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Solid State Relays: A Basic Overview

Author: Apogeeweb Date: 11 Jun 2020  1797

solid state relay


Solid-state relays(SSRs) have unparalleled advantages over other relays, because it can make and break the circuit without contact or spark. In addition, with the progress of technology, the maturity of manufacturing and the decline in price, solid-state relay has become widely used day by day. At the same time, its position in the global market is becoming more and more important.

This article will introduce what is a solid-state relays, its structure and working principle, solid-state relay wiring, advantages and disadvantages, and the difference between soild-state relay and conventional relay.

Solid-State Relay

Figure 1. Solid-State Relay


I Introduction

II What is a Solid-State Relay(SSR)?

III Structure and Working Principle of SSRs

  3.1. Structure

  3.2. Working Principle

IV SSR Wiring, Advantages and Disadvantages of SSR

  4.1. SSR Wiring

  4.2. Advantages of SSRs

  4.3. Disadvantages of SSRs

V Difference between SSRs and Conventional Relays

  5.1. A Brief Introduction to Conventional Relays

  5.2. Differences between SSRs and Conventional Relays

  5.3. Reasons for Choosing SSRs

VI One Question Related to SSR

II What is a Solid-State Relay(SSR)?

Solid-state relay (hereinafter abbreviated as "SSR") is a new type of contactless switching device composed entirely of solid-state electronic components, which makes use of the switching characteristics of electronic components (such as switching transistor, bi-directional thyristor and other semiconductor devices) to achieve the purpose of turning on and off the circuit without physical contact and spark, so it is also called "contactless switch".


SSR is a four-terminal active device, in which two terminals are input terminals and the others are output terminals. It not only has the function of amplifying and driving, but also has the function of isolation, so it is very suitable for driving high-power switching actuators. Compared with electromagnetic relays, SSRs are more reliable, have a longer life, faster speed, and has less interference with the outside world, so this is the reason why it has been largely used.

What is a Solid-state Relay?

III Structure and Working Principle of SSRs

3.1. Structure

A solid-state relay consists of three parts: input circuit, isolation (coupling) and output circuit.

3.1.1 Input circuit

According to the type of input circuit, the input circuit can be divided into the DC input circuit, AC input circuit and AC/DC input circuit. Some input circuits also support TTL/CMOS and have the function of positive and negative logic control and inversion, which makes it easier to connect with TTL/CMOS circuits.


For control signals with a fixed control voltage, a resistive input circuit is used. The control current is guaranteed to be greater than 5mA, for the large range of changes in the control signal (such as 3~32V) is the use of a constant-current circuit, to ensure that the current in the whole range of voltage changes in more than 5mA reliable operation.


3.1.2 Isolation (coupling)

For solid-state relays, there are two ways to isolate and couple the input and output circuits: photoelectric coupling and transformer coupling. Photoelectric coupling usually uses photodiode-phototransistor, photodiode-bi-directional light-controlled thyristor, photovoltaic cell to realize the isolation control between the control side and the load side; High-frequency transformer coupling is the use of input control signals generated by a self-excited high-frequency signal coupled to the secondary, after detection and rectification, logic circuit processing to form a driving signal.


3.1.3 Output circuit

The power switch of SSR is directly connected to the power supply and the load terminal to realize the on-off switching of the load power supply. The main use of high-power crystal transistor (switch-Transistor), unidirectional thyristor (Thyristor or SCR), bidirectional thyristor (Triac), power field-effect transistor (MOSFET), insulated gate bipolar transistor (IGBT). The output circuit of the solid-state relay can also be divided into DC output circuit, AC output circuit and AC / DC output circuit. According to the type of load, it can be divided into DC solid-state relay and AC solid-state relay. For DC output, bipolar devices or power field-effect transistors can be used. For AC output, two thyristors or one triac are usually used. AC solid-state relay can be separated into single-phase AC solid-state relay and three-phase AC solid-state relay. Ac solid-state relays can be divided into random AC solid-state relays and zero-crossing AC solid-state relays according to the timing of turn-on and turn-off.


3.2. Working Principle

SSR can be divided into two types of AC type and DC type according to the occasion of use. They are used as load switches on AC or DC power sources, and cannot be mixed. The following uses the AC SSR as an example to illustrate its working principle. Figure 1 is a block diagram of its working principle. The components ①~④ in Figure 1 form the main body of the AC SSR. From an overall perspective, the SSR has only two input terminals (A and B) and two output terminals (C and D).

Working Principle of SSR

Figure 2. Working Principle of SSR

When working, as long as a certain control signal is added to A and B, you can control the "on" and "off" between C and D, so as to realize the function of "switch". The function of the coupling circuit is to provide a channel between the input and output terminals for the control signals input at the A and B terminals, but electrically disconnect the (electrical) connection between the input terminal and the output terminal in the SSR, to prevent the output from affecting the input.


