### Author Topic: Driving an optocoupler LED at different supply voltages  (Read 408 times)

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#### Giaime

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##### Re: Driving an optocoupler LED at different supply voltages
« Reply #25 on: Yesterday at 07:09:38 pm »
with only 100mV to work with, 1.8V LDO

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

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##### Re: Driving an optocoupler LED at different supply voltages
« Reply #26 on: Yesterday at 09:41:18 pm »
with only 100mV to work with, 1.8V LDO

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

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

#### Zero999

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##### Re: Driving an optocoupler LED at different supply voltages
« Reply #27 on: Yesterday at 11:34:52 pm »
Do you see on what part of text I have reacted? Do you get it?

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.
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