Rules of thumb for LED heat sink size? - The Planted Tank Forum
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post #1 of 14 (permalink) Old 11-21-2015, 02:59 AM Thread Starter
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Rules of thumb for LED heat sink size?

I've been reading through DIY LED threads, some of which were written long before LED aquarium lighting was really a "thing". Really neat reading if anybody's interested. Anyway, I'm trying to come up with estimated costs for a couple ideas, but I'm not sure about heat sink size.

One cheap option from heatsinkusa.com is 2" wide, with 12 fins that rise about 3/8" above the bottom plate. But I'm not sure how much surface area would actually be required. Is there a rule of thumb?

One idea I'm considering is a single strip using that size sink, with 2-3w diodes spaced 2-3" apart. You think that without a fan, and not in an enclosed space, that would be sufficient, or do I need longer fins? At max, that's 72w of power over 48", though probably less, and it's very possible that even if I did go through with this, I'll do it for a shorter tank.

This is one strip I'm considering:
2.079" Wide Extruded Aluminum Heatsink - HeatsinkUSA
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post #2 of 14 (permalink) Old 11-21-2015, 07:39 AM
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Quote:
Originally Posted by kevmo911 View Post
I've been reading through DIY LED threads, some of which were written long before LED aquarium lighting was really a "thing". Really neat reading if anybody's interested. Anyway, I'm trying to come up with estimated costs for a couple ideas, but I'm not sure about heat sink size.

One cheap option from heatsinkusa.com is 2" wide, with 12 fins that rise about 3/8" above the bottom plate. But I'm not sure how much surface area would actually be required. Is there a rule of thumb?

One idea I'm considering is a single strip using that size sink, with 2-3w diodes spaced 2-3" apart. You think that without a fan, and not in an enclosed space, that would be sufficient, or do I need longer fins? At max, that's 72w of power over 48", though probably less, and it's very possible that even if I did go through with this, I'll do it for a shorter tank.

This is one strip I'm considering:
2.079" Wide Extruded Aluminum Heatsink - HeatsinkUSA



Never found any good "rules of thumb" for heat sinks. Even different Al alloys have different thermal transfer properties..

There are "do the math" type approaches though if you have the necessary parameters..
Heat Transfer Resulting from Laminar Flow Over an Isothermal Plate

(for plain bar stock, no fins btw.)
But really not very "fun"...

Actually this is not too bad:
Considerations for heat sink design or selection:
  • Use heat sinks with the largest surface area (A) that is physically or economically feasible. As a general rule of thumb, for a well-ventilated heat sink, there should be 10 square inches of heat sink for every 1 watt of power dissipated. The use of pinned heat sinks, however, is not recommended.
  • Orient heat sink fins in a manner that allows hot air to flow upward and away from the heat sink and cold air to flow onto the surfaces of the heat sink.
  • Avoid constricting the airflow.
  • If possible, use natural convection to transfer heat from the heat sink to the ambient. Doing so avoids potential reliability issues of fans.
  • If natural convection is insufficient, then consider using fans, heat pumps, or liquid cooling elements that can dramatically increase the convection heat transfer coefficient and hence dramatically increase heat transfer.
  • Use heat sinks with surfaces that have high emissivity values.
  • Radiation is energy transfer by electromagnetic waves. By analyzing Equation 4, guidelines for enabling heat transfer through radiation can be developed.
  • Radiation heat transfer has a very strong dependency on temperature. The hotter the heats sink, the more significant the heat transfer through radiation. However, as the maximum case temperature of the LED Array is 105°C, heat transfer due to radiation is very small when compared to other heat transfer modes.
Effective Thermal Management of LED Arrays | DigiKey

12x (.464 x2) (fin area) + 2 x 2 (base area) =15 appox sq inches of al per 2x2 sq of heat sink..Which , using the above "guide" would only allow 1.5W of heat per 2" sq area of the heat sink..

