The solid state relay SSR is a new type of non-contact switching device composed entirely of electronic devices. It uses the switching characteristics of electronic components (such as switching transistors, triacs and other semiconductor devices) to achieve contactless and spark-free switching. And the purpose of disconnecting the circuit. Use photoelectric coupler to realize electrical isolation between input and output.
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 the three-phase load, which can easily control three-phase motors, heaters and other three-phase loads.
2. Single-phase solid state relay. It has two input control terminals and two output terminals. After adding a DC or pulse signal to the input terminal to a certain current value, the output terminal turns from off state to on state.
The three-phase common type SSR uses three bidirectional thyristors as the three-phase output switch contacts, and the three-phase enhanced type uses three sets of anti-parallel one-way thyristors as the three-phase switch output contacts. The enhanced type should be selected for capacitive load and inductive load.
According to the occasion of use, it is divided into zero-crossing trigger and random conduction type. Among them, the zero-crossing trigger type has more applications, which is equivalent to a switch and has low radio frequency interference. Random conduction type, phase shift trigger, mostly used to adjust voltage, there is interference.
The solid-state relay has the following characteristics: It can use weak control signals to implement non-contact on and off for loads with currents of tens of amperes to hundreds of amperes. It has been extended to occasions where electromagnetic relays cannot be used, such as computer terminal interfaces, program control devices, corrosive and humid environments, and certain explosion-proof locations. The disadvantages are the conduction voltage drop, large leakage current, poor AC and DC versatility, poor temperature resistance and overload capability. Pay attention to the following issues during use.
①Rated current should be selected reasonably, and sufficient margin must be left. Pure resistive loads can be selected at 2 times or more of the load current. For cold resistive loads (cold tungsten halogen lamps, inductive, capacitive loads and heavy impact loads, they can be selected at 5 to 7 times the rated current of the load.
②The heating and heat dissipation of the solid state relay will generate heat due to the voltage drop in its conduction. Apply conductive pad silicone grease to the bottom plate of the solid state relay above 10A and the mounting surface of the radiator, and use a fan for cooling when the output current is large.
③Overcurrent of solid state relay. In actual operation, the load short circuit can easily cause the relay to be damaged. The overcurrent protection can choose fast fuse or fast type circuit breaker, which is generally selected at 1.5 times of the rated current.
④There is a certain leakage current when the "contact" of the solid state relay is disconnected, so that the output circuit cannot be truly powered off. Pay attention to the safety of maintenance.