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The wire wound resistor or wirewound resistor is used in a variety of applications and in particular it is used as a power resistor where larger amounts of power need to be dissipated.
As the name indicates, the wire wound resistor consists of a resistive wire that is wound around a former of nonconductive material. Normally the resistive wire is insulated, so that adjacent wires do not short together.
In view of their construction and properties, wire wound resistors are often used as power resistors, dissipating significant amounts of heat, although their properties mean that they can be used in other applications as well.
What is a wire wound resistor?
The wire wound resistor was one of the first forms of resistor to be used. The basic structure of the wirewound resistor has changed little over the years.
The resistor is constructed from resistance wire that is wound around a central core or former, that is usually made of ceramic.
Once wound, end caps are pressed onto the core and the resistance wire is welded to them to make proper contact. Finally the assembly is encapsulated to protect it from moisture and physical damage. The construction means that these components can tolerate high temperatures and as a result they are used as high power resistors in many circmumstances - but check the ratings to ensure they are sufficiently high power for the application.
The resistance of wire wound resistors is determined by a number of factors:
- Length of resistance wire
- Diameter of the resistance wire
- Resisitivity of the resistance wire
To give a perspective of the figures involved, a 30 metre length of small diameter copper wire may have a resistance of only a few ohms. By contrast, using some resistance wire - a popular type is nickel chromium - the wire can be made only 30 cm long. This makes it viable to wind on a typical former that may be used in an electronic circuit.
If tight tolerance wire wound resistors are required, then the resistance wire chosen can have a lower resistance making the wire longer, and enabling its length to be cut more accurately as a proportion of the total length. If necessary the resistance can be trimmed on an individual basis.
Wirewound resistors come in a variety of packages and many are particularly suited for power resistor applications - some are contained within ceramic packages, while others are available within metallic packages that can be bolted to a metallic chassis or other forms of heat sink.
Wirewound resistors are still used quite extensively. They are particularly used as high power resistors where larger levels of power need to be dissipated. They are widely used in many power applications; not only are they physically larger than many other types of resistor, but having a wire conductor and a ceramic former, they are for more suited to being a high power resistor than other types more widely found.
Unfortunately their construction means that they are much more expensive than the much smaller resistors normally used.
Wirewound resistors are also used where very close tolerance and high temperature stability is needed. The wire typically has a better temperature coefficient of resistance than many other types of resistor, although many these days are very good.
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