Principles of Slow Release Relay Design

Slow release relays are built and adjusted to provide a time delay between the opening of the coil circuit and the release motion which restores the contacts to their unoperated condition. They are used to assure a desired sequence of circuit operation, as, for example, in maintaining a closed path through the slow release relay's contacts during the pulses sent in dialing a digit, and opening this path during the much longer interval between digits. Slow release relays constitute a minor but significant part of the relay population in an automatic central office: about ten per cent of the total. For economy in manufacture, installation, and use, slow release relays are made as similar to the ordinary or general purpose relays with which they are used as their special requirements permit. Each general structural relay developed for telephone work has had a variant form for slow release use. Thus the Y type1 relay is the slow release form of the U type relay2 widely used in the Bell System, while the AG relay is the slow release variant of the recently developed wire spring3 relay. The circuit functions of most slow release relays permit considerable variation in release time if the minimum delay specified is assured. This 187