My Flashlight gets Hot so It Must Be Good

[jki]

I see quite a few posts where people say that the thermal management in their flashlight must be good, because their flashlight gets hot on high output. This is not an accurate statement!
If your flashlight gets hot, then it is also very hot inside. Not good. If there is a good thermal path (low thermal resistance) between the inside and the outside of the flashlight, then the difference in temperature inside-to-outside is maintained at a lower delta.
Take two flashlights, one with excellent thermal path from inside to outside, and one with a poor thermal path. Let's say they both run hot to the touch on the outside. All this means is that the poorly designed light is running hotter on the inside. (For simplification, I am ignoring the ability of the outside of the flashlight to get rid of heat to the surrounding environment, be it to the user's hand (conduction), or to surrounding air (convection/radiation). The ability to remove heat externally is also important, but not necessary for this discussion).
A well-designed flashlight should minimize heat generation to begin with. This includes use of high efficiency LEDs (high lumens/watt) and high efficiency regulation. (Achieving high efficiency in regulation is made more difficult if a wide range of battery voltage inputs must be accomodated).
If high efficiency can be achieved in converting DC to light, then add in good heatsinking and a low thermal resistance to the outside of the flashlight, the flashlight will run cooler, not hotter.
A couple of flashlights that I have that achieve a high level of efficiency include the Fenix P1-CE-Q2, and the Lumapower MRV. Both of these can run flat out and only get warm; never hot. Hot is not good!

 

[CodeOfLight]

If you are running a very bright LED flashlight, and you have some idea of the milliamps being run through the LED, and have a good idea that this will generate some heat, and the outside of the light gets hot or very warm pretty quickly, then that IS a good thing. It's exactly what I am looking for in thermal conductivity. Don't forget, the LED itself can run all day long at temperatures that are too hot for us humans to touch.

I am pretty sure that a lot of folks here are well versed in thermal management, measuring current delivered to the LED and in general the amount of heat that should generate. They expect to see that heat reflected on the outside of the light and are looking for it. I know I am. When I put the light in "Turbo" mode, I am looking to see how long it takes the head of the light to start warming up. This gives me a good indication of a good thermal transfer contact point with the LED. It does not really matter to me just how hot it gets, unless it is too hot to touch. If it warmed up quickly in the first place, then I know that the temperature differential between the LED and it's heatsink is not enough for me to worry about frying the LED. I have yet to fry one.

 

[Bullzeyebill]

You are both right. The MRV isn't getting too hot and it isn't running at 1.2 amps to the led either. The light running on "burst" (flupic type) may well be running at 1.2 amps or higher and you do want to know that the heat is getting away from the led quickly. The MRV is fairly large headed and has mass to provide a good heat sink. A light that comes to mind that does not manage heat well is the light weight L4 (KL4). The KL4, lux V puts out a lot of heat fairly quickly and keeps on getting hotter, thermal efficiency is poor and light output will suffer, dropping as the heat builds up, often within seconds of being turned on, so "not good".

The MRV will probably be able to sustain 1.2+ amps and not get hot to the touch. The KL4 should be used only for short periods of time and only hand held; same for a light on burst that becomes hot quickly.

Bill

 

[moon lander]

also keep in mind your light could be hot due to ineficiency in the board. i have a AA light that gets quite hot, and it is reputed to have an ineficient board that makes alot of heat.

heres a rule of thumb that i follow: if the star of the led is too hot to hold for a minute without burning you (or causing significant discomfort) , then the led is being damaged and needs better heatsinking or lower current. does that sound about right?

 

[partipilo]

The Mini-Mag LED is awful at conducting heat away from the LED. If you ever saw one disassembled, you'd almost feel bad for the LED as if it were an abused helpless person.

 

[turbodog]

These things can take more abuse than you think. I've got a kl4 that will reach about 155F. It's done that for many hundreds of runs with no damage. It will shorten the life a little, but the lifespan is STILL longer than you will ever want.

