Optocouplers. Optocouplers device and principle of operation of optocouplers What is an optocoupler

Instructions

If an optocoupler, the serviceability of which is indicated under, is soldered into the board, you need to disconnect it, discharge it electrolytic capacitors, 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, phototriacs). They are also pinned. Therefore, it is necessary to find information about the type and pinout of the optocoupler either in a reference book or datasheet, or in the circuit diagram of the device where it was installed. Often, the pinout of the optocoupler is printed directly on 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 and connect it, observing the polarity, into the chain consisting of the source DC voltage several volts, a resistor designed so that the current through the radiation receiver does not exceed the permissible limit, and a multimeter operating in current measurement mode at the appropriate limit.

Now put the optocoupler emitter into operating mode. To turn on the LED, pass through it in direct polarity a direct current equal to the rated one. Apply the rated voltage to the incandescent lamp. Using caution, connect the neon lamp or light-emitting capacitor to the network through a resistor with a resistance of 500 kOhm to 1 MOhm and a power of at least 0.5 W.

The photodetector must react to the inclusion of the emitter with a sharp change in mode. Now try turning 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 react to every change in the control signal. If the optocoupler has an open optical channel, make sure that the reaction of the radiation receiver 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 this, solder the optocoupler back into the board or replace it with another one. Continue repairing the device that includes an 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 connected to each other, which allows the device to be used for galvanic isolation of circuits.

Instructions

Connect the measuring circuit to the photodetector of the optocoupler in accordance with its type. If the receiver is a photoresistor, use a regular ohmmeter, and the polarity is not important. When using a photodiode as a receiver, connect the microammeter without a power source (positive to the anode). If the signal is received by a phototransistor of the n-p-n structure, connect a circuit of a 2 kilo-ohm resistor, a 3-volt battery and a milliammeter, and connect the battery with the positive side 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 of a 3 V battery and a 6 V, 20 mA light bulb, connecting it with the positive side to the anode of the dinistor.

In most optocouplers, the emitter is an LED or an incandescent light bulb. Apply the rated voltage to an incandescent light bulb in either polarity. You can also submit 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 kOhm resistor (positive to the anode).

Optocoupler- This functional device, which consists of a photoemitter, a photodetector and a light guide and, during operation, converts optical signals into electrical ones, and electrical ones into optical ones.

Appointments. IN electrical diagram The optocoupler performs the function of a communication element, in one of the links of which information is transmitted optically. This is the main purpose of the optocoupler. If feedback is provided between the elements of the optocoupler, the optocoupler becomes an optical device suitable for amplifying and generating electrical and optical signals.

Classification. Optocouplers are most often classified according to the type of optical communication. There are optocouplers with internal and external optical coupling. Optocouplers with internal optical communication are also divided according to the type of internal communication. There are optocouplers with internal direct optical coupling and optocouplers with internal feedback optical coupling. They are also classified by type feedback. There are optocouplers with internal positive optical feedback and optocouplers with internal negative optical feedback. As will be shown below, the main element that determines functionality optocoupler is a photodetector. Therefore, optocouplers are also classified according to the type of photodetector. There are resistor, diode, transistor, thyristor and combined optocouplers.

Rice. 1. Conditional images optocouplers: a - transistor; b - diode; c - resistor; g - with a composite transistor; d - thyristor; e - differential; g- diode-transistor

Conventional images and symbols. Conventional images of optocouplers in the diagrams are shown in Fig. 1. The symbols of optocouplers in the texts combine seven symbols indicating
material, class and subclass of the device, frequency range of operation, serial number of development, division into parametric groups. For example, the designation AOD130A means: gallium arsenide diode optocoupler, operating frequency range 1, development serial number 30, parametric group A.

