I agree with your assessment of this video, as far as the accuracy of the relative temperatures of the wires. However, it's still true that the center of the resistor is likely to be much hotter than the leads. Whenever I've had to deal with something like this, I've ended up attaching a thermocouple (or thermocouples) at the relevant points to directly measure the temperatures at the interesting points to validate expectations. I really wish I could find the excellent appnote or discussion in a textbook somewhere that I specifically remembered discussing the whole thermal environment around leaded power resistors, and how to deal with these issues, including how to predict the temperature at the leads. On Tue, Jan 23, 2018 at 4:53 PM, Sean Breheny wrote: > Nice demo video but please be aware that thermal images of unpainted > metallic parts can be misleading. A perfect mirror-like surface would > actually show the temperature of the surroundings, not its own temperatur= e. > A real typical clean metal surface will show a weighted average of its > surroundings and its own temperature, with the contribution from its own > temperature being about 30% of what a perfect blackbody would radiate at > the same temperature. A tarnished or oxidized metal surface can have an > emissivity which is much closer to ideal, typically 60 to 80%. However, i= t > is best to always use an organic material when measuring temperature > thermographically because almost all organic materials are within 10% of = a > true blackbody so you don't have to worry about calibrating the camera fo= r > your particular surface. > > To make a long story short, the leads of a resistor will usually appear > cooler than they really are in a thermal image. > > On Tue, Jan 23, 2018 at 4:55 PM, Forrest Christian (List Account) < > lists@packetflux.com> wrote: > > > Since this thread seems to be still alive, I'll add my $0.02: > > > > The degrees rise figure is the typically body temperature, not the > > temperature of the leads at the board, which be much cooler. Note > that > > on page 10 of the datasheet you linked it shows recommended lead > > termination bending, effectively offseting the resistor off of the boar= d > by > > 8mm. I was trying to find a good thermal camera picture to show this, > the > > closest was at https://www.youtube.com/watch?v=3D-e-qIjttIIY&t=3D134s ,= at > > 1:57 > > in, there is a thermal camera pic which shows the leads basically being > > cold, while the resistor body itself being hot. I wouldn't be surpris= ed > > to find that when mounted as recommended, the units will not desolder > from > > the board. > > > > Personally I try to run things as cool as possible, if I was engineerin= g > > for 5W dissipation, I would use at minimum a 7.5W if not a 10W resistor= .. > > Unless I was *very* sure of the engineering. > > > > Also, be mindful that the rating is at 40*C ambient *around the > resistor*. > > If you don't provide adequate airflow around the resistor to eliminate= a > > thermal air hotspot around the resistor, you're actually going to have = an > > even lower rating. > > > > > > > > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 *Forrest Christian* *CEO**, PacketFlux Technologies, Inc.* Tel: 406-449-3345 | Address: 3577 Countryside Road, Helena, MT 59602 forrestc@imach.com | http://www.packetflux.com --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .