What is a Zener Diode
What Is A Zener Diode
Find out more about the Zener diode
electric circuit | educationDiodes are electronic components which will let current flow in just one direction. They are used for example in
PV Solar Panel installations to ensure that current can flow into the
battery bank when it is sunny, but not escape through the solar panel when it is very cloudy or at night.
What is a Zener Diode
A
Zener Diode is a special kind of diode which permits current to flow in the forward direction as normal, but will also allow it to flow in the
reverse direction when the voltage is above a certain value - the
breakdown voltage known as the
Zener voltage.
The
Zener voltage of a standard diode is high, but if a reverse current above that value is allowed to pass through it, the diode is permanently damaged.
Zener diodes are designed so that their zener voltage is much lower - for example just 2.4 Volts. When a reverse current
above the Zener voltage passes through a Zener diode, there is a controlled
breakdown which does not damage the diode. The voltage drop across the Zener diode is equal to the Zener voltage of that diode no matter how high the reverse bias voltage is above the Zener voltage.
The illustration above shows this phenomenon in a Current vs. Voltage graph. With a zener diode connected in the
forward direction, it behaves exactly the same as a standard
diode - i.e. a small voltage drop of 0.3 to 0.7V with current flowing through pretty much unrestricted. In the reverse direction however there is a very small
leakage current between 0V and the Zener voltage - i.e. just a tiny amount of current is able to flow. Then, when the voltage reaches the
breakdown voltage (Vz), suddenly current can flow freely through it.
Uses of Zener Diodes
Since the voltage dropped across a
Zener Diode is a known and fixed value, Zener diodes are typically used to regulate the voltage in
electric circuits. Using a
resistor to ensure that the current passing through the Zener diode is at least 5mA (0.005 Amps), the circuit designer knows that the voltage drop across the diode is exactly equal to the
Zener voltage of the diode.
NEW Click here to visit
Zener-Diode.co.uk for a site dedicated to
zener diodes, their characteristics, usage, and other information.
Zener Diode Voltage Regulator Circuit
A
zener diode can be used to make a simple
voltage regulation circuit as pictured above. The output voltage is fixed at the zener voltage of the zener diode used and so can be used to power devices requiring a fixed voltage. Click here to find out more about the
Zener Diode Voltage Regulator and how you go about selecting the resistor and zener diode.
NEW The above mentioned article on
zener diode voltage regulators has now been updated and a handy
calculator has been added to help you select the correct
resistor and
zener diode values and power ratings.
Zener Diodes in Series With Loads
If you have a
regulated fixed voltage - say 12 Volts from a desktop PC power supply, and you want to power something requiring a lower voltage, it is possible to simply place a
zener diode in series with the load device. You would choose a diode with a zener voltage equal to the supply voltage minus the voltage drop across the load.
For example, if you have a 1 Watt 6 Volt lightbulb to power from a 12 Watt regulated power supply, a 6.2V
zener diode could be placed in series with the bulb giving the bulb 5.8V, or you could overpower the bulb a little using a 5.6V zener diode and dropping 6.4V across the bulb.
Heat would be generated in the zener diode so it is essential to calculate the power lost in it so a suitably rated diode could be chosen.
Using a 5.6V zener diode, and knowing the bulb draws a current of around one-sixth of an Amp, we can calculate the power loss in the diode (with
Ohm's Law) to be just under 1 Watt. Therefore a standard
1.3 Watt zener diode should be up to the job.
Circuits with Zener Diodes
Zener diodes are key components in the circuits presented in the following articles:
Make a Simple Battery Status Monitor,
LM741 Voltage Indicator, and
High Capacity Alternative to 9V Battery.
Buying Zener Diodes
Starter mixed packs of
zener diodes are available cheaply in the
REUK Shop.
Article Last Modified: 10:20, 10th Feb 2009Comment on this Article
If you have any comments on this article, please email them to
neil@reuk.co.uk.
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