DIY LED Build - Anodized Heatsink

[howyegettinon]

Hi,

All my parts and lights have arrived and I am about to start building my own LED lights.

My question is what is the MAX LED's I can safely use on my heatsink. I don’t have cooler fans at the moment but may look into it, if there is a big increase.

So how many LED's can I use without and with a cooling fan.

I will be using two Black Anodized Aluminium heatsinks 30cm x 12cm x 2cm

3x Mean Well ELN-60-48D 60W 48V 1.3A Power Supply LED Driver (Dimmable)

The lights that I have - I ordered extra as they were cheaper to buy in 10's
5x 3W 420nm - 425nm Super Actinic Violet Blue
20x CREE XP-E 3W Royal Blue LED 450-455nm
10x CREE XP-E 3W Warm White
10x CREE XP-E 3W Cool White


Also I will be using Arctic Silver Premium Silver Thermal Adhesive - is there any issues using this on anodized heatsinks?
Would there be any benefit to sand down the area for LED's to stick directly to the aluminium?

[mcgyvr]

The only way to really know is to build it and monitor temperatures via thermocouples for a minimum of 1 hour. As it can easily take that long to reach thermal equilibrium.

Typically one can estimate it IF all thermal resistances are known. But as this application involves multiple heat sources and one LED can easily effect the other the calculations can be fairly complex.

Temperature profiling is the ONLY way to know for sure.

No need to sand.. Just apply a VERY thin layer of adhesive (read the datasheet for the recommended application thickness)

[perkint]

What heat sinks are they? The dimensions are a very small part of the story. Depending on the profile, a 30x12x2 heat sink may have a thermal capacity varying massively!

Tim

[uncleof6]

Originally Posted by howyegettinon

Also I will be using Arctic Silver Premium Silver Thermal Adhesive - is there any issues using this on anodized heatsinks?
Would there be any benefit to sand down the area for LED's to stick directly to the aluminium?

My experience with heatsinks lay primarily in high power transistor applications. However, the same principles and rules that apply to these devices apply to all semi-conductors, including LEDs. The article I will link you to even devotes a paragraph to "led lighting" and how these lighting system generally break many of the rules, with a negative effect.

As far as using thermal adhesives, I won't use them, because part of the equation is pressure, and with an adhesive, you get 0 pressure. This just increases the thermal resistance.

Black anodizing is an excellent finish on heatsinks for improving radiation ability, and preventing corrosion. Anodizing results in significant electrical but negligible thermal insulation. It need only be removed from the contact area if electrical contact is needed. The anodized surface does allow for easy mounting of devices that require electrical isolation in low voltage applications, however before getting bold make sure the anodizing meets the specifications for the voltage in use.

If interested in learning the mysteries of heatsinks, more than you ever cared to know about heatsinks:

http://sound.westhost.com/heatsinks.htm

[rwb500]

Originally Posted by perkint

What heat sinks are they? The dimensions are a very small part of the story. Depending on the profile, a 30x12x2 heat sink may have a thermal capacity varying massively!

Tim

yes, this. with some more info on the heatsinks we can provide a guess.

[howyegettinon]

http://www.ebay.com/itm/321340702389...%3AMEBIDX%3AIT

Thats the heatsink I will be using, regarding presure I would much rather use a drilled heatsink but as my first project i will stick to the thermal adhisive for now may look at drilled ones once I move to a bigger tank and light.

[oreo57]

Originally Posted by howyegettinon

That's the heatsink I will be using, regarding pressure I would much rather use a drilled heatsink but as my first project i will stick to the thermal adhesive for now may look at drilled ones once I move to a bigger tank and light.

No need any fans for 15x3W or 40x1W LED

Seems like they determined about 40W is max for non-fanned.. I wouldn't doubt their recommendation too much..except to err on the conservative side w/ fewer LED's or run the LED's at a lower mA output..

As to anodizing itself.. I have no reason to doubt this at this point:

The primary reason to anodize is to produce a cosmetically uniform and very abrasion resistant surface. The coating is very thin but very durable and has negligible impact on the thermal conductivity. There will be a slight increase in emissivity, but it is negligible for most heat sinks. Only very hot, very large heatsinks with no forced air and small or no fins will be helped. Basically if you put any effort into improving convection it will be so much more important than radiation it doesn't matter.

