Path: ...!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: Thu, 30 May 2024 13:37:32 +0000 From: john larkin Newsgroups: sci.electronics.design Subject: Re: Optocoupler datasheets Date: Thu, 30 May 2024 06:37:32 -0700 Message-ID: <050h5jlbdtnavt2aoo037j9p89eu4613af@4ax.com> References: <66574685$0$2363143$882e4bbb@reader.netnews.com> <30bfd151-0f05-5761-1ef9-ae5bc4a3c3b2@electrooptical.net> User-Agent: ForteAgent/8.00.32.1272 MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit Lines: 70 X-Trace: sv3-c5206CLZ9vkGTYzyf6mg5d80IWO45QWt7SL5jxekhbKgwznAt70JfqdwBwJZ3zhBSUnB/slBVGffxUV!aoEG95l/bT1XBwE64vCHvkJMZtuUxfLQ3yqaksPtiUNV07LMComr6cjy7H6efk/q8W/NuqYBTRs+!aa4Upw== 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: 4067 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.