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i want to choose a zener diode so that i can be used as a voltage regulator.how to choose a suitable zener diode?
let say if i want a voltage regulation of 5V,i need to choose a zener diode with zener voltage Vz=5V?
its the operating voltage not the regulating voltage what you have mentioned you need a step down transformer or a voltage regulator ic which are readily available to attain perfect regulation of your choice
A zener diode can be used, within it's power limits, to provide a relatively constant voltage. The voltage rating of the zener is the voltage you want to deliver.
A resistor is placed in series with the zener and connected to an appropriate DC source. The DC source is higher in voltage. The regulated voltage is taken across the zener. The series resistor is selected so that with no load the dissapation of the zener does not exceed it's rating - because it will conduct if the DC source is higher than it's zener voltage.
As the load is introduced less power will be dissapated by the zener however at some point the voltage drop across the series resistor will be such that the zener will no longer conduct and you will loose regulation.
As with other regulators, the regulated voltage will not be perfectly regulated as the load changes but it will be very well regulated, relatively speaking.
Usually this method is limited to fairly low current applications.
The voltage drop of 5 volt and lower voltage zeners varies some with current. Zeners of 6.2 volts and above regulate much better as a rule. The temperature coefficient of 5 volt zeners is about zero and increases about 1.09 millivolts per degree C per volt zener voltage.
You are right philipsi that for 5V you would use 5V zener.
Actually you will have to get whatever is closest match
to voltage you need. In this case that yould be zener
with 5.1V (I don't think anyone makes actual 5.0V zener).
You also need resistor in series with the zener. This is where
"excess" voltage is burnt. For example if you use 9V battery,
resistor will take the 4V while zener will maintain 5V.
Even as battery voltage drops to 7V, zener will keep the 5V
so resistor will drop only 2V (2+5=7).
Such circuit would not be very usefull without load. But when
you add load, it is not hard to bring zener circuit out of balance.
Zener is usually used in fairly narrow range so if you plan to
power some circuit using zener as regulator and you experience
difficulties, you can always return to voltage regulator chips such
as famous LM340 or 7805. They provide not only better regulation
and in wider operating range, they will waste less power than
zener circuit (might be important if you plan on using batteries).