Re: Optocoupler datasheets

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