This article will talk about relay contact protection and input circuits of devices sensitive to voltage and current surges in DC and AC circuits using:

• RC chains;
• diode circuit;
• diode-zener diode circuit;
• varistor circuit.

When turning on and off various electrical equipment, the current in the electrical circuit, as a rule, differs from the steady-state value. In this case, the magnitude of the scatter is several times. Below are diagrams of current changes when different typical types of loads are turned on.

When an inductive load is turned off, a self-inductive emf occurs (from several hundred to several thousand volts). Such a voltage surge can damage the switching element or significantly reduce its service life. If the current in these loads is relatively small (a few amperes), then the effect of self-induced emf on the contacts commutating the inductive load can lead to a corona discharge or arc.

This, in turn, can lead to the appearance of oxides and carbides on the contacts. The impact of self-inductive EMF can also damage a device that shares a power circuit with an inductive load.

For example, an electronic time relay connected in parallel with a powerful intermediate relay may be damaged or unstable if measures are not taken to protect against self-induction EMF.

When an electric arc occurs between the contacts, the contact points are destroyed due to the transfer of material from the contacting surfaces. This leads to welding of the contacts and a change in the shape of the contacts and, as a result, to an increase in the contact resistance.

An increase in contact resistance leads to an increase in heat generation at the contact site, its oxidation and, as a result, to complete loss of contact.

To preserve contact life and protect loads, various protection methods are used.

Protection of contacts and input circuits of devices sensitive to voltage and current surges in DC and AC circuits.

An interference suppression RC chain (network snubber, network damper, RC SNUBBER NETWORKS, RC element) is a device used to suppress voltage surges (Surge suppressors) in electrical circuits, a device for suppressing surge voltages.

The use of RC circuits smoothes out and limits switching overvoltages on the elements of relay control circuits, reduces sparking at the contacts of the control relay and thereby increases its switching life. Preventing or minimizing sparking at relay contacts reduces the intensity of electromagnetic radiation generated during switching moments, which provides the necessary noise immunity when operating sensitive electronic circuits.

An RC circuit consists of a capacitor and a resistor connected in series. The capacitor must absorb the energy of current and voltage pulses and provide protection against potentials generated by the inductance during disconnection and contact bounce. The dielectric of the capacitor used in the snubber circuit must withstand the magnitude of the overvoltage. The resistor must be of a non-inductive type to ensure high performance of the snubber and conduct the impulse noise current. Spark discharges and induced noise generated during switching must be effectively absorbed by the RC circuit.

When controlling electromagnetic devices with significant inductance (for example, solenoids of solenoid valves, coils of electromagnetic starters, relays and contactors), it is recommended to use noise-suppressing RC circuits in accordance with the diagram shown in Fig. 1.

Rice. 1. Inclusion of a noise-suppressing RC circuit in the contactor control circuit. a) circuit without RC circuit; b) circuit with a connected RC circuit

Detailed oscillograms taken in the control circuit of a real ATS are shown in the figures below.

In Fig. Figure 2 shows an oscillogram of the 220 V voltage on the coil of the control relay in a circuit without a noise-suppressing RC circuit, in accordance with Fig. 1a. The circuit uses an ABB ESB 20-11 contactor. The voltage surge when the control relay contacts are turned off was +2200 V (1 division = 1000 V).

Rice. 2. Oscillogram of the voltage on the control relay coil in a circuit without a noise-suppressing RC circuit.

In Fig. Figure 3 shows an oscillogram of the 220 V voltage on the control relay coil in a circuit with an installed noise-suppressing RC circuit, in accordance with Fig. 1b. The circuit uses an ABB ESB 20-11 contactor. There is no voltage surge when the control relay contacts are disconnected (1 div = 1000 V).

Rice. 3. Voltage oscillogram on the control relay coil in a circuit with an installed noise-suppressing RC circuit.

Rice. 4. Method of connecting the RC circuit to the contactor

Note. The use of a noise-suppressing RC circuit with the specified parameters leads to a slight increase in the shutdown time of the contactor/magnetic starter. This delay ranges from 0.05 to 0.015 s, depending on the contactor type. In most applications, the increase in latency is negligible.

