Power Resistor Derating Curve

MIL-R-39007 Power Wire Wound Resistor

Resistor Power ratings are normally specified at +25^oC in the data sheet and must be reduced as the resistor temperature increases. A derating chart is often used, with derating starting at 25^oC; however wire wound resistors [Resistor Derating Curve below] may operate up to 250⁰C, but at a very low power dissipation. 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-39007 resistors are being used which define the type and size of the resistor [Type RWR]. The type remains the same with each deatiled specification, but the sizes change based on power rating at 25C. Note that the vertical axis is Wattage Ratio [derating factor] and sets the recommended maximum power of dissipation at 100%, up to 25 degrees centigrade temperature.

MIL-R-39007 Power Wire-Wound Resistor Derating

When higher ambient temperatures exist or when resistors are mounted in enclosures which limit ventilation, the power 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-39007 style resistor is 250⁰C in still air.

Some recommendations may apply a Derating Factor [percent Rated Load] of something lower than 100 percent. For example a general rule-of-thumb of 80 percent may be applied to the power dissipation regardless of the air temperature [up to the point of power reduction]. So even as the curve falls as the temperature rises, an 80 percent max power rule would be applied. If the curve indicates 30 percent of maximum, the 80 percent rule would be applied to that [80 percent of 30 percent of the maximum].

Resistor Spacing; component spacing or resistor grouping, will also effect device derating. The derating of a resistor must be further reduced if it's effected by nearby components or resistors that are radiating heat which would effect the surrounding ambient temperature. As derating curves only account for the resistor being tested; that is, the equation only accounts for heat generated internally. 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. An appropriate combination of resistor spacing and resistor power rating must be chosen if this is to be assured. Other wise derate to a greater degree than shown in the chart.

Altitude; also has an effect on resistors. However most designs would not have an issue, as few designs are used with aircraft systems. As a rule power resistors may use full power up to 5000 feet, that is after any derating based on heating. Above 5000 feet the resistor should be derated an additional 10% for each addition 10,000 feet of altitude; in addition to temperature derating. Note that the figures for derating to altitude is just an example and is not covered in the spcification, check the manufacturers data sheet for a derating curve. A derating curve only covers sea-level, so account for an increase in altitude if it becomes a design issue.

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.

Mounting; Resistor mounting may also be defined in the data sheet, although it's not very common. Component pads of a particular size or shape or thermal vias [for 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].

Resistor Shape; All the resistors in the M39007 series have the same cylindrical shape. However different series of resistors may have a different body type or contour and size for the same resistance. The resistors within the M39007 do change size based on power rating, but keep the same basic shape. 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. This particular military standard defines the shape of the RWR resistor body.