Carbon Resistor and its Structure

What are Carbon Resistors?

There are many materials used to produce resistance particularly metals and alloys like Nichrome, brass, platinum and tungsten alloys. However, most of these metals have low electrical resistivity, unlike carbon resistor, which makes it difficult to produce high resistances without becoming bulky [remember: Resistance ∝ {Length ×Resistivity}]. However, they can produce highly accurate values of resistance and hence are used usually to calibrate and compare resistances.

For most practical purposes, however, carbon resistors are preferred. This is because they are cheap to produce, compact and can be directly printed onto circuit boards (like the computer processors in phones and tablets). They also reproduce resistance fairly well within practical requirements. Compared to metal wires which are expensive to produce, carbon is abundantly available making it cheap. (Ask the price of a resistor in an electronic component store, you can get a dozen for just 2 rupees!).


The internal structure of a carbon resistor is shown in the image below.

Carbon Resistors

Uses of Carbon Resistors

The carbon resistor contains carbon coated onto a ceramic core. A spiral is etched on the deposited carbon which turns it into a wire wound on a ceramic core. Depending on the resistance required, pitch, diameter and length of the carbon spiral vary. Nickel caps are attached to both ends of the core so as to create a good contact between the carbon and lead. The leads are soldered onto the nickel caps and the entire resistor is coated with lacquer for electrical insulation.

For small resistances required for small currents, the metal caps act as heat sinks to carry away heat dissipated by the resistor. For larger current requirements, a metal heat sink is separately attached to carry away excess heat and to prevent the resistor from burning up.
Carbon resistors are specially colour-coded to identify the resistance.

To learn more on resistors like its dependence on temperature, the heating effect of current, resistors in series and parallel with the help of interactive video lessons download BYJU’S – The Learning App.


Practise This Question

A spherical planet far out in space has a mass M0 and diameter D0. A particle of mass m falling freely near the surface of this planet will experience an acceleration due to gravity which is equal to