Measuring LED Vf without a Constant Power Supply?

[kosPap]

Well can it be done?

So far I have made this setup (and got different readings from 3 LEDs all marked Q5)

AW17670
3xAMC7135 Diver
LEDs mounted on a CPU Heatsink with fan running.

I measured voltage and current to the LED and they were constant within 2 monutes of.
So knowing these one can go to the datsheer and calculate Vf at 350mAh....
Or are there hidden deamons in that setup?

Data
LED 1 – 3.35 V – 0.92 A – 3.08 W
LED 2 – 3.65 V – 0.89 A – 3.25 W
LED 3 – 3.47V – 0.97 A – 3.66 W

What can one deduct from these?

Thanks, kostas

 

[znomit]

Why not just knock up a 350mA driver with an LM317?
http://users.telenet.be/davshomepage/current-source.htm

 

[uk_caver]

So knowing these one can go to the datsheer and calculate Vf at 350mAh....
Or are there hidden deamons in that setup?

Data
LED 1 – 3.35 V – 0.92 A – 3.08 W
LED 2 – 3.65 V – 0.89 A – 3.25 W
LED 3 – 3.47V – 0.97 A – 3.66 W

What can one deduct from these?

It seems odd to have a higher current on LED 3 with a higher Vf than LED 1.

What power source were you using?

Even if running off a sagging power source, it's hard to see how a later measurement should have a higher current when there's less voltage headroom. (Varying resistance in the ammeter connection?)

Also, if none of your readings are close to the nominal 1050mA of 3x7135s, I'd wonder if there is a problem in your setup of having sufficient voltage to fully drive the LEDs.

If you want to know the Vf at 350mA, why not use 1x AMC7135?

 

[bshanahan14rulz]

Yeah, if you have way too many of those 7135 boards, like most people who have to bulk-order them, just knock some of the chips off, leave one on there, make sure your powersource has enough voltage, and measure Vf that way.

 

[Curt R]

Amp measurements can be misleading using the Amp setting on a VOM. We use a 0.1 ohm 1% 3 or 5 watt
resistor in line with the output to the LED, and measure the voltage drop across the resistor. Then using
ohm's law, a more accurate current reading is obtained. DC voltage measurements using the VOM are
far more accurate.

After testing hundreds of LEDs that go into our lights, there is no linear reasoning behind the results. You can get
different readings of voltage, current drive and Lumen output within the same reel of LEDs, no constant.

Curt

 

[kosPap]

indeed, and taht is the reason I am testing them....

Seems the my setup is sound in priinciple, and have to cater for the amp draw details?