I. Overview
Solid-state relays (SSRs), unlike electromechanical relays, are relays without mechanical movement or moving parts, yet they possess essentially the same functions. An SSR is a contactless switching element composed entirely of solid-state electronic components. It utilizes the point, magnetic, and optical characteristics of these electronic components to achieve reliable isolation between input and output. It leverages the switching characteristics of devices such as high-power transistors, power MOSFETs, single-phase thyristors, and bidirectional thyristors to achieve contactless and spark-free connection and disconnection of the controlled circuit.
II. Composition of Solid State Relays
Solid-state relays consist of three parts: an input circuit, an isolation (coupling) circuit, and an output circuit. Based on 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 also have TTL/CMOS compatibility, positive and negative logic control, and phase inversion functions. The isolation and coupling methods between the input and output circuits of solid-state relays include optocouplers and transformer coupling. The output circuit of a solid-state relay can also be divided into DC output circuits, AC output circuits, and AC/DC output circuits. For AC output, two thyristors or one dual thyristor are typically used.
For thyristors, bipolar devices or power MOSFETs can be used for DC output.
III. Advantages and disadvantages of solid-state relays
1. Advantages of solid-state relays
(1) High lifespan and high reliability: SSR has no mechanical parts and uses solid-state devices to complete the contact function. Since there are no moving parts, it can work in high impact and vibration environments. Due to the inherent characteristics of the components that make up the solid-state relay, the solid-state relay has a long lifespan and high reliability.
(2) High sensitivity, low control power and good electromagnetic compatibility: Solid-state relays have a wide input voltage range, low drive power, and are compatible with most logic integrated circuits without the need for buffers or drivers.
(3) Fast switching: Because solid-state relays use solid materials, the switching speed can range from a few milliseconds to a few microseconds.
(4) Electromagnetic Interference: Solid-state relays have no input "coil", no contact arcing or tripping, thus reducing electromagnetic interference. Most AC output solid-state relays are zero-voltage switches, turning on at zero voltage and turning off at zero current, reducing sudden interruptions in the current waveform and thus reducing switching transient effects.
2. Disadvantages of solid-state relays
(1) The voltage drop after conduction is large. The forward voltage drop of a thyristor or a dual-phase thyristor can reach 1~2V. The saturation voltage of a high-power transistor is between 1~2V. The conduction voltage of a general power MOSFET is also larger than the contact resistance of a mechanical contact.
(2) Semiconductor devices can still have leakage current of several microamps to several milliamps after being turned off, so ideal electrical isolation cannot be achieved.
(3) Due to the large voltage drop of the tube, the power consumption and heat generation after conduction are also large. The volume of high-power solid-state relays is much larger than that of 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 the 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-acting fuses or RC damping circuits. The load of a solid-state relay is significantly related to the ambient temperature; as the temperature rises, its load capacity will decrease rapidly.
IV. Differences between solid-state relays and ordinary relays
The comparison between traditional relays and solid-state relays involves many types, so the following comparison uses electromagnetic relays and their corresponding solid-state relays to illustrate their differences:
1. Structural differences: Electromagnetic relays work by utilizing the attraction force generated between the electromagnet core and the armature in the input circuit; solid-state relays use electronic components to perform their functions without mechanical moving parts, and the input and output are isolated.
2. Differences in working methods: Electromagnetic relays utilize the principle of electromagnetic induction, using the force of an electromagnet to control the opening and closing of the circuit. Therefore, when DC current is connected to the coil, the contacts can accept both AC and DC current. 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 classified as DC input-AC output type, DC input-DC output type, AC input-AC output type, and AC input-DC output type.
3. Differences in working conditions: Electromagnetic relays use the attraction between the armatures to turn the circuit on and off. Therefore, they have a slow response, are noisy, and have a limited lifespan. Solid-state relays have a fast response, operate without noise, and have a long lifespan.
4. Operating environment: Electromagnetic relays are generally inferior to solid-state relays in terms of factors such as temperature, humidity, atmospheric pressure (altitude), sand and dust pollution, chemical gases and electromagnetic interference.
5. Differences in electrical performance: Compared with corresponding solid-state relays, electromagnetic relays are simpler to drive but consume more power. They have better isolation and short-time overload tolerance, but are not as good as solid-state relays in controlling high current and high power applications. When controlling circuits with frequent actions, their lifespan is not as long as solid-state relays.
