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Driving an optocoupler LED at different supply voltages
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Giaime:

--- Quote from: langwadt on January 16, 2019, 06:47:54 pm ---with only 100mV to work with, 1.8V LDO

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It seems the best solution to me so far, just lower the supply voltage to the minimum expected and calculate the resistor accordingly.
langwadt:

--- Quote from: Giaime on January 17, 2019, 08:09:38 am ---
--- Quote from: langwadt on January 16, 2019, 06:47:54 pm ---with only 100mV to work with, 1.8V LDO

--- End quote ---

It seems the best solution to me so far, just lower the supply voltage to the minimum expected and calculate the resistor accordingly.

--- End quote ---

yeh, but come to think of it, it might be far too slow ..
Zero999:

--- Quote from: Benta on January 16, 2019, 08:43:46 pm ---
--- Quote from: Yansi on January 16, 2019, 08:34:52 pm ---Do you see on what part of text I have reacted? Do you get it?

--- End quote ---

Not really. You've presented a bipolar solution that doesn't really help the OP (far too large voltage drop). Then you've referred to "unobtainium" JFETs, as if it is important to present a current within +/-1% to an optocoupler. After that you lost me.

The JFET solution is not precise, but will work down to almost 0 V voltage drop.

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I'm not convinced. The JFET constant current based sources I've found require a few volts for devices with a current limit of 4mA.
http://njsemi.com/datasheets/J500%20-%20J511.pdf
https://my.centralsemi.com/get_document.php?cmp=1&mergetype=pd&mergepath=pd&pdf_id=1N5283-5314.PDF
http://www.mccsemi.com/up_pdf/CLD20B(DO-214AA).pdf
https://atcsemitec.co.uk/wp-content/uploads/2018/08/Semitec-CRDs-webpage.pdf

Benta:
In my experience, the JFET works as a current source down to appr. 1...2 V drain-source (depends on current). Below that it tends to go resistive, which is not really an issue in this application.
Kasper:
Generally I'd say current regualtor is ideal for driving LEDs but if you want the smallest and easiest solution, this could be one the rare occassions where zener voltage regulator is good.

Look up: EEVblog #908 - Zener Diodes
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