Zener Diode

An ordinary of general purpose diode conducts in one direction. In the reverse direction the diode withstand up to certain limit and don’t conduct. But it we keep on increasing reverse voltage a point will reach at which the diode breaks down and it will conduct in the reverse direction as well. That point is known are zener voltage point. An ordinary diode reached to its zener voltage will breakdown and would be destroyed at that point. Whereas a special diode can be constructed which could withstand this zener voltage level in reverse bias and will not destroy itself at that certain level. These diodes are called zener diodes. They are doped with special materials to obtain zener property.

Zener diode is designed to work at zener voltage at reverse bias in the breakdown region. At breakdown or zener state the diode conducts in the reverse direction and heavy current flows in the reverse direction which is limited with series resistance. Zener diode always maintain a certain voltage drop on itself in the reverse direction due to its reverse conduction property. This property makes it suitable to work in voltage reference circuits and power supply circuits where regulated voltage is needed.

Zener diode has two types of ratings. One is its zener voltage and the second one is the power rating the zener diode can handle. Very common power rating is 400mw. Whereas the zener voltage rating is somewhere from 2v to 110v. Zener diode is made for certain zener voltage rating in the manufacturing process. This is done during its doping process. That voltage rating is normally written on the zener diode body.

Zener diode always maintains its zener voltage value on itself. For example a 6.2v zener diode will only conduct on above 6.2v in reverse. It will not conduct below 6.2v. The current would be limited with external series resistance. This way it will always maintain a 6.2v voltage on itself. The voltage will never cross 6.2v limit. This property of zener diode makes it a voltage reference.

Normally zener diodes are fixed diodes with a certain voltage rating. But some special zener diodes are there which are known as adjustable or programmable zener diodes. Those diodes can be adjusted externally by connected a few resistors to any value within a certain voltage range. Very common of them is TL431 which is very widely used in switching power supplies circuits.

In the figure below a zener diode is connected in parallel to the gauge which is fed by battery through a temperature sensor which acts like a series resistance. The gauge maximum voltage is 7v full scale. In such a case the zener diode must be of 7v voltage rating so that it will limit the voltage on the gauge at 7v and the gauge point would not step over the maximum and gauge would not be burned in due to overshoot at maximum. This way a steady and safe reading could be obtained from a gauge without damaging it in case if high voltage comes on its terminals.

Zener diode typical use in instrumentation gauge.