From: John Larkin
Subject: Re: luxeon star leds
Date: Mon, 25 Nov 2002 09:33:03 -0800
Organization: Posted via Supernews, http://www.supernews.com
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On 25 Nov 2002 09:11:46 -0800, firstname.lastname@example.org (Tim Shoppa)
>Victor Roberts wrote in message news:<3DE03D65.932BF9BF@RobertsResearchInc.com>...
>> Watson 'Atto Parsec' Name wrote:
>> > I don't buy this convection from the top because I'm using the Luxeon
>> > Star/O which has the whole top of the LED covered by the lens assembly.
>> You are certainly entitled to your opinion, but don't you
>> think that the lens assembly will have convective losses
>> when it gets hot? The lens assembly will, of course, get hot
>> under steady state conditions if the metal base is not
>> conected to a heat sink.
>OK, now I think I'm beginning to understand what you're trying to say:
>1. Heat conduction is proportional to temperature difference.
>2. Heat transfer by convection typically is superproportional to to temperature.
>3. After the thing gets hot enough, convection is how most heat loss is done.
>But here comes a bigger problem:
>> Considering that the lens assembly,
>> including its cylindrical holder, has a much greater surface
>> area than the top of the "bare" Luxeon Star
>In order to lose heat by convection through the lens, you've got to
>conduct heat through the lens first. And the lens is a pretty lousy thermal
>conductor compared to any reasonable heat sink.
>I certainly agree that convective cooling can be effective, but from
>an engineering standpoint you do it by first getting a good conductive heat
>sink/spreader and then using forced air if necessary.
>>, the "convective
>> effect" should be more pronounced for a Luxeon with the lens
>> assembly than for one without.
For a high power density thing like the Luxeon, you may as well ignore
topside convection cooling, lens or no lens. That will change things
in the safe direction by maybe a percent or so.