Path: ...!npeer.as286.net!npeer-ng0.as286.net!weretis.net!feeder8.news.weretis.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Phil Hobbs Newsgroups: sci.electronics.design Subject: Re: Optocoupler datasheets Date: Thu, 30 May 2024 14:58:36 -0400 Organization: A noiseless patient Spider Lines: 89 Message-ID: References: <66574685$0$2363143$882e4bbb@reader.netnews.com> <30bfd151-0f05-5761-1ef9-ae5bc4a3c3b2@electrooptical.net> <050h5jlbdtnavt2aoo037j9p89eu4613af@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Injection-Date: Thu, 30 May 2024 20:58:38 +0200 (CEST) Injection-Info: dont-email.me; posting-host="97dd50364f95b78ab508b343dd52e7bb"; logging-data="1917496"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+V/z7kch45MKuDwCBOxQp1" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.0 Cancel-Lock: sha1:19hlKU+N8oRc7F1VNejqCi8bcV4= In-Reply-To: <050h5jlbdtnavt2aoo037j9p89eu4613af@4ax.com> Bytes: 4770 On 2024-05-30 09:37, john larkin wrote: > On Thu, 30 May 2024 11:29:18 -0000 (UTC), Phil Hobbs > wrote: > >> piglet wrote: >>> 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 >>> >>> >>> >> >> 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