What is a Zener Diode

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.

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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

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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:

1. 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.
   
   
2. 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.

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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

Design example 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: Calculate the difference in voltage across the series resistor 12 - 5.1 = 6.9 volts Determine the resistor current. Choose this to be 15 mA. This will allow sufficient margin above the minimum Zener diode current for some variation in the load current. Check the Zener diode power dissipation. At a current of 15 mA and a voltage across the power dissipation is: 15 mA x 5.1 volts = 76.5 mW This is nicely within the maximum limit for the diode Determine the current through the series resistor. This is 15 mA for the Zener diode plus 2 mA for the load, i.e. 17 mA. Determine the value of the series resistor. Using Ohms law this can be calculated from the voltage drop across it and the total current through it: 6.9 / 17 mA = 0.405 kohm The nearest value is 390 ohms Determine the wattage of the series resistor. This can be determined using the value for the current through the resistor and the voltage across it calculated earlier: V x I = 6.9V x 17mA = 117mW The resistor needs to be able to dissipate this level of heat. A quarter watt resistor should be adequate for this.