Does the feeling of hot on flashlight mean it has a good heat dissipation?

[XTAR Light]

When you use a LED flashlight, you will feel it getting hot on the shell, especially the Aluminum flashlight. Is it normal?

Actually, the shell is hot means your flashlight may has a good heat dissipation. If it is not hot on its shell, it is smoldering, so that the LED is easily damaged.

When LED lamp is lighting, it converts electrical energy into light energy. During the conversion process, most of the electrical energy is converted into heat energy, and only a small part is converted into light energy. Therefore, you will feel the shell of the flashlight is hot when it is working, and the heat comes from high-power LEDs. If you feel the heat quickly, that means the internal components of the flashlight have a high thermal conductivity, and the heat is spread quickly and dissipated very well. This is a normal heat dissipation phenomenon.

Conversely, if you don't feel the heat in the shell of your flashlight, it means that the heat is accumulated inside the LED, which is dangerous to the LED. The LED will deplete too quickly due to the high temperature, resulting in a sharp decrease in life. After using a flashlight with poor heat dissipation for a period of time, the core part of the LED will turn yellow and the brightness will drop to about 50%. This is because the temperature causes the LED to age too fast.

High quality flashlight is designed with corresponding heat dissipation and current control to ensure that the LED can work at a proper temperature.

Any more opinions are welcomed!

 

[Swedpat]

Interesting topic.That is as I understand it. But it also depends on the output. The efficiency of a modern LED is high especially at low output because in opposite to an incandescent bulb the LED does not lit up because it's hot. At very low output an LED does not heat up at all. But at high output it heats up and the efficiency becomes lower. An incandescent becomes more efficient with more power and is most efficient before the moment it burns.

 

[richbuff]

I see the point that XTAR is making. The heat is generated inside, and if conductivity to the outside is poor, the surface of the flashlight does not get hot quickly, because the heat is trapped inside, where it will do damage. If instead, conductivity from inside to outside is excellent, the outside of the flashlight will feel hot quickly, because heat is being quickly conducted from the inside to the outside, where it can be dissipated.

Three things: Sinking depends on mass.
Dissipation depends on surface area.
Conductivity from sink to surface is dependent upon proper design.

 

[WalkIntoTheLight]

Interesting topic.That is as I understand it. But it also depends on the output. The efficiency of a modern LED is high especially at low output because in opposite to an incandescent bulb the LED does not lit up because it's hot. At very low output an LED does not heat up at all. But at high output it heats up and the efficiency becomes lower. An incandescent becomes more efficient with more power and is most efficient before the moment it burns.

It also depends on the driver, even at the same output. Linear drivers just burn off excess voltage as heat. FET drivers cause the LED to operate very inefficient, causing heat. Boost/buck drivers do a much better job maximizing efficiency of both the driver and LED, reducing heat.

Most budget lights use linear drivers (7135 chips) and FET drivers. More expensive lights use regulated boost drivers.

 

[likethevegetable]

All things equal except for mounting the MCPCB, I would say hotter aluminium, especially near the body and further from the head, means better heat transfer to the body and dissipation.

The problem in real life is considering LED and driver efficiency, thermal resistance of aluminum used, etc. in a comparison.

 

[Fireclaw18]

I see the point that XTAR is making. The heat is generated inside, and if conductivity to the outside is poor, the surface of the flashlight does not get hot quickly, because the heat is trapped inside, where it will do damage. If instead, conductivity from inside to outside is excellent, the outside of the flashlight will feel hot quickly, because heat is being quickly conducted from the inside to the outside, where it can be dissipated.

Three things: Sinking depends on mass.
Dissipation depends on surface area.
Conductivity from sink to surface is dependent upon proper design.

This.

However, there are even more factors. Conducting heat to the outside of the light only goes so far. You also need to shed heat from the light entirely if you want to sustain any kind of higher output.

For shedding light, some factors which may help:
* Use a material with high emissivity - dark anodized aluminum or tarnished copper works great. Polished aluminum, titanium and copper works poorly.
* Have lots of surface area - this is what heat sink fins are for: they increase surface area to atmosphere so more heat can be shed.
* Have an unobstructed thermal path from the star to the outside of the light. This is one reason why Zebralights are so great at managing heat: they are manufactured from a single piece of aluminum that contains the shelf, head and body. Fewer transitions means more efficient heat transfer.

 

[Julian Holtz]

[] If you feel the heat quickly, that means the internal components of the flashlight have a high thermal conductivity, and the heat is spread quickly and dissipated very well. []

Any more opinions are welcomed!

I do not agree with this statement.

Generally, the LED will produce the same amount of thermal power over a wide temperature range. So, it does not make a difference if it is cooled well or it heats up so that it is barely destroyed, the amount of thermal power produced at a specific drive current is about the same. If there is a difference, then in a way that the forward voltage decreases with temperature, so an LED running hot would produce a bit less thermal power.

This thermal power is dissipated by the flashlight body, as it cannot go anywhere else. It will be dissipated independently from the thermal conductivity of the material. The thermal energy transfer through a thermal conductor depends on the thermal conductivity and the temperature difference between the input and output side.
The outside temperature of the flashlight body will rise until the thermal power will be dissipated by the ambient air. Given that the air temperature and the flashlight body shape stay the same, the flashlight body temperature will only depend on the amount of thermal power needed to be dissipated.

So we have these fixed values:
- identical flashlight body temperature because of:
- identical LED thermal power

These are the variables:
- thermal conductivity of the LED to the body
- temperature difference between LED and body

The laws of thermodynamics dictate (and this is not just an opinion, but a fact) that the LED will run hotter if the thermal conductivity is bad. What will not change is the flashlight body temperature. It is a misconception that the heat of an LED not well cooled can somehow "not reach" the flashlight body. The LED will just heat up to a point where the thermal energy is "forced" into the flashlight body, even if the connection does not transfer thermal energy well.
Therefore, it is impossible to judge from the flashlight body temperature alone if the LED is cooled properly.
If anything, a flashlight with a properly cooled LED will heat up slower as the thermal capacity of the metal inside needed to transfer the thermal power will absorb some of it at first.

tl;dr:
An LED will produce the same amount of thermal power, no matter the demperature it has. This heat will reach the flashlight body surface, no matter what.
The heat transportation can happen at a low temperature difference between the LED and the flashlight body and high thermal conductivity. It can also happen at a high temperature difference and a low thermal conductivity. Which it is of both cannot be judged by the flashlight body temperature.

 

[defloyd77]

Feeling hot does not necessarily mean good heat dissipation. I have noticed with a few of my Lumintop AAA lights that have the same drivers in their heads, but different materials (copper and aluminum Tools and a steel Worm) the better the conductivity of the metal, the LESS hot it gets. Materials with higher thermal conductivity will warm up faster to a lower temperature, whereas materials with lower thermal conductivity warm up slower to a higher temperature.

 

[orbital]

+

There is no such thing as 100% efficiency,,
really only looking for a light to get warm when driven higher.

not hot though

{if I was doing some stupid intense cpu benchmarking and my heatsink was cool to the touch, I'd be concerned}