Zener diode Zener diode schematic symbol Current-voltage characteristic of a Zener diode with a breakdown voltage of 17 volt. Notice Notic e the change of voltage scale between the forward biased (positive) direction and the reverse biased (negative) direction. A Zener diode is a type of diode of diode that permits current in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage". The device was named after Clarence after Clarence Zener, who discovered this electrical property. A conventional solid-state diode will not allow significant current if it is reverse-biased below reverse-biased below its reverse breakdown voltage. When the reverse bias breakdown voltage is exceeded, a conventional diode is subject to high current due to avalanche breakdown. Unless this current is limited by external circuitry, the diode will be permanently damaged. In case of large forward bias (current in the direction of the arrow), the diode exhibits a voltage drop due to its junction built-in voltage and internal resistance. The amount of the voltage drop depends on the semiconductor material and the doping concentrations. A Zener diode exhibits almost the same properties, except the device is specially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage. A Zener diode contains a heavily doped p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. In the atomic scale, this
tunneling corresponds to the transport of valence band electrons into the empty conduction band states; as a result of the reduced barrier between these bands and high electric fields that are induced due to the relatively high levels of dopings on both sides. A reverse-biased Zener diode will exhibit a controlled breakdown and allow the current to keep the voltage across the Zener diode at the Zener voltage. For example, a diode with a Zener breakdown voltage of 3.2 V will exhibit a voltage drop of 3.2 V if reverse bias voltage applied across it is more than its Zener voltage. However, the current is not unlimited, so the Zener diode is typically used to generate a reference voltage for an amplifier stage, or as a voltage stabilizer for low-current applications. The breakdown voltage can be controlled quite accurately in the doping process. While tolerances within 0.05% are available, the most widely used tolerances are 5% and 10%. Another mechanism that produces a similar effect is the avalanche effect as in the avalanche diode. The two types of diode are in fact constructed the same way and both effects are present in diodes of this type. In silicon diodes up to about 5.6 volts, the Zener effect is the predominant effect and shows a marked negative temperature coefficient. Above 5.6 volts, the avalanche effect becomes predominant and exhibits a positive temperature coefficient. In a 5.6 V diode, the two effects occur together and their temperature coefficients neatly cancel each other out, thus the 5.6 V diode is the component of choice in temperature-critical applications.
Modern manufacturing techniques have produced devices with voltages lower than 5.6 V with negligible temperature coefficients, but as higher voltage devices are encountered, the temperature coefficient rises dramatically. A 75 V diode has 10 times the coefficient of a 12 V diode. All such diodes, regardless of breakdown voltage, are usually marketed under the umbrella term of "Zener diode".
INTRODUCTION
A zener diode is used to provide a stable voltage from potential that tends to fluctuates some what. In the case of a mains power supply small changes tend to occur in the mains voltage, with a battery the voltage tends to drop as it gradually becomes discharged, the actual output from a 3V Battery dropping from about 3.2V-2.5V during its working life.The internal resistance is merely the resistance that must inevitable occur within the internal structure of a battery or mains power supply.(In resistance through the secondary winding of the transformer and the rectifier.)The resistance produces a voltage drop, in much the same way as if it wire an actual resistor connected in series with the output of the supply.
THEORY
Zener diodes are very important because they are the key to voltage regulation.The chapter also includes optoelectronics diodes, schottky diodes,varactors,and other diodes. A zener diodes is specially designed junction diode which can operate continuously without being damaged in the region of reverse breakdown voltage. One of the most important application of zener diode is the design of constant voltage power supply. The zener diode is joined in reverse biased to D.C. through a resistance of suitable value. Small – signal and rectifier diodes are never intentionally operated in the breakdown region because this may damage them. A zener diode is different; it is silicon diode that the manufacture has optimized for operation in the breakdown region, zener diodes work best in the breakdown region. Sometimes called a breakdown diode, zener diode is the backbone of voltage regulators, circuits that hold the load
voltage almost constant despite large changes in line voltage and load resistance. Figure shows the schematic symbol of a zener diode ;another figure is an alternative symbol. In either symbol, the lines resemble a”z”which stands for zener. By varing breakdown voltages from about 2 to 200v.These diodes can operate in any of three regions:forward,leakage ,or breakdown. Figure shows the I-V graph of a zener diode. in the forword region, it stars conducting around 0.7V,just like an ordinary silicon diode .In the leakage (beween zero and breadown), it has only a small leakage or reverse current. In a zener diode ,the break has a very sharp knee, followed by an almost vertical V2 over most of the breakdown region,data sheet usually specity the value of V2 at a particular test current IZR .
CIRCUIT A zener diode can be regared as being much the same as an ordinary silicon type, but it reverse breakdown voltage is know quite accurately. In use it connected across the unstablised supply via a current limiting resistor ,in the manner indicated in circuit diagram. Note that the circuit symbol for a zener is slightly different from that for an ordinary diode. Although the zener is reverse-connected ,it conducts because the supply voltage is greater than its reverse breakdown voltage ,causing it to ‘avalanche’.R2 limits the current through the device to a safe level.D2 has a voltage rating of 2.7V,and within a few per cent this is the reading that should be obtained onME1.The avalanche effect of D2 still limits the voltage across D2 to about 2.7V and the reading on ME1 will probably not noticeably increase. In a practical circuit the value of R2 must be made low enough to give the required output current without dropping the voltage to the zener below its rated voltage ,i.e 2.7V and the zener must be capable of handling the power developed across it with no load connected at the output.
WORKING The circuit shows here is quite simple . A3V battery is connected with a series resistor R-2 .ME1 meter is 250 microAmp, thus needs arounds 22 kilo ohms in series to show full scale deflection.A zener diode of 2.7V is connected via push to’on’ switch to shows its effect on meter reading (working as a voltmeter here).When push to ‘on’ switch is momentarily pressed , the full deflection meter shows slight lower voltage equal to the zener breakdown voltage.
COMPONENT USED: Zener diode 2.7c R-1 ………...100 k ohm (variable resistor) R-2 …………100 ohm(fixed type) S-1 ………….push to on switch ME1 ………...Meter(250 microA) Battery ……….3V(DC)
BIBLIOGRAPHY
1. BASIC ELECTRONIC CIRCUITS
2. BASIC ELECTRINICS: D. C. KULSHRESHTHA.
1. ELECTRONIC COMPONENTS MANUAL: PARSAI 2. A. B. C. OF PHYSICS: XII