Incorrect selection of the parameters of the RC noise suppression circuit on the coil leads to slower operation of the contactor in certain operating modes and even greater rattling of its power contacts.

RC chains:

• RC circuit with a capacitor with a capacity of 0.1 μF/630V DC and a resistor with a resistance of 100 Ohm/2 W for voltage - 250/600 V (AC/DC);
• RC circuit with a capacitor with a capacity of 0.47 µF/400 V and a resistor with a resistance of 220 Ohm/2 W - 127/200 V (AC/DC).

Based on materials: wel.net.ua

Those having a discrete (relay, transistor) output are often connected to an inductive load (devices containing an inductor). The occurrence of arc discharges when such electrical circuits are opened has an extremely negative effect on the performance of relay contacts and output stages of sensors, reducing their service life.

In order to eliminate the harmful effects of arc discharges, spark-extinguishing circuits are used, installed parallel to the relay contacts or parallel to the load.

Without going into the physics of transient processes and the causes of arc discharges, we will consider the most effective and widely used spark-extinguishing circuits of direct and alternating current.

DC circuits:

The silicon diode is connected in parallel with the inductive load; when the contacts are closed and in steady state, it does not have any effect on the operation of the circuit. When the load is turned off, a self-induction voltage appears, the polarity of which is opposite to the operating voltage; the diode opens and shunts the inductive load. Diodes are extremely effective at eliminating arcing and preventing relay contacts from burning better than any other spark suppression circuit. This method is also applicable to alarms with transistor output.

Rules for choosing a reverse diode:

The RC circuit is the cheapest and most widely used means of protecting both AC and DC circuits.

Unlike diode circuits, RC circuits can be installed either parallel to the load or parallel to the relay contacts. In some cases, the load is physically inaccessible for mounting spark-extinguishing elements on it, and then the only way to protect the contacts is to bridge the contacts with RC circuits.

Calculation of an RC circuit connected in parallel to the relay contacts:

where C is the capacitance of the RC circuit, microfarads.

I - operating load current, A.

where R is the resistance of the RC circuit, Ohm.

I - operating load current, A.

The easiest way is to use a universal nomogram. Using the known values ​​of the power source voltage U and the load current I, two points are found on the nomogram, after which a straight line is drawn between the points showing the desired resistance value R. The capacitance value C is measured on a scale next to the current scale I. The nomogram provides the developer with fairly accurate data, in the practical implementation of the circuit, it will be necessary to select the closest standard values ​​for the resistor and capacitor of the RC circuit.

RC circuit connected in parallel with the load

Used where it is undesirable or impossible to install an RC circuit parallel to the relay contacts. The following approximate values ​​of the elements are proposed for calculation:

To protect the output transistor stages of the alarms, the RC circuit is connected in parallel with the load.

The influence of arc discharges on the stability of relay contacts is so great that for an engineer, knowledge of the basics of calculation and application of protective circuits is simply a prerequisite.

Spark arresting circuits

To reduce damage to contacts by arc discharges, the following are used:

1. special relays with large contact gaps (up to 10 mm or more) and high switching speed provided by strong contact springs;
2. magnetic blowing of contacts, realized by installing a permanent magnet or electromagnet in the plane of the contact gap. The magnetic field prevents the appearance and development of an arc and effectively protects contacts from burning;
3. spark arresting circuits installed parallel to the relay contacts or parallel to the load.

The first two methods guarantee high reliability due to design measures when developing the relay. In this case, external contact protection elements are usually not required, but special relays and magnetic blowing of contacts are quite exotic, expensive and distinguished by their large size and solid coil power (relays with a large distance between the contacts have strong contact springs).

Industrial electrical engineering focuses on inexpensive standard relays, so the use of spark arresting circuits is the most common method of extinguishing arc discharges on contacts.

