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Understanding wire wound resistor

Wire wound is a type of resistor designed to be used in high power resistor applications or some other use where low noise and power dissipation are required. It is one of the oldest forms of resistor that appear as a wire that is wound around a former of nonconductive material. Normally, the resistive wire is insulated so that the adjacent wires do not short together.

Although, the wire wound resistors have been superseded by newer resistors such as the metal oxide and metal film resistors. However, they are still in used as the resistor of choice in many applications, they are mostly used as a power resistor where a larger amount of power must be dissipated.

Today you’ll get to know the definition, applications, properties, diagram, types, construction, materials, working, inductance, and capacitance of wire wound resistors. You’ll also understand the advantages and disadvantages of wire wound resistors in their various applications.

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What is a wire-wound resistor?

A wire-wound resistor is an electric device that limits or restricts current flow in a circuit. These types of resistors use a wire with high resistivity, wrapped around an insulating core to provide the resistance. This resistive element is the insulated metallic wire that is wound around a core of non-conductive material. The wire material has high resistivity and is usually made of an alloy such as nickel-chromium (Nichrome) or a copper-nickel-manganese alloy called Manganin.

However, the most common material used is ceramic, plastic, and glass. Despite being one of the oldest, wire wound resistors are still manufactured today. This is because they can be produced very accurately and have excellent properties for low resistance values and high power ratings.

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Applications

Wire wound resistors are used in high-power and low-power resistance applications. They are often used in circuit breakers or as fuses. Most manufacturers attach a small spring to one end of the resistor to make it a fusible resistor. However, wire wound resistors are common in circuit breaker applications. They may serve as part of a large circuit breaker device or may act as circuit breakers themselves.

These types of resistors can also be used as current sensors when the inductive effect that is natural is natural to the resistor is enhanced. This inductive reactance can be determined by the inductance of the device and the current flowing through it. The current sensing devices measure the reactance and convert it to the current reading. These can be used in situations where a high current condition may occur, and it is desirable to correct it before tripping a breaker. Some good examples of the applications are large cooling water pumps and freezer units.

Some other applications of wire wound resistors include:

  • Transducer devices
  • Space and defense
  • Audio and video devices
  • Space and defense
  • Current and voltage balancing
  • Current sensing
  • Telecommunication
  • Computers
  • Medical electronic equipment
  • Telephone switching systems

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Properties

The use of wire wounds is so broad to the extent that they are suitable for some particular areas where other resistors will not perform well. Below are the properties that can be seen in wire wound resistors:

Wire wound resistors are capable of dissipating a significant amount of power. The resistors can be designed to bolt to a heatsink in order to dissipate even higher power levels safely – some are rated up to 2.5kW. The use of resistance wire in these types of resistors made them be of great precision, in fact, some have initial tolerances down to 0.005%. some applications such as measuring instruments find this useful. Also, wire wound resistors have very high-temperature stability, especially when compared to other types of resistors. This can be achieved if the resistance wire is used to have a very low-temperature coefficient of resistance, and the final resistor will have a low TCR.

Another key property of a wire wound resistor is long-term stability. With time, all resistors changes in value, but the wire wound resistors have very little chance to change. This is because they are made with stable materials, figures of between 15 to 50ppm/year can occur which is good compared to others. Another key feature is the ability to absorb pulses. The wire-wound resistors are able to withstand high voltage pulses. This is achieved with the help of their high thermal mass and the resilience of the wire in the resistors, giving them the capability of absorbing levels well above their average ratings for short periods. They perform this without sustaining damage or a change in their resistance.

Furthermore, wire wound resistors can be easily customized to have a specific resistance to which a customer required. Because they produce lower noise, they are specifically use in applications that require no noise. The properties of a wire wound resistor can be summarized into the following:

  • High power applications
  • Very high tolerance applications
  • High temperature stability
  • Long term stability
  • Ability to absorb pulses
  • Customizable to the exact resistance
  • Lower noise applications

Diagram of wire wound resistors:

diagram of wire wound resistors

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Types of wire wound resistors

Below are the various types of wire wound resistors:

Precision resistors:

The precision wire wound resistors are used in a number of different applications such as test instrumentation, multimeters, and measuring bridges, calibration equipment, and AF attenuators, etc. power dissipation is not an issue in these applications because their currents and voltage levels are minimum. The resistors also need a low temperature coefficient of resistance, possibly 5ppm/ oC. Also, low term stability needs to be low, possibly less than 40 ppm per year. With these figures, basic resistance tolerance levels of ±0.01% can be chosen. These figures maintain the required resistance for a long period of time.

Power resistors:

These types of resistors are one of the major uses of wire wound resistors used in power applications. their capabilities range up to 1 kW or 2.5 kW. Several types of power wire wound resistors are available, they are categorized by their package and construction.

