Solid state relay (SSR) is a new type of non-contact switching device, which is composed of all electronic devices. It uses the switching characteristics of electronic components (such as switching triode, bidirectional silicon controlled rectifier and other semiconductor devices) to achieve the purpose of non-contact and non sparking circuit connection and disconnection. Optocoupler is used to realize electrical isolation between input and output.

1、三相交流固體繼電器。它集三只單相交流固態繼電器為一體，以兩個輸入控制端對三相負載，進行直接開關切換，可方便地控制三相電機、加熱器等三相負荷。

1. Three phase AC solid state relay. It integrates three single-phase AC solid-state relays, and uses two input control terminals to directly switch three-phase loads, so as to conveniently control three-phase loads such as three-phase motors and heaters.

2、單相固體繼電器。它有兩個輸入控制端，兩個輸出端。輸入端加上直流或者脈沖信號到一定電流值后，輸出端就從斷態轉為通態。

2. Single phase solid state relay. It has two input control terminals and two output terminals. After adding DC or pulse signal to the input to a certain current value, the output will change from off state to on state.

三相普通型SSR是以三只雙向可控硅作為三相的輸出開關觸點，三相增強型是以三組反并聯單向可控硅作為三相開關輸出觸點。電容性負載、感性負載宜選用增強型。

The three-phase common SSR uses three bidirectional thyristors as the three-phase output switch contacts, and the three-phase enhanced SSR uses three groups of anti parallel unidirectional thyristors as the three-phase output switch contacts. The capacitive load and inductive load should be enhanced.

根據使用場合又分為過零觸發和隨機導通型。其中過零觸發型應用較多，相當于開關，射頻干擾小。隨機導通型，移相觸發，多用于調節電壓，有干擾。

According to the application situation, it can be divided into zero trigger and random conduction. Among them, zero crossing trigger is widely used, which is equivalent to switch and has low RF interference. Random conduction type, phase shift trigger, mostly used for voltage regulation, with interference.

固態繼電器它有以下特點:它可以用微弱的控制信號對幾十安至幾百安電流的負載實施無觸點的通斷。現已擴展到電磁繼電器無法應用的場合，如計算機終端接口，程控裝置，腐蝕潮濕環境及某些防爆場所等。不足之處有導通壓降，漏電流大，交直流通用性差，耐溫及過載能力差。使用過程中應注意以下問題。

Solid state relay has the following characteristics: it can use weak control signal to carry out non-contact on-off for load with current of tens to hundreds of A. It has been extended to the occasions where electromagnetic relay can not be used, such as computer terminal interface, program control device, corrosive and humid environment and some explosion-proof places. The disadvantages are conduction voltage drop, large leakage current, poor AC / DC versatility, poor temperature resistance and overload capacity. Attention should be paid to the following problems during use.

①合理選擇額定電流，必須留有足夠的余量。純電阻負載可按大于等于負載電流的2倍選用，對于冷阻性負載(冷光鹵鎢燈，電感性，電容性負載及沖擊大的負載，可按負載額定電流的5~7倍選用。

① The reasonable choice of rated current must leave enough margin. Pure resistance load can be selected according to 2 times of load current. For cold resistance load (cold light tungsten halogen lamp, inductive load, capacitive load and high impact load), it can be selected according to 5 ~ 7 times of load rated current.

②固態繼電器的發熱與散熱，由于其導通有壓降，會產生熱量。10A以上的固態繼電器底板與散熱器安裝面涂抹導墊硅脂，輸出電流大時用風扇冷卻。

② The heating and heat dissipation of solid state relay will produce heat because of its voltage drop. The bottom plate and radiator mounting surface of solid state relay above 10A shall be coated with guide pad silicone grease, and the fan shall be used for cooling when the output current is large.

③固態繼電器的過流。實際運行中負載短路極易造成繼電器損壞，過流保護可選用快速熔斷器或快速型斷路器一般按額定電流的1.5倍選取。

③ Over current of solid state relay. In actual operation, load short circuit is easy to cause relay damage. Quick fuse or quick circuit breaker can be selected for over-current protection, which is generally 1.5 times of rated current.

④固態繼電器“觸點”斷開時存在一定的漏電流，使輸出回路不能真正斷電，注意檢修安全。

④ When the "contact" of the solid state relay is disconnected, there is a certain leakage current, so that the output circuit can not be really cut off. Pay attention to the maintenance safety.