Zener diode and Voltage regulator circuits

Zener Diode

The zener diode is a type of diode which operates under reverse bias that breakdowns when the applied voltage reaches a particular reverse bias voltage or knee voltage. It allows the current to flow as like a normal PN junction diode when it is forward biased and block the reverse flow of current during the reverse bias up to the breakdown voltage. Beyond the breakdown voltage or zener voltage, the zener diode will permit the reverse flow of current and the voltage across the zener (that is the zener voltage) remains constant for a wide range of reverse current.

How zener diode is different from a normal diode?

Normal PN junction diodes are used to allow the flow of current in one direction (forward biased – current flows from anode to cathode) and to block the reverse flow of current (reverse biased – +ve of the supply connected to the cathode and –ve to the Anode); diodes that are used in rectifiers, demodulators and similar circuits.

A normal diode can prevent the reverse flow up to its PIV voltage (Peak Inverse Voltage). So while designing circuit, the diodes are always selected with a high PIV value than the maximum possible voltage in the circuit that might be applied to it in the reverse bias. Because once a voltage applied above PIV and an avalanche breakdown occurs it will permanently destroy the diode junction.

Whereas a zener diode is a specially designed diode with a specified reverse break down voltage with a heavily doped p-n junction. That is, it has a narrow depletion region results in a low breakdown voltage typically in a few volts range.

zener diode vi characteristics curve, Zener diode VI graph

Zener effect is a type of electrical breakdown which occurs when the reverse voltage increases the electric field, enables the tunneling of electrons from the valence to the conduction band. It increases the number of free charge carriers which leading to a sudden increase in reverse current. A zener breakdown occurs before avalanche breakdown which is a temporary breakdown and the junction recovers when the voltage is withdrawn.

Normal PN junction Zener Diode
Designed for unidirectional operation Operates Bidirectionally
Moderately doped High and sharply doped
High and not definite break down voltage. Low and sharp Break down voltage

Zener reference voltage

One of the main application of a zener diode in circuits is it can be used to obtain constant reference voltage values. As the voltage across the zener diode is always a constant for any applied voltage values above the zener voltage, it can obtain a fixed voltage value in a circuit which has a variable or fluctuating power supply. By using zener of different values a wide range of constant voltages can be obtained without the need of an external battery supply or input supply. Because a battery supply or external reference is practically inconvenient to be obtained.

zener diode voltage reference

VR – Voltage across the Resistor, VS – Input voltage, VZ – Zener voltage, IZ – Breakdown current or Current through Zener.

Calculation of resistance to obtain a reference voltage of 3V from a supply voltage of VS = 6V using a zener diode of 3V.

The voltage difference between the VS and VZ is dropped across the resistor, that is VR. The VR = I * R, where I is IZ + IOUT; sum of zener current and current flow to reference.

R = VS – VR / IZ + IOUT  | IOUT – Current driven to the reference input.

Normally, the reference input is given to circuits or components such as IC which has a high input impedance. So in practical case, the current draw from a reference point would be approximately equal to zero.

Even though, let assume an output current from reference point as 10mA and the zener current 10mA. If the input impedance of the circuit connected to the reference is low, then appropriate value of current should be taken for the calculation.

R = 6 – 3 / .01 + .01 = 3/.02
= 150Ω

Zener voltage regulator circuit

It can be understood how a zener diode acts as a voltage regulator by analyzing the below circuit. As the zener diode is connected parallel to the load the voltage across the load will be always equal to the zener voltage.

zener voltage regulator circuit, zener voltage regulator theory

The voltage difference between the input voltage VS and VZ will be dropped across the resistance R. The circuit arrangement and calculation is same as the above reference circuit, but here discuss the circuit to provides a regulated supply to a load with a particular power rating.

VR = VS – VZ

(IZ + IL) * R = VS – VZ.

A minimum zener current of around 10mA needs to be maintained to keep its regulated voltage. The resistor value should be calculated based on the known input voltage and output current value, so as to provide the minimum Zener current under full load condition. Because when the full load has applied the voltage across the Zener should not be dropped below Vz. Vs- Vr should be always equal to the rated value of the zener. As the drop across the R increases with load current, vs- VR should not come lower than Vz. In such condition the output voltage decreases than the required regulated value.

