Solid state relay, device and principle of operation, circuit, connection, control

In the field of electrical engineering, devices based on semiconductors and providing contactless switching of power circuits are gaining popularity. One of these products is a solid state relay, which is used in the industrial field and in everyday life.

What it is? In what cases is its use recommended? What design features are important to know about? How is the device connected and controlled? We will consider these and other nuances in the article.

Definition

A solid state relay is an electronic type device, one of the types of relays in which there are no moving elements. The product is used to supply current or break the circuit by external control (by the action of a small voltage).

A solid state relay (abbreviated as SSR) has a sensor inside that reacts to the supply of a control signal. In addition, the product contains solid-state electronics, including a circuit capable of switching large I.

The device can be installed in AC and DC circuits, often used as a conventional relay. The main difference is that there are no mechanical contacts in the SSR.

Indications for use

Solid state relays are recommended for applications where standard devices fail to meet their obligations. For example, when they melt or burn during the switching process.

With the help of SSR, the reliability of the circuit and the timely supply of voltage to the load are guaranteed. Unlike simple devices, it is not a problem for an SSR to cope with an inductive load.

In addition, a solid state device should be used when there is a shortage of space during the installation process and high requirements for circuit reliability.

Where are?

Solid State Relays are unique devices that require little maintenance once installed. This is where the “set it and forget it” principle comes into play. For example, in simple models, the cleaning of the contact group is carried out with a certain frequency – as a rule, after a certain number of cycles. If the product rarely works, this does not cause problems.

But what about equipment that requires frequent operation – once a second or even more often? An example of such a technique is a machine with solenoid-type valves.

The voltage is supplied through a relay, which has to break up to ten amperes of inductive I. If a contact device is installed, it will have to be replaced every 1-2 months. If you put a solid-state analogue, you can forget about it for many years.

Despite the reliability of operation, SSRs require periodic inspection. Basic recommendations in this matter are given by the manufacturer of the product. As a rule, we are talking about checking the fact of contact closure, the integrity of the case and insulation.

Types of solid state relays

TSRs are conditionally divided according to two criteria – the principle of operation and design features. To simplify the classification, we highlight the following options:

• By the type of control signal – variable or constant I.
• According to the type of main (switched) voltage – direct or variable.
• By the number of phases (for alternating voltage) – one, three.
• By the presence of a reverse – provided, not provided.
• According to the subtleties of the design – on a DIN rail or on the surface.

Intraspecific differences

In addition to the main classification, it is worth highlighting the differences within the existing types of TTR.

The following types are distinguished:

What are the features?

When creating a solid-state relay, it was possible to exclude the appearance of an arc or sparks in the process of closing / opening a contact group. As a result, the service life of the device has increased several times. For comparison, the best versions of standard (contact) products can withstand up to 500 switching. There are no such restrictions in the TTRs under consideration.

The cost of solid state relays is higher, but the simplest calculation shows the benefits of their use. This is due to the following factors – energy savings, long service life (reliability) and the presence of control using microcircuits.

The choice is wide enough to pick up the device taking into account the tasks and the current cost. Commercially available are both small appliances for installation in household circuits and powerful devices used to control motors.

As noted earlier, SSRs differ in the type of switched voltage – they can be designed for constant or variable I. This nuance must be taken into account when choosing.

The features of solid-state models include the sensitivity of the device to load currents. To avoid such a problem during operation, it is important to carefully approach the installation process and install protective devices in the key circuit.

In addition, it is important to give preference to switches that have a working current of two or three times the switching load. But that’s not all.

For additional protection, it is recommended to provide fuses or circuit breakers in the circuit (class “B” is suitable).

Design features

The solid-state relay is based on an electronic board, which includes three main elements – control and decoupling nodes, as well as a power switch. The following parts are used as power elements:

• For constant I – field-type transistors, simple transistors, modular elements of the IGBT class, as well as MOSFET transistors.
• For variable I – assemblies based on thyristors, as well as triacs.

The decoupling of the circuit is provided by optocouplers – products consisting of a device that emits and receives light. A dielectric with a transparent structure is installed between them.

The control node is made in the form of a stabilizing circuit that provides optimal current and voltage levels for the light-emitting element. The voltage at the input of the circuit should be from 70 to 280 volts.

