How to test an optocoupler with a multimeter without desoldering. How to check an optocoupler

How to check an optocoupler with a multimeter?

You won’t be able to simply check the optocoupler with a multimeter. For the simplest test of an optocoupler, you need to apply voltage to its input (according to the diagram), and check the output with a multimeter in diode test mode.

Check procedure

The test is carried out with a conventional tester, switching the device to the range for measuring diodes or resistance.

Connecting a multimeter for testing

How to test a resistor with a multimeter

Optocoupler tester

Radio amateur forums often contain the opinion that since the element is inexpensive, then why is a pc817 test needed? It is enough just to change it on time.

In fact, everything is not quite like that. You need to understand whether the optocoupler has burned out or not in order to conclude whether anything else has been damaged. It happens that new optocouplers burn out because they have a manufacturing defect.

How to test pc817? To do this, ring the light diode using a tester. First, find out if there is a short circuit in the transistor. Afterwards, pass current through the light diode and make sure the transistor opens.

You can create a simple device for testing optocouplers at home. For this you will need:

LEDs - 2 pieces.
Buttons - 2 pieces
Resistors - 2 pieces.

Light diodes must correspond to a current strength of 5 to 20 mA and a voltage of approximately 2 V. In this case, the two resistors should have a resistance of around 300 V.

The tester's power source is a USB port with a voltage of 5 V. But you can also use 3-4 2A batteries. 9-12 V batteries or a power source with the same voltage will also work. Only here you will have to recalculate the resistances of the two resistors.

Now let's look at how an optocoupler works, based on various experiments.

Optocoupler testing device

To more conveniently check the optocoupler, you can use a more interesting circuit. It includes a minimum of components, and assembly takes no more than half an hour.

The optocoupler is powered through an LED, which will light up if the photoemitter is working. The second LED will light up if the photodetector through which current flows to the LED is working.

For clarity, the second version of the diagram was assembled from elements that were at hand. The role of the test subject is played by the optocoupler PC817.

The role of the socket for connecting the optocoupler is performed by the remains of the COM cable. But it is better to use sockets for microcircuits for such purposes, then connecting the optocoupler will become more convenient.

The circuit is powered using an old USB cord. In general, the circuit works properly right away and does not require additional adjustment. If both LEDs are lit, then the optocoupler can be considered working.

Many people will have a question: if the output of the optocoupler is broken, then both LEDs will also light up! In this case, the brightness of the second LED will be much higher, it will be visually very visible.

Checking the optocoupler

To quickly test the optocoupler, I conducted several test experiments. First on the breadboard.

Option on breadboard

As a result, we were able to obtain a very simple circuit for testing the PC817 and other similar optocouplers.

First version of the scheme

I rejected the first option for the reason that it inverted the transistor marking from npn to pnp

Therefore, to avoid confusion, I changed the diagram to the following;

Second version of the scheme

Optocoupler P817

The second option worked correctly, but it was inconvenient to solder the standard socket

SCS-8

for a microcircuit

Panel SCS-8

Third version of the scheme

The most successful

Uf is the voltage on the LED at which the phototransistor begins to open.

in my version Uf = 1.12 Volts.

The result is a very simple design:

Photodiode, phototransistor, photothyristor, optocoupler, types of indicators

View from above

Bottom view

As you can see from the photo, the part is not rotated according to the key.

Using which you can very quickly check the part. In my practice of repairs, of course, not often, but I have come across non-working optocouplers and before I had to bother checking the part when sometimes I came to a dead end during a complex repair.

The final option is very simple.

Checking with a tester

Since the zener diode and the diode have almost the same current-voltage characteristics with the exception of the breakdown section, the zener diode is checked with a multimeter, just like the diode.

The test is carried out with any multimeter in the mode of diode ringing or resistance determination. The following actions are performed:

the switch sets the Ohm measurement range;
measuring probes are connected to the terminals of the radio component;
the multimeter should show units or fractions of ohms if its internal power supply is connected positive to the anode;
By swapping the probes, we change the polarity of the voltage at the terminals of the semiconductor and get a resistance close to infinity, if it is working properly.

