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Thread: Light

  1. #31

    Join Date
    Dec 2009
    Location
    Union City, CA, USA
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    441

    Re: Light

    No, never tried dual-inlet. I did not want to deal with all the extra pipes.

    Very interesting heat sink idea.
    I usually just increase the heat sink size until no fan is needed, but your idea has the advantage of pulling the hot
    air away from the tank as well.

  2. #32

    Join Date
    Dec 2009
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    115

    Re: Light

    It does that too. But it really came about as a cost saving measure.

  3. #33

    Join Date
    May 2010
    Posts
    57

    Re: Light

    Good idea on the heatsink. i think we are over heatsinking stuff normally tho there are some simple calculations to work out the thermal resistance needed of a heatsink. The use of fans greatly increases the the thermal resistance properties turing a 1C/W into .4 etc

    The warms white LEDs do work at growing algae though so i mite give them a try at first and see what the result is for a vertical scrubber

  4. #34

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    Dec 2009
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    Union City, CA, USA
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    441

    Re: Light

    For the curious:
    Heat sinks are rated in Degrees Celsius per Watt.
    LED junction temperature in Celsius = <LED wattage> * <Total System Degrees Celsius per Watt> + <Room temperature in Celsius>
    Total System Degrees Celsius per Watt = Heat sink C/W + LED bonding C/W + LED Star PCB C/W + Thermal adhesive C/W
    To make it worse, heat sinks are very different depending on air flow. Can be many orders of magnitude.

    EXAMPLE:
    Ambient temperature of a normal hot room = 80 deg F = 27C
    For longer life, and optimal light output, best to keep the junction temperature below 80C.
    My LedEngine Deep red.
    Watts = 2
    LED package = 5.5 C/W
    Star MCPCB = 2 C/W
    Ignore thermal compound and such.
    So 80 = 2 * (5.5 + 2 + <heat sink>) - 27
    Solve for heat sink = 3.5 C/W
    That is actually a tough requirement. It takes a decent heat sink to do that, especially with no air flow.

    However, junction temp can go up to 120 without any serious problems. Just shorter life.
    Solve for heat sink = 6.2 C/W. Much easier.

  5. #35

    Join Date
    Dec 2009
    Posts
    115

    Re: Light

    I'm not sure if the used watts of the LED is a perfect indicator. I'm assuming that a 3w led that puts out 100lm runs hotter than a 3w LED that puts out 300 lumens simply because more of the energy is turned into light as opposed to heat. Am I wrong on this? I'm also pretty sure that when you run a 3w LED at 2 watts your running at far more than 2/3rds the light and far less than 2/3rds the heat. I know when you overdrive an LED you certainly don't get 50% more light for 50% more wattage simply because of the junction temp. I know there are some great threads about this on nano-reef.com. thats where Evilcc hangs and a few other LED and electrical dudes.

  6. #36

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    Dec 2009
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    Union City, CA, USA
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    441

    Re: Light

    You are correct.

    With older LEDs, I remember the efficiency was on the order of 10% or so, so 90% heat.
    I checked the LedEngine Deep Reds, and they actually had some of the data.
    About 15% efficient at max 1.5A current.
    Maybe 18% at the low power I am running.
    I cannot find any data on the white ones.

    So yes, that 2W should be less. Probably 1.7, maybe as low as 1.3 or so.

  7. #37

    Join Date
    May 2010
    Posts
    57

    Re: Light

    essentially most white led's are blue (royal blue) LED's with phosphor coatings the cree XPE royal blue LED's at 350mA use 1.12W and output 425mW of light so thats about 38% efficient so only really really putitng out around .7W of heat. when driven at 1A uses 3.5W and outputs around 950mA so is only 27% efficient.

    harder you drive thigns the less efficient they become and more heat they produce. so it makes sense to dirve at lower currents and use more LED's then cost becomes a factor.

    The heatsinks i have been looking at are usually very good although once you load the heatsinks up with a fair amount of wattage they may struggle but from conrad heatsinks some of these while being more expensive then others have heat resistances less then 1 with the ones being made for forced airflow being alot lower around .4-.2. They are fairly expensive though as far as heatsinks go but using 6-8 LED on a heat sink compared with those nice trapozoid heatsinks rygh mentioned mite make them practiable

    http://www.conradheatsinks.com/products/flat_hd.html

  8. #38

    Join Date
    May 2010
    Posts
    57

    Re: Light

    Also i was looking at the centre entrance flow as a T piece using alot of glue then cutting the full length and only using 1 screen if the Tpiece is any good with an approiate amount of glue would be strong

  9. #39

    Join Date
    Dec 2009
    Location
    Union City, CA, USA
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    441

    Re: Light

    Wow, 38% efficient is really good.
    Of course, the phosphor absorption and re-emission is probably not 100% efficient. But I really have no idea what is is.
    So If it is that efficient, that 2W goes to 1.24, so the target would be about 5.6 C/W
    Which is good, since it is on par with the LED/PCB, which is around 7.
    So your fancy heat sink of .84 C/W free air would support up to 6 LEDs.
    FYI : I mounted 3 on each of those cheapo trapezoid ones.

    Agree that a T should be plenty strong.
    Suggest that you can leave a small 1/2" section with no slot, right at the T, then cut a small notch in the screen.
    Partly for strength. But also to deflect the water that is coming directly out of the T.

  10. #40

    Join Date
    May 2010
    Posts
    57

    Re: Light

    good point on the T piece would add to rigidity and give better flow distrubution. i have reworked some of my calcs and have found that 1 fancy heat sink is fine for 15 XPE and 2 XPG all running at .7A about 35 watts of energy

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