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Driving an optocoupler LED at different supply voltages

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fenclu:

--- Quote from: Yansi on January 16, 2019, 07:10:16 pm ---Do you have (any) power supply available at the isolated (LED) side?

--- End quote ---

Unfortunately, no, I don't. I also had an idea to implement some sort of microcontroller voltage doubler, that way I would have a bit higher voltage to play with.

Benta:
The drain current spread of JFETs is usually rated at 1:3, but better selections are available.
Using a JFET with a rated drain current of 3...9 mA would in this case be appropriate, the real spread will be smaller.
The bipolar circuit proposed by Yansi is useless, it has at least 2 V drop. The JFET will work down to 0 V, but only act as a pure current source above ~0.4 V drain-source. Below that it will be resistive, which is not bad in this application.


Yansi:

--- Quote from: CJay on January 16, 2019, 06:32:55 pm ---
--- Quote from: Yansi on January 16, 2019, 05:58:43 pm ---Minimum LED current is not determined experimentally, but defined by the manufacturer. If I remember correct, it is 8mA for 6N137 (please verify in the datasheet)

--- End quote ---
Indeed, I did some experimenting with optocouplers a while back and found forward current through the LED is very imprtant if you need to meet specs for transfer ratio but you can drive them with quite low currents if you don't need them to be high speed.


--- Quote from: Yansi on January 16, 2019, 05:58:43 pm ---Just ask google for "LED current  source" and check the simple transistorized solutions.

I'd bet on this two transistor one to work quite nicely across a decent voltage range up to some decent speeds.



--- End quote ---

That's the bipolar version of the one I linked, it works well with a MOSFET in the T2 position, 2N7000 works.

I built a few dozen for some PWM fading LED strings a few years back, they're good to really quite high frequencies, far higher than I needed. to stop the flicker

--- End quote ---

But using a FET as T2 does not fulfill requirement of low voltage drop across it, minimum operating voltage would become Vgsth+Vbe minimum, which would probably be significantly more, than just 2* Vbe for the bipolar version. (plus the LED Vfwd of course)

Unfortunately both are probably too much for the OP's application.  :-//

Benta:

--- Quote from: Yansi on January 16, 2019, 07:58:39 pm ---
--- Quote from: CJay on January 16, 2019, 06:32:55 pm ---
--- Quote from: Yansi on January 16, 2019, 05:58:43 pm ---Minimum LED current is not determined experimentally, but defined by the manufacturer. If I remember correct, it is 8mA for 6N137 (please verify in the datasheet)

--- End quote ---
Indeed, I did some experimenting with optocouplers a while back and found forward current through the LED is very imprtant if you need to meet specs for transfer ratio but you can drive them with quite low currents if you don't need them to be high speed.


--- Quote from: Yansi on January 16, 2019, 05:58:43 pm ---Just ask google for "LED current  source" and check the simple transistorized solutions.

I'd bet on this two transistor one to work quite nicely across a decent voltage range up to some decent speeds.



--- End quote ---

That's the bipolar version of the one I linked, it works well with a MOSFET in the T2 position, 2N7000 works.

I built a few dozen for some PWM fading LED strings a few years back, they're good to really quite high frequencies, far higher than I needed. to stop the flicker

--- End quote ---

But using a FET as T2 does not fulfill requirement of low voltage drop across it, minimum operating voltage would become Vgsth+Vbe minimum, which would probably be significantly more, than just 2* Vbe for the bipolar version. (plus the LED Vfwd of course)

Unfortunately both are probably too much for the OP's application.  :-//

--- End quote ---

Who's talking about using a FET as T2? Not me!

I'm talking about using one single JFET as a constant current source. No resistors, no diodes, no nothing. Get it?

Yansi:
Do you see on what part of text I have reacted? Do you get it?

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