From: Spehro Pefhany
Subject: Re: current source for array of infrared leds
References: <firstname.lastname@example.org> <email@example.com>
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Date: Wed, 27 Nov 2002 21:46:32 GMT
NNTP-Posting-Date: Wed, 27 Nov 2002 16:46:32 EST
On Wed, 27 Nov 2002 21:45:33 +0100, the renowned "Frank Bemelman"
>I guess it would, but it would require a relative
>precise voltagesource and a smallish resistor - at
>least that what my gut feeling says. Another brand
>of leds may spoil the entire thing beyond control.
Yes... could be, you won't find useful guarantees
on the data sheets in any case, IMHO.
>That's why I wanted that current-source, and increase
>the reference a bit should the total forwardvoltage
You still need some kind of offset of the expected
Vf, right? I know, use a microcontroller with ADC
>This is probably one of my more stupid ideas. I better
>first start measuring the actual change in Vf.
Numbers will help. If the LEDs change by -2mV/K, and
you want a 20% change for, what, 30'C rise (WAG, I
didn't pull out any data sheets), then that's only
60mV. Pretty fussy. What about throwing an extra LED
into the string with a phototransistor and doing
it closed loop? You'd need a trimpot etc. to set the
light level initially, but it would also compensate for
LED aging if they were all the same batch.
"it's the network..." "The Journey is the reward"
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