I *think* that such power derating curves are usually intended for a certain set of conditions - normally the resistor in its typical mounting, surrounded by open air, so that the temperature mentioned in the derating curve is the ambient air temperature, not the resistor surface temperature. Otherwise, that would imply that the resistor needs active cooling in order to use it at its normal rated power. Some components, like MOSFETs, are typically rated this way (i.e., they give you some ridiculous max power dissipation for a *case* temp of 25C, which would only be achievable if you had it attached to a cold plate with liquid nitrogen running through it). However, I'm under the impression that resistors are not rated that way, especially those which are not designed to be heat-sunk. That is to say, if you really are sure that the ambient air will never be more than 25C, then a 10W resistor *can* actually dissipate 10W and live for its full normal rated lifetime. Someone please correct me if I'm wrong! Sean On Mon, Oct 3, 2011 at 11:14 AM, Paul Hutchinson wrote: > When thinking about using a power resistor as a heater don't forget the > power derating for temperature. The datasheet linked from the DigiKey pag= e > has the derating specs a bit out of place (just above electrical instead = of > below power). The spec is linear 100% @ 25C to 0% @ 275C, so @ 150C the > power handling is only 50% (5 Watts for the 10 Watt version). > > Paul Hutch > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .