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For a circuit such as this, people have suggested on this site, to add a zener diode to prevent the MOSFET from blowing up by protecting against gate overvoltage. I want to know that: if the voltage at the gate is 12V or 0V, then what could be the possible reasons of overvoltage?Example

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What you need is called a "flyback diode". Do NOT use a zener diode for that as these are generally too slow ! What you want is a diode connected in reverse across the motor, like this:

enter image description here

Ignore that here they're using 5V and an NPN transistor instead of 12V and a MOSFET, the same principles still apply.

Note that the diode appears to do nothing when the transistor is conducting or not. That is correct, the diode does nothing in those cases.

But when the motor is ON and you switch OFF the transistor, there will be a lot of magnetic and mechanical energy still in the motor present. That energy will try to escape and generate a high and negative voltage across it's terminals. This voltage can be enough to damage your MOSFET ! But if you have placed the flyback diode it will start to conduct and short this high voltage thereby protecting your MOSFET.

You can use almost any 1 A diode as a flyback diode. The 1N4001 - 1N4007 series diodes are cheap and suitable for this purpose.

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    \$\begingroup\$ This protects against excessive drain voltage, not excessive gate voltage. In the question, the gate is driven from the 12V rail your flyback diode dumps the inductive spikes into. So you also need protection against overvoltage on the 12V rail - including decoupling, but also means to sink excessive voltage (a battery will, a PSU will generally not, a zener or tranzorb may be necessary) \$\endgroup\$
    – user16324
    Commented Mar 16, 2016 at 16:24
  • \$\begingroup\$ On it's own, that schematic does not make much sense anyway. Replacing the Source-Drain with a piece of wire gives very similar functionality. I was assuming something more sensible was used to control the gate. \$\endgroup\$
    – Bimpelrekkie
    Commented Mar 16, 2016 at 20:04

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