ALLNET Brick'R'knowledge Diode Zener Diode Z 3.9V 1W
If a normal diode is operated in the reverse direction, no current initially flows. When a certain voltage is reached, the diode breaks down and current flows again.
In the case of the Zener diode, this breakdown voltage is specially cultivated and precisely defined and is referred to as the so-called Zener voltage, which is stated together with the diode in the identifier.