Should I spray my heatsink black, or leave it silver?

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Hi,

I'm making a heatsink for my robots components. It'll be actually a simple plate of alluminium, cut down and drilled to certain mech. specifications. Now, will it be better (in terms of dissipating heat) if I spray it black (normal car spray, eloxation is not possible), or leave it silver (default surface)?

Ofcourse, I'd prefer it to be black, due to aesthetical reasons, however, if it would perform (considerably) better, I'd keep it normal.

Google gave me everything I wanted to know about cooling my PC with massive slabs of copper, homemade water cooling and even this: http://www.markusleonhardt.de/en... and I've seen that a lot of people leave the side of the component normal, and paint the rest of the heatsink black. Is that better?

What do you think?

The parts on the heatsing (isolated, don't worry ;-) ): 7805 converting 16-12V to 5V at approx. 100-200mA, 78S12 (or some low drop equivalent, any tips?) Converting 16V to 12V at max 1.8A.

Thanks,

David

PS: I know this is more of a mechanical question, or maybe physical, or something like that, but it has to do with electronics and not with AVRs, so the Electronics forum is a good place IMHO. :-)

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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In my opinion this is not a good idea. The paint will insulate the heatsink and could cause the components to run hotter. How much hotter is unknown. You could probably anodize it black and that should still allow the heat sink to work effectively. You can probably find what you need with google to anodize it at home.

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you have to use paint that can take the heat and make a good thermal connection with your chips connected to it. Normal heatsinks are anodized not painted. It's a bit different method (the color is not 0.5mm thick, but more like 2 atoms thick) and it costs a bit more than painting.

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If you spray paint your heat sink you will in effect. be placing a layer if thermal insulation around that heat sink, reducing its efficiency.

Looking cool may be visually appealing but, proper functionality is cooler!

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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Yes, black should radiate better. But, paint causes two problems. One is the boundary between the paint and the metal (which causes thermal resistance). The other is that paint, itself, is a poor heat conductor.

Now, if you were black anodizing, that might be beneficial.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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What an interesting question!

I believe that anodizing is quite simple but a little hazardous, using sulphuric acid or can be done with caustic soda, but I've never done it myself! Follow this with a dark dye in cold water then seal it in boiling water.

Are there any experts out there with home anodizing/dying (black) experience?

Good one David.

73's
Roy
VK5ASY

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Probably more important that painting for thermal efficiency, would be to use a thermal compound paste between the regulator and the heatsink.
And orientate the heat sink so that the fins are vertical and air can rise in a chimney effect.
A heat sink is only as good as the airflow over it.
Radiation is minor compared to convection. ( I hope that is not too opinionated!)

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Are you prejudiced against "black" heatshrink?

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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No indeed! Long polymer chains rule, but we are discussing a lump of ali, and whether painting it is cool - or hot.

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I agree with the importance of getting good thermal contact between the heat source and the heat sink. I suspect that this is far more important than color.

If the heatsink is oriented horizontally, then if there is any prevailing air flow direction, you want the heat sink oriented so that the air flows down the length of the fins. If it is vertical, then you want the fins vertical so that convective air flow is upward along the channels between the fins.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Thanks guys for the replies. So, in summary:

Spraying the heatsink is BAD, cause of thermal resistance of the paint.

Black things radiate heat better, though this advantage would be nullified, cause the paint acts as a isolation.

If the paint wasn't paint, but more like eloxation (most likely what you are refering to as anodization, it's a term we use here for making aluminium go black), then it would be best.

All this is less relevant than airflow. The heatsink will be placed like this:

           A
           |
 Robot direction forward

   H bbbbbbbbbbbbbbb 
   H b  Battery    b
   H b   pack      b
   H bbbbbbbbbbbbbbb


H - heatsink

In the end, I guess I'll do a test. If the heatsink will be cool at full operation, I'll paint it black abit.

Thanks.

David

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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The book by Motorola (AKA use our semiconductors and we will stop producing them!!!!) Titled: Linear/Switchmode Voltage Regulator Handbook, Theory and Practice, 1982, Has a table of Typical Emissivities of Common Surfaces.

Reproduced below.

SURFACE EMISSIVITY
Aluminium, Anodised 0.7 - 0.9
Alodine on Aluminium 0.15
Aluminium, Polished 0.05
Copper, Polished 0.07
Copper, Oxidised 0.70
Rolled Sheet Steel 0.66
Air Drying Enamel (any color) 0.85 - 0.91
Oil Paints (any color) 0.92 - 0.96
Varnish 0.89 - 0.93

So for me, I would use a Black Oil Paint, or anodized finish. The thickness of a painted finish is less likely to introduce a significant thermal impedance between the device and the heatsink, certainly much less than a silpad. Always use Thermal Compound to improve Case to Heatsink thermal transfer.

Ron.

 

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Here's another link on the topic with some useful information.
http://www.diyaudio.com/forums/s...

Tom

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> So for me, I would use a Black Oil Paint, or anodized finish.

That completely leaves out of consideration that unless your
heatsink is glowing dark red, the majority of the heat is transported
by convection, not by radiation. As many people outlined above,
the thermal resistance of oil paint would reduce the convection.

Jörg Wunsch

Please don't send me PMs, use email if you want to approach me personally.

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daqq wrote:

In the end, I guess I'll do a test. If the heatsink will be cool at full operation, I'll paint it black abit.

