# R2 Forward Voltage Test 36 Pieces



## cmacclel (Jun 24, 2008)

I just received 36 R2's and tested the VF on them. Power Supply was set to 3.5v, to he right is the milliamp draw at 3.5v. This should be informative for people asking about direct drive. Each LED can be very different. As you can see below from the LED's tested that *ONE* specific LED can draw 4x the current as the same BIN LED from the same reel.


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## Erasmus (Jun 24, 2008)

I got around the same results, apparently there's not much consistency between LEDs from the same reel. Your LEDs actually were 1 chain of 36 emitters which I had to cut in 4 for shipping. I have 10 of these R2's myself and my readings at 350mA go from 3.22V to 3.57V.

I wonder how this comes. When the first Q5's were released there was the problem of extremely high Vf's, although Cree added a bond wire which should lower the Vf, especially at high currents. I remember the good old days when P4 was the highest flux bin, but Vf readings were very constant AND low (I remember I tested 10 P4's at 350mA going from 3.15V to 3.22V). Apparently the extra bond wire didn't help that much... I suppose there's some other change which increases the Vf?

Even though the Vf's are not very consistent on these R2's, I really love them. Compared to the P4 they give a huge brightness boost and even compared to Q5's they look brighter to the naked eye. Fortunately my light meter is back on it's way to me so I can take measurements 

Cheers to you, Mac! And happy modding with these R2's


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## ViReN (Jun 24, 2008)

cmacclel said:


> I just received 36 R2's and tested the VF on them. Power Supply was set to 3.5v, to he right is the milliamp draw at 3.5v. This should be informative for people asking about direct drive. Each LED can be very different. As you can see below from the LED's tested that *ONE* specific LED can draw 4x the current as the same BIN LED from the same reel.




Excellent Info :thumbsup:. May I request some thing?

What would be the variation in Vf if constant current of 350mA is supplied.


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## Erasmus (Jun 24, 2008)

ViReN said:


> Excellent Info :thumbsup:. May I request some thing?
> 
> What would be the variation in Vf if constant current of 350mA is supplied.


I don't have any test results at hand, but with Mac's LEDs the difference can be up to around .4V Just an educated guess but it will be quite accurate.


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## koala (Jun 24, 2008)

hi mac,
I remember you did a similar test sometime ago? Was it cree Q2? Anyway can you tell us more about your test method? How did you mount the R2? And when was the readings taken after connecting them to the power supply. I have 8 pcs from Eramus but i haven't test them yet.

I would like to see if someone could measure the output per watt regardless the vf of the led. Hopefully this will tell the difference in brightness when driven at the same power.


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## Yapo (Jun 24, 2008)

So is it better to have a lower current draw? are the outputs the same? or would they still be relative to the current draw/voltage?


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## Erasmus (Jun 24, 2008)

Yapo said:


> So is it better to have a lower current draw? are the outputs the same? or would they still be relative to the current draw/voltage?


It's more efficient to have a lower Vf at a certain current. Example : 2 LEDs both put out 115 lumen each at 350mA. LED A has a Vf of 3.2V, this makes a total power consumption (UxI) of 1.12W and thus an efficiency of 102.68lm/W. LED B has a Vf of 3.5V, total power consumption is 1.225W and thus an efficiency of 93.87lm/W. The .3V lower Vf of LED A makes it 9.3% more efficient than LED B. 

Output of these Cree XR-E's is specified at 350mA current, regardless of the voltage at that current, and is between 114 and 121 lumen.

I can maybe put my test results on line at the end of the week. I test it with a constant current source of 350mA. I push the LEDs on a black anodized big aluminum heatsink and I take a reading after the LED is connected for 10 seconds.


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## cmacclel (Jun 24, 2008)

koala said:


> hi mac,
> I remember you did a similar test sometime ago? Was it cree Q2? Anyway can you tell us more about your test method? How did you mount the R2? And when was the readings taken after connecting them to the power supply. I have 8 pcs from Eramus but i haven't test them yet.
> 
> I would like to see if someone could measure the output per watt regardless the vf of the led. Hopefully this will tell the difference in brightness when driven at the same power.


 

My test method is crude and simple. The LED's are placed on a sheet of aluminum with a piece of paper underneath them to isolate the contacts. The I have 2 pointy probes that a manually touch to the LED and then look at my variable power supply to check the current draw.

Mac


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## cmacclel (Jun 24, 2008)

ViReN said:


> Excellent Info :thumbsup:. May I request some thing?
> 
> What would be the variation in Vf if constant current of 350mA is supplied.


 

3.2v 
3.5v

Mac


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## LukeA (Jun 24, 2008)

I guess this is the price we pay for Cree's tight flux bins: the lottery of Vf being up to 10% different.


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## saabluster (Jun 25, 2008)

cmacclel said:


> I just received 36 R2's and tested the VF on them.


