Palladium Telephone Contacts
Palladium Telephone Contacts
A New Series of Miniature Wire Spring Relays
Relays have been called the building blocks of telephone systems, and in spite of the liberal use of semiconductor devices in the exchanges now building, very large numbers are still needed both for circuit functions as well as for making connections to existing equipment.
The relays for the central offices of the future will, however, need to be smaller than in the past—not only on account of the increasing pressure on space in telephone plant but especially so that they may be suitable for mounting on the printed wiring boards which carry the small diodes and transistors.
To meet these demands a new family of miniature wire spring relays has been designed by the Bell Telephone System and the Western Electric Company of America. Their development and characteristics are described by C. B. Brown (Bell System Technical J., 1967, 46, (1), 117–147), who provides a fascinating account of the efficient way in which the many special requirements have been fulfilled. The method of ensuring constancy of contact pressure, independent of inevitable variations in the angle between the spring wires during assembly, is particularly noteworthy.
In choosing a contact material the long experience of the telephone industry in the use of all the common contact materials in many shapes and sizes was available, and provided firm evidence that palladium has always given the best all-round performance in their switching devices. It has further been established that a thin layer of gold on one only of each set of opposing contacts is completely effective in suppressing polymerisation effects.
The detailed design of the contacts is shown in the diagram. Each circuit is controlled by two independent spring-supported movable contacts operating as a pair against one fixed contact. With twin contacts the risk of “open circuit” failure is virtually negligible. The fixed contact is formed with a cylindrical surface to reduce the possibility of dust particles lodging between the operating faces and is made from a sandwich material. This has a nickel core coated on each side with palladium with thin outer layers of gold. At the centre, the combined thickness of palladium and gold amounts to 0.009 inch.
The movable contacts are made from solid palladium strips, 0.020 inch wide and 0.010 inch thick, welded longitudinally to the phosphor-bronze spring wires.
Adjustment and maintenance of the contact assemblies have been made simple and straightforward and the contacts are readily cleaned by moving a strip of lint-free parchment paper, moistened with a solvent, between the surfaces.
With a non-inductive load or with proper protection on inductive loads, the life of these contacts is confidently expected to be equal to or greater than the mechanical life of the relay—at least 200 million operations.