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、三相交流固体继电器。它集三只单相交流固态继电器为一体，以两个输入控制端对三相负载，进行直接开关切换，可方便地控制三相电机、加热器等三相负荷。

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、单相固体继电器。它有两个输入控制端，两个输出端。输入端加上直流或者脉冲信号到一定电流值后，输出端就从断态转为通态。

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是以三只双向可控硅作为三相的输出开关触点，三相增强型是以三组反并联单向可控硅作为三相开关输出触点。电容性负载、感性负载宜选用增强型。

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.

根据使用场合又分为过零触发和随机导通型。其中过零触发型应用较多，相当于开关，射频干扰小。随机导通型，移相触发，多用于调节电压，有干扰。

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.

固态继电器它有以下特点:它可以用微弱的控制信号对几十安几百安电流的负载实施无触点的通断。现已扩展到电磁继电器无法应用的场合，如计算机终端接口，程控装置，腐蚀潮湿环境及某些防爆场所等。不足之处有导通压降，漏电流大，交直流通用性差，耐温及过载能力差。使用过程中应注意以下问题。

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合理选择额定电流，必须留有足够的余量。纯电阻负载可按大于等于负载电流的2倍选用，对于冷阻性负载(冷光卤钨灯，电感性，电容性负载及冲击大的负载，可按负载额定电流的5~7倍选用。

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固态继电器的发热与散热，由于其导通有压降，会产生热量。10A以上的固态继电器底板与散热器安装面涂抹导垫硅脂，输出电流大时用风扇冷却。

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固态继电器的过流。实际运行中负载短路极易造成继电器损坏，过流保护可选用快速熔断器或快速型断路器一般按额定电流的1.5倍选取。

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固态继电器“触点”断开时存在一定的漏电流，使输出回路不能真正断电，注意检修。

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.