If the heat sink keeps cool at full operation you might not need a heat sink, your heat sink is overdimensioned, or the thermal coupling between the heat source and sink is bad.

You can in principle dimension heat sinks. However, with a home-made heat sink you lack one important information, the thermal resistance of the heat sink. This is a value (in Kelvin per Watt, smaller is better) which you get for commercial heat sinks. In fact, it is THE value characterizing a heat sink.

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Jörg wrote:

Quote:
That completely leaves out of consideration that unless your heatsink is glowing dark red, the majority of the heat is transported by convection, not by radiation.

Well think of the case of oil filled or water filled radiators used home heating. They are not too hot and do a great job of radiating heat around a room. Far better than a fan heater that just heats the air.

If you cant get a black anodised heatsink, mask the area where the semiconductors will be mounted & paint it black. That should resolve the additional thermal resistance between the case & heatsink.

Ron.

 

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Jörg wrote:

Quote:
That completely leaves out of consideration that unless your heatsink is glowing dark red, the majority of the heat is transported by convection, not by radiation.

Well think of the case of oil filled or water filled radiators used in home heating. They are not too hot and do a great job of radiating heat around a room. Far better than a fan heater that just heats the air.

If you cant get a black anodised heatsink, mask the area where the semiconductors will be mounted & paint it black. That should resolve the additional thermal resistance between the case & heatsink.

Ron.

 

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AFAIK, the reason why anodized aluminum has better transmision of heat is because anodization increases air contact surface by a means of rugosity. And usually aluminum is black anodized, but this is secondary due the low radiation/convection ratio, since temperature is too low.

Good thermal contact between components and heatsink, and airflow are much more important, specially the later.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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Don't forget that for radiation, 'black' is the new black if and only if it's black at infrared frequencies.

As others have pointed out, the heat transfer is by conduction to the heatsink, and then by conduction to the surface layer of air, and then by convection to move the air away carrying the heat with it. Radiation from the surface must be a minor component of the total.

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rberger wrote:

> Well think of the case of oil filled or water filled radiators used
> in home heating. They are not too hot and do a great job of radiating
> heat around a room.

No, like David's heat sink, they distribute most of their heat by
convection as well, not by radiation. You can feel their radiation
when you stand 1 meter away from them, but you can feel them heating
up the entire room even in the opposite corner.

Jörg Wunsch

Please don't send me PMs, use email if you want to approach me personally.

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Quote:
No, like David's heat sink, they distribute most of their heat by convection as well, not by radiation.

Noted.
Quote:
You can feel their radiation when you stand 1 meter away from them

Show me a heatsink that has a 1 meter clearance.

Objects gain and loose heat at a rate of k*DeltaTemperature. It takes longer for an object at 30 degrees to drop 1 degree in a 25 degree environment than an object at 60 degrees to drop 1 degree in the same environment.

That said, convection is simply a poor mans effort of forced air cooling. Airflow is the major factor in moving heat from a surface efficiently.

In the absence of airflow, Radiation is all that is left, as it leads to convection of air, therefore emissivity is still very important.

If the heatsink does not radiate heat, the air around it would not be heated to allow convection!

Ron.

 

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daqq wrote:
[...]
In the end, I guess I'll do a test. If the heatsink will be cool at full operation, I'll paint it black abit.

Seems reasonable to me. But since we all know you're open-minded for dangerous experiments, why not try to anodize it?
I've done it during my chemistry lessons in school, it was pretty easy. You'd have to imply some safety measures (gloves, visor), though. If it does not turn out the way you want it, you can still paint it.

Btw., anodizing is the english term for 'Eloxieren' as it is called in german.

Ingo

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I did the calculation for heat radiated from Aluminium using an emissivity of 0.1 from http://www.engineeringtoolbox.com/radiation-heat-emissivity-aluminum-d_433.html and using the T^4 relation (Stefan Boltzmans law for black body radiation).

I get 46W per m^2 at 30C and about twice that at 100C. Of course if your heatsink is near another surface it gets its heat reflected back and when that surface heats up it gets to be a radiator sending the heat back to you

A good surface emissivity is 3 times better.

I get 800 w/m^2 for a 70 degree difference from

http://www.frigprim.com/online/natconv_heatsink.html

Klave

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Quote:
Seems reasonable to me. But since we all know you're open-minded for dangerous experiments, why not try to anodize it?
I must say, that it IS tempting, however, chemstry was never my science. Although, this is a good time as any to start. :-D I'll think about it. Although mom probadly won't be too happy about sulphuric acid in the house ;-) If I disolve the carpet, I'll say, "The AVRFREAKS made me do it!" :-D

Thanks for the suggestions.

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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Don't do this indoors! No, really! DON'T!
The vapors will kill you.
you have to use either a very well ventilated room or move to outdoors.

Anyway! good luck ;)

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http://www.focuser.com/atm/anodi...

He makes it look complicated while claiming it's simple ;)

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There really must be a better (safer) electrolyte than sulfuric acid.

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Most likely BS, but: Instead of spraying it, how about using some CD writing pen (alcohol based pen, I don't know the correct term for it)? wouldn't it make a much thinner layer of paint? The surface will never be touched, so the mechanical resistance of the layer isn't a worry.

Thanks so far guys.

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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http://electrochem.cwru.edu/ed/e...

Phosphoric or Oxalic acid would be far more friendly than sulfuric acid. Would be even better if you could do this with orange juice or some other citrus fruit, but I doubt that will work correctly.