Path: ...!local-2.nntp.ord.giganews.com!local-3.nntp.ord.giganews.com!Xl.tags.giganews.com!local-1.nntp.ord.giganews.com!nntp.supernews.com!news.supernews.com.POSTED!not-for-mail NNTP-Posting-Date: Fri, 31 May 2024 10:47:50 +0000 From: john larkin Newsgroups: sci.electronics.design Subject: Re: Optocoupler datasheets Date: Fri, 31 May 2024 03:47:50 -0700 Message-ID: References: <66574685$0$2363143$882e4bbb@reader.netnews.com> <30bfd151-0f05-5761-1ef9-ae5bc4a3c3b2@electrooptical.net> <050h5jlbdtnavt2aoo037j9p89eu4613af@4ax.com> <56th5jl9dinht3hjdff841pslvfuu1643c@4ax.com> User-Agent: ForteAgent/8.00.32.1272 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 107 X-Trace: sv3-HIM/2xdI8teZL84IG1C8GALCiwJxRNOK4k662c10RoMYYOds8WUMT0FD6tOAxqH3BCnH3hedOiyi1R5!Ao0qLCAVndJ7YIaBkm4V7U9O909IDPsihE/da7easJ77jVHU1/r1tREcq0wvWes/wMovYZ1x19Dd!q3UVcQ== 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: 5646 On Fri, 31 May 2024 00:12:30 -0000 (UTC), Phil Hobbs wrote: >john larkin wrote: >> 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. >> >> >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!)