Overview of Solid State Relays
2024-07-14 3473Solid state relay (SSR) is a new type of contactless switching device composed entirely of solid-state electronic components. It utilizes the switching characteristics of electronic components such as switching transistors, bidirectional thyristors, and other semiconductor devices to achieve the purpose of connecting and disconnecting circuits without contact or spark. Therefore, it is also known as a "contactless switch". Solid state relay is a four terminal active device, with two terminals as input control terminals and the other two terminals as output controlled terminals. It has both amplification and isolation functions, making it suitable for driving high-power switch type actuators. Compared with electromagnetic relays, it has higher reliability, no contacts, long lifespan, fast speed, and low interference to the outside world. It has been widely used.
SSRs can be divided into two categories based on their usage scenarios: AC and DC. They are used as load switches on AC or DC power sources respectively and cannot be mixed. Taking the communication type SSR as an example to illustrate its working principle, Figure 1 is its working principle block diagram. Components ① to ④ in Figure 1 constitute the main body of the communication SSR. Overall, the SSR only has two input terminals (A and B) and two output terminals (C and D), and is a four terminal device.
When working, as long as a certain control signal is added to A and B, the "on" and "off" between the two ends of C and D can be controlled to achieve the function of "switch". The function of the coupling circuit is to provide a channel between the input/output terminals for the control signal input to A and B, but also to electrically disconnect the (electrical) connection between the input and output terminals of SSR to prevent the influence of the output terminal on the input terminal. The component used in the coupling circuit is the "optocoupler", which is sensitive in action, has high response speed, and has a high insulation (withstand voltage) level between the input/output terminals; Due to the fact that the load on the input end is a light-emitting diode, it is easy for the input end of SSR to match the input signal level. When in use, it can be directly connected to the computer output interface, which is controlled by the logic levels of "1" and "0". The function of the trigger circuit is to generate a triggering signal that meets the requirements and drive the switch circuit ④ to work. However, due to the fact that the switch circuit will generate radio frequency interference and pollute the power grid with high-order harmonics or spikes without a special control circuit, a "zero crossing control circuit" is specially designed for this purpose. The so-called 'zero crossing' refers to when a control signal is added and the AC voltage crosses zero, the SSR is in the on state; And when the control signal is disconnected, the SSR needs to wait for the junction point (zero potential) of the positive and negative half cycles of the AC power before it becomes disconnected. This design can prevent interference from high-order harmonics and pollution to the power grid. Absorption circuit is designed to prevent the impact and interference (even misoperation) of bidirectional thyristors caused by spikes and surges (voltage) transmitted from the power supply. Generally, "R-C" series absorption circuit or nonlinear resistor (varistor) is used.
Specialized solid-state relays can have short-circuit protection, overload protection, and overheating protection functions, and can be packaged with combinational logic to achieve the intelligent module required by users, which can be directly used in control systems.
Solid state relays have been widely used in computer peripheral interface devices, constant temperature systems, temperature control, electric furnace heating control, motor control, CNC machinery, remote control systems, and industrial automation devices; Signal lights, dimming, flashing lights, lighting stage lighting control system; Instruments and meters, medical equipment, photocopiers, automatic washing machines; Automatic fire protection, security systems, as well as switching switches for power capacitors as compensation for grid power factors, etc., are widely used in explosion-proof, moisture-proof, and corrosion-resistant environments such as chemical and coal mines.
Solid state relay is a non-contact electronic switch with isolation function, which has no mechanical contact parts during the switching process. Therefore, in addition to having the same function as electromagnetic relay, solid state relay also has the characteristics of logic circuit compatibility, vibration resistance, mechanical impact resistance, unlimited installation position, good moisture-proof, mildew proof and anti-corrosion performance, excellent performance in explosion prevention and ozone pollution prevention, low input power, high sensitivity, low control power, good electromagnetic compatibility, low noise and high operating frequency.
(1) There are no mechanical components inside the SSR, and the structure adopts a fully sealed injection method. Therefore, the SSR has the advantages of vibration resistance, corrosion resistance, long life, and high reliability, with a switch life of up to 10.1 million times;
(2) Low noise: The AC type SSR adopts zero crossing triggering technology, which effectively reduces the voltage rise rate dv/dt and current rise rate di/dt values on the line, minimizing the interference of SSR to the mains power during long-term operation;
(3) The switch time is short, about 10ms, and can be applied in high-frequency situations;
(4) Optoelectronic isolation is used between the input circuit and the output circuit, with an insulation voltage of 2500V or above;
(5) Low input power consumption, compatible with TTL and COMS circuits;
(6) There is a protection circuit at the output end;
(7) Strong load capacity.
(1) High lifespan and high reliability: Solid state relays have no mechanical components and rely on solid devices to complete contact functions. Due to the absence of moving parts, they can operate in high impact and vibration environments. The inherent characteristics of the components that make up solid-state relays determine their long lifespan and high reliability.
(2) High sensitivity, low control power, good electromagnetic compatibility: Solid state relays have a wide input voltage range, low driving power, and can be compatible with most logic integrated circuits without the need for buffers or drivers.
(3) Fast conversion: Solid state relays use solid-state devices, so the switching speed can range from a few milliseconds to a few microseconds.
