# How to interpret derating charts for resistors

A data sheet for a 750 ohm 5W 5% resistor:

http://www.parts-express.com/pedocs/specs/xicon-cement-power-resistors-spec-sheet-specifications.pdf

What does this resistor's derating chart say about what happens above 150 degrees C? Undefined or just not guaranteed (but likely to follow its linear descent to 0% load at 275 degrees C)? Or something else?

[Edit: Corrected "F" to "C"]

• Mostly likely untested. The dashed line is interpolated from the previous data. IE mileage may vary. Feb 8 '17 at 15:25
• You have not read carefully. The chart is in C, not F. The datasheet says the operating range is from -55 to +155C ambient temperature. I wouldn't plan to operate it outside that range. If you are going above 155C ambient temperature, you should look at some type of heater element wire, not a resistor. Although there are resistors that can handle it if you look around. Feb 8 '17 at 15:52
• I have never seen Fahrenheit units listed in the context of power ratings on a datasheet. I think graphs and ratings will always be exclusively in C. Feb 8 '17 at 15:54
• @mkeith: "You have not read carefully. The chart is in C, not F." Sorry, it was my typo. Corrected. Thanks. Feb 8 '17 at 16:00
• OK, well, like I said, I would never use a component outside its recommended temperature range. I am pretty sure they set it for a reason. That is an AMBIENT temp, not a surface temp. Nothing else in your circuit is going to survive a 155C ambient temp anyway. Probably not even the circuit board will survive at that temp for an extended period. Feb 8 '17 at 16:05

The chart is showing you how much power the resistor can safely dissipate as a function of ambient temperature. At 70 °C or below is can dissipate full rated power. Above that, its max allowable dissipation goes down until it can't dissipate anything anymore at about 270 °C.

The solid versus dotted line probably means that other specs are violated above 150 °C ambient, but they show you the full power dissipation relationship anyway, all the way down to can't dissipate anything more without getting cooked.