Optocoupler PC817 operating principle and very simple testing. Tester for checking optocouplers. Radio engineering, electronics and DIY circuits Pc 817 how to find out where the first leg is
An optocoupler is an electronic device consisting of a light source and a photodetector. The role of the light source is performed by an infrared LED with a wavelength in the range of 0.9...1.2 microns, and the receiver is performed by phototransistors, photodiodes, photothyristors, etc., connected by an optical channel and combined into one housing. The operating principle of an optocoupler is to convert an electrical signal into light, and then transmit it through an optical channel and convert it into an electrical signal. If the role of a photodetector is performed by a photoresistor, then its light resistance becomes thousands of times less than the original dark one; if it is a phototransistor, then the effect on its base creates a similar effect as when current is supplied to the base of a conventional transistor, and it opens. Typically, optocouplers and optocouplers are used for the purpose of galvanic isolation
This probe is designed to test a large number of types of optocouplers: optotransistors, optothyristors, optosimistors, optoresistors, as well as the NE555 timer chip, the domestic analogue of which is
Modified version of the probe for testing optocouplers
The signal from the third pin of the 555 microcircuit through resistor R9 is supplied to one input of the diode bridge VDS1, provided that the working emitting element of the optocoupler is connected to the Anode and Cathode contacts, in which case current will flow through the diode bridge and the HL3 LED will blink, provided that The photodetector is working properly, VT1 will open and HL3 will light up, which will conduct current, while HL4 will blink
This principle can be used to test almost any optocoupler:
The multimeter should show about 570 miles volts if the optocoupler is working in the diode continuity mode, because in this mode about 2 volts come from the tester probes, but this voltage is not enough to open the transistor, but as soon as we apply power to the LED, it will open and we will see on the display the voltage that drops across the open transistor.
The device described below will show not only the serviceability of such popular optocouplers as PC817, 4N3x, 6N135, 6N136 and 6N137, but also their response speed. The basis of the circuit is a microcontroller of the ATMEGA48 or ATMEGA88 series. The components under test can be connected and disconnected directly into the switched on device. The test result will be shown by LEDs. Thus, the ERROR element lights up when there are no connected optocouplers or their inoperability. If the element is working properly, the OK LED will light up. At the same time, one or more TIME LEDs will light up, corresponding to the response speed. So, for the slowest optocoupler, PC817, only one LED will light up - TIME PC817, corresponding to its speed. For fast 6N137s, all four LEDs will be lit. If this is not the case, then the optocoupler does not correspond to this parameter. The speed scale values of PC817 - 4N3x - 6N135 - 6N137 have a ratio of 1:10:100:900.
Microcontroller fuses for firmware: EXT =$FF, HIGH=$CD, LOW =$E2.
The printed circuit board and firmware can be downloaded from the link above.
Many of us have often had to deal with the fact that because of one failed part, the whole device stops working. To avoid misunderstandings, you should be able to quickly and correctly check details. This is what I am going to teach you. First, we need a multimeter
Bipolar transistors
Most often, transistors burn out in circuits. At least for me. It is very easy to check their functionality. To begin with, it is worth ringing the Base-Emitter and Base-Collector transitions. They must conduct current in one direction, but not allow it to flow in the opposite direction. Depending on whether the PNP is a transistor or an NPN, they will conduct current to the Base or from the Base. For convenience, we can imagine it in the form of two diodes
It is also worth ringing the Emitter-Collector transition. More precisely, these are 2 transitions. . . Well, other than that, that’s not the point. In any transistor, no current should pass through them in any direction while the transistor is off. If voltage is applied to the Base, then the current flowing through the Base-Emitter junction will open the transistor, and the resistance of the Emitter-Collector junction will sharply drop, almost to zero. Please note that the voltage drop across the transistor transitions is usually not lower than 0.6V. And prefabricated transistors (Darlingtons) have more than 1.2V. Therefore, some “Chinese” multimeters with a 1.5V battery simply cannot open them. Don’t be lazy/stingy to get yourself a multimeter with “Krona”!
