Page 113 - Testing Electronic Components
P. 113
Testing Opto-Isolator
A Motorola 4N35 Optocoupler
A lot of electronic equipment nowadays is using optocoupler in the
circuit. An optocoupler or sometimes refer to as optoisolator allows two
circuits to exchange signals yet remain electrically isolated. This is
usually accomplished by using light to relay the signal. The standard
optocoupler circuits design uses a LED shining on a phototransistor-
usually it is a NPN transistor and not PNP. The signal is applied to the
LED, which then shines on the transistor in the IC.
The light is proportional to the signal, so the signal is thus transferred to
the phototransistor. Optocouplers may also comes in few module such as
the SCR, photodiodes, TRIAC of other semiconductor switch as an
output, and incandescent lamps, neon bulbs or other light source. I also
came across two led and two phototransistors in a package in the power
supply of a NEC printer. In this book I will explain only the most
commonly used optocoupler which is the combination of LED and
phototransistor. See the optocoupler IC schematic diagram below:
113
