TheFishMan65
New member
60/40 is the recommended when mixing XP-G and XP-E, but it is all personal taste. I have not heard enough on adding blue or green, but I have heard positives on neutrals.
DIY LEDs - The write-up
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der_wille_zur_macht
Team RC
I don't know if I'd make that sort of general restriction. I've run lots of LEDs on U-channel at typical spacings and power levels (i.e. an LED every 2 - 3", 700mA) with a fan and things are totally fine. Without a fan, yeah - I'd be really worried and would scale back. Clearly though there are lots of variables that are going to come in to play. But given the performance I've seen from home-built U-channel "heatsinks" I really have come to the conclusion that the heatsink-usa stuff is wildly overkill, expensive, and heavy.
TheFishMan65
New member
MosMike,
"Rule of thumb: 10 cm2 of heatsink surface per 1 W". First time I remember hearing this. Could you please tell me more?
"Rule of thumb: 10 cm2 of heatsink surface per 1 W". First time I remember hearing this. Could you please tell me more?
The Grim Reefer
Ready for some NOBALL!!!
I run 2x1 1/8th channel with the LED's inside the channel and temp isn't an issue. You can find the stuff online.
katchupoy
New member
This is what I used. Runs directly on 12 volts, and can be dimmed (voltage selector) You can cut and solder it for max flexibility. And also sealed for protection with water splash or whatever.
http://www.autozone.com/autozone/ac.../_/N-25pt?counter=8&itemIdentifier=22380_0_0_
$20.00
TheFishMan65
New member
I am apparently missing something.
The XP-G produce 139 lumens at 350 ma. At 700 ma it produce 180% as much or 250 lumens and runs at 3.2 volts so 250 / (3.2 * .7) = 111.6 lumens per watt.
The XM-L produce 280 (if you get the best) lumens at 700 ma. At 700 ma it runs at 2.9 volts so 280 / (2.9 * .7) = 137.9 lumens per watt.
I thought someone said they were twice as good or at least way better. If you look at lumens at base current it appears that way, but not if you do the math. So did I miss something or am I remembering wrong? they are 25 more efficient which is a good thing.
The XP-G produce 139 lumens at 350 ma. At 700 ma it produce 180% as much or 250 lumens and runs at 3.2 volts so 250 / (3.2 * .7) = 111.6 lumens per watt.
The XM-L produce 280 (if you get the best) lumens at 700 ma. At 700 ma it runs at 2.9 volts so 280 / (2.9 * .7) = 137.9 lumens per watt.
I thought someone said they were twice as good or at least way better. If you look at lumens at base current it appears that way, but not if you do the math. So did I miss something or am I remembering wrong? they are 25 more efficient which is a good thing.
TheFishMan65
New member
Oh that is where the 2x comes from. For those that care.
The XR-E produce 107 lumens at 350 ma. At 700 ma it produce 170% as much or 181.9 lumens and runs at 3.6 volts so 181.9 / (3.6 * .7) = 72.2 lumens per watt
The XR-E produce 107 lumens at 350 ma. At 700 ma it produce 170% as much or 181.9 lumens and runs at 3.6 volts so 181.9 / (3.6 * .7) = 72.2 lumens per watt
I was thinking the same...
I never understood why add almost 25 lbs to my canopy to cool down something that 1) doesn't get too hot, 2) can be easily achieved by a $4 few ounces heavy fan(s).
Since I am not putting the LED's too close together, I'm actually planning on using any type of sturdy flat metal sheet (metal screen?) as my heat sink - cheap and available at Home Depot.
TheFishMan65
New member
The disadvantage of fans is they cost money to run and can be noisy. Long term is it cheaper to get a heatsink or run a fan. I don't know.
IMHO do not use screen. You will not get good thermal transfer from the star. Now if you could only punch holes away from the start that may help. A large sheet will not have much flow in the center. Do you have a break? Maybe you could make your won U channel?
IMHO do not use screen. You will not get good thermal transfer from the star. Now if you could only punch holes away from the start that may help. A large sheet will not have much flow in the center. Do you have a break? Maybe you could make your won U channel?
My experience is just the opposite, however I'm usually running them at 1A, I'd be very worried running them on U channel. I'm actually replacing a 8X8 heatsink USA heatsink with one that has twice as many fins that are an inch taller (and I'm using a fan).
This was from an old Cree document. They determined that a 1"x1" flat surface is sufficient for 1W LED without fans. I repeated the experiment - the LED was too hot, so a 1.25x1.25" surface is a safe margin.
Of course with active cooling U-channels are sufficient for 2-3W per LED, but I prefer passive cooling. Also we should not forget that hot LEDs are less efficient than cool. For example, @350mA with a die temp of 35C the loss is 3%, @700mA with a die temp of 75C the loss due to overheating is 12%. This means that such losses will be higher in an XM-L @>1000mA vs. XP-G of the same cost.
It should be noted, that this swap is suitable only for active cooling. Heatsinks for natural convection (=passive) must have large gaps between fins. BTW, the best type for natural convection is the pin heatsink, like this one:
http://www.aavidthermalloy.com/technical/papers/pdfs/elliptical.pdf
der_wille_zur_macht
Team RC
I follow you until this statement. What data do you have that supports XM-L running hotter/less efficient than XP-G under given conditions?
kcress
New member
How can you state vastly different currents and their efficiencies as 'temperature' efficiencies? Apple and oranges.
TheFishMan65
New member
MosMike,
There is another possibility open to us. Drill a few holes to allow upward air flow. In most of our cases the fins/pins are pointed upward and the internal pins have trouble getting air flow. Imagine if a hole is drilled in the middle. This lets air flow up through the hottest part.
Go back here and look at the last picture. You can see how the heatsink I found has slot cut in it periodically (just luck that it matches a star size).
Do you have any documentation or opinion on whether this is better or worse?
There is another possibility open to us. Drill a few holes to allow upward air flow. In most of our cases the fins/pins are pointed upward and the internal pins have trouble getting air flow. Imagine if a hole is drilled in the middle. This lets air flow up through the hottest part.
Go back here and look at the last picture. You can see how the heatsink I found has slot cut in it periodically (just luck that it matches a star size).
Do you have any documentation or opinion on whether this is better or worse?
I can't get around not using a fan due to the space I have available in the canopy. I found really cheap ($3 ea. shipped) 120mm fans on eBay claimed to make less than 17 db of noise.
Thanks for the input on the screen. I'll use sheets instead then.
I'm planning on having 2 panels, 12" x 25" each, with 2 fans side by side horizontally in the middle blowing air downward (cold air would enter through far sides of panels).
Very simple:
XM-L T6 - $7.90 280lm @700mA 35lm/$1
XP-G R5 - $4.90 139lm @350mA 28lm/$1
The 7 lumen per dollar difference can be "destroyed" by the fact that XM-L die is running hotter at a higher current. Actually an active cooling of the LEDs can increase the lumen output by 5-10%.
This is better. In reality extruded heatsinks work better when the heatsink is perpendicular to the ground and the fins generate convection currents.
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