On the power connection page, it is suggested that we use zener diode for protection when using back up power supply. The connection suggested is to connect zener across the +5v and GND of the servo rails directly.
However when I read about zener diode, then the circuit to control voltage is suggested as follows.
Notice the resistor in series with the zener. I understand that this resistor will consume the voltage which is excess to the required voltage. However in the connection suggested for PXHawk, there is no resistor. In this case how is it able to regulate the voltage? My knowledge of electronics is limited, so I would appreciate an explanation of this.