Rice. 1. Effective protection significantly extends the life of contacts:

Theoretically, many physical principles can be used to extinguish the arc, but in practice the following effective and economical schemes are used:

1. RC circuits;
2. freewheeling diodes;
3. varistors;
4. combined circuits, for example, varistor + RC circuit.

Safety circuits can include:

1. parallel to inductive load;
2. parallel to the relay contacts;
3. parallel to the contacts and the load at the same time.

In Fig. Figure 1 shows a typical connection of protective circuits when operating on direct current.

Diode circuit (DC circuits only)

The cheapest and most widely used circuit for suppressing self-induction voltage. The silicon diode is connected in parallel with the inductive load; when the contacts are closed and in steady state, it does not have any effect on the operation of the circuit. When the load is turned off, a self-induction voltage appears, the polarity of which is opposite to the operating voltage; the diode opens and shunts the inductive load.

The diode should not be assumed to limit the reverse voltage at the forward voltage drop of 0.7-1 V. Due to finite internal resistance, the voltage drop across the diode depends on the current through the diode. Powerful inductive loads are capable of developing pulsed self-induction currents of up to tens of amperes, which for powerful silicon diodes corresponds to a voltage drop of about 10-20 V. Diodes are extremely effective at eliminating arc discharges and protecting relay contacts from burning better than any other spark extinguishing circuits.

Rules for choosing a reverse diode:

1. The operating current and reverse voltage of the diode must be comparable to the rated voltage and current of the load. For loads with an operating voltage of up to 250 VDC and an operating current of up to 5 A, the common 1N4007 silicon diode with a reverse voltage of 1000 VDC and a maximum pulse current of up to 20 A is quite suitable;
2. the diode leads should be as short as possible;
3. the diode should be soldered (screwed) directly to the inductive load, without long connecting wires - this improves EMC during switching processes.

Advantages of the diode circuit:

1. low cost and reliability;
2. simple calculation;
3. maximum achievable efficiency.

Disadvantages of the diode circuit:

1. diodes increase the turn-off time of inductive loads by 5-10 times, which is very undesirable for loads such as relays or contactors (contacts open more slowly, which contributes to their burning), while diode protection only works in DC circuits.

If a limiting resistance is connected in series with the diode, then the effect of the diodes on the turn-off time is reduced, but additional resistors cause higher reverse voltages than protective diodes alone (the voltage across the resistor drops according to Ohm's law).

Zener diodes (for AC and DC circuits)

Instead of a diode, a zener diode is installed parallel to the load, and for alternating current circuits, two zener diodes connected in back-to-back series. In such a circuit, the reverse voltage is limited by the zener diode to the stabilization voltage, which somewhat reduces the influence of the spark-protective circuit on the load shutdown time.

Taking into account the internal resistance of the zener diode, the reverse voltage on powerful inductive loads will be greater than the stabilization voltage by the amount of the voltage drop across the differential resistance of the zener diode.

Selecting a zener diode for the protection circuit:

1. the desired limiting voltage is selected;
2. the required power of the zener diode is selected taking into account the peak current developed by the load when a self-induction voltage occurs;
3. the true clamping voltage is checked - for this purpose experiment is desirable, and when measuring voltage it is convenient to use an oscilloscope.

Advantages of zener diodes:

1. less turn-off delay than in a diode circuit;
2. Zener diodes can be used in circuits of any polarity;
3. Zener diodes for low-power loads are cheap;
4. The circuit operates on alternating and direct current.

Disadvantages of zener diodes:

1. less efficient than in a diode circuit;
2. powerful loads require expensive zener diodes;
3. For very powerful loads, a circuit with zener diodes is technically unrealizable.

Varistor circuit (for AC and DC circuits)

A metal oxide varistor has a current-voltage characteristic similar to a bipolar zener diode. Until the limiting voltage is applied to the terminals, the varistor is practically disconnected from the circuit and is characterized only by microampere leakage currents and internal capacitance at the level of 150-1000 pF. As the voltage increases, the varistor begins to open smoothly, shunting the inductive load with its internal resistance.

With very small sizes, varistors are capable of discharging large pulse currents: for a varistor with a diameter of 7 mm, the discharge current can be equal to 500-1000 A (pulse duration less than 100 μs).

