Search This Blog

Friday, May 6, 2016

Windows releted software link

http://tinyurl.com/ganoff7
http://tinyurl.com/w10pgan
http://tinyurl.com/7offgan
http://tinyurl.com/chrm222
http://tinyurl.com/w10pkey
http://tinyurl.com/actvtro
http://tinyurl.com/actvtrk
http://tinyurl.com/actvtrw
http://tinyurl.com/winxphm
http://tinyurl.com/w7hp6
http://tinyurl.com/w7hp32b
http://tinyurl.com/hitmn32
http://tinyurl.com/hitman64
http://tinyurl.com/hitman32lic
http://tinyurl.com/revolic
http://tinyurl.com/hitman64patch
http://tinyurl.com/glinks7
--------------------------------------------------
Windows 8 Pro x64

XKY4K-2NRWR-8F6P2-448RF-CRYQH

https://docs.google.com/file/d/0B20oanfbJhRFbGQyMG41X3lXNEU/edit
--------------------------------------------------
Windows 8 Pro x86

XKY4K-2NRWR-8F6P2-448RF-CRYQH

https://docs.google.com/file/d/0B0Z8WSKCYoPtbk9uVDZLZlFtRUk/edit
--------------------------------------------------
Windows 7 Ultimate SP1 x86

https://docs.google.com/file/d/0B0d0zF7ysLrnNnppY1pna3dLRHc/edit
--------------------------------------------------
Windows 7 Ultimate SP1 x64

https://docs.google.com/file/d/0B0d0zF7ysLrnUHhKZkJGbEw1bXc/edit
--------------------------------------------------
Windows 7 Home Premium x64

https://docs.google.com/file/d/0BwhlmRj8mI2Oa2U3Um9VSmtaY3M/edit?pli=1
--------------------------------------------------
Windows 7 Home Premium x86

https://docs.google.com/file/d/0BwhlmRj8mI2OaEI3Y01OVEVmanM/edit?pli=1
--------------------------------------------------
Windows 8.1 KMSAuto Activator

https://drive.google.com/file/d/0B_GEZ0n8KyPaOEkwQ094NXVqVzQ/view?usp=sharing
--------------------------------------------------
Microsoft Office 2011 for Mac Standard Edition v.14.4.4 Volume License SP3

https://docs.google.com/file/d/0B7m0l6_mpLehSTdlbS1GdGk1V3M/edit
--------------------------------------------------
Microsoft Office 2013 Pro Plus x64

https://drive.google.com/file/d/0B_GEZ0n8KyPaX0Zmek00S0tSbW8/view?usp=sharing
--------------------------------------------------
Microsoft Office 2013 Pro Plus x86

https://drive.google.com/file/d/0B_GEZ0n8KyPabDVwMGVTUExzZXM/view?usp=sharing
--------------------------------------------------
Malware Bytes

https://www.malwarebytes.org/mwb-download/
--------------------------------------------------
Trojan Remover

http://www.simplysup.com/tremover/download.html
--------------------------------------------------
Hitman Pro 3.7.x (x64) Patch

https://docs.google.com/file/d/0B7jkw2rnDxh4YVo2TlNRRHBmbW8/edit
--------------------------------------------------
Windows 8.1 Pro Update 1 x86

https://drive.google.com/file/d/0B_GEZ0n8KyPab3pBR0I1TjNyejg/view?usp=sharing
--------------------------------------------------
Windows 8.1 Pro Update 1 x64

https://drive.google.com/file/d/0B_GEZ0n8KyPaRHd5ZFlzVzJONmM/view?usp=sharing
--------------------------------------------------

http://home.mcafee.com/root/landingpage.aspx?lpname=17224_vsp_trial&affid=707&culture=en-US&legacylangcd=en-us  MCAFEE














Wednesday, February 10, 2016

Types of Resistors



Types of Resistors


Like all electronic components, resistors are also available in different sizes, shapes and types. These variations in resistors bring advantages in some types and limitations in other types. This in turn makes some resistors suitable for some applications rather than others. Hence selection of correct resistors should be done with great care.

Resistors Types Based on Composition

Carbon Composition Resistors

Carbon Composition Resistors are commonly used resistors which are manufactured at low cost. This is because of the simpler construction process. They are generally called carbon resistors. The main composition is finely ground carbon along with ceramic clayacting as a binding agent.This is covered in a plastic case and the leads are made of tinned copper. The proportions of carbon and clay are the factor in determining the resistive value. Resistance is higher when the quantity of carbon is lesser.


