# Bronze heatsink ???



## mnm99 (Jan 27, 2015)

I was wondering if the conductivity of Bronze for cooling underwater lights would be sufficiant. I know Aluminum has a much better rating, but these would be under water. I'm looking at a 7/8" thick 4" round puck. The center would be milled out 1/4" leaving the rest to transfer the heat. I'm using 24 3w led's encapsulated in epoxy. Do you think the Bronze will work good enough? My other option is 5086 Marine grade Aluminum, but I think the Bronze will last longer in saltwater.


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## more_vampires (Jan 27, 2015)

http://en.wikipedia.org/wiki/List_of_thermal_conductivities

Aluminum 200-250
Brass 125ish
Copper 350-400

Beyond the raw numbers of thermal conductivity is the surface area of the sink in contact with the water. I think that's the bigger part of the equation.

My impression? Yeah, probably could get away with it. More surface can trump thermal conductivity.


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## mnm99 (Jan 27, 2015)

more_vampires said:


> http://en.wikipedia.org/wiki/List_of_thermal_conductivities
> 
> Aluminum 200-250
> Brass 125ish
> ...



It's Bronze not brass. The Bronze is a little worse than brass at around 50ish.


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## more_vampires (Jan 27, 2015)

Hmm, yeah. It'd take even more of a lot of fins and surface area to the water...


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## SemiMan (Jan 27, 2015)

more_vampires said:


> http://en.wikipedia.org/wiki/List_of_thermal_conductivities
> 
> Aluminum 200-250
> Brass 125ish
> ...




Remember you don't need very deep grooves, a la heatsink fins to almost double the surface area. 

You are pouring the epoxy right onto the domes of the LEDS? Generally considered a no-no for long life. The soft silicone is to prevent thermal stress. Encase it in a hard brick, and you lose the benefit not to mention interaction of the hardeners in the Epoxy with the silicone.

Semiman


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## more_vampires (Jan 27, 2015)

...suppose it's futile to ask what temperature the water is. Got some family worked "heavy water" power generation tech, was a "banana suit" type person at one point.

Don't mind me. Proceed.


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## DIWdiver (Jan 27, 2015)

There are many types of bronze, including by some definitions, brass. What alloy are you talking about? If you know the thermal conductivity of it is around 50 W/m-K, that's sufficient.


What part of the puck is exposed to water?


If the back of the puck is exposed to water, then you might calculate, just as a R.O.M. (rough order of magnitude) the thermal resistance of a 1/2" square of bronze 5/8" thick. That's guessing that each LED has at least 1/2" spacing to the one next to it, and the 7/8" puck machined out 1/4" deep leaves 5/8" thickness. In truth this probably underestimates the area of the thermal path by quite a bit, which means my calculations will be too high.


c = conductivity = 50 W/m-K
A = area of thermal path = 1/2" x 1/2" = 0.125m x 0.125m
L = length of thermal path = 5/8" = 0.159m
R = thermal resistance, K/W = L/(A*c) = 0.159m/(0.125m*0.125m * 50 W/m-K) = 0.2K/W


Since your power is 3W, the temperature differential is 3W * 0.2K/W = 0.6K


This is less than one degree C (1K = 1 degree C), or close to one degree F. 


If the back is not exposed to water, and only the edge is, the calculation is very different. The value might be, R.O.M, 10 times higher. Still pretty low.


I'd say the bronze is okay.


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## lucca brassi (Jan 28, 2015)

> I'm looking at a 7/8" thick 4" round puck.


 = *1,62 kg*:thinking: ( OK- for pond lights )


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## BeastFlashlight (Jan 29, 2015)

more_vampires said:


> http://en.wikipedia.org/wiki/List_of_thermal_conductivities
> 
> Aluminum 200-250
> Brass 125ish
> Copper 350-400.


Man copper really crushes aluminum


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## mnm99 (Jan 29, 2015)

Copper cost 8X Aluminum... I know, wanted to use it then saw the price!


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## more_vampires (Jan 29, 2015)

SemiMan said:


> Remember you don't need very deep grooves, a la heatsink fins to almost double the surface area.



Reminds me how polishing the aluminum/magnesium engine of a BMW motorcycle voids its warranty. The rough textured surface dramatically increases surface area for air cooling.

