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 -----Original Message----- From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of= Bob Blick Sent: Wednesday, 17 January 2018 5:28 PM To: Microcontroller discussion list - Public. Subject: Re: [EE] Resistor temperature rise A funny thing, though, is that I originally was using 2 watt resistors(meta= l oxide and also a different brand) and got these 5 watt wirewound ones hop= ing for a little cooler running. But comparing the graphs in their datashee= ts I noticed that, at 2 watts, the 5 watt and 2 watt resistors both showed = very similar temperature rise. I would have thought the larger size of the = 5 watt resistors would have had some effect. I have not measured the actual= temperature rise though, who knows. Obviously the wirewound resistors can = withstand a higher operating temperature than the metal oxide ones. Long te= rm operation is not high power, I just wanted a little extra headroom for t= emporary overloads. Cheerful regards, Bob ________________________________________ From: piclist-bounces@mit.edu on behalf of Sean B= reheny Sent: Tuesday, January 16, 2018 4:08 PM To: Microcontroller discussion list - Public. Subject: Re: [EE] Resistor temperature rise As you know, Bob, many component vendors do practice fuzzy specsmanship. However, I think resistors are rather mild in this case. For example, it is= standard practice for MOSFET vendors to rate their FETs according to the a= mount of current they could carry if you somehow magically kept the case at= 25 deg C.This means that they only have to account for the junction to cas= e thermal impedance as well as the heating in the leads of the package. In real life you will NEVER get away with that much current for more than a= split second. As I understand it, for through-hole resistors, the rating is the max power= the resistor can dissipate continuously, in free air (i.e., not mounted to= a PCB) with the surrounding air at 25 deg C, for some specified lifetime, = without changing its resistance value by more than X percent. A good and generally accepted rule of thumb is that you can use a resistor = at up to half its rated power, continuously, soldered to a PCB, provided th= at there are no other significant sources of heat in the vicinity of the re= sistor AND the air temperature in the enclosure (if there is one) is not mu= ch above 25 C even considering the dissipation from the resistor. This incl= udes the PCB traces leading to the resistor - they must be large enough tha= t they have negligible heating from the current passing through them. I had= a board once where the (low value current sense) resistors lifted the trac= es off the board because I failed to take into account that the traces and = the resistors would both generate heat. In other words, the resistor alone = was within its specs and the traces were sized properly according to PCB tr= ace width calculators but both of those ratings assume that their contribut= ion is by far dominant but in this case, the heat from the resistor heated = the copper jus! t enough that it went into thermal runaway at the continuous operating cur= rent. On Tue, Jan 16, 2018 at 4:17 PM, Bob Blick wrote: > So basically they sell 5 watt resistors but if you solder them you=20 > can't push more than 2.5 watts out of them? I've looked at a lot of=20 > datasheets lately, and none of them make any mention at all about=20 > connection or mounting methods. > -- http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/chang= e your membership options at http://mailman.mit.edu/mailman/listinfo/piclis= t --=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 .