The role of solid state relays
Solid-state relays are actually non-contact switching devices with relay characteristics that use semiconductor devices to replace traditional electrical contacts as switching devices. Single-phase SSR is a four-terminal active device, of which two input control terminals, two output terminals, and between input and output. For optical isolation, after the input terminal adds a DC or pulse signal to a certain current value, the output terminal can be converted from an off state to an on state. The dedicated solid state relay can have the functions of short circuit protection, overload protection and overheat protection, and the combination logic curing package can realize the intelligent module required by the user, which can be directly used in the control system.
Features of Solid State Relays
Solid-state relays are non-contact electronic switches with isolation function. There are no mechanical contact parts during the switching process. Therefore, in addition to the same functions as electromagnetic relays, solid-state relays are also compatible with logic circuits, resistant to vibration and mechanical shock, and have unlimited installation positions. , 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.

Advantages and disadvantages of solid state relays
First, the advantages of solid state relays
1. High service life and high reliability: SSR has no mechanical parts, and has solid-state devices to complete the contact function. Since there are no moving parts, it can work in high shock and vibration environments. Due to the inherent nature of the components that make up the solid state relay The characteristics determine the long life and high reliability of the solid state relay;
2. High sensitivity, low control power and good electromagnetic compatibility: solid state relays have a wide input voltage range and low driving power, and are compatible with most logic integrated circuits without buffers or drivers;
3. Fast switching: the solid state relay uses solid state, so the switching speed can be from a few milliseconds to a few microseconds;
4. Small electromagnetic interference: The solid state relay has no input “coil”, no arcing 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 zero current. turn off, reducing abrupt interruptions in the current waveform, thereby reducing the effects of switching transients.
Second, the disadvantages of solid state relays
1. The voltage drop of the tube after conduction is large, the forward voltage drop of the thyristor or bi-phase thyristor can reach 1~2V, and the saturation pressure of the high-power transistor is between 1~2V. The conduction electric ancestor is also larger than the contact resistance of the mechanical contact;
2. After the semiconductor device is turned off, there can still be a leakage current of several microamps to several milliamps, 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 high;
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 no effective measures are taken, the work reliability will be low;
5. Solid-state relays are highly sensitive to overload, and must be protected by fast fuses or RC damping circuits. The load of solid-state relays is obviously related to the ambient temperature. When the temperature rises, the load capacity will drop rapidly.