Calculation and installation of varistor protection:

1. are set by the safe voltage limits on the inductive
   load;
2. the current supplied by the inductive load during self-induction is calculated or measured to determine the required varistor current;
3. According to the catalog, a varistor is selected for the required limiting voltage; if necessary, varistors can be installed in series to select the required voltage;
4. it is necessary to check: the varistor must be closed over the entire range of operating voltages at the load (leakage current less than 10-50 μA);
5. The varistor must be mounted on the load according to the rules specified for diode protection.

Advantages of varistor protection:

1. varistors operate in AC and DC circuits;
2. normalized limiting voltage;
3. negligible impact on shutdown delay;
4. varistors are cheap;
5. Varistors ideally complement RC protective circuits when working with high load voltages.

Disadvantage of varistor protection:

1. when using only varistors, the protection of relay contacts from an electric arc is significantly worse than in diode circuits.

RC circuits (for direct and alternating current)

Unlike diode and varistor circuits, RC circuits can be installed both parallel to the load and parallel to the relay contacts. In some cases, the load is physically inaccessible for mounting spark-extinguishing elements on it, and then the only way to protect the contacts is to bridge the contacts with RC circuits.

The principle of operation of the RC circuit is based on the fact that the voltage across the capacitor cannot change instantly. The self-induction voltage is pulsed in nature, and the pulse front for typical electrical devices has a duration of 1 μs. When such a pulse is applied to the RC circuit, the voltage on the capacitor begins to increase not instantly, but with a time constant determined by the values ​​of R and C.

If we assume the internal resistance of the power source to be zero, then connecting the RC circuit in parallel with the load is equivalent to connecting the RC circuit in parallel with the relay contacts. In this sense, there is no fundamental difference in the installation of spark-extinguishing circuit elements for different switching circuits.

RC circuit parallel to relay contacts

The capacitor (see Fig. 2) begins to charge when the relay contacts open. If the time of charging the capacitor to the arc ignition voltage on the contacts is selected greater than the time of divergence of the contacts to a distance at which an arc cannot occur, then the contacts are completely protected from the occurrence of an arc. This case is ideal and unlikely in practice. In real cases, the RC circuit helps, when the circuit opens, to maintain a low voltage at the relay contacts and thereby weaken the influence of the arc.

Rice. 2. protective elements can be connected both parallel to the contacts and parallel to the load:

When only one capacitor is connected in parallel to the relay contacts, the protection circuit also works in principle, but the discharge of the capacitor through the relay contacts when they are closed leads to an inrush of current through the contacts, which is undesirable. In this sense, the RC circuit optimizes all transient processes both when closing and opening contacts.

RC circuit calculation

The easiest way is to use the universal nomogram shown in Fig. 3. Based on known power supply voltage U and load current I find two points on the nomogram, after which a straight line is drawn between the points showing the desired resistance value R. Capacitance value WITH is counted on a scale next to the current scale I. The nomogram provides the developer with fairly accurate data; during the practical implementation of the circuit, it will be necessary to select the closest standard values ​​for the resistor and capacitor of the RC circuit.

Rice. 3. The most convenient and accurate nomogram for determining the parameters of the protective RC circuit (and this graph is already more than 50 years old!)

Selecting a capacitor and resistor for the RC circuit

The capacitor should only be used with a film or paper dielectric; ceramic capacitors are not suitable for high-voltage spark-proof circuits. When choosing a resistor, you must remember that it dissipates a lot of power during the transient process. It can be recommended to use resistors with a power of 1-2 W for RC circuits, and you should definitely check whether the resistor is designed for high pulsed self-inductance voltage. It is best to use wirewound resistors, but metal film or carbon ones filled with ceramic compounds also work well.

Advantages of RC circuit:

1. good arc extinction;
2. no influence on the turn-off time of the inductive load.

Features of RC circuit: the need to use high-quality capacitor and resistor. In general, the use of RC circuits is always economically justified.

