1. Overview
Solid -state relays ( SSRs) are contactless electronic switches with relay characteristics, developed by combining discrete electronic components, integrated circuits (or chips), and hybrid microcircuit technology. They feature long lifespan, high reliability, fast switching speed, low electromagnetic interference, no noise, and no sparks, and can be widely used in industrial automation fields such as aerospace, marine, home appliances, machine tools, communications, chemical, and coal mining.
2. Classification of solid-state relays (based on output load power supply)
2.1 AC Solid State Relay
2.1.1 Classified by switch method
a. Zero-crossing conduction type (abbreviated as zero-crossing type) b. Random voltage conduction type (abbreviated as random type)
2.1.2 Classified by output switching element
a. Bidirectional thyristor output type b. Unidirectional thyristor anti-parallel type (enhanced type)
2.1.3 Classified by installation method
a. Pin type: For circuit boards, generally for low current applications. b. Mount type: Can be equipped with a heat sink and fixed to a metal base plate, for high current applications.
2.2 DC Solid State Relay
2.2.1 Classified by input terminal
a. Optically isolated type; b. High-frequency magnetically isolated, transformer coupled.
2.2.2 Classified by output terminal
a. High-power transistor b. Power MOSFET
2.3 AC/DC Solid State Relays
a. Photovoltaic coupler b. Magnetic isolation
3. Principle Analysis of Typical AC/DC Solid State Relays
3.1 Principle Analysis of AC Solid State Relay (Zero-Crossing Type)
A solid-state relay consists of three parts: an input circuit, an isolation (coupling) circuit, and an output circuit. When a signal is applied to the control terminal of the input circuit, the phototransistor inside the IC1 optocoupler conducts. The series resistor R1 limits the current of the input signal to prevent damage to the optocoupler. An LED indicates the input control signal, and VD1 protects the optocoupler IC1 from damage caused by reversed polarity of the input signal.
V1 acts as an AC voltage detector in the circuit, enabling the solid-state relay to turn on when the voltage crosses zero and turn off when the load current crosses zero. When the phototransistor IC1 is off (when there is no signal input at the control terminal), V1 obtains base current through R2, causing it to saturate and conduct. This clamps the gate trigger voltage UGT of the SCR to a low potential and keeps it in the off state. Ultimately, this results in the BTA bidirectional thyristor not receiving a trigger pulse at the gate control terminal R6 and remaining in the off state.
When the phototransistor IC1 is turned on (when there is a signal input at the control terminal), the operating state of the SCR (Syrenees Rectifier) is determined by the AC voltage zero-point detection transistor V1. If the power supply voltage is divided by R2 and R3, and the voltage at point A is greater than the zero-crossing voltage (VA > VBE1), V1 is in a saturated conducting state, and both the SCR and BTA (Bilibili Thyrenees A) are in a turned-off state. If the power supply voltage is divided by R2 and R3, and the voltage at point A is less than the zero-crossing voltage (VA > VBE1), V1 is in a turned-off state, and the SCR receives a trigger signal through R4 and turns on, thereby triggering the BTA through R6 and turning it on as well, thus controlling the power supply to the load. If the control terminal signal is turned off at this time, the load current will decrease to the holding current IH of the BTA bidirectional thyrenees, at which point it will automatically turn off, cutting off the power supply to the load.
AC zero-crossing solid-state relays are characterized by turning on when the voltage crosses zero and turning off when the load current crosses zero. Their maximum turn-on and turn-off times are half a power supply cycle, resulting in a complete sine wave on the load. This correspondingly reduces the impact on the load. Furthermore, the radio frequency interference generated in the corresponding control circuit is also greatly reduced. The operating waveforms of the zero-crossing and random types are shown in Figures 2 and 3, respectively .
In the input control circuit, resistor R1 is connected in series at the input terminal of the IC1 optocoupler to limit the current of its LED, which indicates the input control signal. VD1 protects against the reverse bias voltage at the input terminal. When there is no signal input at the control terminal, the phototransistor in the IC1 optocoupler is in a cutoff high-resistance state. V1 obtains its base current through R2, causing it to saturate and conduct, which in turn causes V2, V3, and V4 to be in the cutoff state, thus turning off the solid-state relay.
