Zener Diode as Voltage Regulator

When the zener diode is forward biased the diode starts conducting when the voltage exceeds above the cut in voltage of the diode. The characteristics of the zener diode in forward biasing is  similar to the normal p-n junction diode.

When the reverse voltage is applied the zener diode has high resistance and negligble current pass through the diode if the applied reverse voltage is less than the breakdown voltage of the zener diode. If the reverse voltage is increased and when the voltage is equal to the reverse breakdown voltage of the diode, the zener diode starts conducting.

The phenomenon of flowing of the current through the zener diode when the applied voltage is equal to or more than the specified breakdown voltage of the zener diode is called the zener breakdown.

If the voltage is further increased above the specified zener breakdown voltage the current through the diode increases but the voltage across the diode remains fairly constant. The zener diode is available in the range of 1 to 200 volts.

Designing of Zener Voltage Regulator

The circuit diagram of the voltage regulator is as shown below.

The followings two conditions must be meet to function the zener diode as a voltage regulator.

1. The Zener diode must be connected in the reverse bias.
2. The supply voltage Vs must be more than the zener breakdown voltage

In the circuit above, the Zener diode D is connected in the reverse bias. The resistance Rs is connected in the series of the diode to limit the current flowing through the diode.The zener diode maintains the constant output voltage Vo regardless of the variation in the input voltage or variation in the load current. Let us understand how the zener diode in the above circuit maintains the constant output volttage with variation in the supply voltage and/or the variation in the load current.

Case1- When the output is not connected to the load

When output is not connected to the load the the supply current I is equal to the Zener current. The resistance is connected in the series of the diode to limit the zener current in the safe operation region. The correct selection of the series resistance is must for operation of Zener diode as a voltage regulator.

If the resistance value is selected too high the current through the Zener diode will be too low to pass the current through the Zener diode and , in this condition the output voltage V0 will be equal to the supply voltage Vs. The output voltage will be equal to the Zener Voltage only if the Zener starts conducting. The Zener current(Iz) should not exceed above the maximum current rating of the diode.

Rs(Min) = (Vs-Vz)/ Iz(max)

Case2- When the input supply varies in the specified range

When input voltage is maximum in the specified voltage range, the increased voltage leads to increase in the Zener Current(Iz). In the reverse VI characteristics of the Zener diode, if the reverse voltage is increased above the Zener breakdown voltage, the current through the Zener diode increase and the voltage across the diode remains constant.

I = I_z + I_L

The increased supply voltage cause increase in the Zener current. Thus the input supply current I gets increased in the same proportion to the increased Zener current and, the load current and voltage across the load ( I_L ^x Rs = Vz)  remains fairly constant.

When the input supply voltage is minimum in the specified voltage range, the Zener current decrease and consequently the input supply current I decrease. The decreased Zener current should not be less than specified minimum Zener Current(Iz-Min.) else the zener conduction will stop. The output voltage and load current remains constant if the current flowing through the diode is equal to or more than the rated minimum Zener current(Iz-min).

Case3- When the load current increase 

With an increase in the load current(I_L),the Zener current(Iz) gets decreased and thus the total current(I) remains constant. The Zener current may reach up to minimum Zener current(Iz-min). If the Zener current decrease below the rated minimum Zener current(Iz-min) the Zener diode behaves as a open circuit.

With decrease in the load current(I_L),the Zener current(Iz) gets increased and thus the total current(I) remains constant. The Zener current may reach up to maximum Zener current(Iz-max). If the Zener current increase above  the rated maximum Zener current(Iz-max) the Zener diode may get permanently damaged.

Selection of Circuit Components:

Selection of Series Resistance:

Selection of Zener Diode:

The following data of the Zener diode should be known.

• Zener minimum current
• Zener maximum current
• breakdown voltage or Zener Voltage
• Power rating of the Zener diode