To switch loads in alternating current circuits, circuits using powerful field effect transistors. This class of devices is represented by two groups. The first group includes bipolar transistors with an insulated gate - IGBT. The Western abbreviation is IGBT.
The second, most numerous, includes traditional field-effect (channel) transistors. This group also includes KP707 transistors (see Table 1), on which the load switch for a 220-volt network is assembled.
Primary AC power is a very dangerous thing in all respects. Therefore, there are many circuit solutions that avoid managing network loads directly. Previously, isolation transformers were used for these purposes; now they have been replaced by a variety of optocouplers.
Transistor switch with optical isolation
The scheme, which has already become standard, is shown in Figure 1. 
This scheme allows galvanic isolation of control circuits and the circuit of the primary 220 volt network. A TLP521 optocoupler is used as a decoupling element. You can use other imported or domestic transistor optocouplers. The scheme is simple and works as follows. When the voltage at the input terminals is zero, the optocoupler LED does not light up, the optocoupler transistor is closed and does not bypass the gate of the powerful switching transistors. Thus, at their gates there is an opening voltage equal to the stabilization voltage of the zener diode VD1. In this case, the transistors are open and operate in turn, depending on the polarity of the voltage in at the moment time. Suppose there is a plus at the output pin of circuit 4, and a minus at terminal 3. Then the load current will flow from terminal 3 to terminal 5, through the load to terminal 6, then through the internal protective diode of transistor VT2, through the open transistor VT1 to terminal 4. When the polarity of the supply voltage changes, the load current will flow through the diode of transistor VT1 and the open transistor VT2. Circuit elements R3, R3, C1 and VD1 are nothing more than a transformerless power supply. The value of resistor R1 corresponds to the input voltage of five volts and can be changed if necessary.
The entire circuit is made in the form of a functionally complete block. The circuit elements are mounted on a small U-shaped printed circuit board, shown in Figure 2. 
The board itself is attached with one screw to an aluminum plate with dimensions of 56x43x6 mm, which is the primary heat sink. The powerful transistors VT1 and VT2. The corner holes are aligned both in the board and in the plate and serve, if necessary, for attaching the unit to another more powerful heat sink.
This article will focus primarily on optical isolation of an analog signal. Will be considered budget option. Also, the main attention is paid to the speed of the circuit solution.
Methods for decoupling an analog signal
A short review. There are three main methods of galvanic isolation of an analog signal: transformer, optical and capacitor. The first two have found the greatest application. Today there is a whole class of devices called isolating amplifiers or isolating amplifiers (Isolated Amplifier). Such devices transmit a signal by means of its conversion (the circuit contains a modulator and a signal demodulator).
Fig.1. General circuit of isolating amplifiers.
There are devices both for transmitting an analog voltage signal (ADUM3190, ACPL-C87) and specialized ones for connecting directly to a current shunt (SI8920, ACPL-C79, AMC1200). In this article we will not consider expensive devices, however, we list some of them: iso100, iso124, ad202..ad215, etc.
There is also another class of devices - decoupling optical amplifiers with linearizing feedback (Linear Optocoupler); these devices include il300, loc110, hcnr201. The operating principle of these devices is easy to understand by looking at their typical connection diagram.
Fig.2. Typical circuit for isolating optical amplifiers.
You can read more about isolation amplifiers: A. J. Peyton, W. Walsh “Analog Electronics on Op-amps” (Chapter 2), the document AN614 “A Simple Alternative To Analog Isolation Amplifiers” from silicon labs will also be useful, there is a good one there comparison table. Both sources are available on the Internet.
Special optical signal isolation microcircuits
Now to the point! First, let's compare three specialized microcircuits: il300, loc110, hcnr201. Connected according to the same circuit:
Fig.3. Test circuit for il300, hcnr201 and loc110.
The only difference is in the ratings for il300, hcnr201 R1,R3=30k, R2=100R, and for loc110 10k and 200R respectively (I selected different ratings to achieve maximum performance, but not go beyond the permissible limits, for example, in terms of emitting diode current ). Below are oscillograms that speak for themselves (hereinafter: blue - input signal, yellow - output signal).
Fig.4. Oscillogram of il300 transient process.
Fig.5. Transient waveform hcnr201.
Fig.6. Transient waveformloc110.
Now let's look at the ACPL-C87B microcircuit (input signal range 0..2V). To be honest, I spent quite a long time with her. I had two microcircuits in stock, after I got an unexpected result on the first one, I handled the second one very carefully, especially when soldering. I assembled everything according to the scheme indicated in the documentation:
Fig.7. Typical diagram forACPL— C87 from the documentation.