When a signal is input to the control terminal, the phototransistor in IC1 is turned on, causing V1 to be cut off. This turns on V2, V3, and V4, turning on the solid-state relay and applying power to the load. The output of the DC solid-state relay is turned on when the input signal is applied and turned off when the signal disappears.
In addition, high-power, low-voltage DC solid-state relays generally use power MOSFETs instead of power transistors for their output switches in order to reduce input power.
4. Precautions for selection and use
4.1 When selecting solid-state relays for low-current printed circuit boards, since the lead terminals are made of high thermal conductivity materials, soldering should be carried out at a temperature of less than 250℃ and for a time of less than 10 seconds. If the ambient temperature is taken into consideration, derating may be considered if necessary. Generally, the load current should be controlled within 1/2 of the rated value.
4.2 Selection of SSR based on various load surge characteristics
Many controlled loads generate large inrush currents upon switching on. Because the heat cannot dissipate quickly enough, this can damage the internal thyristor of the SSR. Therefore, users should analyze the surge characteristics of the controlled load before selecting a relay. The relay should be able to withstand this inrush current while maintaining steady-state operation. Table 2 shows the derating factors (at room temperature) for various loads when selecting a relay.
If the selected relay needs to operate in environments with frequent operation and high requirements for lifespan and reliability, then the value in Table 2 should be multiplied by 0.6 to ensure reliable operation.
Generally, the above principles should be followed when selecting relays. For low voltage applications requiring minimal signal distortion, DC solid-state relays using MOSFETs as output devices can be selected. For AC resistive loads and most inductive loads, zero-crossing relays can be selected, which can extend the life of both the load and the relay, and also reduce its own radio frequency interference. When used for phase output control, random-type solid-state relays should be selected.
4.3 Influence of ambient temperature
The load capacity of solid-state relays is significantly affected by ambient temperature and their own temperature rise. During installation and use , good heat dissipation should be ensured. Products with a rated operating current of 10A or higher should be equipped with a heat sink, and products with a rated current of 100A or higher should be equipped with a heat sink and a fan for forced cooling. During installation, ensure good contact between the bottom of the relay and the heat sink, and consider applying an appropriate amount of thermal grease to achieve optimal heat dissipation.
If the relay operates at high temperatures (40℃～80℃) for a long time , users can consider derating it to ensure normal operation based on the maximum output current and ambient temperature curve data provided by the manufacturer.
4.4 Overcurrent and Overvoltage Protection Measures
When using relays, overcurrent and load short circuits can cause permanent damage to the internal output thyristor of the SSR. To protect against this, consider adding a fast-acting fuse and an air switch to the control circuit (when selecting a relay, choose one with output protection, a built-in varistor snubber circuit, and an RC buffer to absorb surge voltage and improve dv/dt withstand capability). Alternatively, connect an RC snubber circuit and a varistor (MOV) in parallel at the relay output to achieve output protection. The selection principle is to use a 500V-600V varistor for 220V and an 800V-900V varistor for 380V.
4.5 Relay Input Circuit Signal
If the input voltage or input current exceeds the specified rated parameters during use , consider connecting a voltage divider resistor in series at the input terminal or a shunt resistor in parallel at the input port to ensure that the input signal does not exceed its rated parameter value.
4.6 In practical use, the control signal and load power supply are required to be stable, and the fluctuation should not exceed 10%; otherwise, voltage stabilization measures should be taken.
4.7 During installation and use, keep away from sources of electromagnetic interference and radio frequency interference to prevent relay malfunction and loss of control.
4.8 When a solid-state relay is open-circuited and there is voltage at the load terminal, there will be a certain leakage current at the output terminal. This should be taken into account during use or design.
4.9 When replacing a solid-state relay that fails, try to select a product with the same model or technical parameters as the original to ensure compatibility with the original application circuit and guarantee the reliable operation of the system.
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