What Is a Carbon Composition Resistor?

There was once a time when the carbon composition resistor was as common as the D-cell battery. A high-end example is shown in Figure 1. This picture shows a collection of carbon comp resistors as installed in a Tektronix Type CA plug-in module. The most prominent carbon composition (carbon comp) resistors are in the upper right of the picture with the orange-orange-orange-silver color bands. This module is from a type 547 oscilloscope designed in the late 1960s. We use the term high-end as the oscilloscope was state of the art for its time and sold for the cost of a new car.

Tech Tip: For additional resistor information, please visit this authoritative page. There you will find a complete description of resistor types with clear descriptions to help you categorization and source these essential components.

Figure 1: Close up picture showing a collection of carbon composition resistors installed in a vacuum tube Tektronix oscilloscope module. This is a work of art with silver soldered ceramic standoff terminals.

Tech Tip Nearly all the resistors shown in Figure 1 have 4 color bands. As an example, do you know how to determine the resistance and tolerance of the orange-orange-orange-silver resistor? If not, click here for DigiKey’s online resistor color code calculator.

Figure 2: Picture of DigiKey's resistor calculator.

Figure 2 Picture of DigiKey’s resistor calculator.

As the name implies, the carbon composition resistor is formed by a carbon element. As an analogy, think of resistance as a piece of pencil lead. The pencil lead is the resistive element, and the brown resistor exterior (Figure 1) is like the wood surrounding the resistor. The challenge with carbon composition resistors is to manufacture a core with a specific resistance for a specific size. For example, the previously mentioned orange-orange-orange-silver resistor is designed to dissipate 2 W of heat. To understand the difficulty, imagine baking a resistor with this recipe:

  • start with a fine-grained powdered insulation material
  • add a heat-activated binding agent that can be thermally cycled thousands of times without losing cohesion
  • add carbon powder to taste thereby establishing the desired resistance for the final product
  • mix thoroughly to evenly distribute the materials as any variations in particle distribution will result in inconsistent resistance
  • extrude into a thin rod (like a pencil lead)
  • bake till done
  • cut to length
  • add the connecting wires to the end
  • grind away a small amount of material to trim the resistor
  • encase in an electrically insulating yet thermally conductive material
  • test the final resistance in batches or individual if higher tolerance parts are desired
  • add color bands

There are many steps to this process. Quality control of the materials is essential. The extrusion process and physically connecting the wires is prone to error. The resulting tolerance is not particularly tight as indicated by the previously mentioned silver band. Also, this is a relatively expensive manufacturing process by today’s standards.

On a related note, we should mention that the resulting porous material is susceptible to damage by the elements. Moisture can damage the carbon mandrel-to-wire bond. It’s also been my experience that carbon composition resistors subject to prolonged heat exposure can drift - far out of tolerance - sometime being open with no apparent damage.

Tech Tip: Manufacturing techniques change over time. The carbon composition resistor was once popular because it was inexpensive. Today, manufacturing techniques have changed allowing resistors such as metal film to be manufactured with less expense, improved reliability, and greater quality control as evident by the tighter tolerance.

Carbon composition resistor popularity

The carbon composition resistor remains a popular item. This especially true for folks restoring or even construction older equipment with period-correct components. Vacuum tube audio and guitar amplifier certainly account for the largest consumption of the carbon composition resistors. To be sure, there is something to be said for this nostalgia – as a collector of old equipment such as that Tektronix oscilloscope, I will not judge. Instead, I will encourage you to safely experiment with these vintage components.

Carbon composition resistor availability

DigiKey’s selection of carbon composition resistor is limited. Consider the common 10 kΩ offerings. At the time of this writing, 35 values were listed. Many of these have been designated as obsolete with zero inventory. For those in stock, the price ranges from $0.62 to $5.06 each. For example, here is a NTE Electronics 10 kΩ, 10% 1 W resistor.

Alternatives to carbon composition resistors

For the period-correct restoration, there can be no substitutions. There are only a few viable sources. We search for new resistors such as the previously mentioned NTE component, we can search for New Old Stock (NOS) on online auctions, or we can pull functional resistors from donor equipment.

On the other hand, if we relax our standards and allow new technology to be used, we find a wide variety of excellent options. That 1 kΩ 1 W resistor could be replaced by any one of the 263 DigiKey options.

In fairness, many of these film type resistors will perform better than their antique counterparts. The tolerances are tighter, modern devices are less susceptibility to environmental factors, they have lower noise, and a lower cost.

Yet, I do understand, the new technology doesn’t look right in your period correct guitar amplifier. It doesn’t sound the same, and it doesn’t smell the same. The experience of working with these older components along with the period’s vacuum tubes is certainly rewarding.

Best Wishes,

APDahlen

P.S. To learn more about resistors and their construction, please visit this authoritative page. There you will find a detailed description of all DigiKey resistor types including the carbon composition.

P.P.S. If you are building vintage equipment, please consider incorporating terminal strips or terminal boards such as the one shown in Figure 3. These accessories will improve your circuit layout.

Figure 3: Picture of a terminal board commonly used for clean circuit construction in vintage equipment.

Figure 3: Picture of a terminal board commonly used for clean circuit construction in vintage equipment.

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