Power ratings defined in data sheets are normally specified at +25oC and must be reduced as the resistor temperature increases, see the derating curve above. A derating chart is often used, with derating starting at 70oC. although the exact temperature that a part should begine to be derate depends on the actual resistor style and type. 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.
| Wire Wound Temperature |
metal Film Temperature |
|
| 100% | 70oC | 70oC |
| 80% | 110oC | 85oC |
| 60% | 150oC | 100oC |
| 40% | 190oC | 120oC |
| 20% | 240oC | 140oC |
| 10% | 260oC | 145oC |
As these are average guide-lines, they may change with other IC packages.
The Derating chart is providing what is called a Dearting Factor. Which is a general reduction in the power capability of a component by some percentage of the maximum permissible power dissipation. A derating factor is a guideline and changes, as does best practices between companies. So some companies may use those numbers, while other companies may use different numbers. Not only could the derating factor change, but some companies might change the temperature at which a derating factor begins.
Resistor Network Manufacturers, Resistor Manufacturers, Potentiometer Manufacturers
There are a number of possible IC packages, this is a guide, always check the data sheet for the device being used. Note the chart above starts to derate at 25C, but only because below 25C the device is being operated at 120% of rated wattage. With any derating curve, this graph applies to still air and it applies to a Fixed Film Resistor Network in a DIP package.
Note the curve shown above relates to MIL-PRF-83401 [Resistor Networks]. The Power Rating for this series is given at an ambient temperature of 700C, so for temperatures below that value the device may be operated above the maximum power rating [above 100 percent]. So at 250C the resistor network may operate at 125% of maximum power.
The reason this curve provides a power rating above 100 percent is because many DOD specifications define an operating point of 70 degrees C, and not 25 degrees C as would be standard for commercial parts. So this particular component, which happens to be a resistor network, actually has a higher power rating at 25C, than at 70C. This situation would not normally occur for a commercial part which is specified or designed to operate at a temperature of 25C.
Derating guidelines for other components; Guideline for Derating Electronic Components
Resistor Network Package Styles, SIP Package Style. Resistor Network Schematics
