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Anyone have links to the V-I curves for blue or white LEDs?
Thanks.
Hi there,
I'd be a little careful however of any data sheets i look at when it comes
to white LEDs. Some of them are much older than the newest LEDs out there.
This translates to a lower average voltage than shown on the data sheet.
Of course also to be considered is the LED aging, which can reduce
LED forward voltage drop over time even at the same current.
It's significant in LEDs like the white Luxeons, being -0.2v or greater.
A bunch of people complained about this in the past on other web sites.
After some hours (a surprisingly short time like hours) their flashlights
would get brighter and hotter.
it's hard to say unless of course the resistance relative to
the overhead voltage is high enough to act more or less as a constant
current as the voltage of the LED(s) changes over time.
In other words, if the supply voltage is two times the LED voltage it
will probably work ok for a long time, but if the supply voltage is very
close to the LED voltage (to make it more efficienct of course) then
we can expect problems as the LED ages because the current will
change much, and it's a negative voltage changes so that means
higher current.
Then again if the constant current regulator has to drop more
voltage, it's going to get hotter too.
I've guessed how much the temperature of an LED changes by measuring
it's foward voltage with constant current as the LED self heats after
turn on. I can get a rough idea what temperature the die reaches.
Hi again,
I've used that figure yes. What i was doing was trying to find out how
well my make shift thermal compound was working on a high power LED.
I tested one with more or less regular high temp epoxy and another one
with epoxy that was made just for heat sinking, and they both came out
fairly close. I was surprised how well the regular epoxy worked, but then
i made sure to keep the layer very very thin, which as im sure you know
means less distance for the heat to conduct through, and so it still keeps
the LED cool enough even at full power (with a decent heat sink area).
Perhaps you've had some experience, or maybe i should make another
thread about this?