Other diodes: Diode types
Zener diodes or voltage reference diodes are used in a variety of circuits to enable them to provide a voltage reference. They can also be used in other circuits apart from just providing a voltage reference.
There are many circuits that use Zener diodes, ranging from very simple Zener diode circuits, up to far more complicated ones.
A few examples of Zener diode circuits are given below along with some circuit design hints and tips.
Simple Zener diode circuit providing reference voltage
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 in the Zener diode circuit 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.
Circuit design example
Take the case where a Zener diode circuit 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.
This simple Zener diode circuit is widely used as an easy method of providing a voltage reference.
Zener diode circuit for PSU with series transistor
The very simple Zener diode circuit providing the shunt regulator function as shown above is not particularly efficient and is not practicable for many higher current applications. One solution is to utilise a Zener diode circuit that uses a transistor buffer that acts as a series pass transistor. A simple circuit is shown below and here the transistor is used as an emitter follower.
When utilising this Zener diode 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 – about 0.6 volts for a silicon transistor.
Zener diode circuit for overvoltage protection
Another form of Zener diode circuit is an overvoltage protection circuit. This Zener diode circuit uses the Zener diode in a slightly different way, detecting the breakdown current through the diode once a certain voltage has been reached.
While power supplies are normally reliable, the effects of the series pass transistor or FET failure can be catastrophic. If the series pass device fails as a short circuit, 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.
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, i.e. the Zener diode breakdown voltage, the Zener diode will start to conduct, the SCR will fire and the fuse will be blow.
Zener diode 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.
- Buffer the Zener diode circuit with an emitter or source follower circuit: 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 circuit 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 circuit using just a resistor is adequate for many applications, but a more effective current source can provide some improvements in the circuit performance as the current can be maintained almost regardless of any variations in supply rail.
- 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.
- 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. If this minimum current is not supplied then the diode will not conduct properly and the whole circuit will not operate.
- 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.
Zener diodes are very easy to use and as a result there is a wide variety of different Zener diode circuits. When used with a few precautions, they operate well, but occasionally they can cause a few issues, but following the hints and tips mentioned above should help avoid most issues..
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