MIL-R-83401 Resistor Network
Resistor Network Derating; the curve below shows the derating curve for a MIL spec 83401 resistor network [MIL-PRF-83401]. The specification defines packages which include a DIP, SIP and Flat Pack style resistor networks and resistor arrays.
Regardless of the package style, the MIL-PRF-83401 resistor power ratings are specified to operate up to +25oC before any power reduction is required. However above 25C the power must be reduced as the temperature increases, until 70C is reached. The derating chart shown below details the changes in derating starting at 70oC. 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 resistor size operating at
conservative temperatures and power ratings. In this case MIL-R-83401 resistors are being used which define the type and size of the resistor package. Note that the vertical axis is percent rated Power and sets the recommended maximum power dissipation at ambient temperature.
MIL-R-83401 Resistor Network Derating
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-83401 style resistor is 1250C in still air.
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 DOD calculated 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 percent
Definitions
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. See Spacing.
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. Note that the figures for derating to altitude is just an example, check the manufacturers data sheet for a derating curve.
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 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. This specification defines DIP, SIP and Flat pack styles. Note that if mounting instructions are provided in the data sheet they will account for reducing the hot-spot temperature on the resistor, mounting instructions for high-vibration environments may be completely different.
Square Shape; This particular body style uses a plastic DIP package; however different series of resistors are defined in these specifications using different body types and sizes for the same resistance. Body size or style effects temperature rise because the size of the radiating surface is changing. Check the data sheet for the absolute derating recommendation.
Spacing; 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. In other words near-by resistors will add heat to each other.
