Electrical Resistor | Electrical Resistance

Introduction of Resistor

Resistor is an electrical two terminal passive circuit component that offers opposition to the flow of current. In electrical circuits they are used for reducing the current and to lower the voltage in any particular portion of the circuit.

Electrical Resistance Definition

Electrical resistance is defined as the property of resistor to oppose the flow of current. It is generally represented by Letter “R”. The S.I Unit of resistance is ohm, symbolized by the Greek letter omega (Ω). The symbol of resistance is shown below

Circuit View Point of Resistance

According to ohm’s Law, current in a conductor/ resistor is directly proportional to the voltage applied across the ends of conductor. \\Suppose we have a resistor of resistance R Ω, let the voltage applied across the terminals of the resistor is “V” and current flowing in the resistor is “I” then

\;\;\;\;V \propto I\\\;\\\;\;\; V = IR\\\;\\\;\;\;R=\frac{V}{I}\;\;\;\;\;\;\;(1)

Energy Viewpoint of Resistance

The resistor converts the electrical energy into heat energy. Let we have Resistor of Resistance “R”, Voltage across the end of Resistor is “V” and Current flowing in the resistor is “I”. Then Power (P) absorbed by the resistor is
P = VI = (IR)I = I² R
This is a Joule’s Law. which states that the number of electrons colliding with the atoms of a circuit elements to produce heat is proportional to the square of the current. The corresponding amount of energy converted into heat in the time interval of t₂ – t₁ is\\\;\\\;\;\;\;\;W=\int_{t_{2}}^{t_{1}}I^{2}Rdt

The above equation is applied whenever the current “I” is a time varying quantity. when Current “I” is constant quantity, the above equation becomes \\\;\\\;\;\;\;\;W=I^{2}Rt\\\;\\Where t = t₂ – t₁ \\\;\\\;\;\;\;\;R=\frac{W}{It}Ω

Geometrical Viewpoint of Resistance

Resistance of conductor directly depends on length of conductor, nature of material and inversely proportional to the area of cross section.Consider the conductor having length “L”, area of cross section “A” as shown in figure.

The resistance (R) of conductor will be \\\;\\\;\;\;R=\frac{\rho L}{A}\\Where ρ = Resistivity of material

Resistivity or Specific Resistance

It is defined as resistance offered by a conductor of unit length and unit area of cross section. It is independent of dimensions of conductor. It depends on nature of material and temperature.The unit of resistivity is ohm-meter.

Effect of Temperature on Resistance

Resistance varies with temperature. The resistance of conductor increases with increase of temperature while the resistance of insulators and semi-conductors decreases with increase of temperature.

The variation of resistance with the variation in temperature is given by formulae below\\\;\\\;\;\;R_{2}=R_{1}\left [1+\alpha_{1}(T_{2}-T_{1})\right ]\\Where “R₂” represents resistance at temperature “T₂” and “R₁” resistance at temperature “T₁”. “α₁” represents temperature coefficient of resistance at temperature “T₁”. 

The value of “α” for various metals at temperature 20⁰C are given below

Types of Resistors

The various types of resistors are as follows

Carbon Composition Resistor:- In This type of resistor, the resistive material is carbon-clay composition & leads are made of copper. The resistor is enclosed in plastic case to prevent the entry of moisture and other harmful elements from outside. These resistors are cheap and common type of resistor used. They are highly sensitive to temperature variations. Such resistors are available in range from few ohms to 22MΩ having tolerance range of ±5 to ±20%. The power dissipation capacity ranges from 0.1 to 2 Watts. Such resistors have tendency to develop noise due to passage of electric current from one carbon particle to another. Such resistors are used where cost is main consideration rather than performance requirements. They are mainly used in electronic circuits.

Wire Wound Resistor:- In these resistors a wire is wound around a insulating cylindrical core. Usually wires are of constantan (60% Copper, 40% Nickle) and Manganin. The complete wire wound resistor is coated with an insulating material such as backed enamel. They are less sensitive to temperature variations. These resistors are more accurate than carbon resistors. Tolerance ranges from 0.01% to 1%. They can be used for high power applications (5W to 200W).

The main drawback of the wire wound resistor is the inductance that arises because of their wound coil like structures. At high frequencies this often makes the ordinary wire wound unstable. To overcome this problem, a dual thread wire is wound in one direction and the other half in the opposite direction. The inductance of two halves cancel each other.

Meta film & Carbon film resistors:- The carbon film is constructed by depositing a thick film of resistive material( Pure carbon or some metal) on a insulating (glass, ceramic or other insulating) substrate.
Metal film resistors range in value upto 10,000MΩ and are much smaller in size than wire wound resistors. The problem of inductance of wire wound resistors is overcomed. These resistors have low noise, make them best suitable for use in low level amplifiers and computers. They are much costlier.
Carbon film resistors give lower tolerances and smaller values of resistances than metal film resistors.

Color Coding of Resistors

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 Coding is one way to represent the value of the resistance of resistor along with the tolerance.
There are several color bands to specify the value of resistance. They even specify tolerance, reliability and failure rate. The number of bands vary from three to six. The Values of Various colors are shown below

To Remember the color sequence, remember the sentence ” BB ROY of Great Britain has Very Good Wine” where the first bold letter of each word corresponds to the first letter of the color.

In case of 3 band code, the first two indicate the value of resistance and the third band acts as multiplier. The Tolerance of three band colour system is generally 20%.
Consider the Resistor below

First Color is Black, it Value is zero (0) from above color chart
Second Color is Red, Its value is 2
Third color is Yellow, Its Value is 4
The value of Resistor = 02 \times 10^{4}=20kΩ

For Four color band Resistor as shown below

First Colour is Brown, it Value is 1 from above color chart
Second Color is Orange, Its value is 3
Third color is Green, Its Value is 5
Fourth Color is Brown, its tolerance value ±1%
Thus the Value of Resistor is = 13 \times 10^{5} = 1.3MΩ \pm 1%

For Five color band Resistor as shown below

First Colour is Brown, it Value is 1 from above color chart
Second Color is Red, Its value is 2
Third Color is Black, Its Value is 0
Fourth Color is Brown, its Value is 10
Fifth Color is Red it has Tolerance Value of ±2%
Thus the Value of Resistor = 120 \times 10=1.2k\Omega\pm 2%

The 6th band for a 6-band resistor is the temperature coefficient. This indicates how much the actual resistance value of the resistor changes when the temperature changes

The 6 color band resistor is shown below

First Colour is Brown, it Value is 1 from above color chart
Second Color is Red, Its value is 2
Third Color is Black, Its Value is 0
Fourth Color is Brown, its Value is 1
Fifth Color is Red it has Tolerance Value of ±2%
sixth band is Red, It has TCR value 50PPM
Thus the Value of Resistor = 120 \times 10^{1}=1.2k\Omega\pm 2% and 50PPm

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