Anyone done an LED Heat-study?

Hi,
I was wondering if anyone had done/has equipment to do a heat-generation study on either Cree or SSC LEDs.
The reason I am asking is to find out what kind of configuration is best thermally speaking: Will 1SSC running at 1A generate more than three times the heat of one running at .33A? (The answer is almost certainly Yes, with the decrease in efficiency above .35A, however by how much?)
I would be interested to know if there is a near-linear relationship, or a point after which heat generation increases disproportionately.

-Xiorcal

The heat produced by an LED is basically the power dissipated by the LED, or If * Vf. So you need to know the input current and Vf to calculate how much power it's dissipating. No special equipment needed, just a DMM.

Like you surmised, one LED driven at some current X will always produce less light than 3 of the same LED each driven with a current X/3. The reasons are two-fold: 1) the Vf increases with current, and 2) efficiency drops as current rises (independent of reason 1).

Here are measurements from my Seoul P4:


Compare the 310 to 980mA values (it's pretty close to an even 3X comparison).

At 310mA, you get 86 lumens, but at 980, you only get 213. Drive 3 at 310, and you get a total of 258 lumens for only 3.03W, vs. 213 for 3.61W

So a single P4 at 980mA = 213 luments for 3.61W
3 P4s at 310mA each = 258 lumens for 3.03W

Using 3 lower driven P4s gives you 21% more light while using 16% less power, or around 40% more efficient.

The downside is the higher expense for more LEDs, larger physical space needed, more complicated optics (depending on the application), and more complicated driver and wiring.

The upside is less heat to deal with, higher efficiency, longer LED lifespan, lower power draw, and less color shifting due to high current in the LED.

my dad works at dupont and one of the groups there that he works closely with is working with cree on developing a layer to go directly under the emitter that is not conductive but is very good at conducting heat. leds have to produce heat to work but the quiker you can get it away from the emmiter the better so thats what this layer will do just a side note

BenCox said:

my dad works at dupont and one of the groups there that he works closely with is working with cree on developing a layer to go directly under the emitter that is not conductive but is very good at conducting heat. leds have to produce heat to work but the quiker you can get it away from the emmiter the better so thats what this layer will do just a side note

Click to expand...

I had been wondering if there's anything that conducts heat well and electricity poorly.

LukeA said:

I had been wondering if there's anything that conducts heat well and electricity poorly.

Click to expand...

Diamond

Tom

evan9162 said:

Here are measurements from my Seoul P4:

Code:

Current (ma)	30	130	310	670	980	1260

Vf (volts)	2.71	3.03	3.26	3.53	3.68	3.81

Pd (watts)	0.08	0.39	1.01	2.37	3.61	4.80

Lumens		N/A	38	86	163	213	251

Lumens/Watt	N/A	95	84	68	59	52

Click to expand...

How would this particular P4 would respond to direct drive from 2 x 1.5v Alkaline cells? (No resistor.)

Could a direct driven P4 be used for an efficient, moderate output, long runtime light? (Without a series reistor it should have better efficiency.)

Maybe it would start at about 100ma and 30 lumens, then decline gradually?

Also, how would this P4 respond to direct drive from two Lithium AA cells?

I don't have figures handy for internal resistance of alkaline and lithium cells.

.

To BenCox --


Good luck to your father in his quest to make our lights even better !


Oh, and welcome to CandlePowerForums.


I think you're gonna' like it here.

ringzero said:

How would this particular P4 would respond to direct drive from 2 x 1.5v Alkaline cells? (No resistor.)

Could a direct driven P4 be used for an efficient, moderate output, long runtime light? (Without a series reistor it should have better efficiency.)

Maybe it would start at about 100ma and 30 lumens, then decline gradually?

Also, how would this P4 respond to direct drive from two Lithium AA cells?

I don't have figures handy for internal resistance of alkaline and lithium cells.

.

Click to expand...

a) Using 2 alkalines, current would start somewhere in the 100-200mA range

b) Using 2 1.5V lithiums, current would start out in the 300-400mA range.

