# CREE XML-T6 LED emitter low current at tailcap



## ljw2k (May 1, 2011)

Hi all and just a quick question in regards to the figures i got when i measured the Current at the tailcap on my CREE XML-T6 LED emitter DX Drop in *HERE* and i got the below results:

Low : 165 mA
Mid : 725 mA
High : 1.65 A

It was running on fully charged *Troch Light 3.8v 26650(Protected) 4000mAh Rechargeable Li-ion http://www.lck-led.com/p822/Troch-L...0mAh-Rechargeable-Li-ionTro/product_info.html*


----------



## hellokitty[hk] (May 1, 2011)

Input voltage needs to be higher.


----------



## ljw2k (May 1, 2011)

It is in a 2D mag at the moment but i do have a 3D mag i might try it in on 32600 batteries .

What Lumens would i be getting at my highest current draw 1.65A 

Would i notice a difference in brightness if i used the 3x 32600 batteries @: 11.1 Volts and would that be closer to the 3A DX rating.

I thought i would have got the max Brightness and Lumens at anywhere between the manufatures spec 5.8~12V


----------



## Justin Case (May 1, 2011)

Unless you are looking at direct drive or say a linear regulator, tail current draw is not the same thing as LED drive current. I assume this DX drop-in has some sort of buck driver, based on the stated specs in the link you provided (Working voltage: 5.8~12V).

If you fed the drop-in 2xLi-ion at a nominal 7.4V and assume that the XM-L Vf is 3.35V at 3A drive and the driver efficiency is 85%, then

0.85*7.4V*Ibatt = 3.35V*3A = 10.05W

Thus, Ibatt = 1.60A, which looks suspiciously close to what you measured.

If you feed the drop-in 3xLi-ion at 11.1V nominal, your tail current measurement will *decrease*, not get closer to 3A.


----------



## ljw2k (May 1, 2011)

Ok just looked at the data sheet http://www.cree.com/products/pdf/XLampXM-L.pdf and what you say above is correct and i have learnt something by your reply but can you explain how you got these figures " 0.85*7.4V*Ibatt = 3.35V*3A = 10.05W Thus Ibatt = 1.6A as i am trying to get my head around it so i know for future reference.

Also why do people measure at tailcap and wouldn't it be better to get a true reading at the Emitter end of what it is actually pulling in ( Amps ).

Am i also correct to say that DX website states 3000mA at max is that at the emitter side ? and is there a formular to work out what lumens it is throwing out at the above readings.

Appreciate your help and i am all johnny 5 .....more input all ears


----------



## Justin Case (May 1, 2011)

Power from the driver = Power consumed by the LED

0.85 is 85%, which is a moderately high, but still reasonable, driver efficiency value.
7.4V is the nominal voltage for two Li-ions in series.
3A is the advertised drive current.
3.35V is the datasheet value for the XM-L's forward voltage (Vf) at 3A drive current (If).

Driver power sent to the LED is driver efficiency*Vbatt*Ibatt.
LED power consumed is Vf*If.

Thus, 0.85*7.4*Ibatt = 3.35V*3A. Solving for Ibatt gives you 1.6A.

People measure tail current probably because 1) it is easy to do, 2) it suggests what sort of run time they might get, and 3) it gives them an idea as to how hard the cells are being driven.

Measuring drive current at the LED requires access to the LED, which is not always possible (the head may be threadlocked and thus sealed). You have to measure the current in series with the LED, which generally requires that you de-solder the connection to hook your DMM into the circuit. That can be a hassle. If you screw up your connections with a boost driver and feed it an open load, you often will kill the driver.

I have no idea what DX's advertising copy is supposed to mean. All I can do is assume that the 3A refers to drive current.

To estimate lumens from drive current, refer to the LED's datasheet. There is typically a graph of relative luminous flux vs drive current. Find the value of relative luminous flux for the drive current of interest and apply that RLF factor appropriately. For the XM-L, at 3A, RLF ~325%. Multiply that by the lumens at the standard forward current. For the XM-L, you can see from the graph that this current is 700mA (that's where RLF is 100%). You also can infer this from the fact that the lumens rating for a T6 XM-L is given as 280 lumens at 700mA forward current.