Carbon resistors can be manufactured in wide range of values ranging from 1Ω to a high value as 22 MΩ. Due to its low cost, they are used in circuits where cost is a criterion rather than the performance.
The advantages of carbon resistors are its ability to remain undamaged from high energy pulses, available at very low cost and at all local vendors and good durability. The disadvantages are high sensitivity to temperature, unstable noise properties and stability issues when hot.
Carbon composition resistors are suitable for high frequency applications as they have low inductance. They are easily affected by humidity and hence the tolerance is only 5%. They also have a low-medium range power rating i.e. < 5W.

Film Type Resistors

Film type resistors are manufactured by a process called film deposition technique. This technique involves deposition of a film of resistive material on an insulating substrate. Film type resistors are again classified in to sub types based on the resistive material used which are Metal Film, Carbon Film and Metal Oxide Film. In metal films, nickel metal is used as resistive element and tin oxide in case of metal oxide films. Once the film is deposited on the insulating material, it is cut into a spiral helix pattern with the help of a laser.


The resistive value is controlled or maintained by controlling the thickness of the film that is deposited.
Carbon film resistors are better than carbon composition resistors. Carbon film resistors are used in applications where the operating voltage and temperature is high like laser and radar.
Metal Film type resistors have much higher tolerance and better temperature stability when compared to carbon resistors. Hence they are used in applications like active filters where low temperature coefficient and tight tolerance are required.
Metal oxide resistors have much better temperature stability and better surge current capacity.

Thin Film Resistors

Thin film resistors are manufactured by depositing a resistive layer on an insulating base like ceramic. The thickness of the resistive film is equal to or smaller than 0.1 micro meters.
Vacuum deposition is the technique used to deposit the resistive film on the ceramic. The resistive material which is often an alloy of nickel and chromium called Nichrome is sputtered on an insulator base which is ceramic. This process will create a uniform film of 0.1micrometer thick. The thickness of the metallic film can be controlled by controlling the time of sputtering. Patterns are created by laser trimming process on the dense and uniform layer to create and calibrate the resistive path and resistance value.
Thin film resistors can be produced as SMD resistors or axial leaded resistors. Because of their high tolerance and low temperature coefficient, thin film resistors are used in precision applications.

Thick Film Resistors

In thick film resistors, the thickness of resistive film is nearly 1000 times thicker than that in thin film resistors. The main difference between thick film and thin film resistors is the procedure for applying the resistive film. The resistive film in thick film resistors is made from a mixture of a binder, carrier and metal oxide. Glass frit bonding is used to bind the mixture. Carrier is the extract of organic solvent and oxides of iridium or ruthenium are used. This mixture is made as a paste and the resistive film is produced by applying this paste on to a ceramic base using stencil and screen printing process.
Thick film resistors can be used in applications where less cost is important, high power is handles and high stability is important.

Wire Wound Type Resistors

Wire wound resistors are most precise and high power rated resistors. The construction of wire wound resistors involves a winding of thin metal or metal alloy wire around an insulating substrate. Generally the metals used are manganin or constantan and a nickel chromium alloy which is also called as nichrome is used in case of metal alloy. The resistive value can be varied by varying wrap pattern, diameter, length and type of alloy.
The resistance tolerance of wire wound resistors is as tight as .005% and the power ratings are in the range of 50W-300W. These are precision wire wound resistors. In case of power resistors, the tolerance is 5% and the power rating is in the range of kilo watts.
They are limited to low frequency applications because of the nature of their construction. Since there is a metal wire wound as a coil around an insulator, they act as inductors. This results in reactance and inductance and when used in A.C circuits there is a chance of phase shift when operated at higher frequencies.
There is a possibility to overcome this limitation by winding each half of wire in different directions. This will cancel each other’s inductive effect. These resistors are called as Non-Inductive Wire Wound Resistors. Normally the cost of wire wound resistors is higher when compared to carbon composition resistors. In high frequency applications Non-Inductive Wire Wound Resistors can be used but their cost is more than normal wire wound resistors.
Wire wound resistors are used in many applications. Some of them are circuit breakers, transducers, temperature sensors and current sensors.

