It is often useful to provide protection against accidental reverse polarity for your circuits. This brief review will explore three simple methods for adding this protection to your projects. For a more in-depth tutorial, see this article
Simply using a diode as shown in the first circuit is often a good approach. The advantages are simplicity and cost. The disadvantages are larger power loss for larger circuit loads and a substantial voltage drop. Normal rectifier diodes typically drop around 0.8 volts. If your circuit draws little power and can handle such a drop, then the blocking diode will work for you.
You can improve this circuit somewhat by using a Schottky diode. It has a lower voltage drop - usually about 0.6 volts, but you can get some lower than that. There is one potential problem with using Schottky's though. They have more reverse current leakage, so they may not offer sufficient protection. I suggest avoiding using Schottky diodes for reverse protection.
A greatly improved protection circuit can be provided by using a pnp transistor as a high-side switch as shown in the second circuit. The saturated voltage is much lower than it is with diodes, so the voltage drop and power loss are much lower.
The limitations of this approach is the fact that there is some power loss from the base current, and that loss is constant regardless of the circuit's current power draw. In circuits where a very low quiescent current is typical, this approach could greatly increase it. For circuits which are usually active and draw modest amounts of power, this simple type of protection is hard to beat.
For the ultimate in low voltage drop and high current capability, replacing the PNP transistor with a P-channel FET as shown in the third circuit can't be beat. Please note that the FET is actually installed in the reverse orientation as it would normally be used. This direction is so that the slight leakage current through the FET's intrinsic body diode will bias the FET on when the polarity is correct and block current when reversed, thus shutting off the FET.
If the supply voltage is less than the FETs maximum gate to source voltage, you only need the FET, without the diode or resistor. Just connect the gate directly to ground. If after checking your FET's spec sheet, you find that Vcc could exceed the maximum Vgs, then you must drop the voltage between the gate and the source. The circuit shown does exactly that. By inserting a zener diode with a voltage less than the maximum Vgs, it limits the voltage to a safe level. Calculate the resistor value to provide enough current to properly bias the zener diode chosen.
Each of these circuits offer a different set of advantages and disadvantages. I have listed them in order of increasing complexity and cost. In choosing what is best for your circuit, examine what your voltage and power needs are. Then use the simplest circuit that will suffice for those needs.