Wirewound resistors are commonly made by winding a metal wire around a cylindrical core of ceramic, plastic, or fiberglass core. The ends of the wire are soldered or welded to two caps, attached to the ends of the core. The assembly is protected with a layer of paint, molded plastic, or an enamel coating baked at high temperature. The wire leads are usually between 0.6 and 0.8 mm in diameter and tinned for ease of soldering. For higher power wirewound resistors, either a ceramic outer case or an aluminum outer case on top of an insulating layer is used. The aluminum-cased types are designed to be attached to a heat sink to dissipate the heat; the rated power is dependent on being used with a suitable heat sink, e.g., a 50 W power rated resistor will overheat at around one fifth of the power dissipation if not used with a heat sink.

The interior of a wirewound resistor consists of a ceramic or fiberglass base wound with resistor wire to the desired resistance value. The wire ends are pressed or brazed to the caps. The outstanding characteristic of this type of resistor is the very high surface temperature it can take, up to + 450 °C, which makes it very tough indeed. Their areas of application are comparable to those of the composition resistor, with the reservation that the high frequency characteristics of the wirewound resistor are substantially worse.

Because wirewound resistors are coils they have more undesirable inductance than other types of resistor, although winding the wire in sections with alternately reversed direction can minimize inductance.

Wire wound resistors are a type of power resistor and are very accurate. Wirewound resistors are available as fixed, or adjustable to be used as a rheostat or potentiometer.

Typical applications for wirewound resistors include device requiring high current handling capability, heat dissipation and resistance stability and accuracy.

A wirewound resistor is made of metal resistance wire, and because of this, they can be manufactured to precise values. Also, high-wattage resistors can be made by using a thick wire material. Wirewound resistors cannot be used for high-frequency circuits. Coils are used in high frequency circuits. Since a wirewound resistor is a wire wrapped around an insulator, it is also a coil, in a manner of speaking.

In Wire-wound Resistors, there are two broad categories here: power resistors and precision resistors.

Precision Wire-wound Resistors

The Precision Wirewound is a highly accurate resistor with a very low TCR and can be accurate within .005% tolerance. A temperature coefficient of resistance (TCR) of as little a 3 part per million per degree Celsius (3ppm/°C) can be achieved. Low tolerances, high stability, and low TCRs are the fortes of precision wire-wound resistors. Tolerances as low as ±0.005% are available. Common resistance stability is ±100 ppm/year, and some companies manufacture resistors with stabilities of ±20 ppm/year. The TCRs of precision wire-wound resistors are the best available and as low as as ±1 ppm/°C.

However Precision Wirewound components are too expensive for general use and are normally used in highly accurate DC applications. The frequency response of this type is not good. When used in an rf application all Precision Wirewound Resistors will have a low Q resonant frequency. The power handling capability is very small. These are generally used in highly accurate DC measuring equipment, and reference resistors for voltage regulators and decoding networks.

The accuracy is maintained at 25°C (degrees Celsius) and will change with temperature. The maximum value available is dependent upon physical size and is much lower than most other types of resistor. Their power rating is approximately 1/10 of a similar physical size in a carbon composition. They are rated for operation at +85°C or +125°C with maximum operating temperature not to exceed +145°C. This means that full rated power can be applied at +85 (125) °C with no degradation in performance. It may be operated above +125 (85) °C if the load is reduced. The derating is linear, rated load at +125(85) °C and no load at +145°C.

Life is generally rated for 10,000 hours at rated temperature and rated load. The allowable change in resistance under these conditions is 0.10%. Extended life can be achieved if operated at lower temperatures and reduced power levels. End of life requirements are generally defined by the manufacturer or in some case by user specification. Some degradation in performance can be expected.

In some cases, particularly if the tolerance is very low and the TC is low, the rated power is reduced to improve resistor stability through life. Precision Resistors regardless of type, are designed for maximum accuracy and not to carry power. The materials used in these resistors are highly stable heat treated materials that do change under extended heat and mechanical stress. The manufacturing processes are designed to remove any stresses induced during manufacture. There is little detectable noise in this type of resistor. The stability and reliability of these resistors is very good and their accuracy can be enhanced by matching the absolute value and the temperature coefficient over their operating range to achieve very accurate voltage division.

Power Wire-wound Resistors

“Power” generally means 1 Watt and higher, and since P = V2/R, this most often implies low resistance. Their physical construction is designed to dissipate the heat. They have excellent high-energy pulse handling capabilities. Wire-wound resistors are available at values below 1 Ω, where other technologies are often not available. These resistors are bulky when compared to other resistors, and are tend to be expensive. Power wire-wound resistors typically have 1% tolerance, and have good long-term stability. Even power precision wire-wound resistors often have low TCRs.

Power Wirewound Resistors are used when it is necessary to handle a lot of power. They will handle more power per unit volume than any other resistor. Some of these resistors are free wound similar to heater elements. These require some form of cooling in order to handle any appreciable amount of power. Some are cooled by fans and others are immersed in various types of liquid ranging from mineral oil to high density silicone liquids. Most are wound on some type of winding form. These winding forms vary. Some examples are ceramic tubes, ceramic rods, heavily anodized aluminium, fibreglass mandrels, etc.

To achieve the maximum power rating in the smallest package size, the core on which the windings are made must have a material with high heat conductivity. It may be Steatite, Alumina, Beryllium Oxide, or in some cases hard anodized Aluminium. Theoretically, the anodized Aluminium core has a better heat conductivity than any other insulated material, with Beryllium Oxide being very close. There are specific problems with the anodized aluminium cores such as nicks in the coating, abrasion during capping and controlling the anodized thickness. There are various shapes, oval, flat, cylindrical, and most shapes are designed to optimize heat dissipation. The more heat that can be radiated from the resistor, the more power that can be safely applied.