What does each of the basic crowbar circuit's components do?
Fuse F1


The fuse can be in various locations, as long as it is in the path of the current that the crowbar shorts. It could even be in the ground lead, though that is generally not done because it would disconnect the user ground from the source ground.

It could be argued that putting it before the regulator is better, because then it also protects against overheating when the regulator shorts internally.

I would select a fast fuse, but note that it must fuse at or below the guaranteed short-sircuit current that the regulator can supply (for a short time).


Zener Diode D2:


The purpose of the zener is to conduct (only) when the voltage is above the trigger level. A TVS has a totally different purpose. The crowbar circuit has no effect on the regulators output voltage, that happens wehn you put a zener between the COM terminal of the regulator and the ground. A zender has a somewhat 'soft' characteristic, is not rated for a lot of current and it does not short, hence just a zener is not a good crowbar.


Resistors R1 &amp; R2:


The purpsoe of R1 is to leak the zener's leakage current to ground without triggering the thyristor. I guess R2 is to limit the SCR's gate current to a safe value.

R1 should be low enough so the zener's leakage at the nowmal voltage does not cause the SCR to trigger.

R2 should be high enough that the (short term) maximum trigger current is not exceeded with the highest expected (over) voltage.


Capacitor C4:


My guess is that C4 is to 'short' transients that could otherwise trigger the SCR. I would not choose an elcotrolytical (polarized) type, otherwise I think it is not critical.


SCR


The SCR will not be very critical, if you choose one that can handle the expected voltage and the fuses fusing current it will be OK. Check that the drop voltage at that current must be &lt;&lt; 5V, an SCR rated for mains voltage might not be a good choice.

I guess you could use a thyristor, but most thyristors are meant for mains use so they would not be optimal.


Limitations, disadvantages and draw backs:


This is a rather simple PSU: linear (will keep you warm in the winter). The crowbars trigger voltage is not very well defined (a voltage reference + opamp could do a much better job). 

You could also ask why you need the crowbar at all, the only failure mode it seem to protect against is an internal short of the 7805 from input to output. Is that a likely failure mode, worth the extra components (which will have their own small contribution to the failure rate!)?