Zener Voltage Reference Diode Tutorial

- a summary or tutorial covering the essentials of the Zener diode or voltage reference diode used in many power supply and other circuits.

Zener diodes are widely used as voltage reference diodes in electronics circuits. Zener diodes allow simple voltage regulator circuits to be made, and in addition to this they are cheap and easy to manufacture.

Zener diodes have been available for many years, and nowadays they are widely used in many areas of electronic circuits. Their obvious use is within power supply regulators, but they can be used as a reasonably stable reference voltage in many electronics circuits. In addition to this, they can be used to remove peaks in waveforms that may not be required. In one specific instance they can be used to remove spikes that may damage a circuit or cause it to overload.

Although the term Zener diode is widely used to describe diodes used as voltage references, the Zener effect that gives them their name is used in all diodes as seen later. Accordingly they should probably more correctly be termed voltage reference diodes.

In view of the many applications for Zener diodes, they are used in many areas of electronics circuits - not just in power supplies which is probably their most obvious used.

Zener diode history

Zener diode history can trace its roots to the developments that occurred as semiconductor materials science started to come to the fore. Although early detectors such as cat's whisker, point contact didoes had been available since around 1905, much work on semiconductors and semiconductor diodes was undertaken during and after the Second World War.

The first person to describe the electrical properties utilised by the Zener diode were described by Clarence Melvin Zener (born 1 Dec 1905, died 15 July 1993).

Clarence Zener was a theoretical physicist who worked at Bell Labs and as a result of his work, Bell named the Zener diode after him. He first postulated the breakdown effect that bears his name in a paper published in 1934.

Clarence Zener was known for his emphasis on theoretical work, no preferring to work on practical problems within the arena of applied physics it liking to undertake practical work himself. Instead he preferred to work on practical problems within the arena of applied physics.

Although he was a very successful theoretical physicist, he did not consider himself particularly qualified to work on purely theoretical physics problems.

Zener diode basics

Zener diodes or as they may sometimes be called, reference diodes operate like an ordinary diode in the forward bias direction. They have the normal turn on voltage of 0.6 volts for a silicon diode. However in the reverse direction their operation is rather different. For very low voltages, like a normal diode they do not conduct at all. However once a certain voltage is reached the diode "breaks down" and current flows. It can be seen by looking at the curves for Zener diodes that the voltage is almost constant regardless of the current carried.

Zener diode characteristic

Zener diode characteristic

Although the voltage reference diode is normally referred to as a Zener diode, there are two different breakdown mechanisms that can occur:

  • Zener effect:   This effect predominates below 5.5 volts.
  • Impact ionisation:   This effect predominates above 5.5 volts.

The result of both breakdown effects is the same and design engineers do not need to design their circuits differently in any major way. The main difference is that the two effects have different temperature coefficients.

Zener diode symbol

To differentiate a Zener diode from a normal signal diode the circuit symbol is modified slightly. The Zener diode symbol has a small "tag" applied to the bar of the diode symbol to identify its function.

Zener diode symbol used in circuit diagrams

Zener diode symbol used in circuit diagrams

The Zener diode or voltage reference diode is widely used throughout electronics circuits. The Zener diodes or reference diodes can be used as discrete devices, or they may be used within integrated circuits. As such Zener diodes provide an essential building block for many circuits - one which could not easily be overcome if they were not available for some reason.

By Ian Poole

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