Yes, you are probably right about this. Lowering the assumed efficiency would mean there was actually more thermal power flowing through the host and would lower the thermal resistances values that I derived above.
I don’t know what you mean by this.
Technically, the idea of thermal resistance is only valid in steady state systems, which a flashlight heating up is not. But as I noted, the temperature differences between the shelf and the head were approximately constant as the host heated up, so I think these measurements can approximately be used to infer the shelf temperature (and junction temperature) if you know the temperature of the head and the thermal power flowing from the LED.
I’ll just add that while 150C may be the maximum junction temperature; according to Cree’s LM80 testing, you should keep the junction temperature below 100C if you don’t want the LED to wear out prematurely. Ideally, keep it below 85C for a long life-span. Above 100C, and you’ll never get the rated ~50,000 hours to 70% original brightness. You might get about 5,000 hours. Still a long time, though.
So, it looks like from your analysis, if you assume a 1.5C/W to 2.0C/W total thermal resistance, then a typical 1,000 lumen light which produces 5W of heat (and 5W of light), the junction point should only be about 7.5C - 10C warmer than the head of the light?
My guess would have been quite a bit more than that. Is the built-in thermal protection that some lights use, only for the purpose of not burning the user?
No. It produces 8w of heat, 3w of light. And 10-15C is difference inside led, difference between led die and led thermal pad.
Yes, built-in thermal protection protects your hands only. Big lightening systems work for years, but component are too hot to contact them.
Yes, approximately, unless I missed something in my analysis. Note that the XHP50 junction to solder point resistance is 1.2C/W, but the XPL thermal resistance is 2.2C/W
I guess so, and as you noted, to increase the life of the LED and other components in the light.
I don’t quite understand. Is this so the thermal power flow is well defined?
With a flashlight in air, only when the temperature profile has stabilized can you precisely measure the thermal resistance of the head. Before it is stabilized one cannot precisely define what the value of the heat flow through the head is, because energy is going into heating the head. When the temperature profile is stabilized you know that the heat flow through the head is equal to the thermal power produced by the LED.
Regardless of whether the measurement I did measured the true thermal resistance of the head (I think it is close, though), it can be used empirically to infer the temperature of the shelf if you know the head temperature.
The calculations depands on what do you want to count.
If you want to know temperature difference between two parts or two places of one part, you need to exclude heat transfert to air.
Heat transfert to air difference from lots of extra factors, it need to be calculated separately.
You can use heat transfer to air to set experimental conditions. For example, with a flashlight in air that has stabilized in temperature, the thermal power transferred to the air is equal to the thermal power produced by the LED. So then you know the heat flow through the head, and if you measure the temperature difference between the shelf and head, you can calculate the thermal resistance.
However, this is assuming that all the heat produced by the LED is transferred through the head. In my measurement, the shelf was exposed to air and so some heat was transferred directly to the air instead of flowing through the head. So you are correct that this would cause an error, but the surface area of the exposed shelf is quite small so the error would be small. (only the bottom of the shelf was exposed. The reflector and lens were in place during the measurements)
This was another thought, a while back, along the same lines (it’s hard to find heatsinks that don’t have nasty sharp edges)
The earlier picture, the shiny metal thing with big fins and a little central hole, is part of a 3D printer.
Lots of heat-dissipation hardware out there, just not much for flashlights.
I will continue in this thread bacause it is more suitable as I think.
Here is Osram LUW CEUP.
It is made special for car headlight application.
It can be powered up to 1.5A (5W) according to the datasheet: LEDs, Lasers, Infrared Components, Detectors and VCSEL | OSRAM
Top bin provides up to 500 lumens from 1sq.mm die (XHP 70 - 4000/16=250 lumens per 1sq.mm )
There is no thermal pad.
All heat (5W*85%=4.25W) is transfering through two electrical pads that are 2*0.45*1.65=1.48sq.mm. (XHP70 thermal pad is 26.3sq.mm)
Heat flux is about 2.87W/sq.mm (about 1W/sq.mm for XHP70).
Rth is 5.7…7.3C/W (XHP70 Rth is 0.9C/W).
BMW, Opel, Mercedes, Audi are using Osram leds for car headights.
All they have at least 100000km warranty.
All they will have BIG problems if even one person will crush because of lightening problems.
Does anybody wants to say that their engineers are far away from “dream team” developers from company with head office in Shenzhen?
Perhaps nice to know, I did a test on the latest Oslon Black Flat a while ago, that undoubtedly has the same die as the LUW CEUP . The high surface brightness compared to any other led was indeed found in practice.
I’m not sure where the leap was made from a 105C max of a Cree emitter then the test emitter is a Lattice Bright. Lot of assuming going on.
