Zener diodes are semiconductor diodes which have been manufactured to have their reverse breakdown occur at a specific, well-defined voltage (its “Zener voltage”), and that are designed such that they can be operated be operated continuously in that breakdown mode. The zener diode is like a rectifying diode in that it allows current to flow in the foward direction. It differs from a rectifying diode, however, in that its reverse-direction breakdown voltage is much lower than that of an ordinary rectifying diode. It is very heavily doped during manufacture. The large number of extra current carriers allows the zener diode to conduct current un the reverse direction. This reverse bias current would destroy a normal diode, but the zener is made to operatet this way. The specified zener voltage rating of a zener diode indicates the voltage at which the diode begins to conduct when reverse biased. Commonly available Zener diodes are available with breakdown voltages (“Zener voltages”) anywhere from 1.8 to 200 V.
The schematic symbol for a Zener diode is shown above– it is very similar to that of a regular diode, but with bent edges on the bar. The Zener still conducts electricity in the forward direction like any other diode, but also conducts in the reverse direction, if the voltage applied is reversed and larger than the Zener breakdown voltage.
A typical application might be as above: A 10 V Zener diode (type 1N4740) is placed in series with a resistor and a fixed 12 V power supply. The resistor value is chosen such that several mA flow through it and through the Zener, keeping it in its breakdown region. In the circuit above, there is 10 V across the Zener diode, and 2 V across the resistor. With 2 V across a 400 ohm resistor, the current through that resistor (and the diode, in series) is 5 mA.
– Evil mad scientist web. “Basic : Introduction to Zener Diodes”. 24 May 2014. http://www.evilmadscientist.com/2012/basics-introduction-to-zener-diodes/
– Petruzella, Frank D. Industrial Electronics. 1996. McGraw-Hill International Editions. pg. 131.
– Picture : http://upload.wikimedia.org/wikipedia/commons/d/df/Zener_Diode.jpg