Saturday, December 3, 2016

Basic Electronics on the Go - Resistor Power Rating

From  http://www.electronicshub.org/resistor-power-rating/

Resistor Power Rating


Introduction

Resistors can be rated based on two values. First they can be  based on the resistance of the resistor. Second they can be  based on the power in Watts that a resistor can dissipate safely.

The power rating of a resistor can be defined as the loss of electrical energy in the form of heat in a resistor when a current flows through it in the presence of a voltage.

Resistors can be used in any circuit based on the requirement in any combination of current and voltage. These different combinations of currents and voltages are selected such that the dissipating power rating of a resistor does not exceed the resistor power rating, which indicates the amount of power a resistor can convert into heat without causing any damage to it. The resistor power rating can also indicate the amount of power a resistor can absorb without causing any damages.
The Resistor Power Rating is also called as Resistor Wattage Rating. The Resistor Power Rating is defined as “the amount of heat a resistor can dissipate for an indefinite time period without affecting or degrading its performance.”
The resistor power rating is measured in Watts which are the units of power.

Since the power dissipation is dependent on the size of the object, the resistor power rating can vary from a value as small as one tenth of a Watt to a large value as hundreds of Watts based on the size, temperature and construction procedure of resistor.

Generally the temperature used to define the power rating is ambient temperature or room temperature. Generally, most of the resistors have their maximum power rating at an ambient temperature of  70oC  or less.


Power Dissipation

Resistors are basic electrical components which obey Ohm’s Law. When a voltage V is applied between the leads of the resistor of resistance R, a current I flows through it. This current I is given by
I = V / R
The movement of electrons is the cause for this current and they are accelerated by the electric field due to the applied potential. These accelerated electrons which possess kinetic energy, try to move towards the positive side of the material and in this process they collide with atoms and lose their energy. And the result of this collision is conversion of electrical energy to heat.
The rate of loss of energy or power dissipation can be calculated from the formula P = I × V.


From Ohm’s Law, we have V = I × R
Therefore P = I2 × R

And P = V2 / R


Power Resistors



Some resistors are designed for larger power dissipation. These are called Power Resistors. Resistors with power rating of at least 5W come under power resistors. The material used for construction of power resistors must be of high thermal conductive in nature. Power resistors often come with heat sink which helps them in dissipation of heat.

Wire wound power resistors are common, but they can also be found in other types. If Nichrome alloy based wire wound resistors are used with proper non-conductive enamel paint, they can withstand temperatures up to 4500C.

Another type of resistors used to withstand large amounts of currents is Grid Resistors. Grid Resistors can withstand a current up to 500 Amps and can have a resistance values as low as 0.04 Ω. The construction of Grid Resistors includes two electrodes with large metal stripes connected between them in the form of a matrix. Grid resistors are used as grounding resistors, brake resistors and harmonic filters for electric substations.

Another type of power resistors are Water Resistors. The construction includes tubes which carry saline solution with electrodes connected at both ends of the tube. The concentration of saline solution or salt water will determine the resistance. Because of the presence of water in the tubes, water resistors provide large heat capacity, which in turn result in high power dissipation.



Power resistors can also be made in the form of Surface Mounted Devices. Because of their small size, the power dissipation capacity of SMD resistors is less than grid type resistors and water resistors. Usually the power dissipated by SMD resistors is in the order of few Watts.
The range of power dissipated by different types of power resistors are as follows


  • SMD Resistors 5 W or less
  • Helical Wound 50 W or less
  • Edge Wound 3.5 KW or less
  • Grid Resistors 100 KW or less
  • Water Resistors 500 KW or less

Power rating Examples

1. For example, to choose an appropriate power rated resistor of resistance 800 Ω and a supply voltage of 12 V. The available power ratings are 0.25 W, 0.5W and 1 W.
Power P in the resistor is equal to P = V2 / R
Therefore P = (12)2  / 800 = 0.18 W.
Hence a resistor of Power rating 0.25 should be used.

