Power Resistors: Electronics Parts and Functions

There are different uses for low-power and high-power resistors

What to Know

  • Power resistors are used in electronics to dissipate energy by controlling current flow and voltage.
  • The power rating of a resistor defines how much power a resistor can safely handle before it begins to suffer permanent damage.
  • Most electronics applications use low-power resistors, typically 1/8th watt or less. High-power resistors are rated at 1 watt or better, including kilowatt range.

This article explains how these resistors work and includes a look at a variety of resister types.

Power Resistor Basics

The power dissipated by a resistor can be found using Joule's first law (Power = Voltage x Current). The dissipated power is converted to heat and increases the temperature of the resistor. The temperature of a resistor will keep climbing until it reaches a point where the heat dissipated through the air, circuit board, and the surrounding environment balances the heat generated.

Depending on the required wattage, a device may need a high-power resistor to prevent overheating but keeping the temperature of a resistor low is necessary to handle greater currents without degradation or damage.

Operating a power resistor above its rated power and temperature can result in severe consequences, including shifts in resistance value, reduced operating life, open circuits, or even electrical fires. To avoid such failures, power resistors are often derated based on expected operating conditions.

Power resistors are usually larger than their counterpart components. The increased size helps to dissipate heat and is often used to provide mounting options for heatsinks. High-power resistors are also available in flame-retardant packages to reduce the risk of a hazardous failure condition.

High Power Resistor
Harke / Wikimedia Commons / CC BY-SA 3.0

High-Power vs. Low-Power Resistors

Most electronics applications use low-power resistors, typically 1/8th watt or less. However, applications such as power supplies, dynamic brakes, power conversion, amplifiers, and heaters often demand high-power resistors. Generally, high-power resistors are rated at 1 watt or greater. Some are available in the kilowatt range.

Power Resistor Derating

The wattage rating of power resistors is specified at a temperature of 25C. As the temperature of a power resistor climbs above 25C, the power that the resistor can handle safely begins to drop. To adjust for the expected operating conditions, manufacturers provide a derating chart which shows how much power the resistor can handle as the temperature of the resistor goes up.

Since 25C is typical room temperature, and any power dissipated by a power resistor generates heat, running a power resistor at its rated power level is often very difficult. To account for the impact of the operating temperature of the resistor, manufacturers provide a power derating curve to help designers adjust for real-world limitations. It is best to use the power derating curve as a guideline and stay well within the suggested operating area. Each type of resistor will have a different derating curve and different maximum operating tolerances.

Several external factors can impact the power derating curve of a resistor. Adding forced air cooling, a heatsink, or a better component mount to help dissipate the heat generated by the resistor will allow it to handle more power and maintain a lower temperature. However, other factors work against cooling, such as the enclosure keeping the heat generated in the ambient environment, nearby heat-generating components, and environmental factors such as humidity and altitude.

Types of High-Power Resistors

Each type of power resistor offers different capabilities for different resistor applications. Wirewound resistors, for example, come in a wide variety of form factors, including surface-mount, radial, axial, and chassis-mount designs for optimal heat dissipation. Non-inductive wirewound resistors are also available for high-pulsed power applications. For very high-power applications, such as dynamic braking, nichrome wire resistors are ideal, especially when the load is expected to be hundreds or thousands of watts. Nichrome wire resistors can also be used as heating elements.

Common types of resistors include:

  • Wirewound resistors
  • Cement resistors
  • Film resistors
  • Metal film
  • Carbon composite
  • Nichrome wire

Different resistor types may come in various form factors such as:

  • DPAK resistors
  • Chassis-mount resistors
  • Radial (standing) resistors
  • Axial resistors
  • Surface-mount resistors
  • Through-hole Resistors