Tuesday, October 25, 2011

Working of Zener diode as voltage regulator

Voltage regulator is actually a circuit which receive a D.C. voltage at input fluctuating up and down in a range, depending upon the characteristics of the zener diode used and the other components, and gives an constant voltage (app.) at the output which can be, further, fed into some other circuits e.g. amplifier, oscillator etc where a constant supply of D.C. voltage, of fixed value, is required for well functioning.

Basic principle of working is non linearity of the diode i.e. resistance of the diode does not follow ohm's law according to which the voltage varies linearly with current through resistor and resistance remains constant for all values of the voltage applied across the resistor. While in case of the diode resistance drops as the voltage increases across it. So, the actual characteristic of diode that is used for voltage regulation is its variable resistance (i.e. its voltage dependence). As the diode, being used as voltage regulator, works in reverse bias, so, it is its breakdown voltage which decides the output voltage which can also be as low as 5V which can be easily obtained from a zener diode but not a rectifier diode.  

Working of rectifier: When the fluctuating voltage Vin is applied at the input as shown in the figure it passes through resistor R and diode so the applied voltage gets divided between the resistor R and the diode such that voltage across zener diode is Vz (break down voltage) and that across resistor is (Vin – Vz) i.e. the applied voltage gets divided between the resistor and the diode by the law of potential division depending upon the resistance ratio. Now when positive fluctuation occurs i.e. voltage increases slightly same increase appears across diode, let it be (Vz+v), but as only a very small increase in voltage across diode causes its resistance to drop so in against to the increase in voltage it resistance gets reduced but that of the series resistor remains constant and thus resistance ratio gets chanced. According to the new ratio the new increased voltage is now so divided such that voltage across diode decreases from (Vz+v) to about Vz. The same process occurs each time the input voltage fluctuates.

No comments:

Post a Comment