Path: ...!Xl.tags.giganews.com!local-1.nntp.ord.giganews.com!nntp.supernews.com!news.supernews.com.POSTED!not-for-mail NNTP-Posting-Date: Thu, 30 May 2024 21:52:39 +0000 From: john larkin Newsgroups: sci.electronics.design Subject: Re: Optocoupler datasheets Date: Thu, 30 May 2024 14:52:39 -0700 Message-ID: <56th5jl9dinht3hjdff841pslvfuu1643c@4ax.com> References: <66574685$0$2363143$882e4bbb@reader.netnews.com> <30bfd151-0f05-5761-1ef9-ae5bc4a3c3b2@electrooptical.net> <050h5jlbdtnavt2aoo037j9p89eu4613af@4ax.com> User-Agent: ForteAgent/8.00.32.1272 MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit Lines: 86 X-Trace: sv3-51pQMDmfkxfON6tIBx6KUt5cr7hCJZV9zXQ0+2txMrBrdqr07zO4NDKhFJHM6qjoytlzssyBCqoU7L5!0Dk/0FKTKhaZwcoyyRKm+sDb+M6Y9yDiYXxG+Tvl0wtRMDaf1flRmB5UGxkdaRvQb7HBLjqP1T+A!oB0srA== X-Complaints-To: www.supernews.com/docs/abuse.html X-DMCA-Complaints-To: www.supernews.com/docs/dmca.html X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 Bytes: 4756 On Thu, 30 May 2024 14:58:36 -0400, Phil Hobbs wrote: >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 I did test a Cree white LED for speed. It hit my detector response of about 7 ns, phosphor included. I was surprised.