Page 115 - Testing Electronic Components

have 2 readings then the LED have become shorted. The testing method
is exactly the same when you are checking a normal diode. The LED
mainly connected internally to pin 1 and 2 of the optocoupler IC.

To check the phototransistor, set your meter to times 1 ohm range and
place your black probe to the base of the transistor and the red probe to
collector and emitter. It should show 2 similar readings. Then move your
black probe to collector and red probe to base and emitter of the
transistor. It should not register any reading. The last step is to place your
black probe to emitter and the red probe to base and collector of the
transistor. Again it should not register any reading in the multimeter.

An Optoisolator in Electronic Board

Now turn your meter selector to times 10k ohm range to measure the
collector and emitter of the transistor. It should have no reading on one
way and the other way should have a slight reading. Which mean the
meter's pointer will moved a little bit up from the infinity scale of the
analogue meter. If you get two readings then the optocoupler is faulty.
One of the most famous part numbers of optoisolator is the 4N35.

If you want to know more about the internal diagram of any optocoupler
IC, I recommend that you check from the Philip ECG semiconductor
master replacement guide book for the correct datasheet. From the
schematic it is easier to describe whether it is a phototransistor,
photodiode, SCR or Triac type at the output of the optocoupler IC. Once

115