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A Zener Diode by Muhammad Arif Rattar

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ZENER DIODE A Zener Diode is a special kind of diode which permits current to flow in the forward direction as normal, but will also allow it to flow in the reverse direction when the voltage is above a certain value - the breakdown voltage known as the Zener voltage. The Zener voltage of a standard diode is high, but if a reverse current above that value is allowed to pass through it, the diode is permanently damaged. Zener diodes are designed so that their zener voltage is much lower - for ex
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  ZENER DIODE A Zener Diode is a special kind of diode which permits current to flow in the forwarddirection as normal, but will also allow it to flow in the reverse direction when thevoltage is above a certain value - the breakdown voltage known as the Zenervoltage.  The Zener voltage of a standard diode is high, but if a reverse current above thatvalue is allowed to pass through it, the diode is permanently damaged. Zenerdiodes are designed so that their zener voltage is much lower - for example just 2.4Volts. When a reverse current above the Zener voltage passes through a Zenerdiode, there is a controlled breakdown which does not damage the diode. Thevoltage drop across the Zener diode is equal to the Zener voltage of that diode nomatter how high the reverse bias voltage isabove the Zener voltage.  The illustration above shows this phenomenon in a Current vs. Voltage graph. Witha zener diode connected in the forward direction, it behaves exactly the same as astandard diode - i.e. a small voltage drop of 0.3 to 0.7V with current flowing throughpretty much unrestricted. In the reverse direction however there is a very smallleakage current between 0V and the Zener voltage - i.e. just a tiny amount of current is able to flow. Then, when the voltage reaches the breakdown voltage (Vz),suddenly current can flow freely through it. USES OF ZENER DIODES  Since the voltage dropped across a Zener Diode is a known and fixed value, Zenerdiodes are typically used to regulate the voltage in electric circuits. Using a resistorto ensure that the current passing through the Zener diode is at least 5mA (0.005Amps), the circuit designer knows that the voltage drop across the diode is exactlyequal to the Zener voltage of the diode. ZENER DIODE VOLTAGE REGULATOR CIRCUIT  A zener diode can be used to make a simple voltage regulation circuit as picturedabove. The output voltage is fixed at the zener voltage of the zener diode used andso can be used to power devices requiring a fixed voltage ZENER DIODES IN SERIES WITH LOADS If you have a regulated fixed voltage - say 12 Volts from a desktop PC power supply,and you want to power something requiring a lower voltage, it is possible to simplyplace a zener diode in series with the load device. You would choose a diode with azener voltage equal to the supply voltage minus the voltage dropacross the load. For example, if you have a 1 Watt 6 Volt lightbulb to power from a 12 Wattregulated power supply, a 6.2V zener diode could be placed in series with the bulbgiving the bulb 5.8V, or you could overpower the bulb a little using a 5.6V zenerdiode and dropping 6.4V across the bulb. Heat would be generated in the zenerdiode so it is essential to calculate the power lost in it so a suitably rated diodecould be chosen.Using a 5.6V zener diode, and knowing the bulb draws a current of around one-sixthof an Amp, we can calculate the power loss in the diode (with Ohm's Law) to be justunder 1 Watt. Therefore a standard 1.3 Watt zener diode should be up to the job.
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