A sudden surge or a spike is basically a sharp rise in the voltage lasting not more than a few milliseconds but enough to cause damage to our precious equipment almost instantly.
It thus becomes imperative to stop or block these from entering vulnerable electronic gadgets like our personal computers.
Commercial spike busters are though available pretty easily and cheaply too, cannot be trusted and moreover have no reliability test arrangement so it becomes just a "assuming" game, until it's all over.
The circuit of a Simple AC Mains Surge Protector Device below, which shows how to make a simple homemade AC mains surge protector device is based on very simple principle of "speed breaking" the initial jolt through components who are well equipped in the field.
A simple iron resistor and MOV combination are more than enough to provide the protections we are looking for.
Here R1 and R2 are 5 turns of iron wire (0.2mm thick) over a 1 inch diameter air core each followed by an appropriately rated varistor or an MOV connected across them to become a full fledged spike protector system.
Surge entering the input of spike are effectively tackled and the "sting" absorbed in the course by the relevant parts and a safe and clean mains is allowed to go through the connected load.
Using an NTC and MOV together for added surge protection:
The following image shows how two different surge suppressor devices could be tied up with the mains line for achieving a double edged surge protection.
The NTC here enables an initial switch ON surge protection by offering a higher resistance due to its initial lower temperature, but in the course of this action its temperature begins increasing and it begins allowing more current for the appliance until a normal working conditions achieved.
The MOV on the other complements the NTC output and makes sure that in case the NTC is unable to stop the surge onslaught correctly, it switches ON itself shorting the residual high surge content to ground and as a result establishing a safest possible supply for the connected load or the appliance.