Resistors Types Based on Termination and Mounting

SMD Resistors

Surface Mount Devices (SMD) are produced as a result of a technique called Surface-mount Technology (SMT). In this technique the components are placed directly on the printed circuit board. SMD Resistors are also developed similarly. The development of Surface-Mount Technology and Surface Mount Devices is a result of requirement of smaller, faster, cheaper and more efficient components by PCB manufacturers.
SMD resistors are smaller than their through-hole counterparts and are generally rectangular but sometimes oval in shape. These rectangular chips have very small metal leads or metalized areas at either ends which are used to make contact with the PCB and therefore eliminating the need for holes on PCB and wire leads on resistors.
A single SMD resistor is as shown in the figure.


SMD resistor on a PCB


SMD resistors consist of an insulator substrate which is generally ceramic and a layer of metal oxide film is deposited on this substrate. The value of resistance is determined by the thickness of the film.
Because of their small size they are suitable for circuit boards. They have very little inductance and capacitance and can perform well at radio frequencies.

Through-hole Resistors

Through-hole is a mounting technique where the components are inserted into holes that are drilled on a PCB. For this purpose, the electronic component consists of small metallic leads. All the resistors with leads coming out of them for contact purpose come under Through-hole resistors. Through-hole resistors are available in carbon composition resistors, carbon film resistors, metal film resistors, metal oxide resistors, wire wound resistors and many others.
Apart from discrete components, through-hole resistors can be found as pack of resistors with the usage of Dual in-line package and Single in-line package techniques.

These SIP and DIP resistors are generally used in resistor ladder networks, pull-up and pull-down networks, bus terminators etc.

Resistors Types Based on Shape

Resistors are classified based on their physical shape. They are Square chip, leadless, open and sealed type in the category of Surface–mount technology. Radial and axial type resistors come under leaded type category.
Square chip type includes SMD resistors.
Leadless type resistors are round chip resistors and metal electrode leadless faceor MELF resistors.
Open type are general wire wound resistors.
Radial lead type resistors are vertical taping compatible. Axial lead type resistors are those in which the leads come out of the body axially.

Resistors Types Based on Power Ratings

Resistors can be classified based on their power ratings. The power rating of a resistor is a benchmark used to indicate the maximum permissible power through a resistor for uninterrupted operation at a specified temperature. Beyond this power, the resistor gets hot and may burn up. If a resistor is rated as 0.25W, then a maximum power of .25W can be fed to it.Hence it is important to find the power in a circuit.
Power is rate of doing work. In electrical terminology, power is the rate at which energy is transferred by a circuit. Here the energy is electrical energy.
Power P = VI = V2/R = I2R watts.
When a resistor with known resistance and a fixed supply voltage is present then the power is calculated with this resistance along with supply voltage.
For example a resistor of resistance 400Ω and voltage of 12V is used to power up an LED. The power is calculated as
P = V2/R = (12)2 / 400 = 0.36W
Then a resistor of a power rating of 0.50W should be used.
The standard available power ratings are 0.25W, 0.5W, 1W, 2W, 5W, and 25W.Generally resistorsare available with power ratings up to 500W.

Other Types of Resistors

Fixed Resistors

As the name indicates fixed resistors are those which have a predefined or fixed value of resistance. When the term resistor is used, it generally refers to fixed resistor. Ideally fixed resistors should work independent to changes in temperature, voltage and frequency. This is not possible practically as all resistor materials have temperature coefficient which leads to temperature dependency. The stray capacitance which is present in all resistors will result in impedance and hence the actual resistance will be different from expected.
Fixed resistors are available in different sizes, shapes, leaded, leadless, etc. They can be manufactured based on carbon composition type, carbon film type, metal film type, metal oxide film type, wire wound type, SMD, etc.

Variable Resistors

When there are fixed resistors, there is a scope for resistors with resistance value that is not fixed. Variable resistors are those in which the value of resistance can be varied or adjusted.
The working of a variable resistor can be explained with the help of following diagram.

The resistance path is provided by track and the terminals of device are connected to track. Wiper is used to increase or decrease resistance through its motion.

Potentiometer

A potentiometer or pot is an electro mechanical resistor with three terminals and is the most commonly used variable resistor.


The two terminals on the either end will deliver a constant resistance which is the formal resistance. The terminal in the center is movable and is called Wiper. This movable wiper maintains contact with the resistive surface. The resistance between first terminal and the wiper plus the resistance between wiper and second terminal is equal to the formal resistance of the device. The name potentiometer is given to this device as it adjusts voltage using voltage divider principle.
While wiper is a rotating contact, some potentiometers have continuously adjustable tapping points which are contacted with the third terminal called tappers and they also act as continuously adjustable voltage divider. The best application is their use in tuning circuits an in radio receivers.