The result is the same. I soldered ceramic capacitors directly near the power pins, changed the op-amp (of course, tested it on other circuits), reassembled the circuit, etc. Here's the rub: the output signal has significant fluctuations.
Fig.8. Transient waveformACPL— C87.
Despite the fact that the manufacturer promises a noise level of the output signal of 0.013 mVrms and for option “B” an accuracy of ±0.5%. What's the matter? There may be an error in the documentation, since 0.013 mVrms is hard to believe. Not clear. But let's look at the Test Conditions/Notes column opposite Vout Noise and at Fig. 12 of the documentation:
Fig.9. Dependence of the noise level on the magnitude of the input signal and the frequency of the output filter.
Here the picture becomes a little clearer. Apparently the manufacturer is telling us that we can suppress these noises through a low-pass filter. Well, thanks for the advice (ironic). Why did they turn all this out in such a cunning way? Most likely it’s clear why. Below are the graphs without and with the output RC filter (R=1k, C=10nF (τ=10µS))
Fig. 10. Transient waveformACPL— C87 without and with output filter.
Application of general purpose optocouplers for signal isolation
Now let's move on to the fun part. Below are the diagrams that I found on the Internet.
Fig. 11. Typical scheme of optical isolation of an analog signal using two optocouplers.
Fig. 12. Typical scheme of optical isolation of an analog signal using two optocouplers.
Fig. 13. Typical circuit for optical isolation of an analog signal using two optocouplers.
This solution has both advantages and disadvantages. The advantage is the higher insulation voltage; the disadvantage is that two microcircuits can differ significantly in parameters, so by the way it is recommended to use microcircuits from the same batch.
I assembled this circuit on a 6n136 chip:
Fig. 14. Oscillogram of the transient process of decoupling at 6N136.
It worked, but slowly. I tried to assemble it on other microcircuits (such as sfh615), it works, but it’s also slow. I needed it faster. In addition, the circuit often does not work due to self-oscillations that occur (in such cases they say the ACS is unstable))) Increasing the value of capacitor C2, Fig., helps. 16.
One friend recommended a domestic optocoupler AOD130A. The result is obvious:
Fig. 15. Oscillogram of the transient decoupling process on the AOD130A.
And here is the diagram:
Fig. 16: Isolation diagram for AOD130A.
One potentiometer is needed (RV1 or RV2), depending on whether the output signal is less or greater than the input signal. In principle, it was possible to put only one RV=2k in series with R3=4.7k, or even leave only RV2=10k without R3. The principle is clear: be able to adjust around 5k.
Transformer signal decoupling chip
Let's move on to the transformer option. The ADUM3190 microcircuit is available in two versions for 200 and 400 kHz (I have the ADUM3190TRQZ for 400), there is also a microcircuit for a higher insulation voltage ADUM4190. Note that the case is the smallest of all – QSOP16. Output voltage Eaout from 0.4 to 2.4V. In my microcircuit output voltage the offset is about 100 mV (visible in the oscillogram in Fig. 18). Overall it works well, but personally I am not entirely satisfied with the output voltage range. Assembled according to the diagram from the documentation:
Fig. 17. ADUM3190 circuit from the documentation.
Some oscillograms:
Fig. 18. ADUM3190 transient waveform.
Results
Let's summarize. In my opinion the best option is a circuit for domestic ADO130A (where did they get them?!). And finally, a small comparison table:
| Chip | tr+delay (oscillation), µs | tf+delay (oscillation), µs | Range voltage, V | Voltage insulation, V | Noise (oscillatory) mVp-p. | Price** per piece, r (05.2018) |
|---|---|---|---|---|---|---|
| IL300 | 10 | 15 | 0-3* | 4400 | 20 | 150 |
| HCNR201 | 15 | 15 | 0-3* | 1414 | 25 | 150 |
| LOC110 | 4 | 6 | 0-3* | 3750 | 15 | 150 |
| ACPL-C87B | 15 | 15 | 0-2 | 1230 | nd | 500 |
| 6N136 | 10 | 8 | 0-3* | 2500 | 15 | 50 |
| AOD130A | 2 | 3 | 0.01-3* | 1500 | 10 | 90 |
| ADUM3190T | 2 | 2 | 0.4-2.4 | 2500 | 20 | 210 |
*- approximately (according to the assembled circuit with performance optimization)
**- the price is average according to the minimum.
Yaroslav Vlasov
P.S. AOD130A produced by Proton OJSC (with their logo engraved in a black case) is good. The old ones (from the 90s in a brown case) are not suitable.
