Practical Application And Wiring Diagrams Of Solid State Relays

Classical starters and contactors are gradually becoming a thing of the past. Their place in car electronics, household appliances and industrial automation is taken by the solid state relay, a semiconductor device without any moving parts.

Devices have different designs and wiring diagrams, which determine their application areas. Before using a device, it is necessary to understand its principle of operation, learn about the peculiarities of functioning and connection of different types of relays. Answers to the designated questions are described in detail in the presented article.

Content of the article:

  • Design of a solid state relay
  • The principle of operation of a TTR
  • Classification of solid state relays
    • By the number of phases connected
    • By the type of operating current
    • Design features
    • By the type of control circuit
  • Benefits and drawbacks RTDs
  • Possible wiring diagrams
  • Practical applications of the devices
  • Conclusions and useful video on the subject

Design of a solid state relay

Modern solid state relays (SSRs) are modular semiconductor devices, which are power switches.

The key operating units of these devices are triacs, thyristors or transistors. RTDs have no moving parts, which makes them different from electromechanical relays.

Practical application and wiring diagrams of solid state relays

The size of the solid state relay depends very much on the maximum allowable load and the heat dissipation capability through heat transfer and convection (+)

The internal construction of these devices may vary very much depending on the type of load being controlled  and the electrical circuit.

The simplest solid state relays include these parts:

  • fuse input unit;
  • trigger circuit;
  • optical (galvanic) isolation;
  • switching unit;
  • safety circuits;
  • load output unit.

The RTD input unit is the primary circuit with a resistor connected in series. A fuse is optionally built into this circuit. The task of the input node is to accept the control signal and transmit the command to the switches switching the load.

At AC, galvanic isolation is used to separate the control circuit from the main circuit. The operation of the relay depends to a large extent on its design. The trigger circuit responsible for processing the input signal can be included in an optical isolation node or placed separately.

A protective node prevents overloads and errors, because if the device breaks down, the connected equipment can also fail.

The main purpose of solid state relays is to short-circuit/open the electrical network with a weak control signal. Unlike their electromechanical counterparts, they are more compact and do not produce the characteristic clicking noise during operation.

The operating principle of TTRs

The operation of a solid state relay is quite simple. Most TTRs are designed to control automatics on 20-480V networks.

Practical application and wiring diagrams of solid state relays

Optical decoupling allows the creation of control signals of minimum power, which is critical for sensors operating from autonomous power supplies (+)

In the classic version, the device housing includes two contacts of the switched circuit and two control wires. Their number can be varied by increasing the number of connected phases. Depending on the presence of voltage in the control circuit, the semiconductor elements turn the main load on or off.

A special feature of solid state relays is the presence of a non-endless resistance. Whereas contacts in electromechanical devices are completely disconnected, in solid-state relays the absence of current in the circuit is provided by the properties of semiconductor materials.

Therefore, small leakage currents may occur at elevated voltages, which may adversely affect the operation of connected equipment.

Classification of Solid-State Relays

The applications of relays are diverse, so their design features may differ greatly, depending on the needs of the particular automatic circuit. TTRs are classified according to the number of phases connected, the type of operating current, their design features, and the type of control circuitry.

Number of phases connected

Solid state relays are used both in home appliances and in industrial automation with an operating voltage of 380 V.

These semiconductor devices are therefore divided into:

  • single-phase;
  • three-phase.

Single-phase TTRs allow operation with currents of 10-100 or 100-500 A. They are controlled by an analog signal.

Practical application and wiring diagrams of solid state relays

Wires of different colors should be connected to the three-phase relay to allow proper wiring when installing the equipment

Three-phase solid state relayscan handle a current range of 10-120A. Their construction implies a reversible principle of operation, which ensures the reliable regulation of several electric circuits simultaneously.

Often three-phase RTDs are used to power an asynchronous motor. Fast fuses must be included in its control circuitry because of the high inrush currents.

Based on operating current

Solid state relays cannot be set or reprogrammed, so they can only operate normally within a certain range of mains electrical parameters.

TDTs can be controlled by electrical circuits with two types of current depending on the needs:

  • constant;
  • variable.

Analogously, RTDs can be classified by the type of active load voltage. Most relays in household appliances operate with variable parameters.

Practical application and wiring diagrams of solid state relays

Direct current is not used as the main power source in any country in the world, so relays of this type have a narrow scope

Direct control current devices are highly reliable and use a 3-32V voltage for regulation. They can withstand a wide temperature range (-30...+70°C) with no significant change in characteristics.

Active regulated relays have a 3-32V or 70-280V control voltage. They feature low electromagnetic interference and high actuation speed.

Design features

The solid state relays are often installed in a common apartment electrical panel, so many models have a mounting block for DIN rail mounting.

In addition, there are special heat sinks placed between the RTD and the supporting surface. These allow cooling the device under high loads while maintaining its performance.

Practical application and wiring diagrams of solid state relays

The relays are mounted on DIN rails mainly through a bracket which has the additional function of carrying away the excess heat generated during operation

With a thermal compound between the relay and the heat sink, a contact area increase and a heat dissipation increase is recommended. There are also TCRs designed to be fixed to the wall with conventional screws.

By control circuit type

Not always the operating principle of an adjustable relay requires it to operate instantaneously.

This is why manufacturers have developed several TCR control circuits that are used in different applications:

  1. Control \"through zero\". This type of solid state relay control assumes actuation only at a voltage value of 0. Used with capacitive, resistive (heaters) and weak inductive (transformers) loads.
  2. Instantaneous. Used when the relay needs to operate suddenly when the control signal is applied.
  3. Phase. Assumes regulation of the output voltage by changing the parameters of the control current. It is used to vary the heating or lighting level continuously.

