When normal grinding diodes “decompose”, they are usually destructive. However, it is possible to build a special type of diode that can handle the failures without succeeding completely. This type of diode is called the zener diode.
When forward zener diodes behave the same as standard diode diodes: they have a forward voltage drop that follows the “diode equation” and is about 0.7 volts. In inverting / overturning mode, they do not perform until the applied voltage reaches or exceeds the so-called zener voltage, at which point the diode is capable of conducting substantial current, and in this way will try to limit the low voltage over it, that voltage point zener. As long as the power dissipated by this reverse current does not exceed the thermal limits of the diode, the diode will not be affected.
Zener diodes are manufactured with zener voltages ranging from a few volts to hundreds of volts. This zener voltage changes slightly with temperature and, as well as common values of resistance to carbon composition, can be anywhere from 5% to 10% of the error in the manufacturer’s specifications. However, this stability and accuracy is generally good enough for the zener diode to be used as a voltage regulating device in the common feed circuit.
Please note the orientation of zener diode in the above circuit: the diode is reverse-biased, and intentionally so. If we were dealing the “normal” diode so that we were biased in front, it would only drop by 0.7 volts, just like a regular diode. If we want to exploit the inverse breakdown properties of this diode, we need to operate it in the reverse bias mode. As long as the supply voltage remains above the zener voltage (12.6 volts in this example), the low voltage across the zener diode will remain at about 12.6 volts.
Like any semiconductor device, the zener diode is temperature sensitive. Excessive temperature will destroy a zener diode and, because both voltage drops and leads the current, it produces its own heat according to the Joule Law (P = IE). Therefore, you must be careful to design the regulator circuit so as not to exceed the power dissipation of the diode power. Quite interestingly, when zener diodes fail due to excessive power dissipation, they usually do not work rather shortly than open. An unsuccessful diode in this mode is easily detected: it lowers almost zero voltage when it is biased or as a piece of wire.