As for the load voltage, its value is up to 480 volts. The location of the appliance (before or after the SSR) does not matter.

As a rule, the device is mounted after the load and then connected to the ground. With this version of the circuit, it is possible to protect the internal elements from the flow of short-circuit current (it will flow through the ground wire).

Operating principle

Knowing the design features of a solid state relay, it is easier to understand the principle of its operation. Two signals interact in the device – control and controlled, which is ensured due to galvanic isolation.

In some SSR models, this function is taken over by an optocoupler. The voltage that controls the device is also applied to the LED.

The glow of the latter goes to the photodiode, which leads to the appearance of current, turning on the MOS or thyristor to control the connected device.

In addition, in the process of creating a circuit, it is allowed to use special optoelectronic devices – opto- and photothyristors.

Differences and advantages of solid state relays (in comparison with electromechanical)

When choosing a SSR, the buyer has a number of questions – why overpay for a solid state relay, what are its advantages over standard electromechanical devices. Let’s highlight the main advantages:

• Small dimensions, which eliminates the problems with finding a place for installation.
• No noise and vibration. This is important if the device is installed in rooms where there are people.
• High switching speed.
• Long service life due to the absence of wear on the mechanical and electrical parts.
• Constant output impedance that does not change over its lifetime. In addition, the contact groups are not subject to oxidative processes.
• There are no sudden voltage changes during the switching process.
• There are no sparks, which expands the scope. Its installation is allowed at facilities where there are increased risks of explosions and fires.
• Low sensitivity to external factors, for example, the appearance of magnetic fields, vibrations, increased levels of dust or magnetic fields.
• High resistance level between output and input.
• Low energy consumption.
• A large number of switching, which is not limited by the manufacturer. In reality, it reaches 10⁹.

Disadvantages

In addition to the positive qualities of solid state relays, it is worth highlighting a number of disadvantages:

• In the open form, the product heats up due to the high resistance in the pn junction circuit. To avoid negative consequences in devices that pass through themselves increased currents, it is required to provide cooling.
• When closed, the resistance increases and a reverse leakage current appears (measured in mA).
• When removing the current-voltage characteristic, its non-linear nature is noticeable.
• Some types of solid state relays require strict polarity when connecting output circuits. This applies to those devices that are designed to operate in direct current conditions.
• In the event of a breakdown, the risk of overlapping contacts at the input is high. The reason may be the breakdown of the power key. For comparison, the contacts of classical relays (in case of failure) remain open.
• Protection against erroneous operation caused by voltage surges is required. This is due to the high speed of operation.
• Solid state relays pass current through the return path with a slight delay, which is due to the use of semiconductor elements in the circuit.

Selecting a Solid State Relay

When buying a TTR, it is worth considering a number of features of the device, which will help you make the right choice. For comparison, classical devices are able to withstand overloads that occur for a short time and do not exceed one and a half or two times the rated current.

If you approach the issue of operation correctly, the usual cleaning of contacts is enough.

In the case of solid state relays, the situation is worse. If the rated current parameter is exceeded by 1,5 or more times, the device can be thrown away. That is why, when choosing an SSR for powering a resistive load, it is worth taking a current margin of two to four times.

If the product is planned to be used in the starting circuit of blood pressure, this indicator should be increased by six to ten times. With this approach, you will have to overpay, but it increases the service life of the connected device and the reliability of its operation.

Selection options

When buying a solid state relay, you should pay attention to the following parameters:

• The cost is from 100 to 12 thousand rubles.
• The number of phases is one or three.
• Maximum load current – from 10 to 500 A.
• Switched voltage level. Four ranges are possible here – from 5 to 220 V (direct current), from 24 to 380 V, from 48 to 480 V, from 24 to 480 V. The last three ranges are typical for devices operating on alternating current.
• Control signal – AC (80 to 280 V, 100 to 280 V), DC dock (3 to 32 V), resistance 0 to 560 kΩ (2 W), analog signals – current 4 to 20 mA or voltage from 0 to 10 V (constant voltage).

Recommendations for selection

In order to choose the right solid state relay, as well as to be sure of its high-quality and reliable operation over a long service life, it is important to focus on the following aspects.

