Department of Electronics and Communication Engineering,

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Subject Code ECE 101/102 BASIC
ELECTRONICS COURSE MATERIAL For 1st 2nd
Semester B.E. (Revised Credit System)
DEPARTMENT OF ELECTRONICS COMMUNICATION
ENGINEERING
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Zener Diode

• Zener diode is heavily doped P-N junction diode
• Designed to operate in reverse breakdown region
• Each zener diode has specific breakdown voltage
  (VZ). Value of VZ depends on doping level
• Zener diodes are available with VZ ranging from
  1.8V to 200V, power ratings from 250mW to 50W
• Symbol of zener diode

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Zener diode
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Zener Diode characteristics

• V-I characteristics

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Zener Diode characteristics

• V-I characteristics
• When zener diode is forward biased, it acts like
  ordinary diode i.e., until certain voltage V?
  is reached, current is zero, then afterwards,
  current rises exponentially
• When zener diode is reverse biased, until the
  breakdown voltage is reached, current is zero or
  negligible
• When reverse voltage equals zener voltage,
  current rises exponentially in reverse direction
• After the breakdown has occurred, voltage across
  zener diode remains almost constant at VZ, only
  the current increases with the increase in
  applied reverse bias

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Zener Diode characteristics

• PZM VZ.IZM
• where PZM is Maximum Power dissipation across
  the zener diode
• Zener diode is always connected such that it is
  reverse biased and it is in zener breakdown region

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Zener Diode characteristics

• Equivalent circuits of zener diode
• Forward Reverse Breakdown
• Note RZ is usually very small, hence it can be
  neglected

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Zener diode circuit

• Analysis of networks employing zener diodes is
  similar to analysis of ordinary diodes
• First, the state of the diode (ON, OFF or
  breakdown) must be determined, followed by
  substitution of correct model
• Figure shows a simple circuit employing a zener
  diode

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Zener diode circuit

• Step 1
• Remove the zener diode from the circuit, and
  calculate the voltage V across the resulting open
  circuit
• If V VZ, the zener is ON, appropriate
  equivalent model is substituted
• If V lt VZ, the zener is OFF, open circuit
  equivalent is substituted
• Step 2
• Substitute the appropriate equivalent model and
  solve for the desired unknown
• Suppose that we get V VZ, then zener is in
  breakdown region, and equivalent model is VZ
  (neglecting RZ)

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Zener diode circuit

• IZ ( IR IL) where,

,
,
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Zener diode circuit

• Problem
• For the zener network, Vi 16 V, RS 1 K ohm,
  VZ 10 V and RL 3K ohm. Determine Vo, IZ and PZ
  .
• Repeat for RL 1.2K ohm

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Line Regulation

• Line Regulation is the capability to maintain a
  constant output voltage level on the output
  channel of the power supply despite the changes
  in the input voltage level
• Problem on Line regulation
• In a Zener network, RS 120?, RL 250? and VZ
  5V. Find the minimum and maximum current
  flowing through zener when input varies from 9V
  to 15V.

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Load Regulation

• Load regulation is the capability to maintain a
  constant voltage level on the output channel of
  the power supply despite the changes in the load
• Problem on Load regulation
• For a Zener network, RS 10?, VZ 10V, Vi
  25V. Find the minimum and maximum current
  through zener when RL is varied between 10? and
  100?

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End of module 5