Carbon Film Resistor

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The carbon film resistor, as the name indicates is made from a film of carbon on a ceramic former.

The carbon film resistor took over from the carbon composition resistor, especially as transistor technology started to take over.

The carbon film resistor provides better performance in many respects than the carbon compositing resistor, but this resistor type was overtaken by metal oxide film and metal film resistors which gave even better levels of performance.

What is a carbon film resistor

The carbon film resistor is formed using a ceramic carrier rod onto which a thin layer of pure carbon is deposited as a film. It is the thin carbon film that acts as the resistive element.

To enable the carbon film resistor to have the correct resistance, a helical cut is normally made into the film. This increases the length of the path and also reduces the width of the resistive element.

The resistance is obviously governed by the length of the path, the width of the resistive element, and also the thickness of the deposited carbon film.

In view of the fact that the resistive element forms a coil, these resistors have an inductance and this may affect their operation in RF circuits. The value may reach several µH, although uncut resistors are available and the level of self-induction for these components can be around 0.01 µH.

The capacitance may be around 0.5pF.

The use of pure carbon means that carbon film resistors have a negative temperature coefficient which is higher than that of carbon composition resistors. Typical values may be between -1x10^-4 Ω/°C and -8x10^-4 Ω/°C.

A further property of the carbon film resistor is that, using pure carbon, the noise produced by the resistor is significantly less than that produced by a carbon composition type.

Carbon film resistor manufacture

carbon film resistors are manufactured using a deposition process. The ceramic rod or carrier is placed in an environment where the temperature and pressure are high. Hydrocarbon gas, typically methane or benzene is passed over the rods at a temperature of around 1000°C. At this temperature the hydrocarbon gas breaks down and a thin film of carbon is deposited onto the rods.

Once the rods have been obtained, end caps are placed onto the rods to connect to the carbon film, and the resistance value may be trimmed by making a helical cut in the carbon.

Once complete, the resistors are coated in a resin to protect the resistive element from handling as well as increases of moisture and other contaminants.

Typical carbon film resistor specifications

Typical performance figures for carbon film resistors are given below as a guide to the performance

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