Preset

Preset is a variable resistor which is used in occasional adjustment conditions.
Generally presets are mounted on printed circuit board and are adjusted using the rotary control present on top of it with the help of a screw driver. In contrast to potentiometers where the resistance varies linearly, the resistance in preset varies exponentially. The symbol of a preset is as shown below.
Fig: Preset symbol
Fig: Preset symbol


Presets are made available in single turn and multi turn operations. Presets are used in designs where the value of the resistance is set in the circuit during the time of production. Due to their sensitivity, presets are often used in sensing circuits like temperature or light sensing.

Rheostat

A rheostat is a two terminal variable resistor. In rheostat, one end of resistive track of a variable resistor and its wiper terminal are connected to the circuit. This connection will limit the current in the circuit according to the position of the wiper.

Rheostats are used to control the resistance without interrupting the flow of current. Because of this significant flow of current, rheostats are made as wire wound resistors.
Rheostats are used in applications where current is more important than power rating. They are generally used in tuning circuits and power control applications.

Light Dependent Resistor (LDR)

Light Dependent Resistors or Photo resistors are light sensitive resistors whose resistance varies according to the intensity of the light incident on them. The symbol of Light dependent resistors is
Fig: LDR Symbol
Fig: LDR Symbol
Light dependent resistors are made of semiconductors with high resistance. In absence of light or in dark, the resistance of light dependent resistors is very high usually in the range of Mega Ohms (MΩ). When light is incident on the surface of light dependent resistors, photons fall on the semiconductor material and the valence electrons of the semiconductor are excited to conduction band. For the valence electrons to jump to conduction band there should be enough energy in the photons. Therefore the light incident should exceed a certain frequency and the number of free electrons depends on frequency of light. The free electrons will conduct current and hence lowers the resistance.
Based on the semiconductor material used,Light dependent resistors are divided into intrinsic and extrinsic. Intrinsic Light dependent resistors use undoped or pure semiconductors like silicon. There should be enough energy in the photons to excite the entire band gap. Therefore intrinsic light dependent resistors are used for shorter wavelength or higher frequency photons.
On the other hand, Extrinsic Light dependent resistors use semiconductor materials with impurities in them. These impurities are called dopants and generally boron or phosphorous are used. These impurities create an intermediate energy band which is closer to the conduction band. Hence the energy required to excite these electrons is less. Lower energy photons i.e. longer wavelength or lesser frequency like Infra-red are suitable for extrinsic light dependent resistors.

Network Resistors

Network resistors are single package resistors with two or more resistors. They generally come in Single in-line package or Dual in-line package.

Resistor networks are used to reduce the board space, improve reliability, reduce solder connections and improve tolerance matching. Generally resistor networks are used in resistor ladders, bus terminators and small computer system interface terminators.
They are available as both surface mount devices and through-hole devices.

Varistor

Varistor is portmanteau of variable resistor. It is an electronic component with non-linear current voltage characteristics like diode. The resistance in varistor is changed according to the change in voltage across it. This makes it a voltage sensitive device hence it is also called Voltage Dependent Resistor. Generally varistors are made from semiconductor materials.


The resistance of varistor is very high under normal operating conditions. But the resistance decreases dramatically when the voltage increases beyond the rated value of varistor.
Metal oxide Varistors are most common type of varistors. Grains of Zinc oxide are used because it provides P-N diode characteristics. Hence it is used to protect electronic and electrical circuits from over voltage surges.

Introduction to Resistors

Introduction to Resistors

What is Resistor?

A resistor is a device in which electricity cannot pass through it easily. When certain amount electricity is allowed to pass through a resistor, the electrical energy is changed into another form.The other form of energy is usually light or heat. The working principle of bulb is that electricity is passed through the filament usually tungsten, which is a resistor. The energy is converted to and released as light and heat.
The resistor is an electrical component which creates a resistance in the flow of electric current.
Figure 1: Resistor

A resistor is a basic electrical component found in almost all electronic circuits and electrical networks. A resistor is two terminal passive electrical component. It is a passive component as it consumes energy from a source (active component).
Although resistors are generally used to reduce the flow of current or lower the levels of voltage in a circuit, they are used in many electronic circuits for many purposes. Some of them are the basic current flow limitation ability, to provide a biasing condition to some of the active elements like transistors or to act as a terminating device in transmission lines.
Practically resistors are discrete components of various forms but are also implemented on integrated circuits.

