OPTOCOUPLERS

The combined package of a LED and a photodiode is called an optocoupler. It
is also called an optoisolator or an optically coupled isolator. The Fig. shows the basic circuit  of  an  optocoupler.  It  has  LED  on  the  input  side  and  a  photodiode  on  the output side.

The source V1 and series resistance R1 decide the forward current I1 through    the LED. Thus LED emits the light. This light is incident on a photodiode. Due to this,
a reverse current is set up in the output circuit. This current produces a drop across
the  output  resistance  R2.  The  output  voltage is  the  difference  between  the  supply voltage V2 and the drop across the resistor R2

Now if  input  voltage is  changed, the  amount  of  light emitted by  LED  changes. This varies  the  reverse  current  in  the  output  circuit  and  hence  the  output  voltage.  The output voltage is thus varying in step with the input voltage. This coupling between
LED  and  photodiode is  hence  called  optocoupler.  As  the  name suggests  this  device can couple an input signal to the output circuit.

Characteristics of Optocoupler

Following are the important characteristics of an optocoupler

i) Current transfer ratio

ii) Isolation voltage i

ii) Response time

iv) Common mode rejection.