Hi Stephen, I was hoping to buy a little time and prevent scorching and self-desolderin= g by using 5W instead of 2W resistors. But when I compared the spec sheets = between the two resistors, at 2 watts the temperature rise is very similar.= I don't really see how that can be, the 5W resistor is larger so I'd assum= e it would run cooler at the same power levels. In some configurations there will be a thermal cutout on the power transist= ors, we'll see whether that activates before or after the resistors fail or= do board damage. Thanks, Bob ________________________________________ From: piclist-bounces@mit.edu on behalf of stephe= n.forrest Sent: Monday, January 22, 2018 3:02 PM To: piclist@mit.edu Subject: RE: [EE] Resistor temperature rise Damned physics! ;o) Time is the key here - you mention headroom for temporary overloads. What d= etects the overload? What interrupts the overload? How soon does it operate= ? What is the normal load? Assuming that the normal load keeps the temperature rise within acceptable = bounds, the time taken to interrupt the overload will determine the tempera= ture rise you see in the resistor during the fault. If that time is indefin= ite, you need to deal with the excessive temperature through heatsinking or= airflow or both. It is hard to get around P =3D I^2R. The thing the 5W buy= s you is some time. The greater heat capacity of the body, the greater surf= ace area, etc. etc. means the time to reach critical temperature will be lo= nger. Given that this is for surviving fault conditions, I would be reluctant to = add heatsinking just for that (COG, reliability, manufacturability etc.). R= ather , add earlier detection and interruption of the fault. If critical, w= ith some redundancy (COG ;o) ). As an aside, while it does provide extra cooling to the resistor, the anoth= er (main?) reason for mounting power resistors off the PCB is to limit scor= ching of the board and possible delamination. This can happen well before s= older melts. Stephen --=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 .