Zener is not an obsolete device as some designers think. Even though voltage regulator ICs are used widely for precision voltage regulation, Zener diodes are still in use to drop voltage at some sections of the circuit board. Zener is a simple semiconductor device and looks like the glass rectifier diode 1N4148. It generates a fixed voltage output depending on its value. It is designed to operate in the reverse breakdown region. It is always connected in the reverse biased mode with the cathode to the positive supply. Zener diode requires a series resistor to limit current across it, without which the device will be damaged. Zener diodes are available in various voltage ratings from 1.1 volt to 68 volt. Power Zener diodes are also available for industrial applications. They can handle high voltage and current. It’s lead identification is similar to other diodes. The lead near the black or white band is cathode which goes to the positive supply.
The principle of Zener breakdown is simple. If a silicon diode is reverse biased, a point reached where its reverse current suddenly increases. The voltage at which this occurs is known as “Avalanche or Zener “value of the diode. Zener diodes are specially made to exploit the avalanche effect for use in ‘Reference voltage ‘regulators.
A Zener diode can be used to generate a fixed voltage by passing a limited current through it using the series resistor (R).
The Zener output voltage is not seriously affected by R and the output remains as a stable reference voltage. But the limiting resistor R is important, without which the Zener diode will be destroyed. Even if the supply voltage varies, R will take up any excess voltage.
The value of R can be calculated using the formula
R = Vin – Vz / Iz
Where Vin is the input voltage, Vz output voltage and Iz current through the Zener
In most circuits, Iz is kept as low as 5mA. If the supply voltage is 18V, the voltage that is to be dropped across R to get 12V output is 6volts. If the maximum Zener current allowed is 100 mA, then R will pass the maximum desired output current plus 5 mA. So the value of R appears as
R = 18 – 12 / 105 mA = 6 / 105 x 1000 = 57 ohms
Power rating of the Zener is also an important factor to be considered while selecting the Zener diode. According to the formula P = IV. P is the power in watts, I current in Amps and V, the voltage. So the maximum power dissipation that can be allowed in a Zener is the Zener voltage multiplied by the current flowing through it. For example, if a 12V Zener passes 12 V DC and 100 mA current, its power dissipation will be 1.2 Watts. So a Zener diode rated 1.3W should be used.
This is a constant voltage power supply. See the Zener is providing a fixed output voltage to the base of the NPN transistor so that the output from the emitter of the transistor will be constant.
Here a Zener is used at the last section of a circuit to drop 9 volts to 3 volts for the melody generating IC UM66. Low voltage melody and siren generators require 3 volts because these devises are used in toys operating in 3 volts. Here the Zener is used because 3 volt regulator IC not available. The 220 ohms ( R8) resistor is the current limiter for the Zener and the output voltage ( 3 volts) is taken from the junction of R8 and the cathode of Zener.
The UM 66 IC gives sufficient output signals, so a single transistor is sufficient for amplifying the signals to get loud sound. To limit the strength of the output signals R10 is used. It can be between 100 ohms to 1K. It is necessary to get clear sound from the speaker, otherwise the speaker will vibrate. The value of R10 can be selected by trial and error because the IC types vary. Some versions do not need the resistor.
Here a 5.1 volt Zener is used to provide 5 volt for the Infrared module TSOP 1738. Instead of the Zener, we can use a 7805 voltage regulator which is more reliable. R1 limits the current through the Zener. TSOP 1738 requires a fixed 5 volt for its operation. The voltage range is between 4.5 volt to 5.5 volts. Below 4.5 volts, the device will not work, and above 5.5 volts, it will be destroyed.
Here the Zener provides a fixed 12 volts at the Non inverting input of the voltage comparator IC. The voltage at the inverting input is variable as adjusted by VR1. So by keeping the voltage at the Non inverting input fixed, through VR1, it is possible to turn on/ off the output of IC. R2 is the current limiter for the Zener which is selected as IK slightly higher value.
This is an over voltage protection circuit. Here the Zener is used for another application. Input voltage is 5 volts which directly passes to the output. A 6.2 volt Zener is used to control the SCR. Normally the Zener will not conduct because the input voltage is 5 volt. If the input voltage increases above 6 volts, Zener conducts and SCR fires. This drops the output voltage.
Zener can be used to make High/Low voltage cut off circuits. Two 5.1 volt Zener are used to control the relay driver transistors T1 and T2. VR1 and VR2 adjust the breakdown of Zener which in turn controls the relay.
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