MIL-R-39009 Fixed Wire Wound Resistor
Resistor Power ratings are normally specified at +25oC in the data sheet and must be
reduced as the resistor temperature increases. A derating chart is often
used, with derating starting between 25 and 70oC; wire wound resistors [Resistor Derating Curve below] may operate up to 700C before any derating is required. Since parameters are
application dependent, power de-rating curves or charts should be
considered general rather than absolute, and only used as a guideline. Note in this case that MIL-PRF-39009 resistor derating starts at 25C and operates up to 250C.
The safest designs use the largest physical size operating at
conservative temperatures and power ratings. In this case MIL-R-39009 resistors are being used which define the type and size of the resistor [Type RER]. Note that the vertical axis is Rated Power and sets the recommended maximum power dissipation at 25C, down to 10% rated power at 250C.
MIL-R-39009 Fixed Wire-Wound Resistor Derating
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. Note that the maximum permissible ambient temperature for a MIL-R-39009 style resistor is 2500C in still air. However the wattage rating for this power resistor drops to zero at 250 degrees centigrade.
The Derating Factor [percent Rated Load] shown in the table below 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 [Power Ratio] for resistors, regardless of the ambient temperature.
Resistor Stress Ratio = Operating Power / Rated Power = 80%
MIL-R-39009 Fixed Wire-Wound Resistor Guidelines
Note that the vertical axis above is Power Ratio [derating factor] and sets the recommended maximum power dissipation at 60%, regardless of the temperature. This chart is a rule-of-thumb which varies by company.
Related Definitions
Near-by 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. See Spacing.
High Altitude; also effects resistor power ratings. Resistors may indicate full power up to 5000 feet in altitude, than be reduced by 10 percent for each additional 10,000 feet of altitude. However this is just a rule of thumb; check the manufacturers data sheet for a chart to indicate maximum power vs altitude.
Air Flow; or forced air cooling; Resistor derating curves or equations are routinely related to 25C; how ever 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 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 the device by other components. This resistor package uses the metal body of the package as a fined heat-sink.
Mounting Fins; This style of high power resistor is designed to mount to a metal base or the metal side of a chassis. The aluminum body of the resistor is designed to dissipate heat into the metal surface its mounted to. The size of the metal mounting surface area will also effect the amount of derating required. For additional information on the required surface area, refer to MIL-R-39009 Chassis Area Derating Resistor mounting may also be defined in the data sheet. Check to insure that mounting instructions are given in the data sheet.
Odd Shape; different series of resistors use different body types and sizes for the same resistance, note the mounting fins used with this particular power resistor. 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 derating recommendation.
Foot-print Spacing; Because this style of resistor uses mounting tabs, the tabs set the closest distance two resistors may be mounted next to each other, in a row for example. In addition MIL-R-39009 also defines a resistor with mounting leads or terminals, which than set the front to back spacing [as in a bank of components]. Using the graphic to the left; the mechanical drawing shows [male] terminal leads to the left and right of the component. The graphic also shows two off-set [female] mounting tabs on either side of the resistor body. So because these resistors have both the mounting tabs and terminals it will not be possible to place these resistors so close to each other that spacing should be a problem.
