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 mostcommonly- 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 throughout the rest of this page.

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

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

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

References

ResistorGuide—Wirewound resistors

Image credits:

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