(4) Low electromagnetic interference: Solid state relays do not have input "coils", contact arcing and rebound, thus reducing electromagnetic interference. Most AC output solid-state relays are zero voltage switches that conduct at zero voltage and turn off at zero current, reducing sudden interruptions in current waveforms and thus minimizing switch transient effects.
(1) After conduction, the voltage drop of the transistor is large, and the forward voltage drop of the thyristor or bidirectional thyristor can reach 1-2V. The saturation voltage drop of high-power transistors is also between 1-2V. Generally, the conduction resistance of power field-effect transistors is also higher than the contact resistance of mechanical contacts.
(2) Even after the semiconductor device is turned off, there can still be leakage currents ranging from microamperes to milliamps, so ideal electrical isolation cannot be achieved.
(3) Due to the large pressure drop of the tube, the power consumption and heat generation after conduction are also high. The volume of high-power solid-state relays is much larger than electromagnetic relays of the same capacity, and the cost is also higher.
(4) The temperature characteristics of electronic components and the anti-interference ability of electronic circuits are poor, and their radiation resistance is also poor. If effective measures are not taken, the reliability of operation will be low.
(5) Solid state relays are highly sensitive to overload and must be protected against overload using fast fuses or RC damping circuits. The load of solid-state relays is significantly related to the ambient temperature, and as the temperature increases, the load capacity will rapidly decrease.
(6) The main shortcomings are the presence of on state voltage drop (requiring corresponding heat dissipation measures), off state leakage current, inability to use both AC and DC, limited number of contact groups, and poor indicators such as overcurrent, overvoltage, voltage rise rate, and current rise rate.
Solid state relays consist of three parts: input circuit, isolation (coupling), and output circuit.
According to different categories of input voltage, input circuits can be divided into three types: DC input circuits, AC input circuits, and AC-DC input circuits. Some input control circuits also have compatibility with TTL/CMOS, positive and negative logic control, and anti equalization functions, which can be easily connected to TTL and MOS logic circuits.
For control signals with fixed control voltage, a resistive input circuit is used. Control the current to ensure it is greater than 5mA. For control signals with a large range of variation (such as 3-32V), a constant current circuit is used to ensure reliable operation with a current greater than 5mA throughout the entire voltage variation range.
There are two types of isolation and coupling methods for the input and output circuits of solid-state relays: photoelectric coupling and transformer coupling. Photoelectric coupling usually uses photodiodes phototransistors, photodiodes bidirectional photo controlled thyristors, photovoltaic cells, to achieve isolation control between the control side and the load side; High frequency transformer coupling is the process of coupling a self-excited high-frequency signal generated by an input control signal to the secondary, which is then detected, rectified, and processed by a logic circuit to form a driving signal.
The power switch of SSR is directly connected to the power supply and load terminal to achieve on/off switching of the load power supply. The main types of transistors used include high-power transistor (switching transistor), unidirectional thyristor (SCR), bidirectional thyristor (Triac), power field-effect transistor (MOSFET), and insulated gate bipolar transistor (IGBT). The output circuit of solid-state relays can also be divided into forms such as 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 relays and AC solid-state relays. Bipolar devices or power field-effect transistors can be used for DC output, while two thyristors or one bidirectional thyristor are usually used for AC output. And 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 conduction and disconnection.
How to use an appropriate radiator
Except for solid-state relays with a rated current of 1-5A directly installed on printed circuit boards, all others should be equipped with appropriate heat sinks, and the SSR base plate and heat sink should be coated with thermal conductive silicone grease, tightly in contact with each other, and tightened with screws.
Below are some recommended specifications of heat sinks for SSR, for users' reference. With different usage conditions, users can make appropriate adjustments.
How to protect SSR
A、 Overcurrent protection. SSR is a semiconductor power device that is extremely sensitive to temperature changes. Overcurrent can damage SSR, and fast fuses are usually used. But it is necessary to understand its protective characteristics, know the relationship between its melting current and time, and correctly choose the fast melting that is suitable for the SSR nominal current.
B、 Add RC absorption circuit. Adding an RC circuit not only prevents overvoltage, but also benefits the improvement of dv/dt. Suggested R is 20-100 Ω, power is 2-5W, C is 0.1-0.47uf, and withstand voltage is 250-630v The SSR nominal current is set to an upper limit of 100 Ω for R and a lower limit of 0.1uf for C. Conversely, R is set to a smaller value and C is set to a larger value.
C、 Overheating protection
SSR overheating can lead to a decrease in characteristics, and in mild cases, it can cause permanent damage due to loss of control. It is recommended to install a temperature control switch near the SSR base plate, with a temperature control point between 75 and 80 ℃
D、 Connect inductor L in series in an inductive load. In inductive loads, SSR is usually damaged due to high current change rate di/dt. The inductance of L depends on its size and cost.
How to choose the model and specifications of SSR
The main focus is on selecting solid-state relays (SSRs) with appropriate rated currents. Unless otherwise specified, the same applies to power modules such as rectification and controllability.
Select the rated current of SSR according to different load types. Resistive loads, inductive loads, and capacitive loads have higher instantaneous currents at the beginning of start-up. Even if it is purely resistive, due to its positive temperature coefficient, the resistance value is small in the cold state, resulting in a larger starting current. When the electric furnace is first connected, the current is 1.3-1.4 times that of the stable state. When the incandescent lamp is turned on, the current is 10 times the steady state. Some metal halide lamps not only have a turn-on time of up to 10 minutes, but also have pulse currents up to 100 times the steady-state.