Agree, fins and grooves aren't the only method. I had a brass CPU heat sink in the 90s that just plain didn't do it's job. So hot! Removing its coating and bead blasting didn't bring it to the performance of a rough-cast aluminum sink so I junked it.


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## FRITZHID (Jan 29, 2015)

I have this kicking around if you're interested......


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## Epsilon (Feb 2, 2015)

DIWdiver said:


> c = conductivity = 50 W/m-K
> A = area of thermal path = 1/2" x 1/2" = 0.125m x 0.125m
> L = length of thermal path = 5/8" = 0.159m
> R = thermal resistance, K/W = L/(A*c) = 0.159m/(0.125m*0.125m * 50 W/m-K) = 0.2K/W
> ...


Conversion from inches to meters is not correct (can happen) . I did not review the equations further, only recalculated the outcome.


c = conductivity = 50 W/m-K
A = area of thermal path = 1/2" x 1/2" = 0.*0*125m x 0.*0*125m
L = length of thermal path = 5/8" = 0.*0*159m
R = thermal resistance, K/W = L/(A*c) = 0.0159m/(0.0125m*0.0125m * 50 W/m-K) = *2*K/W


Since your power is 3W, the temperature differential is 3W * 2K/W = *6K*
========

6K is still nothing, but as DIWdiver said, it depends on where the puck is in contact with the water. If only the edge is in contact, this will mean a longer thermal path for the LEDS in the center.
If that is the case, I would recommend to use something different than bronze. Brass minimum (which conducts heat more than twice as good) and preferably something else. 

I'm not an expert in marine stuff, so maybe brass and copper are a no-go. What I do know is that metal ships often (if not always) have a sacrificial metal attached (zink/aluminimum), which is corrodes easier than the metal you are trying to protect (iron). If you are using aluminium, then the only sacrificial metal which is easily available is magnesium. Coating it might be good idea .


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## DIWdiver (Feb 2, 2015)




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## more_vampires (Feb 2, 2015)

Don't feel bad, DIW. I do it too. We still love you, sir.


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## mnm99 (Feb 2, 2015)

more_vampires said:


> Don't feel bad, DIW. I do it too. We still love you, sir.



Since you guys are so smart is there a calculation on what kind of temperature I should be getting off the backside when it's submerged in water? 24 3 watt led's with a 5/8" H x 3.25" W Aluminum heat sink? Oh and w hen I said you were so smart it was a compliment not a wise *** comment! Thanks for all your help. I wish I could post pictures easy here. I finished the light and had it running for almost 2 hrs under water and it's still going....


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## DIWdiver (Feb 2, 2015)

Congrats on getting the light working! Must be a thrill. Always is for me.

Never thought you were being a smart-***. You are always very respectful.

Sharing pictures isn't really that hard. It takes a while to get the hang of it, but after you've done it a few times, it gets easier.

First, you need an account at some sharing site. There are many free options available. I tried Picasa and found it to be too much of a PITA. Not intuitive at all. I already had a dropbox account, so tried that. Eventually figured out the trick. Photobucket seems popular, and allows you to share multiple pictures in such a way that viewers can scroll through them. I haven't used it, so can't comment. YouTube links are also popular. Can't comment on that either, except that posting video seems really cool.

I'm sure others have easier/better/much more sophisticated methods, but here's what I do:

1. Take a good picture, and get the picture on my PC.
2. Open in MS Paint, crop to show just what's interesting. Re-size to no more than 800x800 pixels to meet forum rules. Save with a new name. I usually save as a JPG, which seems to be fine. If you really wanted the best picture possible, BMP format is it. It just takes more data.
3. Copy to Dropbox. Once it's there, right click and select "Share dropbox link". This copies a link (URL) to the picture into your clipboard.
4. Go to CPF and open a new post. Type some text, or not. Wherever you want to insert the picture, put the cursor there. Click "insert image". Hit ctl-v. This pastes the URL of your picture from your clipboard into the dialog box. 
5. The trick to using Dropbox is that you have to change the URL. The part that says "www.dropbox.com" has to be changed to "dl.dropboxusercontent.com".
6. Click OK.

You're done! Photo should appear in your post editing window.


