In an app a white Luxeon emitter (1W ) is run at very low currents (20 to 40mA) for extended periods of time.
Originally it was not recommended to run these below ~200mA else current crowding, channeling and whatever could lead to dramatic output variations over time.
Has this been improved? What output variation may be expected at these current levels?
(Modulating the led is not an option at this time)
Check out the data sheet. That should give you the information you need to use the LED in the circuits you design. If the component characteristics do not fit the circuit topology that you can provide then select another component that can.
- mkaras
I doubt that Lumileds will publish the type of data you need since this is not the intended use of this device. Since low current operation will not damage the LED there is no reason why you shouldn't run the LED in this mode if that is what is required for your application. (I assume your application also has periods of high current operation.) However you may have to run your own tests to get the data you need.
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There's a data sheet at:
According to the data sheet "Driving these high power devices at currents less than the test conditions may produce unpredictable results and may be subject to variation in performance."
It looks like these devices require about 100 to 350mA to operate correctly (depending on color?).
Why not use a photodetector of some sort to determine the level and put the dammned LED in a feedback loop?
when leds were run at very low currents could occurr apparently randomly over time. This radomness included very short times such that for some the effect was instantaneous - i.e. there was a very rapid drop off in efficacy at low current levels when measured from 'new' : others would drop off after a time (which could be very prolongued).
I wondered if this has been 'cured'.
This was all many years before such huge leds as the Luxeons were thought of and I wondered if the Luxeon datasheet reference to avoid running at low currents was due to this type of effect or other, stable, mechanisms which cause the efficacy to fall off at very low currents but which are not time dependant - which allows them to be selected out on test with some assurance.
Unfortunately such schemes, especially those employed in equipment used outside, tend to measure ambient more than anything else. Just have a look at ............. no shan't say.
Do you have a reference for this? Normally issues with output variations over time have to do with high currents (which is what current crowding refers to) and high temperatures (channeling).
I think they're point out that operating a batch of their LEDs at low current densities can lead to unmatched part-to-part light output, since they don't all have the same light-level vs current characteristic. Hence the suggestion of using PWM for dimming. No doubt whatever light vs current curve an LED has it keeps, so you could sort LED into similar groups.
I'd say you should test these LEDs to see if they have the same old problems and report back to us. You could also call them.
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Thanks,
- Win
PWM to get an average brightness is not a big deal - a 555 and a few discretes.
But why design in a 1W emitter when you need only something 10% or 20% the size? What's wrong with a smaller, cheaper LED or two that's happy at 20mA or 40mA?
In my experience the degradation you'll see will be brightness variations/efficiency reduction by 50% or more and a shift in wavelength (color) and these parameters will vary with time.
Tim.
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