After I burn one Raspberry Pi3 due the DC step down regulator issue (12V spike) , I search the web for some solution.
That circuit what you see here, can be found on many websites. I just replace the adjustable resistor : between +5V and 2.2k resistor, with 470 one and also added 1k resistor from thyristor`s gate to ground.
On the diagram, the thyristor act as shunt to ground and the fuse will blow preventing the unit behind. On the video you can see that pin is not connected as the diagram. Instead, for test purpose it is connected to the voltmeter to see when the circuit kick in.. When I made it on PCB, I use 3.15A-230V fast fuse, and again with the bench supply set to be without restriction it work as meant to be.
