The wirewound resistor manufacturing process varies widely. The manufacturing process and choice of materials to be used depend on the way the resistor will be used in a circuit. However, all wirewound resistors are made by winding the resistance wire around a non-conductive core.
The resistance wire is usually a nickel-chromium alloy, and the core is often ceramic or fibre-glass. A coating such as vitreous enamel is used for protection. Spiral winding has capacitive and inductive effects that make it unsuitable for applications higher than 50kHz. Often, other winding techniques are used to reduce the undesired high frequency effects.
For high tolerance requirements, resistance value is measured to determine the exact cut-to-length of the wire. To create a high resistance, wire diameter needs to be small and the length long. Therefore wirewound resistors are mainly produced for lower resistance values.
For low power ratings, a very thin wire is used. Handling of the wire is, for this matter, critical. Any damage may sever contact. After winding, the wire is well protected from access to moisture to prevent electrolytic corrosion.
Next to precision, there are also wirewound resistors with high power rating for 50W or more. These have a different construction. Compared to other resistor types such as metal film, wire diameter is relatively big and therefore more robust.
The resistive element exists out of an insulated metallic wire, which is wound around a core of non-conductive material. The wire material has high resistivity, and is usually made of an alloy such as nickel-chromium (nichrome) or a copper-nickel-manganese alloy called manganin.
Common core materials include ceramic, plastic and glass.
Wirewound resistors are the oldest type of resistors that are still manufactured today. These can be produced very accurately, and have excellent properties for low resistance values and high power ratings.
These are mainly produced with alloys, since pure metals have a high temperature coefficient of resistance. However, for high temperatures, pure metals such as tungsten are used.
Properties of the most common alloys are given in Table I.
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