Rice. 2. Main elements of optocouplers with internal (a) and external (b) optical connections

Structure. An optocoupler with internal optical coupling is a quadripole (Fig. 2, a), which consists of three main elements: a photoemitter (light sources) 1, a light guide 2 and a light receiver (photodetector) 3, placed in a common sealed, light-proof housing. An optocoupler with external optical coupling is a two-terminal device that has one optical input and one optical output (Fig. 2, b). It consists of a photodetector 3, an amplifier 4, a photoemitter 1 and does not have a light guide. In modern optocouplers, injection diodes (LEDs) are predominantly used as photoemitters, less often - luminescent capacitors, and photoresistors, photodiodes, phototransistors, and photothyristors are used as photodetectors. To achieve high
parameter values, it is not enough to use highly efficient photoemitters and photodetectors. It is necessary to ensure their coordination in terms of spectral characteristics, speed,
dimensions, temperature characteristics. Matched optocoupler pairs are the elements given in Table. 3.4. The optocoupler light guide (optical medium) has a triple purpose: to minimize losses during energy transfer from the photoemitter to the photodetector, to ensure high values ​​of galvanic isolation parameters, and to create a structurally integral device. As an optical medium, polymer optical adhesives and varnishes are predominantly used, which have high adhesion to semiconductor crystals, good dielectric properties, high elasticity, and low cost. At the same time, they have significant disadvantages: the refractive indices of these materials ( n≈ 1.5) differ significantly from the refractive indices of silicon and gallium arsenide ( n≈ 3.2-3.4) the spectral characteristics of polymers have many dips in the near-IR region due to the resonant absorption of OH, CH 3, CH 2, NH groups, which, with significant fiber sizes, can affect the light output; Polymer light guides are characterized by aging.

Table 3.4. Matched photoemitter-photodetector pairs

If the rigidity of the optocoupler is ensured by the structural elements, then Vaseline-like silicone lubricants that do not dry out can be used as an optical medium. Chalcogenide glass ( n≈ 1.8..3.0). Its disadvantage is low adhesion to semiconductors, high fragility, poor insulating properties ( p= 10 9 … 10 11 Ohm cm), low resistance to thermal cycles. Real designs of optocouplers (Fig. 3) are designed not only to provide extremely high values ​​of the defining parameters, but also to expand the functionality of these devices.

Robot. The operation of an optocoupler with internal direct optical coupling can be illustrated using its electrical circuit (Fig. 4, a), from which it can be seen that the input and output signals of the optocoupler are electrical. There is no electrical connection between its elements, but there is an optical connection. When applied to the input of an optocoupler electrical signal The photoemitter is excited, the luminous flux of which enters the photodetector through the light guide. An electrical signal is generated at its output, which indicates that a transformation has taken place in the optocoupler according to the electrical signal - optical - electrical circuit.

Rice. 3. Types of optocouplers: optocoupler in a DIP package (a), high-voltage (b), energy (c), optocoupler with a plastic hemisphere (d), optocoupler (e), reflective optocoupler (e): 1 - photoemitter; 2 - photodetector; 3 - light guide; 4 — body; 5 - external terminals; Me - metal electrodes

Rice. 4. Electrical circuit (a) and transfer characteristic (b) of an optocoupler with internal direct optical coupling

In an optocoupler with internal positive feedback, the photodetector and the light source are connected in series (Fig. 5, a). It has two inputs (optical and electrical) and two similar outputs.

Rice. 5. Electrical circuit (a) and current-voltage characteristic (b) of an optocoupler with internal positive feedback optical coupling

There is an electrical connection between its elements. Structurally, the optocoupler is designed so that part of the original light flux falls back into the photodetector. This leads to a decrease in resistance, an increase in the brightness of the glow, a further decrease in resistance. This process is incremental and continues until the change in resistance significantly affects the amount of current or voltage supplied to the light source. To do this, it is sufficient that the following condition be met:

When,

where, and are the minimum resistance of the photodiode and the resistance of the light source; and - input and input maximum currents optocoupler; and - initial and
output maximum brightness.
In practice, this mode of operation of the optocoupler is called the “On” state. The “off” state corresponds to the following condition:

The transition of the optocoupler from the “off” state to the “on” position occurs abruptly and is accompanied by an avalanche-like change in current and brightness in the electrical and optical circles.
In an optocoupler with internal negative optical feedback, the photodetector and the light source are connected in parallel (Fig. 6, a). It also has two inputs (electrical and optical) and two similar outputs. There is also an electrical connection between its elements. Structurally, the optocoupler is designed so that part of the original light flux falls back into the photodetector. This leads to a decrease in the resistance of the photodetector and more and more shunting of the light sources, as a result of which it begins to shine weaker.