Judging by several recent posts, it would be nice to cover what galvanic isolation is and why it is needed. So:
Galvanic isolation- transfer of energy or signal between electrical circuits without electrical contact between them.
Now, let's look at some examples :)
Example 1: Network
Most often people talk about galvanic isolation in relation to mains power, and here's why. Imagine that you grabbed the wire from the socket with your hand. Your “connection” from an electrical point of view looks like this:And, yes, the leakage current of the slippers is quite enough for you to feel a “blow” when you touch the “phase” network wire. If the slippers are dry, then such a “blow” is usually harmless. But if you stand barefoot on a wet floor, the consequences can be very dire.
It’s a completely different matter if there is a transformer in the circuit: 
If you touch one of the terminals of the transformer, no current will flow through you - it simply has nowhere to flow, the second terminal of the transformer hangs in the air. If, of course, you grab both terminals of the transformer and it produces enough voltage, then it will screw you up anyway.
So, in this case, the transformer provides galvanic isolation. In addition to the transformer, there are a lot more different ways transmit a signal without creating electrical contact:
- Optical: optocouplers, fiber optics, solar panels
- Radio: receivers, transmitters
- Sound: speaker, microphone
- Capacitive: through a very small capacitor
- Mechanical: motor-generator
- You can still imagine
Example 2: Oscilloscope
There is a really mega-classic way to blow up half a circuit. There is even a corresponding one on the forum. The thing is, many people forget that the oscilloscope (and many other equipment) is connected to ground. Here's what the full picture looks like when connecting an oscilloscope to a circuit powered directly from the mains:
Remember - once you connect something to a circuit, it becomes part of the circuit! This is also true for various measuring equipment.
The correct way to measure something in a circuit like this is to connect it through a 220->220 isolation transformer: 
Ready-made transformers 220->220 are quite difficult to find. Therefore, you can use so-called shifters. A flip is two transformers, for example 220->24, turned off in series like this: 
You've probably seen what this looks like in practice: 
Inverters are even better than one 220->220 transformer.
- They provide half the capacitance between input and output
- The middle part can be grounded, and thus it is very good to filter out interference from the network
- You can turn on 3 transformers, and then you can get 440 or 110 volts
Song
A long time ago I even wrote a song on the topic of galvanic isolation. The song is under the spoiler.The song, its lyrics and explanations
I recorded this mini-song when I was working on various audio electronics. One friend made a tube guitar gadget and, thinking that the transformer that turns 220 into 220 was completely useless, threw it out of the circuit, for which he paid. I thought that this was quite the theme for a metal mini-song.
Hello Oldfag! Your browser does not support html5! Update yourself!
You didn't install an anode transformer
Powered directly from the network
There was a battery under my foot
And you grabbed the guitar with your handCurrent pierces the mortal body
Mortal flesh wriggles
You can't open your hand
You're alone and no one can helpTearing and burning
Electrons squeeze your heart
Will it fight or will it subside?
Safety, remember, comes first.
By the way, besides the denouement in this little song there are two more good pieces of advice:
- Yes, all work with mains voltage must be performed by at least two people.
- When an electric shock occurs, the hand contracts, therefore, it is better to first touch the devices with the back of your right hand.
Conclusion
Naturally, the topic of denouement does not end there. For example, it is very difficult to transmit fast signals through an interchange. But more on that a little later.International Rectifier company - developer and manufacturer power electronics since 1947 - produces a huge range of opto-relays for all kinds of applications. The most popular of them can be divided into the following groups:
- Fast acting (PVA, PVD, PVR);
- General purpose (PVT);
- Low voltage medium power (PVG, PVN);
- High voltage powerful (PVX).
PVA33: fast acting relay
for signal switching
AC Relay Series PVA33— single pole, normally open. Designed for general analog signal switching purposes.
The operating principle of the device is as follows (Fig. 1). The voltage applied to the relay input causes current to flow through the gallium arsenide LED (GaAlAs), resulting in an intense glow of the latter. The light flux hits an integrated photovoltaic generator (IGG), which creates a potential difference between the gate and the source of the output switch, thereby transferring the latter to a conducting state. Power MOSFET transistors (HEXFET - patented IR technology) are used as power output switches. In this way, complete galvanic isolation of the input circuits from the output circuits is achieved.
Rice. 1.
