Bikelight heat disapation vs speed
- Thread starter Hmmm
- Start date
Help Support Candle Power Flashlight Forum
BrianMc
Enlightened
- Joined
- Nov 4, 2009
- Messages
- 940
How much heat is moved is a function of air temp and humidity. Hot day air is not as effective as cold humid night air. Also, unless you are dropping power at slower speeds, you need to design for the worst (slowest) not best (fastest) air cooling speed. It is very hard to model this well even in very simple lights so the usual DIY approach is to stay above certain minimal values arrived at by trial and error.
The minimum for not overheating to hand burning temperatures and lower light output appears to be about 1 square inch per watt of total consumption though this may not be enough. Heat output with P7's should be about 15% less, but the driver efficiency and heat output play here too. With good airflow at temperatures of about 20 degrees C or lower, the 1 square in per watt is pushing at the lower limit. A full copper liner to spread the heat fast over the whole area of the light is heavy, but I found it helps push this limit. Upping to 1.5 inches allows for some less than perfect junctions and thermal path in a light and for higher temperatures. Up it to 2 to 2.5 square inches per watt if you are going to be sitting in dead calm for a while, I am not sure how much you need in Alice Springs in the height of the Aussie summer with the light as a daytime running light, black anodized, and in full sun, but you have the basic idea.
Not as nice as a neat formula, but that should help some.
The minimum for not overheating to hand burning temperatures and lower light output appears to be about 1 square inch per watt of total consumption though this may not be enough. Heat output with P7's should be about 15% less, but the driver efficiency and heat output play here too. With good airflow at temperatures of about 20 degrees C or lower, the 1 square in per watt is pushing at the lower limit. A full copper liner to spread the heat fast over the whole area of the light is heavy, but I found it helps push this limit. Upping to 1.5 inches allows for some less than perfect junctions and thermal path in a light and for higher temperatures. Up it to 2 to 2.5 square inches per watt if you are going to be sitting in dead calm for a while, I am not sure how much you need in Alice Springs in the height of the Aussie summer with the light as a daytime running light, black anodized, and in full sun, but you have the basic idea.
Not as nice as a neat formula, but that should help some.
Last edited:
find_bruce
Newly Enlightened
- Joined
- May 5, 2011
- Messages
- 84
Somehow I suspect the rider will overheat before the light - have you heard the expression "mad dogs & englishmen go out in the midday sun"
But I did get your point, thanks
I ride consistently 25+mph so I would only use full brightess when riding that fast, I can dial down the brightness if I feel it heat up. Does the humidity realy matter for a light? Its not like evaporation plays an important role. I personaly only use my bikelight at night.
Last edited:
BrianMc
Enlightened
- Joined
- Nov 4, 2009
- Messages
- 940
I find a daytime running light has reduced right hooks. Drivers here are very sloppy and not bike aware. I used to be able to average your speed. Even slower, they misjudge my speed. A light helps reduce this.
Air is a fluid. It has a heat capacity. Riding in the rain adds water cooling. At the same temperature, air with more water has a higher heat capacity, and if you are pushing the limits it would have a small effect.
Always the tradeoff. Cooler equals larger area or more air flow for the same area. Larger area equals more weight.
The LED junction will run about 20 C hotter than ambient with a good path and good air flow. At 30C air temp, 50 C junction temp and a 10-20% output reduction in lumens. If you don't have enough area and air flow, then the heat is not moved out of the light as fast as it is made and the air interface will become hotter and increase transfer until the heat trasnfer equals the production. I can tell you from expereince 0.7-0.8 square inches per watt with excellent thermal path, requires a very good air flow or a cool night. I'm using 1 sq in/watt minuimum from now on.
Hope this helps.
Air is a fluid. It has a heat capacity. Riding in the rain adds water cooling.
At the same temperature, air with more water has a higher heat capacity, and if you are pushing the limits it would have a small effect. Always the tradeoff. Cooler equals larger area or more air flow for the same area. Larger area equals more weight.
The LED junction will run about 20 C hotter than ambient with a good path and good air flow. At 30C air temp, 50 C junction temp and a 10-20% output reduction in lumens. If you don't have enough area and air flow, then the heat is not moved out of the light as fast as it is made and the air interface will become hotter and increase transfer until the heat trasnfer equals the production. I can tell you from expereince 0.7-0.8 square inches per watt with excellent thermal path, requires a very good air flow or a cool night. I'm using 1 sq in/watt minuimum from now on.
Hope this helps.
