From: John Woodgate
Subject: Re: current source for array of infrared leds
Date: Mon, 2 Dec 2002 22:49:26 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Tue, 3 Dec 2002 07:30:34 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Frank Bemelman
wrote (in <firstname.lastname@example.org
uronet.nl>) about 'current source for array of infrared leds', on Mon, 2
>That's why I wanted to start with a currentsource, and compensate
>the current, when the voltagedrop sets in. Doesn't need to be
>perfect, just a bit better than a simple currentsource.
If you use an 'over-simple' current source, i.e. too low a voltage and
series resistor for a 'real' current source, you get the compensation
effect mentioned by Harry. As the diode drop voltage goes down with
temperature, the current goes up a bit. But having no external
resistance at all seems to me to be a very dodgy solution.
For example, take a 4 V supply, a diode drop of 2 V and a current of 20
mA. The resistor is 100 ohms. Now suppose the diode drop falls to 1.8 V.
the current is now 22 mA. 10% drop in voltage, 10 % increase in current.
But nothing needs to be held to +/-0.25%.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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