How to check the health of the optocoupler. Optocoupler PC817 working principle and very simple test
Instruction
If the optocoupler, the serviceability of which is set to, is soldered to the board, it is necessary to turn it off, discharge electrolytic capacitors on it, and then unsolder the optocoupler, remembering how it was soldered.
Optocouplers have different emitters (incandescent lamps, neon lamps, LEDs, light-emitting capacitors) and different radiation receivers (photoresistors, photodiodes, phototransistors, photothyristors, photosimistors). Also they are pinned. Therefore, it is necessary to find data on the type and pinout of the optocoupler either in the reference book or datasheet, or in the circuit of the device where it was installed. Often, the pinout of the optocoupler is applied directly to the board of this device. If the device is modern, you can almost certainly be sure that the emitter in it is an LED.
If the radiation receiver is a photodiode, connect the optocoupler element to it, turn it on, observing the polarity, in a chain consisting of a source constant voltage a few volts, a resistor designed so that the current through the radiation receiver does not exceed the allowable one, and a multimeter operating in the current measurement mode at the appropriate limit.
Now enter the emitter of the optocoupler into operating mode. To turn on the LED, pass through it in direct polarity. D.C., equal to the nominal. Apply rated voltage to the incandescent lamp. Carefully connect a neon lamp or a light-emitting capacitor to the network through a resistor with a resistance of 500 kΩ to 1 MΩ and a power of at least 0.5 W.
The photodetector must respond to the inclusion of the emitter by a sharp change in mode. Now try switching the emitter off and on several times. The photothyristor and photoresistor will remain open even after the control action is removed until their power is turned off. Other types of photodetectors will respond to each change in the control signal. If the optocoupler has an open optical channel, make sure that the reaction of the radiation detector changes when this channel is blocked.
Having made a conclusion about the state of the optocoupler, de-energize the experimental setup and disassemble it. After that, solder the optocoupler back to the board or replace it with another one. Continue repairing the device that includes the optocoupler.
An optocoupler or optocoupler consists of an emitter and a photodetector separated from each other by a layer of air or a transparent insulating substance. They are not electrically interconnected, which makes it possible to use the device for galvanic isolation of circuits.
Instruction
Connect the measuring circuit to the photodetector of the optocoupler in accordance with its type. If the receiver is a photoresistor, use an ordinary ohmmeter, and the polarity is unimportant. When using a photodiode as a receiver, connect a microammeter without a power source (positive to the anode). If the signal is received by an n-p-n phototransistor, connect a circuit of a 2 kilo-ohm resistor, a 3-volt battery and a milliammeter, and connect the battery with a plus to the collector of the transistor. If the phototransistor has p-n-p structure, reverse the polarity of the battery connection. To check the photodinistor, make a circuit from a 3 V battery and a 6 V, 20 mA light bulb, connecting it with a plus to the anode of the dinistor.
In most optocouplers, the emitter is an LED or an incandescent bulb. Apply the rated voltage to an incandescent bulb in either polarity. You can also apply an alternating voltage, the effective value of which is equal to the operating voltage of the lamp. If the emitter is an LED, apply a voltage of 3 V to it through a 1 kΩ resistor (positive to the anode).
I needed a simple way to test optocouplers. I don’t often “communicate” with them, but there are times when it is necessary to determine whether the optocoupler is to blame? .. For these purposes, I made a very simple probe. Weekend Design.
Appearance probe: 
The scheme of this probe is very simple: 
Theory:
Optocouplers (optocouplers) are in almost every switching power supply for galvanic isolation of the circuit feedback. The optocoupler contains a conventional LED and a phototransistor. Simply put, this is a kind of low-power electronic relay, with contacts for short circuit.
The principle of operation of the optocoupler: When through the built-in LED passes electricity, the LED (in the optocoupler) starts to glow, the light hits the built-in phototransistor and opens it.
Optocouplers often come in Dip packages
The first leg of the microcircuit, according to the standard, is indicated by a key, a dot on the microcircuit case, it is also the anode of the LED, then the numbers of the legs go in a circle, counterclockwise.
The essence of the test: Phototransistor, when light from an internal LED hits it,
goes into an open state, and its resistance will decrease sharply (from a very high resistance, to about 30-50 ohms.).
Practice:
The only drawback of this probe is that for testing it is necessary to unsolder the optocoupler and install it in the holder according to the key (I have the test button as a reminder - it is shifted to the side, and the optocoupler key should look at the button).
