WHAT IS ZENER DIODE?
Zener Diode
Definition:
- is a PN junction device that is designed to operate in the reverse breakdown region.
- is more heavily doped than the ordinary PN junction diode.
- the breakdown voltage of a Zener diode is set by controlling the doping level during manufacture.
- available having breakdown voltages of 1.8 to 200V with power ratings from 1/4 to 50W.
- Since in the breakdown region the voltage across the Zener diode is constant, it may be employed as a voltage reference or constant voltage device.
- The operation of a Zener diode in forwarding biased conditions is the same as a PN junction diode.
What are the characteristics of the Zener diode?
- The circuit arrangement to plot the reverse characteristics of the Zener diode is shown in Fig.
- Here the resistance 'R' is used to limit the current through the Zener diode.
- The volt-ampere characteristics of a Zener diode are shown in Fig.
- The forward bias characteristics of the Zener diode are the same as that of the ordinary PN junction diode.
- In reverse bias, as the reverse voltage (VR) is increased the reverse saturation current (IR) remains extremely small up to the knee of the curve.
- If the reverse voltage increased further, breakdown occurs and the current increases rapidly.
- The reverse voltage at which the breakdown occurs is known as the reverse breakdown voltage.
Voltage Regulator
The voltage regulator is a part of dc power supply. The voltage regulator is a circuit that takes the dc voltage with ripples and provides a constant de voltage as output. A good voltage regulator is one, which provides the same de voltage in spite of the changes in the input dc voltage and load currents.
Based on how the regulating element is connected to the load, voltage regulators are classified as,
1. Series regulator.
2. Shunt regulator
In a series regulator, the regulating element is in series with the load, and the regulation is done by varying the voltage across the series element. In a shunt regulator, the regulating element is in shunt with the load, and the regulation is done by varying the current through the shunt element.
Zener diode as the voltage regulator
Fig shows the circuit diagram of a Zener diode as a voltage regulator. Since the Zener diode is connected in parallel with the load, the circuit is known as a shunt regulator. A resistance R2 is connected in series with the Zener diode to limit the current. For proper operation, the input voltage V; must be greater than the Zener diode breaks down voltage. Note that the Zener diode operates in the reverse breakdown region.
The analysis of the Zener diode as a voltage regulator can be carried out in three ways.
They are
(i) The input voltage V; and the resistor Rį fixed,
(ii) Fixed Vi, variable Ri,
(iii) Fixed Ri, variable Vi.
APPLICATIONS OF ZENER DIODE
Zener diodes are used in commercial and industrial applications. Some important applications are discussed below.
1.As a voltage stabilizer
The circuit diagram of the voltage stabilizer using Zener diode is shown in fig. The Zener diode is connected across the load in the reverse direction. The resistor R is connected in series with the circuit, which absorbs the voltage fluctuations. This circuit provides a constant voltage to the load irrespective of the change in supply voltage and loads current.
2. For meter protection
Zener diodes are generally employed in multimeters to protect the meter movement against damage from accidental overloads. For the protection of meter, the Zener diode is connected in parallel with it, as shown in the fig. In case of accidental overload, most of the current will pass through the Zener diode, thus protecting the meter movement from damage.
3. For waveshaping
Zener diodes are used to convert sine waves into almost square waves. For this, the circuit is arranged as shown in fig. During positive as well as negative half-cycles, when the voltage across the diode is below the Zener breakdown value, they offer a high resistance path and input voltage appears across the output terminals.
However, the input voltage increases beyond the Zener breakdown value, the Zener diodes offer a low resistance path and conduct a large current. Thus an input sine wave is clipped-off at the peaks and almost a square wave appears at the output.
SPECIFICATIONS OF ZENER DIODE
a) Zener breakdown voltage (Vz)
The voltage at which a Zener diode breaks in the reverse bias condition is known as Zener breakdown voltage. The value of breakdown voltage depends upon its doping. More doping gives lesser breakdown voltage.
b) Tolerance
The range of voltages about the breakdown voltage in which a Zener diode conducts in the reverse direction is called tolerance.
c) Power rating (Pzm)
The maximum power which a Zener diode can dissipate (or handle) without damage is known is power rating. Commercially available Zener diodes have power ratings from 0.25W to more than 50W.
d) Maximum current rating (Izm)
The maximum value of current which a Zener diode can handle at its rated voltage without damage is known as its maximum current rating.
e) Zener resistance
The opposition offered to the current flowing through the Zener diode in the operating region is known as Zener resistance.
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