Please note that some modern transistors have a diode built in parallel with the Collector-Emitter circuit. So it’s worth studying the datasheet for your transistor if the Collector-Emitter rings in one direction!
If at least one of the statements is not confirmed, then the transistor is not working. But before you replace it, check the remaining parts. Perhaps they are the reason!
Unipolar (field-effect) transistors
A working field-effect transistor should have infinite resistance between all its terminals. Moreover, the device should show infinite resistance regardless of the applied test voltage. It should be noted that there are some exceptions.
If, during testing, you apply the positive probe of the test device to the gate of an n-type transistor, and the negative probe to the source, the gate capacitance will charge and the transistor will open. When measuring the resistance between drain and source, the device will show some resistance. Inexperienced repairmen may mistake this behavior of the transistor for its malfunction. Therefore, before “testing” the drain-source channel, short-circuit all the legs of the transistor to discharge the gate capacitance. After this, the drain-source resistance should become infinite. Otherwise, the transistor is considered faulty.
Please also note that in modern high-power field-effect transistors there is a built-in diode between the drain and source, so the drain-source channel behaves like a regular diode when tested. In order to avoid annoying mistakes, remember the presence of such a diode and do not mistake it for a transistor malfunction. You can easily check this by scrolling through the datasheet for your copy.
Capacitors are another type of radio components. They also fail quite often. Electrolytic ones die most often; films and ceramics deteriorate somewhat less frequently. . .
To begin with, the boards should be examined visually. Typically, dead electrolytes swell and many even explode. Take a closer look! Ceramic capacitors do not inflate, but they can explode, which is also noticeable! They, like electrolytes, need to be called. They should not conduct current.
Before starting an electronic test of a capacitor, it is necessary to perform a mechanical check of the integrity of the internal contact of its terminals.
To do this, it is enough to bend the leads of the capacitor one by one at a slight angle, and carefully turning them in different directions, as well as slightly pulling towards yourself, to make sure that they are motionless. If at least one terminal of the capacitor rotates freely around its axis, or is freely removed from the housing, then such a capacitor is considered unsuitable and is not subject to further testing.
Another interesting fact is the charge/discharge of capacitors. This can be seen if you measure the resistance of capacitors with a capacity of more than 10 µF. It is also present in smaller containers, but it is not so noticeably expressed! As soon as we connect the probes, the resistance will be a few ohms, but within a second it will increase to infinity! If we swap the probes, the effect will repeat.
Accordingly, if a capacitor conducts current or does not charge, then it has already passed into another world.
Resistors are the most common on boards, although they do not fail very often. It’s easy to check them, just make one measurement - check the resistance.
If it is less than infinity and not equal to zero, then the resistor is most likely suitable for use. Usually, dead resistors are black - overheated! But black ones can also be alive, although they should also be replaced. After heating, their resistance could change from the nominal one, which would have a bad effect on the operation of the device! In general, it is worth ringing all the resistors, and if their resistance differs from the nominal value, then it is better to replace it. Please note that ±5% deviation from nominal is considered acceptable. . .
In my opinion, it is easiest to check diodes. We measured the resistance, with a plus at the anode, it should show several tens/hundreds of ohms. We measured it with a plus on the cathode - infinity. If not, then the diode should be replaced. . .
Inductance
Rarely, but still, inductors fail. There are two reasons for this. The first is a short circuit of turns, and the second is an open circuit. It is easy to calculate a break - just check the resistance of the coil. If it is less than infinity, then everything is OK. The resistance of inductors is usually no more than hundreds of ohms. Most often several dozen. . .
