What is a Foil Resistor? The metal foil resistor has the best precision and stability properties of all resistor types. The foil is made of an alloy of usually Nichrome with additives. It is mounted on a ceramic carrier with high heat conductivity. The foil has a thickness of only several micrometers. The desired resistance value is achieved by a photoetched resistive pattern in the foil. The metal foil resistor has a low Temperature Coefficient of Resistance (TCR), good long term stability, low noise, low capacitance, fast thermal stabilization and no inductance. The low TCR is one of the most important parameters that influence the stability. This means that the resistance value will vary only a small amount as the ambient temperature and the resistor’s internal temperature changes. Over a range from 0 till 60 deg C, a typical value for TCR is around 1 ppm per deg C. This depends on the construction (thermo-mechanical effects) and the properties of the foil material. In the planar foil the pattern is made parallel to reduce inductance (max 0.08 microH). Foil Resistor Definition A foil resistor is a high precision component to limit electric current. The opposition to current flow is provided by a very thin piece of metal. Metal Foil Resistor Characteristics The excellent properties for precision and stability are due to a combination of several characteristics, which are following from the design principles of the metal foil resistor. Temperature Coefficient, Resistance (TCR) – Foil resistors achieve a low TCR by taking advantage of two characteristics of the foil. The resistance of the foil naturally increases as temperature increases. The resistor is manufactured so that rising temperature causes compression of the foil. This makes the resistance drop as temperature rises. The total effect is one of very little resistance change as...
Resistors are produced with a wide variety of materials and manufacturing processes. Each resistor material has its typical properties and specific areas of use. The main types that are used in electrical engineering are summed below. Wirewound (WW) 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%. Carbon Composition (CCR) The resistive element is made from a mixture of fine carbon particles and a non-conductive ceramic material. The substance is pressed in a cylindrical shape and baked. The resistance value depends on the dimensions of the body and the ratio between carbon and ceramic material. More carbon means a lower resistance. Carbon composition resistors are remarkably reliable, but have a poor accuracy with a maximum tolerance around 5%. Until the 1960s they were the standard for general applications. They quickly lost market share as other resistor types came on the market with better properties for tolerance, voltage coefficient, temperature coefficient, stability and finally cost. However, their ability to withstand high energy pulses and their high reliability makes them still useful for certain applications. Examples are power supplies and welding controls. Carbon Film A thin, pure carbon film is deposited on an insulating cylindrical...