Carbon Resistor and Its Structure, Parts of a Carbon Resistor, Application of Carbon Resistors (For CBSE, ICSE, IAS, NET, NRA 2022)

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Carbon Resistor and Its Structure

  • The carbon composition resistor is a type of fixed resistor that reduces or restricts the electric current flow to a certain level.
  • Now-a-days, carbon composition resistors are rarely used because of its high cost and low stability. The carbon composition resistors are also called carbon composition or carbon resistors.
  • They are made from a solid cylindrical resistive element with embedded wire leads or metal end caps.
  • It comes in different physical sizes with power dissipation limits commonly from 1 watt down to watt.

What Are Carbon Resistors?

  • Their resistive element is manufactured from a mixture of finely ground carbon dust or graphite (similar to pencil lead) and a non-conducting ceramic (clay) powder to bind it all together.
  • Materials used to produce resistance particularly are metals and alloys like nichrome, brass, platinum, and tungsten alloys.
  • The mixture is moulded into a cylindrical shape with metal wires or leads are attached to each end to provide the electrical connection as shown, before being coated with an outer insulating material and colour coded markings to denote its resistive value as shown in figure below.

Carbon Resistors

Carbon Resistors
  • But most of these metals have low electrical resistivity, unlike carbon resistor, which makes it difficult to produce high resistances without becoming bulky. As we Know that,
  • So, from above they can produce highly accurate values of resistance and hence are used usually to calibrate and compare resistances.
  • For practical purposes 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 well within practical requirements. Compared to metal wires which are expensive to produce, carbon is abundantly available making it cheap.

Parts of a Carbon Resistor

  • Resistors can be constructed out of different materials. The different parts of a carbon resistor are:
    • Ceramic Core
    • Nickel Cap
    • Lead
    • Carbon Film
    • Protective Lacquer

Application of Carbon Resistors

  • Carbon composition resistors are modeled to withstand high energy pulses, while having a relatively small size.
  • For the same reason the carbon composition resistor is used in many applications nowadays.
  • Applications includes
    • The protection of circuits (surge or discharge protection)
    • Current limiting,
    • High voltage power supplies,
    • high power or strobe lighting
    • welding
  • An example of an application is a medical defibrillator.
    • The sensitive measurement equipment attached to the patient needs to be protected against the high energy pulses of around 30 Joule.
    • Carbon composition resistors are applied in the equipment or the leads and have to withstand all pulse energy
  • The carbon resistor has carbon coated onto a ceramic core.
  • A spiral is etched on the deposited carbon which changes it into a wire wound on a ceramic core.
  • Depending on the resistance required, pitch, diameter and length of the carbon spiral changes.
  • Nickel caps are attached to both ends of the core to create a good contact between the carbon and lead.

Electric Resistivity for Resistive Material

Electric Resistivity for Resistive Material
MaterialResistivity / m
graphite
amorphous carbon

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