Stating the proven principles of accurate thermal analysis is hardly "muddling the knowledge pool".
No one has suggested that increased resistance leads to increasing heat generation in an infinite spiral as you mention. This would obviously be nonsense, and is not predicted by theory or observed in practice. As I mentioned in an earlier post, increased resistance DOES lead to increased heat generation, which DOES in turn cause further resistance increase ...but this does not spiral on to infinity for the very reason that the system is nonlinear. Thermal issues ARE highly nonlinear. What DOES happen is that a new higher temperature plateau is reached where (as I wrote in my original post on this subject): "The final temperature that the "system" stabilizes at, is reached when the logarithmically increasing (i.e. also very non-linear) heat transfer to the environment caused by increasing temperature, balances increased heat being generated." The bottom line is that heat transfer issues are unavoidably complex and require iterative solutions for accurate answers. That is why thermal analysis software is so hideously expensive, and requires such long times and high computer "horsepower" to converge on an accurate solution. While it may be possible to arrive at an approximate solution, for a limited set of parameters under a narrow subset of conditions by using rule-of-thumb simplification, it doesn't change the fact that an accurate solution is unavoidably far more complicated than you are presenting, and such a simplification must be understood to be just that: a simplification. Bob Wilson TIR Systems Ltd. Vancouver. -----Original Message----- From: Sam Davis [mailto:sda...@ptitest.com] Sent: May 14, 2002 8:57 AM To: emc-p...@majordomo.ieee.org Subject: RE: Constant for Change of Resistance formula. OK, I've avoided jumping in on this exhausted thread, but here's my take on the situation. It's obvious that more than a few of the posters have not performed this test. When you cannot answer a question with more than a theory, don't throw it out there as fact. You muddle the knowledge pool. First off, why go to the trouble of performing the CoR measurements on a connector, when you can fairly accurately (and much more simply) use a thermocouple. If you decide you need the accuracy of the Change of Resistance measurement, the equipment used for this test is specialized and highly precise, (it's a milliohm meter, not your standard DMM). The meters I've used have been capable of measuring fractions of milliohms, down to microohms. And you do end up measuring the resistance of the leads. That's why you measure them separately, in the ambient, and subtract them from the system resistance in the formula to get the resistance of the EUT only. Also, there is specially designed equipment to perform this test while the EUT is energized, but the normal method is to run the EUT until thermal stabilization, disconnect power, and measure the resistance as it drops over time, and extrapolate back to time 0. The smaller the EUT, the faster you need to get the first measurement, and subsequent measurements because within seconds, the EUT could drop significantly, making your extrapolation inaccurate. And about the resistance to temp rise to resistance rise to temp rise - if it went on infinitum, all conductors (not just those under test) would eventually ignite. This only happens when you allow too much current. Sam Disclaimer - Sorry if I stepped on any toes, but I've got big feet. ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list" ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list" ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
