Heat question

[videoman]

Hello, which setup would produce less heat on the heatsink, each setup to produce 900 lumens. 3-XPG's driven 300 lumens output each or 6 -XPG's driven 150 lumens output each? all setups on the same size heat sink and using identical XPG's. It is for a lamp and not a flashlight. Thanks

 

[AnAppleSnail]

Hello, which setup would produce less heat on the heatsink, each setup to produce 900 lumens. 3-XPG's driven 300 lumens output each or 6 -XPG's driven 150 lumens output each? all setups on the same size heat sink and using identical XPG's. It is for a lamp and not a flashlight. Thanks

To get an exact answer, you would do some mumbo-jumbo with the luminous efficacy charts from Cree and get a number - better, you could figure out how much heat you're likely to be able to dissipate. But broadly speaking, the next lumen from a given emitter is going to take more watts than the previous one.

 

[videoman]

Efficacy from wattage is not really my concern at the moment, as I can sacrifice efficacy for less heat.All things being equal, I am looking for the coolest (heat wise) option, to get that 900 lumen output.From what I read and understood so far, using more leds at a lower output will be advantageous in many ways, but also for heat issues ? or is it that heat is the same for the same total lumen output regardless of leds used? I fail to see on the Cree spec sheet the temperature behind the emitter at different lumen output. If 2 emitters at 150 lm each dissipate both together even 10% LESS heat than 1 at 300lm, that would be ideal.The reason I must know is that I am experiencing thermal shutdown on the driver with 3 leds when they are on just after 20 minutes. I would imagine that using 6 at reduced power will produce less heat just enough so that I can avoid that situation.

 

[Th232]

If my understanding is correct, yes, the 6 LED option will produce less heat.

Heat produced is tied into the efficiency of the LED, basically energy in = light energy out + heat energy out. LEDs are more efficient at lower currents, and will therefore produce more light per watt and less heat per watt.

 

[videoman]

Thanks a million !!

 

[syc]

Check out the second graph in jtr1962's R5 test. Its a graph of lumens per watt at different current levels for the XPG R5, you can see that the efficiency drops from about 140 l/w down to 115 l/w when you push the current from 350mA (150l) to 800mA (300l)

Looks like you're generating about 18% more heat with 3 led's vs 6 leds.

 

[Ekke]

You also get more lumens because of the smaller die temperature. If you have same cooling for each led..

EDIT: so "on the same size heat sink" there isn't that extra advantage..

 

[LukeA]

Down to about 50mA per die, more LEDs at lower current will give you more proportionally more output with proportionally less heat, at the expense of throw in most cases though.

Because the larger number of LEDs at low current will use less power than fewer LEDs at higher current, a heatsink of a given size will have to dissipate less power, and so will stay cooler. Also, the area for the LEDs to transfer heat to the heatsink will be greater, which will further reduce the temperature.