I've lurked around here for the last couple years. Recently I have had a bit more time that I can spend to move beyond bolt-on mods. There's alot to learn on the diode and electronics side and I've begun to ask some questions. I thought I might give a little back in the exchange.
In my "day job" I deal pretty regularly with heat management. I've picked up a bit of experience over the years that might be helpful to some board members. Before writing this, I ran some searches here and found that while the topic comes up occasionally, there are alot if points missing in the dialog.
As I read through threads and examine the various builds, it's very obvious that the heat generated by these various components when pushed to their limits becomes substantial- and often times a barrier. In more than a couple builds, I see things being done that while logic might lead one to believe might help in the management of heat, they are actually detrimental. I hope some of this information is helpful.
Heat-
Understanding the barriers to good thermal management takes a basic understanding of the forms of heat and how it's moving. In the case of a flashlight, you have two things going on. Conductance and emittance. In simple terms, when you heat an object, you excite its molecules. As they crash into each other it creates heat. That heat moves through the mass of our piece of metal as conductance. Conductance is the energy of one molecule transferring it's energy to the one next to it. This is best exhibited by heating the end of a metal rod. When you start, and if you are applying enough heat, the end you're heating get's hot quickly. If you bring it to a temperature and hold it there, the other end of the rod will begin to get hot. I'm guessing everyone here has experienced this with a bench grinder or sweating a copper pipe. This is called equilibrium. Eventually every molecule in that rod will be at the same temperature. This is only going to happen in the real world if you do this experiment in a vacuum.
That brings us to our next facet which is emittance. This is primarily what you're feeling when you hold your hand close to the end of that heated metal rod. This is no longer the transfer of energy at the molecular level. This is the conversion of that energy into electromagnetic radiation. The hotter you get the metal, the higher the frequency of radiation. That's why at lower temperatures that piece of steel is cherry red and at some point it becomes white. Again, in our real world, we are experiencing both. If you hold your hand close enough, part of what you'll feel is the conductance of heat through the air. But air isn't a very good conductor so you don't have to pull back far to reduce that conductance dramatically. This is all going on inside of your flashlight every time you turn it on. And there are simple things that will allow it to operate at better efficiency.
Materials-
The preponderance of these custom builds and mods are builds done in aluminum. Some of the higher end stuff integrates copper. A few are done in titanium. Each of these metals have obvious benefits and drawbacks- and a few maybe not so obvious.
Aluminum is a wonderful material in that it has a strong strength to weight ratio, it's relatively easy to come by inexpensively. But one of the best benefits to aluminum is it's corrosion resistance. It's corrosion resistance comes from aluminum's propensity of rapid corrosion. Aluminum exposed to oxygen begins to oxidize very quickly. This reaction creates aluminum oxide- a crystalline ceramic. After a very thin film forms (a couple nanometers) the oxide seals the surface, preventing further oxidization. It takes more than the relatively low reactivity of the air in our atmosphere to penetrate that coating. Acids and bases (salt) certainly cause continued oxidization. Apply a little electricity and a few of these other elements and you have anodizing. Here is where our "problem" surfaces. Aluminum oxide is a very poor thermal conductor. The average layer of oxidization in the anodization on a flashlight body can create a barrier in the magnitude of 10 to 30 times that of bare aluminum. Anodizing is a very effective thermal conductance inhibitor. That's bad. But not always. Anodizing is one of the most thermally emissive materials. It's about as close to black-body as you can get. And that's good. That's why alot of heat sinks are anodized (color doesn't matter- black-body is just the thermodynamic term). So inside your flashlight, if that new heat sink you just machined is fitted into an anodized body, it has the equivalent of a thermal blanket in between. Even a piece of bare aluminum isn't much better unless it's had the layer of oxidization removed before fitting the parts. And even then there are steps necessary to prevent future oxidization. The outside being anodized- good thing.
Copper is another material I have seen used. Let's look at copper as if it were a highway. Our copper highway is four lanes wide. A little traffic moves absolutely unimpeded. So does moderate traffic. Heavy traffic moves right along pretty well. But if there's only one toll booth open at the end, it could be 30 lanes wide and eventually it will fill up. Our one-lane aluminum highway with one toll booth open is letting just as many cars out as 4 lanes of copper. Copper will transfer heat rapidly but if it isn't able to disperse that heat, once the mass is saturated (equilibrium) it has no benefits beyond aluminum. In a flashlight design, if you haven't figured out how to allow the heat to conduct away from the mass, it really doesn't matter how fast you are initially moving heat as it's going to slow down pretty soon. If you want to run your light for a few minutes, you'll never see a problem. If you want your light to weigh 23 pounds, you'll never see a problem. But if you want a small, light flashlight that will run for extended periods, you have to move heat. Copper and aluminum react with each other and each material oxidizes so unless addressed you have the same issues as your aluminum-aluminum interface.
