Investigation of Slow Release Relays Using Reed Switches

Abstract

This research concerns the design of reed relays having fast-operation and slow-release characteristics without resorting to external timing circuits. In an electromagnetic relay of this type, additional iron is required to offset the inherently low permeance of the reed switch, if a long time constant is to be obtained. In the presence of this additional iron, means must be introduced to ensure that rapid closure of the contacts is possible, on the application of excitation. To this end, a second winding, in addition to the operating coil, is included in the relay, and the reed switches are located in the leakage field between these windings. This is the fundamental departure from normal relay practice, in that the flux across the operating gap is only a small fraction of the main flux. A wide range of values of release m.m.f. is normally encountered in commercially available reed switches, and this gives rise to a corresponding range of release times. The relay which forms the subject of this research embodies a feature whereby, in a multi-contact version, the release of the contacts may be synchronised. It is shown that for given operating conditions an optimum design exists for minimum relay volume. A technique for achieving this design is developed in the thesis. Experimental relays are described and their measured performance is shown to agree well with the theory on which the design is based.