Zener Diode|What is a Zener Diode ?|All about Zener diode

Zener Diode : It is a reverse-biased heavily-doped silicon (or germanium) P-N junction diode which is operated in the breakdown region where current is limited by both external resistance and power dissipation of the diode. Silicon is perferred to Ge because of its higher temperature and current capability , when a diode breaks down, both Zener and avalanche effects are prese although usually one or the other predominates depending on the value of reverse voltage. At voltages less than 6 V, Zener effect predominates whereas above 6 V, avalanche effect is predominant. Strictly speaking, the first one should be called Zener diode and the second one as avalanche diode but the general practice is to call both types as Zener diodes.

Zener breakdown occurs due to breaking of covalent bonds by the strong electric field setup in the depletion region by the reverse voltage. It produces an extremely large number of electrons and holes which constitute the reverse saturation current (now called Zener current, I) whose value limited only by the external resistance in the circuit. It is independent of the applied voltage. Avalanche breakdown occurs at higher reverse voltages when thermally-generated electrons acquire sufficient energy to produce more carriers by collision.

Zener Diode VI Characteristic

A typical characteristic is shown by Fig.1 in the negative quadrant. The forward characteristic is simply that of an ordinary forward-biased junction diode. The important points on the reverse characteristic are:

Vz = Zener breakdown voltage

Iz min = minimum current to sustain breakdown

Iz max = maximum Zener current limited by maximum power dissipation.

Zener Diode
Fig. 1 Zener Diode

Since its reverse characteristic is not exactly vertical, the diode possesses some resistance called Zener dynamic impedance. However, we will neglect it assuming that the characteristic is truly vertical. In other words, we will assume an ideal Zener diode for which voltage does not change once it goes into breakdown. It means that Vz remains constant even when Iz increases considerably.

The schematic symbol of a Zener diode and its equivalent circuit are shown in Fig, 2 (a). The complete equivalent circuit is shown in Fig. 2 (b) and the approximate one in Fig. 2 (c) where it looks like a battery of Vz volts.

The schematic symbol of Fig. 2 (a) is similar to that of a normal diode except that the line representing the cathode is bent at both ends. With a little mental effort, the cathode symbol can be imagined to look like the letter Z for Zener.

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Zener Voltages

Zener diodes are available having Zener voltages of 2.4 V to 200 V. This voltage is temperatuce dependent. Their power dissipation is given by the product Vz Iz …….. maximum ratings vary from 150 mW to 50 W.

Zener Diode Biasing

For proper working of a Zener diode in any circuit, it is essential that it must.

  1. be reverse-biased:
  2. have voltage across it greater than Vz.
  3. be in a circuit where current is less than Imax.
Zener Diode identification
Fig. 2

Zener Diode Identification

Physically, a Zener diode looks like any other diode and is recognized by its IN number such as IN 750 (1O W power) or IN 4000 (high power). Fig 54.2(d) shows a picture of a zener diode with V = 4.7V.

uses

Zener diodes find numerous applications in transistor circuitry. Some of their common uses are:

  1. as voltage regulators;
  2. as a fixed reference voltage in a network for biasing and comparison purposes and for calibrating voltmeters;
  3. as peak clippers or voltage limiters;
  4. for metre protection against damage from accidental application of excessive voltage;
  5. for reshaping a waveform.

Voltage Regulation

It is a measure of a circuit’s ability to maintain a constant output variations are to be regulated. In Fig. 3 the Zener diode is reverse-connected acros Vin . When p.d, acros The load resistance R, acrosS which a constant voltage v diode is greater than Vz it conducts and draws relatively large current through the series resistance R.

The load resistance RL acreoss which constant voltage Vout is required, is connected in parallel with the diode. The toral current I passing through R equals the sum of diode current and load current i.e. I= Iz + IL.

It will be seen that under all conditions \displaystyle \large V_{out}=V_{z}

. Hence, \displaystyle \large V_{in}=IR+V_{out}=IR+V_{z}.
Zener Diode Voltage Regulation
Fig. 3 Zener Diode Voltage Regulation

Case 1.

Suppose \displaystyle \large R_{L}

is kept fixed but supply voltage \displaystyle \large V_{in} is increased slightly. It will increase I. This increases in I will be absorbed by the Zener diode without affecting \displaystyle \large I_{L}. The increase in \displaystyle \large V_{in} will be dropped across R thereby keeping \displaystyle \large V_{out} constant Conversely if supply voltage \displaystyle \large V_{in} falls, the diode takes a smaller current and voltage drop across R is reduced, thus againt keeping \displaystyle \large V_{out} constant. Hence, when \displaystyle \large V_{in} changes, I and IR drop change in such a way as to keep \displaystyle \large V_{out}(=V_{z}) constant.