The advantages of such a solution compared to conventional electromechanical and reed relays are a significant increase in service life and speed, reduction in power losses, and minimization of size. These benefits improve the quality of products developed for a variety of applications, such as signal multiplexing, automated test equipment, data acquisition systems, and others.
The voltage level that the relays of this series are capable of switching lies in the range from 0 to 300 V (amplitude value) of both alternating and direct current. In this case, the minimum level is determined (at constant current) by the resistance of the channel of the output transistors, which averages about 1 Ohm (maximum up to 20 Ohms).
The dynamic characteristics of the device are determined by the on-off time, which is about 100 μs. Thus, the guaranteed relay switching frequency can reach 500 Hz or more.
The maximum frequency of the switched signal depends mainly on frequency characteristics transistors used and for MOS switches reaches hundreds of kilohertz. The relays are supplied in 8-pin DIP packages and are available in two versions: through-hole and surface mount.
PVT312: telecommunication relay
general purpose
Photoelectric relay PVT312, single-pole, normally open, can be used on both direct and alternating current.
This solid state relay is specially designed for use in telecommunication systems. Relay series PVT312L(suffixed with "L") use active current limiting circuitry, which allows them to withstand transient current surges. PVT312 is available in a 6-pin DIP package.
Application: telecommunication keys, triggers, general schemes switching
Connection diagrams can be of three types (Fig. 2). In the first case, two chip keys are connected in series. Due to the symmetry, this allows the resulting circuit to switch alternating voltage. This type of circuit is called a type “A” connection. Type “B” differs in that only one of the two chip keys is used. This allows you to switch a larger, but only direct current. In the third option (type “C”), the keys are connected in parallel, thereby increasing the maximum possible current value.
Rice. 2.
PVG612: low voltage medium voltage relay
power for AC
Photoelectric Relay Series PVG612 - unipolar, normally open solid state relays. Compact devices The PVG612 series are used for isolated switching of currents up to 1 A with voltages from 12 to 48 V AC or DC.
Relays of this type are interesting in that they are capable of switching relatively large (for of this type devices) alternating currents, while maintaining the speed inherent in MOS transistor solutions.
PVDZ172N: low voltage medium
power for DC
Relays of this series (Fig. 3), unlike those described above, are designed for switching currents only of constant polarity with a power of up to 1.5 A and a voltage of up to 60 V. For example, these relays are used in controlling lighting devices, motors, heating elements, etc. .d.
Rice. 3.
PVDZ172N Available in normally open, single-pole design in 8-pin DIP packages.
Other possible applications: audio equipment, power supplies, computers and peripherals.
PVX6012: for heavy loads
For large low frequency loads, IR offers photoelectric relay PVX6012(Fig. 4) (single-pole, normally open). The device uses an output switch based on bipolar transistor with an insulated gate (IGBT), which made it possible to obtain a low voltage drop in the on-state and low loss currents in the closed state at sufficiently high speed operation (7 ms - on/1ms - off).
Rice. 4.
The PVX6012 is available in a 14-pin DIP package, which, interestingly, uses only four pins - this solution allows for better cooling of the device.
Main applications include: test equipment; industrial control and automation; replacement of electromechanical relays; replacement of mercury relays.
PVI: photo insulator for external
high power keys
Devices in this series are not relays in the proper sense of the word. That is, they are not able to commute large energy flows with the help of small ones. They only provide galvanic isolation of the input from the output, hence their name - photoelectric insulator (Fig. 5).
Rice. 5.
Why is such a “underreliance” necessary? The fact is that PVI series devices produce an electrically isolated constant voltage, which is sufficient to directly drive the gates of high-power MOSFETs and IGBTs. In fact, this is an opto-relay, but without an output switch, for which the developer can use a separate transistor suitable for its power.
PVIs are ideal for applications requiring high current and/or high voltage switching with optical isolation between control circuitry and high power load circuits.
In addition, the series insulator PVI1050N contains two simultaneously controlled outputs, which makes it possible to connect them in series or in parallel to provide a higher control current (MOC) or a higher control voltage (IGT). Thus, in fact, you can get an output signal of 10 V / 5 μA when connected in series and 5 V / 10 μA when connected in parallel.
The two outputs of the PVI1050N can be used separately, provided that the potential difference between the outputs does not exceed 1200 VDC. The input-output isolation is 2500 VDC.
Devices of this series are produced in 8-pin DIP packages and are used in organizing the control of powerful loads, voltage converters, etc.
