Normal diodes allow current to flow in only one direction. If a reverse breakdown voltage is exceeded in the other direction, and current is forced to flow, the diode can quickly become destroyed. Zener diodes function differently in that they allow current to flow freely (with a small voltage drop) in the forward biased direction in the same manner as a non-Zener diode. In the other, reverse-biased direction, however, current can flow freely only after exceeding a designed voltage threshold, known as the Zener voltage. This happens without damage to the component.
Zener diode symbol and polarity diagram
Typical Zener voltages might be at values like 2.7V, 3.9V, or 9.1V, but can range from less than a single volt to hundreds of volts. Zener diodes are constructed with a heavily doped junction between the p and n semiconductor elements, with the specified breakdown voltage set by this doping process.
Uses of Zener diodes: Zener diode as a voltage regulator
A common project would be using a Zener diode as a voltage regulator, with a resistor and a Zener diode arranged in series. The output voltage in this case is simply the Zener voltage, as long as the input is at a higher voltage level than the Zener diode’s breakdown voltage:
Zener diode as a voltage regulator
Zener diode voltage shifter
Swapping out the position of the Zener diode and resistor in this circuit instead produces a voltage shifter, where the voltage output is lowered by an amount equal to the Zener voltage.
Back-to-back Zener diodes as waveform clipper
Zener diode waveforms
Two Zener diodes can also be placed back-to-back in the voltage regulator circuit, instead of the single component, forming a waveform clipper. In this circuit, an oscillating signal is restricted from going over the Zener voltage (3.3V in the oscilloscope screencaps below) on both the positive and negative directions, creating an oscillation that appears to be “clipped off” on the top and bottom.
Unregulated 5V oscillating signal and signal “capped by 3.3V Zener diode waveform clipper
±2V oscillating signal exhibits the same waveform with or without an 3.3V Zener clipping setup
Note that when a ±2V signal is applied, nothing changes when a clipper circuit is applied, because it’s below the Zener diode breakdown voltage. When it's restricted on the ±5V signal, the top isn’t chopped off sharply, but it's still a bit rounded on the top.
While this rounding could be due at least in part to my measurement setup, it’s important to note that using a Zener diode in this setup produces an output that does vary to some extent with the input voltage. Whether this is acceptable depends on the application. Another option for voltage regulation comes in the form of low-dropout regulators, or LDOs, as we elaborated upon in this article.
Waveform images were produced by the Digilent Analog Discovery 2 all-in-one signal generator/scope, as outlined here, and here, for an all-in-one Rasbperry Pi setup. Shop Arrow.com for your Zener diode needs designed to fit your next project.
