Zener Diode Circuits & Applications

- 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 used in many circuits in a variety of ways.

The most common Zener diode circuit is one in which the Zener diode is used as a voltage reference element. This type of circuit uses the constant voltage as a reference in one of a variety of forms of power supply circuit.

There are other Zener diode circuits and applications. They can be used to limit voltages, preventing surges from damaging electronics circuits.


Simple Zener diode ciruit for voltage regulator

When used in a regulator cirrcuit, the Zener diode must have the current entering it limited. If a perfect voltage source was placed across it, then it would draw excessive current once the breakdown voltage had been reached. To overcome this the Zener diode must be driven by a current source. This will limit the current to the chosen value.

In a practical circuit, the simplest form of current source is a resistor. This will limit the current taken by the Zener diode and ensure that the operating position of the diode remain approximately constant.

Simple circuit of a Zener diode shunt regulator
Simple circuit of a Zener diode shunt regulator

The value of the series resistor is simple to calculate. It is simply the voltage across the resistor, divided by the current required. The level of Zener current can be chosen to suit the circuit and the Zener diode used.


Resistor value (ohms)     =     (V1 - V2)   /   (Zener current + Load current)


Where:
V1 is the input voltage
V2 is the Zener diode voltage

This form of regulator circuit is known as a shunt regulator, where the regulating element in the circuit is placed in parallel with the load. The voltage appearing across the load is controlled by the Zener diode allowing a portion of the current to flow through the Zener and bypass the load to maintain the voltage across it. Shunt regulators are normally seen as being very inefficient for large levels of power, but for low power levels they are very effective. The Zener diode can be used as a shunt regulator to produce a stable reference voltage, which can then be used by a series regulator to produce the required stable voltage output. This technique is effectively used in analogue regulated power supplies.


Zener diode circuit for PSU with series transistor

The very simple shunt regulator shown above is not particularly efficient and is not practicable for many higher current applications. The solution is to utilise a Zener diode circuit that uses a series pass transistor. A simple circuit is shown below and here the transistor is used as an emitter follower.

Zener diode circuit for a simple regulated power supply
Zener diode circuit for a simple regulated power supply

When utilising this circuit, the current required from the Zener resistor potential diver should be calculated. This is the emitter current from the transistor divided by the gain.

When choosing the Zener diode voltage, it should be remembered that the emitter voltage will be lower than the Zener voltage by the amount of the base-emitter voltage - 0.6 volts for a silicon transistor.


Zener diode circuit for overvoltage protection

Another form of Zener diode circuit is an overvoltage protection circuit. While power supplies are normally reliable, the effects of the series pass transistor or FET can be catastrophic if it fails by forming a short circuit. In this case the full unregulated voltage would be placed onto the circuits using the regulated power. This could destroy all the chips being powered.

One solution is to use a crowbar circuit. When this form of circuit detects an overvoltage situation it fires an SCR. This quickly holds down the output voltage and in the instance shown, it blows a fuse that disconnects the input source power.

SCR overvoltage crowbar circuit
SCR overvoltage crowbar circuit

The circuit operates by firing the SCR when the overvoltage is detected. The Zener diode is chosen to have a voltage above the normal operating voltage - sufficient margin not to fire under normal operating conditions, but small enough to allow current to flow quickly when the fault condition is detected.

Under normal operating conditions the output voltage is below the reverse voltage of the Zener diode and no current flows though it and the gate of the SCR is not fired.

However, if the voltage rises above the allowed voltage, the Zener diode will start to conduct, the SCR will fire and the fuse will be blow.


Circuit tips

The Zener diode is a very flexible and useful circuit component. However, like any other electronics component, there are a few hints and tips which enable the best to be made of the Zener diode. A number are listed below.

  • Choose correct voltage for best stability:   In applications where stability with temperature changes is required, the Zener voltage reference diode should be chosen to have a voltage of around 5.5 volts. The nearest preferred value is 5.6 volts although 5.1 volts is another popular value in view of its proximity to 5 volts required for some logic families. Where different levels of voltage are required, the 5.6 volt Zener can be used and the surrounding electronics can be used to transfer this to the required output value.
  • Buffer the Zener diode circuit with an emitter or source follower:   To keep the voltage from the Zener diode as stable as possible, the current flowing through the Zener diode must be kept constant. Any variations in current drawn by the load must be minimised as these will change the current through the Zener diode and cause slight voltage variations. The changes caused by the load can be minimised by using an emitter follower stage to reduce the current taken from the Zener diode circuit and hence the variations it sees. This also has the advantage that smaller Zener diodes may be used.
  • Drive with constant current source for best stability:   Another way of improving the Zener stability is to use a good constant current source. A simple resistor is adequate for many applications, but a more effective current source can provide some improvements as the current can be maintained almost regardless of any variations in supply rail.
  • Ensure sufficient current for reverse breakdown:   It is necessary to ensure that sufficient current is passed through the diode to ensure that it remains in reverse breakdown. For a typical 400 mW device a current of around 5 mA must be maintained. For exact values of minimum current, the datasheet for the particular device and voltage should be consulted.
  • Ensure maximum limits of current are not exceeded for the Zener diode:   While it is necessary to ensure sufficient current is passed through the Zener diode, the maximum limits must not be exceeded. This can be a bit of a balancing act in some circuits as variations in load current will cause the Zener diode current to vary. Care should be taken not to exceed the maximum current or the maximum power dissipation (Zener voltage x Zener diode current). If this appears to be a problem, an emitter follower circuit can be used to buffer the Zener diode and increase the current capability.

When used to their best, Zener diodes can provide very high levels of performance. They often exceed the performance required, but in view of their ease of use and low cost, they provide a very effective option to use.

By Ian Poole


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