An 12V emitter designed for car headlights compares how to the Cree emitter we use in our flashlight? What is the point of bringing in facts that are outside the parameters of the test?
This last question also applies to the external power supply used for testing. We use cells, they drain, rather rapidly in some applications, so the constant heat source does not exist for us in the real world use scenario and all that testing is dirty water down the drain.
Perhaps thermal probes placed at strategic locations would help test an actual light under simulated conditions, which would still be inaccurate because we use a light under a wide variety of ambient relationship… downright cold to 110 degree F ambient, humid to dry, even wet in rain. Gloves not taking heat from the light as compared to bare hands acting as some form of external sink. Our real world use situation changes nearly constantly, rendering the tests invalid.
In the end, we have the worlds top scientists feverishly working to predict the next move of a woman… we all know what happens next in THAT scenario.
The Osram LUW CEUP has a normal forward voltage around 3V like many of the leds we use in flashlights, and in fact could be fitted in a flashlight and have great performance.
The point being made by kiriba I think is that you do not neccesarily need a dedicated thermal pad and lots of copper to achieve a good output.
(my personal opinion is that if the leds are severely overdriven, like some do at BLF, a thermal pad is not a luxury, but if the thermal design is good, chunks of copper are not needed)
I won’t comment on the Osram LED just mentioned, but I will respond to the comments regarding my measurements.
The main result of my measurements is an approximate value of the thermal resistance of a flashlight head. One can approximate the total thermal resistance by adding all of the individual thermal resistances (LED junction to solder point, solder point to shelf, shelf to surface of head). Then if you approximate the thermal power coming from the LED, you can approximate the LED junction temperature if you know the head temperature. Admittedly these are very back-of-the-envelope type calculations.
However, the calculation is general. It is not critical that I used a lattice bright emitter and not a Cree; this would affect the efficiency and the thermal power that results from use. I made approximations of the efficiency above that are probably not correct. The LED efficiency is indeed a large source of uncertainty in the calculations above.
The ambient conditions outside the flashlight are also not critical (at least to the level of accuracy that I’m concerned with). The result of the analysis is that for a given thermal power output from the LED, the temperature difference between the LED junction and the flashlight head is approximately constant. The ambient conditions will affect how fast heat is dissipated from the head and consequently the temperature of the head. So on a cold day the head will be less hot during use than on a hot day, say, by 20 degrees. Then the LED junction will also be 20 degrees cooler than on a hot day.
So, indeed the analysis is simple and approximative. It is more a framework for understanding what determines how hot the LED junction gets.
All information was taken from manufacture`s datasheet
You use in your flashlights just that emitter that manufacture wants to. Non of them was made for flashlights and most of them are not suitable. If manufacture dont want you to use their production, you will never get that leds.
Those osram leds have Vf=3.5 :person_facepalming:
Agree
Agree
Disagree. Lights is just piece of metall, there is no artificial intelligence inside. All proccesed can be emulated in advance.
The only thing I wanted to say, if you first made, then think, you will make wrong conclusions which are not usefull for other users.
You are showing Nitecore, there is copper pcb, good, and some other piece of copper under pcb, good too, and thermal glue between them :zipper_mouth_face: .
You shouldn`t be a doctor of science to count that this glue will decrease thermal achievements given by copper plates more than one time.
Rt= R1+R2+R3……Rn
R1 is internal led thermal resistance. If your led have neutral thermal pad, this means that there is dielectic layer inside led, with 10, 20 or even 30 degrees between led die and led themal pad. Mainly depends on die size.
R2 is led thermal pad to pcb central pad resistance. Mainly depends on thermal pad size.
R3 is pcb thermal pad to pcb low side resistance. Here I need to say that we are making most simple model, we approximate themal flux that it is same in hole sections so (because we will suppose thermal trasfer from pcb to shelf through thermal paste in next step) if you will try to measure temperature difference you need to take measurements not in the center of pcb low side, move you sensor closer to the edge.
If PCB is DTP, resistance depends on pcb material and size.
If PCB is non-DTP, it=R3A+R3B+R3C+R3D, where
R3A is resistance from pcb thermal pad low side of copper layer (depens on copper layer thickness)
R3B is resistance from copper layer to dielectic layer (depends on copper layer area)
R3C is resistance of dielectic layer (depends on pcb size, dielectric quality (straight) and dielectric thickness (opposite))
R3D is resistance from base material top to base material bottom (mainly depends on pcb material and size)
Here we need to remember that we have approximate themal flux so our pcb does not work as heat distributor from central to the edges.
