Re: Optocoupler datasheets

On Thu, 30 May 2024 14:58:36 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2024-05-30 09:37, john larkin wrote:

On Thu, 30 May 2024 11:29:18 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
 

piglet <erichpwagner@hotmail.com> wrote:

On 29/05/2024 17:39, Phil Hobbs wrote:

On 2024-05-29 11:56, piglet wrote:

bitrex <user@example.net> 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:
>
<https://electronics.stackexchange.com/questions/136928/under-what-conditions-does-an-optocoupler-work-fastest>
>
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.