METHODS OF SWITCHING

Demand is for devices in which control occurs when crossing zero. The advantage of the method is to eliminate the interference that is created during the switching process.

The minus of the option is the interruption of the output signal and the inaccessibility of the use of SSR in a circuit with a highly inductive load. The main application of this switching type is suitable for resistive type load.

In addition, SSRs are used for low inductive and capacitive loads.

PHASE CONTROL

The advantage of the phase technique is that the regulation process runs smoothly and without breaks. Thanks to this, it is possible to change the output voltage (correct the power parameter). The disadvantage of the method is the appearance of interference at the time of switching.

The method is suitable for resistive control circuits that involve heating, for variable resistive and inductive loads (IF emitters and transformers, respectively). This also includes lighting control (connecting incandescent lamps).

LOAD DATA (NATURE AND TYPE)

When choosing, pay attention to the load current. The reliability and duration of operation of the installed TTR depends on it. It is important that the device has a margin of I.

When buying, it is worth considering not only the operating current, but also the currents that occur during the start-up process and exceed the nominal parameter several times. According to the manufacturers, the SSR withstands a tenfold current overload for a short time – up to 10 ms.

If a solid state relay is installed to energize the heater (resistive load type), I must exceed the rated load current by 35-40%.

If you plan to connect an inductive load (an electric motor), it is worth considering the starting currents, which in this case exceed the rated current by 600-1000 percent.

Let’s summarize the recommended current:

• For heating elements – 30-40% more than I nominal.
• For blood pressure – 6-10 times.
• For incandescent bulbs – 8-12 times.
• EM relay – 4-10 times.

AVAILABILITY OF COOLING

In the selection process, it is worth considering the factor of temperature reduction. It was noted above that a solid state relay tends to overheat when high currents are passed. Excess heat that is released during operation is transferred to special cooling radiators.

The SSR is capable of conducting the current specified by the manufacturer if the temperature does not exceed 40 degrees Celsius. In the case of an increase in the parameter, the ability to pass I decreases – by 20-25 percent when heated for every ten degrees. Consequently, when the TTR is heated to 80 degrees Celsius, it is not able to pass current – the product breaks.

The temperature of the device is influenced by many factors, including the installation location, season, load, airflow, and others. If the device is used to connect powerful equipment, for example, to start IM, it is recommended to provide additional cooling.

To solve this problem, a radiator with large dimensions is placed. To increase the efficiency of blowing, a fan is installed.

PROTECTION

Another nuance that should be taken into account in the process of buying a solid model is the presence of the necessary protection. The following options are available here:

• Built-in RC circuit providing protection against erroneous operation when used in an inductive load circuit.
• Protection against short-term power surge from the load. For these purposes, manufacturers install varistors. The latter are selected taking into account the size of the switched U (from 1,6 to 2). It should be noted that modern relays are able to withstand significant overvoltage without the use of varistors. For such devices, overload on I carries great risks.
• Current overload protection. To solve this problem, fuses made on the semiconductor principle are used. They are selected taking into account the magnitude of the current. So, I etc. is up to 30% of the rated current of the relay. In this case, the switching device itself must have a sufficient margin. In the presence of starting currents, this nuance must be taken into account. The considered method provides reliable protection against overload. Installation in a circuit of automata does not make sense, because solid-state relays withstand overload for 10 ms, after which they burn out. The trip time of the circuit breaker is higher.
• Ensuring the reliability of the device at low load currents (equal to leakage currents). To solve this problem, the installation of shunt resistors is required, which are mounted in parallel to the load.

Add-ons to work correctly

If there is interaction with an inductive load during operation, problems may occur. That is why when using solid state relays in combination with transformers, bells, electric coils and other similar devices, an RC circuit is required in parallel to reduce the effect of back-EMF.

In addition, the presence of such a circuit will reduce the total inductance of the connected device and facilitate the operation of the SSR.

Short circuit protection

In case of damage to the insulation in the target and for other reasons, a short circuit may occur. To avoid damage to the SSR, special fuses are used. They are designed for use in combination with solid products.

