Determining Vf

[4sevens]

Hi everybody,

Quick question. Does measuring the two leads of the Luxeon show the actual Vf that it is driving at?

For example, I measured my Lux V (WX1S) across the two leads and it
reads 6.12v. The S bin specifies 5.91-6.39.

Is 6.12v the actual Vf? Can I assume it's within safe specifications?
It is supposedly be driving at 1A with a Fatman.

David

 

[bindibadgi]

The LED will take care of making the Vf right, just as long as the battery/drive circuit provides enough. It will only "use" the voltage it needs.

It's the current you need to be careful with, the LED will just keep sucking it if there is enough, and you can let out all the magic smoke. The fatboy should take care of that though.

 

[CM]

Vf is specified at a given current through the diode. For a 5W Luxeon, the specified current is 700mA. The voltage at the LED when driven at this current is Vf. The voltage you're measuring is on the low side if it is really being driven at 1A. I suspect it's probably being driven close to 700mA.

CM

 

[gadget_lover]

What CM said /ubbthreads/images/graemlins/smile.gif

However, if the Vf for your light is at the lowest end of the range for that bin (5.91 volts) it should pull 700ma at 5.91 volts. If you raise the voltage of the circuit the current will increase. Somewhere on CPF one of the people who really knows noted that on average, you can expect approximately 100ma increase in current for every 1/10 of a volt increase over the Vf. So it's possible that you are driving it above 700ma.

Did I remember to say that these numbers are only valid at 25 degrees C?

So to determine the Vf, you need a heat sink to keep it cool, a DMM configured as an ammeter to measure the current and a DMM configured as a volt meter to read the voltage.

Daniel

 

[4sevens]

I waited for the flashlight to cool to room temp and then
measured current in series and it read 950ma. I measured the voltage and it read 6.21v. This WX1S should be 5.91v-6.39v. I wonder how much lumileds specs will vary from star to star with the same bins.

remeber this is george80's fatman which is current driven, so putting
a meter in series shouldn't matter.

 

[Doug S]

[ QUOTE ]
4sevens said:
Hi everybody,

Quick question. Does measuring the two leads of the Luxeon show the actual Vf that it is driving at?

David

[/ QUOTE ]

Short answer is Yes. This is the actual Vf *for the current and die temperature* being measured. It *is not* the Vf the emitter would have if measured under the conditions specified by Lumileds for binning purposes. Binning is done at a die temperature of 25C and a current of 350 mA for the 1W emitter and 700 mA for the Lux III and Lux V.

 

[3rd_shift]

That's about 6 watts for that lux 5 watter. /ubbthreads/images/graemlins/icon15.gif
If I recall correctly, the fatman can be set for 700 milliamps.
Then remeasure the voltage.
A lux 5 can be a little overdriven with a really good aluminum, or copper heatsink.
Good luck.