Zener diodes are a form of semiconductor diode that are widely used in electronics circuits as voltage references. Zener diodes provide a stable and defined voltage and as a result Zener diode circuits are often used in power supplies when regulated outputs are needed. Zener diodes are cheap and they are also easy to use and as a result they are used in many applications and many circuits.
Zener diode basics
Zener diodes are sometimes referred to as reference diodes as they are able to provide a stable reference voltage for many electronics circuits. The diodes themselves are cheap and plentiful and can be purchased in virtually every electronics components store.
Zener diodes have many of the same basic properties of ordinary semiconductor diodes. They conduct in the forward direction and have the same turn on voltage as ordinary diodes. For silicon this is about 0.6 volts.
In the reverse direction, the operation of a Zener diode is quite different to an ordinary diode. For low voltages the diodes do not conduct as would be expected. However, once a certain voltage is reached the diode "breaks down" and current flows. Looking at the curves for a Zener diode, it can be seen that the voltage is almost constant regardless of the current carried. This means that a Zener diode provides a stable and known reference voltage.
Zener diode markings
There are many styles of package for a Zener diode. Some are used for high levels of power dissipation and others are contained within surface mount formats. For home construction, the most common type is contained within a small glass encapsulation. It has a band around one end and this marks the cathode.
It can be seen that the band around the package corresponds to the line on the diode circuit symbol and this can be an easy way of remembering which end is which. For a Zener diode operating in its reverse bias condition the band is the more positive terminal in the circuit.
Zener diode markings, symbol and package outlines
How a Zener diode works
The Zener diode is particularly interesting in the way that it operates. There are actually two mechanisms that can cause the breakdown effect that is used to provide the voltage reference effect:
- Zener breakdown: Although the physics behind the effect is quite complicated, it can be considered that this effect occurs when the electric field within the semiconductor crystal lattice is sufficiently high to pull electrons out of the lattice to create a hole and electron. The electron then moves under the influence of the field to provide an electric current.
- Impaction ionisation: Again this effect occurs when there is a high level of electric field. Electrons are strongly attracted and move towards the positive potential. In view of the high electric field their velocity increases, and often these high energy electrons will collide with the semiconductor lattice. When this occurs an electron may be released, leaving a hole. This newly freed electron then moves towards the positive voltage and is accelerated under the high electric field, and it to may collide with the lattice. The hole, being positively charged moves in the opposite direction to the electron. If the field is sufficiently strong sufficient numbers of collisions occur so that an effect known as avalanche breakdown occurs. This happens only when a specific field is exceeded, i.e. when a certain reverse voltage is exceeded for that diode, making it conduct in the reverse direction for a given voltage, just what is required for a voltage reference diode.
It is found that of the two effects the Zener effect predominates above about 5.5 volts whereas impact ionisation is the major effect below this voltage.
The two effects are affected by temperature variations. This means that the Zener diode voltage may vary as the temperature changes. It is found that the impact ionisation and Zener effects have temperature coefficient in opposite directions. As a result Zener diodes with reverse voltages of around 5.5 volts where the two effects occur almost equally have the most stable overall temperature coefficient as they tend to balance each other out for the optimum performance.
Zener diode circuits
The most basic Zener diode circuit consist of a single Zener diode and a resistor. The Zener diode provides the reference voltage, but a series resistor must be in place to limit the current into the diode otherwise a large amount of current would flow through it and it could be destroyed.
The value of the resistor should be calculated to give the required value of current for the supply voltage used. Typically most low power leaded Zener diodes have a maximum power dissipation of 400 mW. Ideally the circuit should be designed to dissipate less than about half this value, but to operate correctly the current into the Zener diode should not fall below about 5 mA or they do not regulate correctly.
Basic Zener diode circuit
Take the case where a Zener diode is used to supply a regulated 5.1 Volt rail consuming 2 mA, from an input voltage supply of 12 volts. The following easy steps can be used to calculate the resistor required:
Zener diode overview
A Zener diode is a particularly useful component for the design of electronics circuits. As a result many millions of Zener diodes are used each year in the construction of electronics equipment both as discrete components as well as components contained within large integrated circuits. Although voltage reference integrated circuits are available that provide very high degrees of accuracy and temperature stability, for most uses a straightforward Zener diode of more than satisfactory and will provide a much cheaper solution. Accordingly it helps to know what is a Zener diode, how one works, and the basics of a Zener diode circuit.