Innovation for Every Engineer.

What is a Solid State Relay? Basic Introduction



In this article, we will present you a comprehensive introduction to solid state relay, covers from its definition, characteristics, structure, pros and cons, and some problems you might encounter with during using SSR and so on.



Catalog

I. What is a Solid State Relay?

1.1 Brief Introduction

1.2 Structure of Solid State Relay

1.3 Characteristics of Solid State Relay

1.4 Difference Between Solid State Relay & Normal   Relay

II. Pros and Cons of Solid State Relay

III. Common Problems of Solid State Relays

IV. Maintenance Method of Solid State Relay

V. Application of Solid State Relay

FAQ


I. What is a Solid State Relay?


1.1 Brief Introduction

The solid state relay (SSR) is a non-contact switch composed of microelectronic circuits, discrete electronic devices, and power electronic power devices. It is a component of a full electronic circuit combination. It depends on the electromagnetic and optical characteristics of semiconductor devices and electronic components. Its isolation and relay switching functions.

 

This video tells briefly what solid state relay is.

Compared with the traditional electromagnetic relay, the solid-state relay is a relay without machinery and no moving parts, but has essentially the same functions as the electromagnetic relay.

 

Solid state relays are widely used in industrial automation control, such as electric furnace heating systems, familiar control machinery, remote control machinery, motors, solenoid valves and signal lights, flashers, stage lighting control systems, medical equipment, photocopiers, washing machines, fire protection systems, etc. It works reliably, has no contact, no spark, long life, no noise, no electromagnetic interference, fast switching speed, and achieves the purpose of directly driving a large current load with a tiny control signal.



1.2 Structure of Solid State Relay

The solid state relay is composed of three parts: input circuit, isolation (coupling) and output circuit.

 

1. Input circuit:

According to the different types of input voltage, the input circuit can be divided into three types: DC input circuit, AC input circuit and AC/DC input circuit. Some input control circuits are also compatible with TTL/CMOS, positive and negative logic control and inverting functions, and can be easily connected with TTL and MOS logic circuits.

 

For a control signal with a fixed control voltage, a resistive input circuit is used. The control current is guaranteed to be greater than 5mA. For the control signal with a large variation range (such as 3~32V), a constant current circuit is used to ensure reliable operation of the current greater than 5mA within the entire voltage variation range.

 

2. Isolation and coupling

The input and output circuits of solid state relays can be isolated and coupled in two ways: photoelectric coupling and transformer coupling: photoelectric coupling usually uses photodiodes-phototransistors, photodiodes-bidirectional light-controlled silicon controlled thyristors, photovoltaic cells, to achieve control side and load side Isolation control; high-frequency transformer coupling is a self-excited high-frequency signal generated by the input control signal is coupled to the secondary, detected and rectified, and processed by a logic circuit to form a drive signal.

 

3. Output circuit

The power switch of the SSR is directly connected to the power supply and the load side to realize the on-off switching of the load power supply. Mainly use high-power transistors, unidirectional thyristors (or SCR), bidirectional thyristors (Triac), power field effect transistors (MOSFET), and insulated gate bipolar transistors (IGBT).

 

The output circuit of solid state relay can also be divided into DC output circuit, AC output circuit and AC/DC output circuit. According to the load type, it can be divided into DC solid state relay and AC solid state relay. Bipolar devices or power FETs can be used for DC output, and two thyristors or one bidirectional thyristor are usually used for AC output. The AC solid-state relays can be divided into single-phase AC solid-state relays and three-phase AC solid-state relays. 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.



1.3 Characteristics of Solid State Relay

The solid state relay is a non-contact electronic switch with isolation function, and there are no mechanical contact parts during the switching process. Therefore, in addition to the same functions as electromagnetic relays, solid state relays also have logic circuit compatibility, vibration resistance and mechanical shock resistance, unlimited installation location, and good moisture, mildew and corrosion resistance. It also has excellent performance in explosion protection and prevention of ozone pollution. It also has the characteristics of low input power, high sensitivity, low control power, good electromagnetic compatibility, low noise and high operating frequency.

