There must be something I don't understand about brass and why it is used for a pill.
The thermal and electrical conductivity are less than half of aluminum's ( http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html http://www.kp44.org/ftp/ElectricalConductivityOfMaterials.php ), but the cost is nearly 3x as much (I just did a quick google of scrap prices).
Although of minor concern in most flashlights, using dissimilar metals (aluminum body, brass pill) introduces the possibility of galvanic corrosion.
I suppose it might be easier to solder, and is a more durable surface for threads/contacts, but neither benefit requires making the whole pill brass.
I've purposefully avoided brass drop-ins - is there something I am missing?
There are other advantages of brass. It's easy for machining and has much better "specific heat" parameter than aluminium. Specific heat tells how much energy (heat) can material absorb within given temperature rise.
Specific heat of brass is ~2.5x smaller than aluminium, so brass need ~2.5x more energy (heat) to achieve the same temperature as the aluminium (of the same weight). Now, if you take into acount "specific weight" of both materials, you'll find that brass has ~3x greater specific weight.
So... The same pill made of brass will be 3x heavier than the pill made of aluminium. Now, add to this specific heat and in result, you'll get that in case of brass pill, you need ~7.5x more energy to heat it up to the same temperature as aluminium pill. So basically, brass has much greater "thermal capacity" (despite much worse "thermal conductivity").
but, in something like an emitter, heat buildup Kills. The last thing you want is to have heat stay in the emitter. You want to get the heat out absolutely as fast as possible. That’s the reason for Copper and Aluminum as a secondary alternative. Brass is the worst of the three to use for anything that has to get rid of the heat fast. Have you ever seen a CPU that uses a brass heatsink? There’s a good reason why and it’s not the weight, it’s the bad thremal conductivity. While that brass pill is slowly heating up, the emitter is slowly dying due to excessive heat buildup.
That is the very reason for the bi-metal heatsinks used in CPU cooling. The copper core absorbs the heat much faster, while the aluminum fins radiate the heat more efficiently.
But in real life it has little effect if the size and thermal paths are right.(tight fit and good surface contact to outer body)
I run a XML direct drive off 1x18650 drawing around 3,2 A measured with a shoddy DMM with long thin cables. It sits in a home made brass pill that has good contact surface to the light body. And I run it from freshly charged down to the protection kicks in with no stops. And I've yet to see as much as a hint of change in the light temperature.
I've applied heat goop under the emitter star but that is all I've done. As the effectiveness of the emitters rise the cooling need drops thats all.
I “mostly” agree.. I think the ideal set up is the emitter sitting on the largest aluminum base… ie the 20mm emitter base that most use, that pulls the heat the fastest from the emitter, even faster than copper…((EDIT) -see my comments in another post below... I stand corrected... but with coments :) ) If it heats up… GOOD –that’s what it is meant to do…after this would preferably be a little bit more aluminum, perhaps a double or triple base or a small pill to further -keep the heat flowing quickly away… –THEN copper to sustain the heat transfer… too much aluminum with the lack of a heat transfer out and the aluminum quickly maxes out.. There is simply no good way to get rid of this heat with just aluminum or the inefficient “fins” on some flashlights.I have a custom made solid pill/reflector in aluminum and solid copper –in a big P60 drop-in size… running triple XPG’sand have come to the conclusion ideally some combination of the two would be better… the aluminum pill quickly does its job very well… gets hot quick…but then there is nowhere for the heat to go after that.The copper heats up much more slowly but never seems to max out so I don’t know if it is pulling enough heat from the leds… seems too, but I can be sure.So this is where the brass pill may make a bit of sense… if you can’t have copper and you can’t get enough “Mass” of aluminum then somewhere in the heat cycle you want something with some longer absorption time… brass being the cheap alternative, with the easy machining and soldering properties already mentioned.
All this said with this in mind… most of the XPE up to the single XML lights are typically fine in most aluminum or brass pills… run at 1-2A… it is only recently that this is going to be an issue when we start pushing triple XML’s at 2-3 Amp each… then O yah… we want a chunk of aluminum and copper in there.
(EDIT)O-I should mention… Get the heat away from the emitter…For sure!- but not too far away that it is too hot into your hand… you want some MASS there to hold it while you can use the light…the right balance here…Also if your light does get HOT in your hand… generally this should be viewed as a GOOD thing… the heat is being pulled from the emitter, just wanted to say that typically this goes against common perception that a flashlight is “bad” because it gets hot fast…
definitely. a high specific heat would be good for maintaining a lower external temperature (ie the device won't feel as hot to the touch for a certain quantity of heat), but that doesn't on its own doesn't help cool something creating heat.
I've worked both in a lathe and mill (although much less than I've worked w/ steel). Brass is easier, but by how much (imho) depends on the alloy. It is much easier to thread. The better alloys of aluminum cost more than pure Al, and have lower thermal conductivity. In fact, many have a thermal conductivity that is much closer to brass than pure aluminum http://www.engineersedge.com/properties_of_metals.htm
so, you make a good point, about why they use it. Either they're saving on manufacturing costs or there are enough people convinced it is a good heat sink to make it profitable.
