Resistor Derating Curves

Resistor Power ratings are normally specified at +25oC and must be reduced as the resistor temperature increases. This series of metal Film resistors does not need to derated until 70oC [Resistor Derating Curve below]. 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. MIL-PRF-39017 resistors are capable of full load operation at an ambient temperature of 70�C.

MIL Spec Resistor Derating Curve for MIL-PRF-39017 components
MIL-R-39017 Resistors, Derating Curve

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. Also; if the resistor is operated at an altitude of 5000 feet or over than additional power considerations must be accounted for by reducing the maximum power dissipation by an additional 10 percent for each 10,000 feet over 5000 feet.

Resistor Derating Chart
% Rated load
Wire Wound
metal Film
100% 70oC 70oC
80% 110oC 85oC
60% 150oC 100oC
40% 190oC 120oC
20% 240oC 140oC
10% 260oC 145oC

Derating Resistor Networks, Resistor Manufacturers, or Potentiometer Manufacturers

The Derating Factor [percent Rated Load] may also be applied as shown in the table above as a general rule-of-thumb, and used instead of a derating curve. Many companies and organizations provide a derating factor or stress ratio as a design guideline, really a hardware design guideline that must be followed. So if the stress factor is 80 percent than that equals the operating power over the rated power.

Mil Spec MIL-R-39017 Resistor Derating Factor Curve
MIL-R-39017 Resistors, Type RLR Derating Factor

Note this derating factor graph uses Power Ratio vs Temperature for metal Film Resistors.

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], or no forced air cooling. 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.

Mounting; Resistor mounting may also be defined in the data sheet. Normally the resistor is mounted using the leads of the device, with the body raised just above the board. 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. 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; Example,
TO-126, TO-220, TO-247 package styles.

Component Shape; or body size effects temperature rise because the size of the radiating surface is changing. The MIL-PRF-39017 specification defines four different sizes depending on power dissipation, all having the same shape as the resistor to the right. 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.

Component Spacing; When resistors are mounted side-by-side, in rows or in banks, they should be so spaced that, taking into consideration the restricted ventilation and heat dissipation by the nearby resistors, and none of the resistors in the bank or row exceeds its maximum permissible hot-spot temperature. Resistor 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. An appropriate combination of resistor spacing and resistor power rating must be chosen if this is to be assured. Derating curves only account for the resistor being tested and assume ambient temperature.


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-55182 Derating Curve
MIL-R-83401 Derating Curve

Definition of Resistor Terms

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

How to Derate;
Electronic Components with Guidelines

Fixed FIlm Resistor Axial Lead Package Style
Film Axial Lead Resistor

Resistor Reliability;
In high vibration environments
resistors should not be mounted
in the stand-off position above the PCB.
Secure the resistor to the PCB.

PC motherboard

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Modified 12/31/11
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