Can somebody show me the modified diagram of this circuit?
How to make this circuit as regulated SMPS?
How to integrate an optocoupler?

 

It is already regulated through the feedback winding, though fairly crudely. You cannot really improve on that with this very old and simplistic circuit. Why would you want to build your own when these can be bought for a couple of USD?

If you must build your own, look at the TNY200 series of SMPS controller chips - a single chip replaces the bulk of those parts with far better performance and supports secondary feedback via an optocoupler. See here for an example

 

Adding feedback regulation to that circuit is like trying to add cruise-control to a Model T Ford, but if you really want to build that circuit, then it's easier to just add an LDO regulator on the output to provide a stable voltage.
That will reduce the efficiency some, but doubt that's a problem with a line powered circuit.
It will also reduce the output ripple voltage.

 

“is like trying to add cruise-control to a Model T Ford”
Good analogy, Cruts!

To the TS, as others have already mentioned, it is better to start with a circuit which is designed to implement solid feedback for tight regulation.
However, if you already have got this supply, an LDO on the output would be probably the way to go. It will also reduce ripple.
But you would require that the actual output voltage to be between 1/2 to 1 volt higher than the desired output. Can you measure what is the actual output voltage at zero and full load?

 

Like the others have explained, it is already a regulated supply. So now the questions: How much better regulation is actually required?? And at what percent of the maximum rated load capacity?? AND How close to that maximum cpcity are you arread running it???
OR, does your application require remote sensing, which this system can't easily provide??
One more detail is about the application's budget: Mostly, you only get what you pay for, end remote sensing that is less temperature dependent costs more.

 

I unerstand.
Is it possible to make this SMPS as variable?
Is it possible to make this circuit as varable from 0 to 12V (1A)?

 

Is it possible to make this circuit as varable from 0 to 12V (1A)?

This type of supply needs a pretty constant load.
It will not work far from the 12V it was designed for.
Working with the power line is dangerous.
You can get a laptop power supply from the used junk store for every little. They are often 21V or something. Get almost any awlwort.
Next build a DC to DC switcher that drops the 21V or 24V to 0 to 24V. Now you are only working with low voltages and current limited power. Much safer.
Here is a little board with a meter. Working ready to go. If you found a 25V 1.5A power supply this would give you 2V to 20V at 2A supply. Replace the 10k pot with a panel mount pot and put it in a small box.

 

Making the supply variable is possible, by adjusting the bias on the transistor a bit, as well as adjusting the feedback circuit.
You will notice that I DID NOT SAY it would be simple or easy.

 

I understand. Thanks to all.