Do I have enough heatsink for 4 CXA3070

I have 2 heatsinks that I want to put together, that gives me a surface area of 11"x11". Can I safely put 4 100w (CXA 3070) on it? I was thinking about using 4 120mm 12v fans on top of it. I saw a video on YouTube that showed a guy making a led light that had no fans on the heatsink, and used the exhaust to vent the heat. Can a inline exhaust fan actually do that without burning out the cobs? Would that be considered a attractive light setup, or is it still passive?

Growdad54 said:

I have 2 heatsinks that I want to put together, that gives me a surface area of 11"x11". Can I safely put 4 100w (CXA 3070) on it? I was thinking about using 4 120mm 12v fans on top of it. I saw a video on YouTube that showed a guy making a led light that had no fans on the heatsink, and used the exhaust to vent the heat. Can a inline exhaust fan actually do that without burning out the cobs? Would that be considered a attractive light setup, or is it still passive?

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Doubtful, unless they are an inch or two thick....do they have fins, are they solid blocks of aluminium?
what is the mass, aka how much does it weigh.....

Why CXA? Vero C's would handle that much easier....Running CXA 3070 past 75w's imho is a chore!

Their about 1" thick with fins. I chose the 3070 because of the watts. I'll have to get back to you on the weight.what about removing the heat with the exhaust?

cxa's are old tech, you can run LED's a lot hotter than most people here do, but there is an efficiency cost.

Growdad54 said:

Their about 1" thick with fins. I chose the 3070 because of the watts. I'll have to get back to you on the weight.what about removing the heat with the exhaust?

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How about a pic of the sinks?

The original pic from eBay. I'm a newbie when it comes to LEDs. My cab uses a 250w hps with a cool tube that vents out the cab.Trying to bring down the power consumption. I was thinking that it would be cool if I could vent out the heat off the heatsinks through the exhaust. I haven't got the cobs or the driver yet, so it's still in the planning stages.

Growdad54 said:

The original pic from eBay. I'm a newbie when it comes to LEDs. My cab uses a 250w hps with a cool tube that vents out the cab.Trying to bring down the power consumption. I was thinking that it would be cool if I could vent out the heat off the heatsinks through the exhaust. I haven't got the cobs or the driver yet, so it's still in the planning stages.

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You could certainly vent the heat as you suggest. How effective that will be is anybody's guess. I'm gonna say that heatsink is probably not going to be good for 200W passive though. 100W maybe. But with a couple of120mm fans on it, you could probably do it.

a heatsink with fan works great...i have 3590's @ 50w only small 4" sinks for three years [and not big fans]..the extra air movement is also good
the cobs stay really cool too

I use inline exhaust passive cooling works great for me but I use pcb GrowGreen not cobs

Thanks for the input guys, your help is much appreciated.On one of growmau video's he said that a active cooling needs only 6"x 6" per cob to cool properly,I thought by adding two together I would get all most a 12" x 12" surface. Would this type of cooling still be considered passive?

I'm pretty good at wiring and building, so i thought I'd try my hand at making a led grow light out sheet metal, that vents though the light. Would adding two 120mm intake fans on the back of the light make a difference? Who wants to pay a thousand for the same type of light.

You can use simple $10 to $15 PC heatsinks.

I was thinking about using single heatsinks on each light mounted on a aluminum 12" x 12" sheet. My build would be 12" x 12" with a 4" vent attachment on each side. Was thinking about putting up a build,so you can see how it's made.

Abiqua said:

what is the mass, aka how much does it weigh..

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Mass is not a factor.

Abiqua said:

Doubtful, unless they are an inch or two thick..

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Thickness has a negative effect on thermal management. If you look at the equation for heat transfer by conduction one coefficient is distance. Distance being the distance the heat flux must travel which in the formula is expressed as L (length) See blue circles below.




And here is the formula for the above. Notice that L is in the denominator meaning as L increases the conduction qx decreases. Where qx is the heat transfer from the LED, through base heatsink to the air.

Growdad54 said:

what about removing the heat with the exhaust?

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Calculating the heat transfer with your heatsink is complicated. I have attached a PDF with the formulas needed to calculate the heat transfer. If you have the heatsink and CoBs in hand, just try it and monitor the temperature.

In the CXA 3070 datasheet there is a graph on page 2 with the max case temperatures based on current. The max case temperature would be 90°C for operating at 100W.

Cree also states the max temperature of the Light Emitting Surface being 135°C.
http://www.cree.com/led-components/media/documents/ds-CXA3070.pdf If you have an infrared thermometer gun, you can monitor the LES temperature.

There is more information (a must read) in the CX Family LED Design Guide
http://www.cree.com/led-components/media/documents/CXA_design_guide.pdf

You can use the heatsink calculator provided by USA Heatsink:
https://www.heatsinkcalculator.com/calculator_heatsinkusa.html
You can also create a free use account on https://www.heatsinkcalculator.com




I found from this calculator the heatsink you specified will dissipate about 100 Watts with a heatsource temperature of 100°C.

The best practice is to have an effective passive heatsink without the exhaust air flow. Then if the exhaust fan fails the LEDs will not be damaged. So shoot for a case temperature of 90°C with out the fan then use the fan as well.

Given all that it is highly unlikely you can operate four of these CoBs at 100W. You would need substantial thermal management to dissipate 400 watts. Cree has a reference design where they use passive heatsinks for 550 Watts. I estimated the cost of those heatsinks to be over $200.
http://www.cree.com/led-components/media/documents/HorticultureReferenceDesign.pdf

Mass is definitely a factor......if you had the same heatsink, but one was 2x as dense....the "L" is increased.

