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NNTP-Posting-Date: Fri, 31 May 2024 10:47:50 +0000
From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: Optocoupler datasheets
Date: Fri, 31 May 2024 03:47:50 -0700
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On Fri, 31 May 2024 00:12:30 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>john larkin <jl@650pot.com> wrote:
>> On Thu, 30 May 2024 14:58:36 -0400, Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>> 
>>> 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
>> 
>> I did test a Cree white LED for speed. It hit my detector response of
>> about 7 ns, phosphor included. I was surprised.
>> 
>> 
>Yes, some LEDs are much faster than others. 
>
>We sell a LED-based pulsed light source that has <6 ns rise and fall times,
>using any  of three part numbers at different wavelengths.
>
>With a fancy $20 LED, it gets down to 2 ns. 
>
>Speedup caps , reverse bias, and so on do zilch to speed it up. 
>
>Cheers 
>
>Phil Hobbs 

Why are IR LEDs so much faster? A 10 GBPS SFP transceiver module costs
$16 from Amazon (with Prime free shipping!)