With the social and economic development, constantly improve people's living standards, the relay in the works, machinery, electronics, electrical play an important role in the field, the relay is a kind of electronic control devices, it has a control system (also known as the input circuit) and control system (also known as the output circuit), usually used in automatic control circuit, it is actually a smaller current to control a larger current "automatic switch." It plays in the automatic adjustment circuit, security protection, such as the role of the conversion circuit. 70s from the 20th century, before and after the development of a solid-state relay. Solid-state relay is a two input terminals for the other two output terminals for the four-terminal devices, the use of isolation among the input and output devices to achieve electrical isolation. With the widespread application of solid-state relay has normally open contacts and a solid-state switches, switch the birth of this field of science and technology has a very important role, is of universal relevance. This article in the normally open contacts on the basis of the study a normally closed contact of the solid-state switch, in order to adapt to market and engineering requirements.
Keywords：normally closed contact, solid state relays, switches