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To make sure that the zener diode is working properly, switch the multimeter to the resistance measurement range in kilo-ohms and take a measurement.

With a working device, the readings should be in the range of tens and hundreds of thousands of ohms. That is, it passes current like a regular diode.

PC817 connection diagram

It is common, as for all transistor optocouplers. The input current must be limited. You can use a resistor for this. The output current must also not be exceeded.

Pc817 connection circuit 372 can be seen in the figure:

PC817 specifications in Russian

PC817 LED parameters:

Forward current - 50 mA.
The maximum forward current is 1 A.
The voltage in the opposite direction is 6 V.
Dissipated power - 70 MW.

Parameters of phototransistor PC817

Its parameters are as follows:

The voltage between collector and emitter is 35 V.
Emitter-collector - 6 V.
Collector current - 50 mA.
Dissipated collector power - 150 mW.

We must not forget about one more important parameter. It is called the current transfer ratio, CTR. Its unit of measurement is %. In the optocoupler designation in the pc817 datasheet, it corresponds to the letter following the main code, as in other pc817 optocouplers and semiconductors.

How to test a bipolar transistor with a multimeter

Testing a bipolar transistor with a multimeter allows you to identify a faulty component or determine the location of the terminals (collector K, emitter E and base B). To know how to check the functionality, it is necessary to imagine an analogue of the transistor circuit in the form of two back-to-back (pnp) or reverse (npn) diodes connected with a middle point, which is equivalent to the base terminal. And the remaining two are identical to the emitter and collector pins. For direct conduction transistors, the cathodes are connected to the base (“sticks” according to the diagram), and the anodes (“arrows”) are connected to the reverse conduction. When you connect a red diode (positive wire) to the anode and a black diode to the cathode, the tester will show some value on the indicator. If it is very small, it means that the diode being measured is broken. And if it’s very large, then the diode is broken.

Normal values for the resistance of the emitter or collector junction lie in the range of 0.4 - 1.6 kOhm, depending on the specific transistor. By connecting the transistor terminals in pairs with the multimeter probes, pairs of terminals “B-E” and “B-K” are determined. The K-E transition resistance is always very high. If the pair is not found or the resistance of the collector-emitter junction is small, then the transistor is faulty. It is worth considering that the collector resistance in relation to the base is always less than the resistance of the B-E junction, which will help determine the pinout of a serviceable part.

The above is true both when checking a direct conduction transistor and an npn structure transistor. In the latter case, measurements are carried out with the tester wires connected in reverse polarity.

Tsokolevka

With the PC817 pinout, everything is more or less clear. The device is housed in a 4-pin DIP package. It uses both surface and hole mounting.

There is a depressed dot on one of the contacts that points towards the anode of the light diode from the inside. The legs are numbered clockwise. The second one is the cathode. Pins number 3 and 4 are the emitter and collector.

Modern versions of the device have been tested with good results in accordance with international standards. They are safe to use and difficult to ignite, as stated in the pc817 datasheet.

How to test a field effect transistor

For field-effect transistors, the terminals are called drain (C), source (I) and gate (G). Despite the fact that the physics of operation differs from bipolar, when checking for serviceability, you can also use the diode equivalent of the circuit.

The p-type field effect transistor test circuit is similar to the pnp test. Before checking, it is necessary to connect all the terminals to discharge the junction capacitances. The resistance when connecting probes to pairs of terminals “C, Z” and “I, Z” should be shown only in one of the directions. We connect the black probe to the “C” terminal, and the red one to the “I” terminal. The value of the shown resistance (400-700 Ohms) must be remembered. After this, we connect the red wire to the gate for a second, thereby opening the transition. After this, we measure the transition resistance. Its decrease indicates that the transistor has partially opened. Now we also connect the black wire to terminal “Z” and close the transition. Restoring the initial value of the transition resistance indicates the serviceability of the radio component. The difference between testing an n-type field device is only in changing the polarity of connecting the probes of the device.