PVR13: double fast acting relay
The main feature of this series is the presence of two independent relays in one housing (Fig. 6), each of which can be connected as type “A”, “B”, or “C” (for an explanation of the types, see above in the description of PVT312). Maximum switching voltage 100 V (DC/AC), current 300 mA. Otherwise, this relay is close in scope and characteristics to PVA33 and is also intended for switching analog signals mid frequency(up to hundreds of kilohertz).
Rice. 6.
Available in 16-pin DIP packages with pins for through-hole mounting.
The main characteristics of IR optoelectronic relays are presented in Table 1.
Table 1. Parameters of IR optoelectronic relays
| Characteristics | PVA33 | PVT312 | PVG612N | PVDZ172N | PVX6012 |
|---|---|---|---|---|---|
| Input characteristics | |||||
| Minimum control current, mA | 1…2 | 2 | 10 | 10 | 5 |
| Max. control current for being in the closed state, mA | 0,01 | 0,4 | 0,4 | 0,4 | 0,4 |
| Control current range (current limitation required!), mA | 5…25 | 2…25 | 5…25 | 5…25 | 5…25 |
| Maximum reverse voltage, V | 6 | 6 | 6 | 6 | 6 |
| Output characteristics | |||||
| Operating voltage range, V | 0…300 | 0…250 | 0…60 | 0…60 (constant) | 280 (AC)/400 (DC) |
| Maximum continuous load current at 40°C, A | 0,15 | - | - | 1,5 | 1 |
| A conn. (post or variable) | - | 0,19 | 1 | - | - |
| In connection (fast.) | - | 0,21 | 1,5 | - | - |
| With connection (fast.) | - | 0,32 | 2 | - | - |
| Maximum pulse current, A | - | - | 2,4 | 4 | not a repeat. 5 A (1 sec) |
| Resistance in open state, no more, Ohm | 24 | - | - | 0,25 | - |
| A conn. | - | 10 | 0,5 | - | - |
| In connection | - | 5,5 | 0,25 | - | - |
| With connection | - | 3 | 0,15 | - | - |
| Resistance in closed state, not less, MOhm | 10000 | - | 100 | 100 | - |
| Turn-on time, no more. ms | 0,1 | 3 | 2 | 2 | 7 |
| Shutdown time, no more, ms | 0,11 | 0,5 | 0,5 | 0,5 | 1 |
| Output capacitance, no more, pF | 6 | 50 | 130 | 150 | 50 |
| Voltage rise rate, not less, V/µs | 1000 | - | - | - | - |
| Other | |||||
| Electric strength of insulation “input-output”, V (SCR) | 4000 | 4000 | 4000 | 4000 | 3750 |
| Insulation resistance, input-output, 90 V DC, ohms | 1012 | 1012 | 1012 | 1012 | 1012 |
| Input-output capacitance, pF | 1 | 1 | 1 | 1 | 1 |
| Maximum contact soldering temperature, °C | 260 | 260 | 260 | 260 | 260 |
| Operating temperature, °С | -40…85 | -40…85 | -40…85 | -40…85 | -40…85 |
| Storage temperature, °C | -40…100 | -40…100 | -40…100 |
-40…100 | -40…100 |
Application of Optoelectronic Relays IR
Control systems. In ACS interfaces, one of the pressing problems is the organization of communication between the control and switched circuits, ensuring reliable galvanic isolation. That is, it is necessary to organize the transmission of information (for example, a signal to an actuator) without electrical contact. One of the first devices of this kind were electromechanical relays, in which information was transmitted via a magnetic field. However, the presence of mechanical parts led to sparking contacts and low performance of such systems.
The use of signal transmission through a light flux (optoelectronic relays) in automated control system interfaces (Fig. 7) compared to electromechanical switches provides higher reliability, switching speed, durability, and better weight and size indicators; and the advantage in comparison with electronic switches is the absence of a common point and mutual influence of circuits during switching.
Rice. 7.
The presence of galvanic isolation in the control system is one of the important properties of the switch, because allows you to create separate control flows, which, in turn, makes it possible to ensure electrical independence of the information and executive zones of the system. Optical galvanic isolation isolates microelectronic control equipment from high-current and high-voltage circuits of peripheral execution devices, which leads to increased noise immunity, service life and reduced price of such equipment.
Rice. 8.
One more necessary function in measuring equipment is the switching of operating modes (measurement range, gain, type of connection, etc.), which was previously performed mechanically. For example, to measure voltage, a voltmeter is connected to the circuit in parallel, while to measure current, you need serial connection measuring equipment with circuit. In some instruments, to implement such a switch, it was necessary to use another input, mechanically switching the measuring line. This is quite inconvenient in the case of frequent changes in the measured parameter, so the use of optoelectronic relays can effectively solve this problem, significantly increasing the ease of use of the device.
