A wire wound resistor consists of a winding of resistive wire wrapped around a non-conductive cylinder. This is the oldest type of resistor. Although we use wire wound resistors these days. Because these resistors provide high accuracy, better stability. Also, they can handle much more power than other types of resistors available for fixed resistor purposes. These resistors play a crucial role in applications ranging from precision measurement to high-power industrial systems.
Construction and Design of Wire Wound Resistor
Constructionally, a wire wound resistor includes several key components. Let us discuss one by one:
Core Material
It is typically a ceramic, fiberglass, or plastic-made tube. Obviously, it provides the basic structure of the resistor. The use of ceramic core dominates due to its superior heat dissipation property.
Resistive Wire
We wrap carefully a resistive wire of metal alloy. The metal alloys we use for the purpose are generally nichrome (the alloy of nickel and chromium), manganin (the alloy of copper, manganese, and nickel), or constantan (the alloy of copper and nickel). The resistivity of the alloy used for making the coil, the length of the wire of the coil, and also its diameter altogether determine the resistance value of the wire wound resistor.
Winding Pattern
We wind the wire in a helical pattern. With the spacing and number of turns, we achieve the desired resistance of the resistor. Some designs use a bifilar winding technique, where the wire is folded back on itself to reduce the inductance. In other words there is a layer of turns, and these turns will return with opposite polarity to neutralize the inductive effect of the coil.
Protective Coating
After we complete the winding, we use varnish, enamel, silicone, or epoxy resin to coat and cover the winding, protecting it from moisture. This cover also gives protection from mechanical damage and environmental factors.
Termination
Then we attach metal caps or leads to the ends of the wire for external electrical connections.
Power Resistors
A wire wound resistor is specially designed to dissipate large amounts of heat. In other words, a wire wound resistor can handle power from one watt to even up to several kilowatts sometimes. Some designs use aluminum heat sinks to accelerate the heat dissipation.
Precision Wire Wound Resistors
These resistors offer exceptional accuracy with a tolerance as small as ±0.005%. Additionally, these resistors have excellent temperature stability. In other words, they have a very small temperature coefficient of resistance. That is why we always use this type of resistor in calibration equipment and precision measurement devices.
Adjustable Wire Wound Resistors
In addition to fixed resistance, variable-type wire-wound resistors are also available in the market. We normally call them rheostats or potentiometers. Essentially, they have a sliding contact. By moving or sliding the sliding contact on the wound wire, we can adjust the resistance of the resistor as per the requirement.
Chassis Mount Resistors
Heavy-duty units are designed for mounting directly to the metal chassis with large heat sinks for maximum heat dissipation rate.
Characteristics of Wire Wound Resistors
Resistance Range
The range of wire-wound resistors varies from a fraction of an ohm to several megaohms. Although we most commonly use the wire wound resistors for low resistance applications, like 0.1 ohm to 200 kilohms.
Power Rating
Power rating is one of the main attributes of a wire-wound resistor. The power rating ranges from a fraction of a watt to several kilowatts. We use fractional-watt wire-wound resistors for precision measurements, whereas we use wire-wound resistors of several kilowatts for industrial power applications.
Tolerance
The tolerance typically ranges from ±0.005% to ±5%. Obviously, it is superior to most other resistors.
Inductance
The winding of wire creates inductance, which is why it is not suitable for high-frequency applications. Often, we use bifilar winding to neutralize its inductive effect.
Disadvantages
Despite many advantages, this type of resistor has some drawbacks.
The main drawback is its size and weight. Because of their large size and heavy weight compared to carbon composition resistors, carbon film resistors, or metal film resistors, they are not suitable for use in small electronic devices.
The coiled wire or winding produces unwanted inductance. This problem limits the high-frequency applications for this type of resistor.
This is the most expensive type of resistor among all other types of fixed resistors.
It takes longer to reach thermal equilibrium compared to film resistors.