In most cases the voltage that is induced in the coil can not exceed 6V, and it does not have risk to damage any of the elements. But sometimes, under certain conditions - in a strong magnetic field or a sharp magnetization coil (sudden skidding to a receiver) can form a peak above the maximum voltage that can kill CPU. To prevent the risk of damage from surges, we applied the following schematic.
If the voltage is below 5.1V zener diode (D1) is blocked. The base of the transistor (Q1) is "GND" and also blocked.
At a time when the supply voltage jump over 5.1V. Zener diode opens and unlocks respectively transistor (Q1). On (R7) forms a voltage drop with power sufficient to load the coil so that the supply voltage drops below 5.1V.
Automatic overvoltage protection
原文：http://www.cnblogs.com/shangdawei/p/4823936.html