Resistor Derating Curve

MIL-R-55182 Power Resistor

Resistor Power ratings are normally specified at +25^oC and must be reduced as the resistor temperature increases. A derating chart is often used, always with Still-Air. Since parameters are application dependent, power de-rating curves or charts should be considered general rather than absolute, and only used as a guideline or recommendation. The most conservative or safest designs use the largest physical size operating at conservative temperatures and power ratings. This style of film resistor will operate up to 125C before any derating is required. However the maximum power dissipation is reached at 175C, for types RNC, RNR, RNN [MIL-R-55182].

MIL-R-55182 Resistor Graph

When higher ambient temperatures exist or when resistors are mounted in enclosures which limit ventilation, the wattage dissipation of any resistor should be reduced so that the maximum hot-spot temperatures permissible for the resistor is never exceeded under the most severe combination of temperature conditions.

Derating Resistor Networks, Resistor Manufacturers, or Potentiometer Manufacturers

The Derating Factor [percent Rated Load] shown in the table above is a general rule-of-thumb and can vary depending on the company or organization providing the guideline. In this particular case the Department of Defense [DOD] generated the permissible rated load over temperature. Stress Ratio is another term used to describe the derating factor. NASA uses 80% as a stress ratio for resistors, regardless of the ambient temperature.
Resistor Stress Ratio = Operating Power / Rated Power = 80 percent

Definitions
Near-by heat radiators; will also effect how much power a device may safely dissipate. The maximum power a resistor may dissipate must be further reduced if it's effected by near by components or resistors that are also radiating heat which would effect the surrounding ambient temperature. Derating curves only account for the resistor being tested, with the nearest resistor being 1 inch away during testing.

Air Flow; Resistor derating curves or equations are normally somehow related to 25C; how ever what is not always stated is that the figures are for still air [Free Air], with no circulation other than that caused by the heat of the resistor. Forced air will allow a resistor to operate above what is shown in the derating curves. Free Air rating is also called Full Rating, and Maximum Power Rating. Because resistor bodies may be smaller than other components on the printed wiring board, any forced air added to the system may bypass the resistor as it's diverted around or over the device by other components.

Resistor Mounting; Resistor mounting may also be defined in the data sheet. Component pad sizes of a particular size or shape or thermal vias [High-Power Resistors] may be required for the device to comply with the derating curve provided by the data sheet. Check to insure that mounting instructions are given in the data sheet, for example mounted by the resistors leads some distance above the printed wiring board. Some derating curves may also specify the board type [as in FR4], but this is less common for resistors.

Resistor Shape; different resistor series use different body types and sizes for the same resistance. Body size effects temperature rise because the size of the radiating surface is changing. Correct derating of a particular resistor series does not directly relate to another family that uses a different body shape, regardless of the resistance value.

Resistor Interval; When resistors are mounted in rows or banks they should be positioned so that the body of one resistor does not touch the resistor next to it, so that ventilation is not restricted by the nearby resistors, and none of the resistors in the bank or row exceeds its maximum permissible hot-spot temperature. An appropriate combination of resistor spacing and resistor power rating must be chosen if this is to be assured.

Altitude is another consideration that must be accounted for in high-power and high-voltage applications. As a general rule consider any numbers provided in a data sheet to relate to sea level, or maybe up to 5000 feet in altitude. If a design does need to work at altitude than reduce the maximum power the resistor will be allowed to dissipate by 10 percent for each additional 10,000 feet above 5000 feet. High voltage arcing could also be a problem, which may require resistors or near-by traces to move apart.