280 lumens * 325% = 910 lumens. This is the estimated lumens at the emitter. You will have losses from the reflector or optics, the glass window, LED junction heating, blockage by the bezel, etc. Multiply the emitter lumens estimate by say 1/2 or 2/3 or 3/4 or whatever to get an estimate of the out the front lumens.


----------



## ljw2k (May 2, 2011)

That explains alot thankyou for your time and knowledge Justin and even though it is not all sunk in yet it does make alot more sense.


----------



## ljw2k (May 5, 2011)

Ok put together another 2D Mag today with the below items and it is pulling virtually the same current as the drop in module from DX

*Cree XLamp® XM-L LED - U2*
http://www.lck-led.com/p865/Cree-XL...mm-Board-U2-Group,[email protected]/product_info.html

*Constant Current LED Driver Board - 3 Modes, 5.5-12V 3.0A*
http://www.lck-led.com/p501/Constan...3-Modes,5.5-12v,3.0A,P7,MCE/product_info.html


*Troch Light 3.8v 26650(Protected) 4000mAh Rechargeable Li-ion*
*http://www.lck-led.com/p822/Troch-L...0mAh-Rechargeable-Li-ionTro/product_info.html*


----------



## Dsoto87 (May 5, 2011)

Are you still only using 1 cell? Looks like you need to be using a second cell judging by the drivers input voltage


----------



## Justin Case (May 5, 2011)

ljw2k said:


> Ok put together another 2D Mag today with the below items and it is pulling virtually the same current as the drop in module from DX
> 
> *Cree XLamp® XM-L LED - U2*
> http://www.lck-led.com/p865/Cree-XL...mm-Board-U2-Group,[email protected]/product_info.html
> ...


 
Ok, so what's the problem? As described above, when you run 2xLi-ion at a nominal 7.4V and a buck driver of approx efficiency of 85% to drive an XM-L at 3A and a Vf spec of about 3.35V, you get a calculated tail current around 1.6A, which is what you seem to be measuring.

Tail current is not the same as drive current for your buck driver case.


----------



## ljw2k (May 5, 2011)

Using 2 cells and as Justin says it is Tailcap current which would be different at the led so both will be pulling around 2.8-3A at the LED i think correct me if i am wrong.


----------



## ljw2k (May 5, 2011)

A few pics of what i have done so far :


----------



## ljw2k (May 7, 2011)

Would i benefit from using a linear regulator over the buck driver i am using at the moment if so what are those benefits.

Ssorry for all the quetions ...> Johhny 5 more input all ears and all that.


----------



## Justin Case (May 7, 2011)

7135 linear regulator recommended max input voltage is 6V. 2xLi-ion exceeds that by a lot. Stick with the buck driver. Much more flexible in terms of Vin, as long as you can feed it the minimum Vin to reach regulation.


----------



## 350xfire (May 8, 2011)

Yeap 2 LiIon. I run mine with that and get 1.5 amps at 7.4 volts. Need higher voltage.


----------



## Justin Case (May 8, 2011)

350xfire said:


> Yeap 2 LiIon. I run mine with that and get 1.5 amps at 7.4 volts. Need higher voltage.


 
What do you run with 2 Li-ion? A buck driven XM-L? 1.5A at the tail?

Tail current is not the same thing as drive current.


----------



## 350xfire (May 8, 2011)

Justin Case said:


> What do you run with 2 Li-ion? A buck driven XM-L? 1.5A at the tail?
> 
> Tail current is not the same thing as drive current.


 
Yes, buck driver... And yes, I agree about the tail not being the drive, but should relatively be close minus losses and such!


----------



## ljw2k (May 8, 2011)

So the only way to measure true current draw is after the driver in series with the LED.

Do you think i wil be getting the maxumin out of the LED 900-1040 Lumens with the Buck driver and batteries that i am using.


