Zener diode is same as the normal PN diode but specially designed to operate in reverse bias. In forward bias, the characteristics of zener diode are same as standard PN junction diode. Zener diode is a simplest type of voltage regulator. The voltage at which zener diode conducts in reverse bias is called as “zener voltage (Vz)”. In case of normal PN diodes, diode damages at the break down voltage but these diodes are designed to operate in reverse breakdown voltage.
Following Image gives better idea about the zener diode symbol.
The principle of zener diode is “Zener break down”. These diodes are heavily doped in order to have narrow depletion region. When reverse voltage (Vr) applied to the zener diode exceeds the break down voltage Vb, the process called “avalanche breakdown” occurs in the depletion layer and starts flow through the device and limits the reverse voltage. And the point at which current starts flowing through the device can be controlled accurately in the doping stage of semiconductor devices.
Characteristics of Zener Diode:
From the above V-I characteristics of zener diode, we can say that in forward bias region Zener diode is identical to that of Normal PN junction diode. The Forward voltage of zener diode at room temperature with a current of 1mA is around 0.6V. In reverse bias (positive supply of the battery connects to the cathode terminal and anode is connected to the ground pin) zener diode acts as an open circuit and only small current in order of uA flows through the diode. This leakage current which flows through the zener diode in reverse bias before the breakdown is called as “reverse saturation current”. If the applied reverse voltage exceeds the breakdown voltage then break down occurs. In breakdown region zener diode provides almost constant voltage and the current through the zener increases rapidly. At certain high current level power dissipation of zener becomes excessive and device may be destroyed.
Zener diodes are available in the market with breakdown voltage ranges from 2.4V to 200V. 2.4V, 2.7V, 3.0V, 3.3V, 3.6V, 3.9V, 4.3V, 4.7V, 5.1V, 5.6V, 6.2V, 6.8V, 7.5V, 8.2V, 9.1V, 10V, 11V, 12V, 13V, 15V, 16V, 18V, 20V, 22V, 24V these are the some zener diodes in the market. Zener diodes have power rating Pz. The maximum current through the zener diode Iz(max) = Pz/Vz.Zener diodes that are available in the market with different power ratings are ¼ w, 0.4w, 0.5w, 1w, 2w, 3w, 5w, etc.
Zener diodes are used to stabilize or regulate the voltage source against load or supply variations as it provides constant voltage in the reverse bias even though input voltage is varying after the breakdown.
A regulator is used to provide constant output voltage to the load connected in parallel with the regulator. Zener diode regulates the output voltage until the zener current falls below Iz(min) in breakdown region.
Now let us see how zener diode works and zener diode working principle.
Working of Zener Diode as Voltage Regulator:
Zener diode is used in regulators to produce regulated DC voltage under varying load current conditions. By passing small current through the zener diode from the input source, via current limiting resistor Rs, zener allows sufficient current to maintain the voltage drop.
The above circuit is the zener shunt voltage regulator. In this circuit resistor Rs connected in with the diode to limit the current through the zener. In this regulator circuit cathode terminal of the zener diode is connected to the positive rail and anode is connected to ground so zener is connected in reverse bias.
When there is no load the load current is zero (IL = 0) and total supply current passes through the zener diode (Is=Iz). Hence maximum power is dissipated from zener diode. Even the load is connected if the current limiting resistor value Rs value is small then large current flows through the zener and maximum power dissipation occurs. So we need to take care while selecting the current limiting resistor.
Now let us see how to calculate the value of current limiting resistor Rs. The value of current limiting resistor is the ratio of voltage drop across the resistor to total current flowing through the resistor.
Rs = (Vout – Vs) / (Iz + IL)
When load is connected, voltage across the load is equal to the zener voltage as they are in parallel. The current through the zener diode must greater than the Iz(min) and supply voltage Vs must be greater than the zener voltage Vz for effective stabilization.
The problem with the zener diode voltage regulator circuit is that sometimes it may generate noise while regulating the voltage. It is not a big problem in most of applications but it is better to use a large value decoupling capacitor across the output to produce additional smoothing.
Applications of Zener Diode:
- Zener diode is used in voltage regulator circuits to provide regulated DC voltage.
- Used in wave form clippers to remove unwanted portion of signal.
- Zener diode is used in voltage comparator applications to produce reference voltage.
- Used in electronic circuits to protect the circuit from high voltages.
- Used in power supply circuits.
Also get an idea about Different Types of Diodes and their Characteristics.
3 commentaires:
there are more applications of zener diode than this that you wrote. You should try to make more tests and projects
You have given a good explanation on Zener diode. Here is the application of Zener diode as Voltage regulator
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