control springs also serve the purpose of leading the current in and out of a coil. The entire moving system is statically balanced for all deflection positions by three weights.
(i) The pointer springs
(ii) Pivots are fixed to the coil assembly by means of a pivot basis
(iii) Entire movable coil element supported by jewel bearings.
The scale markings of the basic DC PMMC instrument are usually linearly spaced because the torque is directly proportional to the coil current. The basic PMMC instrument is not suitable for AC measurements unless the current is rectified before reaching the coil.
ADVANTAGES AND DISADVANTAGES OF PMMC
Advantages of PMMC:
1. They have low power consumption (25 μw to 200 μω).
2. They have no hysteresis.
3. They can be modified with the help of shunts and resistors to cover a wide range of currents and voltages.
4. Since the operating fields of such instruments are very strong. they are not significantly affected by the stray magnetic fields.
5. They possess a high torque/weight ratio
Disadvantages of PMMC:
1. Some errors may set in due to the aging of control springs and the permanent magnet.
2. Friction due to jewel-pivot suspension.
3. They cannot be used for AC measurements.
4. These are costlier than moving iron instruments.
PRINCIPLE AND WORKING OF RECTIFIER-TYPE VOLTMETER
Working Principle: Rectifier uses a Silicon or Germanium diode to convert alternating current (AC) to a series of unidirectional current (DC) pulses, which produce positive deflection when passed through a PMMC instrument. Rectifier type voltmeter is used to convert AC input voltage into DC output voltage.
