How do you factor heat-rise/soak in resistor power ratings? - Page 1
Forum section: electronics / beginners

author: bitbanger
author_rank: Regular Contributor
timestamp: February 27, 2019, 02:56:06 pm
content: Hi Folks -This question came to mind today and seemed like an interesting one to post.Say I have a series resistor on a DC line, with a power rating sized for steady-state operation. I model an inrush current as an exponential rise and log. fall.How does one size a carbon film, or wire-wound resistor for this inrush current? If I calculate current area under the curve, doI account for some 'soak' factor of the resistor in order to size it?Some wire-wound power resistors will have an overload rating ("5x power rating for 5sec) but this isn't always the case. Is there a general approach for this?Any thoughts are welcome!Thanks,James

author: David Hess
author_rank: Super Contributor
timestamp: February 28, 2019, 05:19:45 am
content: There are a couple of different limitations:1. Maximum breakdown voltage may limit peak power.2. Peak power may also be limited by formation of hot spots.3. Peak power also depends on thermal capacity which I think is what you are getting at.For power semiconductors this is more common but I have seen it for power resistors also. Look for a graph showing the relationship between peak power and pulse width.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 28, 2019, 05:59:14 am
content: When you get into (very) short time power overload factors it becomes ... difficult.Some resistors have specified and tested ratings, others may have no pulse power rating at all but may be just as good.I've attached a document about pulse rating from Vishay's CRCW-HP.https://duckduckgo.com/?q=CRCW-HP&t=opera&ia=webThese go upto 1kW for an 0805 resistors (Overload factor about 4000) for pulsewidths of 1us, and at such power ratings the maximum voltage becomes a serious limit.

author: bitbanger
author_rank: Regular Contributor
timestamp: March 05, 2019, 12:18:37 pm
content: Awesome info, thanks! Glad in a way it isn't as straight forward as I worried it was.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 06, 2019, 06:28:40 am
content: Yup, somewhere between energy handling and power handling, or both with respect to pulse width (for some typical pulse shape).There should be a short-duration overload spec, X times rated load for Y seconds or something like that. That gives you a rough idea of what kind of abuse it can handle.Mostresistors have a t^(-1/3) to t^(-1/2) relationship between energy handling and time (or the inverse for power). Of course, if you need to be sure, don't take my word on this, keep flipping through datasheets until you find one with the curve!This reflects the amount of time required for that heat to diffuse into the bulk of the part. Thick film is better than thin film; solid wire (wirewound) is even better; and bulk cermet / composite is better still.A very small resistor might have ~constant energy above a certain time scale (i.e., a pulse heats it up entirely, and it then cools down only very slowly), but energy handling almost always drops at short scales due to construction, or localized heating, or even skin effect.Tim