In an optocoupler with external optical coupling, the input and output signals are optical. Its elements are connected to each other by electrical communication.

Rice. 7. Electrical circuit (a) and transfer characteristic (b) of an optocoupler with external optical coupling

When an optical signal is applied to the input of the optocoupler, the resistance of the photodetector decreases, as a result of which the current through the photoemitter increases and, accordingly, the brightness of its glow increases.

Properties. The properties of optocouplers determine their characteristics and parameters. There are incoming, outgoing, current-voltage and transfer characteristics; their type is largely determined by the electrical circuit of the optocoupler and the nature of the existing optical connections. For optocouplers with internal direct optical coupling, the transfer characteristic expressing
dependence of the output electrical signal on the input. For them, any change in the current or voltage of photoemission is accompanied by corresponding changes in the brightness of its glow, the resistance of the photodetector and the output current of the optocoupler. Therefore, its transfer characteristic, expressing the dependence of the output current on the input current, has the form shown in Fig. 4, b. It can be seen that an optocoupler with internal direct optical coupling can be considered as an element of variable resistance, the value of which is determined by the input current or input voltage. For optocouplers with internal positive optical feedback, the main one is the input current-voltage characteristic; its specific feature is the presence of a section with a negative differential resistance, where the voltage drops and the current increases. By appearance it resembles the current-voltage characteristics, electromagnetic relay or trigger (Fig. 5, b).
For optocouplers with internal negative optical feedback, the main one is also the input current-voltage characteristic. Its appearance is shown in Fig. 6, b. Analysis of the shape of the curve shows that with the same spectral composition of the input and output radiation, a monochromatic increase in the light flux is observed. If the spectral composition of the input and output radiation is different, then radiation transformations are observed. An optocoupler with external optical coupling plays the role of an optical signal amplifier (Fig. 7).

The system of optocoupler parameters contains parameters of four groups:
1. Parameters describing the input characteristics of optocouplers.
2. Parameters that describe the initial characteristics of optocouplers.
3. Parameters describing the transmission characteristics of optocouplers.
4. Parameters describing galvanic isolation of optocouplers.

Since the input to optocouplers are LEDs or electroluminescent capacitors, and the output is photodiodes, phototransistors, photoresistors, photothyristors, only the parameters of the last two groups are specific to optocouplers. The degree of influence of the photoemitter on the photoreceiver (transmission characteristic) is determined:
— current transfer coefficient used for diode and transistor optocouplers;

- the ratio of dark resistance to light resistance: or the value of light resistance, which is used for resistor optocouplers;
- minimum input current, which ensures rectified input characteristics, which is used for thyristor optocouplers.

These also include parameters characterizing the inertia of the optocoupler in pulsed mode (on and off times and ) and in high-frequency mode (limiting frequency). The quality of galvanic isolation in statics and dynamics is determined by setting the voltage and resistance of the galvanic isolation (coupling) and the pass-through capacitance (coupling capacitance).
Transistor optocouplers are characterized by the greatest circuit flexibility, have a high current transfer coefficient, but are relatively slow in response ( ). Particularly large values ​​(up to 600 ... 800%) are achieved in an optocoupler with a composite transistor. Diode optocouplers produced primarily using p- And n-photodetectors, characterized by high performance , but the value for them is a few percent, so amplification of video images is necessary.
Diode integrated optocouplers, which are manufactured using planar technology using GaAs-sweet lights and Si - p - i - n-photodiodes separated by a glass immersion medium ( n= 2.7), like diode non-integrated optocouplers, have high performance and low current transfer coefficient (units of percent). The location of their transmitting characteristics on the coordinate plane, which determine the current transfer coefficient, significantly depends on temperature (Fig. 8). The insulation resistance between the output and the input, which determines the degree of isolation according to DC, is 10 8 ... 10 12 Ohm. Quality of solution alternating current depends on the flow capacity, amounts to units nФ.

