Resistor Power ratings are normally specified at +25oC and must be reduced as the resistor temperature increases. A derating chart is often used, with derating starting at 70oC [Resistor Derating Curve above]. 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 some cases component derating starts at 25C, while wire-wound devices may start a bit higher but also operate up to 3000C.
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 temperature permissible for the resistor is never exceeded
Resistor Power Dissipation
under the most severe combination of temperature conditions.
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 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 for resistors, regardless of the ambient temperature.
Resistor Stress Ratio = Operating Power / Rated Power = 80%
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, MIL-PRF-32159 Derating
There are many military specifications that deal with different types of resistors; listing of Resistor MIL Specs
There are also resistors that may be qualified by NASA, Goddard Space Flight Center [GSFC] and be found by the Goddard Designator G311P683 [example].
Refer below for GSFC Derating guidelines.
Near-by resistors; or component grouping will effect resistor derating. A resistors power dissipation 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 [of the resistor being effected]. Derating curves only account for the resistor being tested and assume no other near-by components that would effect the temperature of the component.
Altitude; resistors must also 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. Also check the manufacturers data sheet for a derating curve or maximum power recommendations.
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.
Component Mounting; Resistor mounting may also be defined in the data sheet. However; mounting to prevent over-heating my contradict mounting requirements due to vibration issues. 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. Large power resistors should be mounted to the metal chassis for heat dissipation. More on Resistor Mounting.
...... Surface Mount In general through-hole devices are larger than Surface Mount Devices [SMD], but not in every case. So in general a through-hole resistor will dissipate more heat than a surface mount resistor. However a DPAK style resistor will dissipate more heat than a number of through-hole style resistors [DPAK info below].
...... Through-hole Mount Almost always dissipate more heat or operate at a higher temperature than a surface mount resistor, but it depends on size, so always refer to the data sheet for the correct data.
Component Shape; different series of resistors use different body types and sizes for the same resistance. 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. Surface mount Wire Wound devices [example 2010 SMD] tend to function just as normal surface mount resistors and should be derated starting at 25C. A 2010 SMD device is always the same size; while a larger through-hole wire wound resistor will normally operate to a higher temperature before derating.
...... DPAK [three terminal surface mount device (SMD)] Two surface mount leads form the resistor while the square body acts as a heat sink. The DPAK resistor is derated via case temperature and will operate at full load up to a case temperature of 25C. Above 25C the device should be derated by 0.23W/�C. The conductive body must be mounted to a copper pad on the Printed Wiring Board [PWB]. The Resistor element is electrically insulated from the heat sink [body or case]. The heat sink allows DPAK styles to operate at higher wattages than normal SMD resistors; however there cases are much larger than a normal SMD resistor.
Spacing; Consider proximity to other heat sources as well as self-heat. 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.
GSFC guidelines for derating resistors [Goddard Space Flight Center]
How to Derate Military Style Resistors
1/ Compute the resistor's derated power level by multiplying its nominal power rating by the appropriate derating
factor for ambient temperatures belowT1. If the resistor is operated above T1, derate linearly from the T1 power
level to the zero power level at T2. Exposing the resistor to temperatures exceeding T3, even under no load
conditions, may result in permanent degradation.
2/ The maximum applied voltage shall not exceed the lesser of the following: (1) 80% of the specified maximum voltage rating, or (2) (PR)1/2
P = Derated power (Watts)
R = Resistance of that portion of the element actually active in the circuit.
This voltage derating applies to dc and regular ac waveform applications. For pulse and other irregular waveform applications, consult the manufacturer.
3/ Determine the zero power temperature (T3) from the applicable detail specification. Compute the derated zero power temperature (T2) from the following formula:
T2 = DF(T3-T1) + T1, where:
T2 = Derated zero power temperature
DF = Derating factor
T3 = Zero power temperature
T1 = Rated power temperature
4/ Determine the rated power, the rated power temperature (T1), and the zero power temperature (T3) from the manufacturer's specification. Calculate the derated zero power temperature (T2) as per the previous note.
Note, styles starting with 'G' are NASA qualified, 'R' are MIL qualified.