At 3W each diode would need a 2"X4" area....BUT not all watts used is converted to heat...and not all LEd's are running at "rated" watts either (it will depend on drive current and voltage )
I do know that at 36" 7x3W (1000mA current) LED's will only heat 1 1/2" bar stock (no fins) to 120F.. Way below any critical diode temp..(even considering heat AT the diode will be higher)



Hmmm.. you may need to double everything.. fins have 2 sides..So a 2x2 sq may be fine for 3W..take your pick....

THEN there is this:
Quote:
If you want the LEDs to run a long time and be the most efficient, then 12 sq in per watt is a good "conservative" place to work. This will work for pretty much any LEd.

The other end of the thermal area needed - about 1 in sq per watt - is on the aggressive side, and will result in higher die temperatures, somewhat shorter life, and slightly less efficient. To use this much smaller approach, you need to be willing to acept some limitations, such as:

a) Your wife saying "wow - that is pretty warm". It won't be burning hot, but definitely warm.

b) Probably should limit yourself to Lumileds LEDs like a K2 or Rebel, as they are more heat tolerant.

Fins may or may not help you much with general lighting, depending a lot on how it is mounted. The way fins help, is that that the hot fin warms the air, and the air rises away from it, carrying away the heat. (or a fan blows the air through it) If you design limits air flow around the fins in any way, they are not very helpful at all.

Once again, as a quick and dirty "rule of thumb", you can assume the surface area of a fin contributes about 1/3 to the total area. Example - 3 in sq of fin is about like 1 in sq or flat area attached to the LED.

BTW - I was trained as an engineer and did plenty of thermal calculations with greater accuracy than this, but getting more detailed than this will not really help you that much more.

There is a certain amount of trial and error involved unless you are willing to hire a thermal design engineer to do the calculations. With modern software, these calcs are just amazingly accurate. Certainly I would do this if it were more than just a goof around the house hobby project.
http://www.candlepowerforums.com/vb/...for-power-leds

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Last edited by jeffkrol; 11-21-2015 at 08:10 AM. Reason: edit
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post #3 of 14 (permalink) Old 11-21-2015, 02:55 PM
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My first DIY lighting research claimed 9" sq. per 1 watt.
I like 10 better.
Due to using salvaged materials I have never reached 10.
Usually end up around 6.5" sq. per watt.

Always added fans but have yet to really need them.


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post #4 of 14 (permalink) Old 11-21-2015, 03:28 PM
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Quote:
Originally Posted by Maryland Guppy View Post
My first DIY lighting research claimed 9" sq. per 1 watt.
I like 10 better.
Due to using salvaged materials I have never reached 10.
Usually end up around 6.5" sq. per watt.

Always added fans but have yet to really need them.
Highest density w/ success was 1" x1" X 33" L channel 1/8 aluminum bar stock w/ 15 3W LEd's at 1000 mA
That is 1W at 1.4 sq inches.


not that it matters, but since the bar was laying around I thought I'd do a bit of real world testing..
Sooo I fired it up using 30v 1050mA driver.
Array is 5 in series/3 parallel strands.

So about 17v at 350mA ....17.85W total...

Bar is 75F
1800 LUX at 6" ......Guesstimating 28PAR...
66 sq in/ 17.85W = 3.7sq.in./Watt

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Last edited by jeffkrol; 11-21-2015 at 04:36 PM. Reason: edit
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post #5 of 14 (permalink) Old 11-21-2015, 06:40 PM
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Quote:
Originally Posted by kevmo911 View Post
This is one strip I'm considering:
2.079" Wide Extruded Aluminum Heatsink - HeatsinkUSA
The price of $.32 per inch is quite palatable.
@ 48" $15.36, maybe just double up on heat sink material.
Split the # of lights on 2 pieces?

Are you a fan of evilbay and china?
Saw some good deals there in the past.


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Last edited by Maryland Guppy; 11-21-2015 at 09:52 PM. Reason: mistake on foot vs. inches
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post #6 of 14 (permalink) Old 11-21-2015, 09:30 PM Thread Starter
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Quote:
Originally Posted by Maryland Guppy View Post
The price of $.32 per foot is quite palatable.
@ 48" $15.36, maybe just double up on heat sink material.
Split the # of lights on 2 pieces?