The element used in the coupling circuit is the "optical coupler", which has high sensitivity, high return speed, and high tolerance between the input and output terminals. Because the input terminal is a light-emitting diode, it makes it easy for the input end of the SSR to match the input signal level.


When in use, it can be directly connected with the computer output interface, that is, it is controlled by "1" and "0". The function of generating power lines is to generate signals that meet the requirements and to turn on the work of circuit 4, but because the off-line lines do not add special control circuits, they produce dry radiation and use pollution generators such as high-order waves or spikes, so a "zero-crossing control circuit" is specially built for this purpose.


The "zero-crossing" means that when the control signal is added and the AC is over zero, the SSR is in the state; and after opening the control signal, the SSR has to wait for the junction point (zero potential) between the positive half-cycle and the half-cycle of the AC cycle before the SSR becomes stable. This design can prevent the interference of high-frequency waves and the pollution of electricity.

The absorption circuit is designed to prevent the spike and surge (surge) from the power source from turning on the switching devices to the switching and operation of the controllable silicon tube (or even the operation). It is generally used to use an "R-C" series absorption circuit or non-destructive resistor (thermistor resistor). 


IV SSR Wiring, Advantages and Disadvantages of SSR

4.1. SSR Wiring

When the output of the relay is electrified to the coil and the output voltage is wired according to the load voltage, the contacts will close and the lamp will light up after energizing, as shown in the figure below.

SSR Wiring

Figure 3. SSR Wiring

Looking at the physical wiring diagram for the solid-state relay below, you can see that the equipment has parameters 1, 2, 3, 4, and 60A. 60A of which represents the indicator light in action (there are two states, on and off). The INPUT word in the middle of 3 and 4 indicates the incoming terminals, and the OUTPUT word in the center of 1 and 2 indicates the outgoing terminals. Therefore, be careful not to mistakenly connect when wiring. 3 and 4 are used as trigger signals to control the on and off actions of 1 and 2. The SSR in this physical wiring diagram cannot be regulated, and some can adjust the DC voltage to adjust the voltage of the 3 and 4 outlets.

SSR Physical Wiring Diagram

Figure 4. SSR Physical Wiring Diagram

The physical wiring diagram of the solid-state relay used for electromechanical equipment is as follows, but it is generally widely used in the chemical industry, coal mine, and other fields, and requires explosion-proof and corrosion resistance.

SSR Wiring Diagram

Figure 5. SSR Wiring Diagram

SSRs are non-contact switching devices with relay characteristics that use semiconductor devices as switching devices instead of conventional electrical contacts. The single-phase SSR is a 4-terminal active device that includes two input terminals and two output terminals. Opto-isolated, after connecting the input terminal to a specific current value with a DC or pulse signal, you can change the output terminal from the off state to the on state.

SSR Physical Wiring Diagram

Figure 6. SSR Physical Wiring Diagram

4.2. Advantages of SSRs

  • Long Life Expectancy and High Reliability

The solid-state relay has no mechanical parts, solid-state device completes the contact function. It has no moving parts and can operate in high shock and vibration environments. The components of solid-state relays, due to their unique characteristics, determine the longevity and high reliability of solid-state relays.


  • High Sensitivity, Low Control Power, Good Electromagnetic Compatibility

solid-state relay has a wide input voltage range, low driving power, compatible with most logic integrated circuits without additional buffers or drivers.


  • Fast Transitions

Solid-state relays use solid-state devices, which allow switching speed from milliseconds to microseconds.


  • Low electromagnetic Interference

solid-state relays have no input "coil" and no ignition arc and rebound, which reduces electromagnetic interference. Most AC output solid-state relays are zero voltage switches that turn on at zero voltage and turn off at zero current, reducing sudden interruptions in the current waveform and thereby reducing switching transient effects.

4.3. Disadvantages of SSRs

  • After conduction, The tube voltage drop is large, the forward voltage drop of SCR or bi-directional silicon can reach 1-2V, and the saturation voltage drop of the high power transistor is also between 1-2V. The on-resistance is higher than the contact resistance of mechanical contacts.
  • Even after the semiconductor device is turned off, there can still be several microamperes to several milliamperes of leakage current, so ideal electrical insulation cannot be achieved.
  • Because of the large tube voltage drop, large power consumption and calorific value after conduction, the volume of a high-power solid-state relay is much larger than that of the electromagnetic relay with the same capacity, and the cost is also high.
  • The temperature characteristics of electronic components and the interference resistance of electronic circuits are poor, and the radiation resistance is also poor. If no effective measures are taken, the operating reliability will be low.
  • Solid-state relays are more sensitive to overload and must be protected from overload by fast fuses or RC damping circuits. A load of a solid-state relay is clearly related to ambient temperature: as the temperature increases, the load capacity will decrease rapidly.
  • The main drawbacks are the presence of voltage drops in the on-state(need corresponding heat dissipation measures), leakage currents in the off-state, AC and DC not universally usable, a small number of contact groups. In addition, indicators such as overcurrent, overvoltage, voltage rise rate, and current rise rate are poor.


