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Electronics devices and circuits >> Passive components >> Resistors >> Carbon composition resistor

Carbon composition resistor

Carbon composition resistor definition

The carbon composition resistor is a type of fixed resistor that reduces or restricts the electric current flow to a certain level.

These resistors are the most commonly used resistors in the 1960’s and earlier. However, 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.

Construction

The carbon composition resistors are made from a solid cylindrical resistive element with embedded wire leads or metal end caps. The cylindrical resistive element of the carbon composition resistor is made from the mixture of carbon or graphite powder and ceramic (made of clay). The carbon powder acts as the good conductor of electric current.

The carbon composition resistors are made from a solid cylindrical resistive element with embedded wire leads or metal end caps.

The solid cylindrical resistive element is covered with plastic to protect the resistor from outside heat. The leads made of copper are joined at two ends of the resistive element. In the early days, no insulating materials are used in the carbon composition resistors. Hence, even at low temperature, heat easily enters into the resistor and damages it.

The carbon composition resistors are available with different resistance values ranging from one ohm (1Ω) to 22-mega ohms (22 MΩ).

Resistance of the carbon composition resistor is depends on three factors: amount of carbon added, length of solid cylindrical rod, and cross sectional area of the solid cylindrical rod

The resistance of the carbon composition resistor is depends on the amount of carbon added, length of solid cylindrical rod and cross sectional area of the solid cylindrical rod.

Amount of carbon added

The resistance of the carbon composition resistor is inversely proportional to the amount of carbon added.

We know that carbon is the good conductor of electricity. Hence, if more amount of carbon is added, more amount of electric current flows and only a small amount of electric current is blocked. Thus, the carbon composition resistor with more carbon has low resistance. If less amount of carbon is added, only a small amount of electric current flows and more amount of electric current is blocked. Thus, the carbon composition resistor with less carbon has more resistance.

Length of solid cylindrical rod

The resistance of the carbon composition resistor is directly proportional to the length of solid cylindrical rod.

The cylindrical rod with long length provides high resistance to the electric current because the free electrons have to travel large distance. Hence, the possibility of collision of electrons with the atoms is high. Thus, a large number of free electrons collide with the atoms. The large number of free electrons, which collides with the atoms loss their energy in the form of heat, and the remaining small number of free electrons moves freely. The small number of free electrons, which moves freely, will carry the electric current. Therefore, only a small amount of electric current flows through the carbon composition resistor.

The cylindrical rod with short length provides low resistance to the electric current because the free electrons have to travel only a small distance. Hence, the possibility of collision of electrons with the atoms is less. Thus, only a small number of free electrons collide with the atoms. The small number of free electrons, which collides with the atoms loss their energy in the form of heat, and the remaining large number of free electrons moves freely. The large number of free electrons, which moves freely, will carry the electric current. Therefore, a large amount of electric current flows through the carbon composition resistor.

Cross sectional area of the solid cylindrical rod

The resistance of the carbon composition resistor is inversely proportional to the cross sectional area of the cylindrical rod.

The cylindrical rod with large cross sectional area provides large space for the free electrons to move freely. Hence, the possibility of collision of free electrons with the atoms is low. Thus, a large amount of electric current flows through the carbon composition resistor.

The cylindrical rod with small cross sectional area provides only a small space for the free electrons to move. Hence, the possibility of collision of free electrons with the atoms is high. Thus, only a small amount of electric current flows through the carbon composition resistor.

Noise generated by carbon composition resistor

The carbon composition resistor generates two types of noise: Johnson noise or thermal noise and current noise.

Johnson noise

The Johnson noise is the thermal noise generated by the charge carriers due to thermal agitation.

Current noise

The current noise is the noise caused by internal changes in the resistor when current is flowing through it.

Advantages and disadvantages of carbon composition resistor

Advantages of carbon composition resistor

  • The major advantage of carbon composition resistor is its ability to withstand at high energy pulses.
  • The cost of carbon composition resistor is low.

Disadvantages of carbon composition resistor

  • Poor stability: Poor stability is the major disadvantage of carbon composition resistor. Even within a short period, resistance of carbon composition resistor will change rapidly.
  • Generate high noise
  • Low accuracy
  • The most common problem occurred in the carbon composition resistor is water absorption. When the carbon composition resistor observes water, the resistance may increase or decrease.

Applications of carbon composition resistor

The various applications of carbon composition resistor include:

  • Current limiting
  • Protection of circuits
  • High voltage power supplies
  • Welding controls
  • Carbon composition resistors are used in high frequency applications.
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