The short circuit between turns is somewhat more difficult to calculate. It is necessary to check the self-induction voltage. This only works on inductors/transformers with windings of at least 1000 turns. It is necessary to apply a low-voltage impulse to the winding, and then short-circuit this winding with a gas-discharge light bulb. In fact, loving IN. The pulse is usually applied by lightly touching the CROWN contacts. If the IN eventually blinks, then everything is fine. If not, then there is either a short circuit in the turns or very few turns. . .
As you can see, the method is not very accurate and not very convenient. So first check all the details, and only then sin on the short circuit of the turns!
Optocouplers
The optocoupler actually consists of two devices, so it is a little more difficult to test. First, you need to ring the emitting diode. It should, like a regular diode, ring in one direction and serve as a dielectric in the other. Then you need to apply power to the emitting diode and measure the resistance of the photodetector. This can be a diode, transistor, thyristor or triac, depending on the type of optocoupler. Its resistance should be close to zero.
Then we remove the power from the emitting diode. If the resistance of the photodetector has increased to infinity, then the optocoupler is intact. If something is wrong, then it should be replaced!
Thyristors
Another important key element is the thyristor. He also likes to get out of order. Thyristors are also symmetrical. They're called triacs! It's easy to check both.
We take an ohmmeter, connect the positive probe to the anode, and the negative probe to the cathode. The resistance is infinity. Then we connect the control electrode (CE) to the anode. The resistance drops to about a hundred ohms. Then we disconnect the UE from the anode. In theory, the thyristor resistance should remain low - the holding current.
But keep in mind that some “Chinese” multimeters can produce too little current, so if the thyristor is closed, it’s okay! If it is still open, then remove the probe from the cathode, and after a couple of seconds attach it back. Now the thyristor/triac should definitely close. Resistance is infinity!
If some theses do not coincide with reality, then your thyristor/triac is not working.
A zener diode is actually a type of diode. This is why it is checked in the same way. Note that the voltage drop across the zener diode, with a plus at the cathode, is equal to its stabilization voltage - it conducts in the opposite direction, but with a larger drop. To check this, we take a power supply, a zener diode and a 300...500 Ohm resistor. We turn them on as in the picture below and measure the voltage on the zener diode.
We gradually increase the voltage of the power supply, and at some point, the voltage on the zener diode stops increasing. We have reached its voltage stabilization. If this does not happen, then either the zener diode is not working, or the voltage needs to be increased further. If you know its stabilization voltage, then add 3 volts to it and apply it. Then increase it and if the zener diode does not begin to stabilize, then you can be sure that it is faulty!
Stabilizers
Stabilizers are one of the types of zener diodes. Their only difference is that when connected directly - with a plus on the anode, the voltage drop across the stabistor is equal to its stabilization voltage, and in the other direction, with a plus on the cathode, they do not conduct current at all. This is achieved by connecting several diode crystals in series.
Please note that a multimeter with a supply voltage of 1.5V will not physically be able to set the stabilist to, say, 1.9V. Therefore, we turn on our stabistor as in the picture below and measure the voltage on it. You need to apply a voltage of about 5V. Take the resistor with a resistance of 200...500 Ohms. We increase the voltage by measuring the voltage on the stabistor.
If at some point it stopped growing, or began to grow very slowly, then this is its stabilization voltage. He is a worker! If it conducts current in both directions, or has an extremely low voltage drop in direct connection, then it is worth replacing. Apparently it burned down!
Checking various types of cables, adapters, connectors, etc. is quite simple. To do this you need to call your contacts. In a loop, each contact must communicate with one contact on the other side. If the contact does not ring with any other contact, then there is a break in the loop. If it rings with several, then most likely there is a short circuit. The same goes for adapters and connectors. Those with a break or short circuit are considered defective and cannot be used!
Microcircuits/ICs
There are a great variety of them, they have many pins and perform different functions. Therefore, checking the microcircuit must take into account its functional purpose. It is quite difficult to accurately verify the integrity of the microcircuits. Inside, each represents tens to hundreds of transistors, diodes, resistors, etc. There are hybrids in which there are more than 200,000,000 transistors alone.
