About Optoisolator IC and The Right Way To Test It
Optoisolator IC or Optocoupler
(board location marked as “IC”) is widely used in many electronic circuits such as in the Power supplies (TV,
Monitor, Computer), Modem, Medical Equipment and etc. The Opto-isolator is simply a package that contains both
an infrared light-emitting diode (LED) and a photodetector such as a photosensitive transistor, silicone diode,
Triac, transistor Darlington pair, or silicon controlled rectifier (SCR) as seen from the photo below.
Among all the Optoisolator ICs, the LED/Phototransistor type is the most commonly used in the consumer
Optoisolator IC allows two circuits
to exchange signals yet remain electrically isolated. Let’s take the function of Optoisolator IC (part of error
signal feedback) that were found in switch mode power supplies as an example. The phototransistor acts as an
output device while the LED acts as an input device. The light generated by the LED is determined by the level
and potential of the DC error voltage applied to the LED’s by the error detection circuit. When the LED is
emitting light (inside the Optoisolator IC), the phototransistor is conducting. That means if the LED light
intensity is great, the phototransistor will conduct even more and vice versa (decreases and increases its
resistance proportionally) thus controlling the input to the oscillator in Power IC (through feedback pin as
seen from the below figure ).
The Feedback Circuit In SMPS
The end result causes the
oscillator’s frequency to change in response to the error signal feedback and alters the drive signal to
compensate for the output voltage change. Remember, this comparison/compensation occurs continually and provides
a closely regulated output voltage.
Note: The feedback circuit also provides isolation
between the cold ground (LED) side and the hot ground (phototransistor) side of the power supply. In some
designs, the error signal feedback is developed from the hot side secondary winding of the power supply and
requires no isolation as seen from figure below.
The error signal feedback is developed from the hot side secondary winding
Please observe the Power IC (IC901)
pin number 7 written as F/B which means feedback. Tracing backward from this pin, it will lead you to the
feedback winding at pin 7 of T901.
If the Optoisolator IC have
problems like an open LED or a shorted/leaky phototransistor, the power supply would blink, or produce lower
voltage or even shutdown after the power supply is turned “On”.
Beside monitoring the output
voltage sampling for regulation and provide ground isolation, the Optoisolator IC circuit in the power supply
also provide System control micro for power on/off, Over current protection and energy saving (by shutting down
the high voltage) in Monitors if the VGA signal is not connected to the CPU computer. That’s why sometimes you
could see more than one Optoisolator IC in a power supply circuit.
Why some power supply does not use Optoisolator
Its all about cost saving. The
primary sensing (hot side feedback-figure 3) is cheaper but the output regulation is less accurate. It is used
especially for the low end market (low power, low budget). Secondary sensing (the regulation circuit that
consist of Optoisolator IC, TL431 IC and some other components) is more expensive but has a higher performance.
It is used especially in the medium and high end market.
4 and 6 pins Optoisolator IC
There are 4 and 6 pins Optoisolator
IC found in the market which is the most commonly used. There are also Optoisolator ICs that have many pins. No
matter how many pins an IC have, always refer it to the datasheet so that you will know what type of
photosensitive device used and also how many components in it.
Once you know what type of
components and how many components in it then you can use the right way to test it.
Note: Optoisolator ICs can come in dual in line package or in SMD type.
Testing Optoisolator IC
Since there are so many types of
Optoisolator ICs in the market thus I could not cover all of it. I’m only showing to you how you can test the
one that is commonly used and found in the market which is the LED/Phototransistor type.
1) Set your analog meter (using
Sunwa meter) to X10 K ohm. Place your test probes on pin 1 and 2 (measuring the internal LED) and you should get
one reading when testing it either way. If you get two readings or no reading at all, then the internal LED have
problem and need to be replaced. You can use this method too to test any silicone diode.
2) From the datasheet, you would
know which pin is the base, collector and emitter. Now set your meter to X1 Ohm and place the black probe to the
base (pin 6) and the red probe to collector and then to the emitter. You should get a low resistance when the
red probe touches on the collector and emitter pin. If you do not get any reading or you get only one reading,
that means the internal phototransistor have problem. Now, set your meter to X10 K ohm and measure the collector
and emitter pin and you should get one high resistance reading. If you get one high resistance reading when
testing it either way, that means the phototransistor is good.
Here is one interesting question
that I frequently asked by my ERG member “What if there is no reading at
all when tested it both ways with the test probes-is the IC bad?”
The answer is you have to confirm
it by using another method because certain Optoisolator IC phototransistor has very high ohm of resistance thus
the meter can’t get any reading. In order to solve this problem you need to get another similar analog
Connect the two meters in series
like a battery. The black probe of one meter is connected to the red probe of another meter. You can joint the
probes with the help of an alligator clip. Both meters set to X10 K ohm range to increase the resistance that it
can measure. Now, test again the IC with the probes and it should show a high resistance reading. If by using
this method and you still cannot get any reading, that means the phototransistor already open
Assuming if you do not have a spare
meter and do not wish to invest on another one you can use a variable DC regulated power supply to check if the
Optoisolator IC is functioning well or not. Connect a 330 ohm ¼ watt resistor to pin 1 of the IC, now place the
positive supply to the other end of the resistor as shown from the photo below. The negative supply is connected
to pin 2. Next, set your analog meter to X10 K ohm, place the black probe to pin 4 (emitter) and red probe to
pin 5 (collector).
Turn on the DC power supply and
slowly increase the voltage from zero volts to few volts. For a good Optoisolator IC, you could see that the
resistance gradually increase or decrease depending on the volt setting. The higher the volt you set, the lower
is the resistance. Similarly, the lower the volt you set, the higher is the resistance. If you get an
intermittent reading or no reading with this testing method, the phototransistor is considered to have
For your information, the 4 pins
Optoisolator IC does not have the base pin but testing is the same. Just place the test probes on collector and
emitter pin and follow the above steps. If there are few transistors in a single package IC, you can test the
Note on substitution- By referring to datasheet
downloaded from the Internet; you can find equivalent part number for it. The famous 4N35 (6 pins) and PC123 (4
pins) part number can be easily used to substitute on many different types of Optoisolator IC part number. This
4N35/PC123 IC is quite common and can be easily found from any electronic shop.
Conclusion- You can directly
replace the Optoisolator IC (since it is cheap) but knowing the right way to check and measure the IC will give
you a greater satisfaction especially when the fault can be identified. About checking other types of
Optoisolator IC (such as the SCR, TRIAC, Darlington transistor photodetector type), I will leave it to you to
come out with your own way of testing. You will be happy if you could find solution to test on such Optoisolator
ICs. That’s all for now and hope to send another good article to you next month.
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