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Wednesday, 16 October 2019

Thermistor vs Varistor

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Thermistor :
Thermally sensitive resistor is called thermistor. The temperature coefficient of semi conductors covers wide range 5% per degree C for NTC thermistors to 60% / °C for PTC thermistors. This property leads to their primary application as temperature sensors. Negative temperature coefficient (NTC) thermistors are those whose resistance decreases with increase in temperature. Positive temperature coefficient (PTC) thermistors are those whose resistance increases with the increase in temperature.

The NTC thermistors are made from the oxides of certain metals such as Copper, Manganese, Magnesium, Zinc and Titanium. Most often a mixture of several oxides is used to obtain the requisite property. They are prepared by ceramic techniques in the form of discs, short rods and bits. The properties will depend not only on those of the constituents and their purity but also on their fineness to which they have been grounded, the pressure of the press moulding and the firing condition. They are used to compensate for change in resistance of electrical circuit relays, valves etc., caused by the variation of the temperatures. They are extensively used to measure power in very high frequency test sets.


PTC thermistors are currently made out of two types of materials. The first type uses the semi conductors germanium and silicon and possess a low temperature co-efficient of resistance in the region of 0.7/°C. The second type of material is based on semi conducting Barium titanate and possesses a large temperature coefficient of resistance of the order of upto 70% per-°C. These are frequently called switched PTC thermistors due to the use of the large resistance change that occurs over a small temperature range to switch off over heated equipment. One of the major application of thermistor is for over temperature protection of electric motors. Normal practice is to embed one thermistor in each phase winding, then to connect the three thermistors in series to 2 transisterised control circuit operating power cut-out relay. The second important applicant is as a current limiter in T.V degauging circuit.

Varistor :

In this type the resistance of the resistor varies with the applied voltage. These are divided broadly into two main classes.

(a) Devices with symmetrical (non rectifying) voltage versus current characteristics.
(b) Those with non symmetrical voltage versus current characteristics.

The area of contact devices made from copper oxide, selenium belong to non symmetrical varistor category whereas the device from silicon carbide fall into symmetrical varistor category. The current relation for these is I = KVn where

I is the current.
n is the non linearity exponent dependent on the quality of the material.
K is constant dependent on the quality of the material.

Silicon carbide obtained by heating a mixture of quartz, sand with carbon electrically to a temperature of 2000°C is the prime composition of the varistor and disc moulds are made from the mixture of Silicon carbide, graphite and glass. Elements are made by baking these mould discs. These materials are used to protect high voltage equipment and power lines from over voltages and lightning surges in the form of lightning arresters. In electronic field they serve as voltage stabiliser, arc suppression, motor speed control etc.


Metal Semiconductor Rectifiers

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Copper Oxide Rectifiers and other metal semiconductor rectifiers are explained below.

Copper Oxide Rectifiers :

It is a plate of 99.98% pure copper on which a film of cuprous oxides is produced by a special process at high temperature. One side of the plate is cleaned of cuprous oxide and an electrode is soldered directly to copper. The second electrode is soldered to cuprous oxide film. When positive potential is applied to the oxide layer and negative to the copper, it corresponds to forward biasing a P-N junction. The rectifiers, for many kinds of measuring circuits and instruments, can be obtained by arranging the copper plate elements in stacks. Figure shows the arrangements in the copper oxide rectifier.

These rectifiers have low permissible current density and they are not used for power supply purposes. These rectifiers are protected against moisture by giving their elements a coating of insulating varnish and are often sealed in hermetic containers for protection against moisture. These rectifiers completely fail at sufficiently high reverse voltage. These are cheaper than silicon rectifiers and have better frequency response and do not have distortion in the rectified wave form.


Selenium Rectifier :

These rectifiers use 99.9% pure selenium. Purity is very important in respect of permissible current density and wider working temperature range as compared to copper oxide rectifier. They are used in battery charging and electro plating supplies. The typical selenium rectifier are shown in Figure.

Germanium Rectifiers :

The thermionic valves have been replaced by P-N junction diodes. Be and Si are used in P-N junction rectifiers. It is easier and simpler to produce germanium monocrystals. Germanium melts at 958°C and therefore easier to purify and maintain free from impurity. Germanium rectifiers have limited working temperature from - 50 to + 70°C. Continuous operation at 60°C may cause thermal ageing and deterioration of electrical properties. At low temperature there is a considerable drop in the permissible reverse voltages. It can operate at high current densities and reverse voltages with about 98% efficiency.