Another advantage of anodization is that it can produce a very thin electrically insulating layer. It is possible to use this as the insulation between a heat sink and the tab of a TO-220 transistor. This is generally not done by itself because if you do manage to scratch through the anodization layer you can get a short circuit. If you are careful with manufacturing and test for continuity after you assemble it can be fine, but if you are likely to dismount and remount the transistors it is too risky.

Then again:

In natural convection a black or dark colored heatsink will perform 3% to 8% better
than an aluminum heatsink in its natural silverish color. This is due to the fact
that dark colors radiate heat more efficiently.

In forced air applications, surface color does not increase a heat sink's performance
due to the increase in convection. The color would provide cosmetic benefits only.

or:
http://www.eevblog.com/forum/project...h;attach=11032


http://www.irf.com/technical-info/appnotes/an-1057.pdf

[perkint]

45W would seem a fairly small amount of power for a heat sink that size, but then it does have a very thin base and the usual issue of 45W of one LED will put out a damn site more heat than 45W of a different LED, so it isn't the most helpful figure when it comes to heat sinks. What temp have they considered acceptable when coming up with that measure? What efficiency LEDs?

Tim

[mcgyvr]

Originally Posted by perkint

45W would seem a fairly small amount of power for a heat sink that size, but then it does have a very thin base and the usual issue of 45W of one LED will put out a damn site more heat than 45W of a different LED, so it isn't the most helpful figure when it comes to heat sinks. What temp have they considered acceptable when coming up with that measure? What efficiency LEDs?

Tim

FYI.. a heatsinks "performance" is directly related to surface area.. not "thickness"

45W does not seem a small amount of power at all for a LED heatsink of that size and natural convection (no fan/forced air).
Typically one would only shoot for a junction temp of 40-60 deg C max over ambient (assuming 25 deg C ambient) to ensure long LED life. Anything over that and lifespan decreases drastically.

A heatsink from a "professional" company will always state its thermal resistance.
As I stated above when all thermal resistances are known its fairly easy to estimate if one heatsink is sufficient for the heat load. But as with most "low cost/chinese" products they very rarely state a thermal resistance.

[rwb500]

yours looks similar to some of the Rapid LED heatsinks.

http://www.rapidled.com/as-is-6-x-10...num-heat-sink/


I have built a few fixtures with these. The one that I built for me (and is on at the moment) has 26 3W CREE diodes, all running at about half power. it is in open air with no fans and is barely warm to the touch (sides and tops of fins, there is a plexi shield on the bottom).

[perkint]

Originally Posted by mcgyvr

FYI.. a heatsinks "performance" is directly related to surface area.. not "thickness"

45W does not seem a small amount of power at all for a LED heatsink of that size and natural convection (no fan/forced air).
Typically one would only shoot for a junction temp of 40-60 deg C max over ambient (assuming 25 deg C ambient) to ensure long LED life. Anything over that and lifespan decreases drastically.

A heatsink from a "professional" company will always state its thermal resistance.
As I stated above when all thermal resistances are known its fairly easy to estimate if one heatsink is sufficient for the heat load. But as with most "low cost/chinese" products they very rarely state a thermal resistance.

I sort of agree with you, but - the amount of metal involved (which is obviously related to the base thickness) also affects both how much energy the heat sink can absorb before increasing temperature and also the number of thermal pathways for the heat to conduct away from the LED base. And yes, a junction temp of 40-60°C would be OK (i don't personally agree that this would be OK over ambient as the actual LED could easily be 20°C above the junction temp and that is getting a little hot!), but the listed item doesn't state that it is based on that!

And I certainly agree with you on thermal capacity - personally, for a critical application like main lights for a reef, i would not buy a heat sink that didn't come with a quoted °C/W...

Tim

[oreo57]

Originally Posted by mcgyvr

A heatsink from a "professional" company will always state its thermal resistance.

sooo where is this number for the "Makers" heat sink people love to use..????

[oreo57]

Originally Posted by mcgyvr

F
Typically one would only shoot for a junction temp of 40-60 deg C max over ambient (assuming 25 deg C ambient) to ensure long LED life. Anything over that and lifespan decreases drastically.

