The series of articles consists of three parts:

Interference in circuits.

During normal operation of an electronic device, interference may occur in the circuit.

Interference can not only interfere with the normal operation of the device, but also lead to its complete failure.

Rice. 1. Interference in the useful signal.

You can see the interference on the oscilloscope screen by including it in the part of the circuit under study (Fig. 1). The duration of interference can be either very short (a few nanoseconds, so-called “needles”) or very long (several seconds). The shape and polarity of the interference also varies.
The propagation (passage) of interference occurs not only along the wire connections of the circuit, but sometimes even between parts of the circuit that are not connected by wires. In addition, interference can overlap and add up to each other. Thus, a single weak interference may not cause a malfunction in the device circuit, but the simultaneous accumulation of several weak random interferences leads to incorrect operation of the device. This fact makes the search and elimination of interference many times more difficult, since it takes on an even more random nature.

Sources of interference can be roughly divided:

• External source of interference. A strong electromagnetic or electrostatic field source near the device may cause the electronic device to malfunction. For example, a lightning discharge, relay switching of high currents or electric welding.
• Internal source of interference. For example, when you turn on/off a reactive load (an electric motor or an electromagnet) in a device, the rest of the circuit may malfunction. An incorrect program algorithm can also be a source of internal interference.

To protect against external interference, the structure or its individual parts are placed in a metal or electromagnetic shield, and circuit solutions with less sensitivity to external interference are also used. The use of filters, optimization of the operating algorithm, changes in the construction of the entire circuit and the location of its parts relative to each other help against internal interference.
What is considered very elegant is not the indiscriminate suppression of all interference, but the deliberate direction of them to those places in the circuit where they will fade out without causing harm. In some cases, this path is much simpler, more compact and cheaper.

Assessing the probability of interference in circuits and ways to prevent them is not a simple task, requiring theoretical knowledge and practical experience. But nevertheless, we can firmly say that the probability of interference increases:

• with an increase in switched current or voltage in the circuit,
• with increasing sensitivity of parts of the circuit,
• with an increase in the performance of the used parts.

In order not to redo the finished design due to frequent failures, it is better to become familiar with the possible sources and paths of interference at the circuit design stage. Since about half of all manifestations of interference are associated with “bad” power supply, it is best to start designing a device by choosing a method for powering its parts.

Interference in power supply circuits.

Figure 2 shows a typical block diagram of an electronic device, which consists of a power source, control circuit, driver and actuator.
Most of the simplest robots from the series on this site are built according to this scheme.

Rice. 2. Joint power supply of the control and power parts.

In such circuits we can conditionally distinguish two parts: control and power. The control part consumes relatively little current and contains any control or computing circuits. The power section consumes significantly more current and includes an amplifier and termination load.
Let's look at each part of the circuit in more detail.

Rice. 2 a.

Power supply(Fig. 2 a.) can be “batteries” or a mains transformer power supply. The power supply may also include a voltage stabilizer and a small filter.

Rice. 2 b.

Control circuit- this is part of the circuit (Fig. 2 b.), where any information is processed in accordance with the operation of the algorithm. Signals from external sources, for example, from some sensors, can also come here. The control circuit itself can be assembled using microcontrollers or other microcircuits, or using discrete elements.

Communication lines they simply connect the control circuit to the driver-executive device, that is, these are just wiring or tracks on a printed circuit board.

Rice. 2nd century

Actuator(Fig. 2 c.) is often a mechanism that converts an electrical signal into mechanical work, such as an electric motor or electromagnet. That is, the actuator converts electrical current into another form of energy and usually consumes a relatively large current.

Rice. 2 years

Since the signal from the control circuit is very weak, so driver or amplifier(Fig. 2 d) is an integral part of many schemes. The driver can be made, for example, using only a transistor or a special chip, depending on the type of actuator.


As a rule, the main source of strong interference is the actuator. The interference that appears here, having passed through the driver, spreads further along the power bus (The interference in Fig. 2 is shown schematically by an orange arrow). And since the control circuit is powered from the same power source, there is a high probability that this interference will affect it as well. That is, for example, an interference that appears in the motor will pass through the driver and can lead to a failure in the control circuit.
In simple circuits, it is enough to place a large capacitor of about 1000 μF and a ceramic 0.1 μF capacitor in parallel with the power source. They will act as a simple filter. In circuits with consumption currents of about 1 ampere or more, to protect against strong interference of complex shapes, you will have to install a bulky, complex filter, but this does not always help.
In many circuits, the easiest way to get rid of the effects of interference is to use separate power supplies for the control and power parts of the circuit, that is, the use of the so-called separate power supply.
Although separate power supply is used not only to combat interference.

Separate meals.

In Fig. Figure 3 shows a block diagram of a certain device. This circuit uses two power supplies. The power part of the circuit is powered from power supply 1, and the control circuit is from power supply 2. Both power sources are connected by one of the poles; this wire is common to the entire circuit and signals are transmitted relative to it along the communication line.

Rice. 3. Separate power supply for the control and power parts.

