Carbide Resistors
Carbide resistors are block types made from carbide-containing materials that are created by mixing powdered carbon with powdered ceramics or other temperature-resistant electrical insulating materials and organic binders. Electric current flows through a web-like network of carbon particles that contact each other throughout the bulk material; by increasing the proportion of insulator used, this conductive web becomes finer and the resistivity of the material increases. Early examples were simply two electrically wound wires on a cylinder, which were then painted to indicate resistance values and provide a degree of protection. More modern examples are typically encapsulated in a phenolic resin housing that provides a mounting feature for the leads and a container for the carbide material with characteristics similar to a pencil lead.
They remained in common use for decades around the 1960s for cost reasons, although the advantage has largely disappeared since then. Carbide resistors are bulky; today they are probably ten times larger than similar grades in other technologies. They are not precise. Current examples have tolerances as low as 5%, but that figure refers only to variations in tolerances in manufacturing. It does not take into account other factors that may affect the measured value of the product:humidity/humidity may cause a tolerance variation of about 10%, and temperature may cause a tolerance variation of about 10%. Because of these sensitivities, there is little point in trying to adjust them to exact values during manufacturing. They are the most common resistive technology and are found almost exclusively in through-hole packages. The carbon that forms their composition is flammable, so they often catch fire during sustained overloads or failures, and they have significantly lower maximum operating temperatures than many other resistive technologies.
Nevertheless, due to their block construction, carbide resistors are electrically very robust and tend to be exceptionally resistant to high-intensity, short-term overload events such as electrostatic discharge. In this respect, carbide resistors are typically one to two orders of magnitude more robust than other technologies. Within the limits of their packages, they also exhibit relatively low parasitic inductance, which is often highly desirable in situations involving rapid transients. For limited applications where these strengths are highly valued and other defects can be tolerated, carbon composition resistors are a reasonable choice. However, in most cases, other types of resistors will better serve the application.
Carbon Film Resistors
Carbon film resistors are a thin film type of resistor produced using carbon as the resistive material. A significant improvement over carbide resistors is that : they give up the measurement of electrical robustness in order to improve accuracy, stability, miniaturization, and to improve current noise characteristics. Other thin film type resistors tend to go further in this direction, or do so while obtaining a better price, which may be the preferred choice for current designs. Part of this preference is also due to the fact that carbon is quite flammable, while the general alternatives are less flammable, and it is quite common to avoid fires when there is a problem with the product.
The popularity of carbon film resistors seems to be decreasing as the popularity of through-hole types increases. While the latter is still in use, and other types of thin film resistors are now available in through-hole packages, surface mount carbon film resistors are rare.
Ceramic Resistors
Ceramic resistors are based on their use of resistors formed from ceramic or ceramic composite materials, which is different from other resistor types that may use ceramic materials in their construction, but are not the primary material through which current flows. They are a block resistor very similar to carbide resistors, with similar pulse-tolerant, non-inductive characteristics. In fact, since powdered ceramics are often used as a component of carbide resistors, there is a degree of overlap between the two in terms of classification. However, distinctions can be made based on the conductive materials used and the way in which the synthetics are mixed together; carbide resistors use only carbon as a conductive medium and are joined together using an organic binder, while ceramic synthetics can contain other conductive materials such as metals or metal oxides and are joined together by a sintering or hot-melt process. "Cermet" (metal ceramic) is the term often used for such materials in potentiometers. Ceramic resistors are capable of operating at significantly higher temperatures than the carbide resistors they replace, although they also tend to exhibit a greater temperature coefficient; a combination that often translates into a resistance variation of about 30% over the product's operating range. As a result, they are less suitable for small signal use and are therefore primarily used in applications with power ratings greater than at least 1 watt.
Metal Element Resistors
Resistors with a "metallic" component use large pieces of metallic material as the resistive material and are typically used for resistances well below 1 ohm. They are primarily used in current measurement applications where a stable, known low value resistor can accurately measure large currents without causing excessive voltage drop and power loss. Often referred to as shunt resistors (shunt), they typically have 4-lead termination for Kelvin-type measurements, thus allowing the voltage present across the resistor to be measured with minimal interference or error with the interface resistor at the product connection point.
Metal Film Resistors and Thin Film Resistors
Metal film resistors and thin film resistors are based on similar production techniques in which the resistive component is formed by a thin (typically micron-sized) film layer of metal applied to a ceramic substrate by a vapor deposition process and then trimmed to the desired resistance value. "The distinction between metal film resistors and thin film resistors seems to be a contextual one; in through-hole resistors, "metal film" resistors seem to be more common, where carbon film resistors are alternatives, while in chip format resistors, "thick film" may be a substitute, while "thin film" seems to be the preferred choice. "Thin film" seems to be the term preferred for precisely focused products, while "metal film" seems to be more commonly used for general purpose applications.
Compared to carbon film products, metal film resistors/film resistors have made further advances in accuracy, stability and noise performance, which usually also means further costs mainly for surge events. The total mass of the resistive components is small enough to make such products susceptible to damage from electrostatic discharge, and smaller package sizes and higher resistance values increase the likelihood of electrostatically induced damage. This small component mass also makes thin film resistors particularly susceptible to chemical attack, due to the loss of small amounts of sub-materials or through chemical transformation leading to failure. Overall, thin film/metal film resistors are the technology of choice for applications requiring accuracy and parameter stability at moderate cost. Especially in surface mount package form, they are also preferred for their low inductance characteristics and are often used in high speed circuits.
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