The circuit below can protect from reverse polarity, but I would like to know if it can provide overvoltage protection for any voltage more than 18 volts.
I have chosen a Zener diode with a breakdown voltage of 18 volts so that any voltage greater than 18 volts can flow through the Zener in opposite direction directly to ground.
The P channel MOSFET has a Vds of -30 Volts and a Vgs of +-20 Volts The Zener diode has a breakdown voltage of 18 volts.
A single MOSFET can either protect against reverse polarity or it can protect against overvoltage.
Because of the body diode, the MOSFET can only block voltages over a diode drop in one direction. It can conduct in both directions, but only block in one. From the MOSFET datasheet:
Just to complement to Spehro's answer. You could have simulated the behavior of that circuit straightaway, using the DC-sweep analysis tool of LTspice
As you can see, the output voltage is almost the same as the input voltage, so there is no limiting/protection provided by that circuit.
Of course there is a difference from a real circuit: you omitted to specify any internal resistance for the power source.
This is reasonable if the load draws a current which doesn't cause a relevant voltage drop across that internal resistance, i.e. if the equivalent resistance of the load (Zener+R1 included) is much higher than Ri.
If this is not the case, you will see some limiting action, but not really something that deserves a name of overvoltage protection.
This can also be simulated, adding a .STEP directive to see what happens when the internal resistance Ri changes.
any voltage greater than 18 volts can flow through the zener in opposite direction directly to ground.
Ummm ... no.
The zener does not conduct directly to GND. There is a 100 ohm resistor in series with it to GND. Any voltage greater than Vz will appear across R1, and the output voltage (M1 source) will not be affected. Depending on the output voltage, the input voltage current capability, and the R1 power rating, the resistor might fail.