See how long that 90C j-C temp lasts....

Abiqua said:

Mass is definitely a factor.

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Sorry, I understood what you meant, it was just a technicality. I have seen other posts of yours and I know you know what you are talking about. Technically even though it's not relevant to a heatsink, in thermal dynamics mass is typically used to calculate the time it takes for the temperature to change. Example how long will it take for an aluminum object with with a mass of M to change from a temperature of T1 to T2 with an ambient temperature of Ta. Or how long will it take an ice cube with a mass of M to melt in a volume V of water at temperature T.

For a heatsink mass would be a factor in how much time it would take for the heatsink to reach steady state thermal equilibrium.

Given the thermal dynamic formulas use length width and height for a material with a specific thermal resistance, mass is NOT a factor. If one dimension was unknown and the material was known (e.g.), then the missing dimension could be calculated from the mass. Still the mass is not directly relevant.

Abiqua said:

one was 2x as dense....the "L" is increased.

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Not necessarily. Density may affect the thermal resistance. And if the mass was 2X it would depend on where the new mass came from, like was it added to the fins, or the base or both.

Abiqua said:

See how long that 90C j-C temp lasts...

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My guess would be a millisecond as the temperature quickly rises from ambient past 90°C on it way to a much higher temperature. Keeping a 100W CoB cool is very difficult to do economically. I would use water cooling.

Growdad54 said:

I'm pretty good at wiring and building, so i thought I'd try my hand at making a led grow light out sheet metal, that vents though the light. Would adding two 120mm intake fans on the back of the light make a difference? Who wants to pay a thousand for the same type of light.

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That's what I would suggest.

GrowLightResearch said:

Sorry, I understood what you meant, it was just a technicality. I have seen other posts of yours and I know you know what you are talking about. Technically even though it's not relevant to a heatsink, in thermal dynamics mass is typically used to calculate the time it takes for the temperature to change. Example how long will it take for an aluminum object with with a mass of M to change from a temperature of T1 to T2 with an ambient temperature of Ta. Or how long will it take an ice cube with a mass of M to melt in a volume V of water at temperature T.

For a heatsink mass would be a factor in how much time it would take for the heatsink to reach steady state thermal equilibrium.

Given the thermal dynamic formulas use length width and height for a material with a specific thermal resistance, mass is NOT a factor. If one dimension was unknown and the material was known (e.g.), then the missing dimension could be calculated from the mass. Still the mass is not directly relevant.



Not necessarily. Density may affect the thermal resistance. And if the mass was 2X it would depend on where the new mass came from, like was it added to the fins, or the base or both.

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Produce the math, if we are comparing differences between 6063 Al and Cu, then I would agree, that is certainly density dependent as well as other factors...
Surface area is more important than mass, but if you have mass it will help cooling...seen it done in real time versus on paper......

My guess would be a millisecond as the temperature quickly rises from ambient past 90°C on it way to a much higher temperature. Keeping a 100W CoB cool is very difficult to do economically. I would use water cooling.

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Every one is an expert......why didn't you address the fact that I mentioned Vero C's...No you had to go straight for adding how you are an expert {not actually taking into account real world derivatives...] and that I am wrong but vaguely how.....

The vero c handles 100w at nearly Test current, let alone Max and is very economical to cool....but that is a difference of mass and material in slug makeup, one ceramic and older tighter dies versus the a larger Al slug......

Yes you are correct that older ceramic slugs are less economical in a cooling sense, but that has been common sense for awhile....

Abiqua said:

Every one is an expert.

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I was kidding, saying it was going to get very hot, much hotter than 90°C
BTW 90°C was the operating limit for case temperature at 100W (2.76 Amps).

Abiqua said:

Produce the math,

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I did see previous post. See attached conductionConvectionHeatsink.pdf and
[URL='https://www.rollitup.org/attachments/thermal-management-of-light-sources-based-on-smd-leds-pdf.4046524/?temp_hash=ff35ba9b66d72b4789ebdf8fd6a1c501']Thermal Management of Light Sources Based on SMD LEDs.pdf

Mass is relevant in fluid convection and conduction. Heatsinks are about conduction heat flow to convection and radiation surfaces. No matter how much mass is involved in the conduction heat flow it does no good if there the surface convection and radiation are ineffective.
[/URL]

The very basic thermal conductivity formula. Where q = thermal flux, k=thermal resistance, A = area, ∆T=temperature difference heatsource to heatsink, L=length. No mass, just thermal resistance.



And simplifies to just temperature delta and thermal resistance.





And the example for 250°C delta T, Rth =1.7 W/m K, L=0.15 m
Notice there is NO mass in the equation.

Abiqua said:

but if you have mass it will help cooling..

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Mass will slow things down from getting too hot. If you put a 400 watts of CoB on a 1 kg horizontal plate of copper, eventually you will end up with a very hot plate of copper.

Recently I was running some thermal experiments using a bar of copper 1.5" x 24" x 0.125" ( #2 in this image) with a 20 Watt CoB and it over heated. Problem was while there was plenty of mass there was very little convection (i.e. convection heat transfer coefficient) and very little radiation (poor emissivity and little free convection buoyancy-driven flow). Furthermore heat transfer rate depend strongly on whether the surface is hot or cold and on whether it is facing upward or downward. Free convection (passive) is very ineffective when the hot surface facing downward and the cold surface facing upward.

The entire bar got very hot because the convection and radiation were very ineffective. Great conduction does no good if there is no place to dump the thermal energy.






https://www.rollitup.org/attachments/conductionconvectionheatsink-pdf.4046490/?temp_hash=ff35ba9b66d72b4789ebdf8fd6a1c501