Cooling and heatsinks

Sagermaniac

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Feb 7, 2019
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Ok, so, since im new here maybe im missing something, but so far my experiments have indicated that with enough metal you can overcome the problem of overheating, so im wondering if there is a reason I am not seeing more custom lights with LEDs attached to enormous hunks of metal and/or more fan implementations? is it just because of a size/weight thing or is there something else to it? My next project is going to be something a little more ambitious, im working on modifying a Maglite 6-D cell unit, I want to use either 4 or 5 Cree XHP35's or one of those new massive Cob bulbs, and I was planning on filling like half the length of the unit with solid metal, probably a combination of copper and aluminum, I have not seen anyone else do anything similar, it always seems like people are trying to keep metal content to a minimum and before I started on this I thought I would see if there was something I was missing.
 

andyc

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Jul 25, 2016
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Large chunks of metal will take longer to heat up, but won't necessarily affect the rate of heat loss, particularly if you're talking about filling the existing internal space. So you'll end up able to run at high output for a bit longer, but not really changing the long term sustainable power level by that much.

Really what you need is more surface area, or alternatively as you say a fan to give better heat transfer with the same surface area.
 

Lynx_Arc

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Oct 1, 2004
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It depends on the metal used sometimes larger isn't that much better but rather designs of the heatsink that helps to pull the heat away from the LED and also increase the surface area as air cooling the metal is often a better heatsink than adding more and more metal.
The one reason computer processors have fans on them is the heat builds up faster than putting unreasonably larger heatsinks can handle.
If you look at air heatsinking you will see that fanless heatsinks often do all sorts of things to increase surface area to air contact as much as possible as as sooner or later the metal will be heated up throughout the entire block and the added heat will have to move to the surrounding air or get essentially trapped in the metal. One common method of heatsinking in flashlights is using your hand to help draw heat away from the LED. You also see ribs and such on the sides of lights this increases surface area for air sinking.
 

Sagermaniac

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Feb 7, 2019
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Yeah. so I was thinking a fairly large piece of metal with a small fan, the fan I have on the metal for my XHP70 build is is tiny, I think 30mm, and it keeps it from getting to hot to hold indefinitely
 

DIWdiver

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Connecticut, USA
A large hunk of metal can, in the short run, absorb a large amount of heat. But if you continue to run the light, the metal will continue to increase in temperature, with no upper limit other than that caused by something failing. If the piece of metal is larger, the rate of increase will be lower, but there is still no upper limit.

For a sufficiently short runtime, a hunk of metal can be sufficient. But for longer runtimes, you need to stop the temperature from rising.

The way to stop the heatsink temperature from rising is to remove heat from the heatsink at the same rate as the LED is dumping it in. Larger surface area and faster airflow both help to remove heat from the heatsink. More metal does not necessarily help. Depending on what you do with the extra metal, it can make matters better, worse, or neither.
 

Sagermaniac

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Feb 7, 2019
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One of the interesting methods I figured out is to isolate the head of the flashlight from the rest of the body. According to the Cree data on the XHP70 it can go as far as 300 something F before physical failure, but I think a flashlight would become dangerous to hold at no greater than half that, so if you isolate the head from the rest of the body you can have a small fan providing the additional cooling to keep it below critical, but still run it far hotter than you could comfortably hold in your hand.
 
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