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I would like to add a reverse polarity protection circuit to protect my circuit from reverse polarity.

I tried to use following circuit at this link

enter image description here

With NMOS FET, I have a little problem. If the voltage is too low the FET isn't turning on and the current flow through the internal diode. But I think the diode isn't in a good direction. If the voltage is high enough e.g. >3V, the MOSFET is turning on and current flow through the MOSFET.

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    \$\begingroup\$ You must use a mosfet with a turn on voltage Vgs(th) much lower than the supply voltage. In TI's example they used a Si2312 which has a very low turn on voltage. \$\endgroup\$ – Steve G Oct 6 '16 at 9:50
  • \$\begingroup\$ Both circuits work, but as always, check the parameters. I would suggest looking at Rds max for a voltage higher than your lowest voltage. Rather than Vgs(th). If your lowest operating point is 2.5V, then look for a MOSFET with Rds specified at 2.5 or 1.8V. The other thing to watch out for is Vgs max. Some small low Rds FET's have a max gate voltage of only 8V. \$\endgroup\$ – mkeith Oct 7 '16 at 5:13
  • \$\begingroup\$ And if the gate is directly connected to a battery terminal, you will want ESD protection for the gate. \$\endgroup\$ – mkeith Oct 7 '16 at 5:14

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