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From: piglet <erichpwagner@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: Optocoupler datasheets
Date: Thu, 30 May 2024 09:01:51 +0100
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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