Applications of Power Resistors

Every resistor is rated with its maximum power rating. This power rating indicates the maximum power a resistor can dissipate without causing any damages to itself or the circuit. Some applications require less power dissipation and others require large power dissipation. Power resistors are used in applications where we need to dissipate large power. Some of the applications of Power resistors are
1. Engine brakes in heavy locomotives and trams use power resistors. Locomotives move at high speed and possess high kinetic energy. While stopping these high speed locomotives, their kinetic energy is converted to heat.

Depending on the velocity of the locomotives, the amount of heat generated can be in the order of few Kilo Watts. Classic disc brakes can’t be used as they wear out easily. Hence regenerative brakes or high power resistors in the form of Grid resistors are used in locomotives.

2. Power resistors are used as grounding resistors to limit fault currents, high voltages and act as protective relays. These resistors can be rated up to 8 Kilo Amps.
3. Power resistors are used as load resistors in turbines and Uninterruptable Power Supplies. They can be designed to provide adjustable resistance and can dissipate a power of up to 6 Mega Watts. Because of this high power dissipation, load resistors are equipped with an efficient cooling system to control the temperature and prevent the devices from burning out.





Friday, December 2, 2016

Basic Electronics on the Go - Resistor Color Codes

From http://www.electronicshub.org/resistor-color-code/

Resistor Color Codes


There are many different types of resistors available. In order to identify or calculate the resistance value of a resistor, it is important to have a marking system. Resistor Color Code is one way to represent the value of the resistance along with the tolerance.
The electronic color code is a way of indicating the ratings or values of electronic components like resistors, capacitors, inductors, and others. The electronic color code was developed in the early 1920s by the Radio Manufacturers Association. Color code is used as they are cheap and can be printed on small components as well.
The standards for color coding registers are defined in international standards IEC 60062. This standard describes color coding for axially leaded resistors and numeric code for SMD resistors.
There are several bands to specify the value of resistance. They even specify tolerance, reliability and failure rate. The number of bands vary from three to six. In case of 3 band code, the first two indicate the value of resistance and the third band acts as multiplier.

Three Band Resistor Color Code
The three band color code is very rarely used. The first band from the left indicates the first significant figure of the resistance. The second band indicates the second significant number.
The third band indicates the multiplier. The tolerance for three band resistors is generally 20%. 
For example if the colors on the resistor are in the order of Yellow, Violet and Red from left, then the resistance can be calculated as
47× 10^2± 20 %. This is 4.7 KΩ± 20%.


Four Band ResistorColor Code

Four band color code is the most common representation in resistors. The first two bands from the left are used to indicate the first and second significant digits of resistance. The third band is used to indicate the multiplier.
The fourth band is used to indicate tolerance. There is a significant gap between third and fourth bands. This gap helps in resolving the reading direction. 
For example, if the colors on a four band resistor are in the order Green, Black, Red and Yellow then the value of resistance is calculated as 50 * 10^4± 2 % = 500KΩ± 2%


Five Band ResistorColor Code

High precision resistors have an extra band which is used to indicate the third significant value of the resistance. The rest of the bands indicate the same things as a four band color code.
Therefore the first three bands are used to indicate the first three significant values of resistance. Fourth and fifth bands are used to indicate multiplier and tolerance respectively.
There is an exception when the fourth band is either Gold or Silver. In this case, the first two bands indicate the two significant digits of resistance.
Third band is used to indicate multiplier, fourth band is used for tolerance and fifth band is used to indicate the temperature coefficient with units of ppm/K. 

For example, if colors on a five band resistor are in the order Red, Blue, Black, Orange and Gray then the value of resistance is calculated as 260×10^3± 0.05 = 260 KΩ ± 0.05%.



Tolerance Letter Coding for Resistors

The letter code for tolerance is shown below
  • B = 0.1%
  • C = 0.25 %
  • D = 0.5 %
  • F = 1 %
  • G = 2 %
  • J = 5 %
  • K = 10 %
  • M = 20 %
K and M should not be confused with kilo and mega Ohms.

