What are the advantages and disadvantages of solid state relays

Update:17-11-2021
Solid state relays are non-contact electronic switches with isolation function. There are no mechanical contact parts during the switching process. Therefore, solid state relays have the same functions as electromagnetic relays, but also have logic circuit compatibility, vibration resistance and mechanical shock resistance, and unlimited installation positions , It has good moisture-proof, mildew-proof and anti-corrosion performance, excellent performance in explosion-proof and ozone pollution prevention, low input power, high sensitivity, low control power, good electromagnetic compatibility, low noise and high operating frequency. It has been widely used in computer peripheral interface equipment, temperature regulation, speed regulation, dimming, motor control, electric furnace heating control, electric power and petrochemical, medical equipment, financial equipment, coal, instrumentation, traffic signals and other fields.
Composition of solid state relay
The solid state relay is composed of three parts: input circuit, isolation (coupling) and output 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. The input and output circuits of solid state relays can be isolated and coupled in two ways: photoelectric coupling and transformer coupling. The output circuit of solid state relay can also be divided into DC output circuit, AC output circuit and AC/DC output circuit. For AC output, two thyristors or one bidirectional thyristor are usually used, and for DC output, bipolar devices or power FETs can be used.
Advantages of solid state relays
(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. 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 a few microseconds.
(4) Small 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.
Disadvantages of solid state relays
(1) The voltage drop of the tube after the turn-on is large, 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, general power field effect The on-resistance of the tube is also greater than the contact resistance of the mechanical contacts.
(2) The semiconductor device can still have a leakage current of a few microamperes to a few 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 conduction is 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 the anti-interference ability of electronic circuits are poor, and the ability to withstand radiation is also poor. If effective measures are not taken, the working reliability will be low.
(5) Solid state relays are more sensitive to overload and must be protected against overload with 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.
(6) The main shortcomings are the on-state voltage drop (corresponding heat dissipation measures are required), off-state leakage current, AC and DC are not universal, the number of contact groups is small, and overcurrent, overvoltage, voltage rise rate, current rise rate, etc. Poor indicators.
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