When installing a spark-extinguishing circuit parallel to the AC contacts, with the relay contacts open, a leakage current determined by the impedance of the RC circuit will flow through the load. If the load does not allow leakage current to flow or this is undesirable for circuit design reasons and for personnel safety reasons, then it is necessary to install the RC circuit in parallel with the load.

Combination of RC circuit and diode circuit

Such a circuit (sometimes called a DRC circuit) is extremely efficient and allows you to reduce to zero all undesirable effects of an electric arc on the relay contacts.

Advantages of the DRC circuit:

1. The electrical life of the relay is approaching its theoretical limit.

Disadvantages of DRC circuit:

1. The diode causes a significant delay in turning off the inductive load.

Combination of RC circuit and varistor

If you install a varistor instead of a diode, then the circuit parameters will be identical to a conventional RC spark-extinguishing circuit, but the varistor’s limitation of the self-induction voltage at the load allows the use of a lower-voltage and cheaper capacitor and resistor.

RC circuit parallel to load

It is used where it is undesirable or impossible to install an RC circuit parallel to the relay contacts. The following approximate values ​​of the elements are proposed for calculation:

1. C = 0.5-1 µF per 1 A load current;
2. R = 0.5-1 Ohm per 1 V load voltage;
3. R = 50-100% of load resistance.

After calculating the ratings R and C, it is necessary to check the additional load of the relay contacts that arises during the transient process (charging the capacitor), as described above.

The given values ​​of R and C are not optimal. If the most complete protection of contacts and the implementation of the maximum resource of the relay are required, then it is necessary to conduct an experiment and experimentally select a resistor and capacitor, observing transient processes using an oscilloscope.

Advantages of an RC circuit parallel to the load:

1. good arc suppression;
2. there is no leakage current into the load through open relay contacts.

Flaws:

1. at a load current of more than 10 A, large capacitance values ​​lead to the need to install relatively expensive and large capacitors;
2. To optimize the circuit, experimental verification and selection of elements is desirable.

The photographs show voltage oscillograms across an inductive load at the moment the power is turned off without shunting (Fig. 4) and with an RCE circuit installed (Fig. 5). Both waveforms have a vertical scale of 100 volts/division.

Rice. 4. Disabling an inductive load causes a very complex transient process.

Rice. 5. A properly selected protective RSE chain completely eliminates the transient process

No special comment is required here; the effect of installing a spark-extinguishing circuit is immediately visible. The process of generating high-frequency, high-voltage interference at the moment of opening the contacts is striking.

Photos taken from a university report on the optimization of RC circuits installed in parallel with relay contacts. The author of the report conducted a complex mathematical analysis of the behavior of an inductive load with a shunt in the form of an RC circuit, but in the end, recommendations for calculating elements were reduced to two formulas:

C = I 2 /10

Where WITH– capacity of the RC circuit, μF;I– operating load current, A;

R = E o /(10І(1 + 50/E o))

Where E o– load voltage; IN, I– operating load current, A; R– resistance of the RC circuit, Ohm.

Answer: C = 0.1 µF, R = 20 Ohm. These parameters are in excellent agreement with the nomogram given earlier.

In conclusion, let's take a look at the table from the same report, which shows the practically measured voltage and delay time for various spark-extinguishing circuits. An electromagnetic relay with a coil voltage of 28 VDC/1 W served as an inductive load; the spark-extinguishing circuit was installed parallel to the relay coil.

Inductive loads and electromagnetic compatibility (EMC)

EMC requirements are a prerequisite for the operation of electrical equipment and are understood as:

1. the ability of the equipment to operate normally under conditions of exposure to powerful electromagnetic interference;
2. the property of not creating electromagnetic interference during operation above the level prescribed by standards.

