Simple reverse-polarity-protection circuit has no voltage drop

Common methods of reverse-voltage protection employ diodes to prevent damage to a circuit. In one approach, a series diode allows current to flow only if the correct polarity is applied (Figure 1). You can also use a diode bridge to rectify the input so that your circuit always receives the correct polarity (Figure 2).

The drawback of these approaches is that they waste power in the voltage drop across the diodes. With an input current of 1A, the circuit in Figure 1 wastes 0.7W, and the circuit in Figure 2 wastes 1.4W. This Design Idea suggests a simple method that has no voltage drop or wasted power (Figure 3).
 
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