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 amount 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 case 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 that there are no other significant sources of heat in the vicinity of the resistor AND the air temperature in the enclosure (if there is one) is not much above 25 C even considering the dissipation from the resistor. This includes the PCB traces leading to the resistor - they must be large enough that they have negligible heating from the current passing through them. I had a board once where the (low value current sense) resistors lifted the traces 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 trace width calculators but both of those ratings assume that their contribution is by far dominant but in this case, the heat from the resistor heated the copper just enough that it went into thermal runaway at the continuous operating current. 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 can't > push more than 2.5 watts out of them? I've looked at a lot of datasheets > lately, and none of them make any mention at all about connection or > mounting methods. > --=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 .