Last revised: October 22, 2024
Reviewed by: Scott Orlosky, consulting engineer

Wirewound resistors are power resistors made by winding a metal wire around a core. Resistors are basic passive components which impart electrical resistance for the purpose of protecting or controlling a circuit. For more basic information about resistance, device labeling, and resistors in general, please visit the Resistors Selection Guide.

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.

A basic wirewound resistor. Image credit: Chat with the Designers.org

Pure metals are not commonly used because they tend to have a high temperature coefficient of resistance (TCR), meaning that temperature changes are more likely to change the device's resistance value. While pure metal windings are useful when resistors are intended for use as thermistors, they are not in purely resistive applications. Copper alloys are some of the most commonly-used winding materials due to their low temperature coefficients of around 0.000008 ppm/°C. (For comparison, tungsten's TCR is around 0.0045 ppm/°C.)

When compared to resistors in general, wirewound devices can be produced with high precision, as the wire resistivity and wire length—both of which largely determine the resistance value—can be carefully controlled. In terms of form factor, wirewound resistors may be surface-mounted, as shown in the image at right, or leaded through-hole devices, as shown in the illustration at the prior paragraph.

Winding

Wirewound resistors strongly resemble inductors due to their wire coil and core design. For this reason they exhibit properties of inductors, including inductance and self-capacitance. These two undesirable conditions become worse at high frequencies, to the extent that they interfere with wirewound resistors designed for alternating current (AC) power applications. To mitigate these issues, devices may use one of several special winding methods shown below.

• Bifilar winding involves doubling the wire wrapping; it is very effective at correcting inductance but results in self-capacitance due to the minimal space between doubled wires.
• Flat core winding uses a flat core to increase the distance between wire turns, reducing inductance.
• Ayrton-Perry winding is a specialized method used for precision devices. It also uses a flat core but splits the winding so that close windings experience current flow in opposite directions. This results in low inductance as well as low self-capacitance.

Wirewound resistor winding methods. Image credit: ResistorGuide

Applications

Wirewound resistors are best used in high-power, low-resistance applications. Some common uses are listed below.

Circuit breakers: Wirewound resistors are often employed as circuit breaker fuses by soldering a small spring to one end of the device. If heat buildup reaches critical levels due to overcurrent, the solder melts and activates the spring, opening the circuit. The breaker can be reused by resoldering the spring.

Current sensing: This application leverages the inductive properties of wirewound resistors. The inductive reactance of the device—which changes proportionally with current flow—can be measured and converted to a current reading. Current-sensing resistors are useful in applications that require the measurement and correction of overcurrent before tripping a breaker.

Types of Wirewound Resistors

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

Type	Description	Applications	Image
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.

Table image credits: Riedon | Post Glover | Changzhou Kennon

Standards

Wirewound resistors may be manufactured and used in accordance with various standards and specifications, including:

• MIL-PRF-39005 General specification for fixed wirewound resistors
• BS EN 60115 Generic specification for fixed resistors for use in electronic equipment
• BS EN 140402-801 Fixed low-power SMD wirewound resistors

Wirewound Resistors FAQs

What are the challenges associated with wirewound resistors?

Due to their construction, wirewound resistors can exhibit significant parasitic inductance, especially at high frequencies. This requires special internal wiring patterns and circuit design to mitigate these effects.

How do wirewound resistors compare to other types of resistors?

Wirewound resistors have distinct characteristics that differentiate them from other types of resistors. Here's a comparison based on the information available:

Precision and Stability

Wirewound resistors are known for their high precision and long-term stability. They can be manufactured to very tight tolerances, often less than 0.1% resistance value, and are designed to maintain their values over years of use.

Other types of resistors, such as carbon composition or film resistors, may not offer the same level of precision and stability, especially in high-power applications.

Temperature Coefficient

Wirewound resistors have a low temperature coefficient, meaning they retain their resistance values as temperatures change. This makes them suitable for applications where temperature stability is crucial.

