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Wirewound Resistors

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Wirewound resistors

A 20W ceramic wirewound resistor of 5 ohmA wire wound resistor is an electrical passive component that limits current. The resistive element exists out of an insulated metallic wire that is winded around a core of non-conductive material. The wire material has a 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. Wire wound resistors are the oldest type of resistors that are still manufactured today. They can be produced very accurate, and have excellent properties for low resistance values and high power ratings.

 

The construction of this type of resistor is also very simple. In wire wound resistor a wire of manganin or constantan is wound around a cylinder of insulated material. The temperature coefficient of resistance of these two materials is almost zero. So there would no resistance variation with temperature. The wounded wire is covered with an insulating material such as baked enamel. This cover of insulating heat resistible material is provided to resist the effect of ambient temperature variation. Different sizes and ratings of wire wound resistor can easily be achieved by using different lengths and diameters of the wire. These resistors are easily available for wide range of ratings. The range of resistance values varies from 1 O to 1 MO. Typical tolerance limit of these resistors varies from 0.01 % to 1 %. They can be used for high power applications of 5 to 200 W dissipation ratings. The cost of these resistors is much higher than carbon resistor. Normally wire wound resistor is used where carbon composition resistor cannot meet the purpose because of its limitations.

Characteristics and Materials
While wirewound resistor materials and designs vary, all involve a length of resistive wire wrapped around a ceramic, fiberglass, or plastic core. The wire typically consists of copper or silver alloy, but pure metals such as tungsten are used in high-temperature (> ~1300° C) applications.

 


The main disadvantage of this resistor is the inductor that arises because of its coil like structure. At high frequency the behavior of the circuit may be changed due to its reaction. This problem can be solved if one half of the wire is wound in one direction and other half in opposite direction so that the inductor due to these two halves cancel each other hence net inductive effect of the resistor becomes nil. The non - inductive wire wound resistor is ideal for high frequency circuit but it is costlier than an ordinary one.

Types

Wirewound resistors may be broadly classified into one of the three categories described in the table below.

 

 

Type

Description

Applications


Precision wirewound

Manufactured to very tight tolerances (<0.1% resistance value); designed to hold values over years of use; relatively low temperature threshold.

Attenuators and calibration equipment.

  

Power wirewound

Uses specialty coatings or aluminum armoring for insulation; power range is typically between 4 and 17 W, but may be able to withstand power levels of 1000 W or more; requires low TCR.

High-power applications.

  

Potentiometer

Specialty application for variable resistors; wirewound technology typically employed in potentiometer design.

Low-voltage controls, transducers, analog computing.

 

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