Silicon Rectifiers :

Silicon rectifiers can operate upto 200°C. Its melting point is 1415°C. Modern silicon combines easily with practically all chemical elements and is therefore very difficult to purify and maintain free from impurity. For heavy duty application this type of rectifiers are cost suited. Silicon rectifiers are more sensitive to weak signals when used in high frequency electronic circuits.


Tuesday, 15 October 2019

Energy Band in Solids

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Energy Band Theory in Solids

Electrons orbit around the nucleus at various levels called shells. An electron which orbit around the nucleus of an atom has potential energy, centrifugal energy, rotational energy and magnetic energy and all of these energies together determine the total energy or energy level of an electron. This energy level of an electron is measured in electron volts which are expressed as ‘eV’. An electron volt is defined as that amount of energy gained or lost by an electron when it moves with or against a potential difference of one volt. In any solid material the atoms are very close to each other and so the outermost valence electrons will interact with one another. But two electrons with opposite spin only can have the same energy level. When more atoms are present new levels are to be established. They are very much nearer to one another but they are separated from one another. This group of related energy level in a polyatomic material is called an energy band. Each energy band consists as many separate levels as there are atoms in the material.


About 1026 atoms are contained in a crystal weighing one kg and the width of the band will normally be 0.1 J (atto Joule = 10-18 Joules = 0.6243 eV) but there will be 14 x 1026 levels spread over the above and width 0.1 aJ. Then the spacing of the level will be less than 10-25 aJ. m specified the number of levels in the individual atoms the value of which is 2 in 'b' band and 6 in 'P' band and so on. The energy levels within a band are therefore discrete but of such small separation that the band can be taken as continuous to the first approximation. Energy band for a solid is shown in Figure.
In the Figure the upper most band is the conduction and in which the electrons are free to move by the application of electric field. An atom having many electrons in the conduction band acts as a good conductor of electricity. Just below the conduction band, there are energy levels forming the forbidden band. In the forbidden band no electrons is found in the natural state but electrons may jump from the valence band to the conduction band or vice versa. It should be noted that electrons cannot remain in the forbidden band. For an insulator the width of the forbidden gap is more and for a conductor it is least. In the case of semiconductor, the forbidden gap is between that of conductor and insulator. See Figure above. A little below this level there is valence band which is formed by series of energy levels. The electrons in these levels are called valence electrons. By the application of energy the electrons from this band may be made to move to conduction band.

The two elements commonly used of the fourth column of the periodic system of elements are Germanium and Silicon. The electron configuration of Germanium and Silicon are shown in Figure below.
In the solid state they crystallize into a diamond structure as shown in Figure (a). The schematic two dimensional representations of the electron pair in the diamond structure are shown in Figure (b). The dots represent electrons. In the actual three dimensional structure the electron pair bands emerge from a given atom to the corners of a rectangular tetrahedron, the angles between the bonds being approximately 109°. The bonds between a given atom in this structure and its neighbors are called electron pair bonds.
Figure (a)

Figure (b)
Since each atom has four valence electrons, it has just enough to provide for electron pair bonds with four other atoms. In a pure Silicon or Germanium crystal at absolute zero, there are no electrons which have the freedom to move through the crystal and so behave as insulators. By raising the temperature of the material, the vibrations carried out by the atoms in the lattice are made more violent. Some of the valence electrons may absorb sufficient amount of energy from the lattice vibrations to the released from the bond and once they are free they can move through the crystal constituting conductivity in an applied electric field.

Number Systems in Digital Electronics

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Many number systems are in use in digital technology that represents the digits in various forms.
The different number systems in digital electronics are: 

1. Decimal number system
2. Binary number system
3. Octal number system and
4. Hexadecimal number system


DECIMAL NUMBER SYSTEM:

The decimal system uses ten digits 0 to 9. The base or radix of a number system is defined as the number of digits it uses to represent numbers. Since the decimal system uses 10 digits (0 to 9) its base or radix is 10. The weight of each digit of a decimal number depends on its relative position within the number.

103
102
101
100
      .
10-1
10-2
10-3
 1000             100            10                 1                  .                     0.1             0.01              0.001       

Example: 

3472.65 = 3 x 103 + 4 x 102 + 7 x 101 + 2 x 100 + 6 x 10-1 + 5 x 10-2
= 3 x 1000 + 4 x 100 + 7 x 10 + 2 x 1 + 6 x 0.1 + 5 x 0.01

Symbols used: 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9.
Base (or radix, or weight): 10
Place value: ….. 103, 102, 101, 100 . 10-1, 10-2, 10-3 …..

