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In complex electronic systems, the issue of power resistor derating is often overlooked. The system designer can have many design issues to consider. Passive components, such as power resistors, are often the last consideration. If long-term system performance and reliability are a concern, that is a mistake.

The Impact Of Temperature On Power Resistor Rating

Current flow through a resistor generates heat. Failure to control temperature rise in the resistor film can cause irreversible changes to the resistor value. Or, in extreme cases, complete resistor failure.

Utilising a heatsink arrangement can help, but this has cost and application implications. Regardless, the system designer must carefully select the correct resistor for the application.

Resistor manufacturers assume an ambient temperature of 25C when specifying power ratings. As ambient temperature increases, the resistor’s ability to dissipate heat to the environment decreases.

The challenge is to quantify the actual ambient temperature in the application. Without accurate measurement, the power resistor derating factor can only be an approximation. In safety-critical applications, it is often safer to assume the worst case and select a derating factor accordingly.

How To Derate A Power Resistor

As the name suggests, derating a device means operating it at less-than-rated maximums. The rated load on resistor datasheets is usually quoted at 25C (ambient). At higher temperatures, the power resistor is derated.

Resistor datasheets and/or common industry standards state the linear relationship between temperature rise and derating factor (percentage). Derating curve plots show percentage power rating vs ambient temperature. Generally, the percentage power rating is 100% until 25C. Thereafter, the percentage power rating reduces as temperature increases. Hence, as the temperature of the surrounding environment increases beyond 25C the resistor is derated to a lower power rating.

Many power resistor manufacturer data sheets show a derating curve for the general and special cases. The special case environments include high-altitude applications, resistors within enclosures and resistors grouped together in restricted areas. These environments have their own derating curves.

If power resistor derating is not possible and/or there is no way to control ambient temperature, then the system designer can choose a higher power resistor device. This usually increases resistor size and cost.

Power resistor performance and reliability in high-temperature applications depend on many factors. The resistor and substrate materials used to construct the power resistor, the design and the manufacturing process all have an impact. If in doubt about power resistor derating factors, it is wise to consult a specialist power resistor manufacturer.