Zener diode is a special type of semiconductor that conducts electricity both in forward bias and reverse bias. Normal diodes only allows current to flow through them in forward bias, contrary to this, zeners conducts electricity in forward and reverse bias. The zener diode was invented by Melvin Zener of southern Illinois. This device conducts in reverse bias when a certain amount of current or voltage is met. It conducts backwards without getting spoilt when an amount of voltage is reached which is known as Zener brake down or knee down voltage.
Circuit symbol of a zener diode.
Working principle of zener diodes.
Zener diode conducts backward when a certain amount of voltage pass through it called the knee voltage. However, if a voltage lower than the knee voltage is passed through it in reverse bias no current flows through it. It works just like normal diodes, which when in forward bias, it has a turn on voltage between 0.3 to 0.7V. It's made up of an heavily doped semiconductor material, the heavily doped means refers to an high level of foreing element known as impurities which is added to it, to make it more conductive.
The depletion region of the zener diode is very thin because of the impurities. The foreign element increases the intensity of the electric field across the depletion region of the zener diode even for the small reverse voltage. When no forward or reverse biasing is applied across the zener diode, the electrons remains in the valence band of the P-type material and no current flows through the diode. The band in which the valence electrons place is known as the valence band electron. The electrons of the valence band easily move from one band to another when external energy is applied across it. When the reverse bias applies across the diodes and the supply voltage is equal to the zener voltage then it starts conducting in the reverse bias direction. The zener voltage is the voltage at which the depletion region completely vanish. The reverse bias applies across the diode increases the intensity of the electric field across the depletion region, thus it allows electrons to move from the valence band of the P-TYPE material to the conduction band of N-TYPE material. This transferring of valence band electrons to the conduction band reduces the barrier between the p and N-type material. When the depletion region become completely vanish the diode starts conducting in the reverse biased.
Circuit application of general diodes.
•Voltage regulation.
Zener diodes are use for regulating volatage from a unregulated DC power supply.
Zener diodes, when connected in reverse bias to a DC power source as as shown above reduces the input voltage, no matter how much the voltage is. A Zener diode with a break down voltage of 5v when connected as shown above to a 20v source with a resistor reduces it to 5v. The resistor helps to reduce excess current from the source to the diode, thus it prevent over heating.Zener diodes, when connected in reverse bias to a DC power source as as shown above reduces the input voltage, no matter how much the voltage is. A Zener diode with a break down voltage of 5v when connected as shown above to a 20v source with a resistor reduces it to 5v. The resistor helps to reduce excess current from the source to the diode, thus it prevent over heating.
• over voltage protection.
It is use for protecting circuits from excess voltage. It's use in smart phone charging panels and delicate electronics which can get destroyed by voltage higher than what they need. It's a power electronic circuit that cut out or shut down the supply voltage when current flows more than it preset value. As shown above when current flows through the diode it's regulated to zener breakdown voltage irrespective of the input voltage.
•Zener clipping circuits.
A clipping circuit is a circuit that removes a negative or positive cycle from a alternating current or signal.
Zener diode specifications.
Zener diodes have different specifications, working voltage and current ratings. The specifications i are use for choosing those that will have a good fit.
•Working voltage:
This refers to the reverse voltage, i.e amount of voltage needed for it to conduct in reverse bias. For long lead Zener diodes their Zener breakdown voltage range from 2.4 to 200v. Surface mounted types have a maximum of 47v. More powerful ones can have as much as 1000v. Zener diodes with a knee down voltage less than 10v are usually printed on long lead Zener diodes in form of aVb. a and b represents digits. E.g 3V3 printed on a Zener diode means it has a break down voltage of 3.3V.
•Current rating.
There are maximum and minimum current which zeners diode can take. If excess current flows through a Zener diode in reverse bias it heats up and burns. The maximum current most zeners can take is between 200uA to 200A. While the minimum current for it to operate in the reverse mode is between 5mA to 10mA. •power rating. This refers to the maximum power a Zener can dissipate, it's calculated from the product of the voltage flowing the diode and the current flowing through it.
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Power Electronics