Re: Optocoupler datasheets

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
--
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