The relay is insensitive to high-frequency interference, but the presence of powerful electromagnetic fields near the relay coil affects the on and off voltage of the relay. When installing relays near transformers, electromagnets and electric motors, experimental verification of the correct operation and deactivation of the relay is required. When installing a large number of relays back to back on one mounting panel or on a printed circuit board, there is also a mutual influence of the operation of one relay on the turn-on and turn-off voltage of the remaining relays. Catalogs sometimes give instructions on the minimum distance between relays of the same type, guaranteeing their normal operation. In the absence of such instructions, you can use the rule of thumb, according to which the distance between the centers of the relay coils should be at least 1.5 times their diameter. If it is necessary to tightly mount the relay on a printed circuit board, an experimental check of the mutual influence of the relay is required.

An electromagnetic relay can create a lot of noise, especially when used with inductive loads. Shown in Fig. 4, a high-frequency signal is a powerful interference that can affect the normal operation of sensitive electronic equipment operating near the relay. The frequency of the interference ranges from 5 to 50 MHz, and the power of this interference is several hundred mW, which is completely unacceptable according to modern EMC standards. Spark arresting circuits allow you to bring the level of interference from relay equipment to the safe level required by the standards.

The use of relays in grounded metal cases has a positive effect on EMC, but it must be remembered that when grounding the metal case, most relays reduce the insulation voltage between the contacts and the coil.

Insulation between relay contacts

There is a gap between the open contacts of the relay, depending on the design of the relay. The air in the gap (or inert gas for gas-filled relays) acts as an insulator. It is assumed that the insulating materials of the relay body and contact group are characterized by higher breakdown voltages than air. In the absence of contamination between the contacts, consideration of the insulating properties of the contact groups can be limited to the properties of the air gap only.

In Fig. Figure 6 (a little lower in the article) shows the dependence of the breakdown voltage on the distance between the relay contacts. In the catalogs you can find several options for the maximum voltage between contacts, namely:

1. limit value of voltage constantly applied to two contacts;
2. surge voltage;
3. the limit value of the voltage between the contacts for a certain time (usually 1 minute, during this time the leakage current should not exceed 1 or 5 mA at the specified voltage value).

If we are talking about pulsed insulation voltage, then the pulse is a standard IEC-255-5 test signal with a rise time to a peak value of 1.2 μs and a fall time to 50% of the amplitude of 50 μs.

If the developer needs a relay with special requirements for contact insulation, then information about compliance with these requirements can be obtained either from the manufacturer or by conducting independent testing. In the latter case, it must be remembered that the relay manufacturer will not be responsible for the measurement results obtained in this way.

Relay Contact Materials

The contact material determines the parameters of the contacts themselves and the relay as a whole, such as:

1. current carrying capacity, that is, the ability to effectively remove heat from the point of contact;
2. possibility of switching inductive loads;
3. contact resistance;
4. maximum ambient temperature during operation;
5. resistance of contact material to migration, especially when switching inductive loads on direct current;
6. resistance of contact material to evaporation. The evaporating metal supports the development of the electric arc and deteriorates the insulation when metal is deposited on the contact insulators and the relay body;
7. resistance of contacts to mechanical wear;
8. elasticity of contacts to absorb kinetic energy and prevent excessive chatter;
9. resistance of contact metal to corrosive gases from the environment.

Rice. 7. Each material is designed to operate contacts in a certain range of currents, but can also be used with caution for switching weak signals

Some useful properties of materials are not mutually exclusive, for example, good current conductors always have high thermal conductivity. However, good conductors with low resistivity are usually too soft and easily wear out.

The melting point is higher for special contact alloys (for example, AgNi or AgSnO), but such materials are not at all suitable for switching microcurrents.

As a result, the relay developer settles on a certain compromise between quality, price and dimensions of the relay. This compromise has led to the standardization of the applications of the various relay contacts, as shown in Fig. 7. The areas of application of various materials for contacts are quite conditional, but the designer must understand that when contacts operate at the border of the “allocated” range of currents and voltages for them, experimental verification of the reliability of such an application may be required. The experiment is very simple and consists of measuring the contact resistance of contacts for a batch of relays of the same type, and it is advisable to test not relays that have just come off the assembly line, but those that have been transported and have been in storage for some time. The optimal period of “aging” in a warehouse is 3-6 months, during which time the aging processes in plastics and metal-plastic compounds are normalized.