Zener Diodes

Zener diode is a semiconductor diode, designed to allow current in reverse bias (reverse direction) when the voltage is above a certain value. In forward bias the Zener diode act the same as an ordinary diode and with a voltage of 0.3 to 0.7V allows current to flow freely. In reverse bias, initially the diode will be off (open circuit) but as the reverse voltage increases beyond a certain level (Zener voltage) the unit will breakdown and an avalanche current will result.

In a standard diode, the Zener voltage is high and the diode will be permanently damaged if the reverse current exceeds Zener voltage; but Zener diodes are designed with much lower Zener voltage that allows reverse current to flow without damaging the diode when the reverse current is above Zener voltage.

The Zener diode is always connected in REVERSE BIAS, ie cathode to positive and anode to negative. At the controlled breakdown voltage, determined by the doping levels, the Zener will breakdown, but if the reverse voltage is reduced than the Zener voltage, the diode will block the flow and act like a common diode.

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