Hi,
I checked pcb circuit and found a zener diode (do not have any value on it) is connected on opto 817B collector and emitter pin (see attached picture). I checked it with multimeter and found it resistance was 25E in both condition (probe changed).
IC Viper53 datasheet says
COMP pin
"Input of the current mode structure, and output of the internal error amplifier. Allows
the setting of thedynamic characteristic of the converter through an external passive
network. The useful voltage range extends from 0.5V to 4.5V. The Power MOSFET is
always off below 0.5V, and the overload protection is triggered if the voltage exceeds
4.35V. This action is delayed by the timing capacitor connected to the TOVL pin."
and
"The VIPer53 device delivers a constant current of 0.6 mA on the COMP pin,
creating a constant voltage of 0.6V in R5 and a rising slope across C7. This voltage shape,
together with the operating range of 0.5V to 4.5V provides a soft start-up of the converter."
My question is, what is the value of zener diode?
Is it damaged?