Figure 7. SSR

Difference between SSRs and Conventional Relays

5.1. A Brief Introduction to Conventional Relays

It generally consists of a relay coil and dynamic and static contacts. The movable contact acts through the electromagnetic attraction of the relay coil, thus realizing the connection and disconnection of the circuit. That means, there is a mechanical movement. When the current reaches a certain level, the contacts will spark. Its low price and simple structure can be attractive, but sparks and mechanical movements during the operation have a certain impact on its life span.


The advantages of traditional relays are simple to drive, good insulation, and good resistance to short-term overload.

The disadvantages of conventional relays are large size (cumbersome),  slow response (max ms level), and high power consumption to drive them.

5.2. Differences between SSRs and Conventional Relays

All-solid-state relays use electronic components, so they have many advantages compared with traditional relays, but they also have some limitations to some extent. The following table shows the advantages and disadvantages of solid-state relays and traditional relays.

Conventional Relays



*Low residual output voltage

*No heat sink required


*Can provide multiple sets of contacts and normally open normally closed contacts

*No leakage current

*AC and DC compatible

*Compact size

*Maximum switching frequency is limited (5-10Hz)


*Electromagnetic interference

*Limited contact life

*The switching action cannot be fully synchronized

*Contact bounce

*Poor operating performance of high current, resulting in arc.

*Interface is required to connect with digital circuit

*High control power, usually higher than 200mW




*Low control power, usually 10-50mW

*Synchronous switch

*low electromagnetic interference in synchronous switch mode 

*longer life, 50-100 times than that of traditional relays.

*Fast response time

*No mechanical moving parts

*No mechanical strain

*Compatible with digital circuits

*Anti-vibration, anti-impact

*Anti-corrosion and moisture-proof

*No noise

*There is residual output voltage 1-1.6V

*The output can only be AC or DC, not compatible

*Usually requires heat sink

*Not suitable for small output signals

*There is leakage current

*Only single contact

These two tables show that in conventional switching applications, solid-state relays have no significant disadvantages over traditional relays. By comparison, we have to understand some of the limitations of solid-state relay applications, which will affect our final choice of the type of relay.


Finally, we have to accept the idea that no relay can be used in all applications. The application of relays depends largely on the mechanical and electrical environment, so it is impossible to define a set of accurate selection parameters to guide users to make the best choice of relays. Therefore, the final choice of relays can only be made according to each specific application.

Difference between Solid-state RELAY AND MAGNETIC RELAY

5.3. Reasons for Choosing SSRs

5.3.1. Life Expectancy of Relay

When used correctly, the most important features of solid-state relays are long life expectancy and high reliability. In practical application, the contacts of solid-state relays can be used permanently, while the contacts of traditional relays will be affected by strain, corrosion, bonding and so on. Traditional relays will fail due to the damage of moving parts (springs, electromagnets). The life of solid-state relays is usually 50-100 times longer than that of traditional relays.

5.3.2. Cheap Price

The price is an important factor to consider in the selection of relays. Under the same technical requirements, the initial purchase cost of traditional relays is usually lower than that of solid-state relays. However, this does not take into account the service life of traditional relays and the costs incurred in the future due to monitoring, maintenance and replacement of traditional relays.

5.3.3. Power Control

The sensitivity of traditional relays to control signals is only one-twentieth of that of solid-state relays, that is, in the case of obtaining the same output power, the power required by traditional relays is usually 10-20 times that of solid-state relays. The power required for solid-state relay control is only 200-500mW, and the low power consumption can be directly compatible with digital circuit systems.

5.3.4. Environmental Resistance

Environmental resistance is a very complex concept, but solid-state relays always have an advantage in this respect. A solid-state relay has good mechanical properties because it has no moving parts. The resin packaging shell of the solid-state relay makes it have good shock resistance, impact resistance and corrosion resistance. In addition, humidity has almost no effect on solid-state relays, only slightly reducing their insulation performance. However, traditional relays are very sensitive to humidity, and long-term high humidity will cause corrosion of traditional relays.

5.3.5. Switching Rate

Switching speed is also usually an important factor in choosing solid-state relays or traditional relays. Controlling the response rate is very important, even crucial, in some process control cargo machinery automation applications. In some applications where the special power factor is very low, the traditional relay can not be used. In addition, in some situations where the switch is stable and no jumping is allowed, traditional relays cannot be used.

5.3.6. Electromagnetic Radiation

Solid-state relays can switch the load when the circuit voltage crosses zero, thus limiting the transient phenomenon to a considerable extent and avoiding current surge and electromagnetic radiation. In some situations where the power factor is very low, the switch must be stable and the vibration is not allowed, so solid-state relays must be selected.

solid-state relay

Q: What is a solid-state relay used for?

A: A solid-state relay (SSR) is an electronic switching device that switches on or off when a small external voltage is applied across its control terminals. The relay may be designed to switch either AC or DC to the load. It serves the same function as an electromechanical relay, but has no moving parts.

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