  • Silicone resin encapsulation: these encapsulation formats are designed for lower power resistors. They are compact and can still withstand high temperatures typically up to 300oC, but they not expected to be push to their limit.
  • Vitreous enamel coating: these resistors are specified for operation up to temperatures of ~400°C, but the temperatures are not advisable in most electronic equipment. The coating can insulate perfectly at lower temperature, but it acts bad when the temperature rises toward the top end of the range. The resistance values for these resistors range from ~1Ω up to ~10kΩ or so.
  • Aluminum case: these types of designs are used for highest power level. The resistors have a ceramic core with a silicon resin coating which contained within an aluminum extrusion. It is anodized often with a gold color to ensure good electrical insulation and to passivate the surface. The aluminum case of the resistor is typically manufactured with slight fins and designed to be bolted onto a heatsink. Also, the internals have the ability to conduct as much heat as possible to the aluminum case.

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Construction and material

There are over 20 different series in wire wound construction which can be chosen based on some factors such as mounting, application, and resistance range. The wire used in wire wound resistors governs many of its properties. Different electrical properties such as resistivity, temperature coefficient of resistance, long term stability, maximum operating temperature, etc. can be obtain when different material is used for the wire. Many of the wire materials have familiar names as they have been used for many years as forms of resistance wire and therefore used within wire wound resistors. Wires used are typically copper alloys, a variety of different iron alloys, nickel chromium alloys, silver alloys, and tungsten.

You should know that the construction of wire wound resistor varies widely. The construction method and choice of materials is dependent on how the resistor will be used in a circuit. All wire wound resistors are made by winding a wire around a core. The resistance value is dependent on the resistivity of the wire, the cross section, and the length. Because these parameters can be controlled, a high precision resistance is assured. In high tolerance requirements, the resistance value is measured to determine exactly the length to cut the wire. The wire diameter will be very small and the length will be very long when creating a high resistance.

Therefore, wire wound resistors are often produced for lower resistance values. For low power ratings, very thin wire must be used, although handling a very thin wire is critical. Any damage may separate the wire. After winding, it is well protected from moisture to prevent electrolytic corrosion.

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Watch the video below to learn the working of wire wound resistors:

Inductance and capacitance

The effect of inductance and capacitance of wire wound resistors become more noticeable as the frequency of operation rises. They are very good for low frequency and DC operation. The inductance arises because the resistor is effectively a coil of resistance wire which have an effect on the inductor. On the other hand, the capacitance occurs between the different turns on the coil, etc. so, as the frequency of operation rises above 100kHz, these effects can become significant and alter the operation of the circuit.

The wire wound resistors are normally wound as an ordinary coil on a ceramic former. However, if low inductance and capacitance are required, there is no method that can be use to reduce, although not eliminate the inductance and capacitance. Below are the methods that can be employed to reduce the inductance:

Bifilar winding:

The bifilar winding is one of the adopted methods, in its process two separate wires run together, which are joined at the far end. The concept is by running two wires together, this makes the current to run in exactly opposite directions and the field will cancel. Although it’s not perfect as the inductance reduce significantly, but the two wires are run in close proximity the unwanted capacitance increases.

Ayrton-Perry winding:

The Ayrton-Perry winding is one of the common forms of winding. here, the wire splits into two as soon as the wire enters the resistor coil. One half being wound in one direction and the other half in the reverse sense. This manner of winding, the wires are arranged so that the current flows in opposing directions, thereby reducing the inductance. Also, the wire does not run alongside each other in the same way as with the bifilar winding, thus, there is little increase in the capacitance.

Mostly, self inductance and capacitance is always issue to wire wound resistors. Thus, they are not often used in applications where they are used at high or radio frequencies.

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Advantages and disadvantages of wire wound resistors

Advantages:

The benefits of wire wound resistors include:

  • High power circuits usage
  • Does not produce noise
  • Thermally constant

Disadvantages:

Some limitations still occur despite the advantages. Below are the disadvantages of wire wound resistors in their various applications.

  • Used only for low frequencies because it works as an inductor at high frequencies
  • It is larger in size
  • Cost of resistor is high

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Conclusion

Wire wound resistors are types of resistors designed to be used in high power resistor applications or some other use where low noise and power dissipation is required. They use a wire with a high resistivity, wrapped around an insulating core to provide the resistance. This resistive element is the insulated metallic wire that is wound around a core of non-conductive material. That is all for this article where the definition, applications, properties, diagram, types, construction, materials, working, inductance and capacitance of wire wound resistors are being discussed. You also learned the advantages and disadvantages of this resistor.

I hope you gain a lot from the reading, if so, kindly share with other students. Thanks for reading, see you next time!

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