Yes you are correct and I should have written "ambient temperature" instead of just "temperature" in my first sentence. What I was responding to was the "How do these things fail?" opening that was quoted in my post. I was trying to point to a possible explanation for why the resistor would fail even when the nominal power rating was never exceeded. Let me, hopefully, clarify my point. When you use a resistor as a heating element you will either be unintentionally or, in the case of an enclosed space heater, intentionally increasing the ambient temperature. An increase in ambient temperature causes a decrease in power handling capacity for most if not all resistors. I've seen a number of cases where this effect is why a power resistor used as a heater causes the resistor to fail when used near its nominal power rating. The particular 10 Watt resistor that was pointed out has a derating factor of 0.4% per degree rise in ambient temperature. So that resistor would only have a 9.5 Watt capability with a mere 12.5 degree increase abov= e the rated ambient temperature. If you can ensure that the air entering this resistors heat sink never exceeds the 25 degree ambient specification then it should not fail with 10 Watts applied. Looking at what I just wrote, I'm thinking I may not have clarified the point well. I suck at writing quickly and I'm not going to spend the time i= t takes for me to write well. Paul Hutch > -----Original Message----- > From: piclist-bounces@mit.edu On Behalf Of Sean Breheny > Sent: Tuesday, October 04, 2011 1:00 AM > > 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 page > > has the derating specs a bit out of place (just above electrical instea= d 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 .