The solid state relays differ also in many other, less important, parameters. Therefore, when buying a TDR, it is important to understand the circuitry of the equipment to be connected in order to buy the best possible control device for it.

A power reserve must be provided, because the relay has a service life, which is quickly consumed by frequent overloading.

Benefits and drawbacks of TDRs

TDRs have driven conventional starters and contactors from the market for a reason. These semiconductors offer many advantages over their electromechanical counterparts that lead consumers to choose them.

Practical application and wiring diagrams of solid state relays

The relays for the chips are compact and very limited in their maximum current carrying capacity. They are mainly fixed by soldering of special feet

The advantages are:

  1. Low power consumption (90% less).
  2. Compact size allowing devices to be mounted in a limited space.
  3. High speed start-up and shutdown
  4. Low noise operation, no clicking noise typical of electromechanical relays.
  5. Maintenance free.
  6. Long life due to hundreds of millions of activations.
  7. Based on the wide modification possibilities of the electronics, the RTDs have an extended range of applications.
  8. No electromagnetic disturbance during tripping.
  9. Contacts are avoided by mechanical shock.
  10. No direct physical contact between control and switching circuits.
  11. Capable of load regulation.
  12. Underload protection circuits in pulsed RTDs.
  13. Feasibility in potentially explosive environments.

The above advantages of solid state relays are not always sufficient for normal operation. This is why they have not yet completely superseded electromechanical contactors.

Practical application and wiring diagrams of solid state relays

Efficient heat dissipation is important for the stable operation of high-power solid state relays because the load voltage (+)

TTRs also have drawbacks that prevent their use in many cases.

The disadvantages include:

  1. Unable to operate most devices with voltages over 0.5 kV.
  2. High cost.
  3. Sensitive to high currents, especially in motor starting circuits.
  4. Limitations on use in high humidity environments.
  5. Critical loss of performance at temperatures below 30°C frost and above 70°C heat.
  6. Compact housing leads to excessive heating of the device at consistently high loads, requiring special passive or active cooling devices.
  7. Possibility of melting the device from heat in a short circuit.
  8. Microcurrents in the closed state of the relay may be critical to equipment operation. For example, fluorescent lamps connected to the mains may periodically flash.

Solid-state relays thus have certain applications. In high-voltage industrial circuits, their use is severely restricted because of the imperfect physical properties of semiconductor materials.

But in the domestic appliance and automotive industries, RTDs are well established due to their positive properties.

Possible wiring diagrams

Solid state relays can be wired in a variety of ways. Each circuit is built according to the load to be connected. Additional fuses, controllers and regulating devices may be added to the circuit.

Practical application and wiring diagrams of solid state relays

Because the control and load circuits in the device do not overlap, their electrical characteristics may vary by any parameter (+)

The following are the most simple and common wiring diagrams of the RTD:

  • normally open;
  • connected circuit;
  • normally closed;
  • three-phase;
  • reversible.

Normally open (open) circuit - A relay in which the load is energized when the control signal is present. That is, the connected machinery is in the disconnected state when inputs 3 and 4 are de-energized.

Practical application and wiring diagrams of solid state relays

Before purchasing the relay, decide on the type of initial state required (closed or open) in order to ensure the correct operation of the connected equipment (+)

Normally closed circuit - means the relay in which the load is energized when there is no control signal. This means that the connected equipment is operational when inputs 3 and 4 are de-energized.

There is a solid state relay wiring diagram in which the control voltage and the load voltage are the same. This method can be used simultaneously for DC and AC operation.

Three-phase relaysare connected according to slightly different principles. The contacts can be connected in star, delta or star with neutral.

Practical application and wiring diagrams of solid state relays

The choice of three-phase relay wiring depends largely on the characteristics of the machinery connected to it as a load

Reverse solid state relaysare used in electric motors in their corresponding mode. They are made in three-phase versions and include two control circuits.

Practical application and wiring diagrams of solid state relays

If it is important for the relay to respect the polarity of the contacts, the marking will always indicate where the phase and the zero are connected

Assemble electrical circuits with TRs only after drawing them in advance on paper, because incorrectly connected devices may malfunction by a short circuit.

Practical applications of the devices

The applications of solid state relays are quite extensive. Because of their high reliability and lack of need for regular maintenance, they are often installed in hard-to-reach areas of equipment.

Practical application and wiring diagrams of solid state relays

In many relays, the connection of the control circuit wires requires polarity, which must be considered during equipment installation

The main applications of RTDs, however, are:

  • thermostatic control system with TENS;
  • stable temperature maintenance in processes;
  • transformer control;
  • lighting control;
  • systems for motion sensors, lighting, photo sensors for street lighting etc.
  • Motor control;
  • Uninterruptible power supply.

With increasing automation of home appliances, solid state relays are becoming more and more common, and advances in semiconductor technology constantly open up new applications.

If you wish, you can build your own solid state relay. Detailed instructions are given in this article.

Summary and useful video on the subject

The above videos help to better understand the solid state relays and to learn how to wire them.

Hands-on demonstration of a simple solid state relay:

Learning about the differences and characteristics of solid state relays:

Testing the function and heating of an RTD:

Attempting to wire a circuit from a solid state relay and a sensor can be done by almost anyone.

But planning a working circuit requires a basic knowledge of electrical engineering, because incorrect wiring can lead to an electric shock or a short circuit. But as a result of the right action, you can get a lot of devices useful in everyday life.

Anything to add, or any questions about the wiring and application of solid state relays? Feel free to comment on the publication, participate in discussions and share your own experiences with these devices. The contact form is in the bottom block.

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