Can Vf be measured on a Lux III?

Can you measure the exact Vf of a Lux III? If so how? The Vf range is kind of broad and with some led drivers the Vf range would vary from regulated to un-regulated based on Vin.
Thanks,
Chris
.308

vf is easily measured at luxeon. to get real VF numbers you need to deliver current at spec's

3watt's vf is measured with 700 milliamps load to luxeon. one watt requires 350 milliamps.

I'll deliver rated current at aprox. .3 volt over rated VF.

note there is also an unrealistic temp requirement as well.

VF will raise as current is increased. it's well documented VF will drop after time.

here's a pic testing Ubin for VF and tint for a custom LSH-P build.
Note 3/4in solid aluminum work bench star is clamped to.

what he said minus the watts part, if you really want the EXACT wattage, you need to do the math.
700ma to different Vf things, will be (ever so slightly) differerent wattages.

Cy and Vidpro much thanks for the answers. I have some TW0M lux III's that I am trying to use with a fatman driver - (2) lux III's wired in series. I used the TW0M because I am trying to build a regulated 2C quad lux using Li-ions. The fatman driver must have a Vin less than the Vf total. The TW0M has a Vf of 3.99-4.23. I guess I have one on the low end because The light is going into DD mode when I turn it on. The Li-ions measure up to 4.20v hot off the charger. These were slightly used and the charge I measured was 8.20 v. Am I s.o.l????

with one li-ion per luxIII , you get slight overdrive, with about any of the Vf thingees, hot off the charger, even the higher ones.

will you have a cutoff too??? if you use a boosting type driver, you could discharge your li-ions "to far".
protected cells, or a driver that cutsoff will fix that.
ya see with 2 in SERIES, you can get cell imbalance.

being a cheapskate myself, that is why i DD, use LUXIIIs, have good heat removal, because they match so well.

so are you screwed? or just out of regulation, and a bit brighter for a while /ubbthreads/images/graemlins/grin.gif

The li-ions are 18650 protected from AW.

how did you know it went DD on you? no control of a pot you added?
whats the amps?

Thanks again Vidpro.
Yes, no control of a pot. The current should have been 1.0A if regulated, done thru onboard trimpot of driver by setting resistance. I have done this mod with (1) 18650 and it worked, although it cut off at ~70% brightness which I assumed was the li-ion protection kicking in.

from:
http://www.taskled.com/techfatman.html

"". . . Fatman will no longer regulate and the input voltage will go through the series inductor and schottky diode directly to the load.""

the schotkey diode would offer a BIT of Voltage drop, that could be why this easily worked in a SINGLE setup easier.

(as usual i am guessing things here, just for IDEAS)

can you parellel the 2 li-ions?
the data there says that is a possible method.

and one time, i thought about needing to do that IN a flashlight cylinder, and i thought about connecting the tops (+) by sticking them in with the 'tabs' both in the center and the ends both out. that way to case connection could occur at both ends (- ground), and i would run wire from the center.

also it looks like a lower input voltage, is slightly less efficient (another concideration)

can you do 3 LuxIIIs? /ubbthreads/images/graemlins/cool.gif

Since it is a quad Lux III, at least in theory, you could run all 4 in series off of the fatman. This would be the Vf near the upper limit, but not really over, the Fatman Vout. Some other options, possibly less appealing but workable, would be to add another diode in series with the LEDs, or a low value resistor in series with the cells.

I am a little surprised that you are holding 4.2 V so well under load - that is impressive. This also brings up another, slightly crazy way to deal with the problem, discharge the cells slightly before use.

It's not really the 18650 holding 4.2V so well there are other factors which my friend Chris just slightly overlooked.

The Vf of the Luxeon will drop as the current drops & heat increase.

Here's my experience.

When the batteries are fresh of the charger, under load they will drop the Vin to ~4.1V delivering 2.5A to 4xTW0K in series parallel. Hum... not sure if it's TW0K or TW0J, I'll check later tonight.

After the batteries are 1/2 drained by running for 1/2 hr. When I adjust the pot to minimum setting (eg 1K Ohms for ~100mA) it still WOULD NOT reach that level.

At 100mA my Vf was like 3.4-3.5V which means my Vout is ~ 7V. With the Li-Ion holding at 3.6V it's still above the Vin.

So when you turn down the pot it will get dimmer but not as much as if you're using one cell.

Running 4xLuxIII (H) Vf in series off a fatman will bring about another problem.
You can't run them at high (1000mA) current. @ 1000mA the combined Vf is ~14.8V. Yes that below the 16V Vin max limit spec. But the heat on the converter is ~1W.

It will run for one or two minute but hours at a time I never achieved.

That's why with 2xLi-Ion & 4xLuxIII it's best to DD.
You'll get:
- 1.3A / emitter fresh off the charger.
- 1.2A / emitter rested for a day.
- 600mA / emitter when the batteries are ready for recharge.

Thanks for all the help and answers. I will one day post a long list of lights not to build /ubbthreads/images/graemlins/grin.gif
Thanks,
Chris

another consideration is that the switch to DD is not exactly at Vbat (in) = Vf.. I found this out when i put a Vf of 3.5 lux3 on a single LiON through a fatman... and could dial the current level anywhere from over 1000mA down to about 600... i couldn't figure out why 600 (supposed to go down to 30 or so).. but that was the amount of drive once the Vf requirement was low enough..

In any event 1 cell per lux works fine with Jbins or higher as long as you want to drive them harder than about 600mA.. you just loose dimming control if the Vf is too close.

That said.. to get the same current through a higher Vf emitter takes more power, so it's something to consider.. my 4xlux3 RT4 is putting about 17W through the emitters (dialed back from the 20.5 i started with)... i'm using UX1L emitters.

I have a diode checker on my meter that will tell me the literal 'Vf' of the emitter but that's just the value where current starts to flow.. 'what they said' about measuring the Vf in your case..

hook up the emitters you want to the fatman and just measure the voltage..

setting the current on the fatman is best using the formula and a digital ohm meter, but second-best is measuring the mV from the LED- to the GND and multiplying by 10. (there is a high precision 0.1ohm resistor between those two points.

-awr

I definitely would not call that sense resistor on the fatman high precision....

Keep in mind the Vf drop over time that cy mentioned.

It is important, and has been discussed in several threads.

Thanks again everyone. I think I'm going to have to shelf this idea for a while. I'm swimming in some deep water /ubbthreads/images/graemlins/help.gif /ubbthreads/images/graemlins/icon3.gif /ubbthreads/images/graemlins/wink.gif
Chris
.308

ANDREW

"I have a diode checker on my meter that will tell me the literal 'Vf' of the emitter but that's just the value where current starts to flow.. 'what they said' about measuring the Vf in your case.. "

do you have any guesses of ABOUT what the lower triggering value is on anything?
like the approxomate low triggering voltage on white luxes?

i want to know Basically how low they go. i have a fair idea, but would wonder what you found out.

It's around 2.5V

thanks.