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From: boB <boB@K7IQ.com>
Newsgroups: sci.electronics.design
Subject: Re: Optocoupler datasheets
Date: Thu, 30 May 2024 12:01:15 -0700
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On Thu, 30 May 2024 06:40:36 -0700, john larkin <jl@650pot.com> wrote:

>On Thu, 30 May 2024 08:55:19 +0100, piglet <erichpwagner@hotmail.com>
>wrote:
>
>>On 30/05/2024 03:16, boB wrote:
>>> On Wed, 29 May 2024 22:03:51 +0100, Clive Arthur
>>> <clive@nowaytoday.co.uk> wrote:
>>> 
>>>> On 29/05/2024 16:15, 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)
>>>>
>>>> Not really answering the question, but there's more than one way to skin
>>>> a cat...
>>>>
>>>> https://www.nve.com/Isolators
>>> 
>>> 
>>> You can also turn the LED off a bit faster and reduce off-time
>>> interference/noise by pulling the anode low, activiely.
>>> Not sure how much better it is though.
>>> 
>>> boB
>>
>>
>>I thought IREDs and LEDs are very fast, far faster than the 
>>phototransistor but someone has done as you suggest, see:
>>
>><https://www.edn.com/optocoupler-speed-up-also-reduces-power-consumption/>
>>
>>piglet
>
>
>https://www.dropbox.com/scl/fi/himnbhrq97vj6eplokek5/Faster_Opto_Totem.JPG?rlkey=ste2tfj8v3zpi45wh1wepyaov&raw=1

Interesting approach, John.

Piglet, the first comment says where I first saw this which was from
Avago/HP and their IGBT driver chips.

boB