Electronic – transil and how it works

There are 2 ways to understand this.

1. Replace Current Source with a "Voltage source with a resistor in series (+ve where current goes out)". Now you can imagine that there will be output only if both diodes are blocked, otherwise all current will be grounded.
2. A current is originating from source and it needs a path to ground. It always selects a low resistance path. If N terminal of diode is grounded (logic 0 attached), it is forward bias (ideally there is no +0.7V required). Current takes 0 resistance path irrespective of load and hence 0 output. If any voltage is attached at N terminal (logic 1), it will be reverse bias and so open circuit, current will go to output.

Other than that, practically speaking, HIGH logic state in practice never behaves as ground (0), although at times it may behave as 1.

1) no, your source is an ideal voltage source. Every source in the real world has some series resistance. So in practice less current will flow but often still enough to blow the TVS.

2) Not on its own. When the TVS reaches its limits, what options does it have ? It can only blow up and then either become an open (so now your circuit will get the overvoltage and be damaged !!!) or become a short, more current will flow, the PCB tracks will vaporize.

You need to do something to make the TVS survive !

Solutions are: input series resistors to limit the current. At the maximum expected voltage the TVS will conduct and the resistors will take all the voltage so make sure they are rated for that.

Another solution is a fuse which will blow and disconnect the input.

The best solution is a combination of a fuse and a resistor. There are also resistors which can act as a fuse.

If you can guarantee that there will only be short overvoltage spikes at the input then the resistor only solution is enough. Fuses are slow so for fast transients they do nothing.

Key point is that you need to dissipate the energy of the spike in a controlled way. Resistors are ideal for this.