ResistorSymbols

Generally there are two standards that are used to denote the symbol of a resistor viz.Institute of Electrical and Electronics Engineers (IEEE) and International Electro Technical Commissions (IEC).
The IEEE symbol of resistor is a zigzag line as shown in the below figure.
Figure 2: Resistor Symbol
Resistor IEEE Symbol


The IEC symbol
Figure 3: Resistor Symbol
Resistor IEC Symbol


There are some other symbols of resistors in use , based on the type. Each type has both IEEE symbol and IEC symbol. The types of resistors are potentiometer and variable resistor which is generally known as rheostat.
The IEEE symbol for potentiometer is
Figure 4: Potentiometer Symbol
Potentiometer IEEE Symbol


The IEC symbol for potentiometer is
Figure: 5 Potentiometer Symbol
Potentiometer IEC Symbol


The IEEE symbol for rheostat is
Figure 6: Rheostat Symbol
Rheostat IEEE Symbol


The IEC symbol for rheostat is
Figure 7: Rheostat Symbol
Rheostat IEC Symbol


Resistance

The mechanism of energy flow through a conductor can be described as follows: In the presence of an active source, the passive elements like resistors will always absorb energy and the currents through them will always flow from higher potential to lower potential.
If we apply the same potential difference between the ends of two different but geometrically similar conductors like rods of copper and of glass, it results in different currents. This characteristic of the conductor that results in different currents is its electrical resistance.
The definition of resistance can be derived from the Ohm’s law in its Electromagnetic theory form or Continuum form
J = σ E —-1
Here σ is the conductivity of the material i.e. conductor.
E is the electric field developed along the length of conductor due to flow of electrical energy through the conductor.
If ‘V’ is the voltage drop across the conductor and ‘L’ is the physical length of conductor then
E = V/L —-2
The current density J is resulted within the conductor due to the flow of electrical energy through the conductor.
If ‘I’ is the current flowing through the conductor and ‘A’ is the cross sectional area of conductor, then by the definition of current density
J = I/A —-3
Now combining equations 1, 2 and 3
I/A = σ V/L
V = (L/Aσ) I —-4
The term in parenthesis is constant and let us denotes it by ‘R’.
∴V = R I
This is the Ohm’s law form in circuit analysis.
By the definition of Ohm’s law, the current flowing through a conductor is directly proportional to the potential difference applied.
I ∝ V
The proportional constant is called Resistance parameter of the conductor R.
∴I = V/R
R = V/I
The resistance of a conductor, between its two points is determined, by applying a potential difference V between those two points and measuring the current I .
The unit of resistance is Volts per Ampere and is given the name Ohm (Ω).
∴ 1Ω = 1 volt per ampere = 1 V/A.
From earlier calculations
V = (L/Aσ) I
∴ R = L/(A σ) I
σ is the conductivity of the conductor which is the measure of conductor’s ability to conduct electric current.
1/σ is the reciprocal of electrical conductivity called electrical resistivity denoted by the symbol ρ (rho).
Resistivity is the measure of a conductor’s ability to resist the flow of electric current.
∴Resistance of a material ∝ resistivity of the material.
R = ρL/A Ω
Resistance of a conductor can be defined as the conductor’s opposition to the flow of current through it.
Resistance is a property of an object like conductor. Resistivity is a property of a material from which the object is made.
The definition of resistor can be written as
A conductor whose function is to provide a specified resistance in a circuit.

