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!).

 Resistor

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.

Following is the table with the links related to resistors:

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.

 

Carbon Resistor Important Questions

Q1. What is the use of thermistor?

Ans: It is used for the conversion of temperature into electric voltage.

Q2. What is a rheostat?

Ans: A rheostat is a wire wound variable resistance.

Q3. What is the tolerance of gold colour resistance?

Ans: The tolerance of gold colour resistance is 5%.

Q4. What are the uses of rheostat?

Ans: The uses of rheostat are it can be used as a variable resistor as well as a potential divider.

Q5. What does the absence of the fourth strip in a carbon resistor mean?

Ans: It means that the tolerance limit is ±20%.

Q6. What is the tolerance of silver strip?

Ans: The tolerance of the silver strip is ±10%.

Q7. What is the value of the resistor with colour bands brown, black and red?

Ans: The value of the resistor with colour bands brown, black and red is 1000 ohms.

Q8. Which digit do the following colours denote on a resistor colour band: black, brown, red, orange, yellow, green, blue, violet, grey and white?

Ans: Following are the digits denoted by the different colours:

  • Black=0
  • Brown=1
  • Red=2
  • Orange=3
  • Yellow=4
  • Green=5
  • Blue=6
  • Violet=7
  • Grey=8
  • White=9

Practise This Question

Which of the following is not a natural phenomenon?