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When testing insulated gate field effect transistors, the absence of conductivity between the gate and source is checked. Then we combine the source with the gate. Bilateral conductivity will appear in a depletion type transistor. For enriched type parts the conductivity will be one-sided.

How to make simple optocoupler devices yourself

You can find the pc817 optocoupler in a phone charger or computer power supply, so getting one is not at all difficult. Based on it, a simple LED flasher with a stroboscopic effect is assembled.

You need to have with you:

4.2V power supply part.
Light diode with any color.
Resistors with resistances of 5.6 and 1 kOhm, respectively.
Optocoupler based on transistor pc817.
Capacitor with a capacity of 220 μF and a voltage of 10 V.

The first step is to consider the optocoupler itself. It includes 2 parts that are connected by optical communication. In other words, when voltage is applied to the light diode, the internal transistor opens.

Keep in mind that the point is the first reference contact. Inside the element itself, these contacts are 4. 1 and 2 refer to the input through which the internal light diode is connected. And 3 and 4 are the output.

Using this simplest radio element, an elementary generator with repeating pulses is created. There is no need to configure the circuit and install fully serviceable elements there. Assembly is done by surface mounting without using a board.

Clamp the optocoupler with a clamp and solder 2 resistors.
Then solder the light diode. Keep in mind, it turns on in polarity.
After this, solder the capacitor.
The next stage is the creation of connecting tracks from tinned wires.
Solder the contacts of the part for power supply.
If the flasher begins to perform its function (that is, blink), then it is working properly.
The capacitance of the capacitor is needed directly to regulate the blinking frequency.
If you have problems starting, check that the polarity of all parts is correct. The exception is resistors.

This simple scheme can be applied in different areas.

We all know that in most industrial devices it is very important to detect the isolated mains voltage in time. With the help of insulation, here it is necessary to prevent the flow of current (direct or alternating) from the first half of the structure to the other, and also to ensure the transmission of signal and power.

Thanks to insulation, you can isolate potential differences, make the device resistant to interference and protect it from strong voltage surges. Typically, DC or AC optocouplers are needed to detect voltage. They need to be installed on the path where the signal passes.

Under no circumstances should you touch the board when the device is already turned on. This results in electric shocks.

How to build an AC power detector

The PC817B optocoupler contains an infrared light diode that is coupled optically to a photographic transistor. That, in turn, is placed in a dip package with 4 contacts. The standard insulation voltage at input-output is 5 kV, at collector-emitter - up to 80 V, CTR can be up to 600% at an input current of 5 mA.

The circuit does not require an AC transformer. To reduce the voltage, a series capacitor is used, which is connected directly to a network with a voltage of 220 V. A diode is used to rectify the alternating current voltage, and the final voltage at direct current is corrected by the capacitor.

The zener diode is a pre-regulator to completely protect the circuit. If there is a break on the capacitor, for example, if a resistor accidentally burns out, the capacitor voltage does not exceed 5 V. Therefore, the filter capacitor cannot explode.

The resulting input causes a low output signal during a suitable interface with an external structure where there is a pull-up resistor. If the power is turned off, a high-level output signal appears.

It is possible to create an improved version of such a network detector, which will be adjusted to digital technology. It is clear that the most basic and safe way to determine electricity in the network is using a microcontroller. You can't do without an optocoupler here. To safely connect PC817 such a high voltage (220 V) to an optocoupler, a current limit is needed. Due to its magnitude, the resistor power rating must be taken into account.

For a smooth, stable output of a constant current, for example, if we are talking about a GPIO microcontroller, a slight modification of the circuit is needed. The capacitance of the capacitor does not matter much here. It can range from 2 to 10 µF.

