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A Zener diode is a type of Diode that permits Current to flow in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the rated breakdown voltage or "Zener voltage". A conventional solid-state Diode will not let current flow if reverse-biased below its reverse breakdown voltage. By exceeding the Breakdown voltage, a conventional diode is destroyed in the breakdown due to excess current which brings about overheating. The process is however reversible, if the device is operated within limitation. In case of forward-bias (in the direction of the arrow), the diode exhibits a voltage drop of roughly 0.6 Volt for a typical silicon diode. The voltage drop depends on the type of the diode. A Zener diode exhibits almost the same properties, except the device is especially 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. A reverse-biased Zener diode will exhibit a controlled breakdown and let the current flow to keep the voltage across the Zener diode at the Zener voltage. For example, a 3.2-volt Zener diode will exhibit a voltage drop of 3.2 volts if reverse biased. 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. Tolerances to within 0.05% are available though the most widely used tolerances are 5% and 10%. The effect was discovered by the American physicist Clarence Melvin Zener . 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-volt diode, the two effects occur together and their temperature coefficients neatly cancel each other out, thus the 5.6-volt diode is the part of choice in temperature critical applications. Modern manufacturing techniques have produced devices with voltages lower than 5.6 volts with negligible temperature coefficients, but as higher voltage devices are encountered, the temperature coefficient rises dramatically. A 75-volt diode has 10 times the coefficient of a 12-volt diode. All such diodes, regardless of breakdown voltage, are usually marketed under the umbrella term of 'zener diode'. USES Zener diodes are widely used to regulate the voltage across a circuit. When connected in parallel with a variable voltage source so that it is reverse biased, a zener diode conducts when the voltage reaches the diode's reverse breakdown voltage. From that point it keeps the voltage at that value. In the circuit shown, resistor R provides the voltage drop between U and U. The value of R must satisfy two conditions: # R must be small enough that the current through D keeps D in reverse breakdown. The value of this current is given in the data sheet for D. For example, the common BZX79C5V6 ''BZX79C5V6 data sheet'', Fairchild Semiconductor device, a 5.6 volt 0.5 watt zener diode, has a recommended reverse current of 5 mA. If insufficient current flows through D, then U will be unregulated, and could rise as high as U. When calculating R, allowance must be made for any current flowing through the external load, not shown in this diagram, connected across U. # R must be large enough that the current through D does not destroy the device. If the current through D is I, its breakdown voltage V and its maximum power dissipation P, then . A zener diode used in this way is known as a ''shunt voltage regulator'' (''shunt'' meaning connected in parallel, and '' Voltage Regulator '' being a class of circuit that produces a fixed voltage). SEE ALSO REFERENCE |
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