Re: Optocoupler datasheets

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.