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naperlou
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Blogger
Re: Expanding the use of CFD
naperlou   12/31/2011 5:41:36 PM
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This is a good example of two interesting trends.  One is the use of multiphysics in engineering analysis.  The other is the trend toward smart sensors.  As mentioned, thermal issues are a major cause of failure in LEDs and large scale integrated curcuits.  This is something the EE did not have to spend a lot of time dealing with in the past.  It is good to see Mentor Graphics addressing the issue and taking advantage of new sensor capabilities.  It makes life easier for all of us.

TJ McDermott
User Rank
Blogger
Re: This is sorely needed
TJ McDermott   12/23/2011 6:00:59 PM
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The one saving grace of these traffic signals is, with 75% of the LEDs nonfunctional, the light does in fact still provide enough light to be usable.

I do not know how long they've been in service, but I believe we can safely say it has not been 20 years.

Rob Spiegel
User Rank
Blogger
Re: This is sorely needed
Rob Spiegel   12/23/2011 2:41:30 PM
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That's a very surprising statistic, TJ. At the rate of 75 percent, I would think the city would reconsider either its supplier or the use of LEDs period. Given the difficulty of fixing and replacing traffic lights, I would imagine a more reliable source -- even if more expensive -- would pay for itself.

Beth Stackpole
User Rank
Blogger
Re: LED failures
Beth Stackpole   12/23/2011 7:26:19 AM
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Interesting point about some LEDs being better than others in terms of quality. I suppose that's true with any component. Any rough guidelines engineers need to consider to help them spec the proper LED supplier?

tjones
User Rank
Iron
LED failures
tjones   12/16/2011 12:10:29 PM
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The alterantives to consider for the heavy LED failure rate in the traffic light failure is that teh LEDs themselves may not have been quality LEDs from a reputable supplier. There are a considerable amount of low quality LEDs that do not perform as well as the more expensive offerings. Also, it may be the substrate that they are mounted on, the solder joint and several other failure modes. I worked in the automive industry on LED headlights and tail lights. I am sure you have noticed the dead LEDs in the tail lights of the truck ahead of you. The quality and durability of these LEDs and the assembly as a whole where not very good. They have improved and so will the LEDs and sub-assemblies in the street lights. Unfortunately early adopters are the guinea pigs for QC.

Walt
User Rank
Gold
Re: This is sorely needed
Walt   12/16/2011 9:43:54 AM
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Call me obsessive, but I’ve always thought an LED should last longer than a human if treated correctly.  A continuous light at 80K hours is around 9 years.  If we kept the junction under 80C or some other less severe level, could we get a lifetime (>100 years) from them?  Could they not be “the light that just does not burn out”?

nathatt
User Rank
Iron
Study?
nathatt   12/16/2011 9:15:17 AM
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Can you provide a link to this thermal study?

Charles Murray
User Rank
Blogger
Re: This is sorely needed
Charles Murray   12/15/2011 9:51:04 PM
TJ: A U.S. Air Force study has indicated that about 55% of those failures are caused by thermal issues.  

TJ McDermott
User Rank
Blogger
This is sorely needed
TJ McDermott   12/15/2011 10:15:28 AM
Cost savings has driven the change to LED lighting; they use less electricity and while expensive on the initial purchase will last much, much longer than conventional lights.

I'm in North Charleston, SC commissioning several projects.  North Charleston uses LED traffic signals.  Almost every single red and green LED array in the city has at least five LEDs that don't function.  In some cases, more than 75% of the LEDs do not turn on.  While LEDs do eliminate the single-point failure of incandescent bulbs, 75% failure is unacceptable.

I don't know if it's thermal problems of the LEDs or their circuit boards, environmental aspects of high heat and humidity of this region, or something else. but driving around North Charleston makes me think LED lighting still has bugs to stomped out.

williamlweaver
User Rank
Platinum
Re: Expanding the use of CFD
williamlweaver   12/15/2011 8:37:42 AM
Fantastic synthesis of Theory and Experiment. This interplay between experimentalists and theorists is just what is needed to get an accurate snapshot of the actual system. The theoretical modeling provided by CFD brings us very close to visualizing the system under study, but only when actual measurements are used to carefully calibrate and verify the CFD models. Calculating overall system performance using component values measured in isolation runs counter to the systems reality that the integrated system is often greater than the sum of the parts. Especially when it comes to energy efficiency, I hope we continue to see more stories like this one.

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