The starting current of asynchronous motors is 5-7 times the rated value, and the starting current of DC motors is even higher. Moreover, inductive loads also have a high back electromotive force. This is an indefinite value that varies with L and di/dt. Usually 1-2 times the power supply voltage, which is added to the power supply voltage. There is up to three times the power supply voltage.
Capacitive loads pose greater danger because during start-up, the voltage across the capacitor cannot suddenly change, causing the capacitor (load) to short-circuit. This type of load requires special attention when selecting.
It should be noted that users should not use the surge current value of SSR as the basis for selecting the load starting current. The surge current value of SSR is based on the standard of thyristor surge current. Its prerequisite is half (or one) power cycle. That is, 10 or 20ms. The aforementioned startup process can take as little as a few hundred milliseconds or minutes, and as much as 10 minutes. Please pay close attention to this point.
Selection method
When selecting solid-state relays for use with low current printed circuit boards, due to the high thermal conductivity of the lead terminals, welding should be carried out under conditions of temperature less than 250 ℃ and time less than 10 seconds. If considering the surrounding temperature, it may be necessary to consider derating for use. Generally, the load current should be controlled within 1/2 of the rated value for use.
2. The selection of solid-state relays (SSRs) is influenced by the surge characteristics of various loads. The controlled load will generate a large surge current at the moment of connection, and due to the lack of time to dissipate heat, it is likely to damage the internal thyristor of the SSR. Therefore, users should analyze the surge characteristics of the controlled load before selecting a relay. To ensure that the relay can withstand this surge current while maintaining steady-state operation, refer to Table 2 for the derating factor (at room temperature) under various loads when selecting.
If the selected relay needs to operate in situations with high frequency, lifespan, and reliability requirements, it should be multiplied by 0.6 on the basis of Table 2 to ensure reliable operation.
Generally, the above principles are followed when selecting, and DC solid-state relays using field-effect transistors as output devices can be selected for low voltage requirements with minimal signal distortion; For AC resistive loads and most inductive loads, zero crossing relays can be used to extend the life of the load and relay, as well as reduce their own RF interference. When used as phase output control, random solid-state relays should be selected.
3. The impact of environmental temperature on usage
The load capacity of solid-state relays is greatly affected by environmental temperature and their own temperature rise. During installation and use, good heat dissipation conditions should be ensured. Products with a rated working current of 10A or above should be equipped with a heat sink, and products with a rated working current of 100A or above should be equipped with a heat sink and a fan for strong cooling. During installation, attention should be paid to good contact between the bottom of the relay and the heat sink, and an appropriate amount of thermal conductive silicone grease should be applied to achieve the best heat dissipation effect.
If the relay operates in a high temperature state (40 ℃~80 ℃) for a long time, users can consider reducing the rating based on the maximum output current and environmental temperature curve data provided by the manufacturer to ensure normal operation.
4. Overcurrent and overvoltage protection measures
When using relays, overcurrent and load short circuits can cause permanent damage to the internal output thyristor of SSR solid-state relays. It is recommended to consider adding fast fuses and air switches in the control circuit for protection (relays should be selected with product output protection, built-in varistor absorption circuit and RC buffer, which can absorb surge voltage and improve dv/dt resistance); Output protection can also be achieved by connecting an RC absorption circuit and a varistor (MOV) in parallel at the output terminal of the relay. The selection principle is to use 500V-600V varistors for 220V, and 800V-900V varistors can be used for 380V.
5. Relay input circuit signal
When the input voltage or current exceeds the specified rated parameters during use, it is possible to consider connecting a voltage divider resistor in series at the input end or a shunt resistor in parallel at the input port to ensure that the input signal does not exceed its rated parameter value.
In specific use, the control signal and load power supply should be stable, with fluctuations not exceeding 10%, otherwise voltage stabilization measures should be taken.
7. During installation and use, it should be kept away from electromagnetic interference and radio frequency interference sources to prevent relay misoperation and loss of control.
When a solid-state relay is open circuit and there is voltage at the load end, there will be a certain leakage current at the output end, which should be noted during use or design.
When replacing solid-state relays due to failure, it is recommended to choose products with the same prototype number or technical parameters as much as possible, in order to match the original application circuit and ensure the reliable operation of the system.
Solid state relays for communication can be classified into voltage zero crossing conduction type (referred to as zero crossing type) and random conduction type (referred to as random type) according to their switching modes;
According to the output switch elements, there are bidirectional thyristor output type (ordinary type) and unidirectional thyristor anti parallel type (enhanced type);
According to the installation method, there are two types: needle insertion type (naturally cooled, no need for heat sink) used on printed circuit boards and device type (cooled by heat sink) fixed on metal substrates;
In addition, there are constant current source types with wide range input (DC3-32V) and series resistor current limiting types at the input end.
SSR solid-state relays can be divided into two types in triggering form: zero voltage type (Z) and phase modulation type (P).