People are replacing their led lights every 2-3 years as new stuff comes out. The replaced light hasn't even been broken in good at that point.

 

[greenlight]

I love the title of this thread. So many people make statements like that with nothing to back up their logic. Reminds me of my brother, who likes to make PBJ sandwiches with a quarter of the jar per sandwich, "Hey, peanut butter is healthy, and full of protien." Yeah, but it's half fat, too, fatso.

 

[legtu]

hot/warm is also relative. what might be warm to you can already be considered as hot by others and vise-versa.

 

[Skibane]

A well-designed flashlight should minimize heat generation to begin with.

"Minimizing" heat isn't the same as "eliminating" it. Real-world LEDs produce heat - and the really bright ones produce significant amounts of it.

Since this heat is unavoidable, I'd prefer that as much of it as possible be conducted away from the LED - which implies that a lot of it should end up in the flashlight case.

 

[StefanFS]

You are both right. The MRV isn't getting too hot and it isn't running at 1.2 amps to the led either. The light running on "burst" (flupic type) may well be running at 1.2 amps or higher and you do want to know that the heat is getting away from the led quickly. The MRV is fairly large headed and has mass to provide a good heat sink. A light that comes to mind that does not manage heat well is the light weight L4 (KL4). The KL4, lux V puts out a lot of heat fairly quickly and keeps on getting hotter, thermal efficiency is poor and light output will suffer, dropping as the heat builds up, often within seconds of being turned on, so "not good".

The MRV will probably be able to sustain 1.2+ amps and not get hot to the touch. The KL4 should be used only for short periods of time and only hand held; same for a light on burst that becomes hot quickly.

Bill

The MRV actually delivers 1190 mA to the led with the stock driver and 2 x RCR123/Cr123. My FluPic'd version delivers 1198-1200 mA to the led with one 18650 cell. I had an AMC7135 1400 mA driver in it for a while that delivered 1230-1290 mA to the led. Due to the design and the excellent heatsinking this is not a problem.

 

[Anglepoise]

Three years ago on this site, proper heatsinking of the LED was concidered a high priority. Recently it seems by the posts and the hardware now being used that members are NOT paying enough attention to this important aspect of modding.

It has been my experience that good heatsinking transmits heat to the flashlight body very quickly. Also poor heatsinking will eventually get heat to the outside. Its just about the time taken and the potential damage with overheating that we all should be concerned with.

 

[jki]

Good thermal path design absolutely does pull heat away quickly from the LED. The heat dissipation from the LED is a given. But take a look at the MagLED module for an example of the results of poor thermal conduction. The output of the LED drops to 50% of its initial brightness within just a few minutes.
My LumaPower D-mini on RCR123 looses a noticeable amount of initial brightness after 5-10 minutes or so; by eye about 25% reduction, and it gets pretty warm (not hot). My MRV looses maybe 10% in the same amount of time on an 18650, and gets only modestly warm. And it is putting out more light than the D-mini by measurements of fellow CPF'ers. I would not say that the D-mini has better thermal design than the MRV "because it gets hotter"! That is my point. Actually, the contrary is true: the MRV has the better thermal design (a lot due to the sheer thermal mass of the brass heatsink). Because of better thermal design, the MRV LED runs cooler and does not drop in intensity as much as the D-mini due to efficiency falloff with temperature rise. Hotter is not necessarily better!
Keep in mind that the purpose of good thermal path is to remove heat quickly from the LED, and in fact has an influence on the LED temperature. If the LED is fed with constant current source from the regulator, the hotter the LED is allowed to run, the less efficient it is in converting amps to watts. When it is less efficient, it produces more waste heat instead of light, and the temperature rises. When the temperature of the LED rises, then the heat load increases on the thermal path, and the flashlight gets warmer. It is a vicious cycle.
Which brings me back to my initial post - hotter is not necessarily better, or even good!