Zener diodes are PN junction devices that are designed to operate in the reverse-breakdown region. The breakdown voltage (Vz) of Zener diodes is set by carefully controlling the doping level during manufacture. This breakdown phenomenon is referred to as the Zener voltage or the Zener effect.
The Zener effect functions as follows. When reverse voltage is applied to the PN junction of a diode, there is a rapid avalanche breakdown. This causes the current to reverse direction. The change in current direction accelerates valence electrons within the applied electric field, enough to free them. These free valence electrons collide with and free other electrons, building the avalanche. While this process is taking place, the current changes rapidly. Changes in current are based on changes in voltage, and even a tiny voltage change can have a massive effect on the current. In practical application, the avalanche breakdown depends upon the applied electric field. Changing the thickness of the layer to which the voltage is applied forms Zener diodes. Typical Zener diodes break down at voltages from about 4 volts to several hundred volts.
Functionally, Zener diodes are used as regulators, limiters and to control output switching. When used as regulators, the constant reverse voltage of Zener diodes allows for the regulation of output voltage against both variations in the input voltage from an unregulated power supply and from variations in the load resistance. The current traveling through Zener diodes changes to keep the voltage within the threshold of Zener action and the maximum power it can dissipate. Zener regulators tend to most efficiently function when constant voltage is applied, especially when used in conjunction with regulated power supplies, and for limiter applications.
Zener limiters are constructed with two opposing zener diodes. Each individual diode can limit one side of a sinusoidal waveform to Zener voltage while keeping the other side near zero. When the two opposing Zener diodes are paired, the waveform is limited to Zener voltage on both polarities.
Zener diodes are also used to control output switching. In this application, Zener diodes switch the output between voltages. This is determined by the changes in input voltage through the diode. The output circuit amounts to a Zener regulator that switches from one Zener voltage to the other on transition through the Zener diode.
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