Thermal handling of a resistor - Page 1
Forum section: electronics / projects

author: thexeno
author_rank: Regular Contributor
timestamp: September 20, 2015, 01:08:17 pm
content: Hi!I've found some confusion in datasheets of two kind of resistors. One of them is a 3W from Welwyn, the other is 0.5W from TE Connectivity.Welwyn is here:http://www.farnell.com/datasheets/1449996.pdfAnd its highlight the ambient temperature range to be at max 200°C. Then, provides a graph "Temperature Rise VS Dissipation". Moreover, I assume (it is not sure) that the rise of temperature isaboveambient. This can be even of 300°C. Here the problem: with a given thermal resistance, I can design the system with a rise of 300°C and at the same time being compatible with the ambien temperature range of, let's say, 150°C in my system. In this way I am compliant with the datasheet and using a resistor at 450°C. Is seems strange to me.The same type of specification is given for the 0.5W resistor, except that the graph explicitly mention the "temperature riseaboveambient".Thanks!

author: rs20
author_rank: Super Contributor
timestamp: September 20, 2015, 02:17:42 pm
content: I agree with you that the Welwyn datasheet is poor; I can't see any data on there that would help me definitively figure out the power rating at 150 degrees C ambient either. In particular, there is no absolute maximum "Junction"/element temperature. However, I should point out a couple of things that rule out the idea that using the resistor at 450°C is safe:-- The power rating of 3W is given as "Power rating at 25°C" (ambient).-- The graph temperature rise VS dissipation obviously is above ambient, that what "rise" means. The fact that the line goes to 350°C doesnotimply that the resistor can safely be used at 350°C above ambient for any allowed value of ambient, the line goes that far for the benefit of people using this device at an ambient of -55°C.I'd email them, they should provide a proper derating curve. However, if I were to guesstimate, you can see that the power rating (under different requirements) is given as 2.9W at 25°C and 2.5W at 70°C, and extrapolate from there. Or, see that there's that 3.0W commercial rating at 25°C, figure out that the temperature of the element must be 25°C + 3 * 88 K/W = 289°C, so that must be the maximum element temperature. That leaves a power rating of (289°C - 150°C) / 88 K/W = 1.5W at 150°C ambient.

author: singapol
author_rank: Frequent Contributor
timestamp: September 20, 2015, 05:15:44 pm
content: Quote from: thexeno on September 20, 2015, 01:08:17 pmI think it's unrealistic to expect a electronic circuit to be working at 150C ambient. What the specs say is the temperature of an individual component, in this case it's a resistor. If you look at the power derating graph at maximum temperature the power rating would have been reduced to 10% of the rated power.For small or low rated power resistors and refering specifically to wirewound types the maximum temperature it will survive ( for a short time) is roughly at 200C where the wire will start to melt or disintegrate.What this hints at is the useful lifespand of the resistor. So in practice you would spec the resistor at least 10 times the power rating at the required ambient temperature. This is of importanceas wirewound resistors are employed for their surge capability where the instantaneous voltage and current will be many times the rated specs. So think about the number of on/off cycles this resistor is able to handle will give an appreciation of withstanding ability in addition to it's continuous workload. So in an instant the power rating and temperature would have been reached many times over.

author: rs20
author_rank: Super Contributor
timestamp: September 20, 2015, 10:34:38 pm
content: Singapol,Good job finding the derating curve. I don't know how I missed that. OP, that curve on the second page is your answer.However, Singapol, I find the rest of your message bizarre. Derating bya factor of 10? Presuming that the OP is using the device in an application that will expose it to 10x overloads? If wirewounds are so great at handling surges, why would you need to derate them?

author: singapol
author_rank: Frequent Contributor
timestamp: September 21, 2015, 03:49:52 am
content: Quote from: rs20 on September 20, 2015, 10:34:38 pmThis is to add some insurance but I guess most people will try to save cost and space. BTW when talking about surges whatever quoted is just guess work like an impedance load, some say 6X and some 30Xbut given the number of activation cycles it all become accumulative meaning the resistor is sustainingdamage or what is described as "slow death" failure. Vishay 1% military grade wirewound is quoted as 10K hours life. So it has a useful limit if performance is a factor. So for normal grade 5% types is less depending on application. When there is a surge the stresses on the resistor is greater than most would think.

author: Richard Head
author_rank: Frequent Contributor
timestamp: September 21, 2015, 08:56:34 am
content: If the power resistor is mounted on an FR4 PC board them the PC board will almost certainly limit the allowable temperature rise as the board starts to go brown over time.I have found that with Philips AC05 and similar Dale/Vishay types a good start for max power is about 50% of their stated power capability. And that's assuming you raise the resistor several millimeters above the board. Lifting the component above the board allows a temperature gradient across the leads and helps avoid PC boards going brown and the solder crystalising. If forced cooling is used then obviously the situation changes.I have also found that if the resistor is used in pulse applications (such as in-rush current limiting) then the square cement types are to be avoided, as they tend to go open circuit. If you crack them open you'll find that the resistance wire is wound into a tiny coil with some glass fibre sleeving over it. Not ideal for pulse applications.The best type for pulse applications seems to be the WW type that's wound on a ceramic former (like Welwyn). The ceramic former allows rapid cooling of the wire avoiding very high local wire temperature.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 21, 2015, 11:22:48 am
content: Thermal rating is almost always an absolute limit (e.g. 3W), derated above some limiting temperature, down to zero at rated temperature (e.g. 200C).That way, the device temperature never exceeds its rated temperature, which makes sense.The absolute limit arises due to internal limitations, such as forming hot spots, or the thermal conductivity of internal materials and connections. (For example (not that this applies to most parts), liquid cooled heat transfer is limited by surface boiling, which must be avoided, or the conductivity suddenly becomes very low (vapor blanketed!), and destruction quickly follows. A similar effect might be expected in a solid part due to voids formed by expansion and cracking, though.)So for most part ratings, all you need to find are two points: the maximum power dissipation at maximum ambient or case temperature, and maximum case or hot-spot temperature (which is the ambient temperature where power will derate to zero). If you can find data consistent with this, you should be okay. Even if the datasheet doesn't explicitly say these because it stinks.And if you can't even find that... don't give them your business, select a different part!Tim

