Reed relay
From Wikipedia, the free encyclopedia
As a relay is a switch controlled by an electromagnet, so a reed relay is one or more reed switches controlled by an electromagnet. The contacts are of magnetic material; thus the electromagnet acts directly on them rather than requiring an armature to move them. Sealed in a long, narrow glass tube, the contacts are protected from corrosion, thus are ordinarily plated with silver rather than precious metals. The most common reed relays of the late 1930s through the 80s had two reed switches.
[edit] Memory device
A few million reed relays were used in the 1930s through the 60s for memory functions in Bell System electromechanical telephone exchanges. Usually one of the reeds latched the relay, and the other performed a logic function or more often a memory function. Most reed relays in the Crossbar switching systems of the 1940s through the 1970s were packaged in groups of five. These five reed relays were able to store one decimal digit, encoded in a "two out of five" code for easy validity checking by wire spring relay logic.
[edit] Crosspoint switch
In the Bell System Stored Program Control exchange systems of the 1970s, reed relays were no longer needed for data storage, but tens of millions of them were packaged in arrays for voice path switching. In 1ESS, the cores were made of a magnetically remnant alloy, so the relay could latch magnetically instead of latching electrically. This "Ferreed" method reduced power consumption and allowed both contacts to be used for voice path. The coils were wired for coincident current selection similar to a magnetic core memory, so operating the contacts for one crosspoint would release the other crosspoints in its row and column.
Because the individual crosspoints were more expensive than those of crossbar switches, while the control circuitry was cheaper, reed arrays usually had fewer crosspoints and were more numerous. This required them to be arranged in more stages. Thus, while a telephone call in a typical crossbar exchange like 5XB passed through four switches, a call in a reed system such as 1ESS typically passed through eight.
In the later 1AESS, the reeds themselves were of remnant magnetic material. This "Remreed" design allowed further reduction in size and power consumption. A "grid" of 1024 crosspoints was permanently packaged in a box. Despite the sealed contacts, the lack of precious metals resulted in reed arrays being less reliable than crossbar switches. When one crosspoint failed, the grid box was replaced whole, and either repaired in place or shipped to a repair shop.
Reed arrays passed out of use in the late 1980s, being unnecessary in true electronic (digital) telephone systems. Reed relays continued in their old uses outside the telephone industry.
