Re: LED temperature dependence

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#2395

From: John Popelish
Date: Thu, 25 Apr 2002 13:34

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Adam Seychell wrote:
(snip)
> the output takes several seconds to settle after power is applied, as
> if the LED is heating up and stabilizing. I suspect the LED is
> changing its radiation pattern with temperature, thus altering
> relative
> amounts of light to the feedback and measuring photodiode.
> Currently a mirror is used to reflect part of the light to the
> feedback photodiode. The two photodiodes therefore receive light from
> two different
> spots of the LED. Your idea of using a beamsplitter should get around
> this
> problem because the same light path will be directed to both
> photodiodes.
> 
> As for the circuit I'm using all 100ppm/K components and photodiodes
> are run in shortcircuit mode (i.e across the inputs of a high
> impeadance op-amp).

Are you including an integrator in the brightness feedback loop, to
force the measuring diode to produce a preset amount of signal or just
a fixed gain, to reduce the variations?

-- 
John Popelish

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From: John Popelish <jpopelish@rica.net>
Newsgroups: sci.electronics.design,aus.electronics
Subject: Re: LED temperature dependence
Date: Thu, 25 Apr 2002 09:34:12 -0400
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