BINARY NUMBER SYSTEM:

Electronic circuits can be designed easily with one of the two stable states, either ON (High) or OFF (LOW). ON is represented by a high voltage and OFF by a low voltage. These two states can be represented by the digits 1 (for HIGH) and 0 (for LOW). Thus we have a number system with only 2 digits 0 and 1 and is called the binary number system. These binary digits are called ‘bits’. The electronic elements of a computer system can easily understand the two states 0 and 1. Hence digital computers make use of binary number system in representing information. This is also a positional number system.

23
22
21
20
      .
2-1
2-2
2-3
  8                  4                  2                  1                      .                0.5                0.25               0.125

Example:
1011.01 = 1 x 23 + 0 x 22 + 1 x 21 + 1 x 20 + 0 x 2-1 + 1 x 2-2 = 1 x 8 + 0 x 4 + 1 x 2 + 1 x 1 + 0 x 0.5 + 1 x 0.25 = 11.25 (in decimal)

Symbols used: 0 and 1
Base (or radix, or weight): 2
Place value: ….. 23, 22, 21, 20 . 2-1, 2-2, 2-3 …..

OCTAL NUMBER SYSTEM:

The base of octal number system is 8. It uses 8 digits - 0, 1, 2, 3, 4, 5, 6 and 7. It is also a positional number system. The places to the left of the octal points are positive powers of 8 and places to the right of the octal point are negative powers of 8.

83
82
81
80
      .
8-1
8-2
8-3
512             64                   8                 1                       .                   0.125       0.015625   0.001953125

Example:

72.2 = 7 x 81 + 2 x 80 + 2 x 8-1
= 7 x 8 + 2 x 1 + 2 x 0.125
 = 58.25 (in decimal)

Symbols used: 0, 1, 2, 3, 4, 5, 6 and 7
Base (or radix, or weight): 8
Place value: ….. 83, 82, 81, 80 . 8-1, 8-2, 8-3 …..

HEXADECIMAL NUMBER SYSTEM

This has a base of 16. It uses 16 digits : 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, E and F. Each digit to the left of the hex point has successive negative powers of 16 and to the right of the hex point has successive negative powers of 16. To convert the given hex number to decimal we multiply each digit by its weight and take the sum.

163
162
161
160
      .
16-1
16-2
16-3
4096             256                16               1                       .            0.0625       0.00390625   0.000244140625

Example:

3C.A = 3 x 161 + 12 x 160 + 10 x 16-1
= 3 x 16 + 12 x 1 + 10 x 0.0625
= 60.625 (in decimal)

Symbols used: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F
Base (or radix, or weight): 16
Place value: ….. 163, 162, 161, 160 . 16-1, 16-2, 16-3 …..

Sunday, 13 October 2019

Properties and Applications of Varnishes and Enamels

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Some of the properties and application of Insulating varnishes and enamels are given in Table.

Properties and Applications of Varnishes and Enamels

Material
Curing schedule
Thermal class
Properties
Applications
Gleoresinous varnishes
8 - 12 Hrs E (120°C) at 90°C -120°C
E (120°C)
Flexible, good bonding properties, resistance to humidity acid, alkali
Impregnating varnish suitable for explosion proof encloses & repair purposes.
Oil-modified
synthetic resin varnish
10- 12 Hr 120°C or 8.9 Hrs. 130°C
B (130°C)
- do -
good penetration, low drainage loss.
Impregnating varnish for application of transformers, enamel wires, and explosion proof enclosures.
Solvent-
less varnish
16 Hrs
at 120°C
B (130°C)
High viscosity
curing well in depth giving good bonding & flexible cured mass.
- do -
stator, rotors transformers & for additional moisture protection. All types of electrical equipments.
Glass fibre varnishes
Air drying
B - F (145°C to 150°C)
The film of varnish fixes the glass fibres without deteriorating the suitability of tape.
Impregnation of glass tapes.
Silicon varnish
12 to 16 hrs at 180°C
F & H
(170°C)
155°C continued
operation very good at electrical proper-ties strength, good flexibility humidity resistance.
Situation of humid
atmosphere, fractional motors, marine equipment, explosion proof enclosures Impregnation of glass covered wires.
R.C. varnish
20 min. at 130°C
--
Good resistance to moisture, acid & alkalies.
Protective coating for resistors and electronic components.
Core plate varnish
250°C -300°C by infrared heating (45 - 50 secs)
--
Resistance to punching operation & pressure.
Coating for electrical sheet steel
Stripping Lacquer
1 Hr. Air drying
--
Easily removable after stoving
Coating of parts where impregna-tion with varnish is not desired
Sprodlack
Cures during prebake for equipment
--
Turns brittle under action of heat
Protection of bare machine parts e.g., shafts housing racers etc., before applying impregnating varnish
Conducting varnish
Air drying 15 minutes
--
Good electrical conductivity.
For screening purpose in high frequencies & tele communication engg. In high voltage machines for stress grading purposes
Insulating cement varnish
80 to 120°C for about 1 Hour
--
Excellent adhesive power and retension of adhesion with backing, transparent
Cementing varnish for all purpose in electrical insulation for bonding coil formers, press board etc., used in large size transformers