 

(1) The SSR has no internal mechanical parts, and the structure adopts a fully sealed method of perfusion. Therefore, the SSR has the advantages of vibration resistance, corrosion resistance, long life and high reliability, and its switch life is up to 10.1 million times;


(2) Low noise: AC SSR adopts zero-crossing trigger technology, so the voltage rise rate dv/dt and current rise rate di/dt value are effectively reduced on the line, so that the SSR has minimal interference to the mains during long-term operation;


(3) Its switching time is short, about 10ms, which can be used in higher frequency occasions;


(4) It adopts photoelectric isolation between its input circuit and output circuit, and the insulation voltage is above 2500V;


(5) Its input power consumption is very low, compatible with TTL and COMS circuits;


(6) Its output terminal has a protection circuit;


(7) Strong load capacity.



1.4 Difference Between Solid State Relay & Normal Relay

Ordinary relays are generally composed of relay coils and static and dynamic contacts. The movable contact is actuated by the electromagnetic attraction force of the relay coil to realize the connection and disconnection of the circuit. So there is mechanical movement. When the current reaches a certain level, the contacts will spark. Ordinary relays are cheap and simple in structure, but sparks and mechanical movements during operation will have a certain impact on its life.

 

The advantages of ordinary relays are: simple drive, good isolation, and good short-term overload tolerance.

The disadvantages of ordinary relays are: large size (heavy), slow response speed (up to ms level), and large power consumption to drive the relay.

 

The comparison between traditional relays and solid-state relays, as there are many types involved, the following is a comparison between electromagnetic relays and corresponding solid-state relays to illustrate their differences:

 

1. Structural difference: Electromagnetic relays work by using the suction force generated by the circuit in the input circuit between the electromagnet core and the armature; solid-state relays use electronic components to perform their functions without mechanical moving components, and the input and output are isolated.

 

2. Difference in working mode: Electromagnetic relay uses the principle of electromagnetic induction to control the on-off of the circuit through the power of electromagnet. Therefore, when DC is used to connect the coil, the contacts can pass AC and DC; solid state relays rely on the electrical, magnetic and optical characteristics of semiconductor devices and electronic components to complete their isolation and relay switching functions. Therefore, they are divided into DC input-AC output type and DC Input-branch output type, AC input-AC output type, AC input-DC output type.

 

3. Differences in working status: Electromagnetic relays make use of the suction force generated between the armature to make and break the circuit. Therefore, the action response is slow, noisy, and life is limited; solid state relays have fast response, operate without noise, and have a long life.

 

4. Operating environment: In the influence of temperature, humidity, atmospheric pressure (altitude), sand and dust pollution, chemical gas and electromagnetic interference, electromagnetic relays are generally inferior to solid state relays.

 

5. Electrical performance difference: Compared with the corresponding solid-state relay, the electromagnetic relay is simple to drive, but has large power consumption, good isolation, good short-term overload tolerance, and is not as good as the latter in high-current and high-power situations. And when controlling the circuit with frequent action, the life of the electromagnetic relay is not as long as the latter.

 

In short, traditional relays and solid state relays have their own advantages. The latter is more and more popular because of its reliable operation, no contacts, no sparks, long life, no noise, no electromagnetic interference, and fast switching speed.

 


II. Pros and Cons of Solid State Relay


Pros:

(1) Long life and high reliability: SSR has no mechanical parts and solid components to complete the contact function. Because there are no moving parts, it can work in a high impact and vibration environment. Because of the components that make up the solid state relay The inherent characteristics determine the long life and high reliability of solid state relays.

 

(2) High sensitivity, low control power, and good electromagnetic compatibility: The solid state relay has a wide input voltage range and low drive power, and is compatible with most logic integrated circuits without the need for buffers or drivers.

 

(3) Fast switching: Because solid-state relays use solid-state devices, the switching speed can range from a few milliseconds to several microseconds.

 

(4) Electromagnetic interference: The solid state relay has no input "coil", no arc ignition and rebound, thus reducing electromagnetic interference. Most AC output solid state relays are a zero-voltage switch, which is turned on at zero voltage and turned off at zero current, reducing the sudden interruption of the current waveform, thereby reducing the switching transient effect.