In contrast, resistors made from pure metals tend to have a high temperature coefficient, which can lead to changes in resistance with temperature fluctuations.

Construction and Inductance

Wirewound resistors are constructed by winding a conductive wire around an insulating core, typically ceramic. This construction can lead to significant parasitic inductance, especially at high frequencies, which may require special internal wiring patterns to mitigate.

Other resistor types, such as metal film or carbon film resistors, generally have lower inductance and are more suitable for high-frequency applications.

Power Handling

Wirewound resistors are often used in high-power applications due to their ability to handle large amounts of power. They are available in various wattage sizes and can withstand power levels of 1000 W or more.

Applications

Wirewound resistors are commonly used in industrial applications, circuit breakers, fuses, and high-power settings due to their stability and precision.

What are the challenges associated with using wirewound resistors?

Wirewound resistors, while highly valued for their precision and stability, do present certain challenges that need to be considered in their application:

Due to their construction, which involves winding a conductive wire around an insulating core, wirewound resistors can exhibit significant parasitic inductance. This is particularly problematic at high frequencies, where the inductance can interfere with the resistor's performance in AC power applications. To mitigate these effects, special internal wiring patterns and circuit designs are often required.

Alongside inductance, wirewound resistors can also exhibit self-capacitance due to their coil-like structure. This can further complicate their use in high-frequency applications, necessitating careful design considerations to minimize these effects.

Wirewound resistors are generally larger than other types of resistors, such as film resistors, due to their construction. This can be a limitation in applications where space is a critical factor.

The precision and stability of wirewound resistors often come at a higher cost compared to other resistor types, which might be a consideration in cost-sensitive applications.

These challenges highlight the importance of careful selection and design when incorporating wirewound resistors into electronic circuits, especially in high-frequency or space-constrained applications.

How do wirewound resistors handle high power applications?

Wirewound resistors are well-suited for high power applications due to several key characteristics and design features:

Wirewound resistors are constructed by winding a conductive wire, typically made from copper alloys, around an insulating core, often ceramic. This construction allows them to handle high power levels effectively.

Power wirewound resistors are often housed in metal containers or finned aluminum cases that act as heat sinks. This design helps dissipate the considerable amount of heat generated during high power applications, preventing damage and maintaining performance.

These resistors can withstand power levels ranging from 4 to 17 watts, and in some cases, up to 1000 watts or more. This makes them suitable for high-power applications where other resistor types might fail.

Wirewound resistors are known for their high precision and long-term stability, which are crucial in high power applications to ensure consistent performance over time.

They have a low temperature coefficient of resistance (TCR), meaning their resistance values remain stable even as temperatures change, which is important in high power settings where heat generation is significant.

Although wirewound resistors can exhibit significant parasitic inductance, especially at high frequencies, special internal wiring patterns and circuit designs can be employed to mitigate these effects, ensuring reliable performance in AC power applications.

How do wirewound resistors mitigate issues related to parasitic inductance?

Wirewound resistors mitigate issues related to parasitic inductance through specific design and construction techniques.

Wirewound resistors can exhibit properties similar to inductors due to their coil-like structure, which includes inductance and self-capacitance. To mitigate these issues, special winding methods are employed. These methods help reduce the inductive effects, making the resistors more suitable for applications involving alternating current (AC) power.

The construction of wirewound resistors involves winding a conductive wire around an insulating core. To address the significant parasitic inductance that can arise, especially at high frequencies, special internal wiring patterns are used. These patterns are designed to minimize inductance and ensure reliable performance in AC power applications.

Wirewound Resistors Media Gallery

References

GlobalSpec—Resistor Theory and Technology

ResistorGuide—Wirewound resistors

Image credits:

Chat with the Designers.org |  Precision Resistive Products, Inc. | ResistorGuide | Riedon | Post Glover | Changzhou Kennon