Can I use this Optocoupler circuit instead of LM2931 regulator?

[Willen]

I can't find low droup out regulator on TO-92 package. Only found high droup out regulators in TO-220 case. Can i use a general optocoupler and a general transistor (Or TL431) as a BC....used here? What might be the efficiency? I need 5 V from 6 or 9 or 12V source.

 

[audioguru]

Almost EVERY semiconductor manufacturer makes a 5V low-dropout voltage regulator in a TO-92 case. I used a National Semi LM2931A-5.0 in my FM transmitter only because I had a bag full of them. National Semi also makes a few more.

A 3.3V zener diode is a poor voltage regulator and an LED is also a poor voltage regulator. Their voltage drops when they get warm and when their current drops.
A 6.8V zener diode is a good voltage regulator if the input voltage does not drop too low (it is not low-dropout).

The circuit you found uses the transistor as an on-off switch with nothing limiting the current in the zener diode and LED.

 

[Willen]

Almost EVERY semiconductor manufacturer makes a 5V low-dropout voltage regulator in a TO-92 case. I used a National Semi LM2931A-5.0 in my FM transmitter only because I had a bag full of them.

But may be my life will be end with searching equivalent (replacement) of each and each components . A component which is not used on general TVs and radios, that is impossible to get here!
I said before I have been searching a heatsink of TO-39 from 2 yrs, even can't find! Planning to make myself by melting aluminium in fixed shape! So being mechanical engineer leaving electronics engineer

 

[audioguru]

But may be my life will be end with searching equivalent (replacement) of each and each components . A component which is not used on general TVs and radios, that is impossible to get here!

Here, everything is easily available.

I said before I have been searching a heatsink of TO-39 from 2 yrs, even can't find! Planning to make myself by melting aluminium in fixed shape! So being mechanical engineer leaving electronics engineer

A TO-39 case is obsolete but here there are heatsinks available for a 2N2219 transistor. Use a transistor that is newer and is still made.

 

[Chinoi]

your opto isolator was slow control device .