Path: ...!Xl.tags.giganews.com!local-2.nntp.ord.giganews.com!nntp.supernews.com!news.supernews.com.POSTED!not-for-mail NNTP-Posting-Date: Wed, 29 May 2024 16:39:30 +0000 Subject: Re: Optocoupler datasheets Newsgroups: sci.electronics.design References: <66574685$0$2363143$882e4bbb@reader.netnews.com> From: Phil Hobbs Message-ID: <30bfd151-0f05-5761-1ef9-ae5bc4a3c3b2@electrooptical.net> Date: Wed, 29 May 2024 12:39:29 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Lines: 46 X-Trace: sv3-a8poqlD13DCPzZX8/tIagPN+ZfMYgpS4itO+eqf6wFMVGy6ILJgRMoERNn22qVgS1DMTbvfjSFXVsxF!9143W5HvxVLqhLK1W8mXSakhvsAsXVDNOVUGNhWqbV/Ep2I2TsTK1sBl57cyuR1j5bFQ/2tZT4+D!kyt9gS7KI1FNpnS/mQZWZO8= 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: 3041 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 -- 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