Re: Optocoupler datasheets

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