When the appropriate control signal VIN is applied at the input end, the P-type SSR immediately conducts. When the VIN is revoked and the load current is lower than the holding current of the bidirectional thyristor (AC commutation), the SSR is turned off. The Z-type SSR includes a zero crossing detection circuit inside. When the input signal VIN is applied, the SSR can only conduct when the load power supply voltage reaches the zero crossing zone, which may cause a maximum delay of half a cycle of the power supply. The shutdown conditions of Z-type SSR are the same as P-type, but due to the approximate sine wave working current of the load and low high-order harmonic interference, it is widely used. Due to the use of different output devices, SSR can be divided into ordinary type (S uses bidirectional thyristor elements) and enhanced type (HS uses unidirectional thyristor elements). When an inductive load is added, the bidirectional thyristor conducts before the input signal cutoff t1, and the current lags behind the power supply voltage by 90O (pure inductive). At time t1, the input control signal is cancelled, and the bidirectional thyristor turns off when it is less than the holding current (t2). The thyristor will withstand a reverse voltage with a high voltage rise rate dv/dt. This voltage will be fed back to the gate through the junction capacitance inside the bidirectional thyristor. If the bidirectional thyristor commutation dv/dt index is exceeded (typical value of 10V/s), it will cause a long commutation recovery time or even failure. Unidirectional thyristors (enhanced SSRs) are only limited by the static voltage rise rate (typical value of 200V/s) due to their unipolar working state, so the HS series of enhanced solid-state relays has a 520 times higher commutation dv/dt index than ordinary SSRs. Due to the use of two high-power unidirectional thyristors in anti parallel, the current distribution and heat conduction conditions have been changed, resulting in an increase in SSR output power. In high-power applications, the enhanced SSR outperforms ordinary solid-state relays in terms of voltage and current resistance, as well as product reliability, for both inductive and resistive loads, and meets the basic indicators of imported products. It is an updated product that replaces ordinary solid-state relays.
Selection of Load and SSR
SSR should not be a problem for general loads, but special load conditions must also be considered to avoid excessive surge current and overvoltage, which can cause unnecessary damage to device performance. The "cold resistance" characteristics of incandescent lamps, electric furnaces, etc. cause surge currents at the moment of opening, exceeding the rated working current value by several times. Ordinary SSR can be selected based on 2/3 of the current value. Enhanced SSR can be selected according to the parameters provided by the manufacturer. In industrial control sites under harsh conditions, it is recommended to leave sufficient voltage and current margins.
Some types of lamps may exhibit low impedance at the moment of burning out. Gasification and discharge channels, as well as capacitive loads such as switching capacitor banks or capacitor power supplies, can cause a similar short-circuit state. Further series connection of resistors or inductors in the circuit can be used as a current limiting measure. The opening and closing of the motor will also generate significant surge currents and voltages. The shaking caused by unreliable closing of intermediate relays and solenoid valves, as well as the superposition of capacitor voltage and power supply voltage during capacitor commutation of motors, will generate a surge voltage of twice the power supply at both ends of the SSR.
When controlling the primary of a transformer, the transient voltage on the secondary line should also be considered for its impact on the primary. In addition, transformers may also experience abnormal surge currents caused by saturation due to asymmetric currents in two directions. The above situation makes the application of SSR in special loads somewhat complex. A feasible approach is to use an oscilloscope to measure the potential surge current and voltage, in order to select appropriate SSRs and protective measures.
The controlled load will generate a large surge current at the moment of connection. Due to the lack of time to dissipate heat, it is likely to damage the internal thyristor of the SSR. Therefore, when selecting a relay, users should analyze the surge characteristics of the controlled load and then choose the relay. To ensure that the relay can withstand this surge current while maintaining steady-state operation, refer to Table 2 for the derating factor (at room temperature) under various loads when selecting.
If the selected relay needs to operate in situations with high frequency, lifespan, and reliability requirements, it should be multiplied by 0.6 on the basis of Table 2 to ensure reliable operation.
Generally, the above principles are followed when selecting, and DC solid-state relays using field-effect transistors as output devices can be selected for low voltage requirements with minimal signal distortion; For AC resistive loads and most inductive loads, zero crossing relays can be used to extend the life of the load and relay, as well as reduce their own RF interference. When used as phase output control, random solid-state relays should be selected.
It is a contactless switch device with relay characteristics that uses semiconductor devices instead of traditional electrical contacts as switching devices. The single-phase SSR is a four terminal active device, with two input control terminals and two output terminals. The input and output terminals are optically isolated, and when a DC or pulse signal is added to the input terminal to a certain current value, the output terminal can switch from off state to on state.
AC solid-state relays are divided into voltage zero crossing conduction type (referred to as zero crossing type) and random conduction type (referred to as random type) according to their switching modes
According to the output switch elements, there are bidirectional thyristor output type (ordinary type) and unidirectional thyristor anti parallel type (enhanced type)
According to the installation method, there are two types: needle insertion type (naturally cooled, no need for heat sink) used on printed circuit boards and device type (cooled by heat sink) fixed on metal substrates
In addition, there are constant current source types with wide range input (DC3-32V) and series resistor current limiting types at the input end.
SSR solid-state relays can be divided into two types in triggering form: zero voltage type (Z) and phase modulation type (P).