Application of 2-directional optocoupler

There is another current option - the use of a 2-directional optocoupler. In another way it is called an AC optocoupler. It includes a pair of internal light diodes. They are directed in the opposite direction. One such model is H11AA1.

Thanks to the design of the conceived universal detector, monitoring a high-voltage signal becomes easier. It helps to ensure the formation of a digital output signal with electroplating. There are no expensive elements in the scheme. It can be assembled within an hour.

The project includes 2 important fragments. One of them processes the high-voltage input, the second isolates the low-voltage section from the high-voltage one. And to enhance circuit protection, you cannot do without a fuse and a metal oxide varistor.

The variator is based on metal oxide. It is a resistor that depends on voltage. It is unique and protects circuits from excess voltage. Thanks to him, fluctuations in this indicator are reduced.

Under normal conditions, the varistor has a high resistance, but when the connected voltage increases, compared to the limit of the variator, it immediately decreases. A varistor can be easily connected between phase and zero, but only after the fuse. Then, if the varistor short circuits, the device will be disconnected from the network due to the fuse.

It is possible to use a pull-up resistor for microcontrollers that do not have this element inside. Moreover, using a two-pin jumper, the correction capacitor is turned on or off, if necessary.

The final unsmoothed output signal is not perfectly smooth, but its fluctuations are no more than 500 mV. The input to this optocoupler is connected to the mains voltage, which is processed by a potential capacitance divider circuit. The highest possible switching voltage of the optocoupler is 30 V, and the transistor that is connected to the output of the optocoupler can withstand a current of up to 10 mA.

One example of using a sensor is when it is a reset circuit at the moment it is connected to the network. The second option is an emergency current system, a microcontroller alarm, or a power failure/resume identifier circuit.

Checking a composite transistor with a multimeter

How to test a Darlington transistor? You can check a compound transistor in the same way as a bipolar one, using a digital multimeter with continuity testing of transistors in diode testing mode. The only difference is that the direct voltage to the pair of terminals B-E should be 1.2-1.4 volts. If the existing device cannot provide this, testing is impossible. And then it is better to use a simple probe using a 12 V battery, a 22 kOhm resistor included in the base and a 5 W car light bulb. Next, we connect the “minus” of the source to the emitter, and connect the collector to the lamp. We connect the second terminal of the lamp to the “plus” of the battery. If you connect a resistor to the positive terminal, the light will light up. Now we switch the resistor to “plus” - the light goes out. This means that the transistor being tested is working.

How to check a transistor without unsoldering it from the installation

You can check the transistor with a multimeter after checking the circuit to identify possible short-circuiting of the terminals of the element being tested with low-resistance resistors. If any are found, the part will have to be removed to check. If not, the check is performed using the methods described above, but the reliability of the testing will be low. Sometimes desoldering the base output is enough.

It is better to check field-effect transistors separately from the board. They are very sensitive to static electricity, so it is necessary to use an antistatic wrist strap.

An even easier way to test the PC817 optocoupler

It is clear that using a Chinese tester to test an optocoupler is not the simplest, or rather simple but not the cheapest method. Not everyone has such a device on their household.

Therefore, I bring to your attention a simpler, and most importantly, cheaper optocoupler tester.

It consists of two buttons, two resistors, an LED and a socket for a microcircuit.

Description, characteristics, Datasheet and methods for testing optocouplers using the example of PC817.

Continuing the topic “Popular radio components for repairs of switching power supplies,” we will analyze one more part - optocoupler (optocoupler) PC817. It consists of an LED and a phototransistor. They are not electrically connected to each other, thanks to which, based on the PC817, it is possible to implement galvanic isolation of two parts of the circuit - for example, with high voltage and low voltage. The opening of the phototransistor depends on the illumination of the LED. I will discuss how this happens in more detail in the next article, where in experiments, by feeding signals from the generator and analyzing it with an oscilloscope, you can understand a more accurate picture of the operation of the optocoupler.