At first glance, such a circuit with two power supplies looks cumbersome and complex. In fact, such separate power supply circuits are used, for example, in 95% of all household equipment. Separate power supplies there are just different windings of transformers with different voltages and currents. This is another advantage of separate power supply circuits: several units with different supply voltages can be used in one device. For example, use 5 volts for the controller, and 10-15 volts for the motor.
If you look closely at the diagram in Fig. 3, it is clear that interference from the power part does not have the opportunity to get into the control part via the power line. Consequently, the need to suppress or filter it completely disappears.

Rice. 4. Separate power supply with stabilizer.

In mobile structures, for example, mobile robots, due to their size, it is not always convenient to use two battery packs. Therefore, separate power supply can be built using one battery pack. The control circuit will be powered from the main power source through a stabilizer with a low-power filter, Fig. 4. In this circuit, you need to take into account the voltage drop across the stabilizer of the selected type. Typically a battery pack with a higher voltage than the voltage required for the control circuit is used. In this case, the functionality of the circuit is maintained even when the batteries are partially discharged.

Rice. 5. L293 with separate power supply.

Many driver chips are specifically designed for use in circuits with separate power supply. For example, the well-known L293 driver chip ( Rice. 5) has a conclusion Vss- for powering the control circuit (Logic Supply Voltage) and output Vs- to power the final stages of the power driver (Supply Voltage or Output Supply Voltage).
In all robot designs with a microcontroller or logic chip from the series, L293 can be switched on with a separate power supply circuit. In this case, the power supply voltage (voltage for the motors) can be in the range from 4.5 to 36 volts, and the voltage at Vss can be supplied the same as for powering the microcontroller or logic chip (usually 5 volts).

If the power supply to the control part (microcontroller or logic chip) occurs through a stabilizer, and the power supply to the power part is taken directly from the battery pack, then this can significantly save energy losses. Since the stabilizer will only power the control circuit, and not the entire structure. This - Another advantage of separate power supply: energy saving.

If you look again at the diagram in Figure 3, you will notice that in addition to the common wire (GND), the power section is also connected to the control circuit by communication lines. In some cases, these wires can also carry interference from the power part into the control circuit. In addition, these communication lines are often highly susceptible to electromagnetic influences (“noise”). You can get rid of these harmful phenomena once and for all by using the so-called galvanic isolation.
Although galvanic isolation is also used not only to combat interference.

Galvanic isolation.

At first glance, this definition may seem incredible!
How can a signal be transmitted without electrical contact?
In fact, there are even two ways that allow this.

Rice. 6.

Optical signal transmission method based on the phenomenon of photosensitivity of semiconductors. For this, a pair of an LED and a photosensitive device (phototransistor, photodiode) is used, Fig. 6.

Rice. 7.

The LED-photodetector pair is located in isolation in one housing opposite each other. This is what this detail is called optocoupler(foreign name optocopler), Fig. 7.
If current is passed through the optocoupler LED, the resistance of the built-in photodetector will change. This is how contactless signal transmission occurs, since the LED is completely isolated from the photodetector.
Each signal transmission line requires a separate optocoupler. The frequency of the optically transmitted signal can range from zero to several tens to hundreds of kilohertz.

Rice. 8.

Inductive signal transmission method is based on the phenomenon of electromagnetic induction in a transformer. When the current changes in one of the windings of the transformer, the current in its other winding changes. Thus, the signal is transmitted from the first winding to the second (Fig. 8). This connection between the windings is also called transformer, and a transformer for galvanic isolation is sometimes called isolation transformer.

Rice. 9.

Structurally, transformers are usually made on a ring ferrite core, and the windings contain several tens of turns of wire (Fig. 9). Despite the apparent complexity of such a transformer, you can make it yourself in a few minutes. Ready-made small-sized transformers for galvanic isolation are also sold.
Each signal transmission line requires a separate such transformer. The frequency of the transmitted signal can range from several tens of hertz to hundreds of thousands of megahertz.

Depending on the type of signal being transmitted and the circuit requirements, you can choose either transformer or optical galvanic isolation. In circuits with galvanic isolation, special converters are often installed on both sides to coordinate (connect, interface) with the rest of the circuit.

Let us now consider the block diagram using galvanic isolation between the control and power parts in Figure 10.

Rice. 10. Separate power supply and galvanic isolation of the communication channel.

From this diagram it can be seen that any interference from the power part has no way of penetrating into the control part, since there is no electrical contact between the parts of the circuit.
The absence of electrical contact between parts of the circuit in the case of galvanic isolation allows you to safely control actuators with high voltage power. For example, a control panel powered by a few volts can be galvanically isolated from a phase network voltage of several hundred volts, which increases safety for operating personnel. This is an important advantage of galvanic isolation circuits.

Control circuits with galvanic isolation can almost always be found in critical devices, as well as in pulsed power supplies. Especially where there is even the slightest chance of interference. But even in amateur devices, galvanic isolation is used. Since a slight complication of the circuit by galvanic isolation brings complete confidence in the uninterrupted operation of the device.

International Rectifier, a designer and manufacturer of power electronics since 1947, produces a huge range of opto-relays for a variety 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 MOS 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 the frequency characteristics of the 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.

