In normal diodes only a small current will flow under reverse bias conditions until the voltage becomes so large that there is a catastrophic breakdown of the diode and it is of no further use. With the zener diode the manufacture is such that only a small current flows up to a certain well-defined point, when there is a sudden increase in current flow and the voltage across the diode becomes virtually constant. Typical characteristics are shown in Fig 3.5. Please note the change in scale, between positive and negative, of the vertical axes
Fig 3.5 Characteristic of the zener diode
There are technically two diodes, the zener and avalanche, depending on voltage, but by popular usage 'zener' is used to describe both types. These diodes are manufactured in various fixed values from less than 3V up to about 150V. They can be put in series for greater values. Zener diodes are useful in DC power supplies which need to provide a constant output voltage irrespective of loading and changes in input voltages.