R4 is resistance from pcb to shelf/pill/spacer, depends on solder/glue/paste quality and pcb size
R5 is resistance of shelf. Now hole thermal transfer from pcb area should pass throught smaller area that connects shelf with whole pill/host from sides. Depends on this area and material.
If flashlight has brass pill, R5B is resistance from this shelf connection area to pill threads and depends from pills tube thickness and material.
And also R5C, which is resistanse from threads to threads.
Have no time to continue right now, but as you understand there are more points.
We are not interested in thermal resistance, temperature difference is our target.
Lets imagine that we have full copper light that is solid from milled thetmal pad. Lets approximate that it has ideal thermal trasfer with temperature difference =0C from led to host.
Unfortunately, even with this model we will lost lots of degrees inside led: about 40 ( :exclamation: ) degrees with XP-L at 5A or about 30 degrees with 32W XHP-70.
If we will go to real light and add other resistance and calculate temperature difference, we will see that:
-R2, R3A, R3D, R4 and R5 will gave incomparable less temperature difference in good host, if pcb is not 10mm, copper is not 17mil, base material is not FR-4, pill is not brass and shelf is thicker than 0.5mm, if all are not so bad all this points will give less than 10 degrees total difference so there is no secret in them.
R3B and R3C are main parameters for non-DTP boards. If thermal pad area is not extended to the board and same as led pad size, while dielecric layer is 0.4mm 0.8W/(m*K):
Rt=0.0004/(0.8*0.0033*0.0013)=116.5 for XP-L. Anybody still sure that copper base will help you?
But if thermal pad is connected to 80% area of total PCB, while dielectric is 3.0W/(m*K) 0.1mm:
Rt=0.0001/(3*0.8*0.00025)=0.166 :+1: Even at 5A, you will loose just 3 degrees!
Thats what Im talking about! There is no other parameters that with same (from the first view) technology will give you such difference.
All burned leds - have burned because of:
Internal resistance
Bad design with small thermal pad area on top copper layer
Thick dielectic with bad conductivity
All other parameters gives too small difference to measure any visible influence. There is no situation to imagine that aluminium base is “bottleneck”.
The Nitecore mcpcb is re-flow soldered onto the copper heat sink. Pretty sure I said that before but, well, it’s pretty easily visible in the picture. No thermal paste.
And as far as the manufacturer’s giving me what emitter they want me to use, you’re not 100% correct. I bought a Nichia NVSU333A Ultraviolet emitter rated for 3.8A directly from Nichia and put it in a flashlight. This required much paperwork with Nichia, but we made it happen. 3.8A UV, brightest single UV emitter available and those were not available by retail at all at that time.
Anyway, mostly what you said is true, the emitters we use for flashlights were not designed for use in flashlights, strictly speaking, especially at the power levels we run them.
The idea that a well thought out plan by certified professionals is always the best way to go amuses me. I happen to have seen quite a few failures from top engineers in my time. I’ve also seen totally untrained amateurs produce results that baffled the pro’s.
Whatever, I really don’t care to argue it, I would like to see the stupendous light you are designing put into production so we can all buy it and have the very best. Are you almost done with that? I’m sure it’ll be very successful here. (while that may sound sarcastic I’m actually being serious.)
I’m really not trying to be controversial, really I’m not, but after reading the datasheet on the Osram from above what I gather is a 3.5Vf max forward voltage, a max current of 1.5A, and a max lumens of under 500 lumens at 1500mA. Somehow that doesn’t impress me. I’m probably missing something. Again I stress, I’m not an electrical engineer and I merely build flashlights as a hobby because it relaxes me. My argumentative nature comes naturally (I get it from my Dad) and my stubborn nature is also his fault.
We need argumentative and stubborn people (like you) to prove things different when the ‘calculators’ oversee things that take place in practice, but in most occasions well thought out plans by professionals work out best because those plans were in fact well thought out Despite the fact that failures do occur, in engineering thinking ahead prevents many many more failures.
But of course untrained amateurs beating the professionals make way better movies…
Yes. You are missing 1sq.mm die.
This is most important rate for flashlight use, lm/die area. It is twice more than your XHP-70.
It means that 8 small leds (2 24mm quads that are 6mm height) that gives same rated output that XHP-70, can help you to improve not only optics height (you already had such opportunity with multi-leds), but also optics diameter.
To have same throw (8*500*15kd/lm=60000kd), you will need optics that has not only much more heigh (10 times at least as I think), but also more diameter. This means that in fixed flashlight size you will have LESS throw with SAME lumens. And you wont be able to fix this (while too much throw with small dies can be easily fixed by same size more flood optics.)
)
Despite the fact that failures do occur, in engineering thinking ahead prevents many many more failures.