1. When selecting the solid-state relay used in the printed circuit board with small current specifications, because the lead terminal is made of high thermal conductivity material, the soldering should be carried out under the condition that the temperature is less than 250 °C and the time is less than 10S.
2. The selection of SSR for solid state relay with various load surge characteristics
The controlled load will generate a large inrush current at the moment of switching, and because the heat cannot be dissipated in time, it is likely to damage the internal thyristor of the SSR, so the user should analyze the surge characteristics of the controlled load when selecting the relay, and then select the relay. To make the relay be able to withstand this inrush current under the premise of ensuring steady-state operation, you can refer to the derating coefficient (at room temperature) of various loads in Table 2 when selecting.
If the selected relay needs to work in the occasion with high requirements for more frequent work, life and reliability, it should be multiplied by 0.6 on the basis of Table 2 to ensure reliable operation.
Generally, the above principles are followed in the selection, and the DC solid-state relay using the FET as the output device can be selected when the signal distortion is small at low voltage; For example, for AC resistive loads and most inductive loads, zero-crossing relays can be selected, which can prolong the life of the load and relay, and also reduce its own radio frequency interference. If it is used as a phase output control, a random solid state relay should be selected.
3. The influence of the ambient temperature
The load capacity of the solid state relay is greatly affected by the ambient temperature and its own temperature rise, in the process of installation and use, it should be ensured that it has good heat dissipation conditions, the rated working current of more than 10A products should be equipped with radiators, and products above 100A should be equipped with radiators and fans for strong cooling. During installation, attention should be paid to the good contact between the bottom of the relay and the radiator, and an appropriate amount of thermal grease should be considered to achieve the best heat dissipation effect.
If the relay works at high temperature for a long time (40°C-80°C), the user can consider derating to ensure normal operation according to the maximum output current and ambient temperature curve data provided by the manufacturer.
4. Over-current and over-voltage protection measures
When the relay is used, due to overcurrent and load short circuit, the internal output of the SSR solid state relay will be permanently damaged, and the quick fuse and air switch can be considered in the control loop for protection (the product output protection should be selected for the selection of relay, and the built-in varistor absorption loop and RC buffer can absorb the surge voltage and improve the dv/dt resistance); Output protection can also be achieved by connecting an RC absorption loop and a varistor (MOV) at the output of the relay. The selection principle is 500V-600V varistor for 220V, and 800V-900V varistor for 380V.
5. Relay input loop signal
When the input voltage is too high or the input current is too large to exceed the rated parameters specified in the input, it can be considered to connect the voltage divider resistor in series at the input terminal or connect the shunt resistor at the input port in parallel, so that the input signal does not exceed its rated parameter 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. When installing and using, it should be kept away from electromagnetic interference and radio frequency interference sources to prevent the relay from running out of control.
8. When the solid state relay is open and there is voltage at the load end, there will be a certain leakage current at the output end, which should be paid attention to when using or designing.
9. When the solid-state relay fails to be replaced, the original model or product with the same technical parameters should be selected as far as possible to match the original application circuit and ensure the reliable operation of the system.
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 the switching mode;
According to the output switching components, there are bidirectional thyristor output type (ordinary type) and unidirectional thyristor anti-parallel type (enhanced type);
According to the installation method, it is divided into pin-insert type (natural cooling, no need to have a radiator) on the printed circuit board and a device type fixed on the metal bottom plate (cooled by a radiator);
In addition, the input terminal has a wide range of input (DC3-32V) constant current source type and string resistance current limiting type.
SSR solid state relays can be divided into two types: zero-voltage type (Z) and phase-modulated type (P) in the form of trigger.
