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From: "R. Lewis"
Subject: Re: current source for array of infrared leds
Date: Tue, 3 Dec 2002 21:36:14 -0000
Organization: Nextra UK
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NNTP-Posting-Date: Tue, 3 Dec 2002 21:34:47 +0000 (UTC)
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"John Fields" wrote in message
> What he's saying is that with his stupid way he needs a voltage
> source which can be adjusted to output precisely the current needed
> by the LED at some temperature, (ISTR something about 0.25% of
> something being thrown in there too...) and then that the LED's
> tempco will keep it constantly at that same brightness over a large
> temperature range. But if that doesn't work, then Harry suggests
> that a thermistor (either positive or negative tempco, he's not
> sure) be used to _make_ it work. The original premise was that all
> the LED's in an array should be driven by a constant voltage source,
> so that means that for a 200 LED array each LED would _have_ to be
> driven by its own 1/4% voltage source and thermistor. Patently
I must admit that such an approach is somewhat problematical and may have
been postulated without too much understanding of the principals involved.
I have seen commercial equipment that uses a similar(ish) approach and the
performance over any but the most limited temperature range is as appalling
as would be expected.
Hate to think what happens over the longer term - maybe nothing changes with
time as well as temperature !
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