Further, when the button is pressed (if the optocoupler is intact), both LEDs will light up: the right one will signal that the optocoupler LED is working (the circuit is not broken), and the left one will signal the phototransistor is working (the circuit is not broken). 
(I only had a DIP-6 holder and had to fill in unused contacts with hot melt adhesive.)
For final testing, it is necessary to turn the optocoupler “not by key” and check it in this form - both LEDs should not light up. If both or one of them are on, then this tells us about a short circuit in the optocoupler.
I recommend such a probe as the first one for beginner radio amateurs who need to check optocouplers every six months, a year)
There are more modern schemes with logic and signaling about "exiting the parameters", but these are needed for a very narrow circle of people.
I advise you to look in your “bins”, it will be cheaper, and you won’t waste time waiting for delivery. Can be soldered from boards.
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Do-it-yourself opto-relay test device
The other day I needed to check the opto-relay in large quantities. By assembling this solid state relay tester in half an hour, from a minimum of parts, I saved a large number of time to check optocouplers.
Many novice radio amateurs are interested in how to check an optocoupler. Such a question may arise from ignorance of the device of this radio component. If we consider the surface, then a solid-state optoelectronic relay consists of an input element - an LED and an optical isolation that switches the circuit.
This circuit for checking an optocoupler is simple to the elementary. It consists of two LEDs and a 3V power supply - a CR2025 battery. The red LED acts as a voltage limiter and, at the same time, is an indicator of the operation of the optocoupler LED. The green LED is used to indicate the operation of the optocoupler output element. Those. if both LEDs are lit, the optocoupler test was successful.
The process of checking the opto-relay comes down to installing it in the corresponding part of the socket. This solid state relay tester can test DIP-4, DIP-6 optocouplers and DIP-8 dual relays.
Below I give the positions of the opto-relay in the sockets of the tester and the glow of the LEDs corresponding to their performance.
Description, characteristics, datasheet and methods for testing optocouplers using the example of PC817.
In continuation of the topic “Popular radio components for repairs of switching power supplies”, we will analyze one more detail - optocoupler (optocoupler) PC817. It consists of an LED and a phototransistor. They are not electrically connected to each other, due to which, on the basis of PC817 it is possible to implement galvanic isolation of two parts of the circuit - for example, with high voltage and low. The opening of the phototransistor depends on the illumination of the LED. I will analyze how this happens in more detail in the next article, where in experiments, by applying signals from the generator and analyzing it with an oscilloscope, you can understand a more accurate picture of the operation of the optocoupler.
In other articles, I will talk about the non-standard use of an optocoupler, the first in the role, and in the second. And using these circuit solutions I will assemble a very simple optocoupler tester. Which does not need any expensive and rare devices, but only a few cheap radio components.
The item is not rare and not expensive. But a lot depends on it. It is used in almost every running (I don’t mean any exclusive) switching power supply and acts as a feedback and most often in conjunction with the very popular radio component TL431
For those readers who find it easier to perceive information by ear, we advise you to watch the video at the very bottom of the page.
Optocoupler (Optocoupler) PC817
Brief characteristics:
Housing compact:
- pin pitch - 2.54 mm;
- between rows - 7.62 mm.
The manufacturer of PC817 is Sharp, there are other manufacturers of electronic components that produce analogues, for example:
- Siemens-SFH618
- Toshiba-TLP521-1
- NEC-PC2501-1
- LITEON-LTV817
- Cosmo-KP1010
In addition to the single optocoupler PC817, other options are available:
- PC827 - dual;
- PC837 - built;
- PC847 - quad.
Optocoupler test
For quick check optocouplers, I conducted several test experiments. First on the breadboard.
Breadboard option
As a result, we were able to get very a simple circuit to test PC817 and other similar optocouplers.
The first version of the scheme
I rejected the first option for the reason that it inverted the transistor marking from n-p-n to p-n-p
Therefore, in order to avoid confusion, I changed the scheme to the following;
The second version of the scheme
The second option worked correctly, but it was inconvenient to unsolder the standard socket
under the microcircuit
Socket SCS-8
The third version of the scheme
The most successful
Uf is the voltage on the LED at which the phototransistor starts to open.
in my version Uf = 1.12 volts. 
The result is a very simple design.