Also while selecting the R the power dissipation capacity for the Zener diode should be considered. The Iz should not exceeds the value greater than the maximum permitted current through the zener, which is based on its power rating.

Zener voltage regulator calculation

  • Design a zener voltage regulator circuit to drive a load of 6V, 100mW from an unregulated Input supply of Vmin = 8V, Vmax = 12V using a 6V zener diode?

VS – VZ = VR; 8V – 6V = 2V.

Let take a minimum current Iz as 10mA at full Load.

IL = 0.10/6 = 16mA

Resistor calculation for the above zener voltage regulator circuit,
(IZ + IL) * R = VS – VZ; (10mA + 16mA) * R = 2V
R = 2/93mA = 21Ω

The voltage across the zener remain a constant value VZ for any increase in the input voltages above VZ. The zener and Load resistance is parallel. Hence the VZ = VL; voltage across load will always remain a constant 6V.

Zener Transistor series voltage regulator

A transistor Series Voltage Regulator is basically an emitter follower configuration with a fixed base voltage. The load is connected in series to the transistor, i.e. the load current is connected to the emitter of the transistor. Hence the Load current IL is equal to the base current IE (IE = IL).

zener controlled transistor series regulator, zener voltage regulator experiment

The base voltage is equal to zener voltage VZ, which always remains as a constant for any applied input voltage above value VZ.

IB + IC = IE = IL | IB – Base current.

IL = VOUT / R = (VZ – VBE)/R = P / VOUT. P – Load power rating.

IL = IB β + IB; IB = IL / (1+ β) | β – Amplification factor or hfe.

VS-VZ = (IB + IZ) R

Resistance, R = VS – VZ / (IB + IZ)

VBE – Base-emitter junction voltage around 0.6 to 0.7V.

VOUT= VZ – VBE

Hence a regulated constant output voltage can be obtained at the output from an unregulated supply.

  • Design for a Zener Transistor series voltage regulator circuit to drive a load of 6V, 1w, from a supply of 10V with a ±3V ripple voltage

From the above formula,
IL = 1 / 6 = 167mA

By assuming the value of β as 220 the calculated IB value is 759µA; calculate IB based on the hfe or β value of the transistor using in the circuit.

Assume a minimum Iof 10mA. Then the R = 7/ (10mA + 759µA) = 650Ω

Let’s take a zener diode of 6.8V, then the output voltage, VOUT = VZ – VBE
6.8V – 0.7 = 6.1V

Zener voltage regulator circuit using op-amp

The op-amp voltage regulator circuit consists of a non-inverting amplifier with a fixed reference voltage at the non-inverting input. The negative feedback taken from the output is given to the inverting pin of the op-amp through a voltage divider.

zener voltage regulator circuit using op amp

As similar to a non-inverting op-amp circuit the output generates a voltage value so as to obtain an equal voltage of non-inverting pin at the inverting input. Hence the circuit output voltage adjusts itself so as to obtain a voltage across the R3 equal to value Vz. Unlike from other above circuits, the op-amp voltage regulator has a constant zener current which doesn’t vary with the input voltage changes or load changes.

Here circuit the zener just provides the reference voltage to the op-amp. As the op-amp IC has a high input impedance, the current through R1 will be almost equal to the zener current Iz.

VOUT = VZ * (R2+R3) / R3

= VZ * (1+ R2/R3)

By varying the resistance of R2 and R3 the output voltage of the circuit can be adjusted to the required voltage values. The R3 can also be replaced with a potentiometer to make a variable power supply. In this circuit output voltage values higher than the zener value VZ can be obtained by using a zener voltage as a reference value.

6V op-amp voltage regulator

  • Design for an op-amp voltage regulator circuit to drive a load of 6V,1.2W from an Input supply of 12V with a ±2V ripple voltage, using a 3V zener diode.

Refer the voltage reference circuit to set the resistance R1. Let’s take the value of R2 as 1kΩ.
Using the formula given above for VOUT, the R3 can be calculated as 1kΩ for a 6V output with a 3V zener diode.

Components Required

IC – 741

Resistor – R1 – 150, R2,R3 – 1k

Diode – D – 3V

Transistor – Q – 2N222

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