Rice. 8. Temperature dependence of the transmitting characteristics of a diode optocoupler with internal optical coupling

Rice. 9. Output characteristic of the optocoupler in photovalve mode (max power release point)

One of the important features of diode optocouplers is the ability to operate in photogate mode without applying external voltage to the photodetector (Fig. 9). The optocoupler acts as a control isolated power source. Serial optocouplers in photovalve mode have, as a rule, low efficiency (<0,5 … 1%), но достижения на лабораторных образцах КПД 10 … 15% и
the possibility of battery connection of optocouplers serves as the basis for the creation of a specific group of low-power ( U ≈ 0.5 … 5 V, I ≈ 0.5..50 mA) secondary power sources. Resistor optocouplers are characterized by linearity and symmetry of the initial current-voltage characteristic, the absence of internal EMF, and a high ratio . Therefore, despite its very great inertia and the widespread development of diode and transistor optocouplers, resistor optocouplers retain important independent significance. Thyristor optocouplers are very convenient in “power” optoelectronics. They are equally suitable for switching high-current circuits of radio and electrical equipment. appointments. By controlling such large powers in the load, thyristor optocouplers behind the input are practically compatible with ICs (the value of Iin is tens of milliamps). In addition to the considered types of optocouplers, which are common in industry, those in which MON varicaps, field-effect transistors with a dielectric gate and with a control are used as photodetectors are also of particular interest. p-n-junction, unijunction transistors, avalanche diodes and transistors, Schottky barrier diodes.
Very promising for analog technology are differential optocouplers, in which one photoemitter operates on two identical photodetectors (Fig. 1, e). Elementary ones also include multi-channel optocouplers, which are a set of identical optocouplers in one housing.

Application. Optocouplers with internal optical coupling are widely used in various branches of radio engineering and electronics, computer technology, automation, and electrical engineering. In digital devices they are used to connect devices made on various bases (for example, to interface bipolar ICs with unipolar ones, tunnel diode and transistor circuits, etc.), they are used to control power circuits of motors and relays of direct and alternating currents from low-voltage low-power logic circuits; for connecting logical circuits with computer peripheral equipment; as decoupling elements from ground in power supplies; as low-power relays in electroluminescent information display systems; in control and measuring devices,
directly connected to high-current AC circuits.

Optocouplers, which are suitable for transmitting analog signals, are used as switching elements in telephone lines; in circles of communication of various sensors with a computer; in medical electronics.
Optocouplers with flexible light guides are used to control high-voltage power lines; in measuring systems designed to operate under conditions of strong interference (microwave interference, sparking) in control and monitoring devices for high-voltage electric vacuum devices (klystrons, CRTs, image intensifier tubes, etc.); in the technique of physical experimentation. Optocouplers with an open optical channel (optointerrupting and reflective optocouplers) are indispensable in devices for reading information from punched media as indicators of the position of objects and the condition of their surfaces, as vibration sensors, filling volumes with liquid, etc.

An optocoupler (otherwise called an optocoupler) is an electronic device designed to convert electrical signals into light, transmit them through optical channels and re-convert the signal back into electrical. The design of an optocoupler implies the presence of a special light emitter (in modern devices, light diodes are used for this; previous models were equipped with small-sized incandescent lamps) and a device responsible for converting the received optical signal (photodetector). Both of these components are combined using an optical channel and a common housing.

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).

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;
• , 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 can be called “optorelays” or “”.

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 that the photodetector built into the design has. 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.

Application

There are many areas in which the use of optocouplers is necessary. This breadth of application is due to the fact that they are elements that have many different properties and each of their qualities has a separate area of ​​application.

• Fixation of mechanical impact (devices equipped with an open-type optical channel are used, which can be closed (exert a mechanical impact), which means the device itself can be used as a sensor):
  • Presence detectors (detection of the presence/absence of paper sheets in the printer);
  • End (start) point detectors;
  • Counters;
  • Discrete speedometers.
• Galvanic isolation (the use of optocouplers makes it possible to transmit a signal not related to voltage; they also provide contactless control and protection), which can be provided by:
  • Optocoupler (in most cases used as an information transmitter);
  • Optorelay (mostly suitable for controlling signal and power circuits).

