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From: piglet <erichpwagner@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: Optocoupler datasheets
Date: Wed, 29 May 2024 15:56:18 -0000 (UTC)
Organization: A noiseless patient Spider
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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." ????
> 
> Any suggestions for how to approach methodically/mathematically 
> selecting drive current would be appreciated, thank you! ("Don't bother" 
> a valid option)
> 

Why do you want to saturate the photo transistor? 
If you don’t you can get much higher speeds out of even jelly bean cheap
couplers. Even without a base connection it is possible. 

-- 
piglet