EEVblog Electronics Community Forum

Electronics => Beginners => Topic started by: JeanF on May 18, 2020, 01:42:20 pm

Title: optocoupler CTR calculation
Post by: JeanF on May 18, 2020, 01:42:20 pm

Hi,

I was not sure how to use the various CTR graphs that are included in optocoupler datasheets, and I came across this document https://www.vishay.com/docs/83706/83706.pdf (https://www.vishay.com/docs/83706/83706.pdf) (funnily enough I also found another one by the same manufacturer titled "Guidelines for Reading an Optocoupler Datasheet", at least they now that it can be confusing for beginners  :-+)

and I figured I could do a bit of homework while I'm at it.

Then in Problem 1, step 2, equation 2 (in page 2), where does the I_L = 50 µA come from ? Maybe it has to do with the leakage current of the 74HC04 but in the datasheets I found it's spec'd at +/- 1 µA, which is far from 50, so I'm not sure.

Thanks!

EDIT :


By the way, I was reading this thread on RPi forums https://www.raspberrypi.org/forums/viewtopic.php?f=37&t=54024 (https://www.raspberrypi.org/forums/viewtopic.php?f=37&t=54024) which is about ways to use an optocoupler in the most reliable (and hopefully safe) manner to detect if mains is live or not from a Raspberry Pi or another low voltage device.

Somewhere in the middle, the author (Bud Bennett) says "  So we now know that the output current is 100uA. The CTR of the 4N37 is estimated, using Figures 3 and 5 and iterating a bit, to find IF produces a CTR of about 0.15."

(sorry, can't provide a direct link, please use Ctrl-F to find where it is in the rather lengthy post)

the link to the datasheet he used is broken but I managed to find it with archive.org wayback machine, I've attached it here. (https://www.eevblog.com/forum/beginners/optocoupler-ctr-calculation/?action=dlattach;attach=994098)

Could someone help me replicate his estimation of 0.15? The best way could be to ask him directly but I don't have an account in RPi forums, and replying to a 7yr old thread is often considered bad etiquette...

he is using I_F = 1 mA to reduce LED aging. When I use fig. 3 with I_F = 1 mA, I find the normalized CTR is about 0.9... even multiplied with the worst case 40% CTR @ 100°C, that's about 0.36, quite far from 0.15... and I don't know how to use fig. 5 in this case. Is it possible to extrapolate from the three curves shown?

thanks again   :-+

Title: Re: optocoupler CTR calculation
Post by: David Hess on May 18, 2020, 02:59:01 pm

That looks like the leakage from all sources including the HC logic input and they massively overestimate it just to be sure.  I would have ignored it when operating at 150 microamps.