Zener diodes
A special type of semiconductor device, a Zener diode not only allows current flow in the forward direction, but also enables flow in the reverse direction when the voltage is above a certain value, which is known as the Zener voltage

A special type of semiconductor device, a Zener diode not only allows current flow in the forward direction, but also enables flow in the reverse direction when the voltage is above a certain value, which is known as the Zener voltage