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Thermal Design Comparison of Heat Sinks

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  • Thermal Design Comparison of Heat Sinks

    For LED's which are less sensitive to high heat than many of our laser diodes, I'm seeing advice on the net to have at least 1 square inch of heat sink (no fan) area per watt, some individuals recommend 3 sq. inches per watt. I need to find more info in regard to this to tell me what kind of temperature that amount of heat sink area will keep a laser diode temperature limited to, on a watt per watt basis.

    More info, download Here:

  • #2
    A good YouTube video regarding heat sinks:


    • #3
      I've finally found the figures to show why heat sinks often use a copper core surrounded by aluminum at several times the mass, these figures speak for themselves why:


      Thermal Conductivity of Copper 397.48 W/m*K
      Thermal Diffusivity of Copper 116 mm2/s
      Specific Heat Capacity of Copper 385 J/k*K


      Thermal Conductivity of Aluminum 225.94 W/m*K
      Thermal Diffusivity of Aluminum 91 mm2/s
      Specific Heat Capacity of Aluminum 921 J/k*K

      I'm adding water too, this shows why water is such an effective coolant due to how much heat it can hold:


      Thermal Conductivity of Water 0.603 W/m*K
      Thermal Diffusivity of Water 0.14 mm2/s
      Specific Heat Capacity of Water 4184 J/k*K


      • #4
        I am posting the following here because after an exhaustive search of the net I could not find any information regarding how well a naked heat pipe can cool when used as both the heat pipe and the heat sink itself, in other words, no fins, without the normal heat sink interfaced to the pipe which acts to cool the pipe enough that the end of it (or length) becomes a condenser, but that does not mean the vapor inside pipe will not condense without an external sink coupled to it and keep a device at a low enough temperature without one (if the amount of heat it produces isn't too high). What is key whether this can be done without a heat sink on the end of the pipe is whether the surface area of the heat pipe alone is large enough to keep your device at a low enough temperature all by itself.

        Click image for larger version  Name:	9F762AFA-09B7-4ADA-ACAB-93E03A389C7A.jpeg Views:	1 Size:	143.4 KB ID:	812
        Heat pipes themselves are not normally used to couple a heat producing component directly to the pipe alone without adding a heat condenser to the pipe, but it can be done, as I will show in the next paragraph, below. The normal method is to embed the heat pipe into a metal plate of some kind which has the heat producing component mounted to it and then some distance away attach aluminum fins to the other end of the heat pipe where the heat can be transferred to the air with or without a fan.

        Compared to natural convection, using a fan to force air across or through the heat sink fins allows 3 to 4 times the amount of heat to be removed. This applies to all heat sinks which transfer the heat to the air whether heat pipes are used or not. Just a small amount of airflow at only a few feet or liters per minute can help a lot, more than might be thought, even a small quiet low to moderate amount of airflow producing fan can help to remove far more heat than possible without it.

        The use of naked heat pipes as a heat sink:
        Heat Pipe used for these tests: Advanced Thermal Solutions ATS-HP-F8L300S21W-404

        This experiment is a mock up of the heat produced by a NUGM04T 1.3 watt (1.6 W over driven) 525 nm green laser diode which can produce about 14-15 watts of heat when driven hard.

        I did some tests of a flat 150 mm long 10.5 mm wide x 4 mm thick heat pipe today and found the heat dissipation using only the pipe itself as a heat sink was 5 watts at 58 degrees C., this was without any active air moving across it.

        Without active airflow the heat pipe would keep the resistor below 60 C. while producing 5 watts of heat, but could maintain the same temperature with a very moderate amount of active airflow flowing across the pipe when the resistor power was increased to 15 watts. The airflow was perhaps 15-20 feet per minute from a very cheap slow desk fan placed 12 inches from the heat pipe.

        Orientation Tests:

        Without active (fan etc.) airflow, at these power levels, I found this particular heat pipe made by Advanced Thermal Solutions Inc. does not have a problem dissipating the same amount of heat whether turned 90 degrees to the side, or even upside down, the cooling was about they same. Of course, better if the heat pipe is pointing strait up, but the difference was very minimal in any orientation.


        Note: The resistor used to produce the heat was a small 10 watt aluminum encased power resistor with flat bottom tightly interfaced to the bottom end on one side of the heat pipe with a very thin layer of ceramic heat sink compound between them wrapped in electrical tape to firmly hold it to the pipe.

        The temperature was measured using a 10K thermistor embedded on the resistor with heat sink compound (under the tape and on the side of it) using a Fluke DVM to measure resistance and compare the ohmic reading with a published table to know the temperature. The amount of heat the resistor produced was an approximation using the amount of voltage into the resistor multiplied by the current in amps to calculate watts.

        The heat dissipation capacity of the pipe is far less than what it would be with fins, but naked heat pipes can be used IF large enough or by adding enough of them together on the same plate. My plan is to use them to cool a number of laser diodes all grouped together in one laser pointer without adding a regular heat sink, just using heat pipes soldered into a copper plate and exposed to ambient air.

        PS: If soldering heat pipes, use low temperature SMD solder paste, normal temperature solder must be heated too high and can destroy the heat pipe. Alternatively, thermal epoxy can be used instead of solder, if thin enough won't make much difference compared to solder.

        Summary: Researching heat sinks, I've come to understand heat pipes are by far the best way to transfer heat out of a laser as heat pipes have a hundred times or more the heat conductance of pure copper which itself is considered a good heat conductor. Compared to copper, heat pipes are a heat conductor on steroids, so much better than copper alone, in comparison, copper is a poor conductor of heat so please consider their use with your laser pointer projects which produce a lot of heat. Not every heat sink needs them, but if you are moving a lot of heat, many heat sinks can be improved through their use, even if just embedding a heat pipe inside a solid copper or aluminum block without air fins the heat transfer within the metal from end to end will be greatly improved to more evenly disperse the heat.

        For my own project I will either use three heat pipes per laser diode without a fan, or one per diode with a fan or a shorter lessor number of pipes if using heat sink fins. Next I will experiment with a single heat pipe with a copper fin added to see how much fin is needed with a single pipe to dissipate 15 watts of heat to the air without a fan, as soon as I get the parts. Of course I could just email or call the manufacturer of these pipes asking for assistance, but as I am just a hobbyist and I don't like going to companies asking for freebies, offered or not, I will get the answers on my own. There are online calculators to determine the parameters when using a heat sink with the pipe, but so far I have not found such a calculator which expressly indicates they are useful without a heat sink on the pipe. Perhaps they are if you just enter a small enough sized condenser, but I am not sure they can be used for this so doing real world tests to see for myself.
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