What is a wirewound resistor?
A Wirewound resistor is a resistor made with a conductive wire of a high resistivity. This wire is made of a special alloy, and it is wound on a support tube of refractory material such as ceramics, porcelain, etc.
Note: A refractory material doesn’t allow the conduction of heat, on the contrary it reflects it. The value of a wirewound resistor is determined by the cross-section of the wire, its length and its alloy resistivity.
Why to use wire wound resistors?
Wire wound resistors are used, in most cases, when the power they must dissipate is very high. Once the resistor has been made, it is usually coated with a layer of vitrified enamel.
The longer the wire is and the larger the section of the wire, the greater the heat radiation surface will be and the greater the power dissipation capacity it will be able to withstand.
This type of resistor can be compared to the filament of an incandescent lamp, where the power is transformed into heat (In an incandescent lamp, this power is transformed into light and heat).
Wirewound Resistor values and power
Wirewound resistors are manufactured with values up to 100 Kilohms approximately, because of the physical dimensions problems. The goal is to achieve the biggest dissipation of heat in the smallest possible space. Wirewound resistors can generally dissipate powers from 5 watts up to 100 watts or more.
The last diagram shows a blue refractory tube, and 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.
Wirewound resistor are available as fixed and adjustable to be used as a rheostat or potentiometer.
Olli Niemitalo, CC0, via Wikimedia Commons
Types of wire wound resistors
Precision wirewound resistors
These wirewound resistors are precision resistors, because they are made with a wire whose characteristics are known and a very exact resistance value can be obtained.
This type of wirewound resistor has applications in the field of instrumentation. They are frequently found in multimeters and calibration equipment. Some wirewound resistors have tolerances of up to 0.005%. Power dissipated by these type of resistors is very small.
Power wirewound resistors
This type of resistor can dissipate a great amount of power. Some of them have a heat sink, this allows even more heat to dissipate.
Additional features of wire wound resistor
- Its construction method allows them to be designed to have any value.
- It is very stable over time. Common resistances change their value over time. This type of resistor are very stable over time due to the material with which they are constructed.
- It has great stability against temperature changes. This is achieved using a wire with a low temperature coefficient resistance, in its manufacture.
- It has the ability to withstand large voltage peaks for short times without damage and without changing its resistive value.