Stud mounting Zener diodes
(7)Zener diodes are a family of semiconductor elements and a subgroup of diodes that are asymmetrical conductors, i.e. elements that exhibit different properties depending on their direction of polarity.
Zener diodes, when forward-biased, exhibit properties identical to those of regular semiconductor diodes. This means a ca. 0.7V drop of voltage applied to their outputs. What is different is their behaviour when reverse-biased. In the case of regular semiconductor diodes, exceeding the manufacturer's specified reverse voltage rating causes the diode to break down, which, if the current that flows across the diode at this time is not reduced, will undoubtedly lead to diode failure. In addition, even with limited voltage, the component may still be destroyed due to overheating. However, the value of the reverse voltage for regular semiconductor diodes is not given with great accuracy, so the discrepancy between the value found in the documentation and the actual value may be a few percent.
In the case of a Zener diode, the value of the voltage causing a reverse-biased diode to break down is given with much greater accuracy. It is called the Zener voltage or the avalanche breakdown voltage. If this parameter is exceeded, for a very wide range of current flowing across the diode, the voltage at its outputs remains almost constant, which can be seen very well in the third quadrant of any example characteristic of a Zener diode, in the form of current-voltage dependence. As with regular diodes, if the current at the time of breakdown of a Zener diode is properly limited, and the power dissipated by the diode is not too high, it will not cause damage to the diode. However, Zener diodes are specifically manufactured to have a controlled breakdown, so it is much more difficult to damage them in this way. If a Zener diode is used in an electronic circuit, it is almost always protected against excessive current flow because it is expected to flow in reverse bias quite often or even continuously, unlike in the case of regular semiconductor diodes.
Zener diode – properties and applications
Zener diodes with low reverse voltage conduct current in the reverse direction due to the so-called Zener effect. These diodes feature heavily doped p-n junctions. For elements with high Zener voltage, the so-called avalanche breakdown occurs. It is worth noting that both effects occur for any diode of this type, regardless of the reverse voltage rating, although one of them may be largely dominant.
To better illustrate the behaviour of Zener diodes, we should consider the case of a simple circuit in which a diode of this type will be placed between the outputs of an adjustable power supply, with a properly selected resistor connected in series to one of diode’s outputs, limiting the current flowing across it to an appropriate value, for which voltage stabilisation will be most effective. The resistance value of this resistor is often given in the documentation of the electronic component. When the diode is reverse-biased, as the supply voltage is increased, the voltage difference across the diode outputs will be identical to that indicated by the power supply display. However, if this value is exceeded above the breakdown voltage of the diode, the voltage difference at its outputs will still be equal to the Zener voltage, even if you successively increase the voltage generated by the power supply. We can say that the voltage is stabilised by the Zener diode.
Zener diodes should not, however, be used to stabilise voltage with the intention of powering some electronic circuit. For this purpose, there are dedicated voltage stabilisers such as the popular LM317 stabiliser. The main difference between these two components is the current they can conduct. While in the case of the LM317 stabiliser it is about 1.5A, in the case of Zener diodes these values will be from about 5mA, although in the case of stud mount Zener diodes, which are much larger than their SMD or THT counterparts, it is possible to conduct currents at the level of about 0.5 to 0.6A. However, these values may vary significantly depending on the specific model, as you can find e.g. THT Zener diodes capable of handling even 1.5A.
Due to the above described reasons, Zener diodes are used relatively rarely as a source of stabilised voltage and only for systems drawing small current, whereas their main application is as a source of reference voltage, e.g. in stabilised power supplies, although they are also used as a protection and overvoltage element, e.g. against electrostatic discharge (ESD) or other transient voltage spikes.
Stud mount Zener diodes
Stud mount Zener diodes are a specific subtype of Zener diodes that have one of their connections in the form of a threaded stud, hence they are referred to as “stud mount”. The other terminal is a plate with a hole to be soldered to. Their structure is also specified, indicating which terminal is on the stud (anode [+] or cathode [-]).The two packages in which stud mount Zener diodes are housed include DO4 and DO5. Compared to SMD or THT diodes, they also offer much higher powers that they can dissipate, among which the two most popular values are 10W and 50W, while the Zener voltage for these elements can range from about 3.9V to as much as 200V. It is worth knowing that the variety of these values is significant. Some examples include 4.3V, 4.7V, 5.1V, 5.6V, 6V, 6.2V, 6.8V [...]. As you can see, the scale of reverse voltage ratings for Zener diodes is quite dense, so finding the right component for your electronic circuit should not cause any problems.
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