Zener diode
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Photo diode :

       When a diode is reversed biased, a reverse current flows due to minority carriers. These carriers exist because
       thermal  energy  keeps  on  producing  free  electrons  and  holes.  The  lifetime  of  the  minority  carriers  is  short,  but
       while they exist they can contribute to the reverse current. When light energy bombards a pn junction, it too can
       produce free electrons. Fig 41
In other words, the amount of light striking the junction can control the reverse current in a diode. A photo diode
is  made  on  the  same  principle.  It  is  sensitive  to  the  light.  In  this  diode,  through  a  window  light  falls  to  the
junction. The stronger the light, the greater the minority carriers and larger the reverse current.

Opto Coupler:

It  combines  a  LED  and  a  photo  diode  in  a  single  package.  LED  radiates  the  light  depending  on  the  current
through  LED.  This  light  fallls  on  photo  diode  and  this  sets  up  a  reverse  current.  The  advantage  of  an  opto
coupler is the electrical isolation between the input and output circuits. The only contact between the input and
output is a beam of light. Because of this, it is possible to have an insulation resistance between the two circuits
of  the  order  of  thousands  of  mega  ohms.  They  can  be  used  to  isolate  two  circuits  of  different  voltage  levels.
Fig42.
http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-ROORKEE/BASIC-ELECTRONIC...   5/18/2009


Zener diode
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