Zener diode
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Zener Diode :  Fig35

       The diodes designed to work in breakdown region are called zener diode. The power handling capacity of these
       diodes  is  better.  The  power  dissipation  of  a  zener  diode  equals  the  product  of  its  voltage  and  current.  PZ  =VZ
       IZ  .  When  zener  is  forward  biased  it  works  as  a  diode  and  drop  across  it  is  0.7V  when  it  works  in  breakdown
        region  the  voltage  across  it  is  constant  Vz  and  the  current  through  it  is  decided  by  external  resistance.  Zener
        diode is used for regulating the the dc voltage. It maintains the output voltage constant even through the current
        through changes.
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Zener diode
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                                  fig. 35

To operate the zener in Breakdown region Vs should always be greater then Vz. Rs is used to limit the current.
If the Vs voltage changes operating point also changes simultaneously but voltage across zener is almost
constant. Fig. 37 The first approximation zener diode is a voltage source of Vz magnitude and second
approximation includes the resistance also.
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Zener diode
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(Fig.38) The resistance produces more I*R drop as the current increases.

       The voltage at Q1 is

           V1 = R z +Vz

           At Q 2 = I 2 +R 2 +Vz
Change in voltage is
If zener is used to regulate the voltage across a load resistance. The zener is will work in the breakdown region
only if the thevenin voltage across zener is more than V Z
.
If zener is operating in breakdown region, the series current will be given by
Zener Drop Output point :

        For  a  zener  regulator  to  hold  the  output  voltage  constant,  zener  diode  must  remain  in  the  breakdown  region
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Zener diode
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under  all  operating  conditions,  i.e.  there  must  be  zener  current  for  all  source  voltage  load  currents.  The  worst
case occurs for minimum source voltage and maximum load current.
The critical point occurs when maximum load current equals minimum series current
When  the  zener  diode  operates  in  breakdown  region,  the  voltage  Vz  across  it  remains  fairly  constant  even
though the current Iz through it vary considerably. If the load IL should increase, the current Iz should decrease
by  the  same  percentage  in  order  to  maintain  load  current  constant  Is.  This  keeps  the  voltage  drop  across  Rs
constant  and  hence  the  output  voltage.  If  the  input  voltage  should  increase,  the  zener  diode  passes  a  larger
current,  that  extra  voltage  is  dropped  across  the  resistance  Rs.  If  input  voltage  falls,  the  current  Iz  falls  such
that Vz is constant.

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