Power Resistors: Electronics Parts and Functions

High Power Resistor

Harke / Wikimedia Commons / CC BY-SA 3.0

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 resistors that are rated for 1 watt or greater loads and are available in the kilowatt range.

Power Resistor Basics

The power rating of a resistor defines how much power a resistor can safely handle before the resistor begins to suffer permanent damage. The power dissipated by a resistor can be found easily using Joule's first law, Power = Voltage x Current. The power dissipated by the resistor 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. Keeping the temperature of a resistor low will avoid damage to the resistor and let it handle greater currents without degradation or damage. Operating a power resistor above its rated power and temperature can result in severe consequences including a shift in resistance value, reduction in an operating lifetime, open circuit, or temperatures so high that the resistor can catch on fire or catch surrounding materials on fire. To avoid these failure modes, power resistors are often derated based on expected operating conditions.

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

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 better component mount to help dissipate the heat generated by the resistor will let a resistor 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

Several types of high power resistors are available on the market. Each type of resistor offers different capabilities for different resistor applications. Wirewound resistors are common and are available in a wide variety of form factors, from surface mount, radial, axial, and chassis-mount design for optimal heat dissipation. Non-inductive wire-wound resistors are also available for high pulsed power applications. For very high power applications, such as dynamic braking, nichrome wire resistors, also used as heating elements, are good options, especially when the load is expected to be hundreds to thousands of watts.

  • Wirewound Resistors
  • Cement Resistors
  • Film Resistors
  • Metal Film
  • Carbon Composite
  • Nichrome Wire

Form Factors

  • DPAK Resistors
  • Chasis Mount Resistors
  • Radial (standing) Resistors
  • Axial Resistors
  • Surface Mount Resistors
  • Through-hole Resistors