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From: "Harry Dellamano"
Subject: Re: current source for array of infrared leds
Date: Wed, 4 Dec 2002 07:45:13 -0800
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"Tony Williams" wrote in message
> In article ,
> R. Lewis wrote:
> > Generally an leds resistance (dV/dI) reduces with increasing current,
> Presumably, (when driven by a const-V), thermal runaway
> occurs if/when dV/dI goes negative. AFAIK that can occur
> with diodes, but can it happen with LEDs?
> I would suspect that it depends on the Theta(j-a) of
> the LED, and the source-resistance of the "const-V".
> Some LEDs could have an intrinsic series-R, (which may
> even have a +ve tempco), and this will keep the overall
> dV/dI positive...... Possibly what Harry experienced.
> Other LEDs (old fashioned IR types?) will need some min
> value of (external) series-R in order to keep the dV/dI
> of the (LED+R) combination positive.
> But Frank is going to want a value of series-R that keeps
> the light output constant..... he just has to hope that it
> is higher than the min value required for thermal stability.
> Tony Williams.
Hi Tony, You make a good point that there are many types of LEDs and if
you are going to put them a constant voltage array then a little time on a
curve tracer with a heat gun is well spent. Frank did not tell us how many
rows and columns in his array. It's nice to have more than five columns so
if one LED fails it knocks out less than 20%. In constant current mode the
other columns would increase intensity to take up the slack.
He should also do a Vf sort (binning) so the sum of each column's Vf is
within maybe 50mV at his operating current. In large arrays the LEDs are pre
binned by the vendor (usually off the same wafer) and come in maybe 5 bin
types with 50mV differences. For small lots it's easy to sort with a
constant current and a volt meter.
The tempo of the voltage source is also important and for a robust design
and must be considered in the final equation. That is why I mentioned
positive and negative thermistors because for fine tuning it is as much a
thermal problem as electronic. At high intensity levels the eye is so
insensitive that thermistors are not necessary.
Bottom line, know your LED's I/V curve over it's temp range and a little
math will result in a good nights sleep. Also the LED die is operating at
maybe 150 watts/sq. inch so thermal planes may be needed.
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