Technical question...

Will an XP-G emitter pulling 2A get just as hot (assuming the same flashlight) as an XM-L emitter pulling the same 2A?

Since you said BOTH would be pulling 2A that means both will be dissipating 2A worth of power. The XM-L is more efficient at 2A so it will be brighter than the XP-G at 2A... but temps should be the same barring any differences in emitter heat sinking.

If the XM-L is more efficient than the XP-G at 2 amps, then the XM-L will be cooler because more of the energy will be converted to light instead of heat.

However, the XP-G has a max current of 1.5 amps, so the XP-G might be a lot warmer once it catches on fire. :D

We're just talking about conservation of energy here. If the same amount of energy goes into both and more light comes out of one, the extra energy has go somewhere and that somehwere is heat.

Even though LED's are more efficient than other light sources, most of the energy becomes heat. A little of the energy becomes light. So the XM-L will convert a little more of the energy into light than the XP-G (everything else being heat) and therefore should be just a little cooler. But I don't know if you could measure it.

I have an XP-G that pulls 2.25A at the tail, so that concerns me.

Not sure about the answers so far. Lets say you have an old 110V heater plugged into your wall, putting out X number of BTU's. A newer, more efficient heater might put out X times 120% because it is more efficient. So I'm thinking that a more efficient emitter that throws out more light might also put out more heat at the same amps, but less heat at the same light output.


How's it going to do that without violating the first law of thermodynamics?

See my post about heaters above. If you have a more efficient heater, it can put out more heat with the same input.

Why would emitters be any different? They are just small heaters...

Same current draw, assuming same Voltage, should see Less heat from the XM-L, by a very small amount, because more of the Energy is converted to light as opposed to heat.

Practically, you also need to define what Type of driver your using, its efficiency, and the voltage supply and the forward voltage of the LED.

A Linear driver on full output will give the least difference between the 2 emitters (for a fully charged battery), while a good quality switching driver will see less power being drawn by the XM-L due to the lower forward voltage of the XM-L, and combining this with the slightly higher efficiency of the XM-L, should produce Slightly less heat. We are really talking numbers in the order of 5-10% less heat or something small like that, which in the whole scheme of things, is nothing compared to the differences in efficiencies of the drivers.

Driver efficiencies are very variable depending on battery setups (single or multiple cells) and the corresponding driver (linear or switching) and the quality of the driver (design/components used). Most switching regulators for budget torches are horrendously inefficient, with many of mine anywhere between 50-80% efficient across various voltages.

Conclusion - the difference in heat between XM-L and XP-G (within normal operating ranges) is less significant than appropriately choosing your driver/battery setup, but can make a small but noticeable difference given the right setup (high efficiency switching driver with 2 cells)

Any electric heater is 100% efficient because 100% of the energy it consumes is converted to heat - first law of thermodynamics again. (OK, one might be slightly less efficient than another because it might have a fan attached or it might have a light but these are trivial differences that have nothing to do with the efficiency of the heating mechanism.)

That's kind of what I was thinking, but wanted to just throw it out there and see if I'm off track.

That sounds good in theory, but the reality is there are numerous heaters (and air conditioners) on the market, both electric and ceramic, that put out different amounts of heat at different efficiencies. There are differences in how coils are wound, how reflectors are used, etc.

The first law of Thermodynamics is fixed, I agree. But the fact that one car may get 20mpg with one engine and 25mpg with a different engine (in the same car) means that energy can be converted into something besides heat, and there are variables involved.

That simply isn't possible for electric heaters. It doesn't matter how coils are wound or what kind of reflector is used: 100% of the electrical energy always becomes heat because there is no other outlet for the energy. Of course, different efficiency is entirely possible for air conditioners.

The key thing here is that if electrical energy isn't converted to heat, it must go somwhere. In the LED example, the only other place for it to go is light, so there is a direct trade between heat and light in a more efficient LED.

In a car, not all of the energy in the fuel is consumed and some goes out your tailpipe so there can be differences in efficiency just in the amount of energy that is extracted from the fuel. After that, assuming that you generate the same amount of kinectic energy by moving a car of the same mass, the remaining differences in effiiency must be heat. Of course, one car could have brighter headlamps than another or a loader radio, so they could emit energy in other ways, but if we keep these things constant across cars that we're comparing, any difference not measured at the tailpipe must be a difference in heat produced.

