The most common type of Composition Resistor is the Carbon Resistor, also known as Carbon Composition. It consists of a solid cylindrical resistive element with embedded wire leads or metal end caps to which the lead wires are attached (Figure 1). The carbon composition resistors in the early 20th century have uninsulated bodies where the lead wires were wrapped around the ends of the resistance element rod and soldered.
Figure 1 Structure of the Carbon Composition Resistor
It can be considered as the oldest design and, more often, the cheapest of the general purpose resistors that are used in electrical and electronic circuits. These resistors are made from a molded carbon powder that has been mixed with a phenolic binder to create a uniform resistive body. The body may also be protected with paint or plastic. As seen in the image below, a mixture of finely ground carbon dust or graphite (similar to pencil lead) is used to manufacture their resistive element. A non-conducting ceramic (clay) powder is used to bind the mixture all together along with resin. The carbon granules are mixed with a filler material and inserted into a tubular casing or molded into a cylindrical shape with metal wires or leads attached to each end to provide the electrical connection.
Figure 2 Carbon Composition Resistors
The overall resistive value of the mixture is determined by the ratio of carbon dust to powdered ceramic or the amount of carbon added to the filler mixture. The resistive value is denoted by the color-coded markings to the outer insulating material. Low resistance is a result of higher concentrations of carbon, making it a weak conductor.
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