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Driving an optocoupler LED at different supply voltages

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voltsandjolts:
Maybe microcontroller can measure the supply voltage then use PWM to set LED drive current, along with on-off keying for the comms. Probably lowest BOM cost.

TrickyNekro:

--- Quote from: fenclu on January 20, 2019, 08:59:30 am ---
--- Quote from: TrickyNekro on January 18, 2019, 09:15:26 pm ---
--- Quote from: SiliconWizard on January 18, 2019, 06:16:16 pm ---As a side note, the Vfd of the LED is said to be 1.7V but in the 6N135/136 datasheet I have, Vfd is up to 1.9V max? So 1.8V may not even allow to light up the LED if you're unlucky...

--- End quote ---

Do you have to use the 6N135/6? Cause I can find many parts that are much cheaper and have a lower forward voltage.
Example:
https://eu.mouser.com/Optoelectronics/Optocouplers-Photocouplers/_/N-6x5ji?Ns=Pricing%7c0&No=25&Rl=6x5jiZgjdhqbZ1yuoc3aZ1yuo3qbSGT

And... a more specific example:
https://eu.mouser.com/ProductDetail/Vishay-Semiconductors/VOH1016AB-T?qs=sGAEpiMZZMtd3yBnp8bAgLhQurE%252baQR29IOeeMf%252bGH1H6u1Y7D9ZWg%3d%3d

I am sure if you search a little bit you can find better parts than 6N135/6. These are quite mature parts, that alone makes them expensive.

--- End quote ---

This one needs a supply voltage of at least 3V.


--- End quote ---

Hmmm yes, the isolated side, because you got a amp and gate there, there are others that use only a transistor.
Are you going to run both sides with the same power supply? I thought your problem was the LED!
How fast do you want it to be? Sorry, If I missed an earlier comment or something but something is not adding up.
Cause if you need an optocoupler with a low voltage switch LED there are solutions, as far as I remember you needed it for comms.

Edit: I´ve read your first comment again, the 3V is for the gate side not the LED side. Also if you want it for serial, your best bet is anyhow going with a gated design cause you can be sure that it works.
Or you want to have bidirectional comms? As far as I understood it, it´s for the LED side.

A gated design is also the best for both varying driving current and simplicity. Remember KISSing is always your safest bet.

TrickyNekro:
And in case you did not understand it! Here! Page 3 of Datasheet, you look at the input characteristics!
Your microcontroller is going to be at the input side. As far as you told us anyhow.
Maximum Vf = 1.4V! You can run it down to 300uA in case it is a battery operated project.
Unless you are stuck with a bunch of 6N135/136 for whatever reason, there are many other optocouplers that are far superior for your use case, ready out of the box!

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