When a suitable control signal IN is applied at the input end, the P-type SSR immediately conducts. When the IN is revoked and the load current is lower than the holding current of the bidirectional thyristor (AC commutation), the SSR is turned off. The Z-type SSR includes a zero crossing detection circuit inside. When the input signal IN is applied, the SSR can only conduct when the load power supply voltage reaches the zero crossing zone, which may cause a maximum delay of half a cycle of the power supply. The shutdown conditions of Z-type SSR are the same as P-type, but due to the approximate sine wave working current of the load and low high-order harmonic interference, it is widely used. Due to the use of different output devices, the SSRs of Beijing Lingtong Electronics Company are divided into ordinary type (S, using bidirectional thyristor elements) and enhanced type (HS, using unidirectional thyristor elements). When an inductive load is added, the bidirectional thyristor conducts before the input signal cutoff t1, and the current lags behind the power supply voltage by 90O (pure inductive). At time t1, the input control signal is cancelled, and the bidirectional thyristor turns off when it is less than the holding current (t2). The thyristor will withstand a reverse voltage with a high voltage rise rate dv/dt. This voltage will be fed back to the gate through the junction capacitance inside the bidirectional thyristor. If the bidirectional thyristor commutation dv/dt index is exceeded (typical value of 10V/s), it will cause a long commutation recovery time or even failure. Unidirectional thyristors (enhanced SSRs) are only limited by the static voltage rise rate (typical value of 200V/s) due to their unipolar working state, so the HS series of enhanced solid-state relays has a commutation dv/dt index that is 5-20 times higher than that of ordinary SSRs. Due to the use of two high-power unidirectional thyristors in anti parallel, the current distribution and heat conduction conditions have been changed, resulting in an increase in SSR output power. In high-power applications, the enhanced SSR outperforms ordinary solid-state relays in terms of voltage and current resistance, as well as product reliability, for both inductive and resistive loads.
How to match the user's driver circuit with the input characteristics of SSR
Generally speaking, the input control voltage of SSR is 3.2-32V. Control the current to 5-30mA Usually, SSR input circuits with 1-25A are not constant current source circuits, and the input control voltage is 4-16V. The SSR input circuit with a control current of 5-20mA is connected to a constant current source circuit for larger rated currents. The input control voltage can be between 3.2-32V. In a three-phase circuit, if the user connects the input terminals of three SSRs in series, they hope to provide a control voltage greater than 12V; If the input terminals of three SSRs are connected in parallel, the driving current must be guaranteed to be 50mA. When using a single SSR, the driving current should not be designed to be greater than 6mA at the critical state of 4-5mA.
Horizontal W-shaped and vertical L-type, with small volume, suitable for direct soldering and installation of printed boards. Vertical L2 type, suitable for both circuit board soldering and plug-in installation on circuit boards. When selecting solid-state relays for use with low current printed circuit boards, due to the high thermal conductivity of the lead terminals, welding should be carried out under conditions of temperature less than 250 ℃ and time less than 10 seconds. If considering the surrounding temperature, it may be necessary to consider derating for use. Generally, the load current is controlled within 1/2 of the rated value for use.
K-type and F-type are suitable for installing radiators and instrument baseboards. When installing high-power SSRs (K-type and F-type packages), attention should be paid to ensuring that the contact surface of the heat sink is flat and coated with thermal conductive silicone grease (Meibao T-50). The greater the installation torque, the smaller the contact thermal resistance. High current lead wires require cold pressed solder pads to reduce the contact resistance of the lead wires.
Input circuit
SSR can be divided into resistance type, constant current source, and AC input control type according to the input control method. At present, we mainly provide resistor input type for 5V TTL level. When using other control voltages, current limiting resistors can be selected accordingly. SSR input belongs to current type devices. When the optocoupler thyristor at the input end is fully conductive (in microseconds), it triggers the power thyristor to conduct. When the excitation is insufficient or the triggering voltage is in the form of a ramp wave, it may cause the power thyristor to be at the critical conduction edge and cause damage due to the main load current flowing through the triggering circuit. When the input voltage or current exceeds the specified rated parameters during use, it is possible to consider connecting a voltage divider resistor in series at the input end or a shunt resistor in parallel at the input port to ensure that the input signal does not exceed its rated parameter value.
For example, in the basic performance testing circuit, an adjustable voltage source is used as the input, a 100W light bulb is used as the test load, and the input trigger signal should be a step logic level with strong triggering mode. The standard current for devices provided by foreign manufacturers is 10mA. Considering the full temperature operating range (-40~+70 ℃), stable luminous efficiency, and anti-interference ability, it is recommended that the optimal DC trigger operating current be between 12-25mA.
The SSR input can be driven in parallel or series. When used in series, one SSR is considered to have a voltage of 4V, and 12V voltage can drive three SSRs. In specific use, the control signal and load power supply should be stable, with fluctuations not exceeding 10%, otherwise voltage stabilization measures should be taken.
interfere
During installation and use, it should be kept away from electromagnetic interference and radio frequency interference sources to prevent relay misoperation and loss of control. SSR products are also a source of interference, which can generate radiation or RF interference from power lines through the load when conducting, and the degree of interference varies with the size of the load. The interference generated by incandescent resistance loads is relatively small, and the zero voltage type conducts near the zero crossing zone (i.e. zero voltage) of the AC power supply, so the interference is also relatively small. The method of reduction is to connect inductive coils in series with the load. In addition, cross interference between signal lines and power lines should also be avoided.
When a solid-state relay is open and there is voltage at the load end, there will be a certain leakage current at the output end. When using or designing, attention should be paid to preventing electric shock. When replacing solid-state relays due to failure, it is advisable to choose products with the same prototype number or technical parameters as much as possible, in order to match the original application circuit and ensure the reliable operation of the system.
