A solid state relay is a relay with solid state components (such as semiconductor devices) as the key components. Compared with the traditional mechanical relays, the solid-state relays have many differences and advantages. The following will detail the definition, working principle, structure, advantages and application areas of solid-state relays.
1. Definition of a solid-state relay:
Solid state relay (word abbreviation for sr), the whole composition of solid state relay is composed of solid state electronic components, is a new type of touchless switching device above industrial electrical appliances. What is the contact switch, in fact, is the use of electronic components (such as switching triode, bidirectional silicon control and other semiconductor devices) switch characteristics, can achieve the purpose of no spark to touch on and off the circuit, so it is also known as "contact switch". A solid-state relay is a four-terminal active device, in which two terminals are input control ends and the other two ends are output control ends. It has both amplification drive effect and isolation effect
2. Working principle of solid-state relay:
The solid-state relay is a touch-free switch. Its working principle is based on the conductive characteristics of the semiconductor device. It contains a control circuit and a load loop. When the control signal is applied to the control circuit when the relay is operating normally, the control model passes through the semiconductor element in the control circuit, and then the current can pass through the load circuit to realize the switch operation. In fact, the state of the switch can be controlled by changing the size of the input control voltage.
3. Structure of the solid-state relay:
input：
According to the different categories of input voltage, the input circuit can be divided into DC input circuit, AC input circuit and AC / DC input circuit. Some input control circuits also have with TTL / CMOS, compatible, positive and negative logic control and reverse functions, can be easily connected with TTL, MOS logic circuits.
For the control signal with a fixed control voltage, the resistance input circuit is adopted. Control current is guaranteed at greater than 5 mA. For the control signal with a large variation range (such as 3~32V), the constant current circuit is adopted to ensure that the current works reliably at more than 5 mA in the whole voltage variation range.
Isolation coupling
The isolation and coupling of input and output circuit of solid state relay include photoelectric coupling and transformer coupling: photoelectric coupling usually uses photodiode-phototriode, photodiode-bidirectional light-controlled silicon, photovoltaic cell, realizing isolation control between the control side and load side. High frequency transformer coupling uses high frequency signal coupled to the secondary, rectified by wave check, and the logic circuit to form the drive signal.
output circuit
The power switch of SSR is directly connected to the power supply and the load end, realizing the on and off switching of the load power supply. It mainly uses high-power crystal transistors (switch tube-Transistor), unidirectional (Thyristor or SCR), bidirectional (Triac), power field effect tube (MOSFET), insulated gate bipolar transistor (IGBT). The output circuit of solid state relay can also be divided into DC output circuit, AC output circuit and AC / DC output circuit. According to the load type, it can be divided into DC solid state relay and AC solid state relay. Bipolar devices or power field effects tubes can be used for DC output, and two SCR or one bidirectional SCR are usually used for AC output. The ac solid state relay can be divided into single phase AC solid state relay and three phase AC solid state relay. AC solid state relay can be divided into random AC solid state relay and over-zero AC solid state relay according to the timing of conduction and shutdown.
4. Differences and advantages:
(1) Speed: The response speed of a solid-state relay is faster, because it is controlled by semiconductor devices, and it is a mechanical relay without mechanical components. Therefore, the switching speed of solid state relay can reach the microsecond level, much higher than the millisecond level of mechanical relay.
(2) Life: because there is no mechanical structure, the life of the solid-state relay is longer. Mechanical parts of mechanical relays are prone to wear and are prone to failure after long use, while solid state relays do not have this problem, so they can provide a longer life.
(3) Silence: the mechanical relay in the switching process will produce mechanical impact sound, and the solid state relay has no mechanical motion, the solid state relay is no contact switch, there will be no mechanical relay impact sound. This solid-state relay will not produce noise, more silent.
(4) Volume: the difference above the volume, compared to the mechanical relay volume, the solid state relay volume will be smaller, occupy less space. Has a larger portable nature. The small size of the solid-state relay also enables it to achieve a higher circuit density in a limited space, and it can be better applied in some areas with limited space or high weight requirements.
(5) Reliability: the solid state relay is no mechanical contact, is no contact switch. Solid-state relays do not degrade their performance due to mechanical wear and cannot produce no arc phenomenon, which significantly extends their service life. And the switching speed of the solid state relay is very fast, which can reach a response time of several milliseconds to several microseconds, and has high reliability and long life due to its inherent electronic component characteristics. The structure of the solid state relay is fully sealed, with good vibration resistance and mechanical impact resistance, and is suitable for the work of high impact and vibration environment. The shell also has good moisture-proof, mildew, corrosion prevention properties, suitable for all kinds of harsh environment.
5. Application areas:
Solid-state relays are widely used in various fields, including:
(1) Industrial automation control system: industrial automation control system needs high-speed, efficient switch operation, solid state relay can meet these requirements, and can withstand the industrial environment noise, electromagnetic interference and other adverse factors.
(2) Household appliances: solid-state relays are widely used in household appliances, such as electric furnace, microwave oven, electric water heater, etc. Because of its reliability and long life characteristics, it can improve the service life and working efficiency of household appliances.
(3) Medical equipment: Medical equipment has high requirements for accuracy and reliability, and solid-state relays can provide more stable and precise switch control, so it is widely used in medical equipment.
(4) Power electronics: The application of solid-state relays in the field of power electronics is also gradually increasing, such as frequency converters, inverters and charging piles.
In conclusion, solid state relays are relays using semiconductor devices as key components, with the advantages of fast speed, long life, silence, small size and high reliability. It is widely used in industrial automation, household appliances, medical equipment and power electronics. These features make solid-state relays an ideal alternative to mechanical relays.
