Dear all
Please help me to know what is different:
Transient-voltage-suppression diode and Sidac ???
Can I use P6KE120ca (TVS) replacementMK1V125 =K1200 (Sidac) in this circuit. please see photo .Thank you so much.

 

the tvs is essentially a high current zener. there is no action like a diac with a zener. the diac or sidac has a diferent voltage for conducting and turning off, that makes it oscilate. the zener just goes to a voltage and conducts.

 

The SIDAC will breakdown and conduct heavily until a much lower voltage / current threshold is reached.

Kind of like 'snap-action'

 

but thats not what it takes to oscilate, there has to be a difference between turn on and turn off, kind of liie a neon bulb, turns on at a higher voltage than it turns off as in "relaxation oscilator".

 

Dear alfacliff and Sensacell
Thank you so much for your time helpful me.
Kind regards
minh

 

but thats not what it takes to oscilate, there has to be a difference between turn on and turn off, kind of liie a neon bulb, turns on at a higher voltage than it turns off as in "relaxation oscilator".

I think it's properties are very similar to a neon lamp, the OP's circuit is a relaxation oscillator, it's a repeating strobe light circuit.