# What Is The Most Efficient LED At A Low Current Draw?



## CyberCT (Aug 10, 2012)

I'm looking for which LED has the most efficiency at say 60ma or 2-3 lumens worth of output. I want a flashlight that will have a VERY long runtime on the lowest mode. I have XPG and XML based lights, never looked into the smaller LEDs because I'm more of a fan of flood vs throw, so maybe something like the XPE any better at a low draw?


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## mvyrmnd (Aug 10, 2012)

I suspect the new XP-G2 would be a competitor here.


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## ShineOnYouCrazyDiamond (Aug 10, 2012)

At the levels you are looking for I would say the efficiency of the drive is more important than that of the LED itself. At low levels like what you are talking about the type of driver setup may make more of a difference. The magnetic ring systems have a bit of system overhead, so even at lower and lower currents they still tend to draw the same amount of power. Clickies will usually have less driver overhead and improve on efficiency.

Do some searching and you will see that there are plenty of XP-G and XM-L light that advertise weeks/months on their lowest settings. Take a look at the 4Sevens and Zebralight lights. I think you may find what you are looking for in one of those two.


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## Gunner12 (Aug 10, 2012)

At 20 mA, the XP-E Q5 (not top bin or brightest emitter) would be 6+ lumen already. IIRC, from the White LED lumen testing thread, 20 mA is where the ~1mm x 1mm LEDs hit their max efficiency. Lower and you get less efficiency. What might work for you is a top bin XP-C/XR-C. It's a 0.7mm x 0.7mm die (.49 mm^2). At half the surface area, it should hit max efficiency at 10 mA, giving half the output of the XP-E.

What battery will you be using? IIRC, the LEDs have a Vf of lower then 3 v at these currents.


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## CyberCT (Aug 11, 2012)

Gunner12 said:


> At 20 mA, the XP-E Q5 (not top bin or brightest emitter) would be 6+ lumen already. IIRC, from the White LED lumen testing thread, 20 mA is where the ~1mm x 1mm LEDs hit their max efficiency. Lower and you get less efficiency. What might work for you is a top bin XP-C/XR-C. It's a 0.7mm x 0.7mm die (.49 mm^2). At half the surface area, it should hit max efficiency at 10 mA, giving half the output of the XP-E.
> 
> What battery will you be using? IIRC, the LEDs have a Vf of lower then 3 v at these currents.



I have both eneloops NIMHs, D cell NIMHs, and 18650s so either power source would work for me. What flashlights would you recommend? I'm biased toward Fenix because of their true waterproofness but for something and high quality but of such low output it would be used as a constant light source, not for underwater snorkeling. The XML is less efficient at 80ma than say the XPG at 80ma, am I right?


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## samgab (Aug 11, 2012)

CyberCT said:


> I have both eneloops NIMHs, D cell NIMHs, and 18650s so either power source would work for me. What flashlights would you recommend? I'm biased toward Fenix because of their true waterproofness but for something and high quality but of such low output it would be used as a constant light source, not for underwater snorkeling. The XML is less efficient at 80ma than say the XPG at 80ma, am I right?




I'm not sure if you're asking about raw emitters, or commercial flashlights.
For flashlights, Zebralight have incredibly efficient circuits and they go for weeks on extremely low settings, and also go very hard on the high settings. So any of the ZL flashlights.

As for raw emitters, I don't have specs on the new G2 XP-G, but I have a comparison of the XM-L vs the original XP-G from the Cree product characterization tool:





The other emitters, like XP-E, XR-E, etc, only get _less _efficient and _less _efficacious. 
You can't go to really low levels using this Cree tool, but you can see the trend and project the outcome.

EDIT:
On second thoughts, looking at the XP-E (Green line in the chart below) that might not be the case:




(That's using an R4 XPE, an R4 XPG, and a T6 XML, since XMLs don't go down to R4 bins)
(Raw Data here)


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## CyberCT (Aug 11, 2012)

Wait, so if I'm reading that right, the XML is more efficient than the XPG in all current draws? I thought the XPG was more efficient until 500mah or so.


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## samgab (Aug 11, 2012)

CyberCT said:


> Wait, so if I'm reading that right, the XML is more efficient than the XPG in all current draws? I thought the XPG was more efficient until 500mah or so.



I don't have numbers on the new Generation II XPG yet, but from what I've seen from my hands-on testing and on paper, the XM-L is the most efficient emitter I can find at all current levels (up to 3A).


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## wquiles (Aug 11, 2012)

I created a small size Joule Thief circuit to drive LED's at low current (50-100mA) and found the XM-L to be excellent in this regard. I can drive the LED's at about 50mA with a 1.5 volts, and about 100mA with about 3.0 volts - here is the link for more information on the lights myself and others have built using these small circuits.

Will


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## Gunner12 (Aug 11, 2012)

It's very possible that I'm wrong, and that LEDs get more efficient at lower currents. I haven't done any LED test so I can only refer to things that I've read. Judging by this post, that specific XP-G R5 is less efficient at 10 mA then at 20 mA and 40 mA which is why I said that for max efficiency at low currents, a smaller die might be better. It is possible that the overall efficiency of the XM-L means it is brighter then all other LEDs at this low current though.

Generally, the XM-L should be more efficient then the XP-G.

Zebralights have very low modes and they seem pretty efficient.


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## SemiMan (Aug 11, 2012)

CyberCT said:


> I'm looking for which LED has the most efficiency at say 60ma or 2-3 lumens worth of output. I want a flashlight that will have a VERY long runtime on the lowest mode. I have XPG and XML based lights, never looked into the smaller LEDs because I'm more of a fan of flood vs throw, so maybe something like the XPE any better at a low draw?



