# Power resistor derating

I'm going to use a power resistor which needs to dissipate around 2.5W. I will use a resistor rated at 3W. I have a thermal impedance of 88°C/W. If the ambient is at 40°C, I assume that the resistor will go over 2.5W*88°C/W + 40°C = 286°C

Opening the datasheet, it allow an ambient temperature range -55 to 200°C. Then there is a graph showing the dissipation over the allowed temperature rise, that goes over 300°C.

If the resistor will go over 200°C, is it still ok? Can the resistor handle the temperature? (assuming that the tin will not melt). I should be able, by observing the next graph, which is the derating curve. In other words, how one should use the Temp Rise VS Power graph? Here an example datasheet: http://www.farnell.com/datasheets/1449996.pdf

EDIT: Searching around and viewing other datasheets, I've understood that one should see only the derating curve. It is considered to use resistor with its stated thermal resistance, while the temperature rise can be used to understand if the temperature achieved is suitable for you. In fact, the derating curve is shaded over the 200°C (max ambient) up to 315°C, because it is not guranteed a well behaviour of the materials. All resistor's datasheet are explained like this.

The graph you refer to is the ambient temperature versus derating curve. This doesn't apply to the power dissipated by the resistor BUT it does assume that the ambient temperature arounf the resistor is maintained. For instance if the resistor were in a sealed bix the ambient temperature is inside the box and, if you ran the resistor at 200 degC then pretty soon the local ambient temperature around the resistor would be 200 degC and all bets are off.

You have to provide thermal managemtn for resistors getting this warm and that either means an open construnction enclosure such that air freely moves in and out, a fan or some other form of cooling system.

The problem you have is interpreting the thermal resistance figue of 88 degC per watt - this figure may assume that the resistor is soldered down to a "standard" PCB with some significant copper tracks OR it might apply to the resistor before the leads are cropped. The data sheet doesn't say (I didn't see this bit) so contact the supplier; Welwyn are a reputable firm and their is likely to be further info available from their web site.

If the resistor will go over 200°C, is it still ok?

What does the ambient graph tells you - at 200 deg C you can dissipate 54% of the power - that's 1.62 watts and that would take the resistor up to 200 degC + 88x1.62 degC = 343 degC. Here lies two anomalies: -

• That graph tells you that you cannot dissipate any power in the resistor at an ambient of 315 degC
• The BS CE CC 40-201-002 requirements tell you that you can dissipate 2.5 watts at 70 degC. Given that TR is 88 degC per watt, it implies a resistor temperature of 290 degC.

You ought to clarify these with the supplier.

• Let's assume that I have a given configuration for the thermal resistance, provided by the supplier and coincide with 88°C/W when mounted on my system. You are assuming that when I say "If the resistor will go over 200°C..." it is in a sealed box, therefore the 200°C of derating apply. If this is correct, if I provide a method to keep the internal air temperature at a maximum 40°C, the resistor then can dissipate 2.5W rising to 290°C. If ambient goes up to 70°C, then the resistor can still work at max 320°C. Is it correct? Commented Sep 13, 2015 at 18:14
• @thexeno clarification from Welwyn is required because on one hand the total local temeprature (which includes self heating) can be 315 degC but on the other hand it can only be 290 degC. Clarification required. Commented Sep 13, 2015 at 18:40
• I've required a clarification. In the meanwhile, since it is the first time that I read a datasheet for a resistor, I read the same graphical information for every resistor's ds. It specify the temperature increase above ambient VS the dissipated power. In this case, if Tamb max is 100°C and the allowed increase is 150°C, I can use the resistor at 250°C. I'm quite sure that these assumptions are made assuming a constant ambient temperature of 100°C when the resistor and the system are at thermal regimen. Can you provide a confirmation to this sentence? Commented Sep 19, 2015 at 20:10
• I'm not overly impressed with the confusion brought about by conflicting numbers on the data sheet and you have my sympathy. I'd rather wait to hear what welwyn do actually say. Commented Sep 19, 2015 at 20:21
• They haven't answered. I tried all of their channels, maybe under their point of view it does not worth an answer. Anyway, for now I'm using it with a final "junction" temp of about 280°C. It is running right now. We will see how long :D Commented Oct 4, 2015 at 17:06