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From: "R. Lewis"
Subject: Re: current source for array of infrared leds
Date: Wed, 4 Dec 2002 22:46:09 -0000
Organization: Nextra UK
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NNTP-Posting-Date: Wed, 4 Dec 2002 22:44:35 +0000 (UTC)
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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.
Running the leds at gentle currents - keeping the junction temp sensible-
keeps any runaway problem in hand. Trouble is most want to push the leds for
as much output as they can get.
If subsequently ambient is much increased everything goes pearshaped
notwithstanding the fact that the wavelength also decides to go on holiday.
Generally wavelength shift with temp is not too much of a problem with IR
leds because of their applications but it can be above sheer nuisance value
for conventional (non 'phosphorised') visible leds around the centre of the
spectrum - where we readily see any colour change.
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