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Zener Diodes

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Zener diodes break down at a specified voltage in reverse bias. They are most commonly used to provide an accurate reference voltage.

All diodes allow current to pass in one direction only. In normal use, diodes never suffer a reverse breakdown which would destroy the diode.

Zener Diodes are special because they can break down without being damaged. They do this at a specific voltage (the Zener voltage) which is accurately determined by the manufacturing process. It is possible to purchase Zener diodes with a range of breakdown voltages between about 3.3 up to 200 volts.


Zener Diode Calculations

Diode zener.gif


Resistor Calculation

If the Zener voltage is 4V and the supply voltage is 9V, the potential difference across R will be 5V.

Using Ohm's Law ...


Zener Diode Power Rating

To calculate the Zener diode power rating ...

Power_Rating = (Maximum_Load_Current + Minimum_Zener_Current) x Zener_Breakdown_Voltage

P = (95 x 10-3 + 5 x 10-3) x 4

P = 400mW or 0.4W


Zener Diode Power Calculation

Power = I V        Power = I2 R        Power = V2 / R

This diagram shows a Zener diode operating under typical conditions. In exam questions, the diode is often operating under it's extreme conditions with either zero or maximum load current. In all cases, the total circuit current is the same.


Load Power

Use the load voltage and the load current.

P = I V        P = 0.05 x 4        P = 0.2 Watts

Resistor Power

Use the resistor voltage and the resistor current.

P = I V        P = 0.1 x 5        P = 0.5 Watts

Zener Diode Power

Use the Zener diode voltage and the Zener diode current.

P = I V        P = 0.05 x 4        P = 0.2 Watts

Whole Circuit Power

Use the whole circuit voltage and the whole circuit current.

P = I V        P = 0.1 x 9        P = 0.9 Watts

Notice how all the separate component powers add up to the whole circuit power.


Zener Diode Characteristics

The graph below shows the current/voltage characteristics of a Zener diode. The reverse leakage current has been exaggerated to make it show on the graph. The reverse breakdown voltage varies between different Zener diodes. The range of values goes from a few Volts up to about 200 Volts.

Zener characteristic.gif

If the reverse bias current is less than about 5mA, the Zener voltage will be lower than the expected value. When designing circuits to give a reference voltage using a Zener diode, the minimum Zener current should always be at least 5 mA (check the data sheet). The yellow region in the graph shows where the Zener voltage is not properly established.


Practical Task - LED Characteristics

Diode Characteristics PDF

This circuit can be used to test the behaviour of any low voltage / low current diode. These include typical silicon diodes such as the 1N4001, LEDs and Zener diodes. The activity below shows an LED.

Build this circuit and measure and record the potential difference across the diode and the current through it. Repeat these measurements for the full range of movement of the potentiometer. Plot a graph of your results. Plot your graph as you go and get sufficient readings before you dismantle your circuit.


Your tasks ...

Make sure there are no gaps in your graph where extra measurements should have been taken.


Zener Diode Simulation

For the Falstad Circuit Simulation, CTRL+Click 4 Volt Zener Diode
In options, check European Resistors and uncheck Conventional Current.
Adjust the Potentiometer

Alternatively view Diodes_Zener.txt.
Save or copy the text on the web page. Import the saved or copied text into the Falstad simulator.

Here is the new HTML5 Simulator Site.




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