Carbon Composition Resistor

What Are Carbon Composition Resistors?

A carbon composite or carbon composition resistor is a type of resistor that is made of a mixture of electrically-conducting carbon and a non-conducting binder material, like clay.

The body of a carbon composition resistor is a mixture of carbon, most often in the form of graphene, and a binder material. The more carbon that the mixture contains, the more conductive the resistor (and the lower its’ value).

Once extremely popular, carbon composition resistors have gradually lost market share due to issues with tolerance, noise, and aging as well as cost.

Although they are no longer ubiquitous in electronics, carbon composition resistors remain popular in power-intensive and pulsing applications as well as vintage electronics restoration. Carbon composition resistors are highly popular in restoring or building vintage amplifiers, where their distinctive properties help create the sound audiophiles are looking for.

In this article, we’ll cover how carbon composite resistors function, their positives and negatives versus other types of resistors

How Carbon Composite Resistors Work

Carbon composite resistors consist of a solid body that is made from a blend of conducting particles (carbon) and a non-conducting binder material. Metal leads are then soldered to each side of the resistor, and the body is coated in an insulator (usually plastic) to prevent short circuits.

When the carbon composition resistor is placed in a closed circuit with a power source, electric current will flow through the resistor. Current flows from the resistor leads through the solid carbon composite body. Since the material inside the body is inherently resistive due to the non-conducting binder, the resistor will do its’ job – restricting the flow of current.

A voltage difference will be measured across the resistor (between the resistor leads). This voltage difference is determined by Ohm’s Law:

V = IR
Voltage (volts) = Current (amps) x Resistance (ohms)

The resistance of the carbon composite resistor is determined by the mixture of conducting and non-conducting components. The mixture causes the material to act as a resistor- allowing electric current to pass while restricting it’s flow. The more binder and less carbon in the mixture, the greater the resistance.

Usage of Carbon Composite Resistors

Carbon composition resistors are not the most popular today. They are relatively expensive, have poor tolerance, and may age more poorly than other types of resistors. For these reasons and others, they have largely been overtaken by other types of resistors, like metal and carbon film types.

But carbon composite resistors can still be found in power intensive applications like power supplies and welding gear. This is attributable to the fact that they have a greater conducting volume over which to dissipate thermal losses, so they are suited for high power applications.

Carbon composition resistors are also popular in the field of vintage electronic restoration, where they can help in retaining original character. For this reason, they are still very popular in the restoration of amplifiers, where the unique musical character of the original electronics is often a highly valued component.

Advantages and Disadvantages of Carbon Composition Resistors

Advantages	Disadvantages
Simple design – hard to break	Poor tolerances
Large conducting volume – excellent thermal dissipation	Relatively high cost
Good for restoration	Resistance value drifts significantly over time

The main useful feature of carbon composition resistors is their ability to tolerate high pulses of energy.

As electric current flows through the resistor, the entire carbon composition body conducts the energy. This gives it a much greater volume over which it actually conducts, when compared with a wirewound or film resistor.

The higher volume of the carbon composition resistor allows it to withstand much higher energy pulses than other types of resistors.

History of Carbon Composite Resistors

Carbon composition resistors were very popular 50 or so years ago.

However, the resistance value is not stable enough to be used in many modern applications. Over the course of one year, carbon composition resistors can change value up to 5%. This value also changes with use; a heavily used carbon composition resistor may change up to 15% in one year.

This change is due, in large part, to changes in the material structure caused by thermal stresses. Inside the carbon composition resistor, there are small areas of different composition. As the resistor heats and cools, these areas will expand and contract at different rates. This leads to stresses that can change the material microscopically, leading to a change in the resistance value. Unfortunately this long-term instability is inherent to the material composition of the resistor.

Carbon Composition Resistor vs. Film Resistors

Although there are times when it may be desirable to choose a carbon composition resistor, film resistors generally have superior properties.

Advantages of film resistors over carbon composition resistors include less noise, tighter tolerances, lower capacitance/inductance (better high frequency performance), lower temperature coefficient, better long-term performance.

How Carbon Composite Resistors are Made

A mixture is made of:

Conductive carbon in the form of graphite or ground carbon dust.
Non-conducting ceramic (clay).
Resin.

This mixture is heated to a high temperature and pressed into a rod at high pressure. Connecting wires are pressed into each end of the resistor. The outer body is then insulated to prevent short circuits.

Carbon composition resistors may be made with a larger diameter to allow for greater power dissipation.

The type of carbon has a significant impact on the composition of the mixture. Graphite and amorphous carbon are both commonly used but the resistivity of graphite is lower:

Form of Carbon	Resistivity (Ω / m)
Graphite	4 – 11 x 10^-6
Amorphous	35 – 50 x 10^-6

Conclusion

In this article, we have covered the basics of carbon composite resistors.

While once very popular, CCRs have lost market share to film and other types of resistors. This is mostly due to poor tolerances and long term characteristics. In general, other types of resistors offer much better specifications.

However, carbon composite resistors are still in-demand in power intensive applications and vintage electronic restoration.