 

Cons:

(1) After the solid state relay is turned on, the tube voltage drop is large, and the forward voltage drop of the thyristor or two-phase thyristor can reach 1~2V, and the saturation voltage drop of the high-power transistor is also between 1~2V. Generally, the on-resistance of the power FET is also larger than the contact resistance of the mechanical contacts.

 

(2) The semiconductor device can still have a leakage current of several microamperes to several milliamperes after it is turned off, so ideal electrical isolation cannot be achieved.

 

(3) Due to the large pressure drop of the tube, the power consumption and heat generation after the turn-on are also large, the volume of the high-power solid-state relay is much larger than the electromagnetic relay of the same capacity, and the cost is also higher.

 

(4) The temperature characteristics of electronic components and electronic circuits have poor anti-interference ability and poor radiation resistance. If effective measures are not taken, the working reliability of solid state relays will be reduced.

 

(5) Solid state relays are more sensitive to overload and must be protected by fast fuse or RC damping circuit. The load of the solid state relay is obviously related to the ambient temperature. As the temperature rises, the load capacity will drop rapidly.


III. Common Problems of Solid State Relays


When the solid state relay is open and there is voltage at the load terminal, there will be a certain amount of leakage current at the output terminal. Care should be taken to prevent electric shock when using or designing. When solid state relays fail to be replaced, products with the same original model or technical parameters should be used as much as possible to match the original application circuit to ensure the reliable operation of the system. Among all, overheat, overcurrent and overvoltage are always the common problems you might encounter when using a solid state relay.

 

overheat

When the SSR is turned on, the component will withstand the dissipation power of P=V (tube pressure drop) × I (load), where the effective value of V and the effective value of I are the effective values of the saturation voltage drop and the operating current, respectively.

 

The load capacity of the solid state relay is greatly affected by the ambient temperature and its own temperature rise. It must be based on the actual working environment conditions and strictly refer to the allowable case temperature rise (75°C) at the rated working current. Reasonably select the size of the radiator or reduce the current for use. During installation and use, ensure that it has good heat dissipation conditions, otherwise it will cause loss of control due to overheating, and even cause product damage.

 

Generally speaking, under 10A, an instrument base plate with good heat dissipation conditions can be used, and a product with a rated working current above 10A should be equipped with a radiator.

 

Below 30A, use natural air cooling. When the continuous load current is greater than 30A, the instrument fan must be used for forced air cooling. Products above 100A should be equipped with a radiator and a fan for forced cooling.

 

When installing, pay attention to the good contact between the bottom of the relay and the radiator, and consider applying a proper amount of thermal grease to achieve the best heat dissipation effect.

 

For example, when the relay is working at high temperature for a long time (40℃~80℃), the user can consider derating according to the curve data of the maximum output current and ambient temperature provided by the manufacturer to ensure normal operation.

 

Reasons for overheating of solid state relays:

When the solid state relay is working normally, there is a certain power loss on its internal chip. This power loss is mainly determined by the product of the output voltage drop of the solid state relay and the load current, and is consumed in the form of heat.

 

Therefore, the quality of heat dissipation directly affects the reliability of the solid state relay, and the excellent thermal design can avoid failure and damage caused by poor heat dissipation.

 

Overcurrent and overvoltage

When the relay is in use, the internal output thyristor of the SSR solid state relay will be permanently damaged due to overcurrent and load short circuit. You can consider adding a fast fuse and an air switch to the control loop for protection (the product output protection should be selected when selecting the relay, built-in Varistor absorption circuit and RC buffer can absorb surge voltage and improve dv/dt tolerance).

 

Fast fuse and air switch are general overcurrent protection methods. Fast fuse can be selected according to 1.2 times of rated working current, generally small capacity fuse can be used. Pay special attention to load short circuit, which is the main cause of damage to SSR products.

 

For inductive and capacitive loads, in addition to the internal RC circuit protection, it is recommended to use a varistor in parallel at the output as a combined protection. The area of the metal zinc oxide varistor (MOV) determines the absorption power, and the thickness determines the protection voltage value.

