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Trying to decide between two models and have the following questions:
Vishay’s CMF65 (RN65) MIL Spec power ratings for CHAR E (25ppm) components (up 125 deg.C) are half the rated power (0.25W). Can these model still be run up to 70 deg.C at rated power (0.5W), or do they have to be derated throughout the entire range?
Vishay’s CPF1 Thermal Resistance graph shows about ~32 deg.C heat rise at 0.3W; however, the device’s Thermal Resistance is listed is 85 K/W, which would yield 25.5 deg.C if its linear. Can the 85 K/W value be used in place of the graph?
Re. (1) I understand the question to be whether or not the Char. E parts rated for 0.25W@125°C can be operated under the 0.5W@70°C conditions at which the Char. D variants are qualified. I think the best answer to that would depend on exactly what a person meant by “can this model still be run…”
The different variations are essentially the same part for most purposes, but they’re produced, tested, and marketed on the basis of conformity to different standards. Could you buy a Char. E part and reasonably anticipate that it likely also capable of meeting the requirements set forth for the Class D variant? Yes. Is it Vishay’s problem if they fail to do so? No.
Re. (2), as a general principle the specification tables are to be considered authoritative if there’s ever disagreement between them and the charts, which are commonly under a heading like “Typical operating characteristics.”
That said, it should be understood that when speaking of heat transfer (convection in particular) there are a lot of variables that can cause performance in an end application to differ from the test scenario. Take the figures with a grain of salt, don’t expect more than about 1 significant digit of accuracy out of them, and verify performance in the context of the end application.
Thanks for the response. I am only interested in the specifications in an official capacity. I am not interested in may/may not work, user to test, or experimenting.
If I were to summarize then what you are saying is:
(1) NO. The specification of the resistor is 1/4W up to 125 deg.C. This means there is no Vishay specification that is manufacturer approved for 1/2W below this temperature.
(2) YES. The data sheet technical value takes precedence over graphs so if it is listed at 85 K/W, then that is the official specification used.
That’s a reasonable summary for the most part, but there may be some nuance getting overlooked that’s useful to have in one’s toolbag.
(1) The main issue here is that you get what you order: If you order a part qualified to meet/exceed Char. E, you get a part qualified to meet/exceed Char. E. Not Char. D.
Mil-spec is about demonstrated conformity to military specification, not actual part performance of any specific item. Because the two specifications differ, a single product that meets the joint criteria of both will still be offered as two different part numbers that are not considered interchangeable by Uncle Sam.
(2) Yes on trusting the tables over the charts, but there’s an important difference between characterization data and a “specification”. Because convection heat transfer is affected by user-defined parameters beyond the manufacturer’s control, thermal resistances to ambient should be understood as the former, regardless of how labeled.
