A Zener diode is a special diode that in contrast to a regular diode allows current in both directions.

A Zener diode can be used as a voltage reference or for over voltage protection. Zener diodes have a very useful “side-effect” of maintaining a rather constant voltage in the reverse direction. Like a normal silicon diode, also the Zener diode has a forward voltage drop of about 0.7 volts. The Zener (break-down) voltage and current can be found in the datasheet. A reverse voltage, or zener voltage of 3.3 V or 4.7 Volt for example can be used to protect the input of your micro-controller against voltage spikes.

The Zener diode is used in its “reverse bias” or reverse breakdown mode, i.e. the diodes anode connects to the negative supply! Zener diodes are specially constructed such that the Zener diode has a region in its reverse bias characteristics of almost a constant negative voltage regardless of the value of the current flowing through the diode. This Zener voltage remains nearly constant even with large changes in current as long as the Zener diodes current remains between the breakdown current and the maximum current rating.

A Zener diode has an optimal current in which the Zener-effect works. Too low a current results in drifting Zener voltages. Too high currents translate in heat production, noise, or failure. The ideal Zener current typically is a few mA. Adapt the series resistance accordingly.

Most important specifications

• V_Z: Zener voltage; the (reverse) voltage at which the Zener effect occurs (V)
• Optimal Zener (reverse) current: the current for optimal Zener effect (mA)
• I_ZM: Maximal Zener (reverse) current (A)