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I am planning to build a light from 12 LEDs (CREE XML RGBW). These are 4 diodes on one star - a red, a blue, a green, and a white - with a total forward voltage of 13.9V at 350mA. Is there any kind of "slim profile" (i.e. max. 24-30cm long and max. 3 cm high) heat sink that could dissipate 170W? Also, shouldn't it really be able to dissipate whatever the max. forward Voltage is at 1,000mA (the max current drive these LEDs support), just to be safe? I have looked everywhere, and I haven't found any. I am working on using some of the calculators online, but so far I get different results than expected and I am also having some difficulty identifying some of the parameters (e.g. junction-to-case thermal resistance and thermal resistance of the heat sink in question).

For example, using this Celsia calculator, I set Q = 170, Tcase Max = 100 (the default, I don't know where to find this), Max Ambient T = 30 (because the light will be in open case suspended in air in my house), and I play with different values of Volumetric Thermal Resistance across the range in their table, I get heat sink volume estimates between 121 cm^3 (Rv = 50) and 1943 cm^3 (Rv = 800). A 30cm piece of the MakersLED HeatSink SLIM has a volume of 1801 cm^3. Based on the calculator, that heat sink should most likely be adequate for all 12 of my LEDs. However, the manufacturer recommends no more than 33 W per 1 ft (approx. 30cm) of heat sink. 33 W is very different from 170 W. I'm thoroughly confused. I'd appreciate some help, even if it's just links to resources that will help me figure this out for myself. Thanks.

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  • \$\begingroup\$ "just to be safe" Not if you never plan on running them at maximum brightness. \$\endgroup\$
    – DKNguyen
    Commented Jul 17, 2020 at 5:44
  • \$\begingroup\$ @DKNguyen, yes - you mean I SHOULD use the forward voltages at 1,000 mA, right? (I don't plan to ever run them a max. brightness, but I still want to build it properly.) \$\endgroup\$
    – Jess
    Commented Jul 17, 2020 at 5:51
  • \$\begingroup\$ You don't need to if you are never going to run it that hot. \$\endgroup\$
    – DKNguyen
    Commented Jul 17, 2020 at 5:51
  • \$\begingroup\$ Do consider bringing a fan into your solution. This is what the "Rv" value implies, and as you note yourself it has a tremendous impact on heatsink size (if the heatsink have adequate fin structures). \$\endgroup\$
    – EmilBonnevie
    Commented Jul 17, 2020 at 6:53
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    \$\begingroup\$ @po.pe "the efficiency of an LED is in the range of 70%" - Where are you getting that? A quick peruse of digikey for 95 CRI white LEDs yields efficiencies of 14% - 42%, which sounds typical to me. 70%? Maybe for one second in a Cree lab somewhere, but in practice? I don't think so, not yet. \$\endgroup\$
    – marcelm
    Commented Jul 17, 2020 at 7:43

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Your heatsink volume is wrong. I am not sure how you got \$1801cm^3\$ specifically, but that range implies you just multiplied the external dimensions of the heatsink together but that heatsink is not a rectangular block of solid aluminum with those dimensions. If I do that, I get \$1.5" \times 6.20" \times 12" = 1828.8cm^3\$ Eyeballing the photos, the actual volume of aluminum in the heatsink is probably 1/5th of that.

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  • \$\begingroup\$ Thank you...yes, after a night's sleep I see the problem with fresh eyes and it's very obvious. It's exactly as you said - the online calculator just finds the volume assuming it is solid aluminum, which obviously it is not. As for the calculations, it's because I was using 30cm x 15.8cm x 3.8cm, which is a bad idea (rounding error). Thank you for correcting these mistakes! \$\endgroup\$
    – Jess
    Commented Jul 17, 2020 at 13:58
  • \$\begingroup\$ @Jess And what'd ya know 1/5th of 170W is 34W. Very close to 33W. I swear I eyeballed it instead of working backwards. Volumetric Thermal Resistance is new to me though. \$\endgroup\$
    – DKNguyen
    Commented Jul 17, 2020 at 14:00

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