On the “The diode as rectifier” there has been a talk of the inversely polarized diode behavior; it is seen that, by applying a positive voltage to the cathode than the anode, a current flows only very weak, said “drift current“, until the applied voltage reaches a value such as to trigger the “avalanche effect“. Working in such conditions, a normal diode comes early to destruction by overheating.
However, heavily doping the semiconductor, achieve a similar effect to the effect avalanche, but different for two fundamental aspects:
- the phenomenon can be repeated indefinitely without the diode being destroyed
- the phenomenon is also produced at low voltages, of the order of a few volts
This phenomenon, for which, at practically constant voltage, occurs an abrupt increase in the reverse current, is referred to as “Zener“; as the process depends on the intensity of the applied electric field, it is possible, by setting the thickness of the layer to which the voltage is applied, attract zener diodes which show the avalanche effect at different voltages, in a field which ranges from about 4 volts to several hundred volts.
Thanks to its characteristics, the zener diode is widely used to create mood-stabilizing circuit voltage.
Firstly we observe that, in normal use, while a rectifier diode is traversed by current in the anode-cathode, a zener diode is inserted in circuit with the cathode facing toward the positive, so as to be crossed by a reverse current in the cathode sense -anode.
In figure 1 it is shown the application of both diodes:
- Dr is a rectifier diode, which allows the passage of the direct current Idir, only when the voltage on its anode is positive; They are eliminated all the negative half-waves contained in the AC voltage which comes from the transformer
- Dz is a zener diode, which has the purpose to stabilize the voltage Vcc; when the incoming voltage tends to rise, the current flowing in zener diode increases in proportion: because the same current also passes through the resistor Rz, to the heads of the latter determines a greater voltage drop, which thus compensates for the increase of the input voltage.
In figure 2 it is shown the characteristic operation of the zener diode: when it is subjected to direct voltage, its operation does not differ from that of the rectifier diode; in operation with reverse voltage we see, however, that the avalanche effect occurs at a very low voltage VZ (in this case 5.1V). This voltage is called the “Zener voltage” and is characteristic for the type of diode.
The circuit of Figure 1 is the simplest of the stabilized power supplies; its use is limited to modest loads by absorption, up to a few tens of mA. The Rz of the resistance value can be determined approximately by the formula:( Vi – Vz ) : ( Ic + Iz )
Where Vi is the input voltage, Vz is the voltage of the zener diode (and thus the output voltage), Ic is the maximum current that is required at the output.
Iz is the minimum current that must pass through the zener diode because this can fulfill its stabilizing impact: its value changes from one type of diode, but is about 5 to 10 mA.
A zener diode is therefore characterized in the first place from the voltage at which the avalanche effect occurs (zener voltage); important is then the maximum power that the diode can dissipate without being destroyed: the most common use diodes are suitable in a power range between 0.35 and 1 or 2 W.