Are you a fan of evilbay and china?
Saw some good deals there in the past.
Yeah, that's what I was thinking. And I have no problem ordering through Evilbay. Stevesleds.com uses square tubes they sell with a blower that slides into one end. I think it was like $15 for a tube and another $15 for the blower. Also interesting.

I was trying to do some vague math, and with the 1/3 fin area value jeff's link suggested, plus the plate, it was something like 166 sq in for a max of 2.3 sq in per w (for 24X3w diodes). Not great. Doubling up plates would be decent.
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post #7 of 14 (permalink) Old 11-22-2015, 12:02 AM
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Yeah, that's what I was thinking. And I have no problem ordering through Evilbay. Stevesleds.com uses square tubes they sell with a blower that slides into one end. I think it was like $15 for a tube and another $15 for the blower. Also interesting.

I was trying to do some vague math, and with the 1/3 fin area value jeff's link suggested, plus the plate, it was something like 166 sq in for a max of 2.3 sq in per w (for 24X3w diodes). Not great. Doubling up plates would be decent.
Estimating Parallel Plate-Fin Heat Sink Thermal Resistance « Electronics Cooling Magazine ? Focused on Thermal Management, TIMs, Fans, Heat Sinks, CFD Software, LEDs/Lighting

Quote:
and Afin is the surface area per fin taking into account both sides of the fin.
1/3 seems low.. According to this:
fin heat transfer area:

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post #8 of 14 (permalink) Old 11-22-2015, 08:09 PM
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Quote:
Originally Posted by jeffkrol View Post
Highest density w/ success was 1" x1" X 33" L channel 1/8 aluminum bar stock w/ 15 3W LEd's at 1000 mA
That is 1W at 1.4 sq inches.


not that it matters, but since the bar was laying around I thought I'd do a bit of real world testing..
Sooo I fired it up using 30v 1050mA driver.
Array is 5 in series/3 parallel strands.

So about 17v at 350mA ....17.85W total...

Bar is 75F
1800 LUX at 6" ......Guesstimating 28PAR...
66 sq in/ 17.85W = 3.7sq.in./Watt
temps of the heat sink????

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post #9 of 14 (permalink) Old 11-22-2015, 10:14 PM
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temps of the heat sink????
75............

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post #10 of 14 (permalink) Old 11-22-2015, 10:21 PM
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75............
cool...literally!

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post #11 of 14 (permalink) Old 11-24-2015, 02:26 AM
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Bigger is always better, that being said unless you have 20 3w leds tightly spaced on a heat sink you can get away with using auminum u channel or bar stock, me personally I just use 1 inch or 3 quarter inch thick stock but I usually play with high output leds. I never found I needed a heavily finned heat sink for a 3w system just bar stock

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post #12 of 14 (permalink) Old 11-24-2015, 02:48 AM
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Bigger is always better, that being said unless you have 20 3w leds tightly spaced on a heat sink you can get away with using auminum u channel or bar stock, me personally I just use 1 inch or 3 quarter inch thick stock but I usually play with high output leds. I never found I needed a heavily finned heat sink for a 3w system just bar stock

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Yea, mass vs surface area.. A story as old as the hills..
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post #13 of 14 (permalink) Old 11-30-2015, 05:24 PM
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copy and paste job...I'm not this smart..

Just as the electric current I in a resistor results from the potential difference ΔV across a resistance R in an electrical circuit, thermal power flow PD produces a temperature difference ΔT across the thermal resistance in a thermal circuit.

Solving the thermal equation given in the figure gives T1 = PD • θ + T2. For the problem of heatsink design, T1 is the desired operating junction temperature Tj; T2 is the temperature of the ambient air TA; PD is the power dissipated by the semiconductor device, and θ is the total thermal resistance between the semiconductor junction and the environment; by substitution this results in the thermal equation Tj = PD • θ + TA.

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post #14 of 14 (permalink) Old 11-30-2015, 10:55 PM
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my rule of thumb is that most diy projects have oversized heatsinks. I think this is a good thing as it lowers the temp increasing lifespan of the emitters and lowering the risk of any fan failure causing burn out.
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