The XTE R5 bin hits 180 lumens/watt at 100mA, so probably 170+ at real temperatures when running at low currents.

The data sheet makes it looks like it is quite linear down to 0 current, but I would guess there is some point where the efficiency starts to drop. That point is pretty easy to find. Just adjust current and see where the peak is. That is where you operate for maximum efficiency.

When you want even lower light levels, then you PWM the drive at the maximum efficiency point. Just make sure whatever drive circuit you are using has really low quiescent current.

Semiman


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## AnAppleSnail (Aug 11, 2012)

How vital is strictly maintained output? Try 2 (various alkaleak, NiMH, etc), direct-driving an led. It may drop-in in output, but will be drive-thru efficient.


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## CyberCT (Aug 12, 2012)

Well rumor has it that Cree is going to do an XM-L2 like they did the XP-G2. I'll just wait some months and see. If you can simply swap the LED and the driver, opitcs, etc work just like with the XP-G and XP-G2, Fenix will do doubt come out with something new and improved. IOne of their newer lights, or another manufacturer like Zebralight, will have to have a 3 or so lumen low mode that will last forever. I'll be replacing the LEDs in my two modified Rayovac lanterns with neutral XM-L2s also.


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## Tobias Bossert (Aug 12, 2012)

The Cree Product Characterisation Tool (CPTC) provides you with the required date for currents above 100 mA or above 1/20 of the maximum current which ever is higher. You can compare up to three types at once. The most recent types XM-L U2 and XP-G S2 provide their best efficiency at about 1/20 of their specified maximum current. These modern types show a relatively flat maximum and don't lose so much efficiency at higher currents. Older types like XR-E R2 show a quite different behaviour. Its maximum is sharper and occurs towards lower current levels.
XM-L U2: 165 lm/W @ 150 mA - 160 lm/W @ 350 mA
XP-G S2: 157 lm/W @ 100 mA - 148 lm/W @ 350 mA
XR-E R2: 167 lm/W @ 100 mA - 114 lm/W @ 350 mA

So the most efficient LED at current levels at or just below 100 mA is'nt a recent one, but the old XR-E!

If you need still lower current levels, you should leave DC drive current and go towards PWM. With PWM you can allways hold the peak current at the current of maximum efficiency and modify the average current by the pulse duty factor.


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## A10K (Aug 13, 2012)

Check the PCT for the XB-D and XT-E.
At 100mA an R5 XT-E gets over 182 lm/W. (assuming a 25C temperature, which is reasonable at this power level)
Its got an ugly color dispersion with tighter optics, but if you're worried about color quality driving CREE LEDs at low currents isn't the way to go--many suffer notable tint shifts as the drive current gets lower.


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## CyberCT (Aug 13, 2012)

A10K said:


> Check the PCT for the XB-D and XT-E.
> At 100mA an R5 XT-E gets over 182 lm/W. (assuming a 25C temperature, which is reasonable at this power level)
> Its got an ugly color dispersion with tighter optics, but if you're worried about color quality driving CREE LEDs at low currents isn't the way to go--many suffer notable tint shifts as the drive current gets lower.



Right, and I'd rather go with a neutral white LED instead of cool white because the yellowish color at such a low drive is nicer than the green/blue clor.


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## SemiMan (Aug 13, 2012)

CyberCT said:


> Right, and I'd rather go with a neutral white LED instead of cool white because the yellowish color at such a low drive is nicer than the green/blue clor.



I can't say I have noticed much in the way of color shift with the XTE between 350 and 100mA. If I ever get a new hear ordered for my spectrometer I could run some tests but visually I do not notice much difference. For most Cree, I have not noticed a big change at 100mA and 350mA. Yes, over larger drive currents/die temps. They are certainly not as consistent as rebels.

Semiman


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## CyberCT (Aug 13, 2012)

maybe not but with my Fenix PD31 which runs on a low 3 lumens mode, I swapped the cool white XPG for a neutral white XPG and at 3 lumens wow what a color difference.


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## samgab (Aug 13, 2012)

With those emitters that do amazing levels at super low drive currents, is the tradeoff worth it, considering how much better the newer ones are at higher currents? If it was for a 24x7 nightlight that was going to be running at 60mA on a backup battery for weeks, with no desire to ever use higher outputs, I suppose. But if it's for dual purpose, I'm sure the large difference in efficiency at higher draws makes the small difference at low currents not worth the tradeoff?


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## CyberCT (Aug 18, 2012)

samgab said:


> With those emitters that do amazing levels at super low drive currents, is the tradeoff worth it, considering how much better the newer ones are at higher currents? If it was for a 24x7 nightlight that was going to be running at 60mA on a backup battery for weeks, with no desire to ever use higher outputs, I suppose. But if it's for dual purpose, I'm sure the large difference in efficiency at higher draws makes the small difference at low currents not worth the tradeoff?



I'm looking for a light with just the dedicated purpose of a nightlight / constantly running light in complete darkness. I've modded a few of my Fenix lights and modded a few Rayovac lanterns with diferent LEDs and drivers, but that's my extent. What already manufactured light can I buy that will run at a low lumen mode (2-3 lumens) for weeks? Or if I am to build something somewhat easy, what driver and LED do I use then? What body / base?


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## samgab (Aug 19, 2012)

I'm not sure what sort of body or size or power supply you're looking at, but the SC600 will run at 0.1 lumens for 80 days and nights continuously on a single 18650. Pretty impressive!
Edit: (or ~280 hours continuously at 2.8 lumens, that's using a 2900mAh 18650 cell.)


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## bshanahan14rulz (Aug 20, 2012)

Nichia raijin has been out for a while, might give one of those a shot. Also have a more efficient 5mm version, but don't know if any distributors have it.


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