Introduction to Optocoupler Ic application,
testing, schematic and functions.
A Motorola 4N35 Optocoupler
A lot of electronic equipment
nowadays are 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 article i will explain
only the most commonly used opto coupler which is the combination
of LED and phototransistor. See the optocoupler ic schematic
Symbol or Schematic
The optocoupler usually found
in switch mode power supply circuit in many electronic equipment.
It is connected in between the primary and secondary section of
power supplies. The optocoupler application or
function in the circuit is to:
Monitor high voltage
Output voltage sampling for regulation
System control micro for power on/off
If the optocoupler ic breakdown, it will cause the equipment to
have low power, blink, no power, erratic power and even power shut
down once switch on the equipment.
Many technicians and
engineers do not know that they can actually test the optocoupler
with their analog multimeter. Most of them thought that there is no
way of testing an ic with an analog meter.. Since
we already knew the optocoupler pinout from the schematic diagram,
testing this ic is just the same as measuring a normal bipolar
transistor and LED.
In order to accurately check optoisolator ic, you need to use an
analog multimeter. Test the LED using the times 1 ohm and times 10k
ohms range. It should have one reading when checking both ways. If
you 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
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 emmiter. It should show 2 similar readings. Then move
your black probe to collector and red probe to base and emmiter of
the transistor. It should not register any reading. The last step
is to place your black probe to emmiter 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 emmiter 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 analog meter. If you get two readings then
the optocoupler is faulty. One of the most famous part numbers of
optoisolator is the 4N35 and 4N25.
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 you know which type of components
inside the ic then you can use the necessary testing method to
apply to the ic.
Optoisolator In Power Supply