Carbon Resistor

A carbon resistor is a type of electrical component used to limit the flow of electric current in a circuit. It is made from a mixture of carbon powder and a binding material, which together form a resistive element. The resistance value depends on the proportion of carbon and the size of the resistor. Carbon resistors are widely used due to their low cost, making them suitable for general-purpose applications in various electronic devices.

1.0Definition of Carbon Resistor

• It is a component of an electrical circuit offering certain opposition to the flow of current in that circuit.The resistance offered by different resistors to the flow of current varies from few ohms to millions of ohms.
• Some resistors are wire wound and some are made from carbon.Both can be of two types fixed(having non-variable resistance) and variable (whose resistance can be changed)

2.0Pictorial Symbol of Resistors

1. Fixed Resistor

2. Variable Resistance or Rheostat

3. Carbon Colour Code Resistor

3.0Elements of Carbon Resistor

To make a carbon resistor of fixed resistance,carbon with a suitable binding material is molded into a cylinder.Wire or leads are attached to this cylinder and the whole resistor is encased in a ceramic jacket.

1. Carbon Composition-Carbon is the primary material used to make the resistive element. It provides resistance due to its properties and the way it conducts electricity. The resistivity of carbon can be controlled by varying the proportion of carbon powder mixed with a binder material.
2. Binder Material-The carbon particles are mixed with a binder material, often a plastic resin or clay, which holds the carbon powder together. This allows the mixture to be molded into a solid form and maintains the integrity of the resistor's structure.
3. Resistive Value-The value of the resistor, measured in ohms (Ω), is determined by the ratio of the carbon to the binder, as well as the length and diameter of the resistor. The resistance is controlled by adjusting the amount of carbon in the mixture.
4. Electrode Leads-The resistor has two metal leads (usually made of copper or tinned copper) that are connected to the ends of the carbon resistive element. These leads allow the resistor to be connected to a circuit.
5. Surface or Body-Most carbon resistors have a cylindrical body that is coated with an insulating material, often a colored band or sleeve, which can indicate the resistance value through a color code.
6. Color Bands- Carbon resistors often use a color coding system to represent their resistance values. The bands typically consist of 4 or 5 colored bands that indicate the resistor's value, tolerance, and sometimes its reliability or temperature coefficient.
7. Tolerance- The tolerance of a carbon resistor indicates how much the actual resistance can vary from the labeled value.

Note:The ceramic rod (or ceramic core) serves as an insulating support structure for the resistive material.

4.0Read The Value Of A Carbon Colour Code Resistor

• The value of the resistances of the carbon resistors are indicated by three or four colour bands painted on the bodies of the carbon resistor.These colour bands can be translated into a number by using the standard colour code.
• The first coloured band $B_1$ represents the first figure $(Say\propto )$ and the second band $B_2$ $(Say\beta )$ and the third band $B_3$ represent the number of zeros to be added to the right side after the second figure say (c) to get the total resistance.The fourth band,if present gives the tolerance known as limit of accuracy or permitted variation in the value of resistance ($\gamma$) from 5 % to 10 %.

$R=\left(\alpha \beta \times 10^c\mp \gamma \right)\text{ohm}$

• If fourth colour band does not exist then the tolerance is to be taken as ∓20%

Memory Aid To Learn The Table: Black Brown Rods Of Your Gate Became Very Good When Given Silver Colour.

5.0Sample Question on Carbon Resistor

Q-1. Find Resistance in ohm band colours are given as B1=Yellow, B2=violet, B3=Brown     B4(Tolerance)=∓5%

Solution: $R=(\alpha \beta \times 10^C\mp \gamma )ohm$

              $R=47\times 10^1\mp 5$ $\Omega$