When SSR is conducting, the component will bear the dissipated power of P=V (tube voltage drop) × I (load), where V effective value and I effective value are the effective values of saturation voltage drop and operating current, respectively. The load capacity of solid-state relays is affected by environmental temperature and
The impact of self temperature rise is significant, and it is necessary to strictly refer to the allowable shell temperature rise (75 ℃) at the rated working current based on the actual working environment conditions. The size of the radiator should be selected reasonably or the current should be reduced for use. During installation and use, good heat dissipation conditions should be ensured, otherwise overheating may cause loss of control and even product damage.
Generally speaking, instrument baseboards with good heat dissipation conditions can be used for currents below 10A. Products with a rated working current of 10A or higher should be equipped with a heat sink. For currents below 30A, natural air cooling should be used. When the continuous load current is greater than 30A, instrument fans should be used for forced air cooling. Products above 100A should be equipped with a heat sink and fan for forced cooling. During installation, attention should be paid to good contact between the bottom of the relay and the heat sink, and an appropriate amount of thermal conductive silicone grease should be applied to achieve the best heat dissipation effect. If the relay operates in a high temperature state (40 ℃~80 ℃) for a long time, users can consider reducing the rating based on the maximum output current and environmental temperature curve data provided by the manufacturer to ensure normal operation.
Reasons for heating of solid-state relays:
Solid state relays have a certain power loss on their internal chips during normal operation, which 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 solid-state relay operation, and excellent thermal design can avoid failures and damages caused by poor heat dissipation.
When using relays, overcurrent and load short circuits can cause permanent damage to the internal output thyristor of SSR solid-state relays. It is recommended to consider adding fast fuses and air switches in the control circuit for protection (relays should be selected with product output protection, built-in varistor absorption circuit and RC buffer, which can absorb surge voltage and improve dv/dt resistance); Quick fuses and air switches are common overcurrent protection methods. Fast acting fuses can be selected at 1.2 times the rated working current, and fuses can be used for small capacities. Pay special attention to load short circuits, which are the main cause of damage to SSR products.
For inductive and capacitive loads, in addition to internal RC circuit protection, it is recommended to use varistors in parallel at the output end as combined protection. The area size of metal zinc oxide varistors (MOVs) determines the absorbed power, while the thickness determines the protection voltage value. AC 220V SSR, using MYH12-430V varistor; Select MYH12-750V varistor for 380V; Large capacity motor transformers should use MYH20 or MYH2024 varistors with high current carrying capacity. The selection principle is to use 500V-600V varistors for 220V, and 800V-900V varistors can be used for 380V.
SSR and TSR voltage regulating modules can be triggered by external analog signals to achieve linearly adjustable output voltage. For example, PLC or temperature controller output analog signals: 1-5V, 4-20mA trigger system. Domestic single-phase and three-phase thyristor trigger board, combined with thyristor, can also be adjusted by external analog signals to adjust the trigger board. The trigger board can then trigger the module to achieve linear adjustable output voltage, control the thyristor conduction angle, and achieve voltage regulation.
'AC power regulation' is a commonly used method in Z-type SSRs, which can also achieve PID regulation. Control the number of half waves of AC sine current within a fixed period to achieve power regulation. Analog circuits often use voltage comparators to compare a fixed period sawtooth voltage with the error voltage from the previous stage, and output a square wave for regulation, as shown in Figure 3. Using a timing algorithm on a computer to generate square pulse impulses with adjustable duty cycles. For example, SHIMADEW from Japan and OMRON's SR22, FD20, E5 series intelligent temperature control products, combined with Z-type SSR, achieve adaptive "automatic flipping" control, which generates disturbances through computers and calculates the optimal PID control parameters.
HS series SSR products can be directly used for controlling three-phase motors. The simplest method is to use 2 SSRs for motor on-off control, 4 SSRs for motor commutation control, and no control for the third phase.
When reversing the direction of a motor, it should be noted that due to the inertia of the motor's motion, it must be stopped before reversing to avoid situations similar to motor stalling, which can cause large impulse voltages and currents. In the design of control circuits, it should be noted that there should be no possibility of simultaneous conduction of commutation SSRs at any time. The timing of power on and off should adopt the sequence of first adding and then disconnecting the control circuit power supply, and then adding and disconnecting the motor power supply. Inverter connection cannot be simply used between reversing SSRs to avoid phase to phase short circuit accidents caused by the conduction of another phase SSR while the conducting SSR is not turned off. In addition, the safety, phase loss, and temperature relays in motor control are also protective devices to ensure the normal operation of the system.
When selecting solid-state relays for use with low current printed circuit boards, due to the high thermal conductivity of the lead terminals, welding should be carried out under conditions of temperature less than 250 ℃ and time less than 10 seconds. If considering the surrounding temperature, it may be necessary to consider derating for use. Generally, the load current should be controlled within 1/2 of the rated value for use.
2. Selection of Solid State Relay (SSR) for Various Load Surge Characteristics
The controlled load will generate a large surge current at the moment of connection. Due to the lack of time to dissipate heat, it is likely to damage the internal thyristor of the SSR. Therefore, when selecting a relay, users should analyze the surge characteristics of the controlled load and then choose the relay. To ensure that the relay can withstand this surge current while maintaining steady-state operation, refer to Table 2 for the derating factor (at room temperature) under various loads when selecting.
