question about TL431 and PC817 on SMPS circuit

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Hello

I have a question about using TL431 and optocoupler as feedback. The attached circuit is as an example.

 

I understood like this. 

The way the output signal is fed back to the input to regulate the output is 

 

R8 and R10 are current regulating resistors, 

 

and   2.5 V is fixed by a resistance of 2.4 K,

the reference voltage is formed by PC817,

Depending on the output being large or small from the number ① shown,
Is it correct that the current in the pc817 diode changes?

 

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Hi! AVR

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I'd suggest 1 is the supply voltage. 2 is the voltage sense.

if the current in the pc817 didn't change, then the circuit would not do too much!