Case 2.

In this case \displaystyle \large V_{in}

is fixed but \displaystyle \large I_{z} is changed. When \displaystyle \large I_{L} increases, diode current \displaystyle \large I_{z} decreases thereby keeping I and hence IR drop constant. In this way, \displaystyle \large V_{out} remains unaffected.

Should \displaystyle \large I_{L} decrease, \displaystyle \large I_{z} would increase in order to keep I and hence IR drop constant. Again, \displaystyle \large V_{out} would remain unchanged because

\displaystyle \large V_{out}=V_{in}-IR=V_{in}-(I_{z}+I_{L})R

Incidentally, it may be noted that \displaystyle \large R=(V_{in}-V_{out})/(I_{z}+I_{L})

.

It may also be noted that when diode current reaches its maximum value, I, becomes zero. In that case

\displaystyle \large R=(V_{in}-V_{out})/I_{Z max}

In Fig. 54.7, only one reference voltage level is available. Fig. 54.8 shows the circuits for estab- lishing two reference levels. Here, two diodes having different Zener voltages have been connected in series.

Zener Diode as Peak Clipper

Use of Zener diodes in wave shaping eircuits is illustrated in Fig. 4. The two similar diodes \displaystyle \large D_{1}

and \displaystyle \large D_{2} have been joined back-to-back across the input sine wave voltage of peak value \displaystyle \large _{+}^{-}\textrm{} 25V Both have \displaystyle \large V_{z} = 20 V. As seen, the output is a semi-square wave with a peak value of \displaystyle \large _{+}^{-}\textrm{} 20V.
Zener Diode as Peak Clipper
Fig. 4 Zener diodes as Peak Clipper
Zener Diode as Peak Clipper
Fig. 5 Zener diodes as Peak Clipper

It is well-known that a Zener diodes acts like a “short (or very low resistance) in the forward direction and an ‘open’ in the reverse direction till it goes into breakdown at \displaystyle \large V_{z}. During positive input half-cycle, \displaystyle \large D_{1} is shorted (being forward-biased) but \displaystyle \large D_{2} acts like an open upto 20 V. Théreafter, it goes into breakdown and holds the output voltage constant till input voltage falls below 20 V in the later part of the half-cycle. At that point \displaystyle \large D_{2} comes out of the breakdown and again acts like an open across which the entire input voltage is dropped.

During the negative input half-cycle, roles of \displaystyle \large D_{1} and \displaystyle \large D_{2} are reversed. As a result, the output wave is clipped on both peaks as shown in Fig, 4.

If we increase the peak value of the input signal voltage and use Zener diodes of lesser \displaystyle \large V_{z} value, we can get an almost square output voltage wave from a sinusoidal input wave as shown in Fig. 5.

Meter Protection

Zener diodes are frequently used in volt-ohm-milliammeters (VOM) for protecting meter movement against burn-out from accidental overloads. If VOM is set to its 2.5 V range and the test leads are accidentally connected to a 25 V circuit, an unprotected meter will be burned out or at least get severely damaged.

Zener diode Meter Protection
Fig. 6 Zener diodes Meter Protection

This hazard can be avoided by connecting a Zener diode in parallel with the meter as shown in Fig. 6 (a). In the event of an accidental overload, most of the current will pass through the diode. Two Zener diodes connected as shown in Fig. 6 (b) can provide overload protection regardless of the applied polarity.

Zener diodes as a Reference Element

In many electronic circuits, it is desirable to maintain a constant voltage between two points and use it as a reference voltage for comparing other voltages against it, The difference between the two voltages is amplified and then used for performing some control function. This type of arrangement is used for power supply voltage regulator circuits, measurement circuits and servomechanism circuits.

Zener Diode as a Reference Element
Fig. 7 Zener Diode as a Reference Element

The constant-voltage characteristic in its breakdown region makes a Zener diode desirable for this application. Fig. 7 shows a circuit in which Zener diode is used as a reference element, The reference voltage equals the Zener breakdown voltage. The value of R is so chosen that the diode operates well within its breakdown region. The difference \displaystyle \large (E_{in}-E_{ref})

gives the control output.

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