Great post! The pictures really help you see what going on with these as far as the dispersion of VF. Thank you. Seems like this should have been posted in the "LEDS" forum.:shrug:


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## Burgess (Jun 25, 2008)

Very interesting info.


Thank you for your efforts !

:thumbsup:
_


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## Lynx_Arc (Jun 25, 2008)

Erasmus said:


> It's more efficient to have a lower current draw at a certain Vf. Example : 2 LEDs both put out 115 lumen each at 350mA. LED A has a Vf of 3.2V, this makes a total power consumption (UxI) of 1.12W and thus an efficiency of 102.68lm/W. LED B has a Vf of 3.5V, total power consumption is 1.225W and thus an efficiency of 93.87lm/W. The .3V lower Vf of LED A makes it 9.3% more efficient than LED B.
> 
> Output of these Cree XR-E's is specified at 350mA current, regardless of the voltage at that current, and is between 114 and 121 lumen.
> 
> I can maybe put my test results on line at the end of the week. I test it with a constant current source of 350mA. I push the LEDs on a black anodized big aluminum heatsink and I take a reading after the LED is connected for 10 seconds.


are you sure it is the opposite of that, instead of having a lower current draw at a Vf... have a lower Vf at a set current draw. A lower current draw probably equates to lower output while a lower voltage at a specified current draw points to more efficient I would think.


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## cmacclel (Jun 25, 2008)

saabluster said:


> Great post! The pictures really help you see what going on with these as far as the dispersion of VF. Thank you. Seems like this should have been posted in the "LEDS" forum.:shrug:


 

Thats the forum I was looking for when I posted  I missed it! I'll have it moved!

Mac


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## monkeyboy (Jun 25, 2008)

This is very interesting information. It makes me think that the LED manufacturers should use a much tighter binning structure but then I guess they don't really have CPF modders in mind.


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## LukeA (Jun 25, 2008)

monkeyboy said:


> This is very interesting information. It makes me think that the LED manufacturers should use a much tighter binning structure but then I guess they don't really have CPF modders in mind.



Luxeon and SSC do bin by Vf as well as flux.


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## saabluster (Jun 25, 2008)

Having looked at it a second time I finally got it. You are giving us all the "finger". Real classy man.:shakehead


cmacclel said:


>


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## Greg G (Jun 25, 2008)

No, No...he's just telling us that we're Number 1! :lolsign:


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## nikon (Jun 26, 2008)

For you statisticians out there, 500mA is the mean, median, and mode, and the distribution forms a very nice bell curve. :green:


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## James35 (Jun 26, 2008)

During your 3.5V testing, did you feel that the lumens were the same?


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## cmacclel (Jun 26, 2008)

James35 said:


> During your 3.5V testing, did you feel that the lumens were the same?


 
I try not to look at them when I power them up 

Mac


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## James35 (Jun 26, 2008)

Hehe, yeah, I don't blame you. I would be really curious if the .2 amp LED puts out the same lumens as the .8 amp LEDs. I almost hope it's not the case. Otherwise, we'll end up doing the same thing we do in RC racing when it comes to matching batteries. (Going through a bunch and cherry picking the best).


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## LukeA (Jun 26, 2008)

James35 said:


> Hehe, yeah, I don't blame you. I would be really curious if the .2 amp LED puts out the same lumens as the .8 amp LEDs. I almost hope it's not the case. Otherwise, we'll end up doing the same thing we do in RC racing when it comes to matching batteries. (Going through a bunch and cherry picking the best).



If an emitter is drawing 800mA, it will put out more light than one pulling 200mA.


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## Erasmus (Jun 26, 2008)

James35 said:


> Hehe, yeah, I don't blame you. I would be really curious if the .2 amp LED puts out the same lumens as the .8 amp LEDs. I almost hope it's not the case. Otherwise, we'll end up doing the same thing we do in RC racing when it comes to matching batteries. (Going through a bunch and cherry picking the best).


The current determines the light output, not the voltage. At .2 amps an R2 will put out around 75 lumen of light, at .8 amps around 230 amps.


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## saabluster (Jun 26, 2008)

Erasmus said:


> At .2 amps an R2 will put out around 75 lumen of light, at .8 amps around 230 amps.


Cree has finally solved the energy crisis!


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## znomit (Jun 26, 2008)

I would like to see the best and worst, mounted next to each other, running in series. I don't trust the pointy probes!


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## cmacclel (Jun 26, 2008)

znomit said:


> I would like to see the best and worst, mounted next to each other, running in series. I don't trust the pointy probes!



Each led is tested twice and the probes are accurare.

Mac


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## TMorita (Jun 27, 2008)

Erasmus said:


> It's more efficient to have a lower current draw at a certain Vf. Example : 2 LEDs both put out 115 lumen each at 350mA. LED A has a Vf of 3.2V, this makes a total power consumption (UxI) of 1.12W and thus an efficiency of 102.68lm/W. LED B has a Vf of 3.5V, total power consumption is 1.225W and thus an efficiency of 93.87lm/W. The .3V lower Vf of LED A makes it 9.3% more efficient than LED B.
> 
> Output of these Cree XR-E's is specified at 350mA current, regardless of the voltage at that current, and is between 114 and 121 lumen.
> 
> ...