If the selected relay needs to operate in situations with high frequency, lifespan, and reliability requirements, it should be multiplied by 0.6 on the basis of Table 2 to ensure reliable operation.
Generally, the above principles are followed when selecting, and DC solid-state relays using field-effect transistors as output devices can be selected for low voltage requirements with minimal signal distortion; For AC resistive loads and most inductive loads, zero crossing relays can be used to extend the life of the load and relay, as well as reduce their own RF interference. When used as phase output control, random solid-state relays should be selected.
3. The impact of environmental temperature on usage
The load capacity of solid-state relays is greatly affected by environmental temperature and their own temperature rise. During installation and use, good heat dissipation conditions should be ensured. Products with a rated working current of 10A or above should be equipped with a heat sink, and products with a rated working current of 100A or above should be equipped with a heat sink and a fan for strong cooling. During installation, attention should be paid to good contact between the bottom of the relay and the heat sink, and an appropriate amount of thermal conductive silicone grease should be applied to achieve the best heat dissipation effect.
If the relay operates in a high temperature state (40 ℃~80 ℃) for a long time, users can consider reducing the rating based on the maximum output current and environmental temperature curve data provided by the manufacturer to ensure normal operation.
4. Overcurrent and overvoltage protection measures
When using relays, overcurrent and load short circuits can cause permanent damage to the internal output thyristor of SSR solid-state relays. It is recommended to consider adding fast fuses and air switches in the control circuit for protection (relays should be selected with product output protection, built-in varistor absorption circuit and RC buffer, which can absorb surge voltage and improve dv/dt resistance); Output protection can also be achieved by connecting an RC absorption circuit and a varistor (MOV) in parallel at the output terminal of the relay. The selection principle is to use 500V-600V varistors for 220V, and 800V-900V varistors can be used for 380V.
5. Relay input circuit signal
When the input voltage or current exceeds the specified rated parameters during use, it is possible to consider connecting a voltage divider resistor in series at the input end or a shunt resistor in parallel at the input port to ensure that the input signal does not exceed its rated parameter value.
In specific use, the control signal and load power supply should be stable, with fluctuations not exceeding 10%, otherwise voltage stabilization measures should be taken.
7. During installation and use, it should be kept away from electromagnetic interference and radio frequency interference sources to prevent relay misoperation and loss of control.
When a solid-state relay is open circuit and there is voltage at the load end, there will be a certain leakage current at the output end, which should be noted during use or design.
When replacing solid-state relays due to failure, it is recommended to choose products with the same prototype number or technical parameters as much as possible, in order to match the original application circuit and ensure the reliable operation of the system.
The key technical parameters of solid-state relays are:
Input voltage range
The input voltage range within which solid-state relays can operate at an ambient temperature of 25'c.
Input current
The input current value corresponding to a specific voltage within the input voltage range.
Connected voltage
When this voltage is applied to the input terminal or greater than this voltage value, the output terminal ensures conduction.
Turn off voltage
When the voltage is applied to the input terminal or less than the voltage value, ensure that the output terminal is turned off.
Reverse polarity voltage
The maximum allowable reverse voltage that can be applied to the input terminal of a relay without causing permanent damage.
Rated output current
Maximum steady-state operating current at 25'C environment.
Rated output voltage
The maximum load operating voltage that can be sustained.
Output voltage drop
The output voltage measured at the rated output current when the relay is conducting.
Output leakage current
The current value flowing through the load when the relay is in the off state and the rated output voltage is applied.
Connection time
When the relay is turned on, the time interval between applying the input voltage and the start of the turn-on voltage until the output reaches 90% of its final voltage change.
Off time
When the relay is turned off, the time interval between cutting off the input voltage and turning off the voltage until the output reaches 10% of its final voltage change.
Zero crossing voltage
The maximum starting voltage at which the output terminal of an AC zero crossing solid-state relay can conduct by adding a rated voltage to the input terminal.
Maximum surge voltage
The non repetitive surge (or overload) current that a relay can withstand without causing permanent damage.
Peak value of electrical system
The maximum superimposed instantaneous peak breakdown voltage that the relay output terminal can withstand in the working state of the relay.
Voltage index rise rate dv/dt
The voltage rise rate that the output components of a relay can withstand without causing it to conduct.
working temperature
The normal operating temperature range of a relay when installed according to specifications or without a heat dissipation plate.
Technical terms
Environmental temperature range:
The temperature limit of the surrounding air during normal operation of solid-state relays is usually given under two conditions: operation and storage. The maximum temperature is also limited by the radiator and power factors.
--------------------------------------------------------------------------------------------
Dielectric withstand voltage (unit: V)
The maximum voltage that a solid-state relay can withstand between its input and output terminals, as well as between the input and output terminals and the heat dissipation base plate. Attention: It is not allowed to measure the dielectric withstand voltage between the output terminals of the same input (or output) circuit. They should be short circuited before measurement.
--------------------------------------------------------------------------------------------
Insulation resistance (unit: M Ω):
The resistance value measured by applying a voltage of 500VDC between the input and output terminals of the solid-state relay, as well as between the input and output terminals and the heat dissipation base plate. Attention: It is not allowed to measure the insulation resistance between the output terminals of the same input (or output) circuit. They should be short circuited before measurement.
