# 30 watt COB LED with no fan?



## YellowGTO (Dec 19, 2013)

Hey guys,
I want to build a custom track light in my office. I was wondering if its possible to use 30w LEDs with no fans? I was looking on Ebay at LED heatsinks and they all seem to say you need fans . This LEDs will be used as normal office lights, so they will be on for a few hours. I don't plan on encasing the heatsinks in a shroud or anything, so they will be open to dissipate heat


----------



## M79 (Dec 19, 2013)

Hi mate,

as always: it depends. 

Here is my opinion:

1. First thing is the size of your radiator. If the piece will be of the size of 2 midsized cpu radiators (so lets say 750g of aluminium with some copper core) then it is absolutely no problem. I think I would even try with a 400-500g piece (or even slightly lower but I don`t want be billed for your COB LED). While operating the radiator should be warm, even slightly hot will do, but your led life will be considerably shortened. 
2. Of course the design (optimized for passive convection) and material of the radiator matters - you of course prefer copper (silver, grafen and diamond coating are welcome too  )
3. Placing of the radiator (unblocked convection and fresh air)
4. THE LED. 30w quality led? Or a non labeled cheapo? Best of the best could achieve lets say 16w light output and 14w heat. Worst will do 8w light and 22w heat (numbers out of my head just for general reference). 
5. Led driving: of course the current settings will change the values from point 4. If you drive on very low current then the efficiency will rise, but you will have to use more leds to achieve the same light output.
6. Even led mounting is important. Radiators efficiency rises for higher temps (delta between radiator temp and ambient temp). 
Screwed led would transfer heat to the radiator more effectively, allowing to cool down the precious led core better and transfering the heat to the radiator (making the radiator delta temp bigger). Glued or screwed without thermal paste will let cook the led in its sausage, leaving it hot inside and not allowing to transfer the heat to the radiator. 

I hope I could help.


----------



## YellowGTO (Dec 20, 2013)

M79 said:


> Hi mate,
> 
> as always: it depends.
> 
> ...



Actually you replayed with everything I wanted to know lol. Copper core, screw mount is not a problem. Thanks a lot!


----------



## M79 (Dec 21, 2013)

You're welcome.
Just one more hint: use thicker (few mm) aluminium as your track. Connect the radiator thermally to the track; you`ll be really fine then.


----------



## Epsilon (Dec 22, 2013)

Yes, totally possible.

The thing is: Use heatsink which are designed to be used without a fan. CPU Heatsinks are not designed for that purpose and will not function great in that role. You want the natural convection of the air to do the work and this is less the case with heatsinks designed for usage with a fan.

IE more like this: http://www.lcaudio.com/index.php?page=323
Than :http://www.vantecusa.com/en/product/view_detail/132 (It says "passive" but it really is designed to be put in an air-flow of a 1U server).

The passive heatsinks are always rated in "Degrees/watt", normally in "K/W" which is Kelvin (the same as Celcius but with a different starting point). Which means that for every watt put in, the temperature is above ambient temperature by that much degrees in celcius/kelvin. The lower the number, the better.
I would aim at a operating temperature of around 60degress celcius. Which on hot days, means a 30degC above ambient.

With a heatload of 30W (conservative, it will be less) you will need a heatsink of 1.0 K/W. 

Example: http://www.conrad.nl/ce/nl/product/188013/ which is 0.68 K/W at 200*150*40mm
Example: http://www.conrad.nl/ce/nl/product/188009/ which is 1.07 K/W at 200*75*40mm

Both have a thick base to spread out the heat.
As you can see: considerably larger that a CPU heatsink in size. In this buisness, size and surface area matters, but only if convection can touch it. 

The mass is pretty irrelevant if you are running the light of extended periods of time. Aluminium and copper have similar thermal capacity (not conductivity!). Aluminium is 880J/kg*K, which means that for 0.5kg and 30W heatload, you will reach the 60deg almost in (880*0.5*(60-30) / 30W(= J/s)) = 440seconds. It will lose some heat due to the heatsink working, but the time is far below "a few hours".


----------