On the other hand, in data acquisition systems the need to use opto-relays is often due to the high probability of damage to the sensitive input circuits of the measuring equipment (analog-to-digital and frequency converters). Such undesirable effect may arise, for example, due to the long length of conductors from the primary transducer to the measuring element, which contributes to the induction of electrostatic interference. In addition, both transient processes during switching on/off of the equipment and errors in its use, for example, the presence of a large amplitude input signal during a power outage, can have a significant impact.
All these factors lead to the need to use galvanic isolation. An example is the PVT312L series relay with a built-in active ripple current suppression circuit, which can be effectively used in devices associated with long conductors or operating in difficult electromagnetic conditions (wired environmental monitoring systems of enterprises, industrial measuring transducers).
Telecommunications. The use of opto-relays in the field of communications is also a promising area. There are several unique functions that can be effectively implemented using the advantages of an opto relay. This includes galvanic isolation between the modem and the telephone line to prevent damage associated with electrostatic (including lightning) discharges; implementation of specific functions of telephone equipment (pulse and tone dialing, connection and determination of line status), etc.
Conclusion
In recent years, there has been a trend towards a constant increase in demand for optoelectronic relays from IR. The main consumers of solid-state relays are the industrial giants of our country - instrument-making and transport enterprises, large state corporations Rostelecom, Rosatom, Russian Railways. Manufacturers value convenience and high technical specifications IR relays for industrial applications.
On the other hand, the requirements for the reliability of electronic equipment from the military and aerospace industries are constantly growing. The issue is very relevant, which requires specific technical solutions that will reduce equipment failures during operation. None of the experts doubt that solid-state relays can increase the reliability of special-purpose equipment.
Galvanic isolation. Optocoupler circuit
| An optocoupler, also known as an optocoupler, is an electronic component that transmits electrical signals between two isolated electrical circuits using infrared light. As an insulator, an optocoupler can prevent the passage high voltage along the chain. Signal transmission through the light barrier occurs using an IR LED and a photosensitive element, such as a phototransistor, which is the basis of the optocoupler structure. Optocouplers are available in various models and internal configurations. One of the most common is an IR diode and a phototransistor together in a 4-pin package, shown in the figure.
Certain parameters must not be exceeded during operation. These maximum values are used in conjunction with the graphs to correctly design the operating mode.
On the input side, the infrared emitting diode has a certain maximum forward current and voltage, exceeding which will cause the emitting element to burn out. But even a signal that is too small will not be able to make it glow, and will not allow the impulse to be transmitted further along the circuit. Advantages of optocouplers
Disadvantages of optocouplers
Application of optocouplers
The versatility of optocouplers as elements of galvanic isolation and contactless control, the diversity and uniqueness of many other functions are the reason why optocoupler applications have become computer technology, automation, communications and radio equipment, automated systems controls, measuring equipment, control and regulation systems, medical electronics, visual information display devices. Read more about various types optocouplers, read this document. |
elwo.ru
Galvanic isolation: principles and diagram
Galvanic isolation is the principle of electrical insulation of the current circuit in question in relation to other circuits that are present in one device and improves technical performance. Galvanic insulation is used to solve the following problems:
- Achieving signal chain independence. It is used when connecting various instruments and devices, ensuring the independence of the electrical signal circuit with respect to the currents arising during the connection of different types of devices. Independent galvanic coupling solves problems of electromagnetic compatibility, reduces the influence of interference, improves the signal-to-noise ratio in signal circuits, and increases the actual accuracy of measuring ongoing processes. Galvanic isolation with isolated input and output makes the devices compatible with various devices under complex electromagnetic environment parameters. Multichannel measuring instruments have group or channel isolation. The isolation can be single for several measurement channels or channel-by-channel for each channel independently.
- Compliance with the requirements of the current GOST 52319-2005 on electrical safety. The standard regulates insulation resistance in electrical control and measurement equipment. Galvanic isolation is considered as one of a set of measures to ensure electrical safety and must work in parallel with other protection methods (grounding, voltage and current limiting circuits, safety valves, etc.).
Isolation can be provided various methods and technical means: galvanic baths, inductive transformers, digital isolators, electromechanical relays.
