Diodes are semiconductor devices commonly used for many purposes. In general, you can imagine diode to be a valve that passes current to one direction and stops it to flow back. The first thing that comes in mind – this might be a good choice for reverse voltage protection, but. In reality, things are a bit different. First of all, diodes aren’t perfect devices. They have a so-called forward voltage drop which is about 0.7V for standard diodes. So if insert diode into the power supply, say 5V the after protection you will get 4.3V where part of voltage is lost in the diode. If you want to go this way – better choose Schottky diode instead which has a smaller forward voltage drop. A forward voltage drop occurs when the diode is forward biased what means current flow from anode to the cathode.