The time relay is a control device that realizes delay control by using electromagnetic principle or mechanical principle. It is available in a variety of types, including air damping, electric and electronic, and other types. Air-damped time relays are often used in AC circuits in the early days. They use the principle of air throttling through small holes to obtain delayed action. It consists of three parts: the electromagnetic system, the delay mechanism and the contacts. After the relay sensing component receives the action signal, the relay whose actuator (contact) is delayed for a period of time is called the time relay. Currently, the most commonly used time relay is a large-scale integrated circuit type, which utilizes the principle of RC. To achieve delay action. In the AC circuit, the transformer is often used to step down, the integrated circuit is used as the core device, and the output uses a small electromagnetic relay, so that the performance and reliability of the product are much better than the earlier air damping type time relay, and the timing precision of the product can be Control has also improved a lot. With the popularization of single-chip microcomputers, various manufacturers have successively adopted single-chip microcomputers as the core components of time relays, and the controllability and timing accuracy of the products can be completely adjusted by software, so the future time relays will be completely replaced by single-chip microcomputers.