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 diagram below:

An Optocoupler 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

Ground isolation

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 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 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