Carbon film resistors are a fixed form type resistor. They are constructed out of a ceramic carrier with a thin pure carbon film around it, that functions as resistive material. Carbon film resistors are a significant improvement on carbon composition. However, in comparison to metal film and metal oxide film, the commercially available range steadily decreases. Metal and oxide film are not more expensive to produce, and have overall better properties. Besides carbon filme resistors, Micrhom also manufacture various axial resistors, such as metal fim resistors HPMRY series, PMRY series, EE series and metal oxide resistors, NGT series.
Carbon film resistors are made with a deposition process. At high temperature and under a high pressure, a ceramic carrier is held in hydrocarbon gas. The gas (methane or benzene) is cracked at a temperature of 1000°C. The crystalline carbon is pyrolytically deposited on the ceramic substrate. Because of the precise distribution of the pure graphite without binding, these carbon resistors have a low noise. The desired resistance value can be obtained by choosing the right layer thickness, and by cutting a spiral shape in the carbon layer. The helical cut in the film increases the length of the current path. By decreasing the pitch of the helix, the length of the resistive path increases, and therewith the resistance value increases. Furthermore, by fine tuning the cutting of the spiral the resistor can have a higher accuracy of resistance value. Typical tolerance values for carbon film resistors are 2, 5, 10 and 20%.