Titanium is about as cool as it gets in obtainable metals. I have a Ti paperweight. There's a reason Lockheed built the mach 3-plus SR-71 out of titanium. It barely conducts heat. Ti is great for hypersonic aircraft and hip implants but not so good for high-output flashlights.
Other metals- If copper is so good, brass must be as well, right? I mean, it's mostly copper. Brass is three times less thermally conductive than copper and about two times less than aluminum. Silver is very good- about a few percent better than copper. Both oxidize about the same. The best conductor? Diamond. At the extreme edge of thermodynamic management we're starting to see diamond heat sinks. Back to metals, you have to be careful in your selection. Anything alloyed with copper significantly impeded its thermal abilities. 6061 aluminum is better than 2024. 0 temper 6061 is a bit better that T6. Not enough to worry about. But all things being equal, you might as well get what you can on your side.
Thermal management strategies-
In talking about the thermal conductivity of metals, I missed one. Lead. Lead is a terrible thermal conductor About 1/1oth of copper ans 1/6th of aluminum. If you used common solder paste to affix your diode to that heat sink, you placed a huge barrier in the way of it's heat conductance. Semiconductor manufacturers are now turning to exotic metals to aid in thermal management. One of the most attractive is indium. Done in a similar fashion to furnace brazing, a small piece of indium sheet is placed between the component and the board and the piece is heated. The best part is it melts at about 100c. You could do that with a good hair dryer. Indium is 3 or 4 times more conductive than lead. Here's one that really shocks people- thermal paste is almost worthless as a thermal conductor. Someone showed me once a test of most of the commercially available pastes and ordinary zinc oxide and common grease worked just about as well. One compound works astonishingly well and that's gallium. Gallium is a metal that, like mercury, exists in liquid form at room temperatures. Downside- it's highly reactive with aluminum.. Indium and gallium are so much better because they are molecularly much more similar to the metals used in these components than grease is. We're now looking at this from a mechanical perspective at the micron level. The reason that block of aluminum in your hand doesn't separate into its gazillion separate atoms and disperse into the wind is molecular attraction. If you were to take two blocks of that aluminum and perfectly microfinish the surfaces absolutely flat, touching the two faces together would make one block. They would be inseparable. This isn't possible with our present level of technology. But I've seen something close. It was a laboratory grade surface plate with gage blocks sitting on it. They were so flat and so polished that couldn't be lifted off. They had to be slid to the edge to remove them. This is why these metals work so well. They're bonding at the molecular level. I went through all of this to illustrate a point. If you machine a pill and slide it into an anodized aluminum tube, you're leaving alot on the table. Removing the anodized layer and using thermal compound isn't much better. This is why all the instructions you see tell you to use the absolute thinnest amounts of thermal compounds and adhesives. You really want as much of the metal you can to be in contact. The thermal compounds just fill the voids. So take that heat sink and polish it to the finest possible point that you can. The tube it's going into as well. Then join them in an interference fit- press them together. Your thermal transfer is now many, many times better than a 0.001" gap filled with Arctic Silver. This goes back to our differences between conductance and emittance. A rough finish viewed under a microscope would show that the heat sink is only making physical contact in not alot of places. These are the only points that conductance is occurring. In all the other spots, emittance is at work, and that's way more inefficient. Air is a lousy thermal conductor. I can't make any recommendation on this yet but I've begun to look into aluminum soldering. Ideally our assembly is made of one piece of metal. The diode is on the star with something like indium and the star is bonded to the body in the same fashion. This is out of reach for all but the most advanced fabricators. Aluminum soldering seems to me to be the ideal fashion of bonding a sink into a body. In practice, the heat may be too high. You could certainly do it with a thin sheet of indium. For a few hundred dollars.
To summarize-
-Oxides and surface finishes conduct a fraction of what bare metals do
-Alloys typically conduct much less than pure materials, some significantly more
-Metal-to-metal contact is always the best thermal pat
-Avoid thermal compounds, apply very thinly when used
Alot of this is unimportant in the majority of flashlights. But in the short time I've been around the build and mod side, the more signs I am seeing that these issues will become more prevalent. Batteries are getting smaller while at the same time they are seeing higher current and voltage outputs. Led size is shrinking while at the same time output is rising. Inevitably, technology on the manufacturer side will bring more efficient diodes that generate more light and less heat. In the interim, some of this may help you.
