Resistor Derating Curve

MIL-R-39009 Fixed Wire Wound Resistor

Power Resistor Temperature 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. Note in this case that MIL-PRF-39009 resistor derating starts at 25C.

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-PRF-39009 Resistor Derating Curve
MIL-R-39009 Fixed Wire-Wound Resistor Derating by Area

When mounted on a chassis plate with free air the resistor power rating is;
RER40 = 5 watts, RER45 = 10 watts, RER50 = 20 watts, RER55 = 30 watts. However that power rating is reduced as the chassis mounting area is smaller than 100 square inches as seen in the graph above.

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.

MIL-PRF-39009 Resistor Derating Curve
MIL-R-39009 Fixed Wire-Wound Resistor Derating

Note that the vertical axis above is Power [watts] and sets the recommended maximum power dissipation by chassis mounting area [by resistor style].

Resistors must 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.

Air Flow; 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 and should be higher than many other components. Also this style is normally mounted on a metal plate or chassis wall so components mounted on a near-by printed circuit board should not be an issue.

This high-power body style is designed to mount to a metal base or the side of a metal chassis. The aluminum body of the resistor is designed to conduct heat into the metal surface its mounted to, normally an aluminum plate. The size of the metal mounting surface area will also effect the amount of derating required, see Chassis Area x Power Dissipation above. Resistor mounting may also be defined in the data sheet. Check to insure that different mounting instructions are not given in the data sheet. If these power resistors are not mounted to a metal plate they need to be derated another 40% more than what is shown in the graph above. Mounting is very important because the metal body of the resistor is designed to attach to a metal chassis, or metal plate which then acts as a heat sink. In addition because this style resistor is mounted using nuts and bolts vibration should not be an issue in any high shock usage.

This particular metal style power resistor uses fins and an aluminum body to dissipate heat away from the resistor body. Body size effects temperature rise because the size of the radiating surface is changed. 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.

Resistors should be so spaced so that, ventilation is not restricted and heat dissipation by the nearby resistors does not effect another resistor. Because this style of resistor uses between two and four mounting tabs [and terminals], the tabs set the closest distance two resistors may be mounted next to each other, so spacing is not that much of an issue as with other styles. However; these are high-power resistors, so the more spacing the better. 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; during temperature testing the nearest component is more than an inch away.

Wire-Wound MIL-R-39009 Chassis Mount Resistor
Chassis Mount Power Resistor

MIL Spec Resistors
MIL-R-39005 Derating Curve
MIL-R-39007 Derating Curve
MIL-R-39008 Derating Curve
MIL-R-39009 Derating Curve
MIL-R-39015 Derating Curve
MIL-R-39017 Derating Curve
MIL-R-55182 Derating Curve
MIL-R-83401 Derating Curve

Related Topics
Derating Resistor Networks
Derating Thermistors
Derating Potentiometers
Derating Wirewound Potentiometers
Resistor Sizes and Values
Resistor Terms

How to Derate;
Derating Resistor Networks
Resistor Manufacturers
Potentiometer Manufacturers
Electronic Components with Guidelines

Wire-Wound MIL-R-39009 Chassis Mount Resistor
Power Resistor MIL-PRF-39009

Wire-Wound Fixed Resistor
Chassis Mounted, Power type
Noninductively wound resistor

Wire-Wound MIL-R-39009 Chassis Mount Resistor, Nut-Bolt connection
Power Resistor MIL-PRF-39009
PC motherboard

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Modified 6/13/15
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