Galvanic isolation solution diagrams
During the construction of complex systems for digital processing of incoming signals associated with operation in industrial conditions, galvanic isolation must solve the following problems:
- Protect computer circuits from exposure to critical currents and voltages. This is important if operating conditions involve exposure to industrial electromagnetic waves, there are difficulties with grounding, etc. Such situations also occur in transport, which has a large human influence factor. Errors can cause complete failure of expensive equipment.
- Protect users from harm electric shock. The problem is most often relevant for medical devices.
- Minimize the harmful effects of various interferences. Important factor in laboratories performing precise measurements, when building precision systems, at metrological stations.
Currently, transformer and optoelectronic isolation are widely used.
Operating principle of the optocoupler
Optocoupler circuit
The light-emitting diode is forward biased and receives only light from the phototransistor. This method provides a galvanic connection between circuits that are connected on one side to an LED and on the other side to a phototransistor. The advantages of optoelectronic devices include the ability to transmit communications over a wide range, the ability to transmit pure signals at high frequencies, and small linear dimensions.
Electrical impulse multipliers
They provide the required level of electrical insulation and consist of transmitter-emitters, communication lines and receiving devices.
Pulse multipliers
The communication line must provide the required level of signal isolation; in the receiving devices, the pulses are amplified to the values necessary to start the thyristors into operation.
The use of electrical transformers for isolation increases reliability installed systems, built on the basis of sequential multicomplex channels in the event of failure of one of them.
Parameters of multi-complex channels
Channel messages consist of information, command or response signals; one of the addresses is free and is used to execute system tasks. The use of transformers increases the reliability of the functioning of systems assembled on the basis of serial multicomplex channels and ensures the operation of the device when several recipients fail. Due to the use of multi-stage transmission control at the signal level, high levels of noise immunity are ensured. In the general operating mode, it is possible to send messages to several consumers, which facilitates the initial initialization of the system.
The simplest electrical device is an electromagnetic relay. But galvanic isolation based on this device has high inertia, is relatively large in size and can only provide a small number of consumers with a large amount of energy consumed. Such disadvantages prevent the widespread use of relays.
Galvanic isolation of the push-pull type allows you to significantly reduce the amount of used electrical energy in full load mode, thereby improving the economic performance of the devices.
Push-pull isolation
Through the use of galvanic isolations it is possible to create modern circuits automatic control, diagnostics and control with high safety, reliability and stability of operation.
plast-product.ru
Galvanic isolation. If not an optocoupler, who?
There is such a thing in electronics as galvanic isolation. Its classic definition is the transfer of energy or signal between electrical circuits without electrical contact. If you are a beginner, then this formulation will seem very general and even mysterious. If you have engineering experience or just remember physics well, then most likely you have already thought about transformers and optocouplers.
The article below the cut is dedicated to in various ways galvanic isolation digital signals. We’ll tell you why it’s needed at all and how manufacturers implement an insulation barrier “inside” modern microcircuits.
As already mentioned, we will talk about isolating digital signals. Further in the text, by galvanic isolation we will understand the transmission of an information signal between two independent electrical circuits.
Why is it needed?
There are three main tasks that are solved by decoupling a digital signal.
The first thing that comes to mind is protection against high voltages. Indeed, ensuring galvanic isolation is a safety requirement for most electrical appliances. Let the microcontroller, which naturally has a small supply voltage, set control signals for a power transistor or other high voltage device. This is a more than common task. If there is no insulation between the driver, which increases the control signal in power and voltage, and the control device, then the microcontroller risks simply burning out. In addition, input-output devices are usually connected to control circuits, which means that a person pressing the “turn on” button can easily close the circuit and receive a shock of several hundred volts. So, galvanic isolation of the signal serves to protect people and equipment. 
No less popular is the use of microcircuits with an isolation barrier for interfacing electrical circuits with different supply voltages. Everything is simple here: there is no “electrical connection” between the circuits, so the signal, the logical levels of the information signal at the input and output of the microcircuit, will correspond to the power supply on the “input” and “output” circuits, respectively. 
Galvanic isolation is also used to improve the noise immunity of systems. One of the main sources of interference in electronic equipment is the so-called common wire, often the device housing. When transmitting information without galvanic isolation, the common wire provides the common potential of the transmitter and receiver necessary for transmitting the information signal. Since the common wire usually serves as one of the power poles, connecting various electronic devices to it, especially power ones, leads to short-term impulse noise. They are eliminated by replacing the "electrical connection" with a connection through an insulating barrier. 