In my "day job" I deal pretty regularly with heat management. I've picked up a bit of experience over the years that might be helpful to some board members. Before writing this, I ran some searches here and found that while the topic comes up occasionally, there are alot if points missing in the dialog.
As I read through threads and examine the various builds, it's very obvious that the heat generated by these various components when pushed to their limits becomes substantial- and often times a barrier. In more than a couple builds, I see things being done that while logic might lead one to believe might help in the management of heat, they are actually detrimental. I hope some of this information is helpful.
Heat-
Understanding the barriers to good thermal management takes a basic understanding of the forms of heat and how it's moving. In the case of a flashlight, you have two things going on. Conductance and emittance. In simple terms, when you heat an object, you excite its molecules. As they crash into each other it creates heat. That heat moves through the mass of our piece of metal as conductance. Conductance is the energy of one molecule transferring it's energy to the one next to it. This is best exhibited by heating the end of a metal rod. When you start, and if you are applying enough heat, the end you're heating get's hot quickly. If you bring it to a temperature and hold it there, the other end of the rod will begin to get hot. I'm guessing everyone here has experienced this with a bench grinder or sweating a copper pipe. This is called equilibrium. Eventually every molecule in that rod will be at the same temperature. This is only going to happen in the real world if you do this experiment in a vacuum.
That brings us to our next facet which is emittance. This is primarily what you're feeling when you hold your hand close to the end of that heated metal rod. This is no longer the transfer of energy at the molecular level. This is the conversion of that energy into electromagnetic radiation. The hotter you get the metal, the higher the frequency of radiation. That's why at lower temperatures that piece of steel is cherry red and at some point it becomes white. Again, in our real world, we are experiencing both. If you hold your hand close enough, part of what you'll feel is the conductance of heat through the air. But air isn't a very good conductor so you don't have to pull back far to reduce that conductance dramatically. This is all going on inside of your flashlight every time you turn it on. And there are simple things that will allow it to operate at better efficiency.
Materials-
The preponderance of these custom builds and mods are builds done in aluminum. Some of the higher end stuff integrates copper. A few are done in titanium. Each of these metals have obvious benefits and drawbacks- and a few maybe not so obvious.
Aluminum is a wonderful material in that it has a strong strength to weight ratio, it's relatively easy to come by inexpensively. But one of the best benefits to aluminum is it's corrosion resistance. It's corrosion resistance comes from aluminum's propensity of rapid corrosion. Aluminum exposed to oxygen begins to oxidize very quickly. This reaction creates aluminum oxide- a crystalline ceramic. After a very thin film forms (a couple nanometers) the oxide seals the surface, preventing further oxidization. It takes more than the relatively low reactivity of the air in our atmosphere to penetrate that coating. Acids and bases (salt) certainly cause continued oxidization. Apply a little electricity and a few of these other elements and you have anodizing. Here is where our "problem" surfaces. Aluminum oxide is a very poor thermal conductor. The average layer of oxidization in the anodization on a flashlight body can create a barrier in the magnitude of 10 to 30 times that of bare aluminum. Anodizing is a very effective thermal conductance inhibitor. That's bad. But not always. Anodizing is one of the most thermally emissive materials. It's about as close to black-body as you can get. And that's good. That's why alot of heat sinks are anodized (color doesn't matter- black-body is just the thermodynamic term). So inside your flashlight, if that new heat sink you just machined is fitted into an anodized body, it has the equivalent of a thermal blanket in between. Even a piece of bare aluminum isn't much better unless it's had the layer of oxidization removed before fitting the parts. And even then there are steps necessary to prevent future oxidization. The outside being anodized- good thing.
Copper is another material I have seen used. Let's look at copper as if it were a highway. Our copper highway is four lanes wide. A little traffic moves absolutely unimpeded. So does moderate traffic. Heavy traffic moves right along pretty well. But if there's only one toll booth open at the end, it could be 30 lanes wide and eventually it will fill up. Our one-lane aluminum highway with one toll booth open is letting just as many cars out as 4 lanes of copper. Copper will transfer heat rapidly but if it isn't able to disperse that heat, once the mass is saturated (equilibrium) it has no benefits beyond aluminum. In a flashlight design, if you haven't figured out how to allow the heat to conduct away from the mass, it really doesn't matter how fast you are initially moving heat as it's going to slow down pretty soon. If you want to run your light for a few minutes, you'll never see a problem. If you want your light to weigh 23 pounds, you'll never see a problem. But if you want a small, light flashlight that will run for extended periods, you have to move heat. Copper and aluminum react with each other and each material oxidizes so unless addressed you have the same issues as your aluminum-aluminum interface.
