An introduction to carbon composition resistors Carbon composition resistors (CCR) are fixed form resistors. They are made out of fine carbon particles mixed with a binder (for example clay). After baking it has a solid form. Although carbon composition resistors are widely applied in circuits, the majority of resistors are nowadays made by deposition of a metal or carbon film over a ceramic carrier. Advantages and disadvantages The big advantage of carbon composition resistors is their ability to withstand high energy pulses. When current flows through the resistor, the entire carbon composition body conducts the energy. The wirewound resistor for example, has a much smaller volume of the wire to conduct. So the thermal mass of the carbon composition resistor is much higher, resulting in a higher energy capability. Carbon resistors can be made with a higher resistance than wirewound resistors, and are considerably cheaper. However, the properties are less good in terms of temperature coefficient, noise, voltage dependence and load. Fifty years ago, carbon composition resistors were widely used in consumer electronics. Because of the low stability of the resistance value, this type of resistor is not suitable for any modern high precision application. For example, the resistance value can change up to 5% over a shelf life of one year. With heavy use the value changes even more: up to 15% for a 2000h test at full rating with 70°C. Soldering can cause a 2% change. The reason for this instability is inherent to the design of the resistor. The carbon composition contains materials with different heat expansion properties. When the conducting carbon particles and the nonconducting binder heat up or cool down, stresses arise in the resistor body. The mechanical contact between the conducting particles will change, and this leads to a change in resistance value. Also...
Resistors are produced with a wide variety of materials and manufacturing processes. Each resistor material has its typical properties and specific areas of use. The main types that are used in electrical engineering are summed below. Wirewound (WW) These types are made by winding resistance wire in a spiral around a non-conductive core. The resistance wire is usually a nickel-chromium alloy and the core is often ceramic or fiberglass. A coating such as vitreous enamel is used for protection. The spiral winding has capacitive and inductive effects that makes it not suitable for applications higher than 50 kHz. Often other winding techniques are used to reduce the undesired high frequency effects. Wirewound resistors are essentially produced for high precision or for high power applications. They have low noise, are robust, and are temperature stable. Resistance values are available from 0.1 up to 100 kW, with accuracies between 0.001 and 20%. Carbon Composition (CCR) The resistive element is made from a mixture of fine carbon particles and a non-conductive ceramic material. The substance is pressed in a cylindrical shape and baked. The resistance value depends on the dimensions of the body and the ratio between carbon and ceramic material. More carbon means a lower resistance. Carbon composition resistors are remarkably reliable, but have a poor accuracy with a maximum tolerance around 5%. Until the 1960s they were the standard for general applications. They quickly lost market share as other resistor types came on the market with better properties for tolerance, voltage coefficient, temperature coefficient, stability and finally cost. However, their ability to withstand high energy pulses and their high reliability makes them still useful for certain applications. Examples are power supplies and welding controls. Carbon Film A thin, pure carbon film is deposited on an insulating cylindrical...