Contactless Electric Switch

equipment that connects, disconnects, and transfers current in electrical circuits without mechanically closing (opening) contacts but rather by means of an abrupt change of internal resistance in a control element connected in series with the load. Such elements may be magnetic amplifiers with feedback operating in a relay mode, semiconductor devices that alter their resistance as a function of the strength of a control current, certain semiconductor resistances that change their parameters when heated to a specific temperature, or other devices. Unlike a contact apparatus, in the “disconnect” state a small current flows through a contactless electric switch on account of the large but finite internal resistance of the control element when cut off. In the “connect” state this resistance is sharply reduced but nevertheless is between 10 and 50 times greater than the contact resistance of contact apparatus, so that considerably less current overloading is permissible.

Contactless electric switches are put in the protective circuits of electric networks, in automatic control and regulation systems, and in the low-current circuits of electric equipment. Having no closing and opening electrical contacts, they can operate reliably in chemically active, dangerously explosive, dusty, damp, and other similar surroundings. The use of semiconductor devices increases the speed-of-response (down to several microseconds), increases the frequency of the connect-disconnect cycles, and increases the service life. In order to switch high-current electric circuits, a parallel combination of semiconductor contactless electric switches and contact switches is frequently employed. In this case the contactless switch provides all the advantages of a contactless connection (and disconnection), whereas the contact apparatus provides for the long-time passage of high current as well as ensuring thermal and dynamic stability during short circuits.