Carbon Film Resistor

- the carbon film resistor took over from the carbon composition resistor, providing some improvements in performance over the previous carbon composition type..

The carbon film resistor took over from the carbon composition resistor in the 1960s and 1970s. It too was later superseded by metal oxide film and metal film resistors that were able to provide improved performance at no additional cost.

Nevertheless the carbon film resistor was able to provide a significant improvement in performance for a while and was widely used in the late 1960s and 1970s.

Carbon film resistor structure & manufacture

Carbon film resistors consist of a thin carbon film that is deposited on a ceramic former. Connections and leads are added and the carbon film is covered with a protective layer.

Often these resistors are available in small wattage levels, down to 1/8 watt and have been available as axial leaded varieties since the 1960s.

Carbon film resistors are used less now as superior resistor technologies are available at no extra cost.

The carbon film resistor is manufactured by cracking hydrocarbon onto a ceramic former. Once the formers or substrates are complete, connections are added at either end and the value is adjusted by cutting a helix into the carbon film. The fact that the resistors have a helix cut into them, makes them slightly inductive and the effects of this may be noticed when using them for RF applications.

Kin view of the tolerance to which these devices can be manufactured and their long term stability, tolerance levels down to 5% are normally viable, below this, then the long term tolerance and effect of heat, operation and the like may take them out of their tolerance band.

The temperature coefficient of carbon film resistors is typically between about -100 and -900 ppm/°C.

By Ian Poole

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