 
Whoa whoa hold. Stop.

Your first sentence here:



Erasmus said:


> It's more efficient to have a lower current draw at a certain Vf.


 
does not correlate with the rest of your post:



Erasmus said:


> Example : 2 LEDs both put out 115 lumen each at 350mA. LED A has a Vf of 3.2V, this makes a total power consumption (UxI) of 1.12W and thus an efficiency of 102.68lm/W. LED B has a Vf of 3.5V, total power consumption is 1.225W and thus an efficiency of 93.87lm/W. The .3V lower Vf of LED A makes it 9.3% more efficient than LED B.


 
I agree with the second part, but not the first part.

Continuing your example with LED A and LED B:

Let's say you drive both LED A and LED B at 3.5 volts.

LED A has already been measured at 3.5 volts and 350ma, so it will draw more current at 3.5 volts - let's say 600 ma.

LED B has already been measured to draw 350 ma at 3.5 volts.

So if you look at your first sentence:



Erasmus said:


> It's more efficient to have a lower current draw at a certain Vf.


 
...this would mean LED B is more efficient, because it draws 350 ma at 3.5 volts versus LED A which is 600 ma at 3.5 volts, which directly contradicts the rest of your post.

The first sentence is IMHO badly worded and mostly wrong, because the LED which draws more current at the same Vf has a higher efficiency (and lower resistance) than a similar LED which draws less current at the same Vf.

Toshi


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## Erasmus (Jun 27, 2008)

TMorita said:


> Whoa whoa hold. Stop.
> 
> Your first sentence here:
> 
> ...



Hi Toshi,

Thank you very much for correcting, I indeed made a mistake. I switched the place of current and Vf in that sentence, I corrected it now. The right sentence is "It's more efficient to have a lower Vf at a certain current.". Maybe it's better you also correct your post to avoid misunderstanding for other people?

Thanks again for correcting!!!

Raf


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## Lynx_Arc (Jun 28, 2008)

Erasmus said:


> Hi Toshi,
> 
> Thank you very much for correcting, I indeed made a mistake. I switched the place of current and Vf in that sentence, I corrected it now. The right sentence is "It's more efficient to have a lower Vf at a certain current.". Maybe it's better you also correct your post to avoid misunderstanding for other people?
> 
> ...


efficiency needs to take into account desired operating perameters which may include overdriving or underdriving the LED which can have more of an effect on efficiency too. You should use (light output)/(power output) as a comparison or something like lumens/watts because an LED has higher or lower Vf means nothing if the light output and current needed are unacceptably high. A Low Vf Luxeon 1 may be less efficient than a Q5 cree but may draw more current at the same Vf or less current.


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## znomit (Sep 27, 2008)

Just a note. I've had three shipments of R2s since christmas. All from cutter. 
The first 2 lots(on a quad and a triple MCPCB) were 3.5-3.6V. The recent lot (a quad MCPCB and a single star) were 3.2V. 
Huge improvement. I had to hook up the ammeter today because I didn't believe the readout on the power supply!


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## Nake (Sep 27, 2008)

znomit said:


> Just a note. I've had three shipments of R2s since christmas. All from cutter.
> The first 2 lots(on a quad and a triple MCPCB) were 3.5-3.6V. The recent lot (a quad MCPCB and a single star) were 3.2V.
> Huge improvement. I had to hook up the ammeter today because I didn't believe the readout on the power supply!


 
That's good, I think. I just received 3 from Cutter last week. 

Is there anyway to measure with just a battery and multi-meter?


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## znomit (Oct 12, 2008)

Nake said:


> Is there anyway to measure with just a battery and multi-meter?


Yeah run it for a few min to settle things down, put the meter in series to measure current. Remove meter and reconnect circuit, measure the V on the LED.


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## David Gretzmier (Oct 13, 2008)

ok, so a newbie question- which is better in a direct driven flashlight- a cree r2 LED that is at 3.2 volts or at 3.5? given a lithium rechargable cell puts out 3.7-4.2 volts, I am guessing the LED that is 3.2 volts will be brighter? or, given that a battery puts out a constant voltage that decreases with use, isn't it better to get an led that draws less amps, or what?


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## faklya (Oct 13, 2008)

znomit said:


> Yeah run it for a few min to settle things down, put the meter in series to measure current. Remove meter and reconnect circuit, measure the V on the LED.



Theoretically this is the way, but in practice this could be quite inaccurate, because the meter has it's own internal resistance. This is not an issue while measuring the voltage, but because of the considerable current the internal resistance of the device can lower the current readings. Leds run about 25% dimmer while measuring the current with my 10$ cheap multimeter than when direct driving(or measuring voltage).


I still don't have a good way of measuring wattage (both forward voltage and current) with only one meter and battery.


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