Posted on : Sun , 03 2014 by : virusi
Theory OF OPERATION :
Zener Diode construction :
Zener Diode or “Breakdown Diode” operates like an ordinary diode in the forward bias direction. They have the normal turn on voltage of 0.6 volts for a silicon diode. However in the reverse direction their operation is rather different.
To differentiate a Zener diode from a normal signal diode the circuit symbol is modified slightly. The Zener diode symbol has a small “tag” applied to the bar of the diode symbol to identify its function. The Zener diode or voltage reference diode is widely used throughout electronics circuits. The Zener diodes or reference diodes can be used as discrete devices, or they may be used within integrated circuits. As such Zener diodes provide an essential building block for many circuits – one which could not easily be overcome if they were not available for some reason.
Diode V-I characteristic :
For very low voltages, like a normal diode they do not conduct at all. However once a certain voltage is reached the diode “breaks down” and current flows. It can be seen by looking at the curves for Zener diodes that the voltage is almost constant regardless of the current carried.
Although the voltage reference diode is normally referred to as a Zener diode, there are two different breakdown mechanisms that can occur:
– Zener effect : Under most conditions electrons are contained within atoms in the crystal lattice. In this state they are in what is called the valence band. If a large electric field is placed across the semiconductor this may be sufficient to pull the electrons out the atom into what is called the conduction band. When they are free from the atom they are able to conduct electricity, and this gives rise to the name of the conduction band. For them to pass from the valence band into the conduction band there must be a certain force to pull them free. It is found that once a certain level of electric field is present a large number of electrons are pulled free creating allowing current to suddenly start to flow once a certain reverse voltage is reached.
– Impactionisation : Impact ionisation is very different to Zener breakdown and it occurs when a high electric field is present in a semiconductor. Electrons are strongly attracted and move towards the positive potential. In view of the high electric field their velocity increases, and often these high energy electrons will collide with the semiconductor lattice.When this occurs a hole-electron pair is created. This newly created electron moves towards the positive voltage and is accelerated under the high electric field, and it may collide with the lattice. The hole, being positively charged moves in the opposite direction to the electron. If the field is sufficiently strong sufficient numbers of collisions occur so that an effect known as avalanche breakdown occurs. This happens only when a specific field is exceeded, i.e. when a certain reverse voltage is exceeded for that diode, making it conduct in the reverse direction for a given voltage, just what is required for a voltage reference diode.
The two reverse breakdown effects in the diode have very similar characteristics, but they are not the same. In most cases it is possible to ignore the difference between the two effects and use a diode in the same manner.
Since the diode can conduct “infinite” current above this knee point as it effectively becomes a short circuit, therefore resistors are used in series with the diode to limit its current flow. Exceeding its maximum forward current specification causes the device to dissipate more power in the form of heat than it was designed for resulting in a very quick failure of the device.
PRACTICAL EXAMPLE :
– In your browser allow Java SE 7.
– Lower you java security settings (Go to Control Panel >> Java >> Security and set the security level to medium) .
– Edit Site List (Go to Control Panel >> Java >> Security and click on Edit Site List… and add eagerlearning.org in the list).
Let’s apply an AC voltage to Zener diode in series with a resistor. How the output voltage would look?
If you want to change resistor or AC voltage value than you should double click on the element and insert the desired value. First Diagram is showing Zener Diode IV characteristic, you can play with the resistance value and see how the V-I characteristic is changing. The second diagram shows the I and V that is flowing through the Zener Diode.