Solid State Relay (SSR) is a contactless relay consisting of an input circuit, isolation (coupling), and output circuit.

-Input circuit: According to different categories of input voltage, input circuits can be divided into three types: DC input circuits, AC input circuits, and AC-DC input circuits. Some input control circuits also have compatibility with TTL/CMOS, positive and negative logic control, and anti equalization functions, which can be easily connected to TTL and MOS logic circuits.

-Isolation coupling: There are two ways to isolate and couple the input and output circuits of solid-state relays: photoelectric coupling and transformer coupling. Optoelectronic coupling usually uses photodiodes phototransistors, photodiodes bidirectional thyristors, and photovoltaic cells to achieve isolated control between the control side and the load side; High frequency transformer coupling is the process of coupling a self-excited high-frequency signal generated by an input control signal to the secondary, which is then detected, rectified, and processed by a logic circuit to form a driving signal.

-Output circuit: The power switch of SSR is directly connected to the power supply and load terminal to achieve on/off switching of the load power supply. The main types used are high-power transistor (switching transistor), unidirectional thyristor (SCR), bidirectional thyristor (Triac), power field-effect transistor (MOSFET), and insulated gate bipolar transistor (IGBT).

The output circuit of solid-state relays can also be divided into forms such as DC output circuit, AC output circuit, and AC-DC output circuit. According to the type of load, it can be divided into DC solid-state relays and AC solid-state relays. Bipolar devices or power field-effect transistors can be used for DC output, while two thyristors or one bidirectional thyristor are usually used for AC output. And AC solid-state relays can be divided into single-phase AC solid-state relays and three-phase AC solid-state relays. AC solid state relays can be divided into random AC solid state relays and zero crossing AC solid state relays according to the timing of conduction and disconnection.