Resistor Derating Curve

MIL-R-39008 Power Wire Wound 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, with derating starting at 70^oC for this series of Fixed Carbon Resistors. Since parameters are application dependent, power de-rating curves or charts should be considered general rather than absolute, and only used as a guideline. The safest designs use the largest physical size operating at conservative temperatures and power ratings. In some cases component derating starts at 25C, while wire-wound devices may start a bit higher still.

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%

Pertinent Definitions
Near-by resistors; or resistor grouping, will also effect device derating. A resistors derating must be further reduced if it's effected by near by components or resistors that are radiating heat which would effect the surrounding ambient temperature. Derating curves only account for the resistor being tested and not other components used in the design.

Altitude; resistors must also be power derated based on altitude. Example numbers may indicate full power up to 5000 feet, than derate 10% for each addition 10,000 feet of altitude; in addition to temperature derating. Normal resistor data sheets do not account for an increase in altitude above sea-level when providing electrical parameters. The M39008 resistor series has been cancelled, so it's unclear if the specification called out any altitude numbers.

Air Flow; allows the resistor to operate to a higher temperature or dissipate more power. Resistor derating curves or equations are routinely related to or start at 25 degrees centigrade, with out forced air flow. However what is not always stated is that the figures are for still air [Free Air]. Forced air will allow a resistor to operate above what is shown in the derating curves. Free Air rating might also be 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 resistor by other components, so they resistor body gets no direct forced air.

Body 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. Some derating curves may also specify the board type [as in FR4], but this is less common for resistors.

Fixed Carbon Resistor Mounting

Surface Mount resistor packages having a tab [TO-263] may require the tab attachment point [Pad] to be much larger than the actual size of the tab.
Some through-hole resistors may also have a tab to attach a heat sink; Reference,
TO-126, TO-220, TO-247 package styles.

Resistor Outline; effect how close resistors may be placed to each other. Different series of resistors use different body types, contours 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. Also check the data sheet for the absolute power recommendation.

Spacing Resistors; should also be considered when resistors are mounted in rows or banks of rows. The M39008 series have a concave shape, so at worst case only a small portion of the resistor body would ever touch another resistor mounted nearby. The resistors should be so spaced so that there is no restricted ventilation between resistors and the heat dissipation by the nearby resistors do not effect other resistors. That is, resistor derating assumes only the air temperature and the heat dissipated from within the component, not another component. An appropriate combination of resistor spacing and resistor power rating must be chosen if this is to be assured, not.