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From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: Optocoupler datasheets
Date: Wed, 29 May 2024 08:53:28 -0700
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On Wed, 29 May 2024 11:15:17 -0400, bitrex <user@example.net> wrote:

>Optocoupler datasheets seem like kind of a mess, I try not to use them 
>too often in situations where there's any kind of power budget because 
>other than "shove some relatively huge current through the LED like 5-10 
>mA" it's hard to know what you can get away with.
>
>A light load on the transistor side will definitely reduce the forward 
>current required (and of course slow the speed to a crawl) but who can 
>say by how much while still ensuring the thing will turn on sufficiently 
>to saturate the output?
>
>The CTR varies widely from process variation, varies with temperature, 
>varies with collector emitter voltage, varies with forward current, and 
>the data sheets are full of caveats like "At I_f < 1 mA, note CTR 
>variation may increase" and "Graphs are representative, not indicative 
>of actual performance." ????

Keeps the amateurs away.

>
>Any suggestions for how to approach methodically/mathematically 
>selecting drive current would be appreciated, thank you! ("Don't bother" 
>a valid option)

For on/off, use a logic coupler or an opto SSR. They have well defined
input requirements.

A dual transistor-type optocoupler makes a fun totem-pole logic
coupler, with zero static power dissipation on the isolated side. It
basically always swings rail-rail out and gets faster as LED side
current goes up. Or spike that to save driver power.