Introduction to Carbon Composition Resistors

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.

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.

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.

In the 1960s and earlier, carbon composition resistors were commonly used, but not so popular for general use today because other types have better specifications such as tolerance, voltage dependence, and stress (changes value when stressed with over-voltages). Soldering heat can create a non-reversible change in resistance value if internal moisture content or exposure for some length of time to a humid environment is significant. They can be remarkably reliable if never subjected to over-voltage. Carbon composition resistors do not have the close tolerances of either carbon or metal film types, and they have poor stability with time and were consequently factory sorted to, at best, only +/- 5% tolerance. The tolerance may be up to +/- 20% in the long term.

Usage of Carbon Composition Resistors

Carbon composition resistors are used in applications in which the initial tolerance does not need to be any closer than +/- 5% of the value that is suited for non-high-tolerance devices. It has an advantage of being suited to applications that involve larger voltage pulses than more modern types. Because they are very cheap to make, they are commonly used in electrical circuits such as power supplies and welding controls. For very large tolerances and for more precision and high value tolerances, film type resistors are used instead. These resistors are no longer used in most applications, mainly because of the high price.