10,000hrs is 3.5 years @ 8 hours/day..



http://www.digikey.com/en/articles/t...mal-management

[mcgyvr]

Originally Posted by oreo57

sooo where is this number for the "Makers" heat sink people love to use..????

I said "professional".. Makers is not.

I'm talking aavid thermalloy/wakefield,etc...

Just because you start a Kickstarter thing and can get an extrusion made does NOT make you a professional heatsink company.

[mcgyvr]

Originally Posted by perkint

And yes, a junction temp of 40-60°C would be OK (i don't personally agree that this would be OK over ambient as the actual LED could easily be 20°C above the junction temp and that is getting a little hot!), but the listed item doesn't state that it is based on that!

um.. no.. junction temp will be the hottest.. Junction temp refers to the die junction in the LED package itself. You are thinking case temp.

The "junction" could easily be 20deg C hotter than what you can physically measure on the package of the LED (the case).
As junction to case thermal resistance is stated in the datasheet (for Cree LEDs anyways) its easy to calculate junction temp from the case temp.
I always mount a thermocouple on the body/case of the LED then calculate junction temp to achieve the 40-60deg C rise over ambient (with 25degC ambient).. Or Never let the junction get over 85degC.
And any fixture I've built I don't let get even close to that. My personal goal is 65degC MAX junction temp at 100%

[perkint]

I was thinking junction between LED star and heat sink, not between LED chip and the star, yes. Since that or similar is usually all you can (reasonably) measure!

Tim

[mcgyvr]

Originally Posted by perkint

I was thinking junction between LED star and heat sink, not between LED chip and the star, yes. Since that or similar is usually all you can (reasonably) measure!

Tim

Yeah "junction" is always inside the LED package or IC or whatever itself. I mount a TC on the actual substrate that the LED die itself is mounted on..
http://www.unionledlight.com/ewebedi...2217859501.jpg

[howyegettinon]

WOW! never even thought there could be that much thought and info behind a heatsink, thanks for all the info lads!!

I dont have anything to measure the temperature with, but my results are this....

After powering up all the LED's and running them for about 15-20 minutes the heatsink get too hot to keep you hand on it for too long, so I have ordered some 120mm PC fans to keep them cool. So untill they arive i am stuck just running the blues on their own to keep the temp low.

Here is the link to the build thread if anyone is interested..... http://www.reefcentral.com/forums/sh....php?t=2474729

[oreo57]

Originally Posted by howyegettinon

WOW! never even thought there could be that much thought and info behind a heatsink, thanks for all the info lads!!

I dont have anything to measure the temperature with, but my results are this....

After powering up all the LED's and running them for about 15-20 minutes the heatsink get too hot to keep you hand on it for too long, so I have ordered some 120mm PC fans to keep them cool. So untill they arive i am stuck just running the blues on their own to keep the temp low.

Here is the link to the build thread if anyone is interested..... http://www.reefcentral.com/forums/sh....php?t=2474729

you can decrease the current by 25% in the driver.. Probably would "cool" all channels enough.......
AFAICT you ar already over driving them at 1.3A. Normal rating is 1A AFAICT..

[howyegettinon]

Thanks I have the blue and whites running at 760mA and UV at 650mA but still alot of heat

[zachts]

violets in most cases shouldn't be run over 700mA absolute max! due to higher heat at the junction of those chips.

With the heatsink you linked fans will temendously increase thermal load capacity as they are ribbed fins, which with forced air flow give a large increase in cooling power. The downside is the base of the heatsink is thin which doesn't spread the individual led's heat out as efficiently as a thicker base heatsink so the fans and fins can't remove the heat as fast.

without getting into technical calculations if you can maintain a heatsink temp that allows you to keep your finger on it near the edge of the led star chip where it meets the heatsink for a long time without pain you are in a good place and way under what the led can tolerate long term.......violets often being the exception but Cree can run at temps that would burn you and have no issue. Just better to err on the side of caution and run some fans knowing that the Crees at least will be able to tolerate it for some time should the fans ever fail...

[howyegettinon]

Thanks zachts, I will make some andjustments once I get all the parts together, I am also going to add 2 24w T5's so I have some room to play with dimming the LED's down with out the tank getting too dark