Zener diode

 

The Zener diode is like a general-purpose signal diode. When biased in the forward direction it behaves just like a normal signal diode, but when a reverse voltage is applied to it, the voltage remains constant for a wide range of currents.

Avalanche Breakdown: There is a limit for the reverse voltage. Reverse voltage can increase until the diode breakdown voltage reaches. This point is called Avalanche Breakdown region. At this stage maximum current will flow through the zener diode. This breakdown point is referred as “Zener voltage”.

The Zener Diode is used in its “reverse bias“. From the I-V Characteristics curve we can study that the zener diode has a region in its reverse bias characteristics of almost a constant negative voltage regardless of the value of the current flowing through the diode and remains nearly constant even with large changes in current as long as the zener diodes current remains between the breakdown current IZ(min) and the maximum current rating IZ(max).

This ability to control itself can be used to great effect to regulate or stabilise a voltage source against supply or load variations. The fact that the voltage across the diode in the breakdown region is almost constant turns out to be an important application of the zener diode as a voltage regulator

 

Zener diode as voltage regulator

Zener diode are specially designed PN junction(highly dopped) diode. It works only in reverse break down region. Zener diodes are mainly used as regulator. That means to making consant voltage power supplies.Commonly its power ratings are high so that it is not damaged by high reverse current.Zener diodes are a special type of semiconductor diode– devices that allow current to flow in one direction only –that also allow current to flow in the opposite direction, but only when exposed to enough voltage.Diodes usually come in glass or plastic cylindrical packages, marked with a stripe on one side to indicate polarity. In a perfectly ideal diode, current flows in one direction only, from the anode to the cathode   which is marked with the stripe. The schematic symbol is a triangle pointing towards a bar, where the current flows in the same direction, towards the barred or striped  end. Surface mount versions of diodes tend to follow the same labeling convention, where the cathode end is marked with a broad stripe.

Zener Diodes can be used to produce a stabilised voltage output with low ripple under varying load current conditions. By passing a small current through the diode from a voltage source, via a suitable current limiting resistor (RS), the zener diode will conduct sufficient current to maintain a voltage drop ofVout. We remember from the previous tutorials that the DC output voltage from the half or full-wave rectifiers contains ripple superimposed onto the DC voltage and that as the load value changes so to does the average output voltage. By connecting a simple zener stabiliser circuit as shown below across the output of the rectifier, a more stable output voltage can be produced.

  Characteristics

 

current versus voltage curve for a Zener diode. Observe the nearly constant voltage in the breakdown region.The forward bias region of a Zener diode is identical to that of a regular diode. The typical forward voltage at room temperature with a current of around 1 mA is around 0.6 volts. In the reverse bias condition the Zener diode is an open circuit and only a small leakage current is flowing as shown on the exaggerated plot. As the breakdown voltage is approached the current will begin to avalanche. The initial transition from leakage to breakdown is soft but then the current rapidly increases as shown on the plot. The voltage across the Zener diode in the breakdown region is very nearly constant with only a small increase in voltage with increasing current. At some high current level the power dissipation of the diode becomes excessive and the part is destroyed. There is a minimum Zener current, Iz(min), that places the operating point in the desired breakdown. There is a maximum Zener current, Iz(max), at which the power dissipation drives the junction temperature to the maximum allowed. Beyond that current the diode can be damaged.

 

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