Carbon Film Resistor One of the most used “passive devices” in an electric or electronic circuit is undoubtedly the resistor. Since it has wide range of applications they are found in different varieties. According to the type of resistance they offer, these devices are often classified as fixed or variable resistors. However, the resistors, either fixed or variable are not same in construction and hence are termed according to their type of construction. One of these resistor type is the carbon film resistor, which we would be discussed in this article. What is a Carbon Film Resistor? It is clear from the name “carbon film resistor” itself, that this resistor is made from carbon film. The carbon film is deposited on a ceramic former. This is actually a fixed type of resistor, meaning it provides only a single resistance value. Here the carbon film plays the role of resistive material that restricts the flow of current in a circuit. Thus in a single line we can define a carbon film resistor as: Carbon film resistor is a fixed resistor that uses a carbon film that is deposited on a ceramic former, to restrict the flow of current The figure below shows how a set of commercial carbon film resistors looks like. The accuracy of resistance value offered by this resistor owes to the helical cut that is usually made into the film. This and other construction features are discussed in the “construction” section of the article. Now does this resistor have a particular symbol to represent it schematically, you may wonder. Actually, the symbol is same as that of a resistor, a zig- zag line or a rectangular box (IEC standard). However to differentiate that the used resistor in the circuit is a carbon film resistor, the resistance value of the resistor is prefixed by “CR”. Like for example, if we have used a 120kΩ carbon film resistor, then we would write it as CR120kΩ, to indicate the same. Carbon Resistor – Construction The carbon film resistors are constructed or manufactured using a deposition process. As mentioned earlier, a carbon film is deposited on a ceramic substrate. This carbon film restricts the flow and hence is the imperative part of this resistor. It is due to this reason, the resistor is named “carbon film resistor”. A hydrocarbon such as methane or benzene is cracked at a high temperature of 1000oC where a ceramic carrier. Since a pure graphite(Carbon) is used for distribution on the ceramic substrate without binding the noise produced by the carbon film resistor is low. The role of ceramic substrate is that of an insulator to heat or electricity. Its presence therefore makes this resistor withstand high temperature without much damage. The figure below shows a schematic of the construction of the carbon resistor. Here we observe that the carbon film is helical in shape. As already mentioned it helps in getting accurate resistance out of this resistor. This helps in increasing the effective length of the resistor while decreasing the width of the resistor. The value of resistance is adjusted by increasing/decreasing the length of the helical path (since resistance is directly proportional to the length). An epoxy coat is given to the carbon film for its protection. The two end caps shown in figure are actually metallic and the two connecting leads made of copper are joined at the two ends of these metallic caps. So how does the carbon film layer help in restricting the electron flow? It actually depends on the width of the carbon film layer. For a high resistance...
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