----------



## Justin Case (May 8, 2011)

350xfire said:


> Yes, buck driver... And yes, I agree about the tail not being the drive, but should relatively be close minus losses and such!


 
Why should it be "relatively close"? Only if your driver is less than 50% efficient, which is a really bad driver. Basically, in the ideal case of 100% efficiency, with 2xLi-ion at 7.4V nominal and an XM-L with Vf of 3.3V at 2.8A drive, your Vbatt/Vload ratio is over 2:1 and your tail current vs drive current ratio will be the same. In reality, this is modified by the fact that drivers aren't 100% efficient. So let's assume an average efficiency of say 80%. Thus,

Power delivered by the driver = power consumed by the LED
80% * 7.4V * Ibatt = 3.3V * 2.8A = 9.24W

Thus, Ibatt = 1.56A, which is very close to your figure of 1.5A. If we increase the assumed driver efficiency to 85%, we get Ibatt = 1.5A.


----------



## Justin Case (May 8, 2011)

ljw2k said:


> So the only way to measure true current draw is after the driver in series with the LED.
> 
> Do you think i wil be getting the maxumin out of the LED 900-1040 Lumens with the Buck driver and batteries that i am using.


 
I don't know anything about that driver, but the photo looks like it was soldered by a 1st grader.


----------



## 350xfire (May 8, 2011)

Justin Case said:


> Why should it be "relatively close"? Only if your driver is less than 50% efficient, which is a really bad driver. Basically, in the ideal case of 100% efficiency, with 2xLi-ion at 7.4V nominal and an XM-L with Vf of 3.3V at 2.8A drive, your Vbatt/Vload ratio is over 2:1 and your tail current vs drive current ratio will be the same. In reality, this is modified by the fact that drivers aren't 100% efficient. So let's assume an average efficiency of say 80%. Thus,
> 
> Power delivered by the driver = power consumed by the LED
> 80% * 7.4V * Ibatt = 3.3V * 2.8A = 9.24W
> ...


 
While I agree with the calcs, these are all theoretical calcs. At the end of the day, what matters is that wen I turn the switch on, I have a bad *** light! Not to mention the effects of temperature on those numbers. I have compared with P7, SST-50, Cree 1200 lumen module and this light is within their range. I am not about to take a module apart to try to determine LED Vf and current at the LED and all that stuff. I am happy with what I have and don't need to lose sleep over the exact details. I am not worried about losing 100 lumens due to inefficiencies and such.


----------



## hellokitty[hk] (May 8, 2011)

When I said the input voltage is too low, I thought you were using a single cell.
With two cells 1.6v is normal for a buck, in fact, if you look KD SKU S005296, it even has 1.6A listed as 7.2v current.



350xfire said:


> Yes, buck driver... And yes, I agree about the tail not being the drive, but should relatively be close minus losses and such!


No it shouldn't.


----------



## ljw2k (May 8, 2011)

Justin Case said:


> I don't know anything about that driver, but the photo looks like it was soldered by a 1st grader.


 
Thanks for your kind comments and will that effect the output and what driver would you advise best to drive the Cree X-ML?


----------



## ljw2k (May 8, 2011)

hellokitty[hk] said:


> When I said the input voltage is too low, I thought you were using a single cell.
> With two cells 1.6v is normal for a buck, in fact, if you look KD SKU S005296, it even has 1.6A listed as 7.2v current.
> 
> 
> No it shouldn't.


 

It does indeed but does say below 

*Output Voltage:
2.8A(High)~ 1.38A(Middle)~0.25A(Low) --at any voltage level from 5.5V to 13.2V*


----------



## old4570 (May 8, 2011)

Yes well , product descriptions are not always the best from DX etc ... 

Now , to know how hard your driving the emitter (ball park) people have answered the question ... " Watt's " 
Voltage x Amps = Watts 

What's important here is the Watts .. So it would appear that you are driving the emitter as hard as it needs to be driven .