author: thexeno
author_rank: Regular Contributor
timestamp: September 25, 2015, 12:28:11 pm
content: Thanks to all. I will analyze my results and your ou suggestions as soon as I will build my proto circuit (in other words, when I will find some spare time).I've also send a clarification request to Welwyn, but I don't think that they will reply. But I've used all of their available contacts, who knows...In the meantime, other suggestions/interpretations are well accepted.

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: September 28, 2015, 01:08:55 am
content: A power resistor (not a precision wire wound) will have a maximum hot spot of 350°C, this includes whatever ambient temperature it is in. This temperature occurs at full power rating. At this maximum temperature, the alloy wire will not be above its rated operating temperature but for the alloy to retain its characteristics, the maximum temperature must be observed, too high of a temperature will change characteristics like the temperature coefficient of the resistance. In power resistors this TCR is rather high, in the 200PPM-400PPM/°C range in general depending on exactly what alloy is used. There are some power resistors with lower TCR but they cannot be operated at such high temperatures, usually no more than 200°C. Most power resistors are made with bare wire but there are some which use an enamel coating which can withstand more than 200°C operating temperature.Sand resistors such as the Welwyn you're asking about can not operate at as high a temperature as the enameled power resistors, but in both types, there will be a maximum full power temperature limit from which the power will be derated to zero. Exceeding any of these limits can either change the characteristics of the resistor or cause it to fail if you push it hard enough."And its highlight the ambient temperature range to be at max 200°C. Then, provides a graph "Temperature Rise VS Dissipation". Moreover, I assume (it is not sure) that the rise of temperature is above ambient. This can be even of 300°C. Here the problem: with a given thermal resistance, I can design the system with a rise of 300°C and at the same time being compatible with the ambien temperature range of, let's say, 150°C in my system. In this way I am compliant with the datasheet and using a resistor at 450°C. Is seems strange to me." As I said above, the maximum operating temperature, whether 150°C, 200°C or 350°C INCLUDES the ambient temperature, if your ambient is 50°C, then that is subtracted from the maximum resistor temperature and you must observe the power curve temperature at that point or you risk damage. There should be a power rating curve vs. temperature graph on the data sheet, it tells you just how much power you can apply at the ambient temperature.No you can't design a system with a rise of 300°C because no electronic component will operate reliably in that high of a temperature, vacuum tubes only ran at 200°C-225°C internally and the highest temperature solid state device barely runs 200°C ambient plus internal temperature. Why on earth would you be running a system at an ambient of 150°C in the first place? In no way would you be compliant with any datasheets of electronic components.While it is well and good that you at least came looking for answers, take some time and carefully read the data sheet and if that particular one doesn't seem to answer your questions, try a different manufacturer, all sand or enamel power resistors are similar no matter who the manufacturer is, try looking at Ohmite for example.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2015, 03:25:31 am
content: Quote from: Edwin G. Pettis on September 28, 2015, 01:08:55 amThis is only relevant as a historical note, really: typicalenvelopetemperature limits (= hottest spot on the glass or ceramic) were typically 200-220C for most (commercial life) tubes, from 150C or below for the most sensitive tubes, to 300C for "premium quality" hard glass types in extreme applications (like military transmitters or down-hole drilling sensors). The electrodes within were cooled mostly by radiation (with some help by conduction through pins, or also the fully conduction cooled types), and operated at 300-400C (partly because it takes that much temperature for radiation itself to become usefully conductive, and partly because of the temp rise above the glass*). Special transmitting tubes were made with different plate materials, intended to operate at red heat (graphite or zirconium, usually), which also served as the getter and depended on that temperature in operation.*Actually to put a finer point on it -- radiation goes as T^4, so thermal conductivity goes as 4 T^3. It's not actually meaningful to speak of a temp rise, with radiation cooling, because if you raise the absolute temperature some amount, the peak internal temperature rises much less than that amount, because the conductivity is so much higher at the new equilibrium. Whereas conduction through solids (and immobile fluids) is reasonably proportional, so it's useful to speak in terms of conductivity W/degC; convection cooled heatsinks are somewhere between linear and quadratic, but usually gradual enough that it's still useful (despite the general intractability of convection problems). Physics is interesting.Tim

author: thexeno
author_rank: Regular Contributor
timestamp: October 04, 2015, 05:18:29 pm
content: From Welwyn I still haven't received an answer.Anyway, if I buy a resistor rated at 3W and they say that at 3W I have an increment over the ambient of 300°C, I can't discuss it, I trust them. Morevoer, given the thermal resistance, I actually have an increment of such temperature with this power.Derating? Ok, I'm currently using it at 2.3W, which brings an increment of 264°C, as verified by the graph. They do not provides other info, like the ambient temperature from which these graphs are starting from. So at 20°C I still presum that it works. And I'm not surprised to see that working also on 80°C of ambient.It is running right now at 2.3W, I didn't measured the temperature yet, but, technically speaking, according to my finger is less than 200°C (a water drop initially evaporate but it stops almost immediately... a thing that do not happens at 200°C since the water evaporates more quickly).PS: the resistor is Vitreous Enamelled Wirewound type.