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Thanks to emanjackson for the idea.
This circuit uses transistors for reverse polarity protection. Emanjackson used a diode bridge for reverse polarity protection. It is a simple method, but is useless for low voltage, high current applications. The diode bridge drops around 1.5V (for Si diodes), because of which a huge amount of power is wasted at high currents and the load doesn't get the full supply voltage.
My reverse polarity circuit uses transistors. It's just as simple, but transistors cost more than diodes. The advantage: a tiny power loss in the pass transistor and high current capabilities. The principal is simple. The transistor conducts if the applied input voltage turns it on. That's all. A reverse Vgs voltage* would keep the pass transistor from conducting, thus protecting the circuit from reverse polarity. And when it's conducting, the load virtual gets the entire supply voltage, because of the low ON resistance of the pass transistor.
*By reverse Vgs voltage, I mean a negative Vgs applied to an NMOS and a positive Vgs voltage applied to a PMOS.
PS: MOSFETs are better for high current applications due to their low Rds(ON) value. And NMOSs generally have a lower Rds(ON) compared to PMOSs.
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