Am I lucky? (regarding XP-E Vf)

[ahorton]

I normally consider myself a fairly lucky guy, but after testing 8 XP-Es recently, I'm starting to wonder just how lucky I can get.

All 8 XP-Es I have tested, gave me a Vf of between 2.97 and 3.0V at around 400-500mA.

This is not what I expected from the datasheet which suggests that the Vf should be around 3.2-3.4V

My testing method involves putting a 2 ohm resistor in series with the LED and powering it from a 4.1V Li Ion. Vdrop across the resistor was about 1V and across the LED was 3V in each case. Tested were 4 R2 cool white, 2 P3 neutral, 2 P4 warm. All were mounted to a massive (100x70x3mm) heatsink and tested within the first 5 seconds of operation.

Is my testing method wrong or should I go buy a lottery ticket?

 

[jtr1962]

Nothing at all wrong with your testing method. At first I thought you might have been getting a very low Vf due to poor heatsinking (Vf drops with temperature) but that appears to not be the case.

I'd say go and buy some lottery tickets. :whistle:

Or perhaps Cree has a new process which results in a very low Vf for the XP-E. If others are getting similar results then this is very good news indeed.

 

[Tekno_Cowboy]

How did you measure the Vf?

 

[rmteo]

To determine the Vf (per manufacturer's spec), you need a constant current source (say 350mA) connected to the LED. Then you measure the voltage across the LED - that is its Vf at the selected current.

 

[Sgt. LED]

AH the lower the Vf the longer your runtime.

I like a low Vf, congratulations! Anything under 3 and it's a big winner.

 

[Benson]

To determine the Vf (per manufacturer's spec), you need a constant current source (say 350mA) connected to the LED. Then you measure the voltage across the LED - that is its Vf at the selected current.

Measuring with a ballast resistor works, too -- just then you have less control of what current you're measuring Vf at, but for current over 350mA and getting Vf well under expected values at 350mA, you know you got some good LEDs...

 

[Tekno_Cowboy]

To determine the Vf (per manufacturer's spec), you need a constant current source (say 350mA) connected to the LED. Then you measure the voltage across the LED - that is its Vf at the selected current

I see, I'll have to try and remember that.