LCD TVs, in a small private workshop. This topic is quite cost-effective, and if you deal mainly with power supplies and inverters, it is not too complicated. As you know, an LCD TV, like almost all modern electronic equipment, is powered by a switching power supply. The latter, however, contains a part called . This part is designed for galvanic isolation of circuits, which is often necessary for safety reasons for the operation of the device circuit. This part contains a conventional LED and a phototransistor. How does an optocoupler work? Simply put, this can be described as something like a kind of low-power, with contacts on the circuit. The following is a diagram of an optocoupler:
Optocoupler circuit
And here is the same thing, but from the official datasheet page:
Optocoupler Pinout
Below is the information from the datasheet, in a more complete version:
Optocoupler housing
Optocouplers often come in Dip packages, at least those used in impulse blocks power, and have 4 legs.
Optocoupler in the photo
The first leg of the microcircuit, according to the standard, is indicated by a key, a dot on the microcircuit case, it is also the anode of the LED, then the numbers of the legs go in a circle, counterclockwise.
Optocoupler test
How can I check the optocoupler? For example, as in the following diagram:
Optocoupler test circuit
What is the purpose of such a check? Our phototransistor, when light from the internal LED hits it, will immediately go into the open state, and its resistance will sharply decrease, from a very high resistance, to 40-60 Ohms. Since I need to test these microcircuits, optocouplers regularly, I decided to remember that I am not only an electronics engineer, but also a radio amateur), and collect some kind of probe to quickly check the optocoupler. I went through the diagrams on the Internet, and found the following:
The circuit, of course, is very simple, the red LED indicates the operability of the internal LED, and the green one indicates the integrity of the phototransistor. A search for ready-made devices assembled by radio amateurs, gave out a photo of simple probes like this one:
Device for checking optocoupler from the Internet
This is all very good, of course, but dismantling the optocoupler each time and then soldering it back is not our method :-). A device was required for a convenient and quick check of the optocoupler’s performance, without soldering, plus, at the same time, he also swung at the sound and visual indication :-).
Sound probe - circuit
I previously assembled a simple sound probe according to this scheme, with sound and visual indication, powered by one and a half volts, AA batteries.
Simple sound probe
I decided that this was what I needed, a ready-made semi-finished product right away), opened the case, was horrified by my semi-mounted installation), from the time of the first years, I studied radio engineering. Then he made a board, by cutting grooves in a foil-coated textolite, with a cutter. Please do not be afraid), looking at this collective farm.
Internals and details
It was decided to go by making an analog, a kind of tweezers, for a quick check of the optocoupler, with one touch. Two small strips were cut out of textolite, and in the middle of them, a groove was drawn with a cutter.
PCB contact plates
Then a compression mechanism was needed, with a spring. An old headset from the phone, or rather a clip, for attaching to clothes, from it, was used.
Headset clip
It was just a matter of soldering the wires. and fix the plates on the clip with hot glue. It turned out collectively again, as without it), but surprisingly strong.
Tweezers for measuring homemade
The wires were taken from the connection connectors to motherboard, case buttons system block, and indication LEDs. The only caveat, in the diagram I have a ground on one of the probes from the multimeter connected to the probe, make its contact, if you repeat, be sure to opposite the optocoupler LED power ground, in order to avoid a very fast battery discharge, when the power supply is shorted, to the battery minus . The pinout diagram of the tweezers, I think it would be superfluous to draw, everything is clear and so without difficulty.
The final view of the optocoupler probe
This is how the finished device looks like, moreover, it has retained its functionality of a sound probe, by connecting through standard sockets, probes from a multimeter. The first tests showed that 40 ohm in the open state of the phototransistor between the terminals of the emitter - collector, for such a probe, is somewhat too much. The sound of the probe was muffled, and the LED did not shine very brightly. Although to indicate the performance of the optocoupler, this was already enough. But we are not used to half measures). At one time, I assembled an extended version, the circuits of this sound probe, where measurement is provided with a resistance between the probes, up to 650 Ohms. Below is a diagram of the extended version:
Scheme 2 - sound probe
This circuit differs from the original, only by the presence of another transistor, and a resistor in its base circuit. The printed circuit board of the extended version of the probe is shown in the figure below, it will be attached in the archive.
Printed circuit board to the sound probe
This probe showed itself during testing, quite convenient to use, even in its current version, after an upgrade the other day, the drawback with a quiet sound and a dim glow of the LED will certainly be eliminated. Good luck with your repairs! AKV.
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