electronic repair

The Right Way To Test Optoisolator

An opto isolator or sometimes refer to as optocoupler allows two circuits to exchange signals yet remain electrically isolated.
This is usually accomplished by using light to relay the signal.
The standard opto circuit uses an LED shining on a phototransistor.
The signal is applied to the LED, which then shines on the transistor. The light is proportional to the signal, so the signal is thus transferred to the phototransistor. Optoisolators may also use SCRs, photodiodes, TRIACs of other semiconductor switch as an output, and incandescent lamps, neon bulbs or other light source. In this article i will explain only the most commonly used opto isolator which is the combination of LED and phototransistor. See the diagram.

opto isolator


The optocoupler mainly found in switch mode power supply circuit. It is connected in between the primary and secondary circuit of power supply.
The purpose of using optoisolator in the circuit is to
Monitor high voltage
Output voltage sampling for regulation
System control micro for power on/off
Ground isolation


If the opto isolator breakdown, it will cause the equipment to:
-power blink
-no power
-low power
-unstable power
-power comes and shut down

Many technicians do not know that they can actually check optoisolator with their meters. Most of them believe Ic could not be measure. Since we already knew the internal layout of opto coupler, checking this ic is just the same as checking the LED and normal bipolar transistor.

To be able to accurately check optoisolator you need to use an analog multimeter. Check the LED using the times 1 ohm and times 10k ohms range. It should have one reading on both way. If you have 2 readings then the internal led have become shorted. The testing method is exactly the same when you check a normal diode. The LED mainly connected internally to pin 1 and 2 of the ic.


use for checking the phototransistor, set your meter to times 1 ohm and put your black probe to the base of the transistor and the red probe to collector and emmiter. It should show 2 readings . Then move your black probe to collector and red probe to base and emmiter. It should not have any reading. Lastly put your black probe to emmiter and the red probe to base and collector. Again it should not have any reading.

Now set your meter to times 10k ohm to measure the collector and emmiter. It should have no reading on one way and the other way should have a small reading. Which mean the meter's needle move a little bit up from the infinity scale of the meter. If you get two readings then the opto isolated is spoilt. One famous part number of optoisolator is the 4N35.

Conclusion- If you want more information about the internal layout of any optocoupler, i suggest that you check from the Philip ECG semiconductor master replacement guide . From the diagram it is more easy to describe whether it is a phototransistor, photodiode, scr or triac type. Then use the proper testing method to test them.