-deleted-

Erasmus said:

I can make a calculation using evan9162's results. Maximum luminous efficacy is 683 lm/W. At .31A the SSC outputs 84lm/W or an efficacy of 12.30%. That means to other 87.70% of the energy is heat, being .88577W of heat. At .98A the SSC outputs 59lm/W or an efficacy of 8.64% meaning the other 91.36% of the energy is heat, in this case 3.298W. So in fact the SSC generates 3.72 times more heat when driven at .98A compared to .31A.

Here you go, sir.

Click to expand...

That's not quite correct. Maximum 683 lm/W is only for monochromatic green light at 555nm (src: http://hyperphysics.phy-astr.gsu.edu/hbase/vision/bright.html#c2 )

White light is going to be somewhere in the 250-300 lm/W range given the color spectrum of a blue-pumped yellow phosphor. Let's just pick an easy value like 250.

So at 310mA, 84 lm/W = 33.6%, and at 980mA, 59 lm/W = 23.6%

66.4% of 1.01W = 0.67W going up as heat at 310mA
76.4% of 3.61W = 2.75W going up as heat at 980mA

So 980mA will produce 4.1x as much heat if you take emitted light into account. However, I don't think people generally perform these calculations, as using the full input power as a power dissipation. Even the LED manufacturers still use full input power when calculating junction temperature. Until we get to higher fractions of optical efficiency, it's just easier to use full input power for calculations, as its simpler, and you'll always err on the side of caution.

How would this particular P4 would respond to direct drive from 2 x 1.5v Alkaline cells? (No resistor.)

Could a direct driven P4 be used for an efficient, moderate output, long runtime light? (Without a series reistor it should have better efficiency.)

Click to expand...

Although efficiency will be good as there will be no resistor losses, altogether the light will waste a lot of energy. Once the cells' capacity drops even slightly, its terminal voltage will start to drop -- meaning the light will go almost completely off while the batteries still have a lot of unused capacity.

The best way to use a two-cell would be with a regulated boost circuit set to a low drive current. The boost means the entire capacity will be used. By setting a low current limit though, you get the following benefits:

1) The fact that the current draw is low means the battery won't waste a lot of energy due to internal resistance losses. This should give effectively a higher capacity
2) The LED is most efficient at very low currents

a 200mA light should run for about 12 hours straight off of freshly charged NiMH cells, less off of alkalines (more internal resistance).

evan9162 said:

a) Using 2 alkalines, current would start somewhere in the 100-200mA range

b) Using 2 1.5V lithiums, current would start out in the 300-400mA range.

Click to expand...

Thanks evan9162.

The highest lumens/watt listed in your table is 95.

Have you looked to find the current that produces the max lumens/watt possible for your P4?

I'm interested in building a very simple, efficient, reliable light of modest output. Objective would be to maximize lumen-hours of output with two alkaline cells - modest, but useful output for the longest possible time.

20 lumens for 20 hours is 400 lumen-hours. Maybe starting at 30 lumens on fresh cells and gradually declining to 5 lumens as the cells deplete. It would be great if I could achieve 400 lumen-hours from 2 AA alkies, or close to that figure.


.

2xTrinity said:

Although efficiency will be good as there will be no resistor losses, altogether the light will waste a lot of energy....a 200mA light should run for about 12 hours straight off of freshly charged NiMH cells, less off of alkalines (more internal resistance).

Click to expand...

Thanks 2xTrinity. I was hoping to avoid the boost circuit for greater reliability and efficiency.

10 to 20 lumens is what I'm shooting for. That range is enough output for a backup light for me for outdoors use.

I'd like to build or buy a P4 light that puts out 10 to 20 lumens for the longest possible time.


.

ringzero said:

Thanks evan9162.

The highest lumens/watt listed in your table is 95.

Have you looked to find the current that produces the max lumens/watt possible for your P4?

I'm interested in building a very simple, efficient, reliable light of modest output. Objective would be to maximize lumen-hours of output with two alkaline cells - modest, but useful output for the longest possible time.

20 lumens for 20 hours is 400 lumen-hours. Maybe starting at 30 lumens on fresh cells and gradually declining to 5 lumens as the cells deplete. It would be great if I could achieve 400 lumen-hours from 2 AA alkies, or close to that figure.


.

Click to expand...

I haven't. This P4 is in a Mag 3D, running at 700mA. It's pretty difficult to make measurements (it involves soldering extra wires to the LED).

I think others have found the peak efficiency - it's around 25mA or so.