Titanium is about as cool as it gets in obtainable metals. I have a Ti paperweight. There's a reason Lockheed built the mach 3-plus SR-71 out of titanium. It barely conducts heat. Ti is great for hypersonic aircraft and hip implants but not so good for high-output flashlights.
Other metals- If copper is so good, brass must be as well, right? I mean, it's mostly copper. Brass is three times less thermally conductive than copper and about two times less than aluminum. Silver is very good- about a few percent better than copper. Both oxidize about the same. The best conductor? Diamond. At the extreme edge of thermodynamic management we're starting to see diamond heat sinks. Back to metals, you have to be careful in your selection. Anything alloyed with copper significantly impeded its thermal abilities. 6061 aluminum is better than 2024. 0 temper 6061 is a bit better that T6. Not enough to worry about. But all things being equal, you might as well get what you can on your side.
Thermal management strategies-
In talking about the thermal conductivity of metals, I missed one. Lead. Lead is a terrible thermal conductor About 1/1oth of copper ans 1/6th of aluminum. If you used common solder paste to affix your diode to that heat sink, you placed a huge barrier in the way of it's heat conductance. Semiconductor manufacturers are now turning to exotic metals to aid in thermal management. One of the most attractive is indium. Done in a similar fashion to furnace brazing, a small piece of indium sheet is placed between the component and the board and the piece is heated. The best part is it melts at about 100c. You could do that with a good hair dryer. Indium is 3 or 4 times more conductive than lead. Here's one that really shocks people- thermal paste is almost worthless as a thermal conductor. Someone showed me once a test of most of the commercially available pastes and ordinary zinc oxide and common grease worked just about as well. One compound works astonishingly well and that's gallium. Gallium is a metal that, like mercury, exists in liquid form at room temperatures. Downside- it's highly reactive with aluminum.. Indium and gallium are so much better because they are molecularly much more similar to the metals used in these components than grease is. We're now looking at this from a mechanical perspective at the micron level. The reason that block of aluminum in your hand doesn't separate into its gazillion separate atoms and disperse into the wind is molecular attraction. If you were to take two blocks of that aluminum and perfectly microfinish the surfaces absolutely flat, touching the two faces together would make one block. They would be inseparable. This isn't possible with our present level of technology. But I've seen something close. It was a laboratory grade surface plate with gage blocks sitting on it. They were so flat and so polished that couldn't be lifted off. They had to be slid to the edge to remove them. This is why these metals work so well. They're bonding at the molecular level. I went through all of this to illustrate a point. If you machine a pill and slide it into an anodized aluminum tube, you're leaving alot on the table. Removing the anodized layer and using thermal compound isn't much better. This is why all the instructions you see tell you to use the absolute thinnest amounts of thermal compounds and adhesives. You really want as much of the metal you can to be in contact. The thermal compounds just fill the voids. So take that heat sink and polish it to the finest possible point that you can. The tube it's going into as well. Then join them in an interference fit- press them together. Your thermal transfer is now many, many times better than a 0.001" gap filled with Arctic Silver. This goes back to our differences between conductance and emittance. A rough finish viewed under a microscope would show that the heat sink is only making physical contact in not alot of places. These are the only points that conductance is occurring. In all the other spots, emittance is at work, and that's way more inefficient. Air is a lousy thermal conductor. I can't make any recommendation on this yet but I've begun to look into aluminum soldering. Ideally our assembly is made of one piece of metal. The diode is on the star with something like indium and the star is bonded to the body in the same fashion. This is out of reach for all but the most advanced fabricators. Aluminum soldering seems to me to be the ideal fashion of bonding a sink into a body. In practice, the heat may be too high. You could certainly do it with a thin sheet of indium. For a few hundred dollars.
To summarize-
-Oxides and surface finishes conduct a fraction of what bare metals do
-Alloys typically conduct much less than pure materials, some significantly more
-Metal-to-metal contact is always the best thermal pat
-Avoid thermal compounds, apply very thinly when used
Alot of this is unimportant in the majority of flashlights. But in the short time I've been around the build and mod side, the more signs I am seeing that these issues will become more prevalent. Batteries are getting smaller while at the same time they are seeing higher current and voltage outputs. Led size is shrinking while at the same time output is rising. Inevitably, technology on the manufacturer side will bring more efficient diodes that generate more light and less heat. In the interim, some of this may help you.
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