Optocouplers

The use of transistor or integrated optocouplers is especially important if it is necessary to provide galvanic isolation in a signal circuit or a circuit with low control current. The role of a control element can be performed by three-electrode semiconductor devices, circuits that control discrete signals, as well as circuits with special specialization.

Fig2: Optocouplers 5000 Vrms 50mA.

Parameters and operating features of optocouplers

Based on the exact design of the device, its electrical strength can be determined. This term refers to the value of the voltage that occurs between the input and output circuits. Thus, manufacturers of optocouplers that provide galvanic isolation demonstrate a number of models with different housings:

• SSOP;
• Miniflat-lead.

Depending on the type of housing, the optocoupler generates one or another insulation voltage. To create conditions in which the voltage level sufficient to cause insulation breakdown was high enough, the optocoupler should be designed so that the following parts are located sufficiently far from each other:

• and optical recorder;
• Inner and outer side of the case.

In some cases, you can find optocouplers of a specialized group, manufactured in accordance with international safety standards. The level of electrical strength of these models is an order of magnitude higher.

Another significant parameter of a transistor optocoupler is called the “current transfer coefficient”. According to the value of this coefficient, the device is classified into one category or another, which is reflected in the model name.

There are no restrictions regarding the level of the lower operating frequency of optocouplers: they function well in a circuit with direct current. And the upper limit of the operating frequency of these devices involved in transmitting signals of digital origin is calculated in hundreds of megahertz. For linear-type optocouplers, this figure is limited to tens of megahertz. For the slowest designs, including an incandescent lamp, the most typical role is played by low-frequency filters operating at frequencies not reaching 10 Hz

Transistor optocoupler and the noise it produces

There are two main reasons why the operation of a transistor pair is accompanied by noise effects:

To overcome the first reason, you will need to install a special screen. The second is eliminated through a correctly selected operating mode.

Optorelay

An opto relay, otherwise known as a solid state relay, is usually used to regulate the operation of a circuit with large control currents. The role of the control element here is usually performed by two MOSFET transistors with back-to-back connections; this configuration ensures the ability to operate in alternating current conditions.

Fig. 3: Optorelay KR293 KP2V

Classification of types of opto-relays

Three types of topologies are defined for opto-relays:

1. Normally open It is assumed that the control circuit will close only when the control voltage is applied to the terminals of the light diode.
2. Normally closed It is assumed that the control circuit will open only when the control voltage is applied to the terminals of the light diode.
3. Switching The third topology involves a combination of normally closed and normally open channels.

An opto-relay, like an optocoupler, has an electrical strength characteristic.

Types of opto-relays

• Standard type models;
• Models with low resistance;
• Models with low CxR;
• Models with low bias voltage;
• Models with high insulation voltage.

Fields of application of opto-relays

• Modem;
• Measuring device;
• Interface with the actuator;
• Automatic telephone exchanges;
• Electric, heat, gas meter;
• Signal switch.

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Optocouplers are those optoelectronic devices in which there is a source and a radiation receiver (light emitter and photodetector) with one or another type of optical and electrical connection between them, structurally connected to each other.

Operating principle optocouplers of any kind are based on the following. In the emitter, the energy of the electrical signal is converted into light; in the photodetector, on the contrary, the light signal causes an electrical response.

In practice, only optocouplers have become widespread, which have a direct optical connection from the emitter to the photodetector and, as a rule, all types of electrical communication between these elements are excluded.

According to the degree of complexity of the structural diagram, two groups of devices are distinguished among optocoupler products. An optocoupler (also called an “elementary optocoupler”) is an optoelectronic semiconductor device consisting of emitting and photoreceiving elements, between which there is an optical connection that provides electrical isolation between the input and output. An optoelectronic integrated circuit is a microcircuit consisting of one or more optocouplers and one or more matching or amplifying devices electrically connected to them.

Thus, in an electronic circuit, such a device performs the function of a communication element, in which, at the same time, electrical (galvanic) isolation of the input and output is carried out.

