In our previous topics relating to diodes, we’ve discussed the operation of a Zener
diode as well as its important specifications.
We’ve learned that a Zener diode is a special-purpose diode which works as a normal diode when forward
biased but also permits current to flow when reverse biased, if its Zener voltage is surpassed.
When its Zener voltage is surpassed, it maintains a stable reference voltage across it.
With this characteristic, Zener diodes are often used as a simple voltage regulator and
we’ll go over the practical usage of Zener diodes in this way.
So, what is a Voltage Regulator?
A voltage regulator is a device or a circuit with the purpose of creating a specific voltage
while minimizing output variations with respect to variations in input, temperature, and load
requirements.
The most common use of a regulator are in power supplies, but any circuit that incorporates
regulatory technique to give a controlled output or function can be considered as a
regulator.
Simply put, a regulator “regulates” the voltage to provide a constant DC voltage across
the load.
When a Zener diode is used as a voltage regulator, ideally, it has a constant voltage drop equal
to its nominal Zener voltage.
This constant voltage drop across the zener diode produced by reverse breakdown is represented
by a DC voltage symbol even though the zener diode does not produce a voltage.
This is the ideal characteristic curve of a zener diode.
However, in reality, there exists a Zener impedance and the actual voltage curve is
not perfectly vertical.
There is a change in the Zener current that produces a small change in Zener voltage.
By Ohm’s law, the Zener impedance is equal to the ratio of the change in zener voltage
to the change in zener current.
The Zener impedance is usually specified at the Zener test current.
Most of the time, this zener impedance is a small constant over a full range of Zener
current values.
However, if the Zener diode is operated near the knee of the curve, the Zener impedance
changes drastically.
Therefore, it's better to operate the Zener diode beyond the knee of the curve to get
a more stable Zener impedance.
As mentioned, when a reverse bias that is equal or above the Zener voltage is applied
to the Zener diode, the diode can maintain a stable voltage across it.
With this characteristic, Zener diodes are used to regulate voltage in some circuits.
The Zener diode voltage regulator is very economic as it is very inexpensive, simple
and easy to build.
Before you build your own voltage regulator using Zener diodes, here are some important
parameters to be considered.
These parameters can be found in the datasheet, so it is very important to look at the datasheet
of your Zener diode to ensure proper operation.
First, take note of the Zener diode’s rated breakdown voltage or Zener voltage.
This parameter specifies the amount of reverse-biased voltage that causes the diode to conduct current.
For the diode to operate, the voltage applied across the Zener diode must be equal or greater
than the zener voltage.
Second, make sure that the minimum current is exceeded.
This is to keep the diode in breakdown for voltage regulation.
It is not recommended to operate the Zener diode near the knee current because of the
effect on Zener impedance.
Third, see to it that you do not exceed the maximum current.
Exceeding this maximum current may damage the Zener diode.
Though zener diodes are used a voltage regulators, they have a low efficiency for heavy load
currents.
It is because if the load current is large, there will be considerable power loss in the
series limiting resistance (R).
When the power dissipated on R exceeds its power rating, this will eventually damage
the resistor.
This is a common problem of a Zener diode voltage regulator.
Also, in reality the output voltage slightly varies due to Zener impedance as Vout = VZ
+ IZ ZZ.
Changes in load current produce changes in Zener current.
Consequently, the output voltage also changes.
Therefore, the use of this circuit is limited to only such applications where variations
in load current and input voltage are small.
In general, Zener diode regulators can provide a reasonably stable dc voltage at the output,
but they are not particularly efficient.
Its regulating ability is somewhat limited by the change in zener voltage over a range
of current values, which restricts the load current that it can handle.
For this reason, they are limited to applications that require only low current to the load.
To achieve better regulation and provide for greater variations in load current, the zener
diode is combined as a key element with other circuit components to create a 3-terminal
linear voltage regulator.
Hopefully this theoretical yet practical overview of zener diodes is helpful and either gives
you a good foundation for further usage of zener diodes or clarifies some questions that
you’ve had!
We have a hands-on tutorial for zener diode voltage regulation and show its operation
in the lab.
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