Hello
What is the purpose of R5 (Paralleled with diode of Opto) and C8 (connected through K&R of tl431) in the following circuit ?
In some circuits, a resistor is also connected in series with C8, what is its purpose?
R5 is to allow current to flow through the TL431so it can strike, without it the led in the optocoupler is too high in resistance. C8 is a filter capacitor to stop bouncing on the TL431 when it strikes.
Most switchmode psu have this method when using an optocoupler.