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I was looking for a reverse voltage protection for the 100 ohm load in the circuit using an N-channel MOSFET, this is my design. The Zener diode is for gate voltage and of 5.1 volts, assuming the MOSFET wont turn on if the gate voltage is 0.7 volts, is the schematic topologically correct? (actual MOSFET parameters may not match)enter image description here

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If you are committed to using an n-channel FET, place it in series with the negative side of the voltage source. With the drain to the - input and the source to the load, this orients the internal body diode correctly for blocking a reverse voltage connection when the FET is off.

NOTE: The internal body diode actually is a zener diode, not a simple rectifier. If the reverse voltage is high enough, the diode will conduct. The zener conduction voltage always is greater than the FET's maximum drain-source voltage rating, and should be on the datasheet for the FET.

Here is an excerpt from an old project. When the correct voltage is applied. current through the load and the body diode creates a voltage drop across the load, which drives the source away from the + input and the gate, which turns on the FET, which shorts out the body diode for much better efficiency.

enter image description here

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  • \$\begingroup\$ Update: added turn-on explanation. \$\endgroup\$
    – AnalogKid
    Commented Mar 23, 2021 at 16:15
  • \$\begingroup\$ when there is positive gate voltage will the mosfet conduct in either direction by the channel? \$\endgroup\$
    – Sayan
    Commented Mar 23, 2021 at 16:24
  • \$\begingroup\$ Generally, yes. BUT - "positive" means that the gate is positive with respect to the source, which might not be true in both current directions, depending ob the rest of the circuit. \$\endgroup\$
    – AnalogKid
    Commented Mar 23, 2021 at 17:16
  • \$\begingroup\$ what is mean is I will swap the terminals, now the drain has become source, and it is connected to the negative terminal, and hence applying a positive voltage to the gate, in this situation the MOSFET should be fully functional, won't it? \$\endgroup\$
    – Sayan
    Commented Mar 23, 2021 at 18:12
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    \$\begingroup\$ Yes. With an n-channel MOSFET connected as shown, it will be fully enhanced (on). \$\endgroup\$
    – AnalogKid
    Commented Mar 23, 2021 at 18:42
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Here is a better symbol for the IRF530 (and most discrete n-channel MOSFETs):

enter image description here

If you reverse the 12V, then the body diode will conduct and you'll get most of the -12V across the load.

Swap drain and source on the IRF530 and run it "backwards", leave the zener connected G-S.

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