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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Optocoupler datasheets
Date: Thu, 30 May 2024 14:58:36 -0400
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On 2024-05-30 09:37, john larkin wrote:
> 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.
> 

Yup.  Even with a better photoreceiver, most of the usual speedup tricks 
don't work with LEDs, on account of their diffusion-dominated carrier 
dynamics.

Cheers

Phil Hobbs
-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com