Wirewound Resistor
These types are made by winding resistance wire in a spiral around a non-conductive core. The resistance wire is usually a nickel-chromium alloy and the core is often ceramic or fiberglass. A coating such as vitreous enamel is used for protection. The spiral winding has capacitive and inductive effects that makes it not suitable for applications higher than 50 kHz. Often other winding techniques are used to reduce the undesired high frequency effects. Wirewound resistors are essentially produced for high precision or for high power applications. They have low noise, are robust, and are temperature stable. Resistance values are available from 0.1 up to 100 kW, with accuracies between 0.001 and 20%. Microhm Electronics' wirewound resistors, especillay metal clad NUAL series are popular wire wound resistors for high power application.
Foil Resistor
The foil resistor was invented in the 1960’s and is still the most accurate and stable type today. The resistive element is a thin bulk metal foil that is cemented on a ceramic substrate. The foil thickness is several micrometers. Inherent to the mechanical design, they feature a very low temperature coefficient of resistance. They are used for applications with high precision requirements. MVR2321-4 is a typical metal foil resistor especially suitable for precision measurement.
Carbon Film
A thin, pure carbon film is deposited on an insulating cylindrical core. A spiral cut is made in the film to increase the resistive path. This increases the resistance value and is also a way to control the resistance value more precise. Values from 1Ω up to 10 M Ω are available. The accuracy is a significant improvement compared to carbon composition, but metal and metal oxide film have overall better properties and gained therefore more popularity. In applications that require high pulse stability, special carbon film resistors are used.
Metal Film
Metal film resistors are usually made of Nichrome, but also other materials such as tantalum nitride is used. The resistive film is printed on a cylindrical or flat insulating substrate. The resistive material is a combination of a Ceramic material and a Metal, and therefore these resistors are also referred to as Cermet. Just as with carbon film, the resistance value is adjusted by cutting a spiral pattern in the film. This can be done with an abrasive or a laser. The stability, temperature coefficient and tolerance are better than for carbon film. Typical tolerances are between 0.5% and 2% with a temperature coefficient between 50 and 100 ppm/K. Stability is lower than for wirewound, but the high frequency properties are better. HPMRY series and PMRY series are welllkown for their low TCR, high precision and competitive price.
Metal Oxide Film
They are similar to metal film with the difference that the resistive material is a metal oxide, such as tin oxide. These durable resistors feature a better reliability and stability than metal film resistors. Furthermore the operating temperature is higher. Therefore they are more used in applications requiring a high endurance.
