What Are Carbon-Film Resistors?
Resistors are used extensively in electronic circuits to help modulate electrical activity for various reasons. Depending on their design and material composition, resistors may be used to selectively reduce current flow, divide voltages, terminate transmission lines, and more. One of the most common types of resistors found in applications demanding a high amount of resistance is the carbon-film resistor. In this blog, we will discuss everything you need to know about this important component, including its function and varying applications.
Carbon-film resistors are made using a specialized decomposition process that subjects hydrocarbons to immense temperature and pressure to make them crack, thereby distributing the carbon onto a ceramic substrate. The amount of distributed carbon can be modulated by altering the number of hydrocarbons used, which determines the component's capacity for resistance. After the carbon settles, the resistors typically receive a protective layer of paint, which protects it from temperature fluctuations as well as corrosion.
Since the materials needed to produce carbon-film resistors are generally easy to procure, the overall cost of the component is less compared to other similar products. Their resistance range can vary greatly, with the low end being just above 2 Ohms and the upper limit being 10 Ohms. Carbon-film resistors can also tolerate high voltages and maintain their resistance levels over a long operational lifespan. In addition, these resistors feature a negative temperature coefficient, meaning the resistance value decreases as temperature increases. They are also rated to function over a wide range of powers, including 1/8W, 1/4W, 1/2W, 1W, 2W, and other standard values.
Carbon-film resistors are generally labeled on the outside of their shell with important information. The most common method is direct marking, which includes printing the resistance value of the specific resistor as well as an allowable error, expressed as a percentage. Additionally, some resistors are labeled with a three-digit method, in which the first two numbers represent the deviation value, while the last can be counted as an exponent. Finally, resistors may be labeled with a color-code method, which involves multicolored rings. Each color has a different meaning, allowing a great deal of information about the resistor to be displayed at once. If the configuration includes four colored rings, the first two represent significant digits, the third is an exponent, and the fourth is used to describe the deviation. A similar pattern exists for 5-ringed labels, with the difference being that the first three digits are significant.
Over time, the resistance value for carbon-film resistors may increase, which may be good or detrimental depending on the application. The most common cause of this occurrence is oxidation, which happens when the carbon interacts with the air or moisture in the environment surrounding the component. Another significant cause of gained resistance over time is gas adsorption, which is caused by carbon adsorbing ambient gas due to pressure changes. There may also be direct damage to the film by a degraded protective coating or if the component comes into contact with ions like Na+, K+, and Cl-. If an application requires a higher resistance than what can be provided with the native component, it is possible to scrape the protective paint and the carbon film while continuously testing the resistance until the optimal level is reached. While this method may allow the resistor to be used in a broader range of applications, it also compromises the stability of the resistor and likely leads to a shorter lifespan.
Owing to their affordability, superior longevity, and high-resistance capacity, carbon-film resistors may be found in a number of applications, ranging from handheld devices to X-ray and radar systems. Despite their monetary benefits and reliability, carbon-film resistors produce more noise when compared to metal-oxide resistors and should thus not be used in radio-frequency applications. Finally, it is important to consider the relative size of these two options, with the carbon-film resistor being much larger than its metal-oxide counterpart.
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