--------------------------------------------------------------------------------------------
Peak value of electrical system (unit: V):
Under specified environmental conditions, the input terminal of the solid-state relay is open circuited, and a specific waveform and energy voltage is superimposed on the rated output voltage of the output terminal for one minute of testing. After the experiment, the solid-state relay still meets the requirements.
----------------------------------------------------------------------------
Shutdown time (unit: ms):
The time interval from the moment the input voltage of the normally open solid-state relay is cut off to ensure the shutdown voltage until the output voltage reaches 90% of its final voltage change.
----------------------------------------------------------------------------
Conduction time (unit: ms):
The time interval from the moment the input voltage of the normally open solid-state relay reaches the guaranteed turn-on voltage to the moment the output voltage reaches 90% of its final voltage change.
----------------------------------------------------------------------------
Output leakage current (unit: mA):
The maximum (effective value) off state leakage current flowing between the output terminals when no conduction control signal is applied to the input terminal. Usually refers to the value at the maximum output rated voltage within the entire temperature range. This value is mainly generated by the output buffer.
----------------------------------------------------------------------------
Maximum on state voltage drop (unit: V):
The maximum (peak) voltage drop across the output terminal at full load current at a specified ambient temperature.
----------------------------------------------------------------------------
Transient overvoltage (unit: PIV):
The maximum allowable voltage deviation that a solid-state relay can withstand without causing damage or error while maintaining its off state. Exceeding this transient voltage can cause the solid-state relay to conduct, and if the current condition is met, it is non destructive. The duration of transients is generally not specified and can be on the order of a few seconds, limited by internal bias network power consumption or capacitor ratings.
----------------------------------------------------------------------------
Minimum cutoff dv/dt (static) (unit: V/us):
When no conduction control signal is applied, the output terminal (AC) of the solid-state relay can withstand a non conducting voltage rise rate. Usually expressed as the minimum voltage rise rate at the maximum rated voltage.
----------------------------------------------------------------------------
Maximum repetitive conduction voltage peak (unit: VRMS):
The maximum (peak) off state voltage across the output terminals after applying the conduction control signal for half a cycle and before each subsequent half cycle is about to conduct. This parameter is also applicable to solid-state relays with or without "zero conduction" characteristics.
----------------------------------------------------------------------------
Maximum zero crossing conduction voltage (unit: VRMS) (also known as zero crossing voltage):
The maximum (peak) off state voltage across the output terminals after applying the conduction control signal and before each subsequent half cycle of conduction.
----------------------------------------------------------------------------
Power consumption (at rated current) (unit: W):
The maximum average power consumption mainly caused by the effective voltage drop (power consumption) of the output semiconductor.
Maximum I2t (for selecting fuses) (unit: A2s):
The ability of solid-state relays to withstand maximum non repetitive pulse currents, used for fuse selection.
----------------------------------------------------------------------------
Maximum overcurrent (unit: A):
The maximum instantaneous current that is not allowed to flow for a specified duration is usually expressed as an effective value of 1 second.
----------------------------------------------------------------------------
Maximum surge current (non repetitive) (unit: A):
The maximum instantaneous current that is not allowed to flow for a specified duration, with a typical duration of one cycle of alternating current (10ms), is usually defined as the peak value and the curve of current versus time.
----------------------------------------------------------------------------
Minimum load current (unit: mA):
The minimum load current required for solid-state relays to perform specified operations. It is usually listed together with the maximum load current as the "operating current range".
----------------------------------------------------------------------------
Maximum load current (unit: A):
The maximum steady-state load current capability of a solid-state relay at a specified ambient temperature is also limited by the heat dissipation of the radiator and ambient temperature conditions.
----------------------------------------------------------------------------
Output voltage range (unit: V):
At the specified ambient temperature, the voltage range applied to the output terminal, within which the solid-state relay continues to be in the off or switching state, or in other words, executes the specified state. The frequency value of the line is either included or indicated in units (AC).
----------------------------------------------------------------------------
Minimum input impedance (unit: Ω):
The minimum impedance at a given voltage. As an alternative or supplement to input current, it determines the input power requirements.
----------------------------------------------------------------------------
Reverse polarity voltage (applicable only to DC input) (unit: V):
The maximum allowable reverse voltage that can be applied to the input terminal of a solid-state relay at a specified ambient temperature without causing permanent damage to the solid-state relay. This value is generally determined as the upper limit of the input voltage.
----------------------------------------------------------------------------
Input current (unit: mA):
The current value flowing into the input circuit of a solid-state relay by applying a specified input voltage to its input terminal at a specified ambient temperature.
----------------------------------------------------------------------------
Ensure shutdown voltage (unit: V):
A voltage applied to the input terminal at a specified ambient temperature that ensures the output terminal is in an off state when the input is at or below that value.
----------------------------------------------------------------------------
Ensure the connected voltage (unit: V):
A voltage applied to the input terminal at a specified ambient temperature that ensures the output terminal is in a conducting state when the input is at or above that value.
----------------------------------------------------------------------------
Input voltage range (unit: V):
The voltage range applied to the input terminal at a specified ambient temperature to maintain the output terminal in a "conducting" state. In general, DC inputs include: 3-32VDC constant current input type and 3-14VDC, 10-40VDC resistive input type. Communication input includes: 90-280VAC input type. The lower limit of the input voltage is the so-called guaranteed turn-on voltage, and the upper limit of the input voltage is the so-called reverse polarity voltage (only applicable to DC input).
中文
English