Saturday, 12 October 2019

Reinforced Cement Concrete (RCC) Poles

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Reinforced Cement Concrete (RCC) Poles:


Reinforced Cement Concrete (RCC) poles and prestressed cement concrete (PCC) poles, rectangular or square in cross section, are very popular in rural electrification due to very low maintenance cost, corrosion resistant and free from oxidation. Such poles are provided with holes to reduce their weight and to make climbing on the poles easy. They can be cast at the erection side to minimise the transportation charges. Such poles are good to look at, require negligible maintenance, have insulating properties, last long and possess resistance to chemical action. The drawbacks of such poles are difficulty in their erection due to heavy weight, high initial cost and difficulty in transportation. Moreover R.C.C. poles once they are damaged are not repairable. Skilled labour is required for the fabrication and erection of the poles. R.C.C poles are generally used upto 11 kV single circuit and double circuit distribution lines. The span of such poles is 80 metres to 200 metres.


The raw materials which are required for the above types of poles are mortar and concrete. There are two types of mortar: cement mortar and lime mortar. The strength of the mortar depends on the ratio of cements and water. Mortar is a binding material and cement mortar is stronger than lime mortar. Concrete is a mixture of sand, cement and aggregate in water in different proportions. If steel rods, bars or mesh structures of steel are embedded into it to make it stronger, it is called steel reinforced concrete.

Important Properties of other commonly used Insulating Materials

SI. No.
Insulating Material
Volume resistivity ohms. cm
Breakdown Voltage kV/mm
Dielectric Constant
Max. Operating Temperature
Tensile Strength
1
Asbestos
16 x 10
3 to 4.5
2.5 - 3
500
22 - 28
2
Bakelite
(20-40) x 102
15 - 25
5.7
200
350 - 720
3
Cotton
1011 to 1016
4
--
90
--
4
Empire Cloth
1011 to 1016
15
2
--
--
5
Glass
greater than 107
10 - 40
3.5 - 3.9
500
420
6
Leatheroid Plain
--
14 - 17
--
90
--
7
Leatheroid Varnished
--
16-20
--
95
--
8
Mica
(10 - 200) x107
60 - 200
5 - 8
500
--
9
Micanite
--
12 - 24
--
--
--
10
Paper Varnished
--
20 - 40
4.5
80 - 115
--
11
Paper Varnished
(300 - 500) x 1012
16 - 12
2.5 - 4
95
--
12
Paper Plain
(5 - 100) x 1012
6-12
2 - 2.5
90
--
13
Polythene
3 x 1019
40
2.3
85
120 - 210
14
Porcelain
greater than 1013
35
6 - 7.5
1000
70 - 420
15
Polysterene
1019 - 1020
20
2.5
65
--
16
P.V.C
1012 - 1015
40 - 170
4 - 12
60 - 80
70 - 630
17
Press Board Plain
1012
8 - 14
5
90
--
18
Press Board Varnished
--
10 - 16
7.65
90
--
19
Press Board Oiled
--
28 - 36
3
90
--
20
Quartz (Silica)
(1 -2) x 1016
--
3.5 – 4.5
1100 - 1200
490
21
Resin
5.1 x 1016
11
--
--
--
22
Rubber Plain
(10 -15) x 1017
16 - 28
3.5
--
9 – 2.6
23
Vulcanised
(1 -10) x 1017
12 - 24
2.3 – 3.0
--
210 - 280
24
Silicon Compound
1015-1016
--
2.5 – 4.0
150 - 200
--
25
Wood Impregnated
--
26
4 - 8
--
--