 

For AC 220V SSR, select MYH12-430V varistor; 380V select MYH12-750V varistor; for larger capacity motor transformer, select MYH20 or MYH2024 varistor with large current capacity. The selection principle is to use 500V-600V varistors for 220V, and 800V-900V varistors for 380V.


IV. Maintenance Method of Solid State Relay


1. When selecting solid state relays used on printed circuit boards with low current specifications, since the lead terminals are made of high thermal conductivity materials, the soldering should be carried out under the conditions of a temperature less than 250℃ and a time less than 10S. If the surrounding temperature is considered, If necessary, derating can be considered. Generally, the load current should be controlled within 1/2 of the rated value.

 

2. Selection of solid state relays for various load surge characteristics

 

The controlled load will generate a large inrush current at the moment of switching on. Because the heat is too late to dissipate, it is likely to damage the SSR's internal thyristor.

 

Therefore, the user should analyze the surge characteristics of the controlled load when selecting the relay, and then select the relay. The relay can withstand this surge current under the premise of ensuring steady-state operation. When selecting, refer to the derating factor of various loads in Table 2 (at normal temperature).

 

If the selected relay needs to work in the occasions with more frequent work, high life and reliability requirements, it should be multiplied by 0.6 on the basis of Table 2 to ensure reliable work.

 

Generally, follow the above principles when selecting, and when low voltage requires low signal distortion, you can choose a DC solid-state relay that uses a field effect tube as an output device; for example, for AC resistive loads and most inductive loads, you can choose a zero-crossing relay. Extend the life of loads and relays, and also reduce their own radio frequency interference. For phase output control, random solid state relays should be used.

 

3. The influence of ambient temperature

 

The load capacity of solid state relays is greatly affected by the ambient temperature and its own temperature rise. During installation and use, ensure that it has good heat dissipation conditions. Products with a rated operating current of more than 10A should be equipped with a radiator, and products with a rated operating current of more than 100A should be equipped with a radiator. Equipped with a radiator and a fan for forced cooling. When installing, pay attention to the good contact between the bottom of the relay and the radiator, and consider applying a proper amount of thermal grease to achieve the best heat dissipation effect.

 

For example, when the relay is working at high temperature for a long time (40℃~80℃), the user can consider derating according to the curve data of the maximum output current and ambient temperature provided by the manufacturer to ensure normal operation.

 

4. Overcurrent and overvoltage protection measures

 

When the relay is used, the internal output thyristor of the SSR solid-state relay will be permanently damaged due to overcurrent and load short-circuit. Consider adding a fast fuse and air switch to the control loop to protect it (the product output protection should be selected when choosing the relay, built-in Varistor absorption circuit and RC buffer can absorb surge voltage and improve dv/dt tolerance); RC absorption circuit and varistor (MOV) can also be connected in parallel at the output of the relay to achieve output protection. The selection principle is to use 500V-600V varistors for 220V, and 800V-900V varistors for 380V.

 

5. Relay input circuit signal

 

When in use, when the input voltage is too high or the input current is too large and exceeds its specified rated parameters, consider connecting a voltage divider resistor in series at the input end or a shunt resistor in parallel at the input port, so that the input signal does not exceed its rated parameters value.

 

6. In specific use, the control signal and load power supply should be stable, and the fluctuation should not be greater than 10%. Otherwise, voltage stabilization measures should be taken.

 

7. Keep away from electromagnetic interference and radio frequency interference sources during installation and use to prevent the relay from malfunctioning and out of control.

 

8. When the solid state relay is open circuit and there is voltage at the load terminal, there will be a certain amount of leakage current at the output terminal. Pay attention to it when using or designing.

 

9. When the solid state relay is replaced by failure, try to choose the product with the same original model or technical parameters to match the original application circuit to ensure the reliable operation of the system.



V. Application of Solid State Relay


The dedicated solid-state relay can have short-circuit protection, overload protection and overheat protection functions, and the combination logic solidification package can realize the intelligent module required by the user, which can be directly used in the control system.

