Wirewound resistor
A wire wound resistor is manufactured with a conductive wire with a high resistivity (specific resistance).
The wire has a special alloy and it is wrapped in a support tube made of a refractory material such as ceramic, porcelain, etc.
Note: A refractory material doesn't allow the conduction of heat, on the contrary it reflects it.
The value of a wire wound resistor is determined by the transversal section of the wire, length and the specific resistance of this alloy.
We need to use a wire wound resistor when the power that should be disipated is very high. Once the wire wound resistor has been built it is generally covered with a layer of vitrified enamel.
This kind of resistor is can be compared to a filament of an incandescent lamp. Here the power is transformed into heat. (In an incandescent lamp this power becomes part light and part heat)
The longer the wire is and the larger its section is the greater will be the ability to endure the power of dissipation, since the surface radiation of heat is greater.
Whire wound resistors are manufactered with values up to 100 Kilohms approximately, because of the problems with the physical dimensions. The idea is to achieve the biggest dissipation of heat in the smallest possible space.
Wire wound resistors can generally dissipate powers from 5 watts up to 100 watts or more.
 The diagram shows a blue refractory tube, and the threads of wires surrounding it.
The black spots represent the wires that are coming in and out from the screen forming a reel of a very tight spring around the tube.
Wire wound resistor are available as fixed and adjustable to be used as a rheostat or potentiometer.
Related Links
- Photoresistor - LDR
- Resistors in series and parallel
- Energy dissipation in resistors (Joule's Law)
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