Zener Diodes

The Zener diodes are primarily used as voltage regulator devices. These devices are specific type diodes designed to be operated in the reverse breakdown region. Every zener diode has been manufactures for a specific reverse-breakdown voltage that`s called the zener voltage (VZ).

For understanding the operational aspects of zener diodes, refer to Figure

below. See the symbol being used to represent a zener diode. In this illustration, a 5.6-volt zener has been chosen. Also consider that we can apply a variable DC voltage to the +ve and -ve terminals of the circuit, that should be ranging from zero to 10 volts. Starting at 0 volt, as the voltage level is increased up to 5.6 volts, the zener diode behaves like any other reverse-biased diode. It completely blocks any significant current flow. And, that’s why it represents an almost infinite resistance; the whole applied voltage will be dropped across it. This all discussion holds true up to the point when the voltage we supplied exceeds the rated zener voltage (ZV) of the diode. When this avalanche voltage occurs, the zener diode will abruptly start to conduct current freely. Avalanche point is that pony at which this abrupt operational change happens.

The minimum amount of current flow through the zener diode which is required to keep it in an avalanche mode of operation is called the holding current. The voltage level across the zener diode does not decrease when the avalanche point is reached, but it does not increase by a very significant amount as the supplied circuit voltage is increased by a significant value. Hence, the voltage level across the zener diode is regulated, or held constant.

In order to justify the previous statements, consider the circuit operation in Figure below. at specific input voltage levels. If the input is 5 volts (observing the polarity as illustrated), any significant current flow is blocked by means of zener diode, because its avalanche point will occur when the voltage level across it reaches a level of at least 5.6 volts. If we supply 6 volts, about 5.6 volts will be dropped across the zener, and approximately 0.4 volt will be dropped across R1. Increasing the applied input voltage to 7 volts causes the voltage across R1 to increase to about 1.4 volts when voltage goes to 7 volts, but the voltage across the zener diode will stay at about level of 5.6 volts. If the supplied input voltage is increased up to 10 volts, nearly about 4.4 volts will be dropped by R1, but still the zener diode will continue to maintain its zener voltage of about 5.6 volts. Or say in different way, when the applied voltage to circuit is increased “above” the rated voltage of the zener diode, the voltage across the zener diode will remain reasonably constant and the voltage in excess will be dropped by its associated series resistor, R1.

If a we place a load of some kind parallel with the zener diode of Fig. shown above, the zener diode will keep the voltage applied to the load at a relatively constant level, as long as the applied circuit voltage  not droped below the rated zener voltage – ZV - of the zener diode.

There are two very important parameters in respect of zener diodes are the firstly, zener voltage and secondly, the rated power dissipation. Zener diodes are easily available in voltages of about 3 volts to over 50

volts. If a higher zener voltage is needed, two or more zener diodes can

simply be placed in series. If you need higher zerner volotage simply pace 2 or more zener diodes in series. For example, 51-zener and 39-volt zener can be used to provide 90-volt zener required for an application.  Unusual zener voltages can be obtained by using the same manner.

There is another way of obtaining an odd i.e. non-standardized zener voltage level; this way is that it incorporates the 0.7-volt forward threshold voltage drop of a common silicon diode. When working in this way, the general-purpose silicon diode is placed in series with the zener diode, but it is oriented in the forward-biased direction, and the zener diode will reverse-biased in this case.

The range of standardized power dissipation ratings for zener diodes is 1/2, 1, 5, 10, and 50 watts. Zeners rated at 10 and 50 watts are required to be manufactured in stud-mount casings, and required to be mounted into reasonably appropriate sized heat sinks for gaining maximum power dissipation.