Even after seeing the charts on that last link, I'll probably continue to avoid brass...
Too bad...because I've got a foot long, ~1.5" diameter, hexagonal brass rod here that I don't know what to do with lol
Just wanted to add... for milling.. Copper absolutely sucks, never consistent and always a pain, and you make mistakes far too easy… this from a buddy who is a master and done some work for me… he grimaces every time I mention it.
the thermal conductivity of copper is higher, ie pure copper pulls heat away about 60% faster than pure aluminum
some people think the reverse of what you said (aluminum then copper) is true. their idea is, that since the aluminum reaches a higher temperature, it looses heat faster (larger difference between sink temp and air temp).
the original poster in this thread was of that opinion, but the sixth post explains (at least to my satisfaction) why no combination of aluminum and copper is superior to just copper.
that brass rod is SO heavy. It really surprises anybody that picks it up.
It used to be longer. I made a big punch out of some. Turning a hex rod into round one (on a lathe) isn't much fun. I should have sanded it first but once I had it centered (also not fun) I wasn't about to take it out of the chuck
Efficacy of the emitter does not change Ohms Law. Ohms Law will always run true. For a certain amount of voltage run through components that resist the flow, a certain amount of heat will always be generated. More efficient emitters only mean that for x amount of current and y amount of resistance that they are creating z amount of light output, but the heat is still there. The law does not go away.
Unless you were to put matched emitters into a test station and compare Silver, Copper, Aluminum and Brass and Steel, in the exact same testing method, you have no real way of knowing that the emitter is not being damaged (slowly). Supposedly an emitter that is put in ideal conditions (manufacturer specifications) should still have 75%+ the brightness after 1 year, as it did when new. How would you know when it gets to 75%? Humans can't tell the difference in small (relatively large actually) amounts of change in lumen output, so you (I, us all of us) would not realize that the emitter is dying slowly, since the change is slow enough that we don't notice it.
The fact that emitters can take more voltage and amperage than they are rated for is good engineering and lowballing the ability of the emitters. Again, the problem with all flashlights is that the cooling is passive, not active. So in a manner of speaking, it doesn't matter too much.
In another way, who really cares, since 75%+ of all these lights will fall apart or be trashed within a few weeks to a few months, or sold off to build the next monster, or the next budget light #1000,001 from China, so you are right, it does not really matter unless someone wants to keep that light for the next 50 years and that era is just about gone forever. (which is sad).
My apologies, even though I am not smart about this stuff, I still go off on a tangent once in a while. I get tunnel vision and hang on like a pitbull sometimes.
not for long! I've got two mods planned, one w/ peltier (scavenged from phlatlight) and one w/ vga cooler - I just need to sell some stuff to pay for the emitters/drivers :)
yes and no I think brass is good enough, and the difference might be irrelevant except extremely demanding setups - but I haven't heard a good argument for choosing brass for a pill (if all other factors are equal) so I might as well try and get the most for my money:)
I stand corrected… in regards to specific heat… yes…copper is better… but all things bing equal –I still don’t know what is best…
A couple more thoughts… I do understand that about copper in factual terms, if we are talking the specific heat by numbers -what you say is very true… by mass… to conduct heat -100g of copper is better than 100g or aluminum… the problem is we are comparing often the volume of the metal, similar size pills to similar sized pills… the masses are not equivalent however.A 100galuminum heat sink would be three times the size of the equivalent copper 100g heat sink… in this case… the “heat capacity” of the aluminum pill would be superior to “get the heat away for the emitter base” as fast as possible, aluminum having this better quality, so it is not just about what is touching the LED, even if copper is better.. –but even if copper is better… is it better for this application…after a certain mass is reached –at what point does heat capacity become more important than specific heat… big solid copper heat sinks… generally are not well received, and not just because they are expensive, they just don’t work as good as some combination, Copper will SINK the best.. but it won't get rid of the heat best.
The debate is ongoing… if you need to stay small… then copper to aluminum is best…provided you can get the aluminum dissipated out… but if you “can” be bigger –is a bigger volume piece of aluminum superior… I don’t think we will settle the discussion here.
I stand corrected as you say -that a copper 20mm emitter base will conduct heat better than an aluminum one… but it will also retain that heat longer… so in this application –I simply do not know what is best truth be told…Is it best to have “some” copper first… if so how much? –especially if we are talking critical destructive heat that we could get pushing 3A plus… how long can that copper hold the heat before it becomes critical…. I do know this though… while the aluminum will not conduct the heat From the emitter as fast as copper… it will sure get rid of it from itself faster…than copper… That’s my point about the two different P60 heads I have, and it seems some combination is going to work best… to get the heat absorbed from the emitter fast.. Keep it from further heating up and sustain the radiance of heat out… truth is as I stated.. I simply don’t know between the two in my circumstance –which is better…
I think brass is good enough, and the difference might be irrelevant except extremely demanding setups - but I haven't heard a good argument for choosing brass for a pill (if all other factors are equal) so I might as well try and get the most for my money:)