In the block diagram in Fig. 1 input device is used to optimize the operating mode of the emitter (for example, biasing the LED to the linear section of the watt-ampere characteristic) and converting (amplifying) the external signal. The input unit must have high conversion efficiency, high speed, a wide dynamic range of permissible input currents (for linear systems), and a low value of the “threshold” input current, which ensures reliable transmission of information through the circuit.

Figure 1. Generalized block diagram of an optocoupler

The purpose of the optical medium is to transfer the energy of the optical signal from the emitter to the photodetector, and also, in many cases, to ensure the mechanical integrity of the structure.

The fundamental possibility of controlling the optical properties of the medium, for example, by using electro-optical or magneto-optical effects, is reflected by introducing a control device into the circuit. In this case, we get an optocoupler with a controlled optical channel, functionally different from a “conventional” optocoupler: the output signal can be changed either by input and through the control circuit.

In the photodetector, the information signal is “restored” from optical to electrical; At the same time, they strive to have high sensitivity and high speed.

Finally, the output device is designed to convert the photodetector signal into a standard form convenient for influencing cascades subsequent to the optocoupler. An almost mandatory function of the output device is signal amplification, since the losses after double conversion are very significant. Often the amplification function is performed by the photodetector itself (for example, a phototransistor).

Electrical circuits and output characteristics of optocouplers with a photoresistor (a), photodiode (b) and photothyristor (c): 1 - semiconductor light-emitting diode; 2 - photoresistor; 3 - photodiode; 4- photothyristor; U And I- voltage and current in the output circuit of the optocoupler. Dashed curves correspond to the absence of current in the input circuit of the optocoupler, solid curves correspond to two different values ​​of input currents.

Optocouplers allow solving the same problems as individual emitter-photodetector pairs, but in practice they are usually more convenient, since they already have the optimal characteristics of the emitter and photodetector and their relative position.

If we talk about the most obvious application of an optocoupler, which has no analogues among other devices, it is a galvanic isolation element. Optocouplers (or, as they are sometimes called, optocouplers) are used as communication devices between hardware units at different potentials, to interface microcircuits with different logical levels. In these cases, the optocoupler transmits information between blocks that do not have an electrical connection and does not carry an independent functional load.

No less interesting is the use of optocouplers as elements of optical non-contact control of high-current and high-voltage devices.

It is convenient to use optocouplers to build units for launching powerful thyratrons, distribution and relay devices, power switching devices, etc.

Optocouplers with an open optical channel simplify the solution of problems of monitoring the parameters of various environments and allow the creation of various sensors (humidity, liquid level and color, dust concentration, etc.).

One of the most important is a linear circuit, designed for undistorted transmission of analog signals through a galvanically isolated circuit. The complexity of this problem is due to the fact that linearization of the transfer characteristic in a wide range of currents and temperatures requires a feedback loop, which is fundamentally impossible to implement in the presence of galvanic isolation. Therefore, they take the path of using two identical optocouplers (or a differential optocoupler), one of which acts as an auxiliary element providing feedback (Fig. 6.13). In such circuits it is convenient to use differential optocouplers KOD301A, KOD303A.

In Fig. Figure 6.14 shows a diagram of a two-stage transistor amplifier with optoelectronic coupling. Changing the transistor collector current VT1 causes a corresponding change in the optocoupler LED current U1 and the resistance of its photoresistor, which is connected to the base circuit of the transistor VT2 . On the load resistor R2 highlight

There is an amplified output signal. The use of an optocoupler almost completely eliminates signal transmission from the output to the input of the amplifier.

Optocouplers are convenient for interunit galvanic isolation in electronic equipment. For example, in a galvanic isolation circuit of two blocks (Fig. 6.15), the signal from the block output 1 transmitted to the block input 2 via diode optocoupler U1. If an integrated circuit with a low input current is used as the second block, there is no need to use an amplifier, and the photodiode of the optocoupler in this case operates in photogenerator mode.

