MIL-R-39005 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 at 70oC; however wire wound resistors [Resistor Derating Curve below] may operate up to 1250C 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.
The safest designs use the largest physical size operating at conservative temperatures and power ratings. In this case MIL-R-39005 resistors are being used which define the type and size of the resistor [Type RBR]. Note that the vertical axis is Power Ratio [derating factor] and sets the recommended maximum power dissipation by temperature. MIL-39005 style resistors shall have a power rating based on continuous full-load operation at an ambient temperature of 125C
MIL-R-39005 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-39005 style resistor is 1400C in still air.
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 Defense Department generated the permissible rated load over temperature. The intention of the specification in covering temperature rise is to limit the final hot spot
temperature to 145�C. However, if it is desired to operate these resistors at ambient temperatures greater than
125�C, the resistors should be derated in accordance with the graph above. Stress Ratio is another term used to describe the derating factor. NASA uses 80 percent as a stress ratio for resistors, regardless of the ambient temperature.
Resistor Stress Ratio = Operating Power / Rated Power = 80%
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 do not account for any near by components..
Component Altitude; resistors must also be power derated based on altitude, in addition to temperature derating. Example numbers may indicate full power up to about 5000 feet, followed by a 10 percent reduction in available power for each additional 10,000 feet of altitude above 5000 feet. Check the manufacturers data sheet for a power curve based on altitude.
Component 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.
Resistor Mounting; Resistor mounting may also be defined in the data sheet. Resistors shall be supported by their terminal leads. Resistors shall be so arranged that the temperature of any one resistor shall not appreciably influence the temperature of any other resistor. 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.
Resistor Shape; The MIL-R-39005 specification defines both axial lead and radial lead resistors, a photograph of an axial lead resistor is shown to the left.
Resistor Spacing; When resistors are mounted in rows or banks, they should be so spaced that, taking into consideration the restricted ventilation and heat dissipation by the nearby resistors, none of the resistors in the bank or row exceeds its maximum permissible hot-spot temperature.
Mil Spec resistors are sometimes used without regard to application; They may be used in commercial applications so two different style resistors need not be stocked [assuming a military need to begin with].