How do Zener diodes protect circuits?

A Zener diode is not a specific type of diode, rather, the name describes how a diode is used to direct or channel current away from its usual path. To fully understand what a Zener diode is, let's quickly go back to the basics of diodes.

A diode is a semiconductor consisting of a PN junction. In a forward biased configuration, a positive voltage is applied to the anode (P) and a negative voltage is applied to the cathode (N). When the applied voltage exceeds the necessary threshold (usually 0.7V for silicon diodes), current begins to flow.

When the anode is supplied with a negative voltage and the cathode is supplied with a positive voltage, the diode is said to be in reverse bias. When reverse biased, current is not allowed to flow and the voltage across the depletion region increases. When the reverse bias voltage exceeds the breakdown voltage of the diode, the PN junction is destroyed, allowing current to flow through the broken-down element.

Zener diodes utilize both forward bias and reverse bias behavior. Zener diodes are designed to create an emergency path for current flow in special situations. Under normal conditions, the Zener diode is in a reverse biased state.

Zener diode applications

In electronics, you'll find Zener diodes used extensively to protect components from transient events such as surges and ESD. These events involve sudden rises in voltage and current that can damage pins and semiconductor ICs. Zener diodes are often used to protect Ethernet, RS232/RS485 interfaces, and I/O ports.

The figure above is a typical configuration of Zener diodes used to protect circuits from transient events. Under normal conditions, both D1 and D2 are reverse biased. When a transient voltage appears on the ground connector, D2 activates to direct it away from the circuit side.

Similarly, when the voltage on the connector rises above Vcc, D1 is turned on to divert the excess current away from the circuit. When current is diverted to Vcc, there is a risk that current may flow and damage other components, which explains the placement of the extra Zener diode. The Zener diode will provide a safe path for excess current to flow from Vcc to ground.