How it works
Traditionally, galvanic isolation is based on two elements - transformers and optocouplers. If we omit the details, the first ones are used for analog signals, and the second ones are used for digital signals. We are considering only the second case, so it makes sense to remind the reader who an optocoupler is. To transmit a signal without electrical contact, a pair of a light emitter (most often an LED) and a photodetector is used. The electrical signal at the input is converted into “light pulses”, passes through the light-transmitting layer, is received by a photodetector and is converted back into an electrical signal.
Optocoupler isolation has earned enormous popularity and has been the only technology for isolating digital signals for several decades. However, with the development of the semiconductor industry, with the integration of everything and everyone, microcircuits appeared that implement an insulation barrier at the expense of other, more modern technologies. Digital isolators are microcircuits that provide one or more isolated channels, each of which outperforms the optocoupler in terms of speed and accuracy of signal transmission, level of resistance to interference and, most often, cost per channel.
The isolation barrier of digital isolators is manufactured using various technologies. The well-known company Analog Devices uses a pulse transformer as a barrier in ADUM digital isolators. Inside the microcircuit housing there are two crystals and a pulse transformer, made separately on a polyamide film. The transmitter crystal generates two short pulses at the edge of the information signal, and one pulse at the decline of the information signal. A pulse transformer allows, with a slight delay, to receive pulses on the transmitter crystal through which the inverse conversion is performed.
The described technology is successfully used in the implementation of galvanic isolation; it is in many ways superior to optocouplers, but has a number of disadvantages associated with the sensitivity of the transformer to interference and the risk of distortion when working with short input pulses.
A much higher level of noise immunity is provided in microcircuits where the isolation barrier is implemented on capacitors. The use of capacitors eliminates communication DC between the receiver and the transmitter, which in signal circuits is equivalent to galvanic isolation. 
If the last sentence agitated you... If you felt a burning desire to scream that there cannot be galvanic isolation on capacitors, then I recommend visiting threads like this one. When your rage subsides, note that all of this controversy dates back to 2006. As we know, we will not return there, just like in 2007. And insulators with a capacitive barrier have been produced for a long time, used and work perfectly.
The advantages of capacitive decoupling are high energy efficiency, small dimensions and resistance to external influences. magnetic fields. This makes it possible to create inexpensive integral insulators with high reliability indicators. They are produced by two companies - Texas Instruments and Silicon Labs. These companies use different technologies for creating the channel, but in both cases silicon dioxide is used as the dielectric. This material has high electrical strength and has been used in the production of microcircuits for several decades. As a result, SiO2 is easily integrated into the crystal, and a dielectric layer several micrometers thick is sufficient to provide an insulation voltage of several kilovolts. On one (at Texas Instruments) or on both (at Silicon Labs) crystals, which are located in the digital isolator housing, capacitor pads are located. The chips are connected through these pads, so the information signal passes from the receiver to the transmitter through the isolation barrier. Although Texas Instruments and Silicon Labs use very similar technologies for integrating a capacitive barrier on the chip, they use completely different principles for transmitting the information signal.
Each Texas Instruments isolated channel is a relatively complex circuit.
Let's look at its “lower half”. The information signal is supplied to RC circuits, from which short pulses are taken along the leading edge and falling edge of the input signal, and the signal is reconstructed from these pulses. This method of passing a capacitive barrier is not suitable for slowly changing (low frequency) signals. The manufacturer solves this problem by duplicating channels - the “lower half” of the circuit is a high-frequency channel and is intended for signals from 100 Kbps. Signals below 100 Kbps are processed in the "top half" of the circuit. The input signal is subjected to preliminary PWM modulation with a large clock frequency, the modulated signal is fed to the insulating barrier, the signal is restored using pulses from the RC circuits and is subsequently demodulated. The decision-making circuit at the output of the isolated channel “decides” from which “half” the signal should be sent to the output of the microcircuit.
As can be seen in the Texas Instruments isolator channel diagram, both in low frequency and in high frequency channels differential signal transmission is used. Let me remind the reader of its essence.
Differential transmission is simple and effective way protection against common mode interference. The input signal on the transmitter side is “divided” into two signals V+ and V-, inverse to each other, which are equally affected by common-mode interference of different natures. The receiver subtracts the signals and, as a result, the Vsp interference is eliminated.
Differential transmission is also used in digital isolators from Silicon Labs. These microcircuits have a simpler and more reliable structure. To pass through the capacitive barrier, the input signal is subjected to high-frequency OOK (On-Off Keyring) modulation. In other words, a “one” of an information signal is encoded by the presence of a high-frequency signal, and a “zero” by the absence of a high-frequency signal. The modulated signal passes without distortion through a pair of capacitors and is restored at the transmitter side.
