I'm trying to figure out why different opto's for a midi isolation bridge(4-20mA current loop with isolation) doesn't work for most of the op amps I've tried.
The one that works, but not well is the TLP630. The rise time is aweful but enough so that I can sharping using two mosfet to get it to work.
The others,
H11G3, 4N33 - Darlingtons CNY17, MOC8205
The darlingtons are total crap. I may be using them wrong though but the rise time is about 1/20th of the others. I'm not sure what to do with the base but I have tried various things as seen in other circuits including
Usually, such as connecting the base to V, complete ruins the signal and draws a significant amount of current.
The other opto's don't work well either.
I'm not sure if this is normal behavior and they are just extremely slow or if it the CTR and all the ones I have are too low(although this doesn't explain why the TLP630 works).
Any Ideas what I'm doing wrong?