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NNTP-Posting-Date: Thu, 30 May 2024 13:37:32 +0000
From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: Optocoupler datasheets
Date: Thu, 30 May 2024 06:37:32 -0700
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On Thu, 30 May 2024 11:29:18 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>piglet <erichpwagner@hotmail.com> wrote:
>> On 29/05/2024 17:39, Phil Hobbs wrote:
>>> On 2024-05-29 11:56, piglet wrote:
>>>> 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.
>>>> 
>>> Because unless there's overall feedback, running it unsaturated gives 
>>> you a beta-dependent circuit that's further dependent on the LED 
>>> efficiency, the transparency of the white snot filling the opto package, 
>>> temperature, you name it.
>>> 
>>> Cheers
>>> 
>>> Phil Hobbs
>>> 
>> 
>> Sorry, maybe my language was sloppy. I meant keep phototransistor 
>> collector from bottoming and reduce C-B miller effect. Not necessarily 
>> by rationing photons. Keeping Vce constant by feeding straight into a 
>> transistor base is brutally effective. See the post about halfway down here:
>> 
>> <https://electronics.stackexchange.com/questions/136928/under-what-conditions-does-an-optocoupler-work-fastest>
>> 
>> piglet
>> 
>> 
>> 
>
>If you have the base pinned out, you can do more stuff, true.  But at the
>end of the day you’re still dealing with a phototransistor.  
>
>BITD TI and HP made optos with actual specs, but these days, not so much. 
>
>Linear mode works great when there’s overall feedback, as in your typical
>offline switcher, which has a TL431 to do the actual regulating. 
>
>Cheers 
>
>Phil Hobbs 

A c-b schottky clamp would help, sort of a 74LS photocoupler.

But the really good logic couplers these days aren't optical.