Kory Schroeder, Director of Marketing & Product Engineering, Stackpole Electronics
Carbon comps, as they are called, are now becoming scarcer. It is important to understand the technology behind the carbon comp as well as its strengths and weaknesses so that potential alternatives can be assessed. This document will explore the construction of carbon composition resistors and the performance benefits and drawbacks to this technology along with potential alternatives.
Carbon Composition Technology
Carbon composition resistors are formed from conductive carbon particles, mixed with a ceramic binder. Different resistances can be achieved by adjusting:
· the amount of carbon in the element mixture
· the diameter of the element
· the length of the element
Increasing the ratio of carbon powder to ceramic decreases the resistance value because more conductive particles makes it easier for current to flow thereby lowering the measure of resistance. Increasing the diameter of the resistive element also decreases the resistance value because larger diameter allows for more current paths which act like parallel resistances thereby lowering the effective resistance. Increasing the length of the resistor increases the resistance value since a given length of a carbon and ceramic mixture will have a relatively uniform resistance per unit length, therefore the longer the element, the higher the resistance. Because of physical size limitations for each power rating, changes in element length and diameter are limited as well. The carbon powder to ceramic ratio adjustment is the main factor in carbon composition resistor value design.
The carbon comp resistive element is formed into long rods of the carbon and ceramic binder material. The elements are then cut from the rods at the proper length to achieve the required resistance value. At this point, leads are embedded into the ends of the material and the units are fired to cure them. The element and lead assembly is encapsulated in plastic molding which is normally an epoxy and polyester based resin. The resistors are then tested twice on two separate test stations to ensure value integrity before they are taped and made ready for shipping.
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Figures 2a & 2b: Carbon Composition Pulse Power Capability
Carbon Composition Resistor Benefits and Drawbacks
Carbon comps are well known for their two main strengths: high pulse energy handling and low inductance. The solid carbon / ceramic element has significant element mass which allows it to dissipate large amounts of energy for a given footprint. The graphs shown in Figure 2 show the pulse power handling capability for a 100 ohm 1/4 W and a 10 K ohm 1/2 W carbon comp. The improvement in energy handling would be more dramatic if the 1/2 W data were in a lower resistance value like the 1/4 W part. Still, 80 KW pulse handling for short pulse durations in a resistor that measures only roughly 10mm by 4mm is remarkable. The carbon / ceramic slug has no additional trim, meaning the only inductance this configuration has is in the lead. Figure 3 shows lower resistance values maintain impedance beyond frequencies of 100 MHz.
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Figure 3: Impedance vs. Frequency for Carbon Comps
Carbon Comp Drawbacks
Unfortunately, the list of drawbacks for carbon comps is long and significant. A wide range of factors can affect the resistance value of a carbon comp. Some are avoidable, but some are inherent to the design and materials for carbon comps. For example, carbon comps may shift up to 10% during endurance testing. Most general purpose chip resistors have a maximum shift under the same conditions of less than 3%, for comparison. Carbon comps are also highly susceptible to moisture penetration. Damp heat testing may cause carbon comps to shift up to 10%. For that reason, carbon comps are shipped in a poly-bag with desiccant and are recommended to be used soon after purchase, especially once the bag is opened. However, even in a sealed poly-bag, carbon comps may shift up to 5% in a year. Carbon comps are also very temperature sensitive with a strong negative temperature coefficient. Carbon comps in high resistance values (1 Megohm and higher) may shift up to +20% at -55 °C and up to -14% at +105 °C. Resistance temperature characteristics for carbon comps for various resistance values are shown below in Figure 4. This type of instability makes carbon comps unsuitable for many of today’s precision electronics.
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Figure 4: Resistance Temperature Characteristics for Carbon Comps
Carbon Comp Alternatives
Currently there are some alternatives to carbon comps, but all have their own advantages and disadvantages. Wirewounds may be used since in low resistance values, wirewounds will typically have very robust wire elements with energy handling comparable to carbon comps. Wirewounds have no wear out mechanism so if the surge does not fuse the element wire and the element can cool down, it returns to essentially its starting resistance value. Unfortunately, wirewound resistors are inherently quite inductive. Even non- inductively wound wirewounds will have more inductance than carbon comps. This makes wirewounds unsuitable as replacements if high speed and low inductance are important. Power film resistors, with combinations of metal oxide and carbon film may be viable as carbon comp replacements especially for high resistance, high voltage applications where carbon comps may be unstable. Overall stability for film resistors is many times better than carbon comps as well. But the spiral groove used to trim film resistors means inductance is going to be marginally higher. Also, film resistor elements have far less mass, even those designed for pulse power handling, therefore power film resistors are not viable replacement for carbon comps when high energy handling is critical. Ceramic composition resistors may be a non-inductive alternative for carbon comps as they also provide high energy handling and may operate at high temperatures safely. But ceramic composition resistors are large for their power rating, expensive, and may shift up to 5% under load life testing, short time overload testing, or moisture testing and have limited precision capabilities.
Carbon composition resistors have been around for many years and continue to be used for certain applications where high energy handling and low inductance are more important than precision or stability. Carbon comps have become more difficult to find in the market and design engineers are encouraged to explore the alternatives to carbon comps. Careful review of the voltage, energy, inductance, and stability requirements will allow for the proper replacement part to be chosen for long term reliability.