Resistor Derating Curves

MIL-R-39015 Variable Wire Wound Resistor

Power ratings for these Variable Wire Wound Resistors extend up to 85oC before any power reduction is required. The derating chart shown below indicates that derating starts at 85oC and extends out to 1500C, which is the maximum temperature. Since parameters are application dependent, power de-rating curves or charts should be considered general rather than absolute, and only used as a guideline.

mil-prf-39015 Resistor Derating Curve for Mil spec resistors
MIL-PRF-39015 Variable Wire Wound Resistor Derating

In this case MIL-R-39015 resistors are being used which define the type and size of the resistor [Type RTR]. Note that the vertical axis is Power Ratio [derating factor] and sets the recommended maximum power dissipation with temperature.

Note that there are a number of resistor types defined in MIL-PRF-39015 [MIL-R-39015], this is only one type..

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-39015 [MIL-PRF-39015] style resistor is 1500C in still air.

The Derating Factor [percent Rated Load] shown in the graph 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 generated the permissible rated load over temperature. Stress Ratio is another term used to describe the derating factor.
Resistor Stress Ratio = Operating Power / Rated Power = percent rated load

Resistor Grouping; may also effect component derating. This style allows back-to-back placement [which should be avoided].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 graphs only account for the resistor being tested at a specific temperature.

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, above sea level.

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. Component pad sizes of a particular size or shape may be required for the device to comply with the derating curve provided by the data sheet.

Variable resistor Shape; This series of variable resistor use either a square or rectangular body, as shown by the two graphics.

Component Body Spacing; When resistors are mounted in rows, they should be so spaced so that, taking into consideration the restricted ventilation and heat dissipation by the nearby resistors, none of the resistors in the row exceeds its maximum permissible hot-spot temperature. Because this series of resistor has a square body or rectangular body they should not be placed next to each other with out some amount of air space between each resistor body.

RTR style mil-prf-39015 Variable Resistor
Variable 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-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 and Definitions
Printed Wiring Board Terms

Passive Vendors;
Resistor Manufacturers
Potentiometer Manufacturers

mil-prf-39015 Variable Resistor style RTR12
Variable Resistor

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Modified 6/13/15
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