I guess there's a reason data sheets are (usually) anonymous? ... On 1/17/18, Bob Blick wrote: > A funny thing, though, is that I originally was using 2 watt resistors(me= tal > 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 datasheets= I > noticed that, at 2 watts, the 5 watt and 2 watt resistors both showed ver= y > similar temperature rise. I would have thought the larger size of the 5 w= att > 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 > term operation is not high power, I just wanted a little extra headroom f= or > temporary overloads. > > Cheerful regards, > > Bob > > ________________________________________ > From: piclist-bounces@mit.edu on behalf of Sean > Breheny > 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 pow= er > the resistor can dissipate continuously, in free air (i.e., not mounted t= o > 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 resisto= r > 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 th= e > resistor AND the air temperature in the enclosure (if there is one) is no= t > much above 25 C even considering the dissipation from the resistor. This > includes the PCB traces leading to the resistor - they must be large enou= gh > 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 trace= s > and the resistors would both generate heat. In other words, the resistor > alone was within its specs and the traces were sized properly according t= o > 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. >> > -- > 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 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .