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Would 1212's measure hotter in the center of the chip than the outer?

Discussion in 'LED and other Lighting' started by Joe34, Mar 20, 2017.

  1.  
    Joe34

    Joe34 Active Member

    diagram.png

    Regarding Citizen 1212 Cobs.
    in above diagram where the center red = hottest, and moving outwards from there gets colder temps

    Is the above somewhat accurate? I dont own any chips to test.

    Thanks!
     
    Last edited: Mar 20, 2017
  2.  
    Danielson999

    Danielson999 Well-Known Member

    Since nobody has given you anything I'll bug ya a bit more.

    When we mount cobs on a larger (than the cob) heatsink, the laws of natural convection dictate that the heatsink will cool faster on the outside edges. So since our heatsinks are always hotter the closer you get to the center, one would think that the cob has no option than to be hottest in the center also. The difference in temps would be so minimal you might not be able to measure it.
    This is assuming you mount the cob horizontally. If you mounted it vertically the hot spot would shift towards the upper side of the cob because of the heat exchange occurring as the hot air rises over the sink. I think your diagram is spot on.

    Why are you interested in knowing specifics about this? just curious...
     
    Last edited: Mar 20, 2017
    Moflow and Joe34 like this.
  3.  
    Joe34

    Joe34 Active Member

    Hey, Those heatsink laws sound correct, however Im assuming the cob itself(regardles of heatsink) would be hotter in the center because thats where the largest cluster of leds are.

    Want to know for heat managment purposes for a horizontal mounted cob.
     
  4.  
    CobKits

    CobKits Well-Known Member Rollitup Advertiser

    id say no. the shortest heat dissipation path from the LES to the heatsink is a line perpendicular to the chip/TIM/heatsink interface. provided that the heatsink is of adequate thickness, this path is essentially the samr fo rall dies. ive youve ever looked closely at a chip, the dies are pretty evenly spread over the LES
     
  5.  
    Danielson999

    Danielson999 Well-Known Member

    You can't sidestep thermodynamics. As long as it's mounted horizontally to a heatsink larger than itself, the center is always going to be hotter. Pretend you ran the cob suspended in mid air with no heatsink, convection would suck air from below the cob around the outside edges creating a cooling effect. If you add a heatsink, it's just an extension of the cob and the same principles will apply. Obviously adding forced convection can alter results but the center of your cob is always going to be hottest but I doubt we could measure the difference from one side of the LES to the other. From one side of the heatsink to the other we can definitely measure though.


    Screenshot_133.jpg Screenshot_134.jpg
    [​IMG]
     
  6.  
    CobKits

    CobKits Well-Known Member Rollitup Advertiser

    somebody with a flir cam shoul be able to shoot a cob at low current and confirm. im sure there is a gradient but prob subtle
     
  7.  
    Danielson999

    Danielson999 Well-Known Member

    If anybody wants to read the mother of all heatsink books that includes everything from windtunnels, heatpipes, heatsinks, liquid cooling, design, thermal management and everything you could possibly think about....check out Advanced Thermal Solutions Inc book Qpedia Thermal Management – Electronics Cooling Book. It would take a year or two to read it all and an engineering degree might help but it's full of specific info to LED users. Best book out there. There are 4 chapters, each is expensive AF! Might want to check out the content of each because not all of them are necessary for most of us.

    Sorry 5 chapters, seen HERE
     
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  8.  
    Danielson999

    Danielson999 Well-Known Member

    Yeah, maybe with a larger cob it would be more evident.
     
  9.  
    CobKits

    CobKits Well-Known Member Rollitup Advertiser

    my money is still on the LES is somewhat isothermal. even in your heatsink example the entire area above the chip (near the chip mounting edge) is the same temp
     
  10.  
    Danielson999

    Danielson999 Well-Known Member

    Actually it's not the same temp, it's the same color. It's hard to read but the colors actually represent a heat 'range'. I know what you mean though but you still can't stop the heatsink from sucking the heat out toward the edges, leaving the center warmer. Obviously it will be so minimal which is why I originally asked why he wanted to know.
     
  11.  
    sixstring2112

    sixstring2112 Well-Known Member

    who cares lmao.but seriously,danielson that is some heavy stuff you linked and im still having fun watching that moving pic of the heatsink bongsmilie
     
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  12.  
    frica

    frica Well-Known Member

    I don't think the center of the COB would be the hottest since the diodes inside the board are all spread around evenly. (mostly)

    So thermal draw should be spread evenly around the LES.
    Or just about.

    But if you're talking about the heatsink, the part directly above the LES will be the hottest.
     
    Joe34 likes this.
  13.  
    wietefras

    wietefras Well-Known Member

    It's a Cree CXB3070 and not a Citizen 1212, but I don't see much of a gradient:
    2016-02-03 20.33.12.jpg 2016-02-03 20.34.09.jpg
     
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  14.  
    CobKits

    CobKits Well-Known Member Rollitup Advertiser

  15.  
    Danielson999

    Danielson999 Well-Known Member

    Pretty much exactly what we thought. Maybe less than ½ of a ° variance across the LES. But this does show in an excellent way how natural convection cools the outer edges. You can also see the different colors of white across the LES are kind of random, probably just something to do with the thermal paste thickness.
     
    Joe34 likes this.
  16.  
    Danielson999

    Danielson999 Well-Known Member

    @Joe34 there's a good example of how much cooler the outer edges are for your next build. Pretty drastic difference from the outside of the cob to the center.
     
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  17.  
    Joe34

    Joe34 Active Member

    Great pics Wiete! Its just wat I needed!

    thanks guys
     
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