3.4V Zener Diode

Zener diodes differ from regular diodes in that they have a varying "reverse-breakdown" voltage. That means you apply voltage across them, and when a preset voltage is reached, they start conducting. A common analogy is a "pressure relief valve". They have lots of uses, especially in safety circuits where you want to make sure a voltage signal doesn't exceed a limit that might damage the circuit.

This 1N4728A zener diode is set at 3.4V, making it ideal for protecting the inputs of a 3.3V circuit. specifically,  it has a minimum voltage of 3.315 V and a maximum voltage of 3.465 V, a leakage current of 1 V and a test current of 76 mA.

Specifications:

Voltage - Zener (Nom) (Vz): 3.3V
Power - Max: 1W
Impedance(Max) (Zzt): 10 Ohm
Voltage - Forward (Vf) (Max) @ If: -
Current - Reverse Leakage @ Vr: 100µA @ 1V
Tolerance: ±5%
Mounting Type: Through Hole, Axial
Package / Case: DO-41, Axial
Operating Temperature: -65°C ~ 200°C
Other Names: 1N4728ATRFS