SMD Resistor Code

There are three types of coding systems used to mark SMD Resistors. They are

  • Three digit coding
  • Four digit coding
  • E96 coding
Three Digit Code
In three digit coding, the first two numbers indicate the significant value of the resistance and the third number indicates the multiplier like 10 in case the digit is 1, 100 in case the digit is 2 or 1000 in case the digit is 3 and so on.
Some examples of three digit codes are
450 = 45 * 10^0 = 45 Ω
221 = 22 * 10^1 = 220 Ω
105 = 10 * 10^5 = 1 MΩ
If the resistance is less than 10Ω then the letter R is used to indicate the position of the decimal point. For example
3R3 = 3.3Ω
47R = 47 Ω

Four Digit Code

For more precision resistors, a four digit code is marked on them. The calculation is similar to three digit code. The first three numbers indicate the significant value of the resistance and the fourth number indicates the multiplier.

Some examples under this system are
4700 = 470 * 10^0 = 470 Ω
1001 = 100 * 10^1 = 1 KΩ
7992 = 799 * 10^2 = 79.9 KΩ
For resistors less than 100 Ω, R is used to indicate the position of the decimal point.
For example,
15R0 = 15.0 Ω

E series

Electronic Industries Association (EIA) specified a standard preferred value system for resistors and is named as E series. IEC 60063 is an international standard which defines the preferred number series in resistors (and also for capacitors, inductors and Zener Diodes). The coding is based on the tolerance values and different E series available are
  • E3 50% tolerance
  • E6 20% tolerance
  • E12 10% tolerance
  • E24 5% tolerance
  • E48 2% tolerance
  • E96 1% tolerance
  • E192 0.5, 0.25, 0.1% and higher tolerances
  • E3 coding is no longer in use and E6 coding is very rarely used.
  • The E96 coding system is used for high precision resistors with a tolerance of 1%.
There is a separate coding system in EIA E96 marking system. This system uses three figures for marking. The first two are numerals which indicate the three significant digits of the value of resistance. The third figure is a letter used to indicate the multiplier.

The EIA 96 code scheme for multipliers is shown below
CODE
MULTIPLIER
Z
0.001
Y or R
0.01
X or S
0.1
A
1
B or H
10
C
100
D
1000
E
10000
F
100000


Some examples of EIA 96 coding system are
92Z = 887 × 0.001 = 0.887 Ω
38C = 243 × 100 = 24.3 KΩ


Saturday, November 19, 2016

Basic Electronics on the Go - Types of Resistors

From www.electronicshub.org/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. 



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 clay acting 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.


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 0.1 micrometer 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  and high stability is important.


Wire Wound Type Resistors

Wire wound resistors are the 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 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 lead less face or 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 0.25W 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.


he 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.


However, 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 resistance path is provided by the  track and the terminals of the device are connected to the track. The 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 the wiper. This movable wiper maintains contact with the resistive surface.
The resistance between first terminal and the wiper plus the resistance between the wiper and the second terminal is equal to the formal resistance of the device. The name potentiometer is given to this device as it adjusts the  voltage using voltage divider principle.
The best application is their use in tuning circuits  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. 

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 the rheostat, one end of the 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. 
Light dependent resistors are made of semiconductors with high resistance. In the absence of light or in the 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 the portmanteau of the  variable resistor. It is an electronic component with non-linear current voltage characteristics like the diode. The resistance in the 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  the varistor is very high under normal operating conditions. But the resistance decreases dramatically when the voltage increases beyond the rated value of the varistor.

Metal oxide Varistors are the  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.

Friday, November 11, 2016

Basic Electronics on the Go - Resistors

http://www.electronicshub.org/introduction-to-resistors/


What is a 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, usually light or heat. The working principle of a 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. 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 .



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


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.

Resistivity

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.

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.

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.


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.

An increase in temperature on a conductor will result in an increase in its resistance as the atoms will vibrate more. These vibrating atoms will impede the path of the electrons flowing through.



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.
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 a 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.