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 keeps 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. 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 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 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. This derating chart shows how much power the resistor can handle as the temperature of the resistor goes up. Since 25C is the typical room temperature, and any power dissipated by a power resistor generates heat, running a power resistor at its rated power level is often 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 within the suggested operating area. Each type of resistor has 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 allows 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 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 resistorsCement resistorsFilm resistorsMetal filmCarbon compositeNichrome wire
Different resistor types may come in various form factors such as:
DPAK resistorsChassis-mount resistorsRadial (standing) resistorsAxial resistorsSurface-mount resistorsThrough-hole resistors