Resistance Measurement

Resistors are main components in electric and electronic circuits as they determine the amount of current that flows in a circuit and also the potential at different points in a circuit. Therefore it is important to make sure that the value of resistance is known for a given resistor which is placed in a circuit.
From Ohm’s law, it is easy to calculate the resistance. Ohm’s law relates the Voltage V, Current in the circuit I and the Resistance of the resistor R.
R = V/I
∴In terms of units, 1 Ohm (Ω) = 1 Volt (V) / 1 Ampere (A).
In other terms, a resistor is said to be having a resistance of 1Ω when 1A of current is passed through it for a supply voltage of 1V.
Ohmmeter is a device designed specifically for this purpose. A resistor is connected across the terminals of ohmmeter and the reading of the ohmmeter is the value of the resistance of that resistor along with the resistance offered by the wires used to connect the resistor to ohmmeter.
Even though the resistance of wire is very small, it can’t be neglected. Hence the values measured using an ohmmeter is not accurate.
The next best way to determine the resistance is to make use of both voltmeter and an ammeter in the circuit.
The set up can be as follows
In this method, the readings both current and voltage are taken using the respective devices. If we consider the resistances of the wire then the circuit will be
Since it is a series loop, current will be same at all the points. Because the voltmeter is used to measure the voltage drop across the unknown resistor, wire resistances doesn’t come into picture.
∴RX = Voltmeter Reading / Ammeter Reading
The above setup is good if internal resistance of the voltmeter is larger compared to the RX.
In case of resistance RX is larger than that of internal resistance of the voltmeter then the following setup can be used.
Another technique is used for the measurement of lower resistances. This is called Four Terminal Sensing or Kelvin Sensing.
To measure the lower resistance (< 100Ω), the Kelvin Sensing is used to eliminate the inappropriate influence of contact resistances and wiring resistances.
This connection method for resistance measurement uses separate pairs of current-carrying and voltage-sensing probes to eliminate the influence of contact and test lead resistances which appear in the precious setup which is also called Two Terminal Sensing.
The four terminal sensing method with internal resistances of wire can be depicted as follows:
The resistance of Rx1 can be calculated as follows
The current which passes through the voltage-sensing path or terminals is very less than the current through the resistor RX1. This implies that the voltage drop across the wire resistances in the voltage-sensing path is very less.
∴ I = IRX1 + IVPath ≈ IRX1
Now the voltage across the resistor RX1 is the value of voltmeter VRX1.
∴ RX1 = VRX1 / IRX1

Resistivity

Often the ability of a material to conduct electricity or the electrical transport property of a material is measured by the conductivity of the material.
Electrical Conductivity of a material is the measure of its ability to conduct current.
Resistivity is the reciprocal of conductivity. Resistivity is the measure of a conductors’ ability to resist the flow of electric current.
Derivation
Assume a material of length ‘ L’ and area of cross section ‘A’ and resistance ‘R’.
Resistance to the of the material is directly proportional to the length ‘L’ and inversely proportional to area of cross section ‘A’.
Thus
R ∞ L ,
R ∞ 1/A ,
Combining above two equations
R ∞ L / A
Assume a constant ‘ρ’to eliminate proportionality
So, R =(ρ ×L) /A.
Hence ρ = (R×A) / L.
Thus from above equation materials with low resistivity allows the movement of electrons while those with high resistivity oppose the flow of electrons.
Elements like copper, aluminum will have low resistivity
Units of resistivity are Ohm-meter (Ω-mt).
In mathematical terms, the definition of resistivity is resistance per unit length per unit cross sectional area of the material.
In electromagnetic theory, the term resistivity can be defined as the magnitude of electric field across the material that results in a certain current density.
ρ = E / J
E is the electrical field and J is the current density.

Resistance Example:

If a rectangular block of iron (with ρ = 9.68 x 10-8Ω.m) of dimensions 1.2cm x 1.2cm x 15cm is applied with a potential difference such that the sides are equipotential, find the resistance.
Solution:
L = 15cm = .15m
A = 1.2cm×1.2cm = 1.44× 10-4 m2
∴R = ρL/A
R = (9.68× 10-8Ω.m ×.15m) / (1.44 × 10-4 m2)
R = 1 × 10-4Ω = 1µΩ

Resistance Units

Resistance R = V/I
This results in the units of resistance as volts per ampere. This combination is given a special name called Ohm named after the physicist  Georg Simon Ohm.
∴ 1Ω = 1 volt per ampere
The units of conductance which is reciprocal of resistance is given by 1/Ω and given the name Mho. Mho is Ohm written in reverse. It is given by the symbol ℧. Later this is changed to Siemens (S).
S = Ω-1 = A/V.
The value of Ω can be defined in various forms as shown in the below equation
Where
  • Ω is the resistance
  • V is Volts
  • A is Ampere
  • Kg is Kilogram
  • m is meter
  • s is second
  • C is Coulomb
  • J is Joule
  • S is Siemens
  • F is farad
  • W is Watt

Carbon Resistors

Carbon Composition Resistors are commonly used resistors which are manufactured at low cost. This is because of the simpler construction process. They are generally called carbon resistors. The main composition is carbon clay which is covered in a plastic case and the leads are made of tinned copper. The main advantage of carbon resistors is that they are easily available at very low cost at all local vendors and the durability is good. The only disadvantage is that they are very sensitive to temperature.
Carbon resistors can be manufactured in wide range of values as low as 1 Ω value to a high value as 22 MΩ. Due to its low cost, they are used in circuits where cost is a criterion rather than the performance.

