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NNTP-Posting-Date: Wed, 29 May 2024 16:39:30 +0000
Subject: Re: Optocoupler datasheets
Newsgroups: sci.electronics.design
References: <66574685$0$2363143$882e4bbb@reader.netnews.com>
 <v37j72$171gp$1@dont-email.me>
From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Message-ID: <30bfd151-0f05-5761-1ef9-ae5bc4a3c3b2@electrooptical.net>
Date: Wed, 29 May 2024 12:39:29 -0400
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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

-- 
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