One thing is for sure - if you see external damage to the case, spots from overheating, cavities and cracks on the case, loose leads, then the microcircuit should be replaced - it is most likely damaged in the crystal. A heating microcircuit, the purpose of which does not involve heating it, must also be replaced.
A complete check of the microcircuits can only be carried out in a device where it is connected as it should be. This device can be either equipment being repaired or a special test board. When checking microcircuits, the typical inclusion data available in the specification for a specific microcircuit is used.
Well, that's it, no more fluff for you, and fewer burnt parts!
Using the proposed probe, you can check NE555 (1006VI1) microcircuits and various optodevices: optotransistors, optothyristors, optosimistors, optoresistors. And it is with these radioelements that simple methods do not work, since simply ringing such a part will not work. But in the simplest case, you can test the optocoupler using the following technology:
Using a digital multimeter:
Here 570 is the millivolts that drop at the open junction of the optotransistor. In the diode continuity mode, the drop voltage is measured. In the “diode” mode, the multimeter outputs a pulse voltage of 2 volts, rectangular in shape, to the probes through an additional resistor, and when the P-N junction is connected, the ADC of the multimeter measures the voltage dropping across it.
Optocoupler and IC tester 555
We advise you to spend a little time and make this tester, since optocouplers are increasingly used in various amateur radio designs. And I’m generally silent about the famous KR1006VI1 - they install it almost everywhere. Actually, the 555 chip under test contains a pulse generator, the functionality of which is indicated by the blinking of LEDs HL1, HL2. Next comes the optocoupler probe.
It works like this. The signal from the 3rd leg 555 through resistor R9 reaches one input of the diode bridge VDS1, if a working emitting element of the optocoupler is connected to contacts A (anode) and K (cathode), then current will flow through the bridge, causing the HL3 LED to blink. If the receiving element of the optocoupler is also working, then it will conduct current to the base of VT1, opening it at the moment of ignition of HL3, which will conduct current and HL4 will also blink.
P.S. Some 555s do not start with a capacitor in the fifth leg, but this does not mean they are faulty, so if HL1, HL2 do not blink, short-circuit c2, but if even after that the indicated LEDs do not blink, then the NE555 chip is definitely faulty. Good luck. Sincerely, Andrey Zhdanov (Master665).
A simple way to test optocouplers was needed. I don’t often “communicate” with them, but there are times when I need to determine whether the optocoupler is to blame?.. For these purposes I made a very simple probe. "Construction of the Weekend Hour."
Probe appearance: 
The circuit diagram of this probe is very simple: 
Theory:
Optocouplers (optocouplers) are installed in almost every switching power supply for galvanic isolation of the feedback circuit. The optocoupler contains a conventional LED and a phototransistor. To put it simply, this is a kind of low-power electronic relay with short-circuit contacts.
Operating principle of the optocoupler: When electric current passes through the built-in LED, the LED (in the optocoupler) begins to glow, the light hits the built-in phototransistor and opens it.
Optocouplers are often available in Dip package
The first leg of the microcircuit, according to the standard, is designated by a key, a dot on the body of the microcircuit, which is also the anode of the LED, then the numbers of the legs go along the circumference, counterclockwise.
The essence of the test: Phototransistor, when light from the 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 disadvantage of this probe is that to test it is necessary to unsolder the optocoupler and install it in the holder according to the key (my role as a reminder is the test button - it is shifted to the side, and the optocoupler key must face the button).
Next, when you press the button (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 that the phototransistor is working (the circuit is not broken). 
(I only had a DIP-6 holder and had to fill the unused contacts with hot glue.)
For final testing, it is necessary to turn the optocoupler “off 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 this probe as a first one for beginning radio amateurs who need to check optocouplers every six months or a year)
There are also more modern circuits with logic and signaling of “out of 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 removed from boards.
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