Path: ...!local-2.nntp.ord.giganews.com!local-4.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 11:41:56 +0000 From: john larkin Newsgroups: sci.electronics.design Subject: Re: Optocoupler datasheets Date: Fri, 31 May 2024 04:41:56 -0700 Message-ID: <4qdj5jhh6pd5aar7kcijl7k8ve4g92108e@4ax.com> 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=ISO-8859-1 Content-Transfer-Encoding: 8bit Lines: 123 X-Trace: sv3-UCJTomOS0xrFTPGfoqB73+wjZlX44nhJjvwb0jwztxtWQG/jb1esnY1YFozFe/CY94IXrtYp1NfRXxL!DFWba0aEy+/YrEXNkLoyr80/9k/3BGoZ1QJrpjdTQH0860PO5ig2mvxgM7HrPvi5RdZFFyQs/wOu!Oumigg== 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: 6464 On Fri, 31 May 2024 11:20:53 -0000 (UTC), Phil Hobbs wrote: >john larkin wrote: >> 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!) >> >> > >Those are lasers. The carrier dynamics of a laser running above threshold >are dominated by radiative recombination, which is much quicker. > >Lasers are also designed to avoid the horrible diffusion delay of most >LEDs—lower doping, thinner epi, and so on. > I have noticed that the capacitances of LEDs vary all over the place, like 50:1.