Applications: telecommunications keys, triggers, general switching circuits.

Connection diagrams can be of three types (Fig. 2). In the first case, two chip keys are connected in series. This allows the resulting circuit to switch alternating voltage due to its symmetry. 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. The compact devices of 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 this type of device) alternating currents, while maintaining the operating speed inherent in solutions based on MOS transistors.

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 include 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 an insulated gate bipolar transistor (IGBT), which makes it possible to obtain a low voltage drop in the on state and low loss currents in the closed state at a fairly high operating speed (7 ms on / 1 ms 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 the PVI series devices produce, upon receiving an input signal, an electrically isolated DC voltage, which is sufficient to directly control 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 the PVA33 and is also intended for switching analog signals of medium 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, °C	-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.

Another necessary function in measuring equipment is switching operating modes (measuring 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, the measuring equipment must be connected in series to the 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 if the measured parameter changes frequently, 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 an undesirable effect may arise, for example, due to the long length of the 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 the convenience and high technical performance of 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.

To switch loads in AC circuits, circuits using powerful field-effect transistors have recently begun to be increasingly used. 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 circuit allows for galvanic isolation of the 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 at a given 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. Powerful transistors VT1 and VT2 are also attached to it through heat-conducting paste and mica insulating spacers using screws with bushings. 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.

Modern life is unthinkable without television. In many apartments you can find two and sometimes three television receivers. Cable TV is especially popular. But what if you need to connect several TVs to one antenna cable? It is natural to use a “Chinese” double or even tee.

For example, like this one:

I installed just such a double splitter on two TVs to receive cable TV channels. However, the quality of reception left much to be desired; if the channels of the first meter range were shown tolerably, then the channels of the second and UHF ranges were received with strong signal attenuation. Having disassembled the splitter, I found in it a small double ferrite ring and several turns of single-core wire:

The device is a high-frequency transformer with antiphase winding of the windings. And in theory, it should exclude the mutual influence of the input circuits for receiving the RF signal, but in fact it only weakened it, apparently due to the fact that there was a galvanic connection

I decided to replace the transformer with ordinary ceramic capacitors (red flags) with a nominal value of several picofarads, thereby eliminating this galvanic connection:

My surprise knew no bounds; both TVs were shown as if only one was working, i.e. not the slightest hint of mutual influence and excellent reception on all bands.

The containers fit into the splitter housing:

The only thing I blame myself for is why this idea didn’t come to me earlier.

Introduction

Galvanic isolation (isolation), commonly referred to simply as decoupling, is a method by which individual parts of an electrical system can be at different ground potentials. The two most common reasons for creating decoupling are for safety from faults in industrial-grade products, and where wired communication is required between devices, each of which has its own power supply.

Power decoupling methods

Transformers

The most common form of decoupling is the use of a transformer. When designing a power stabilization circuit where decoupling is required, the isolating part of the design is associated with the need to increase/decrease the voltage level and is not considered as a separate part of the system. In the event that it is necessary to isolate the entire electrical system (for example, many automotive test equipment requires power supplies to be isolated from the AC mains), a 1:1 transformer can be installed in series with the system to provide the necessary isolation.

Figure 1 - Assortment of SMD transformers

Capacitors

A less common method of creating decoupling is to use capacitors in series. Because of the ability of AC signals to flow through capacitors, this method can be an effective way to isolate parts of an electrical system from the AC mains. This method is less reliable than the transformer method because if a fault occurs, the transformer breaks the circuit and shorts the capacitor. One of the purposes of creating galvanic isolation from the AC mains is to ensure that in the event of a fault, the user is safe from a functioning unlimited current source.

Figure 2 - Example of using capacitors to create decoupling

Signal Isolation Methods

Opto-isolators

When a signal is required to pass between two parts of a circuit at different ground potentials, a popular solution is an opto-isolator (optocoupler). The opto-isolator is a phototransistor that opens (“turns on”) when the internal LED is energized. The light emitted by the internal LED is the signal path and thus the isolation between ground potentials is not broken.

Figure 3 - Diagram of a typical opto-isolator

Hall sensor

Another method of transmitting information between electrical systems with separate ground potentials is the use of a sensor based on the Hall effect. The Hall sensor detects induction non-invasively and does not require direct contact with the signal of interest and does not violate the insulating barrier. The most common use of passing inductive information through circuits at different ground potentials is in current sensors.

Figure 4 - Current sensor used to measure current through a conductor

Conclusion

Galvanic isolation (isolation) is the separation of electrical systems/subsystems that may not carry direct current and may have different ground potentials. Isolation can be divided into main categories: power and signal. There are several ways to achieve decoupling, and depending on the project requirements, some methods may be preferable to others.

Case Study

Figure 5 - PoE (Power over Ethernet) project diagram based on the TPS23753PW controller

In the diagram above, several transformers and an opto-isolator are used to create a switching power supply, which is used in Ethernet PD (Powered Device) devices. Connector J2 has internal magnets that isolate the entire system from the PoE source. T1 and U2 isolate the power supply (to the left of the red line) from the regulated 3.3V output (to the right of the red line).