How to measure Vf of LED's

[ODatsBright]

I've searched the site but can't find any discussion on how to measure Vf on the Luxeons (or any LED). I assumed the diode check function on my DMM would get me close but it keeps coming up about a volt low according to the bin code. I've also tried using my variable power supply and it shows even lower reading with visible light coming from the chip. Ramping up the current to 350mA shows way over the bin code on voltage.

So how do you test if your luxeon is within it's bin code?

Thanks

 

[Curious_character]

I've searched the site but can't find any discussion on how to measure Vf on the Luxeons (or any LED). I assumed the diode check function on my DMM would get me close but it keeps coming up about a volt low according to the bin code. I've also tried using my variable power supply and it shows even lower reading with visible light coming from the chip. Ramping up the current to 350mA shows way over the bin code on voltage.

So how do you test if your luxeon is within it's bin code?

Thanks

How much over the bin code voltage are you seeing at 350 mA? At 350 mA, you should be within the bin code voltage if the LED is properly mounted on a good heat sink. If it's not, the voltage will be lower by about 2.2 mV/degree C of chip temperature rise.

I assume you're measuring the forward voltage with a DMM on the volt scale, not on a diode check setting, and directly at the LED terminals. If not, you should be.

How are you measuring the current?

c_c

 

[gadget_lover]

The VF is defined as the voltage at which an LED draws it's specified current at a specified temperature. If it's a lux III the current is .35 amps (350 ma) at room temperature.

Yes, the VF will often be near the edges of the specs.

I measure VF with a variable power-supply, slowly increasing the voltage while measuring the current with a second meter.

Daniel

 

[wasBlinded]

A Lux I has its Vf measured at 350 mA, while a Lux III is measured at 750 mA.

The current regulated power supply should be a good way to measure Vf, but the power supply itself can be reading a higher voltage that what is actually present at the LED as a result of the resistance of your power leads.

Set your PS at the desired current, and measure your voltage at the LED with your DMM. That should give you an accurate Vf for that current.

 

[Long John]

A Lux I has its Vf measured at 350 mA, while a Lux III is measured at 750 mA.

The current regulated power supply should be a good way to measure Vf, but the power supply itself can be reading a higher voltage that what is actually present at the LED as a result of the resistance of your power leads.

Set your PS at the desired current, and measure your voltage at the LED with your DMM. That should give you an accurate Vf for that current.

+1, except 700mA at the LuxIII

Best regards

____
Tom

 

[evan9162]

The diode check function on your DMM will probably only supply 10mA of current or so. You need to supply a constant current equal to the device's rated current in order to properly measure its Vf relative to what it was binned at.

Putting 10mA through a luxeon will only result in a Vf of 2.6-2.8V

 

[ODatsBright]

Ahh, this is better.

A QY0L at .7065A had a Vf of 3.864V, right on target. Had forgotten that a Lux III was 700mA instead of 350, also thought is was a TW0J star, but taking it off the heatsink and actually reading the thing, 'twas not.

Thanks everybody for straightening me out.