Scope of application of the device

They are used in a variety of fields:

As elements of galvanic isolation, optocouplers are used: to connect equipment units between which there is a significant potential difference; to protect the input circuits of measuring devices from interference and interference.
Another important area of application for optocouplers is optical, non-contact control of high-current and high-voltage circuits. Launch of powerful thyristors, triacs, control of electromechanical relay devices. Switching power supplies.
The creation of “long” optocouplers (devices with an extended flexible fiber-optic light guide) opened up a completely new direction for the use of optocoupler products – communication over short distances.
Various optocouplers are also used in radio engineering circuits for modulation, automatic gain control and others.
Impact through the optical channel is used here to bring the circuit to the optimal operating mode, for contactless mode adjustment.
The ability to change the properties of the optical channel under various external influences on it makes it possible to create a whole series of optocoupler sensors: these are sensors for humidity and gas contamination, sensors for the presence of a particular liquid in the volume, sensors for the cleanliness of the surface treatment of an object, and the speed of its movement.
The versatility of optocouplers as elements of galvanic isolation and contactless control, the diversity and uniqueness of many other functions are the reason that the scope of optocoupler applications has become computer technology, automation, communications and radio equipment, automated control systems, measuring equipment, control and regulation systems, medical electronics, devices for visual display of information.

View of the optocoupler from different sides.

Advantages of optocouplers

the ability to provide galvanic isolation between input and output;
for optocouplers there are no fundamental physical or design restrictions on achieving arbitrarily high voltages and decoupling resistances and arbitrarily small throughput capacitance;
the possibility of implementing contactless optical control of electronic objects and the resulting diversity and flexibility of design solutions for control circuits;
unidirectional propagation of information along the optical channel, absence of feedback from the receiver to the emitter;
wide frequency bandwidth of the optocoupler, no limitation from low frequencies;
the possibility of transmitting both a pulse signal and a constant component via an optocoupler circuit;
the ability to control the output signal of the optocoupler by influencing the material of the optical channel and the resulting possibility of creating a variety of sensors, as well as a variety of devices for transmitting information;
the possibility of creating functional microelectronic devices with photodetectors, the characteristics of which, when illuminated, change according to a complex given law;
the immunity of optical communication channels to the effects of electromagnetic fields, which makes them protected from interference and information leakage, and also eliminates mutual interference;
physical, design and technological compatibility with other semiconductor and radio-electronic devices.

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Disadvantages of optocouplers

significant power consumption due to the need for double energy conversion (electricity - light - electricity) and the low efficiency of these transitions;
increased sensitivity of parameters and characteristics to the effects of elevated temperature and penetrating radiation;
temporary degradation of optocoupler parameters;
a relatively high level of self-noise, due, like the two previous disadvantages, to the peculiarities of the physics of LEDs;
the complexity of implementing feedback caused by electrical isolation of the input and output circuits;
design and technological imperfection associated with the use of hybrid non-planar technology, with the need to combine several individual crystals from different semiconductors located in different planes in one device.

Optocoupler structure

Classification of types of optocouplers

There are several characteristics according to which optocoupler models can be divided into several groups.

Depending on the degree of integration:

elementary optocoupler - includes 2 or more elements united by a common housing;
optocoupler integrated circuit - the design consists of one or more optocouplers and, in addition, can also be equipped with complementary elements (for example, an amplifier).

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Depending on the type of optical channel:

Open type optical channel;
Closed optical channel.

Depending on the type of photodetector:

Photoresistor (or simply resistor optocouplers);
Photodiode optocouplers;
Phototransistor (using a conventional or composite bipolar phototransistor) optocouplers;
Photothyristor or phototriac optocouplers;
Optocouplers operating using a photovoltaic generator (solar battery).

The design of devices of the latter type is often supplemented with field-effect transistors, the same generator is responsible for controlling the gate.

Phototriac optocouplers or those equipped with field-effect transistors may be called “optorelays” or “solid state relays”.

