Zener Diode, IC 78XX and 79XX as a Voltage Regulator

 Zener Diode as a Voltage Regulator

Fig. Voltage regulator circuit using Zener diode

The Zener diode is always connected in reverse biased in series in a voltage regulator circuit as shown in above fig.

The Zener diode is selected with a Vz equal to the voltage desired across the load.

The Zener diode acts as a ‘bypass valve’ through which more current flows when an increase in input voltage or a decrease in load current occurs, maintaining the voltage at the output nearly constant at Vz.

The resistance Rs drops the excess voltage from the input to give the desired output; hence this circuit cannot compensate for input voltages smaller than desired output.

The selection of R is made by using the following equation

Rs= (Vin-Vout) / Iin

Where, I = Iz + IL

When RL is not connected,  IL =0

i.e. I= Iz = Iz(max)

Therefore, Zener must be capable of passing all the current I through it.

For Zener to be remaining in breakdown, Iz must not be permitted to fall below Iz(min) to work it as a regulator.

The maximum value of load current is given by

IL (max) + IR- IZ(min)

Iz(max) can be Izm, the maximum value of IZ as limited by maximum power dissipation.

Iz(min) could be Iz near the knee of the characteristics, Izk, but always Iz(min) is kept much longer than Izk because Iz becomes very large at Izk.

 IC 78XX and 79XX as regulator :

     

IC 78XX is the positive voltage IC series.

IC 79XX is the negative voltage IC series.

e.g.- If 78XX is IC 7812, the output variation of D.C. voltage will be from  0 to 12 volt maximum, with unregulated D.C. input voltage to regulator IC equal to 12 or 15 volt max.



The above circuit is mostly used in variable power supplies, In this circuit by adjusting R2 we get desired range of output voltage. This control is available on the front panel of variable power supply.

Moslty the adjustment knob is given on front panel or control panel of variable power supply as “COURSE” or “FINE” adjustment knob.

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