They are easily recognized by the following specifications:

• gR – melting inserts operating in a wide range of I. They are used to protect semiconductors. Today it is one of the fastest devices.
• gS – like the previous fuses, they can work in all I ranges. They are used in case of high load, as well as to protect semiconductors.
• aR – melting inserts that do not have restrictions on I work. They are installed to protect semiconductors from short circuit. The disadvantage of such products is the high price. This is why many people prefer the more affordable B-class machines.

Solid state relay connection

The principle of connection is simple. The device provides control inputs (they are supplied with voltage with strict observance of polarity) and an output for connecting the load. An important point is the quality of the connection. The screw method is used here (soldering is excluded).

To avoid damage to the SSR, it is important to prevent dust from entering the contacts, as well as foreign mechanical elements. It is necessary to provide measures to prevent negative impact on the casing of the device (in the on or off state).

After switching on, do not touch the case, which may be hot. Please note that the SSR is not located near flammable materials. In addition, during the connection process, make sure that the switching is completed without errors.

If, after turning on the product, the temperature rises above 60 degrees Celsius, install a radiator on it for cooling (the reasons and features of this protective measure are discussed above).

If nothing is done, when it reaches 80 degrees Celsius, the device will stop working. Management is carried out by means of a chain with various versions.

SSR and TSR Series Solid State Relays

Today on sale there are models TSR and SSR. Let’s consider them in more detail.

Features

Products have an insulation resistance of 50 MΩ or more when tested with a megohmmeter for a voltage of 500 volts. The input and output insulation is 2 volts strong. The control power is small – 500 Volts * 12A.

It is worth highlighting the minimum emission of electromagnetic interference, which is guaranteed by switching at the zero crossing, as well as a high overload parameter for I. It is allowed to exceed the nominal I by ten times for up to one period.

transcript

The name of the product is as follows – SSR (1) – 40 (2) D (3) A (4) – H (5). The numbers in brackets correspond to the decryption number:

1. SSR or TSR is a solid state relay (single-phase or three-phase, respectively).
2. Load I. The figure corresponds to the current parameter. In our case – 40 A.
3. Input signal. The following options are available here:
   • L – 4 to 20 mA (linear SSR).
   • D – from 3 to 32 V DC I (on and off).
   • V is a variable resistance.
   • A – from 80 to 250 Volts AC I (on and off).
4. Output voltage:
   • D is constant.
   • A is variable.
5. Output voltage range:
   • H – from 90 to 480 Volts (variable).
   • No – from 24 to 380 Volts (variable).

Popular Models

Let’s highlight the popular models of solid state relays for each of the series:

• Three-phase (TSR series) – TSR-25DA, TSR-40DA, TSR-75DA, TSR-25A, TSR-40AA, TSR-75AA.
• Single-phase (SSR series) – SSR-10DA, SSR-25DA, SSR-40DA, SSR-50DA, SSR-75DA.
• Single-phase with output voltage regulation (SSR series) – SSR-10VA, SSR-25VA, SSR-40VA
• Linear single-phase with output voltage regulation (SSR-LA series) – SSR-25LA, SSR-40LA, SSR-50LA, SSR-75LA.
• Single-phase AC-AC and DC-DC type (SSR series) – SSR-10AA, SSR-25AA, SSR-40AA, SSR-05DD, SSR-10DD.

Specifications.

Driving.

Specifications.

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Specifications.

Driving.

Radiator dimensions.

Load power control options

Today, there are two main options for power management. Let’s consider each and them in more detail:

1. PHASE CONTROL. Here, the output signal for I in the load has the form of a sinusoid. The output voltage is set at 10, 50 and 90 percent. The advantages of such a scheme are obvious – the smoothness of the output signal, the ability to connect different types of loads. Minus – the presence of interference in the switching process.
2. SWITCHED CONTROL (DURING ZERO CROSSING). The advantage of the control method is that during the operation of the solid state relay no interference is created that interferes with the third harmonic during the switching process. Of the shortcomings – limited application. This control scheme is suitable for capacitive and resistive loads. Its use with a highly inductive load is not recommended.

Despite the higher price, solid state relays will gradually replace standard devices with contacts. This is due to their reliability, lack of noise, ease of maintenance and long service life.

Having flaws do not have a negative impact, if you correctly approach the selection and installation of the device.