They were invented in 1950s especially for the purpose of reverse breakdown operation by an American Physicist- Clarence Zener. Otherwise, simple diodes were good for general forward biasing but they failed when I-V graph reaches most left break down region. It simply worked perfect in all the biasing reason.
They also look different than other elements on circuit board and are beautifully laminated. Since they work in break down or avalanche conditions, they are usually placed separately from other passive components.
This invention was made to bypass the reverse breakdown. But, surprisingly, Zener Diode finds many interesting uses. Like- a) for to give constant voltage reference [of 4.7 V] with Op-amp b) and to clip and clamp the voltage waveform.
They are connected with positive voltage at cathode with respect to anode. When connected in this way it is said to be reversed biased. For low voltage, the diode acts like a resistor with small linear I-V region. But when a certain voltage is reached [called as zener voltage- also known as reverse breakdown voltage] the voltage does not increases further but essentially any current can flow up to the maximum rating of that diode.
Mostly, we use 1N750 Zener diode. Its rating is: 75mA for 4.7 V zener voltage.
Zener diodes symbol [straight line with tilted ends] is the reflection of their I-V characteristics. We are free to use any of the following symbol since it is conducting in both direction appreciably.
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