Rice. 6.13. Galvanic isolation of the analog signal: 01, 02 – optocouplers, U1, U2 – operational amplifiers

Rice. 6.14. Two-stage transistor amplifier with optoelectronic coupling

Optocouplers and optoelectronic microcircuits are used in devices for transmitting information between blocks that do not have closed electrical connections. The use of optocouplers significantly increases the noise immunity of communication channels and eliminates unwanted interactions of decoupled devices along power circuits and the common wire. Interface circuits using optocouplers are widely used in computing and measuring technology, in automation devices, especially when sensors or other receiving devices operate in conditions that are dangerous or inaccessible to humans.

For example, the implementation of communication between galvanically independent logic elements can be carried out using an optoelectronic switch (Fig. 6.16). An optoelectronic switch can be a K249LP1 microcircuit, which includes a packageless optocoupler and a standard valve.

Optocouplers make it possible to simplify the solution of problems of interfacing blocks with different functional purposes
niya, the nature of the power supply, for example, actuators powered from an alternating current network, and control signal generation circuits powered from low-voltage direct current sources.

A large group of tasks is also represented by the coordination of digital microcircuits with different types of logic: transistor-transistor logic (TTL), emitter-coupling

controlled logic (ESL), complementary metal-oxide-semiconductor structure (CMOS), etc. An example of a matching circuit for a TTL element with an MIS using a transistor optocoupler is shown in Figure 6.17. The input and output stages do not have common electrical circuits and can operate in a wide variety of conditions and modes.

Ideal galvanic isolation is needed in many practical cases, for example, in medical diagnostic equipment, when the sensor is attached to the human body, and the measuring unit, which amplifies and converts the sensor signals, is connected to the network. If the measuring unit malfunctions, there may be a risk of electric shock to a person. The sensor itself is powered from a separate low-voltage power source and connected to the measuring unit through an isolating optocoupler (Fig. 6.18).

Optocouplers are also useful in other cases where “ungrounded” input devices must be interfaced with “grounded” output devices. Examples are

Some tasks may include connecting a teletype communication line with a display, an “automatic secretary” connected to a telephone line, etc. For example, in the circuit for connecting a communication line with a display (Fig. 6.19, A) The operational amplifier provides the required level of signals at the display input. Similarly, you can connect the transmitting console to the communication line (Fig. 6.19, b).

Rice. 6.19. Pairing “ungrounded” and “grounded” devices

Rice. 6.20. Optoelectronic semiconductor relays:

a – normally open, b – normally closed

It is convenient to transmit the amplified signals of the photodetector to actuators (for example, electric motors, relays, light sources, etc.) through optoelectronic galvanic isolation. Examples of such decoupling are two variants of the most common semiconductor relays, open and closed (Fig. 6.20). The relay switches DC signals. The signal sensed by the phototransistor of the optocoupler opens the transistors VT1, VT2 and turns on the load

(Fig. 6.20, A) or disables it (6.20, b).

Figure 6.21. Optoelectronic pulse transformer

A pulse transformer is a very common element of modern electronic equipment. It is used in various pulse generators, power amplifiers of pulsed signals, communication channels, telemetry systems, television equipment, etc. The traditional design of a pulse transformer using a magnetic core and windings is not compatible with technological solutions used in microelectronics. The frequency response of a transformer in many cases does not allow satisfactorily reproducing both low- and high-frequency signals.

An almost ideal pulse transformer can be made on the basis of a diode optocoupler. For example, in the circuit of an optoelectronic transformer with a diode optocoupler there is shown (Fig. 6.21) a transistor VT1 controls the optocoupler LED U1 The signal generated by the photodiode is amplified by transistors VT2 And VT3.

The pulse rise time largely depends on the speed of the optocoupler. Photodiodes have the highest performance p— i— n-st
structures. The rise and fall time of the output pulse does not exceed several tens of nanoseconds.

Based on optocouplers, optoelectronic microcircuits have been developed and produced, containing one or more optocouplers, as well as matching microelectronic circuits, amplifiers and other functional elements.

The compatibility of optocouplers and optoelectronic microcircuits with other standard microelectronic elements in terms of input and output signal levels, supply voltage and other parameters determined the need to standardize special parameters and characteristics.