Fig. 1: Optocoupler device

Optoelectronic devices operate differently depending on which of the two types of directions they belong to:

Electro-optical.

The operation of the device is based on the principle according to which light energy is converted into electrical energy. Moreover, the transition is carried out through a solid body and the internal photoelectric effect processes occurring in it (expressed in the emission of electrons by the substance under the influence of photons) and the glow effect under the influence of an electric field.

Optical.

The device operates through the subtle interaction of solids and electromagnetic radiation, as well as using laser, holographic and photochemical devices.

Photonic electronic computers are assembled using one of two categories of optical elements:

Optocouplers;
Quantum optical elements.

They are models of devices of the electron-optical and optical directions, respectively.

Whether the optocoupler will transmit the signal linearly is determined by the characteristics of the photodetector built into the design. The greatest transmission linearity can be expected from resistor optocouplers. As a result, the process of operating such devices is most convenient. A step lower are models with photodiodes and single bipolar transistors.

To ensure the operation of pulsed devices, optocouplers based on bipolar or field-effect transistors are used, since there is no need for linear signal transmission.

Finally, photothyristor optocouplers are mounted to ensure galvanic isolation and safe operation of the device.

Device Description

The emitter - a packageless LED - is usually placed in the upper part of a metal case, and in the lower part - on a crystal holder - a crystal of a silicon photodetector, for example a photothyristor, is strengthened. The entire space between the LED and the photothyristor is filled with a hardening transparent mass. This fill is covered with a layer that reflects light rays inward, which prevents the scattering of light beyond the working area. The design of the resistor optocoupler differs little from the one described.

Here, in the upper part of the metal body there is a subminiature incandescent lamp, and in the lower part there is a photoresistor based on cadmium selenide. The photoresistor is made separately, on a thin glass-ceramic substrate. A film of semiconductor material, cadmium selenide, is sprayed onto it, and then forming electrodes made of a conductive material (for example, aluminum) are sprayed onto it. The output terminals are welded to the electrodes. A rigid connection between the lamp and the substrate is ensured by a hardened transparent mass. The holes in the housing for the optocoupler leads are filled with glass. The sealed connection between the lid and the base of the housing is ensured by welding.

The current-voltage characteristic (volt-ampere characteristic) of a thyristor optocoupler is approximately the same as that of a single thyristor. In the absence of input current (I=0 - dark characteristic), the photothyristor can only turn on at a very high value of the forward voltage applied to it (800...1000 V). Since in practice the application of such a large voltage is unacceptable, this curve has a purely theoretical meaning.

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If a direct operating voltage is applied to the photothyristor (from 50 to 400 V, depending on the type of optocoupler), the device can only be turned on when an input current is supplied, which is now the control current. The turn-on speed of the optocoupler depends on the value of the input current. Typical switching times t=5…10 µs. The turn-off time of the optocoupler is associated with the process of resorption of minority current carriers in the junctions of the photothyristor and depends only on the value of the flowing output current. The actual value of the shutdown time is in the range of 10...50 µs.

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The maximum and operating output current of a photoresistor optocoupler decreases sharply when the ambient temperature increases above 40 degrees Celsius. The output resistance of this optocoupler remains constant up to an input current of 4 mA, and with a further increase in the input current (when the brightness of the incandescent lamp begins to increase) it decreases sharply. In addition to those described above, there are optocouplers with the so-called open optical channel. Here the illuminator is an infrared LED, and the photodetector can be a photoresistor, photodiode or phototransistor.

The difference between this optocoupler is that its radiation goes out, is reflected from some external object and returns to the optocoupler, to the photodetector. In such an optocoupler, the output current can be controlled not only by the input current, but also by changing the position of the external reflecting surface. In optocouplers with an open optical channel, the optical axes of the emitter and receiver are located either parallel or at a slight angle. There are designs of similar optocouplers with a coaxial arrangement of optical axes. Such devices are called optochoppers.

Optocoupler or optocoupler.