Over current AND reverse polarity protection using a MOSFET

Thread Starter

Nicholas Popich

Joined Sep 9, 2017
Our lecturer posed us this problem. I just can't get it solved.

Design a circuit using a MOSFET which provides reverse polarity protection and ALSO over current protection. Over current protection should activate when current through a load is more than 2A.

Designing reverse polarity protection is easy, adding the over current protection is the problem.

I figured that if I used a 1ohm resistor, and a voltage source ramping up from 0-3V (I know my reversed polarity protection works), I would want the second MOSFET to become an open circuit when the input voltage reaches 2V and higher (creating the 2A current through the load which should not become any higher). Selecting a P-type made sense to me as it is normally closed (how I understand) between source and drain. Where to connect the source and gate is the issue I have not solved in an entire day. I used a PMOS with a threshold voltage of -2V. I also tried using a small current sense resistor with roughly a 50mV drop and wanted to use this to drive the FET, also had no luck with this.


Joined Mar 14, 2008
If you want to detect 50mV of drop, you will need to add an op amp amplifier.
A simpler current limit circuit uses a BJT transistor, which will sense about a 700mV drop.
The details are up to you.


Joined May 20, 2015
I thought that the discussion had reached a dead end. Besides, I did not give a standard solution. It can not be the answer to homework. I cleaned my circuit. I am sorry.


Joined Mar 14, 2008