 

Solid state relays have been widely used in:

(1) Computer peripheral interface equipment, constant temperature system, temperature adjustment, electric furnace heating control, motor control, numerical control machinery, remote control system, industrial automation device;


(2) Signal light, dimming, flasher, lighting stage lighting control system;


(3) Instruments, medical equipment, photocopiers, automatic washing machines;


(4) Automatic fire-fighting, security systems, as well as the switch of power capacitors for power factor compensation of the power grid, etc. In addition, solid state relays are widely used in chemical, coal, and other occasions that require explosion-proof, moisture-proof, and corrosion-proof.


FAQ


1. What is solid state relay and how it works?

A solid state relay (SSR) is an electronic switching device that switches on or off when an external voltage (AC or DC) is applied across its control terminals. It serves the same function as an electromechanical relay, but has no moving parts and therefore results in a longer operational lifetime.


2. What is the difference between a relay and a solid state relay?

The main difference between solid state relays and general relays is that there is no movable contacts in solid state relay (SSR). In general, solid state relays are quite similar to the mechanical relays that have movable contacts. ... SSR provide high-speed, high-frequency switching operations.


3. How fast is a solid state relay?

The SSR output is activated immediately after applying control voltage. Consequently, this relay can turn on anywhere along the AC sinusoidal voltage curve. Response times can typically be as low as 1 ms. The SSR is particularly suitable in application where a fast response time is desired, such as solenoids or coils.


4. Do solid state relays get hot?

All solid state relays develop heat as a result of a forward voltage drop through the junction of the output device. Beyond a point, heat will cause a lowering (or derating) of the load current that can be handled by the SSR. ... Loads greater than 4 Amps will require heat sinks.


5. What causes solid state relay failure?

What are the main causes and solutions of the Solid-state Relays (SSR)'s failures? If an inrush current exceeds the rated making current of the SSR due to the high inrush current of loads such as motors and lamps, SSR output elements are damaged. Consider using an SSR with a higher capacity.


6. Can a solid state relay switch DC?

Solid state relays can be designed to switch both AC or DC currents by using an SCR, TRIAC, or switching transistor output instead of the usual mechanical normally-open (NO) contacts.


7. How do you test a solid state relay with a multimeter?

Using Multimeter: 

1. Set the multimeter in continuity test mode.

2. Place the probes of the multimeter on the coil terminals.

3. If the multimeter beeps (or show any sign of continuity), the coil is electrically closed (good).

4. If the multimeter does not beep, the coil is open & damaged. The relay needs to be replaced.


8. How reliable are solid state relays?

Solid-state relays are the preferred choice for system reliability because they have no moving parts or contacts. Over time, the plating on the contacts inside EMRs can erode. This erosion can cause the contacts to weld shut; therefore they no longer open/close properly, and the relay has to be replaced.


9. Is a solid state relay a transistor?

Solid-State Relay: A sort of hybrid between a conventional relay and a transistor, these relays switch a load using an LED activated by the control circuitry. The LED activates a light-activated MOSFET that controls the load.


10. How do I know if my solid state relay is bad?

Solid-state relays should be checked with an ohmmeter across the normally open (N.O.) terminals when control power is off. The relays should be open, switched to OL, and closed (0.2 , the internal resistance of the ohmmeter) when control power is applied.


11. How do I choose a solid state relay?

When selecting a Solid State Relay, consider: Current rating, as a general rule consider using the relay at no more than 70% of its rated current. Electrical environment,. i(In harsh electrical environments, consider a relay with an line voltage rating above the application line voltage.)


12. Do solid state relays need a diode?

2 Answers. The control side of solid state relays is usually just a LED, sometimes two LEDs back to back, and sometimes with integrated resistor. ... If the relay is on the same board as whatever is driving it, then no inductive kickback diode is needed. It's no different than driving any other on-board LED.


13. Do solid state relays leak voltage?

Solid State relays have leakage. If you want to repeatedly switch something on / off, use them. But when you want the SSR to be fully off, say after pressing an off switch, a mechanical relay should be across the load to take it off the SSR. ... The SSR control is attached to the atmega328 through a 200ohm resistor.