The XM-L has twice the surface area as the XP-G, which explains just about perfectly the difference in brightness without considering thermal transmission. If you powered one T6 XM-L, you'd get almost exactly the same amount of the light, consume almost exactly the same amount of power, and use almost exactly the same surface area, as if you powered to R5 XP-Gs.

Die size might be critical for the lifetime - XP-G 2mm2 as opposed to 4mm2 for the XM-L resulting in higher junction temperatures at same overall wattage for the XP-G (more thermal resistance) - the phosphor is heat sensitive.

Cree XP-G datasheet

thermal resistance: 6°C/W

Cree XM-L datasheet

thermal resistance: 2.5°C/W

Since all energy ends up as heat once we're done with it, there is no such thing as a more efficient heater - heat is where energy goes when there's nothing left.

See heat death.

Very interesting info, guys. Thanks for sharing ;)

I have an electrical engineering degree (although I no longer work in EE so my memory is not 100%) so I'm fairly confident in what I'm about to write...

- TroopLewis, your original question was about "pulling 2A". Do you mean from the battery, or 2A through the LED? If you mean from the battery, then as other people have said, the driver plays a big role in overall performance and efficiency. But for the sake of this discussion, we could say "driver efficiency being equal...", which would answer your question better, correct?

- If we make the above assumption about drivers, and if TroopLewis means 2A from the battery, then the same amount of power is being delivered to the XP-G and XM-L. That is, 2A, at the battery voltage (which of course wouldn't change with different LEDs), minus the power loss in the driver and wires.

- As brted and okwchin said, the XM-L is more efficient than the XP-G. So given the same amount of power delivered to each, a little more is converted to light in the XM-L, but it might not be a very significant amount of heat (I believe a small amount of heat is equivalent to a lot of light, in terms of energy).

- About heaters: Don is right when he says all energy eventually becomes heat. Even light eventually becomes heat. When light hits an object, some energy is reflected back, but some is absorbed, and that absorbed energy heats the object. Dark-coloured objects reflect less light back than light-coloured objects. That's why a black car with a black interior gets much hotter in the sunlight than a white car with tan interior.

- ronparr is right when he says that heaters are 100% efficient, but let me add to what he wrote: all of the power you put into an electric heater will come out as heat, somehow. It may not all come off the heating element though. So if you put 100W into a heater, maybe 80W comes off the heating element. The other 20W goes into making the enclosure hot, the wiring hot, etc. If there's a fan, then some power goes into making air move, but eventually, the air slows down and kinetic energy is converted into heat. Ultimately, the fan's power ends up as heat, as Don implied. Another heater might make 90W come off the heating element, leaving less power to be dissipated in the enclosure, wiring, etc. (did you know that leaving a fun running in an empty room, with doors and windows closed, actually heats up the room? I could never convince my parents of that. The fan consumes power. All of the power eventually becomes heat. A fan cools people, not air. The layer of air next to your skin (the boundary layer) is heated by your body. If the surrounding air is cooler than your body, then the fan will remove the boundary layer, leaving cooler air next to your skin, cooling you. Of course, a fan will cool a room if you blow hot air out a window or door.)

- Be-Seen Triker, you said that the characteristics of the XP-G and XM-L are nearly identical. You're omitting one major property though: the forward voltage drop. If you compare the forward voltages of the two LEDs (someone posted links to the data sheets in a post later than yours), you'll see that the XM-L has a much lower Vf. e.g. at 1A, the XP-G's Vf is about 3.35V. At 1A, the XM-L's Vf is about 2.97V. Since power dissipation = voltage x current, then if trooplewis's original question was about pullling 2A through the LED, then the XM-L would be dissipating at least 10% less power than the XP-G. And at 1.5A, the difference is even larger: around 3.55V vs. 3.1V. But if trooplewis meant 2A from the battery, then my second and third comments above apply.

That's all. Hope I didn't sound too know-it-all-y. And I sure hope I'm right. :)

So peteybaby, is your answer to the OP yes or no?

Short answer is ,no , for the reasons explained above regarding thermal resistance , the different Vf between emitters ...guessing since is out of specs an XP-G at 2A has a Vf of around 3,65V , that`s equate at 7.2 XM-L at 2A has a Vf of 3.18 V, that's = 6.36 W...also run much cooler since the more area the die has allow to transmit the heat to the star way better...surely a XP-G at 1.2 A give much more light than one at 2A..