----------



## Justin Case (May 8, 2011)

350xfire said:


> While I agree with the calcs, these are all theoretical calcs. At the end of the day, what matters is that wen I turn the switch on, I have a bad *** light! Not to mention the effects of temperature on those numbers. I have compared with P7, SST-50, Cree 1200 lumen module and this light is within their range. I am not about to take a module apart to try to determine LED Vf and current at the LED and all that stuff. I am happy with what I have and don't need to lose sleep over the exact details. I am not worried about losing 100 lumens due to inefficiencies and such.


 
Well I have made the measurements only many driver/LED combos and they agree quite well with the calcs.


----------



## Justin Case (May 8, 2011)

ljw2k said:


> Thanks for your kind comments and will that effect the output and what driver would you advise best to drive the Cree X-ML?


 
Not your soldering. I mean the soldering shown on the lck-led.com web site of the driver you are using. Compare it to a quality product such as from TaskLED or The Sandwich Shoppe.


----------



## ljw2k (May 8, 2011)

Thanks for your reply guys this has helped.


----------



## 350xfire (May 8, 2011)

ljw2k said:


> It does indeed but does say below
> 
> *Output Voltage:
> 2.8A(High)~ 1.38A(Middle)~0.25A(Low) --at any voltage level from 5.5V to 13.2V*




Makes sense, nearly twice the amps as the input with 2 LiIon cells.


----------



## zawaprz (Feb 17, 2013)

Hello, 

I'm so new to Cree XML-T6 LED technology... Recently I purchase off internet mine stayting: 
Mouse over image to zoom 

 *Have one to sell?*  Sell it yourself 

 
[h=1]1800 Lumen Zoomable CREE XM-L T6 LED 18650 AAA Flashlight Torch Zoom Lamp Light[/h]

... where description says: 
Max Output brightness : Can come to 1800 lumens [lm]


Of course, it's probably not original Ultrafire (it stays on cover).. 
So I contact guy in China asking if I can put second cell into it (that is another set via extension tube of 3AAA) to increase voltage, as I doubt currently it provides even 1000 lumens... 

In return he wrote me: 
- his flashlight can you either 18550 and 26650 battery
- so sorry you can't do it for 8v because we have contacted the factory and they mention the control board have a chip set which is not allowing the higher voltage as it would burn the board.


========================================

So I am wondering as he stated if I would risk indeed by doubling cell # burn of my flashlight??
Thx!






Justin Case said:


> What do you run with 2 Li-ion? A buck driven XM-L? 1.5A at the tail?
> 
> Tail current is not the same thing as drive current.


----------



## pyro1son (Mar 6, 2013)

Just a quick question, did the drop in require any modding?



ljw2k said:


> Hi all and just a quick question in regards to the figures i got when i measured the Current at the tailcap on my CREE XML-T6 LED emitter DX Drop in *HERE* and i got the below results:
> 
> Low : 165 mA
> Mid : 725 mA
> ...


----------



## Epsilon (Mar 6, 2013)

Welcome. You should open your own thread for this question .

But to begin with the most obvious. A XM-L T6 will never ever produce 1800lumens. 1000 lumens at 3A is what it can produce at the LED max.
Do the 3*AAA Cells deliver this kind of current? Probably not at the required voltage (around 3.4v), so the current and therefor the output will drop below the theoretical 1000lumen even further.


----------



## radha (Aug 25, 2013)

Epsilon said:


> Welcome. You should open your own thread for this question .
> 
> But to begin with the most obvious. A XM-L T6 will never ever produce 1800lumens. 1000 lumens at 3A is what it can produce at the LED max.
> Do the 3*AAA Cells deliver this kind of current? Probably not at the required voltage (around 3.4v), so the current and therefor the output will drop below the theoretical 1000lumen even further.



HI ALL 
I propose to drive a CREE XML-T6 LED with 3x1.2 v NImhD 7500 mah "D" batteries directly,ie 3.6 v.

without any driver. I am assuming that the voltage drop across the LED will be enough to keep the voltage at around 3.1v. Am I right in that , do i need to control the current any way.? Could you please give me the voltage drop for the xml-t6 led.
thanks


----------

