Carbon Composition Resistor

- the carbon composition resistor sometimes called the carbon composite resistor was the resistor technology that was mainly used before the 1960s.

Carbon composition resistors have been available for very many years. They are low cost and have provided useful service over many years.

The carbon composition resistor was the mainstay resistor type in valve equipment where it provided adequate performance and was able to withstand the rigours of the operating environment, both electrical and thermal.

Image of a 1960s style carbon composition resistor
Carbon composition resistor

Carbon composition resistor history

The carbon composition resistor is one of the earliest resistor types to have been available. They have been used for over a hundred years and before the 1960s they formed the mainstay of resistors for electronic equipment. The only other type that was available was the wire-wound format.In the 1960s there was a significant move away from the carbon composition resistors to carbon film, and metal oxide film resistors which were generally smaller and able to provide much higher levels of stability.

Today, it is still possible to obtain these carbon composition types, but they are only rarely used, and in more specialist applications or in restoring old radios.

Carbon composition resistor structure

Carbon composition resistors or carbon composite resistors are manufactured from a mixture of graphite, fine ceramic particles and a resin as a binder. The materials are evenly mixed and pressed into short cylindrical rods which are heated under pressure. Caps with wires attached are placed on the resulting rods as the connecting electrodes.

In the very early days, no coating was applied, but it was found that the mixture was porous and also was easily damaged and therefore a ceramic style coating was normally added.

The resistance of the resistor is governed by the length of the rod and the mixture. Often large numbers of resistors were manufactured and the resistance was measured before the value markings were added as it was not always possible to manufacture the values exactly.

The width was generally used to determine the wattage. The wider, and to an extent the length of the overall package determined how much heat could be dissipated. The larger the overall package the greater the level of heat dissipation. Values of 1/4, 1/2, 1 watt, and in some cases up to 5 watt types were manufactured.

Advantages and disadvantages

For many years, the carbon composition resistor formed the mainstay of the electronics industry in terms of resistors. Although they have some advantages, they also fell into disuse because of their disadvantages as other more advanced techniques became available.

Advantages:

  • Ability to withstand high energy pulses:   The main advantage of the carbon composite resistor is that it is able to withstand high energy pulses. As the resistive element within the component is relatively large, it is able to dissipate the short energy burst more effectively than other resistor types where the actual resistive element is a film or wire where the area of conduction is relatively small and more prone to burning out.

Disadvantages:

  • Stability:   The lack of stability was one of the major drawbacks for the carbon composition resistor. Even when not in use the value could change by as much as 5% in a year. Also heat would change the value. Soldering could alter the value by 2 or 3% and operation at temperatures up to 70°C could alter the value by 15% or more. As a result these resistors normally only had tolerance indications of ±20%. Ten percent tolerance resistors were comparatively rare.
  • Noise:   carbon composition or carbon composite resistors were renowned for the high levels of noise created. This resulted from the granular composition and structure of the resistive element.
  • High temperature coefficient:   These resistors were also renowned for the lack of temperature stability. They could often exhibit stability levels worse than 1000 ppm/° C.

Carbon composition resistors are not widely used these days. Their place as a general purpose resistor has been overtaken by other types; initially it was the carbon film resistor, but latterly the metal oxide film and the metal film resistor, that offer far superior performance in most aspects have come to dominate the resistor marketplace.

Large carbon composition resistors may be used as RF loads because of the power handling capability and also the lack of a helical cut or would wire that will add inductance.

By Ian Poole


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