14) zener diode

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Mand Labs presents

Zener Diode

About Zener Diode

• A usual diode designed to conduct in reverse direction

• Conducts in reverse bias after a certain voltage, known as, Breakdown Voltage (Vz)

• Vz is imprinted on the zener diode

• Vz of zener diode in kit = 5V6 = 5.6 V

• Vz Range: 2 to 200 Volts

Zener in Forward Bias

Works as a normal diode when forward biased

Note: Minimum Forward Bias voltage required to make the zener diode (used in our kit) conducting is 0.99 V

Zener in Reverse Bias

Vcc i (reverse current)

< 5.6V 0

≥ 5.6V High Value

Let, Voltage across zener diode = Vzener

Vz = Breakdown Voltage (Vz) of our diode = 5.6 V

Vz = Vzener

Voltage Regulator

• Use a zener diode to maintain constant voltage across two points

• Zener diode will not allow the voltage to shoot above its voltage breakdown rating in reverse bias mode

• VAB = Breakdown Voltage, if Vs ≥ Breakdown Voltage

Example

• 10k Ω resistor is connected with a 9V battery

• Ideal Voltage Drop across 10k Ω resistor = 9V

Voltage drop will increase, if we increase the value of Vcc

Can we make this

voltage drop

constant ?

Example

Add Zener Diode

• Connect a zener diode in reverse bias and parallel with resistor

• Measure the voltage across the resistor

Voltage Reading

• Voltage across the zener diode = Voltage across the resistor

• Voltage across zener diode = 6.53 V = Breakdown Voltage

Characteristic Curve

• In +ve voltage region:

‣ Threshold Voltage: Minimum forward bias voltage required to open up a diode

‣ Forward current is in mA

‣ Current increases exponentially with voltage

• In -ve voltage region:

‣ Breakdown Voltage: Voltage after which reverse current flows

‣ Reverse current is minimal till Vz and increases exponentially after that

‣ Voltage across zener becomes constant after breakdown

‣ Avalanche Breakdown

Observations from curve

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