When a suitable control signal VIN is applied to the input, the P-type SSR is immediately turned on. When the VIN is revoked, the load current is lower than the triac maintenance current (AC commutation) SSR is turned off. The Z-type SSR includes a zero-crossing detection circuit, and when the input signal VIN is applied, the SSR can only be turned on when the load supply voltage reaches the zero-crossing zone, and may cause a maximum delay in the power supply half cycle. The shutdown condition of Z-type SSR is the same as that of P-type, but it is widely used because the load working current is similar to a sine wave and the interference of higher harmonics is small. SSRs are divided into normal type (S uses bidirectional thyristor) and enhanced type (HS uses unidirectional thyristor) depending on the output device. When an inductive load is applied, the triac is turned on before the input signal cuts off t1, and the current lags at the supply voltage of 90O (pure inductance). The input control signal is withdrawn at T1, and the triac is turned off when the maintenance current is less than (T2), and the thyristor will be subjected to a reverse voltage with a high voltage rise rate dv/dt. This voltage is positively fed back to the gate through the junction capacitance inside the triac. If the bidirectional triac commutation dv/dt index (typical value of 10V/s is exceeded), it will cause a long commutation recovery time or even failure. Since the unidirectional thyristor (enhanced SSR) is in a unipolar state, it is only limited by the quiescent voltage rise rate (typical value 200 V/s), so the commutation dv/dt index of the enhanced solid state relay HS series is 520 times higher than that of ordinary SSR. Due to the use of two high-power unidirectional thyristors in reverse parallel, the current distribution and thermal conductivity conditions are changed, and the SSR output power is increased. In high-power applications, the enhanced SSR surpasses ordinary solid-state relays in terms of inductive load and resistive load withstand voltage, current impulse resistance and product reliability, and has reached the basic indicators of imported products, and is an updated product to replace ordinary solid-state relays.
Choice of load with SSR
SSR should be no problem for general loads, but some special load conditions must also be considered to avoid excessive inrush current and overvoltage, which can cause unnecessary damage to the performance of the device. The "cold resistance" characteristics of incandescent lamps, electric furnaces, etc., cause the inrush current at the moment of opening, which exceeds the rated working current value by several times. Generally, the ordinary SSR can be selected according to 2/3 of the current value. Enhanced SSR can be selected based on the parameters provided by the vendor. In the industrial control site under harsh conditions, it is recommended to leave sufficient voltage and current margin.
Some types of lamps have low impedance at the moment of burn-off. Gasification and discharge channels, as well as capacitive loads, such as switching capacitor banks or capacitor power supplies, can cause similar short-circuit conditions. Resistors or inductors can be further connected in series in the line as a current limiting measure. The opening and closing of the motor will also produce a large inrush current and voltage. Jitter caused by unreliable engagement of intermediate relays and solenoid valves, and the superposition of capacitor voltage and supply voltage when commutating a capacitor commutation motor will produce twice the surge voltage of the power supply at both ends of the SSR.
When controlling the primary transformer, the influence of the transient voltage on the secondary line on the primary should also be considered. In addition, the transformer may also cause abnormal surge current due to saturation due to asymmetry of current in two directions. The above situation makes the application of SSR in special loads somewhat complicated. The feasible way is to use an oscilloscope to measure the potential inrush current and voltage, so as to select the appropriate SSR and protection measures.
The controlled load will generate a large inrush current at the moment of switching, and because the heat cannot be dissipated in time, it is likely to damage the internal thyristor of the SSR, so the user should analyze the surge characteristics of the controlled load when selecting the relay, and then select the relay. To make the relay be able to withstand this inrush current under the premise of ensuring steady-state operation, you can refer to the derating coefficient (at room temperature) of various loads in Table 2 when selecting.
If the selected relay needs to work in the occasion with high requirements for more frequent work, life and reliability, it should be multiplied by 0.6 on the basis of Table 2 to ensure reliable operation.
Generally, the above principles are followed in the selection, and the DC solid-state relay using the FET as the output device can be selected when the signal distortion is small at low voltage; For example, for AC resistive loads and most inductive loads, zero-crossing relays can be selected, which can prolong the life of the load and relay, and also reduce its own radio frequency interference. If it is used as a phase output control, a random solid state relay should be selected.
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