Working of a Resistor

The principle behind the working of resistor can be explained using hydraulic analogy. Let us imagine a pipe with water flowing through it. If we make the diameter of the pipe small, the flow of water is restricted. Now we increase the force of water through the same reduced diameter by increasing the pressure, then the energy will be dissipated in other form. The difference in pressure at both the ends of the pipe is significant. Now we apply this analogy to an electrical system i.e. force applied to water is comparable to current through a resistor and applied pressure is comparable to voltage.
The reason for this change in the form of energy can be explained as follows. The high electrical conductivity of metals is useful as it has free flow of electrons and this flow of charge is called electric current. But when the flow of electrons is not free i.e. restricted, the electrical energy is converted in to other forms like heat in case of poor conductors. This restriction of flow of electrons without completely stopping it is the idea behind resistors. This principle of restricted flow of electrons may not have to be used to get heat as output but has other functions like reduction in voltage or current, emission of light etc.

V-I Characteristics of a Resistor

V-I Characteristics of a resistor are the relation between the applied voltages and the current flowing through it. From Ohm’s law, we know that when the voltage applied across the resistor increases, the current flowing through it also increases i.e. the voltage applied is directly proportional to current. The V-I characteristics graph can be determined from the following circuit
The graph corresponding to the circuit for the function v(t) = R i(t) which is in the form of y = mx is as shown below. To plot the graph, the values of voltage (V) are taken on the y-axis and the values of current (I) are taken on the x-axis. From the graph it is clear that V-I characteristics of a resistor are linear and the value of the resistance at any instance can be determined by the slope of the curve at that instance.
The above specifications are valid in case of a pure resistance i.e. ideal resistor and the temperature is constant. In practical conditions, these values may vary depending on the operating environment and the characteristics might be different from the ideal linear values.

Variation of Resistance with Temperature

The effect of changes in temperature is a change in the value of resistance of the material. The reason for this change is not because of the variations in the dimensions of the material but rather the change in the resistivity of the material.
The flow of current in materials like conductors is the movement of electrons between atoms in the presence of an electric field. This is achieved by applying a potential difference across the conductor. This potential difference will cause the negatively charged electrons to move towards the positive terminal from atom to atom. These electrons which move freely between atoms are called free electrons. The conductivity of a material is dependent on the number of these free electrons in the atom of the material.
When there is a rise in the temperature, the heat will cause an atomic vibration and these vibrations will cause a collision between the free electrons and the electrons in the inner layers of the atom. These collisions will use the energy of free electrons. If more collisions take place, more energy of free electron is used and increases the resistance to flow of current. This is the case in conductors. In case of insulators the resistance decreases with increase in temperature. The reason is the availability of number of free electrons which are released from its captive stage.
In mathematical terms, a fractional change in resistance is directly proportional to the change in the temperature.
∆R/R0∝∆T
Where ∆R is the small change in resistance
∆R = R – R0
R is resistance at temperature T
R0 is resistance at temperature T0
∆T is change in temperature
∆T = T – T0
If we denote the proportionality constant in the above equation as alpha (α)
Then ∆R/R0 = α∆T
Where α is the temperature coefficient of the resistance.
The temperature coefficient of resistance is used to describe the relative change in resistance in association with change in temperature.
If the change in temperature is small then the above equation can be written as
R = R0 [1+α (T-T0)]
If the resistance increases with increase in temperature, then the material is said to be having a positive temperature coefficient. These materials are conductors.
If the resistance decreases with increase in temperature, then the material is said to be having a negative temperature coefficient. These materials are insulators.
The units of temperature coefficient are /0K or K-1.

Resistor Packages

Surface Mount Technology is technique where the components are directly placed on the surface of the circuit board. These components are called Surface Mount Devices. Resistors are also manufactured as SMDs. SMD Resistors are generally smaller with small leads or no leads. For contact purpose, they have metal contacts on either end.
The SMD resistors have two types of packages. They are two-terminal packaging and Metal Electrode Leadless Face packaging.
Initially, they have small leads through which the connections are made. Later, they have no leads, but have a metal contact on either ends of the device for connections.