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## more_vampires (Feb 2, 2015)

Post deleted by request. Perhaps Semiman cares to explain percentage error of actual value versus calculated.

Thanks for politeness. If the values are different, then tell us why. Error analysis? Important Yes/No?


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## SemiMan (Feb 2, 2015)

Let me tell you a story. I once read a wikipedia post that said "This page deleted thereby greatly increasing its accuracy". Same would apply to the post before mine.

Posted by really crappy Tapatalk app that is questionable wrt respect of personal data.


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## mnm99 (Feb 3, 2015)

I tried photobucket. Click on links 

http://s270.photobucket.com/user/mnm1999/media/20150201_075706_resized-1.jpg.html?o=3

http://s270.photobucket.com/user/mnm1999/media/20150202_063846_resized.jpg.html?o=2


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## more_vampires (Feb 3, 2015)

mnm99 said:


>



 Wow, that's really cool! Thanks for the pics!


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## LED Boatguy (Feb 4, 2015)

I made underwater lights for years using 6061 aluminum (5000 series would be better) with the hardest anodizing available. No corrosion to speak of, but barnacles were a bear. The lights would overheat (thermistor set to 140 F) in ~10 minutes and shut off out of the water, but stayed nice and cool when immersed. Channels in the back for water flow was key. Good luck.

Here are some early ones. Those were XP-Gs back then. Imagine the lumens with XM-L2s. These were crammed with electronics and potted with de-gassed optically-clear "epoxy" for lack of a better word. Only one failure out of almost a hundred units made.


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## mnm99 (Feb 5, 2015)

Those look cool. I wish I had a mill..I was thinking about the aluminum and barnacles. If you bottom painted the case it would get even hotter. I see the problem there. I did mine this way for that reason too. I don't have a mill and I'm going to use my bottom paint on the housing, except the heat sink on the back. Well see what happens. It will be in the water for 6 months at a time, and I will be pulling it out a few times during that anyway. On my other post " How hot is too hot" I came up with these numbers
With the water at 21.6*C / 71*F , the light is holding at 46*C / 115*F....Lights running for 1 hr at 12.5v

With the water at 32*C / 90*F , the light is holding at 56*C / 133*F....Lights running for 1.5hrs at 12.5v

I don't have a thermistor. I'm trying to get the specs on the LED's today. Does this seem too hot? Why did you stop making them?


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## LED Boatguy (Feb 5, 2015)

mnm99 said:


> With the water at 21.6*C / 71*F , the light is holding at 46*C / 115*F....Lights running for 1 hr at 12.5v
> 
> With the water at 32*C / 90*F , the light is holding at 56*C / 133*F....Lights running for 1.5hrs at 12.5v
> 
> I don't have a thermistor. I'm trying to get the specs on the LED's today. Does this seem too hot? Why did you stop making them?



That should be fine. Back in the day, I ran torture tests on LEDs, running them for months at a time at temps WAY above the shutdown temps of my devices (140F), and never had one cook.

In lieu of a thermistor, you can use a thermal switch (Mouser, Digikey) and either connect it to a power lead (depending on your amp draw) or the enable pin of your driver (going from memory here).

Why did I quit making these lights? I did what I always try to do: turn it into a profitable company and sell it.


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## SemiMan (Feb 5, 2015)

mnm99 said:


> Those look cool. I wish I had a mill..I was thinking about the aluminum and barnacles. If you bottom painted the case it would get even hotter. I see the problem there. I did mine this way for that reason too. I don't have a mill and I'm going to use my bottom paint on the housing, except the heat sink on the back. Well see what happens. It will be in the water for 6 months at a time, and I will be pulling it out a few times during that anyway. On my other post " How hot is too hot" I came up with these numbers
> With the water at 21.6*C / 71*F , the light is holding at 46*C / 115*F....Lights running for 1 hr at 12.5v
> 
> With the water at 32*C / 90*F , the light is holding at 56*C / 133*F....Lights running for 1.5hrs at 12.5v
> ...




Most high powered LEDs can survive 10's of thousands of hours at 105C+ junction temperatures. That would be decades of life on your boat. At 56C and all metal construction, I can't see how you would be 50C between the exterior of the heatsink and the LED.

If it doubt, put in a resettable thermistor.

Semiman


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