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Subject: Optocoupler datasheets
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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." ????

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