Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: piglet Newsgroups: sci.electronics.design Subject: Re: Optocoupler datasheets Date: Thu, 30 May 2024 09:01:51 +0100 Organization: A patent noisesome spinner Lines: 48 Message-ID: References: <66574685$0$2363143$882e4bbb@reader.netnews.com> <30bfd151-0f05-5761-1ef9-ae5bc4a3c3b2@electrooptical.net> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Injection-Date: Thu, 30 May 2024 10:01:52 +0200 (CEST) Injection-Info: dont-email.me; posting-host="8b3bcd9e7eadf67b0b48c39501c26114"; logging-data="1696050"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+hgIr2DYkJLVC/BL4o+BEq" User-Agent: Mozilla Thunderbird Cancel-Lock: sha1:CEpYFKumNn9N4O/hhcbZ3nP/eOE= Content-Language: en-US In-Reply-To: <30bfd151-0f05-5761-1ef9-ae5bc4a3c3b2@electrooptical.net> Bytes: 3201 On 29/05/2024 17:39, Phil Hobbs wrote: > On 2024-05-29 11:56, piglet wrote: >> bitrex 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: piglet