Download Zener diode

ZENER DIODES
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Zener diode
A Zener diode is a type of diode that permits current
not only in the forward direction like a normal diode,
but also in the reverse direction if the voltage is
larger than the breakdown voltage known as "Zener
knee voltage" or "Zener voltage". The device was
named after Clarence Zener, who discovered this
electrical property.
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Zener diode
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ZENER DIODES
The simplest of all voltage regulators is the
zener diode voltage regulator.
A zener diode is a special diode that is
optimized for operation in the breakdown
region.
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VOLTAGE REGULATION
A voltage regulator circuit automatically maintains
the output voltage of a power supply constant,
regardless of
- a change in the load
- a change in the source voltage
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ZENER DIODE CIRCUIT
 The zener diode is typically connected
reverse biased, in parallel with the load.
 Resistor Rs limits current to zener.
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ZENER DIODE CHARACTERISTICS
In the forward region,
the zener diode acts like
a regular silicon diode,
with a 0.7 volt drop
when it conducts.
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ZENER DIODE CHARACTERISTICS
In the reverse bias
region,
a
reverse
leakage current flows
until the breakdown
voltage is reached.
At this point, the reverse
current, called zener
current Iz, increases
sharply.
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ZENER DIODE CHARACTERISTICS
Voltage after breakdown
is also called zener
voltage Vz.
Vz remains nearly a
constant, even though
current Iz varies
considerably.
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ZENER DIODE RATINGS
A zener spec sheet typically provides
- the maximum power rating Pz
- the nominal zener voltage Vz at test
current IzT
- the maximum DC zener current IzM
Example:
- 1N752 has a power rating of 500mW, a
nominal zener voltage of 5.6V at a test
current of 20 mA, a maximum zener
current of 80 mA.
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ZENER DIODE CIRCUIT EXAMPLE
In a zener diode voltage regulator circuit,
VDC(IN) = 20V, Vz = 9V, Rs = 82, RL = 100
Q: Calculate IL.
A: IL = VL/RL = VZ/RL = 9V/100  = 90 mA
82
20V
9V
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100
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ZENER DIODE EXAMPLE (CONT’D)
Q: Calculate IT.
A: IT = VRs/Rs = [VDC(IN) - Vz]/Rs
= [20 - 9]/82 
= 134 mA
82
20V
9V
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100
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ZENER DIODE EXAMPLE (CONT’D)
Q: Calculate Iz
A: Iz = IT - IL
= 134mA - 90mA
= 44mA
82
20V
9V
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ZENER DIODE EXAMPLE (CONT’D)
Q: If zener Pz = 500mW, is Iz safe ?
A: IzM = Pz / Vz
= 500 mW / 9V
= 55 mA
Since circuit Iz
= 44 mA (< 55mA),
zener is safe
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ZENER DIODE EXAMPLE (CONT’D)
Q: Suppose load changes from 100  to 200 .
Find new IL, Iz, IT. Is Iz safe ?
A: IL = VL/RL = VZ/RL = 9V/200  = 45 mA
20V
9V
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200
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ZENER DIODE EXAMPLE (CONT’D)
A: IT = VRs/Rs = [VDC(IN) - Vz]/Rs
= [20 - 9]/82  = 134 mA (no change)
Iz = IT - IL = 134mA - 45mA = 89mA
Since IzM = 55mA, zener is not safe.
20V
9V
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200
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ZENER DIODE EXAMPLE (CONT’D)
Q: Suppose load changes from 100  to 80 .
Find new IL, Iz, IT. Is Iz safe ?
A: IL = VL/RL = VZ/RL = 9V/80  = 112.5 mA
20V
9V
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80
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ZENER DIODE EXAMPLE (CONT’D)
A: IT = VRs/Rs = [VDC(IN) - Vz]/Rs
= [20 - 9]/82  = 134 mA (no change)
Iz = IT - IL = 134mA - 112.5mA = 21.5 mA
Since IzM = 55 mA, zener is safe.
20V
9V
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80
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ZENER DIODE EXAMPLE (CONT’D)
Q: Suppose zener regulator circuit has no load
connected, is Iz safe ?
A: IT = VRs/Rs = [VDC(IN) - Vz]/Rs
= [20 - 9]/82  = 134 mA (no change)
Iz = IT = 134 mA this is very Unsafe
20V
9V
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ZENER DIODE EXAMPLE (CONT’D)
Q: In the previous example, suppose VDC(IN)
changes from 20V to 30V. Is Iz safe ?
A: IL = 9V/100  = 90 mA
IT = [30-9]/82  = 256 mA
Iz = IT - IL = 256 mA - 90 mA = 166 mA
Very Unsafe
82
30V
9V
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ZENER DIODE EXAMPLE (CONT’D)
Q: Suppose VDC(IN) changes from 20V to 18V.
Is Iz safe ?
A: IL = 9V/100  = 90 mA
IT = [18-9]/82  = 110 mA
Iz = IT - IL = 110 mA - 90 mA = 20 mA
Safe
82
18V
9V
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100
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The End
… Thank You …
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